Welding
design
DIRK PONS
PhD, Tohunga Wetepanga
1
Welding design
by Dirk Pons
Cover image: Space shuttle Discovery lifts off the launch pad. Any engineering
project has both opportunities and threats. Image NASA
https://en.wikipedia.org/wiki/Space_Shuttle#/media/File:STS120LaunchHiResedit1.jpg Public Domain
Author biography
Dirk Pons, PhD, Master of [Business] Leadership, Master of Science (Medicine),
BScEng (Mech), Chartered Professional Engineer (CPEng: IPENZ), International
Professional Engineer. Professor, University of Canterbury, Christchurch, New
Zealand. dirk.pons@canterbury.ac.nz
Edition 7.18 2024
Except where indicated otherwise, this work is licensed
under a Creative Commons Attribution-NonCommercialNoDerivatives 4.0 International License. This license allows
users to copy and redistribute the material in any medium
or format, for non-commercial purposes.
2
Graphical abstract
Rapid cooling in welds
As the weld cooling is extremely rapid, the microstructure does not have
time to fully follow the Iron/Iron carbide phases, and the carbon is trapped in
the ferrite to create a different microstructure called martensite or bainite.
3
Heat treatment
Heat treatment includes preheating before welding, inter-pass heating during
welding, and post-weld heat-treatment.
Preheating is used to slow down the formation of martensite, and to reduce
residual stresses, hence also to reduce distortion.
4
Heat affected zone
The heat affected zone (HAZ) is the region of unmelted parent material either
side of the weld. This region experiences a graduation in temperature, and
hence variations in microstructure.
Close to the weld, the grains are exposed to high temperature and internal
diffusion of elements can take place. This can cause depletion of alloying
elements, with changes to strength and corrosion resistance. Also the grain
size & morphology changes shape. All these effects generally mean that the
HAZ is weaker than the parent material.
The greater the heat input, the greater the physical extent of the HAZ, and this
is undesirable. Preheat also increases the HAZ size.
5
Welding consumables
Consumable are designated by a code. For example a common electrode for
welding steel by MMA might be E6010. The E refers to an electrode, and first
two numbers (e.g. 60 in this case) denote the ultimate tensile strength of the
filler material in kpsi (e.g. 60 000 psi).
• The third digit shows the positions:
• 1electrode can be used in all positions
• 2electrode can be used in the down flat position and also for
making fillet welds in the horizontal position.
6
Electrode selection
• Select an electrode such that the tensile strength of the electrode
matches or exceeds that of the plate.
7
Stainless steel electrodes
Stainless steel electrodes are designated by a number that includes the
composition of the electrode and its usability.
• - 15 Lime type covering. Electrode can be used with DCRP current
only. Can be used in all welding positions.
• - 16 Either a lime type or a titania type covering. Electrode can be
used with either DCRP or AC current. Used primarily in the down flat
and horizontal welding positions.
• Select an electrode with at least as much chrome as the plate, as chrome
is depleted during welding. For example use E308 to weld 304 to itself.
8
Welding Symbols
Symbols belo w the line ap ply to
the arro w side of the jo int.
Fillet weld
To appl y a wel d to the fa r sid e
of the jo int, mirror the symbo l
ab ove the lin e.
Fillet weld
Wel ds on b oth side s of the jo int
are sho wn with b oth symbols.
Fillet weld
Each type of weld has a symbol which is a stylised representation of the weld
groove or the weld. The designer need not show the physical weld on the
drawing as the symbol alone is adequate. Symbols below the line apply to the
arrow side of the joint. Weld symbols are generally an upside-down
representation of what the weld joint or the prepared edges look like.
9
If weld preparation (groove) is required on only one part, then
make a break in the arrow and point it towards the part to be
prepared. This is unnecessary if one part is obviously to be
prepared (eg J groove on T joint), or on double welds.
BEVEL GROOVE
The arrow is broken when it is necessary to show without doubt which side of
the joint must be prepared.
10
In thick materials it will be necessary to have multiple weld passes to complete
the joint. Each pass may be a different type of weld if necessary, and multiple
symbols will be used. It is also permissible to have different welds on each side
of the joint.
11
The weld symbol is assumed to apply the full length of the weld line, until the
changes in direction at the ends.
Specify weld all round if the assembly will be operating in a corrosive
environment: this seals the weld crevice off against moisture. However, be
aware that this practice also decreases the fatigue strength.
Ribs [drg]
12
13
Stresses in Welded Joints
t = 0.707 h
For fillet welds in tension/compression/axial shear, the relationship is
F
F
σ = t L = 0,707h L
In the usual case where fillets are welded on both sides, it is assumed that each
side takes half the total force.
14
The correct throat for purposes of calculation is the shorter dimension (right).
15
The codes provide an allowable stress range for a weld (eg 115 MPa).
There is a different allowable stress for the parent material (plate), eg 155
MPa.
In the case of the butt welds, the joint will have to be designed to the lower
stress.
16
Weld orientation to load
The basis of the fatigue codes
17
Weld fatigue
Fatigue life of welds is not dependent on material properties such as tensile
strength, but rather it depends on the type of microscopic weld defects. In turn
this depends on the type of weld (butt vs. fillet) and orientation (parallel or
transverse to the loading). Thickness of the plate is also a variable.
For infinite life, the stress value at 107 cycles is taken as the endurance limit for
welded steels in air. There are variants on this, with some codes using 2x10 6
and some having no endurance limit at all.
18
Weld classes
This results in the fatigue strength, more accurately the permissible stress
range, being defined by weld categories A-D, irrespective of the tensile
strength or grade of the material.
19
20
21
20 000
to
Category
100 000
cycles
100 000
to
500 000
cycles
500 000
over
to
2 000 000
2 000 000
cycles
cycles
A
276
221
165
165
B
228
172
117
103
C
193
145
97
83
D
165
117
69
62
E
117
83
48
41
F
117
97
76
62
G
103
83
62
55
Table: Allowable stress range 𝑅𝑓 [MPa] as a function of weld category and
desired fatigue life. Adapted from AWS.
22
Aluminium
The fatigue mechanisms are the same as for steels. Allowable stresses are
approximately one third those of steel (i.e. proportional to the ratio of the
elastic moduli). For aluminium, the crack growth rate is an order of magnitude
faster, so these materials have a lower fatigue limit.
23
Principles of Reliable Welded Joints
Types of joint
Butt
Tee
Corner
Lap
Edge
The designer has to specify both the type of joint, and the type of weld to be
used. Just specifying the joint is not enough, as several different welds can be
used for any one joint.
24
Use a better type of joint
Joint type: poor
Joint type: good
The fatigue strength of a joint depends heavily on the type of joint and weld.
This is shown in the weld classes. Changing the joint can allow a better class,
and therefore a higher loading.
25
Type of weld - Ease of preparation
Which is easiest to make?
Types of weld
Single
Double
Fillet
Square
Bevel groove
V groove
J groove
U groove
The common types of weld are the fillet, square, bevel-groove, and V-groove.
The fillet and square weld require no special joint preparation other than a
reasonably straight edge. For most purposes this can be achieved with a flame
cut edge. The bevel- and V-groove welds require more careful preparation to
cut the inclined faces. Usually a flame cut can provide acceptable surface
quality. The J- and U-groove welds need special preparation by machining.
26
Easy access for
electrode helps make a
higher quality weld
Poor quality welds with
slag inclusions are
likely when access is
difficult
Ensure access
27
Cost of weld material
• Material
• Time
• Defects
28
Position
Flat
Horizontal
Vertical
downwards
Transverse
Vertical
upwards
Overhead
29
The best weld profiles are made in the workshop rather than on site, and in the
flat position.
30
31
Bevel angle choices
60
45
30
1/8"
1/4"
3/8"
This is a design choice. The bigger the angle the more accessible the joint is to
the welding electrode, and therefore the faster the joint may be made.
However a big angle also requires more filler material to close, and this costs
more and takes longer to lay down. A 45 o bevel angle is typical
32
Root opening
This is the initial separation between the members. Its purpose is to permit the
welding electrode to get into the root of the joint. If the root is too small, then
there is a possibility of slag being left at the root. If on the other hand the root
opening is too large, then the molten weld material will fall through (this is
called burnthrough).
33
Root face
Root face
Root face
Root face
Root face
The purpose of a root face is to provide extra material to resist burn through.
The amount of root face is a design choice. It should be noted that preparation
of a root face is more complicated than a feather edge. While a feather edge
may be cut with a torch, a root face requires at least two torch cuts, and
possibly also machining.
34
Move joints out of critical regions
Try and move the joint out of the highly stressed region. This may allow it to go
up a class or two, and thereby carry greater load.
b
R>1.25b
Shaped transition piece can
avoid the above case
The example shows replacement of a fillet weld with some butt welds.
Change
the design
35
UNACCEPTABLE DESIGN
Greater throat
Symmetrical
stress in weld
Acceptable designs for STATIC STRESS
Larger chamfer
reduces stress
concentration
Weld away from
stress
concentration
Larger chamfer and
symmetrical
geometry
Acceptable designs for DYNAMIC STRESS
36
Consider using Back gouging
37
Design for the thinner member
Poor
Good
Poor
Good
The weld size is selected on the basis of the thinner member. Weld size on
drawing is described by the leg length, which is not necessarily the throat.
38
Non-load carrying welds
They typically arise where attachments like brackets are required. As there will
always be some load that strays into these welds, they still weaken the
structure compared to an unwelded part. The larger the attachment, the more
loads stray into it, and the greater the weakening effect. The critical areas are
the weld toes and weld ends.
39
Partial strength welds
Fillet weld
Size
Full strength
leg = 0.75 x plate thickness
50% strength
leg = 0.375 x plate thickness
33% strength
leg = 0.25 x plate thickness
The 50% and 33% partial strength welds are used where strength is not critical,
but rigidity is still required.
In many applications a full strength weld is not required. This situation typically
arises in the machine fabrication industry (e.g. a base for a generator set).
Partial strength welds may be permissible. These are made by providing
incomplete penetration, or smaller weld size.
40
Avoiding distortion
Distortion is not entirely avoidable in welding, due to the hot nature of the
process. However there is quite a bit that can be done, by the designer and the
welder, to reduce the effect
TRANSVERSE SHRINKAGE
Weld cross section
12
12
Process
Transverse shrinkage
Two runs
2,3 mm
Five runs, root gouged
two backing runs
1,8 mm
20
Twenty runs
3,2 mm
35
12
41
a
ANGULAR SHRINKAGE
Weld cross section
12
12
Process
Angular shrinkage
Five runs
3,5 deg
Five runs, root gouged
two backing runs
0 deg
20
Twenty runs
13 deg
35
12
42
Reduce weld size, since large welds provide greater shrinkage force when they
cool. This can be done by:
Avoid over design in welding.
Weld in multiple smaller passes.
Use a type of joint requiring less weld material. For example a double V
is better than a single V in this regard.
Consider sub-assemblies, since it is easier to control the distortion in smaller
parts. When the sub-assemblies are joined (welding or bolting), then the
distortion can be accommodated.
Use jigs and fixtures to hold pieces so that they cannot distort so much.
Weld on neutral axis, or symmetrically about it
Control weld sequence, for example:
weld both sides alternatively
back stepping, which is to weld a short length, leave a gap and weld
another piece, then return and weld the gap, and repeat. This is like a
continuous intermittent weld!
first weld the joints where distortion will be the worst
progress towards the unrestrained parts of the joint
43
Safety considerations
44
45
46
Weld Defects
Slag inclusions
Slag inclusions
Undercut
Crater
Centreline crack
Overfill
Parent metal
crack
Porosity
Underbead cracks
Arc strike
Wormhole porosity
Lack of penetration
Lack of fusion
Underfill
47
48
Glossary
penetration: fused depth of the joint
throat: distance from root to weld face
reinforcement: weld material that is proud (a convex weld has reinforcement)
49
Contents
Graphical abstract ......................................................................................................................................................................... 3
Rapid cooling in welds ................................................................................................................................................................... 3
Heat treatment ............................................................................................................................................................................. 4
Heat affected zone ........................................................................................................................................................................ 5
Welding consumables ................................................................................................................................................................... 6
Electrode selection ........................................................................................................................................................................ 7
Stainless steel electrodes ............................................................................................................................................................... 8
Welding Symbols .......................................................................................................................................................................... 9
Stresses in Welded Joints ............................................................................................................................................................. 14
Weld orientation to load ............................................................................................................................................................. 17
Weld fatigue ............................................................................................................................................................................... 18
Weld classes ............................................................................................................................................................................... 19
Principles of Reliable Welded Joints.............................................................................................................................................. 24
Use a better type of joint ............................................................................................................................................................. 25
Type of weld - Ease of preparation ............................................................................................................................................... 26
Cost of weld material .................................................................................................................................................................. 28
Position ...................................................................................................................................................................................... 29
Bevel angle choices ..................................................................................................................................................................... 32
Root opening .............................................................................................................................................................................. 33
Root face .................................................................................................................................................................................... 34
Move joints out of critical regions ................................................................................................................................................ 35
Consider using Back gouging ....................................................................................................................................................... 37
Design for the thinner member .................................................................................................................................................... 38
Non-load carrying welds.............................................................................................................................................................. 39
Partial strength welds ................................................................................................................................................................. 40
Avoiding distortion...................................................................................................................................................................... 41
Safety considerations .................................................................................................................................................................. 44
Weld Defects .............................................................................................................................................................................. 47
Glossary........................................................................................................................................................................................ 49
1
Introduction ............................................................................................... 55
1.1
Welding applications ...................................................................................................................................................... 55
Joint welding .............................................................................................................................................................................. 55
Build-up welding ......................................................................................................................................................................... 55
1.2
Principles ......................................................................................................................................................................... 55
Making a joint ............................................................................................................................................................................ 55
Energy sources ............................................................................................................................................................................ 56
1.3 Welding Processes ........................................................................................................................................................... 57
Open arc welding ........................................................................................................................................................................ 57
Manual metal arc (MMA) ............................................................................................................................................................ 57
Flux cored wire metal arc (FCAW) ................................................................................................................................................ 58
Submerged arc (SAW) ................................................................................................................................................................. 58
Gas shielded arc welding ............................................................................................................................................................ 58
Metal inert gas (MIG) .................................................................................................................................................................. 58
Tungsten inert gas (TIG) .............................................................................................................................................................. 59
Plasma beam welding ................................................................................................................................................................. 59
Gas fusion welding ...................................................................................................................................................................... 59
Oxygen-acetylene welding........................................................................................................................................................... 59
Hot gas welding .......................................................................................................................................................................... 60
Beam welding .............................................................................................................................................................................. 60
Electron beam welding ................................................................................................................................................................ 60
Laser welding ............................................................................................................................................................................. 60
Resistance fusion welding........................................................................................................................................................... 60
Electroslag welding ..................................................................................................................................................................... 60
Resistance pressure welding ...................................................................................................................................................... 60
Spot welding ............................................................................................................................................................................... 61
Seam welding ............................................................................................................................................................................. 61
Resistance butt welding .............................................................................................................................................................. 61
Flash butt welding ...................................................................................................................................................................... 61
50
Projection welding ...................................................................................................................................................................... 61
Other welding .............................................................................................................................................................................. 61
Arc pressure welding ................................................................................................................................................................... 62
Capacitor discharge stud welding ................................................................................................................................................ 62
Friction welding .......................................................................................................................................................................... 62
Diffusion bonding........................................................................................................................................................................ 62
Magnetically impelled arc butt .................................................................................................................................................... 62
Heated tool ................................................................................................................................................................................ 62
Explosive cladding ....................................................................................................................................................................... 62
Thermit welding.......................................................................................................................................................................... 62
2
Microstructures of welding ....................................................................... 64
2.1
Phase transitions.............................................................................................................................................................. 64
Rapid cooling in welds ................................................................................................................................................................. 65
2.2
Heat treatment ................................................................................................................................................................ 66
Carbon equivalent ....................................................................................................................................................................... 67
Preheat ...................................................................................................................................................................................... 67
2.3
2.4
3
Heat affected zone........................................................................................................................................................... 67
Welding consumables ..................................................................................................................................................... 68
Types of welded joints ............................................................................... 70
3.1
3.2
Joint geometries ............................................................................................................................................................. 70
Types of Weld.................................................................................................................................................................. 72
FILLET WELD ............................................................................................................................................................................... 72
BEAD WELD ................................................................................................................................................................................ 72
FLANGE WELDS ........................................................................................................................................................................... 73
SURFACING WELD ....................................................................................................................................................................... 73
GROOVE WELDS.......................................................................................................................................................................... 74
FUSION SPOT WELD .................................................................................................................................................................... 75
PLUG AND SLOT WELDS............................................................................................................................................................... 75
STUD WELDS .............................................................................................................................................................................. 76
RESISTANCE WELDS .................................................................................................................................................................... 77
OTHER WELDS ............................................................................................................................................................................ 78
Scarf for brazing ......................................................................................................................................................................... 78
Friction welds ............................................................................................................................................................................. 78
Ultrasonic welds ......................................................................................................................................................................... 78
3.3
4
Welding Symbols............................................................................................................................................................. 79
Principles of Reliable Welded Joints........................................................... 82
4.1
General considerations ................................................................................................................................................... 82
Use a better type of joint ............................................................................................................................................................. 82
Type of weld - Ease of preparation ............................................................................................................................................... 83
Cost of weld material .................................................................................................................................................................. 84
Position ...................................................................................................................................................................................... 85
Consider accessibility for the welder............................................................................................................................................. 86
Bevel angle choices ..................................................................................................................................................................... 87
Root opening .............................................................................................................................................................................. 87
Root face .................................................................................................................................................................................... 87
4.2
Move joints out of critical regions .................................................................................................................................. 89
Avoid high-stressed regions ......................................................................................................................................................... 89
Tips for Fatigue design ................................................................................................................................................................ 89
Avoid welding at large scale stress concentrators ......................................................................................................................... 90
Minimise the small-scale stress risers ........................................................................................................................................... 90
Consider using Back gouging ....................................................................................................................................................... 90
Optimise the use of natural metallurgical benefits ........................................................................................................................ 91
Design for the thinner member .................................................................................................................................................... 92
4.3
Using stronger materials ................................................................................................................................................. 92
51
4.4
Non-load carrying welds................................................................................................................................................. 93
Partial strength welds ................................................................................................................................................................. 94
4.5
Avoiding distortion ........................................................................................................................................................ 95
Some tips for avoiding distortion ................................................................................................................................................. 96
4.6
Typical problem welds.................................................................................................................................................... 97
Moisture is trapped in a weld and causes corrosion ...................................................................................................................... 97
Fillet welds fail in the throat ........................................................................................................................................................ 97
Welded copies of casting designs are not always successful. ......................................................................................................... 98
Failure of welds near rotating equipment. .................................................................................................................................... 98
Cracked built-up shafts................................................................................................................................................................ 98
Cracks near fabrication aids (e.g. brackets) on structures. ............................................................................................................. 98
Cracks near arc strikes................................................................................................................................................................. 98
Backing strips cause failure ......................................................................................................................................................... 98
Sudden changes in stiffness ......................................................................................................................................................... 99
4.7
4.8
Safety considerations ..................................................................................................................................................... 99
Weld procedure specification (WPS) .......................................................................................................................... 101
Drawings .................................................................................................................................................................................. 101
Procedure qualification record (PQR) ......................................................................................................................................... 102
Purpose of a WPS...................................................................................................................................................................... 102
Content of a WPS ...................................................................................................................................................................... 102
4.9
Repair construction ...................................................................................................................................................... 108
Stress relief............................................................................................................................................................................... 108
Weld dressing ........................................................................................................................................................................... 108
Remelt the weld toe .................................................................................................................................................................. 109
Compressive residual stresses .................................................................................................................................................... 109
5
Weld discontinuities and defects ............................................................. 111
5.1
Introduction ................................................................................................................................................................... 111
Reduce welding flaws ................................................................................................................................................................ 111
5.2
Classification of Weld Discontinuities ......................................................................................................................... 111
Design Related Discontinuities ................................................................................................................................................... 111
Process Related Discontinuities .................................................................................................................................................. 112
Metallurgical Discontinuities ..................................................................................................................................................... 113
5.3
Causes and elimination of common weld discontinuities ......................................................................................... 114
Undercut .................................................................................................................................................................................. 114
Underfill ................................................................................................................................................................................... 114
Lack of Fusion and Lack of Penetration (LOF & LOP) .................................................................................................................... 115
Slag Inclusions .......................................................................................................................................................................... 115
Porosity .................................................................................................................................................................................... 115
Arc strike .................................................................................................................................................................................. 116
5.4
Detection of Weld Discontinuities ............................................................................................................................... 116
5.4.1
Destructive testing ................................................................................................................................................... 116
5.4.2
Non-destructive testing ........................................................................................................................................... 117
Surface inspection ..................................................................................................................................................................... 117
Ultrasonic testing:..................................................................................................................................................................... 117
Radiographic testing ................................................................................................................................................................. 117
Magnetic testing....................................................................................................................................................................... 117
Liquid penetrant ....................................................................................................................................................................... 117
6
Stresses in Welded Joints ......................................................................... 118
Static stresses ........................................................................................................................................................................... 118
6.1
Weld loading ................................................................................................................................................................. 118
Welds in Tension or compression (F) .......................................................................................................................................... 119
Welds in transverse Shear (V) .................................................................................................................................................... 120
Allowable shear stress ............................................................................................................................................................... 120
Welds in axial shear (P) ............................................................................................................................................................. 121
6.2
Load carrying butt and groove welds .......................................................................................................................... 121
Static loading............................................................................................................................................................................ 122
Groove welds ............................................................................................................................................................................ 122
6.3
Load carrying fillet welds.............................................................................................................................................. 123
52
Application ............................................................................................................................................................................... 123
Load cases ................................................................................................................................................................................ 123
Applications to design ............................................................................................................................................................... 124
Weld Throat ............................................................................................................................................................................. 124
Allowable stress for Fillet welds in static loading ........................................................................................................................ 125
Design ratio .............................................................................................................................................................................. 125
How large to make the fillet weld .............................................................................................................................................. 126
Minimum fillet weld size............................................................................................................................................................ 127
Longitudinal load carrying fillet welds ........................................................................................................................................ 127
6.4
6.5
Allowable stress in weld metal ..................................................................................................................................... 129
Groups of Welds in torsion .......................................................................................................................................... 132
Example: Welded attachment in torsion..................................................................................................................................... 137
6.6
6.7
Groups of Welds in bending......................................................................................................................................... 140
Stresses due to misalignment ....................................................................................................................................... 146
Misalignment ........................................................................................................................................................................... 146
Secondary bending stress due to lateral misalignment ................................................................................................................ 147
Secondary bending stress due to angular misalignment .............................................................................................................. 148
Total stress due to misalignment ............................................................................................................................................... 148
Stress magnification factor due to misalignment ........................................................................................................................ 148
7
Weld fatigue ............................................................................................ 149
Fatigue loading ......................................................................................................................................................................... 149
7.1
Design life ...................................................................................................................................................................... 150
Design procedure ...................................................................................................................................................................... 150
Corrosive Environment .............................................................................................................................................................. 152
High temperature Environment ................................................................................................................................................. 152
Low temperature Environment .................................................................................................................................................. 152
7.2
Classes of Joint .............................................................................................................................................................. 153
Pons design code ...................................................................................................................................................................... 153
Weld groups ............................................................................................................................................................................. 155
7.3
Allowable stress range................................................................................................................................................... 165
Pons allowable stress range in fatigue ....................................................................................................................................... 166
AS3990 permissible stress range ................................................................................................................................................ 166
Fatigue categories per AWSD1.1:2000 ....................................................................................................................................... 167
7.4
Theoretical approach to weld fatigue .......................................................................................................................... 169
A generalised approach ............................................................................................................................................................. 170
Permitted stress for finite life..................................................................................................................................................... 170
Plate thickness effect ................................................................................................................................................................ 172
Stress ratio ............................................................................................................................................................................... 172
Fatigue strength ....................................................................................................................................................................... 173
Not sure about this section – ignore for now .............................................................................................................................. 175
8
Deeper physics of weld fatigue cracks ..................................................... 176
8.1
8.2
Fatigue Crack growth ..................................................................................................................................................... 176
Probability of failure ..................................................................................................................................................... 179
Fatigue strengths of other welded materials .............................................................................................................................. 179
Aluminium ................................................................................................................................................................................ 180
Stress concentration factors ...................................................................................................................................................... 180
Principal stress.......................................................................................................................................................................... 180
9
A qualitative understanding of fatigue mechanism in welds .................... 180
Fracture surface ........................................................................................................................................................................ 180
9.1
Location of fatigue cracks............................................................................................................................................. 180
Fatigue failure at weld toe......................................................................................................................................................... 180
Fatigue failure at weld roots ...................................................................................................................................................... 181
Fatigue failure at weld ripples ................................................................................................................................................... 182
9.2
Factors affecting fatigue strength ............................................................................................................................... 183
53
Residual stresses ....................................................................................................................................................................... 183
Strength of filler material .......................................................................................................................................................... 184
Type of joint ............................................................................................................................................................................. 185
Stress concentration effects ....................................................................................................................................................... 185
Size effects ............................................................................................................................................................................... 185
Weld overfill-reinforcement ....................................................................................................................................................... 186
Weld root conditions ................................................................................................................................................................. 187
Backing bars ............................................................................................................................................................................. 187
Weld orientation ....................................................................................................................................................................... 188
Weld ends ................................................................................................................................................................................ 189
Welding on edges ..................................................................................................................................................................... 190
Tubular frames ......................................................................................................................................................................... 190
Weld quality ............................................................................................................................................................................. 190
54
1 Introduction
1.1
Welding applications
Welding is used for either making a joint, or for building up a part. In general
welding can be applied to metals, and the linear (or thermoplastic) polymers.
Joint welding
Joint welding is the joining together of components by welds, to form assemblies
and structures. The competing technologies for making such joints are threaded
fasteners (bolt & nut), rivets, and adhesives.
Advantages of welding compared to other joining technologies:
 overlapping material not required, so joints are lighter
 full strength of the material can be utilised
 more even stress distribution than localised fasteners
 eliminates crevices where corrosion can occur
 smoother surfaces
 provides reliable pressure tight joints
 can be more economical than other fasteners
Disadvantages of welding compared to other joining technologies:
 heat sensitive materials cannot be welded
 relatively less portable
 application limited to metals and some polymers
 introduces distortion/ residual stress to the structure
 internal flaws may not be visible
 requires operator skill
Build-up welding
In this application, a layer of material is deposited onto a surface. It is typically
done to build up worn areas (resurfacing), or to clad surfaces with a layer of
harder or corrosion resistant material (hard facing).
1.2
Principles
Making a joint
There are several ways of making a welded joint:
55

Fusion welding
The points of contact of the two parts are heated until they melt. If necessary a
filler material of similar composition is added, which melts at the same
temperature as the base materials. This is the basis of many common welding
processes such as ‘stick’, TIG and MIG welding. It is important to note that a
molten zone forms at the joint, and so the local metallurgy is one of cast
microstructures. This has certain consequences as will be explained shortly.

Pressure welding
Here the points of contact of the two parts are heated, but not necessarily until
they melt. While hot, the parts are pushed together. This type of welding results
in large plastic deformation at the joint, and a fine grained microstructure. This
is the basis of resistance and forge welding.

Diffusion welding
The parts are heated, while a vacuum (or inert gas) prevents surface oxidisation.
Under light pressure, diffusion occurs between the parts, creating a bond.
Surface cleanliness is necessary. This type of welding produces very little
deformation.

Cold pressure welding
Pressure is applied to the parts while in the cold condition, to create a join. The
pressure needs to be high enough to disrupt the oxide film on the surface of the
parts. There is considerable plastic deformation, and extensive cold working.
Energy sources
Welding requires energy input, and over the years many different sources have
been used. These are:

gas flame

electric arc

electrical resistance at joint

induced current

electrical resistance in slag

relative motion: friction or ultrasound

electron beam

laser beam

exothermic chemical reaction
56
The most commonly encountered heat sources are gas, arc and electrical
resistance.
1.3 Welding Processes
Of the various types of welding processes available, probably the most familiar
processes are Manual Metal Arc (MMA), Metal Inert Gas (MIG) and Tungsten
Inert Gas (TIG).
The main welding processes are open arc, shielded arc, gas fusion, beam,
resistance, and pressure welding. The basic principles of each are discussed next.
Category descriptions are provided to help the understanding, but these
divisions are somewhat arbitrary.
Open arc welding
The following processes use an arc to generate heat. This arc is open, that is it is
visible.
Manual metal arc (MMA)
Process:
An arc passes between the work-piece and an electrode, to melt
the joint area. Electrode is covered externally with a flux coating,
and both melt.
Application: All types of welds and positions. Used mainly on steels, typically
structures. With appropriate electrodes and heat treatment, the
process can be used on other metals.
Other terms:
Commonly just called "arc welding" although this is a poor
term as many of the welding processes use an arc. Sometimes also
called "stick" welding. In other countries this is called (shielded)
metal arc welding (SMAW or MAW).
Details:
The coating has several functions. Firstly it forms a gas which
shields the melting process from high temperature oxidisation.
Secondly it scavenges contaminants from the molten material, and
floats them on the surface. The layer of solidified slag on the surface
performs the third function of protecting the cooling weld from
oxidisation. The coating can also be formulated to increase the
metal deposition rate. Metallurgical reaction between the slag and
the weld deposit can also be used to advantage. There are two types
of electrode coating: cellulose, and mineral. The common
electrodes use cellulose coatings. These can tolerate some moisture
57
in the coating: they even require a little. The mineral coatings are
used for high strength materials, and they must be baked and kept
dry. The reason is that water dissociates into hydrogen during
welding, and subsequently causes hydrogen embrittlement of the
joint. This appears as under-bead cracks in the base material. The
tensile strength of the electrode should meet or exceed the tensile
strength of the base material. Welding current 15-20A/mm2 of core
wire, at 10-45V.
Flux cored wire metal arc (FCAW)
Process:
An arc passes between the workpiece and an electrode, to melt the
joint area. A tubular electrode is used, containing filler and flux. It is
consumed.
Application: Single run fillet welds, and hard facing
Submerged arc (SAW)
Process:
An arc passes between the workpiece and an electrode, to melt the
joint area. Arc is submerged in granular flux. The otherwise bare
electrode is consumed. Various refinements of the process exist.
Application: Butt and fillet joints, and cladding (e.g. for corrosion resistance).
Horizontal position mainly. Suitable for steel, aluminium. Used in
ship building.
Gas shielded arc welding
The following processes use an arc to generate heat. This arc is shielded by an
inert gas, but still visible. The purpose of the gas is to shield the weld deposit
from oxidisation that would otherwise occur at the high temperature.
Metal inert gas (MIG)
Process:
An arc passes between the workpiece and an electrode, to melt the
joint area. Gas is used to shield the hot weld area. Bare electrode
wire (metal) is consumed as it is fed into the weld. There are several
variations of the process, using other gas mixtures.
Application: All joints, all positions. Suitable for most metals, especially steel,
aluminium.
Other terms:
In the USA this is called gas metal arc welding (GMAW).
58
Tungsten inert gas (TIG)
Process:
Arc melts joint area. Electrode (tungsten rod) is not consumed. Filler
rod/wire is fed into molten joint area. Gas (argon and/or helium) is
used to shield heated areas.
Application: Butt and fillet of welds and in all positions. Suitable for steel,
aluminium, stainless steel, copper. High quality weld.
Other terms:
In the USA this is called gas tungsten arc welding (GTAW).
Plasma beam welding
Process:
Shielding gas is energised into a plasma by an arc from electrode
(tungsten rod) to the nozzle wall. Electrode is not consumed. Filler
powder is fed into molten joint area. Plasma arc welding is similar
except that the arc is between electrode and workpiece.
Application: Joints in high-alloy steels, also cladding of high melting temperature
alloys (e.g. carbides).
In all the arc processes the current has important effects on the weld. Higher
current causes greater heat input, and therefore the electrode melts faster, and
the base material melts to a greater depth (i.e. greater penetration).
The polarity is also important: straight polarity (electrode negative) gives greater
melting of the electrode, and therefore greater deposition, but less penetration
into the base material. Reverse polarity (electrode positive) gives the opposite
effect of greater penetration.
Gas fusion welding
A jet of hot gas is used to provide heat to the weld.
Oxygen-acetylene welding
Process:
Jet of hot gas is produced by combustion, and heats joint surfaces.
Rod added for filler. Other gases may also be used. Burnt gas shields
molten pool.
Application: Used on metals and linear (thermoplastic) polymers. Relatively
large heat input to work pieces, so distortion may be a problem.
Suitable for thin sections. Also used for brazing.
59
Hot gas welding
Process:
Jet of hot gas is produced by combustion/ electrical elements, heats
joint surfaces. Rod added for filler
Application: Used on linear (thermoplastic) polymers only, as relatively low
operating temperature.
Beam welding
A beam of energy (not hot gas/plasma) is used to provide heat to the weld.
Electron beam welding
Process:
Electron beam is used to heat material, in a vacuum
Application: Produces high quality welds (good penetration, low distortion, no
contamination). Expensive equipment.
Details:
Considerable penetration due to focused beam. Local heating
is minimal.
Laser welding
Process:
Laser beam is used to heat material. Laser optical ducts are
evacuated, but working beam is in the open and shielded with inert
gas.
Application: Produces high quality welds (good penetration, low distortion, no
contamination). Expensive equipment.
Details:
Can also be used for cutting, e.g. of metal sheet, composites,
fabric. Highly reflective materials may be a problem.
Resistance fusion welding
Heat is generated by electrical resistance.
Electroslag welding
Process:
Slag is heated by the current flowing through it, and heats the
workpiece. The pool of slag is contained by copper shoes. Filler wire
is fed into molten joint area. The filler wire carries the current. No
pressure need be applied.
Application: Vertical upwards butt joints, also cladding
Resistance pressure welding
Heat is generated by electrical resistance, and pressure is used to complete.
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Spot welding
Process:
Two work pieces are pressed together by copper electrodes.
Current between the electrodes melts (or nearly melts) local spots
in the work pieces due to electrical resistance. Pressure is applied
at the same time. Electrode is not consumed. No filler is required.
Application: Joining metal sheet, (maximum about 12 mm total joint thickness).
Typical use is in automobile body fabrication.
Seam welding
Process:
Similar to spot welding, except that electrodes are discs and can
rotate. A continuous weld is made by resistance heating and
pressure from the electrodes.
Application: Joining sheet metal (maximum about 6 mm total joint thickness).
Resistance butt welding
Process:
Two work pieces are machined flat, pressed together, and heated
by the electrical resistance at the interface. Temperature is below
melting point. The parts are then forced together to complete the
join. An upset (or flash) is formed. No filler is required. (Inductive
heating may be used instead.)
Application: Joining round bar.
Other terms:
Upset welding
Flash butt welding
Process:
Two work pieces (may be rough) are pressed together lightly, while
current flows between them. Heat is generated by the arcing. Then
parts are forced together. An upset is formed.
Application: Butt joints, dissimilar metals.
Projection welding
Process:
Two work pieces are pressed together and heated by the passage
of current. One piece has a pressed projection, and so concentrates
the current (and heat). The projection is flattened in the process.
Application: Joining components to sheet.
Other welding
Next are some welding processes that combine features from several divisions,
or which are unique.
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Arc pressure welding
Process:
Arc forms between stud and flat surface. Once sufficiently hot, the
parts are forced together.
Application: Studs onto flat surfaces
Other terms:
Stud welding
Capacitor discharge stud welding
Process:
Capacitors are used to generate arc and resistance heating between
two parts, under pressure.
Application: Studs, and butt welding wires
Other terms:
Stud welding
Friction welding
Process:
Torsional Rubbing of components with axial load, forms plastic
zone. One part rotates in lathe, other is fixed.
Application: Joins dissimilar materials, eg drill bits.
Diffusion bonding
Process:
Clean, shielded surfaces bond at 70% of the melting temperature.
Application: Low distortion. Accommodates heavy sections
Magnetically impelled arc butt
Process:
Arc is moved along joint by magnetic field, then joint forced
together
Application: Welds thin sections.
Heated tool
Process:
Heated tool put against joint surfaces, which soften and are pushed
together.
Application: Suitable for linear plastics. Similar process used in soldering.
Explosive cladding
Process:
Explosive charge forces layer of material onto substrate
Application: Usually used to clad a surface.
Thermit welding
Process:
Aluminium powder is oxidised, to release heat, which melts a
granular filler material. The molten material then flows into the
weld space
62
Application: Used for joining railway tracks in the field.
Many of these welding processes are relatively specialised and uncommon. The
most common are the arc welding processes, and these are discussed in more
detail following.
63
2
Microstructures of welding
2.1 Phase transitions
Typical weldable steels have low carbon, say 0.2% C. A weld cools extremely fast
– faster than a water quench – because of the heat conduction out into the plate.
Hence at cooling the weld initially comprises austenite, with all the carbon
dissolved therein. As the temperature reaches line AB, and then DB, the
austenite transforms to ferrite. Austenite is face-centred cubic, whereas Ferrite
is body-centred cubic. As ferrite only accepts 0.02% C, the rest of the carbon is
pushed out of the grains to form an intergranular iron-carbide called cementite.
This combination of ferrite and cementite is called pearlite.
64
Rapid cooling in welds
As the weld cooling is extremely rapid, the microstructure does not have time
to fully follow the Iron/Iron carbide phases, and the carbon is trapped in the
ferrite to create a different microstructure called martensite or bainite.
Figure: Iron/iron-carbide diagram for steel.
Martensite structures are strong and brittle, and generally undesirable in a weld
(though valued in tool steel). This effect is shown by Time - Temperature Transformation (TTT) diagrams.
65
Figure: TTT diagram for plain carbon steel, with water quench.
The material alongside the weld – the heat affected zone (HAZ) – is heated into
austenite, and depending on the cooling rate will transform to martensite or
bainite.
2.2 Heat treatment
Heat treatment includes preheating before welding, inter-pass heating during
welding, and post-weld heat-treatment. Preheating is used to slow down the
formation of martensite, and to reduce residual stresses, hence also to reduce
distortion.
66
Carbon equivalent
CE is determined as:
C.E. = C% + 6xMn% + 5x(Cr% + Mo% + V%) + 15x(Ni% + Cu%)
The number of members available to conduct away heat is also important. Butt
joints have 2 members, whereas Tee joints have 3 and hence cool faster. Tables
of Thermal Severity Number (TSN) are used to determine whether or not
preheat is needed.
Preheat
For steels with up to 0.25% carbon and less than 25mm thickness, preheat is
not needed up to a carbon equivalent of CE = 0.4, but for greater CE values a
preheat of at least 100 deg C is needed.
If the temperature of the plate drops too low during welding, it may be
necessary to add inter-pass heating.
For steels with high CE, it may be necessary to apply post-weld heat treatment.
The welding engineer determines the temperature and the cooling rate.
2.3 Heat affected zone
The heat affected zone (HAZ) is the region of unmelted parent material either
side of the weld. This region experiences a graduation in temperature, and
hence variations in microstructure.
Close to the weld, the grains are exposed to high temperature and internal
diffusion of elements can take place. This can cause depletion of alloying
elements, with changes to strength and corrosion resistance. Also the grain size
& morphology changes shape. All these effects generally mean that the HAZ is
weaker than the parent material.
The greater the heat input, the greater the physical extent of the HAZ, and this
is undesirable. Preheat also increases the HAZ size.
67
2.4 Welding consumables
Consumable are designated by a code. For example a common electrode for
welding steel by MMA might be E6010. The E refers to an electrode, and first
two numbers (e.g. 60 in this case) denote the ultimate tensile strength of the
filler material in kpsi (e.g. 60 000 psi). Select an electrode such that the tensile
strength of the electrode matches or exceeds that of the plate.
For high temperature applications, it is also important to match the chemical
composition of electrode and plate. For low temperature application, toughness
is important.
The third digit shows the positions:
1electrode can be used in all positions
2electrode can be used in the down flat position and also for making fillet
welds in the horizontal position.
The last two digits taken together indicate the type of current with which the
electrode can be used and the type of covering on the electrode. 18 means that
the electrode operates satisfactorily on DCRP or on AC current.
EXX10 has a cellulose coating, which is suitable for general purposes on plate up
to about 10mm. For thicker sections or higher tensile strength base material, use
EXX18, which has low hydrogen and hence less risk of cracking. EXX12 has a
titania coating and is used for rapid production of lap and fillet welds.
Stainless steel electrodes are designated by a number that includes the
composition of the electrode and its usability.
68
- 15 Lime type covering. Electrode can be used with DCRP current only. Can
be used in all welding positions.
- 16 Either a lime type or a titania type covering. Electrode can be used with
either DCRP or AC current. Used primarily in the down flat and horizontal
welding positions.
Select an electrode with at least as much chrome as the plate, as chrome is
depleted during welding. For example use E308 to weld 304 to itself.
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3 Types of welded joints
The designer has to specify both the type of joint, and the type of weld to be
used. Just specifying the joint is not enough, as several different welds can be
used for any one joint.
3.1
Joint geometries
There are several types of joint which can be made with welding, and for each
there are several welds that can be used.
Butt joint: may use all the groove
welds
Corner joint: may use fillet weld, all
the groove welds, flange and corner
welds. With this type of weld it sit is
important not to allow prying.
T joint: may use fillet weld, all the
groove welds
70
Lap joint: typically uses fillet weld. This
is not a strong joint because of the
induced bending.
Edge joint: uses flange and corner
welds. Used on thin materials
generally.
71
3.2
Types of Weld
There are several types of weld, as shown below. Also given is the symbol used
for the weld.
FILLET WELD
This is a relatively easy weld to make, as it does not require groove
preparation. The weld can be applied to T joints (illustrated below), corner,
and lap. Double sided joints are possible, and are discussed later.
Fillet weld
BEAD WELD
This weld is similar to a fillet weld, in that it is external and needs only limited
penetration. It is largely used as a backing weld, that is it seals the joint so that
molten metal from the other side weld (not shown) does not escape.
BEAD WELD
72
FLANGE WELDS
These are corner and edge welds. Like the fillet weld they do not require joint
preparation. These welds are intended for sheet materials only.
FLANGE: EDGE WELD
FLANGE: CORNER WELD
SURFACING WELD
Welds are sometimes used to build up a surface, without providing any joining
function. Typically a hard weld material is used to provide wear resistance.
Applications include working bits of earth moving equipment, knives (industrial
sizes) and other high wear situations. Worn shafts are also repaired by surface
welding.
73
GROOVE WELDS
All these welds (except square groove butt) require groove preparation, which is
usually done by machining or grinding. Square welds are unsuitable for very thick
materials, as the weld cannot penetrate through the whole joint, hence the use
of various joint-shaping profiles to get the weld material into the core of the
parts.
There are several types of groove welds, depending on the preparation, as
shown below.
74
The flare bevel and flare V welds are a bit deceptive: they are usually not
machined like that, but arise naturally when a cylindrical part is joined to a plate
(or two cylinders together).
Groove welds exist so that the weld may be made to near the root of the joint.
This also means that the welds often require something to prevent the molten
weld material falling out of the joint. This can be provided by a backing strip, or
by a bead weld on the other side.
All the above figures and symbols are for single sided groove welds. These are a
common form of weld in their own right, especially where there is access to only
one side of the joint.
Double Groove welds are also used: these have both sides prepared, usually
though not necessarily in the same way. They offer the advantage of a
complete weld (stronger), use of less welding material (hence lower production
cost), and less distortion.
FUSION SPOT WELD
This type of weld is often used for joining thin sheet, e.g. motor car panels. This
is a spot weld made with external heat input. The other types of spot weld are
the resistance welded spot, and the projection weld. Some standards
differentiate between these types, but the American standards do not. In
general spot welds are like rivets in function.
PLUG AND SLOT WELDS
A plug weld is like a spot made in a prepared countersunk hole. A slot weld is a
linear weld that is made in a prepared countersunk groove. The same symbol is
used for both, although it should be appreciated that the slot weld is long
whereas the plug weld is local.
75
STUD WELDS
If a stud welding gun is used to weld studs to a substrate, then the symbol shown
above may be used. Note that it is unnecessary to show the actual stud in this
case. If other welds, such as fillet, are used, then rather show the fillet weld
symbol.
76
RESISTANCE WELDS
These welds are made by passing an electrical current through the two
substrates. The electrical resistance generates heat, and at the same time the
substrates are pressed together. The process may be used to generate a spot
(local weld) or a linear weld. In the resistance spot weld it is also possible with
pressing to make a raised area on one substrate where the weld is to occur. This
is called a projection weld. The form and dimensions of the projection must be
shown separately on the detailed drawing. The linear version of the spot weld is
called the seam weld, and it is made by a moving electrode tip. There is also a
resistance butt weld, as shown below. This is also called a flash weld.
77
OTHER WELDS
Scarf for brazing
Brazing also uses the butt weld. However as brazing is weaker than welding, it
may be necessary in thin materials to increase the surface area for bonding. Of
course a lap joint achieves this, but alternatively a scarf joint may be used as
shown below. Note that this is a USA designation.
SCARF JOINT
In SOLDERING AND BRAZING the joint is heated, either with a hot tool, or with a
jet of hot gas, but (in contrast to welding) the parent materials are not melted.
The filler material in rod/wire form is introduced to heat area, where it melts and
flows (capillary action) through the joint. The filler must melt at a lower
temperature than the parent material(s), and must be able to wet them. Typical
applications: electrical component assembly (using lead-silver solder)
Friction welds
These welds are made by torsional rubbing of components with axial load. A
typical application is in drill bits, where a tool steel cutting bit is joined to a softer
steel shanks. The resistance butt weld could be an appropriate symbol, with a
note attached.
Ultrasonic welds
This weld is used for plastics. A vibrating tool (called a horn) presses the work
pieces together. The high frequency vibration causes softening of the material,
and a joint is made. The horn may also be used to press an insert into the
polymer. There are no symbols for these welds.
78
3.3
Welding Symbols
Each type of weld has a symbol which is a stylised representation of the weld
groove or the weld. The designer need not show the physical weld on the
drawing as the symbol alone is adequate. Symbols below the line apply to the
arrow side of the joint. Weld symbols are generally an upside-down
representation of what the weld joint or the prepared edges look like. Note
that a weld arrow is broken if necessary to show unambiguously which side of
the joint must be prepared.
Symbols belo w the line ap ply to
the arro w side of the jo int.
Fillet weld
To appl y a wel d to the fa r sid e
of the jo int, mirror the symbo l
ab ove the lin e.
Fillet weld
Wel ds on b oth side s of the jo int
are sho wn with b oth symbols.
Fillet weld
79
The arrow is broken when it is necessary to show without doubt which side of
the joint must be prepared.
If weld preparation (groove) is required on only one part, then
make a break in the arrow and point it towards the part to be
prepared. This is unnecessary if one part is obviously to be
prepared (eg J groove on T joint), or on double welds.
BEVEL GROOVE
In thick materials it will be necessary to have multiple weld passes to complete
the joint. Each pass may be a different type of weld if necessary, and multiple
symbols will be used. It is also permissible to have different welds on each side
of the joint.
Figure: Double weld. One plate has been bevelled, followed by a bevel groove
weld, and then a fillet weld on top.
The weld symbol is assumed to apply the full length of the weld line, until the
changes in direction at the ends.
The weld notation also provides for size of weld, length, pitch, and other
variables. These are summarised in the figure below.
80
Note that only a weld on the arrow side of the joint has been shown, and this
symbol is always shown below the line for the close side. The break refers to the
close side. A weld may be specified for the opposite side of the joint, by putting
the appropriate symbol above the line: symbols above the line refer to the far
side. The parameters will be assumed to apply to this weld also, unless otherwise
explicitly shown.
Specify weld all round if the assembly will be operating in a corrosive
environment: this seals the weld crevice off against moisture. However, be
aware that this practice also decreases the fatigue strength.
81
4 Principles of Reliable Welded Joints
4.1 General considerations
Use a better type of joint
The fatigue strength of a joint depends heavily on the type of joint and weld.
This is shown in the weld classes. Changing the joint can allow a better class,
and therefore a higher loading.
Joint type: poor
Joint type: good
82
Type of weld - Ease of preparation
The common types of weld are the fillet,
square, bevel-groove, and V-groove.
The fillet and square weld require no
special joint preparation other than a
reasonably straight edge. For most
purposes this can be achieved with a flame
cut edge. The square weld is not
recommended in thick materials for its
poor penetration.
Fillet welds also have poor penetration,
which reduces their fatigue strength. It is
not practical to grind fillet welds to make
them smoother.
Types of weld
Single
Double
Fillet
Square
Bevel groove
V groove
J groove
The bevel- and V-groove welds require
more careful preparation to cut the
inclined faces. Usually a flame cut can
provide acceptable surface quality. The JU groove
and U-groove welds need special
preparation by machining. Bevel is easiest as only one face needs preparation.
The groove welds offer best fatigue strength. Often they are ground down.
Consequently, on the basis of ease of preparation, the fillet and square welds
are the best. However the square weld is limited to use on sheet, as there is
insufficient penetration on thicker members.
83
Cost of weld material
While the fillet weld is easiest to prepare, it requires a relatively large amount of
filler material. The figure below shows welds with the same throat, and therefore
the same strength. Note that a single bevel-groove weld has the same area as a
fillet weld. But to get this throat with a fillet weld or bevel-groove weld requires
more material than with a double bevel-groove weld. More filler material costs
more in consumables, and also increases the welding time. Both of these
increase the cost. Furthermore, more filler material often means greater
tendency to distortion. Of course a double groove weld requires more joint
preparation.
Therefore a fillet weld is not always the most economical type to use. Generally
fillet welds are best in thinner plate, and double bevel-groove welds best in
thicker plate. The precise plate sizes for each will depend on the economics of
the situation.
84
Position
The position in which the weld will have to be made is also a factor in selecting
the type of weld. Where possible the designer should try to arrange for welds
to be made in the easier positions. Vertical and overhead welds are more difficult
than horizontal and flat welds. See the figure for weld positions. For example, it
may be possible to replace an overhead double fillet weld with a single bevelgroove weld, as shown in the figure. Sometimes welds can be made in a
workshop, using an overhead crane to reposition the pieces to keep them in the
flat or horizontal position.
Flat
Horizontal
Vertical
downwards
Transverse
Vertical
upwards
Overhead
The best weld profiles are made in the workshop rather than on site, and in the
flat position.
85
Consider accessibility for the welder
The groove welds are slightly more complicated than the fillet welds, and the
designer has to be aware of several additional effects.
Allow
access
to
welding
equipment: don’t require welds at
inaccessible
locations.
Ask
yourself the question ‘Can I
inspect this weld?’. Remember
that welds can weaken structures,
and so if it can’t be inspected,
then how will you know whether
the weld contributes or detracts
from the overall strength?
Consider moving the weld to a
more accessible location, or
changing the design.
Easy access for
electrode helps make a
higher quality weld
Poor quality welds
slag inclusions are
likely when access
difficult
Ensure access
86
Bevel angle choices
This is a design choice. The bigger the angle the more accessible the joint is to
the welding electrode, and therefore the faster the joint may be made. However
a big angle also requires more filler material to close, and this costs more and
takes longer to lay down. A 45o bevel angle is typical.
60
45
30
1/8"
1/4"
3/8"
Root opening
This is the initial separation between the members. Its purpose is to permit the
welding electrode to get into the root of the joint. If the root is too small, then
there is a possibility of slag being left at the root. If on the other hand the root
opening is too large, then the molten weld material will fall through (this is called
burnthrough). Also, more filler material will be required to make the joint.
Typical root openings are shown in the figure.
Root face
The root face is the flat area at the base of a weld. All U- and J-groove welds have
a weld face. Bevel- and V-groove welds may have either a root face or a feather
(sharp) edge. The purpose of a root face is to provide extra material to resist
burn through. The amount of root face is a design choice. It should be noted that
87
preparation of a root face is more complicated than a feather edge. While a
feather edge may be cut with a torch, a root face requires at least two torch cuts,
and possibly also machining.
Root face
Root face
Root face
Root face
88
4.2 Move joints out of critical regions
Avoid high-stressed regions
Try and move the joint out of the highly stressed region. This may allow it to go
up a class or two, and thereby carry greater load. An extension of this idea is to
avoid sudden changes in section: e.g. taper the pieces, or used use shaped
transition pieces (e.g. at T joints).
b
R>1.25b
Shaped transition piece can
avoid the above case
Change the design
Tips for Fatigue design
The figure below shows some designs for joining different thicknesses.
89
UNACCEPTABLE DESIGN
Greater throat
Symmetrical
stress in weld
Acceptable designs for STATIC STRESS
Larger chamfer
reduces stress
concentration
Weld away from
stress
concentration
Larger chamfer and
symmetrical
geometry
Acceptable designs for DYNAMIC STRESS
Avoid welding at large scale stress concentrators
Do not position welds in areas of high stress concentration for the structure as
a whole.
Minimise the small-scale stress risers
Fatigue strength is improved by avoiding stress concentrators. These include:
bead roughness, slag inclusions, cavities, excess reinforcement, undercut. The
designer should consider machining the welds to flush with the surface, where
fatigue is a problem.
Avoid welds that intersect.
Consider using Back gouging
Joints that are welded on both sides may trap some slag inside, which would
weaken the joint. To eliminate this problem, the root may be cut out before the
second side is welded. The process is called back gouging. It can be done with an
angle grinder, by machining, by chipping, or by a special cutting torch. The
purpose is to expose sound weld material. It is also important that the shape of
the groove be such as to provide sufficient access for the electrode. After back
90
gouging, the gouge is welded up. Back gouging is needed where ever a root face
exists, and a full strength weld is desired. A spacer also has to be back gouged.
Optimise the use of natural metallurgical benefits
Orientate parts when cutting from plate: remember that the best mechanical
properties are in the rolling direction.
Avoid welding near the core of structural sections, as this is where the
segregation of (undesirable) alloy elements occurs.
Segregation in core
Avoid welding here
Stiffening rib
Segregation zones
91
Design for the thinner member
The weld size is selected on the basis of the thinner member. Weld size on
drawing is described by the leg length, which for fillet welds, is not the throat.
The figure shows the application of this principle to some joints. Making a larger
weld than necessary is not any stronger as the strength is limited by the thinnest
member. However a large weld costs more, and will usually cause more
distortion of the assembly.
Poor
Good
Poor
Good
4.3 Using stronger materials
Unfortunately this does not help as much as might be first thought. Observe
that the allowable stress in a weld of any class depends on the class alone, and
not on any material properties (other than the assumption that it is steel). This
is because high strength is only an advantage in preventing cracks starting in
the first place, but welds contain abundant crack-like features already.
However there is still advantage in using higher strength materials. Firstly the
codes are conservative and there is actually some advantage to higher material
strength, even if not acknowledged formally.
There is a second advantage to higher strength materials, which comes into
effect in cases where finite life is being considered.
If a finite life can be tolerated, then the use of a stronger material (and
corresponding electrode) permits a higher stress. Bear in mind that at low life
the permissible fatigue stress for a joint is limited by the static tensile strength.
Take for example a weld that needs to have a life of 2x10 5 cycles. A first
material might have a permissible static strength of 100 MPa, and therefore
the fatigue chart shows that the weld detail would need to be G or better.
However even if the weld were B, the permissible stress would still only be 100
92
MPa, and this would determine the weld size. If a second material was used
instead, with a static strength of 130 MPa, then it would be possible to design
the weld for higher stresses. Using a better class weld would mean that higher
weld stresses could be tolerated. This would permit smaller welds and result in
some welding economy.
Higher strength materials also show greater improvements in fatigue strength
when compressive residual stresses can be generated (see below), although
again these effects are not accounted for in the codes.
4.4
Non-load carrying welds
Non-load carrying welds are those that are not intended to carry the structural
loads. They typically arise where attachments like brackets are required. As
there will always be some load that strays into these welds, they still weaken
the structure compared to an unwelded part. The larger the attachment, the
more loads stray into it, and the greater the weakening effect.
The critical areas are the weld toes and weld ends. Putting the weld parallel to
the loading doesn’t change the possibilities of failure at weld ends.
UNDERCUT
Weld toes are critical areas for cracks
93
Fillet welds are commonly used for nonload carrying welds, because of the
simple weld procedure. Single sided
fillet welds are also sometimes used,
but these are particularly vulnerable to
fatigue cracks starting at the root. The
prying (bending) action on this type of
weld is harmful. Where possible, this
type of weld should be avoided, since
there is no such thing as an entirely load
free weld. The one sided weld also
means that the root crack will not be
externally visible when it forms.
For most common safety critical structures,
there is probably a welding code that could
be used. However codes do not exist for
unusual or non-critical applications. In such
cases welding calculations would be
appropriate. As in engineering design
generally, the designer has to choose a
balance between design effort and residual
risk. In many cases the consequences of
failure are trivial and the application does
not warrant large design effort. In other
cases destructive or non-destructive testing
may be sufficient.
W
L
Weld ends are also
critical
Single sided lap joints
with fillet welds are
weak
Partial strength welds
In many applications a full strength weld is not required. This situation typically
arises in the machine fabrication industry (e.g. a base for a generator set).
Partial strength welds may be permissible. These are made by providing
incomplete penetration, or smaller weld size.
The table below gives some common practices for fillet welds for non-critical
applications. It is assumed that the strength of the weld material matches that
of the members.
Fillet weld
Size
94
Full strength
50% strength
33% strength
leg = 0.75 x plate thickness
leg = 0.375 x plate thickness
leg = 0.25 x plate thickness
The 50% and 33% partial strength welds are used where strength is not critical,
but rigidity is still required.
4.5
Avoiding distortion
Distortion is not entirely avoidable in welding, due to the hot nature of the
process. However there is quite a bit that can be done, by the designer and the
welder, to reduce the effect. First, it’s important to understand why distortion
occurs. Consider a single weld, like the V-groove weld, and assume that it is filled
in one pass. The weld bead is molten initially, but soon solidifies. As it solidifies,
it shrinks, as any material would do. To a first approximation the shrinkage is a
constant percentage of the width of the weld. At the root the width of molten
material is zero, so there is zero shrinkage. At the surface, the width is a
maximum, and so maximum shrinkage occurs here. Therefore a shrinkage
gradient is set up, being greatest at the surface. This causes the members to be
pulled together at the weld surface, and we see this as distortion. The distortion
will be in shortening of the assembly, and in bending.
TRANSVERSE SHRINKAGE
Weld cross section
12
12
Process
Transverse shrinkage
Two runs
2,3 mm
Five runs, root gouged
two backing runs
1,8 mm
20
Twenty runs
3,2 mm
35
12
95
The effect in welds made of multiple passes is similar: the first pass solidifies and
pulls the members together as it does. The next pass on top of that applies a
little more closing force on top of that provided by the pass below. In the end
the greatest joint closing force will be where the last pass is made. Of course the
more weld material is molten, the greater the opportunity for distortion, and
reducing weld size is a good way of reducing distortion. The figures below give
some typical values for distortion.
a
ANGULAR SHRINKAGE
Weld cross section
12
12
Process
Angular shrinkage
Five runs
3,5 deg
Five runs, root gouged
two backing runs
0 deg
20
Twenty runs
13 deg
35
12
Some tips for avoiding distortion
Reduce the heat input (e.g. turn down the current). Also consider changing to a
welding process that gives less heat to the base material or a higher deposition
rate (e.g. change from gas to TIG welding).
Reduce weld size, since large welds provide greater shrinkage force when they
cool. This can be done by:
*
Avoid over design in welding.
*
Weld in multiple smaller passes.
96
*
Use a type of joint requiring less weld material. For example a
double V is better than a single V in this regard.
Consider sub-assemblies, since it is easier to control the distortion in smaller
parts. When the sub-assemblies are joined (welding or bolting), then the
distortion can be accommodated.
Use jigs and fixtures to hold pieces so that they cannot distort so much.
Control weld sequence, for example:
*
weld both sides alternatively
*
back stepping, which is to weld a short length, leave a gap and weld
another piece, then return and weld the gap, and repeat. This is like
a continuous intermittent weld!
*
first weld the joints where distortion will be the worst
*
progress towards the unrestrained parts of the joint
Weld on neutral axis, or symmetrically about it
Preset the structure: bend it the opposite direction before welding starts. Avoid
over stressing.
Use double welds (e.g. double bevel-groove, instead of single bevel-groove).
The two sides tend to pull each other straight.
4.6
Typical problem welds
Here are some weld problems that arise often, and are worth some comment.
Moisture is trapped in a weld and causes corrosion
The obvious way to solve this is to seal the weld so that the moisture cannot
get in. This is often done by welding all round. However welding all round has
its problems too: mainly that the fatigue strength of the weld is lowered. It is
difficult to entirely avoid undercutting the base material when welding all
round, especially at edges. Sealing the joint with adhesive sealant is another
option. In some cases it is possible to avoid the problem rather than solve it, by
changing the joint so that water can flow away rather than be trapped.
Fillet welds fail in the throat
97
Fillet welds naturally have lower strength than other types. Failure in the throat
shows that the weld size is possibly inadequate. Also check what the root
conditions are like, since this is where such failures originate. The main
advantage of fillet welds is not strength but that they are convenient to make.
Welded copies of casting designs are not always successful.
One of the reasons often is because penetration is poor. However even if the
penetration is perfect, welds naturally have lower fatigue strengths than cast
material.
Failure of welds near rotating equipment.
Rotating machines generate load cycles very quickly, pushing the design into
the fatigue region. Even the vibration of an engine may be enough. In many
cases these loads are mistakenly ignored.
Cracked built-up shafts
Worn shafts are often built-up by welding. However this often causes hydrogen
cracking below the weld, since the shaft materials generally contain high
carbon content. Practically any steel can be welded, provided you take enough
care. The trick is in the care, and this might involve pre- and post-heat
treatment.
Cracks near fabrication aids (e.g. brackets) on structures.
Contrary to common perception, any component that is welded onto a loaded
member will reduce the fatigue strength of the main member, even if there is
no load on the attachment. The common culprit is a fabrication aid, like some
little support. These have to be ground off completely, otherwise they become
the source of fatigue cracks.
Cracks near arc strikes
Arc strikes on the side of the weld may cause fatigue failure (see previous).
Backing strips cause failure
Backing strips must be welded with as much care as the main weld, otherwise
they become the weakest link in the chain. Backing strips are not used to
reinforce a joint as many people mistakenly believe. In fact they weaken a joint.
Instead they are used to make welding easier.
98
Sudden changes in stiffness
For example, do not support a thin walled tank on a stiff support. Rather
reinforce the tank wall about the support region, to transfer the load gradually
between the shell and the support. Wherever possible, provide a gradual
change in stiffness (this is a general principle of all design, not just welding).
4.7
Safety considerations
It is relatively easy for the designer to specify a weld. Whether the operator can
make that weld adequately and safely must also be considered. Consider the
following safety aspects at the design stage:
 burns: heat, UV radiation
 eye damage: UV radiation
 poisons: fumes, gas, dust,
 respiratory: fumes, gas, dust
 hearing loss: noise
 explosion: fumes, gas, dust
 shock: electrical equipment
 injury: power tools
 radiation: testing equipment
 preheating can be uncomfortable for the welder
 falls from working at height
Risks are amplified when welding in confined spaces.
It is necessary to provide adequate ventilation. For welding in confined spaces,
it is necessary to use forced ventilation or respirators.
99
Personal protective equipment is required, as is careful work planning and
curation of the workspace.
100
Figure: ‘A man welds a railing for a tuk-tuk in his shop in the Yaowarat
(Chinatown) area of Bangkok, Thailand. ’Image Mark Fischer
https://www.flickr.com/photos/fischerfotos/16111909245 CC BY-SA 2.0
4.8
Weld procedure specification (WPS)
Whatever the means of determining weld size, be it code, calculation, or just
guess, there are still many ways to lay down the weld bead. Therefore a
fabricator who wishes to do welding for a client will need to define exactly how
the welding will be done. Also, the fabricator will need to prove competence to
the client. These functions are taken care of in the following documents.
Drawings
The drawings would be done according to accepted standards. Welding symbols
were discussed in the chapter above, and would be included. The welding
symbols are reasonably well standardised worldwide. It is recommended that
the standard used be written on the drawings, to prevent misunderstanding.
101
Procedure qualification record (PQR)
This document is a test report that proves to the client that the fabricator is
capable of successfully making the particular type of weld. It describes the exact
values of all parameters (see WPS above) that were used, who the welder was,
and the results of mechanical tests (ultimate tensile strength, bend tests,
hardness) and inspection (radiography, macro examination).
Purpose of a WPS
A weld procedure specification (WPS) is a welding recipe. It is a document, in
table format, that describes the key welding parameters for use by the welder.
These parameters are the settings of the welding machine, heat treatment,
position of the weld. It is a proven way to make a reliable weld, and hence gives
confidence that the weld will have the intended structural quality.
Content of a WPS
It specifies all the welding parameters necessary, such as: type of welding
process (MMA, MIG etc.), welding automation (manual, semi-automatic, etc.),
shielding gas (100% Argon etc.), electrical parameters (current, voltage,
polarity), position (horizontal, vertical etc.) , base material, material thickness,
joint preparation (ground, gouging, flame cut etc.), filler material, code used,
backing material (insert, weld pass, strip or none), preheating, post weld heat
treatment, joint geometry (root opening, included angle, etc.), and any other
relevant factors. It may give permissible ranges for the parameters. For examples
see AWS.D1.1:2000 Annex E.
Welding Procedure Specification [Document number]
[Date of writing]
Authorised by: [name of engineer]
JOINT DESIGN
Type: Butt/Fillet
Backing: yes/no
Backing material:
Root opening distance:
Groove angle:
Back Gouging: yes/no
102
BASE METAL
Material specification and grade:
Thickness: 6mm-15mm
WELDING METHOD
Type: MMA etc.
Polarity: AC/DCEP/DCEN/PULSED
Electrical current:
TECHNIQUE
Number of weld passes:
Travel speed:
Travel mechanism: manual/automatic
Interpass cleaning: slag removed
WELD POSITION
Position: all positions OH/VU/VD/H/F
FILLER MATERIAL
Material specification: E6010
Consumable size:
Shielding: flux/gas 100% Ar
Shielding flow rate: 10-20 lt/min
HEAT TREATMENT
Preheat temperature:
Interpass temperature: Min xxx, Max xxx
Postweld heat treatment temperature:
Postweld heat treatment time:
103
Manual metal arc (MMA), or stick welding, or shielded metal arc welding
(SMAW or MAW)
Image Weldscientist
https://commons.wikimedia.org/wiki/File:Shielded_Metal_Arc_Welding.jpg
Creative Commons Attribution-Share Alike 4.0 International used unchanged
104
Submerged arc welding (SAW)
Image NearEMPTiness
https://commons.wikimedia.org/wiki/File:Submerged_Arc_Welding.JPG
Creative Commons Attribution-Share Alike 3.0 Unported used unchanged
105
Metal inert gas (MIG) or gas metal arc welding (GMAW).
Image https://www.dvidshub.net/ Public domain
106
Tungsten inert gas (TIG) or gas tungsten arc welding (GTAW)
Image https://www.dvidshub.net/ Public domain
107
Position
Note that structures that are welded all around, like pipes, impose the full variety
of positions as the welder moves round (unless the structure is rotated). For
more description of positions see AWS.D1.1:200 Figure 4.5.
4.9
Repair construction
It is a reasonably common occurrence that a welded structure develops a
fatigue crack and needs to be repaired. Changing the design geometry of the
structure is not usually cost effective, but some method is sought to prolong
the life of the repaired part. The part will probably not be loaded any heavier
than before, so if the strength of the repaired welds can be increased, then the
life will increase. There are several things that can be done about such
situations. In fact the methods can also be applied to original construction, but
this is not common because there is a cost penalty, and the designer cannot
take advantage of the methods to increase the loading under most current
design codes.
Stress relief
Stress relief is a heat treatment process that attempts to alleviate the tensile
residual stresses that always form in a welded joint as it cools. However,
remember that the process is not always reliable. The design codes do not
acknowledge stress relief, so the designer cannot use the process to increase
the loading on joints. Nonetheless it is a useful process where a joint needs to
have special resistance to fatigue, or is being repaired from a past failure.
Weld dressing
The objective here is to grind the weld to a smooth profile that blends into the
base material, also to remove start/stop discontinuities. This is a post-weld
process. Typical tools are an angle grinder or a rotating burr or sanding tube
108
Remelt the weld toe
Heat is used to remelt the weld toe, so that a smoother profile is created at the
toe (junction between the weld and the plate). This is done either by plasma
dressing (less sensitive to positioning), or TIG (arc has to be precisely
positioned).
Weld dressing works by removing some stress concentrations, and thereby
crack initiation has to take place before cracks can propagate. This increases
the fatigue life. Note that weld dressing the face has no effect if the failure is
going to occur in the weld root. Also, any corrosion adds crack-like features
back to the weld, and undoes the benefits of weld dressing.
Compressive residual stresses
A compressive residual stress is useful since an applied tensile load first has
overcome the compressive stress before it can generate a tensile stress. The
compressive residual stress tends to keep cracks closed, and this prolongs the
fatigue life. There are several ways of introducing compressive residual
stresses. Cold working is a common process, and there are various options:
O
manual peening (hammering) of welded or unwelded material, in regions
where fatigue is expected
O
hammer peening, using a pneumatic or electric machine (very noisy)
O
needle peening
O
shot peening (be careful that the surface finish is not degraded)
Prior overloading is a less well known way of creating compressive residual
stress, but it can be a risky process. The structure is loaded greater than the
maximum service tensile load, and in exactly the same way. By overloading the
structure in this way, the most highly stressed parts of it will go into yield while
the rest is still elastic. Once the overload is removed, the elastic parts will pull
back and compress the formerly plastic regions. The method occurs in proof
testing of vessels, but is otherwise not very practical. The risk occurs in that the
structure may fail during the process. Prior overloading is of no value on either
parts that are subject to compression during service, or smooth parts without
stress concentrations. The value of prior overloading is that it automatically
applies a compressive treatment to all the stress concentrations and flaws in
the whole structure, without the fabricator having to individually identify each
of them.
109
An unusual method is Spot heating. The areas of stress concentration are
identified and an nearby area of unwelded material is rapidly heated (eg with a
flame) and then allowed to cool. The heating causes the spot to expand, but
the expansion is limited by the surrounding material. This, plus the high
temperature, puts the heated spot into compressive yield. Once the heat is
removed, the spot cools and shrinks. In doing so it creates tensile residual
stresses inside the spot, and compressive residual stresses outside. The method
can also be used near the active part of existing cracks.
110
5 Weld discontinuities and defects
5.1 Introduction
Modern practice in welding is to avoid the use of the word defect. Defect implies
a fault that must be rectified to make a component fit-for purpose. It is
impossible to produce a weld that is 100% defect free. Engineers that demand
this on drawings and in specifications are firstly expecting the impossible, and
secondly going to result in the welded fabrication becoming prohibitively
expensive to produce.
Fitness-for purpose is a better concept. To this end, the welding industry uses
the word discontinuity rather than defect to describe some imperfection found
in a weld.
The decision as to when the discontinuity becomes classified as a defect (i.e. in
need of repair) is made according to code requirements, customer demands and
the effect of the discontinuity on fitness-for purpose of the component. The
bottom line is that repairing a non-relevant discontinuity will often do more
harm to the component than if it was simply left alone - over and above the fact
that conducting a repair is costly and time-consuming.
Reduce welding flaws
It will be noticed that some of the classes require NDT, and if this is not
provided, then the joint is downgraded. Therefore reduction of welding flaws
(and the means to prove that this has been achieved), is another way of
permitting greater weld stress.
5.2
Classification of Weld Discontinuities
The best example of classification of weld discontinuities is the one which was
developed by the American Society of Metals (ASM). It states that discontinuities
can be classified into one of three groups:
 Design related
 Process related
 Metallurgical
Design Related Discontinuities
These are discontinuities resulting from :
111




Incorrect joint design
Poor structural details
Incorrect choice of joint design and location for a given application
Undesirable changes in cross-section at a joint
Process Related Discontinuities
Some of the typical discontinuities are listed below and shown graphically:
 Slag inclusions
 Porosity
 Undercut
 Centre-line crack
 Under bead cracks
 Lack of Penetration (LOP)
 Craters (found at the end of a weld pass)
 Overfill
 Arc strikes
 Lack of Fusion (LOF)
 Underfill
112
Slag inclusions
Slag inclusions
Undercut
Crater
Centreline crack
Overfill
Parent metal
crack
Porosity
Underbead cracks
Arc strike
Wormhole porosity
Lack of penetration
Lack of fusion
Underfill
Figure: Weld discontinuities of the process variety (Image D Pons).
Metallurgical Discontinuities
These include all types of cracks found as a result of welding, as well as
undesirable micro structures such as hard phases (Martensite) and segregation
(in which alloying elements in the weld do not have sufficient time to mix
completely before solidification). Cracks include:
 HAZ cracks (also known as hard cracks) , which are normally associated
with the presence of Hydrogen
113
 Junction zone cracks, which are caused by differences in thermal
expansion between parent metal and weld metal. As the name implies,
these cracks are found in the zone where parent and weld metals meet.
 Lamellar tearing, which occurs in parent metal, and is the result of
inadequate ductility as it expands and contracts during welding.
 Liquation cracks (better known as hot cracking), caused by segregation
within the cooling weld deposit. These are also found during the welding
of stainless steels.
5.3
Causes and elimination of common weld
discontinuities
Since there are too many types of discontinuities to discuss all of them here, we
will concentrate on a few of the more commonly found examples.
Undercut
This is typically a small notch found at the toe of a weld. It poses serious
problems as regards weld quality goes, as it reduces impact strength, fatigue life
and low temperature service behaviour. It is caused by excessive heat
concentration at the area in which the undercut was found. To minimize
undercut:
 Decrease welding current, travel speed and electrode diameter (any or all
of these)
 Alter electrode angle ( to move heat concentration away from affected
area)
 Avoid weaving the electrode from side to side
 Maintain constant travel speed
 Use proper backing on the joint
Underfill
This implies that the weld cross-section is smaller than that of the adjacent
parent metal, automatically making the weld a point of weakness. To minimize
underfill:
 Reduce voltage
 Reduce travel speed
 Reduce root opening of joint
114
Lack of Fusion and Lack of Penetration (LOF & LOP)
LOF implies that the weld metal did not melt the adjacent parent metal. It occurs
when insufficient heat has been absorbed by the parent metal. When the weld
metal does not penetrate to the bottom of the weld joint, LOP occurs. It is caused
by incorrect welding technique or by inadequate root gap. To minimize LOF and
LOP:
 Increase amperage
 Reduce electrode diameter
 Reduce travel speed
 Change electrode angle
 Ensure adequate root gap
Slag Inclusions
These are found when welding with processes that use fluxes to shield the weld
metal from the atmosphere (Shielded metal arc, Submerged arc, Flux-core arc).
Slag inclusions can occur randomly as isolated inclusions or as long Astringer@
types. Multi-pass welds are more prone to slag becoming trapped in the weld
deposit than single pass welds. Slag entrapment is caused by:
 Inadequate inter-pass cleaning
 Excessively rapid cooling
 Welding current parameters set too low (i.e. insufficient heat)
 Poor joint geometry
 Wrong type of electrode coating
By correcting any and all of these problems, slag entrapment can be avoided.
Porosity
This consists of pores or cavities in the weld deposit as a result of gases evolved
during welding becoming trapped in the weld metal. Elements such as Sulphur
in the parent metal, and the presence of contaminants like paint, grease, oils,
rust and water on the joint preparation, as well as contamination of fluxes on
electrodes, and inadequate or contaminated shielding gases, are all common
causes of porosity. To minimize porosity:
 Ensure that no contaminants are present on joint preparation or in
electrode coating
 Ensure that electrode coatings and shielding gases are moisture free
 Use short arc lengths
 Reduce travel speed
 Increase amperage
115


Provide adequate shielding from atmosphere
Use parent metal with lowest possible Sulphur content
Arc strike
Arc strikes are where the welder attempts to get the arc to form, by tapping
elsewhere than the weld region. These bursts of energy cause extremely rapid
heating and cooling, hence the formation of martensite. Arc strikes are
therefore initiators for fatigue cracks, and should be avoided or ground out or
heat treated.
5.4
Detection of Weld Discontinuities
Quality checks fall into two major groups: destructive and non-destructive.
Non-destructive testing is widely utilized in all types of welding applications,
whereas destructive tests are normally only carried out on components which
are safety-critical or will be under high stress in service.
It is important to realise that there is no such thing as 100% reliable inspection
- at least not in the real world! The accuracy of inspection results depends too
much on the human factor - the inspector themself. Their level of training,
frame of mind, time available for inspection and working and personal
environmental conditions will all influence their performance enormously.
Inspection cannot catch every discontinuity, and moreover adds cost. It is
better to emphasise the prevention of defects by ensuring that sound welding
practices are utilized at all times, and adequate training of welders.
5.4.1
Destructive testing
This includes impact, tensile, side-bend, drop-weight and fatigue tests amongst
others.
U-Bend tests are a common and easy form of inspection. See
https://www.youtube.com/watch?v=1kD1wsc56dk
116
5.4.2
Non-destructive testing
Surface inspection
Check for neatness, porosity, slag inclusions, excessive splatter, excessive
reinforcement, undercut. These checks may be done with the naked eye, and do
not involved any great expense.
Ultrasonic testing:
Internal flaws reflect sound, and the time taken for the echo to return is
displayed on an oscilloscope. The method requires specialised training in order
to set the apparatus and correctly interpret the results. Numerous other echoes
are obtained, and the real flaws have to be distinguished from the artefacts.
Otherwise the method works reasonably well, and is in common use. The
equipment is portable. As with many of the testing methods, one of the
limitations is that flaws must be suitably orientated with respect to the probe.
Or, to put it the other way, it must be possible to inspect the weld from various
angles in order to catch all the flaws, and this is not always possible.
Radiographic testing
This produces a permanent record of the weld, which is useful for certification
purposes. However the method is not without hazard due to the high energy Xrays used. Shields are needed to prevent stray radiation, and so the method can
be relatively cumbersome. Technically, the major limitation is that the X-ray
must be correctly aligned with the flaw in order to show it.
Magnetic testing
Magnetised materials attract particles around cracks. However the method can
only be used on magnetic material, and only detects surface defects. It is a
relatively simple method to use.
Liquid penetrant
Coloured dye penetrates cracks and shows them up. Can only show surface
flaws.
117
6 Stresses in Welded Joints
The designer need to consider the stress in the base metal, and separately the
weld. It is important to differentiate between the stress in the material (base
metal or weld) which depends on the loading and geometry of the structure,
the material strength with depends on the weld type and orientation, and weld
filler material.
For structures that undergo plastic deformation, such as roll-over protection
for vehicles, the weld tensile strength needs to be stronger than the base metal
tensile strength. This is necessary for the incoming loads, and hence energy
absorbance, to be taken by deformation of the entire structure, not only the
welds.
Thus in general the weld material is often selected to give a greater strength
than the base material. Thus static failure is expected in the base material.
However, fatigue failure will still probably occur in the weld, since there are still
metallurgical imperfections in the weld however good the surface might be.
Static stresses
Static loading refers to stresses that do not change with time. This is typical of
dead weights. Structures that are NOT static include those loaded cyclically, or
in shock, or with vibration, or with changing loads. See the fatigue section for
these cases.
6.1
Weld loading
Welds may be loaded with force in one or more of tension-compression,
torsion, bending or shear. Remember that the measurement of weld size is the
throat t. The welds that we shall be considering are the groove and fillet types.
118
F
t throat
Tension/compression load on joint
h leg
V
Transverse (shear) load on joint
fillet weld
P
Axial shear (parallel) load on joint
L
Welds in Tension or compression (F)
Welds that are in tension (or compression) are designed according to the
average direct stress in the throat. For groove- and fillet-welds it is
F
σ=
tL
where
F
tension (or compression) force
t
throat
L
length of weld
Where throat is
𝑡 = 𝑝𝑙𝑎𝑡𝑒 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑓𝑜𝑟 𝑔𝑟𝑜𝑜𝑣𝑒 𝑤𝑒𝑙𝑑𝑠
𝑡 = 0.707 𝑥 𝑤𝑒𝑙𝑑 𝑙𝑒𝑔 𝑙𝑒𝑛𝑔𝑡ℎ 𝑓𝑜𝑟 𝑓𝑖𝑙𝑙𝑒𝑡 𝑤𝑒𝑙𝑑𝑠
NB: Load carrying Fillet welds are not to be used singly for anything safetycritical, as the loading tends to open the root. Instead the joints are welded on
both sides, and it is assumed that each side takes half the total force.
119
Welds in transverse Shear (V)
Welds that are in shear are designed according to the average shear stress in
the throat. For groove welds it is
V
τ=
tL
and for fillet welds
V
τ=
hL
where
V
shear force
h
leg length
t
throat
NB: Load carrying Fillet welds are not to be used singly, as the loading tends
to open the root. Instead the joints are welded on both sides, and it is assumed
that each side takes half the total force.
Allowable shear stress
For static loading the allowable shear stress for weld material in a fillet or
partial penetration bevel-groove weld is
τ = 0,30 Rm
Example of application, for a fillet weld with equal legs, the allowable force is
F = 0,30 Rm A
= 0,30 Rm a.L
= 0,30 Rm 0,707 w.L
If the strength of the weld material is Rm = 100MPa, and the leg length is w =
10mm, and the length of the weld is L = 150mm, then the permissible force on
the joint is
F = 0,30 x 100 x 106 x0,707 x 10 x 10-3 x 150 x10-3 = 31,82 x 103 N
where Rm is the tensile strength of the weld material (minimum value), as
determined from the Table: ‘Permissible stress of weld’.
Knowing the electrode type, the designer can determine the weld metal
strength (see the chapter on welding consumables for details), and then
determine the permissible static shear stress with the above equation. Then
this shear stress is related to the throat area A of the weld and the applied
force F by
τ = F/A
120
from which the weld size or length may be determined, or the permissible
force.
Welds in axial shear (P)
This is loading parallel to the axis of the weld. The shear stress is
P
τ=
tL
for both groove-and fillet-welds. P is the parallel shear force.
t throat
F
Tension/compression load on joint
V
Butt weld
P
Transverse (shear) load on joint
L
Axial shear (parallel) load on joint
NB: Load carrying Fillet welds are not to be used singly, as the loading tends
to open the root. Instead the joints are welded on both sides, and it is assumed
that each side takes half the total force.
6.2
Load carrying butt and groove welds
The butt and groove welds are made through the thickness of the plate. The
weld has nominally the same stress as the parent plate.
The extra height of the weld above the surface is called reinforcement.
However it is anything but that, rather it creates a stress concentration. Hence
there is great advantage in machining welds down to base material. The
trouble and cost of doing so usually prevents this being done. Underfill is also a
bad weld.
121
Static loading
Butt welds can be treated as parent material for strength calculations. Partial
penetration butt welds that are welded from one side only (i.e. single bevel, V,
J. U) must not be allowed to be loaded in such a way as to open the root.
Throat thickness is the minimum depth of penetration. For a U or J weld it is
the depth of the groove. For a Bevel or V weld it is the specified penetration
less 2 mm. For a partial penetration butt weld, the specified penetration must
be at least:
a=2 t
where t is the thickness of the thinner member. However it is not good practice
to use partial penetration butt/groove welds, and these are no allowed in some
standards.
Groove welds
Loading on weld
Groove and butt welds
– FULL PENETRATION
Tension normal to the
effective area of the
weld
Allowable static
stress (strength)
in welded base
material before
any safety factor
Requirements for filler
material
Tensile stress of
base material
Rm
Compression
Tensile stress of
base material
Rm
Filler metal tensile
strength greater than
or equal to base metal
tensile strength
Filler material may be
at most 70MPa weaker
than base metal
Tension or
compression parallel to
the axis of the weld
Tensile stress of
base material
Rm
Filler material may be
at most 70MPa weaker
than base metal
Shear
Lower of 0.3 Rm
Tensile stress of
base material,
and 0.4 Re yield
strength
Filler material may be
at most 70MPa weaker
than base metal
Fatigue
category
Fatigue strength
at 107 cycles
Partial (incomplete) penetration butt welds are a bad idea, as they are difficult
to verify. They are not permitted in some codes. However AWS.D1.1:2000 does
allow incomplete penetration groove welds.
122
6.3
welds
Load carrying fillet
Leg length h
Application
Fillet welds are often used for load
carrying joints, particularly in T joints. The
T joint is relatively common in mechanical
design, since it can join various cut plates
together. The joint does however have
the disadvantage of having a ready-made
crack at its base, in the form of the
incomplete penetration. Of course there
Weld throat and leg
are ways of making the joint so that the
penetration is complete, but these
require an extra effort that might not always be worthwhile.
Throat t
Load cases
A loaded fillet weld develops shear stresses across what is called the throat.
The allowable stress is the strength of the weld filler material, for which you
need to consult tables of electrodes/filler material.
F
t throat
Tension/compression load on joint
h leg
V
Transverse (shear) load on joint
fillet weld
P
Axial shear (parallel) load on joint
L
123
Applications to design
For fillet welds in tension/compression/axial shear, the relationship is
F
F
σ= =
t L 0,707h L
With F force, t throat, L weld length, h leg length.
In the usual case where fillets are welded on both sides, it is assumed that each
side takes half the total force.
In the case of a fillet weld, it is necessary to determine the leg length h, since
this is the dimension shown on the drawing.
Weld Throat
There are two important dimensions in fillet welds, weld throat and leg length.
The throat is the vital dimension for design. It is the shortest distance from the
root to the face. The overfill (reinforcement) is not included. Penetration is also
not counted (except in the submerged arc process). The leg length is the size of
the triangle that fits inside the weld section. Note that the penetration and
reinforcement are neglected. The figure illustrates the legs and throat of a
fillet weld.
Figure: When multiple welds are used, then the total throat is NOT the sum of
the individual ones. Instead, it is necessary to find the new shortest line through
the weld. Sum of individual leg lengths (left) for a compound weld (groove and
fillet), and throat (right). The correct throat for purposes of calculation is the
shorter dimension (right).
124
Note that for equal leg fillet welds (the most common type of fillet weld), the
throat t is related to the leg length h by the following expression.
t = 0,707 h
For other types of weld the relationship may need to be changed.
While design calculations use the THROAT, usually the leg length is more easily
measured than the throat, and therefore it is the LEG that is shown on
drawings. For butt welds the throat is the same as the plate thickness.
Allowable stress for Fillet welds in static loading
The allowable stress is 0,3 Rm where Rm is the ultimate tensile strength of the
weld metal, with a limit of twice the yield strength of the parent metal.
Alternatively, the allowable stress in a fillet weld, is 130 MPa for Grade 43 steel,
and 160 MPa for Grade 50 steel.
Stress in a fillet weld is calculated as the vector sum of the stresses due to the
forces and moments in the weld, based on the throat thickness.
Loading on weld
Allowable static
stress (strength)
in welded base
material before
any safety factor
Requirements for filler
material
Tension normal to the
effective area of the
weld (shear)
Tension or
compression parallel to
the axis of the weld
0.3 Rm Tensile
stress of base
material
Tensile stress of
base material
Rm
Filler material may be
at most 70MPa weaker
than base metal
Filler material may be
at most 70MPa weaker
than base metal
Fatigue
category
Fatigue strength
at 107 cycles
Design ratio
The codes provide an allowable stress for a weld (eg 115 MPa). There is a
different allowable stress for the parent material (plate), eg 155 MPa. In the
case of the butt welds, the joint will have to be designed to the lower stress.
125
However for fillet weld joints, the fillet weld may be designed according to the
allowable weld stress, and the plate may be designed to the allowable plate
stress. Therefore, for fillet welds the stresses in the plate and the fillet are not
necessarily the same.
The design ratio is the ratio of plate stress to weld stress. If the designer
chooses, the weld stress may be arranged to be the same as the plate stress,
that is a design ratio of 1,00. In this case the leg length (for an equal leg fillet
weld, welded both side) would h = 0,707B where B is the plate thickness. This
is a somewhat conservative approach.
How large to make the fillet weld
Fillet welds require some care because of their characteristics under fatigue
loading. The location of failure depends on the ratio of weld leg size to plate
thickness. The optimum is when there is sufficient weld material that the
failure starts at the weld toe rather than at the weld root.
Cracks in the weld root are very difficult to detect, because they often occur
inside, hence failure can occur unexpectedly. In contrast toe cracks are easier
to detect, e.g. by discolouration or dye penetrant.
This requires that the leg length be
h = 0,707 B rd
where B is plate thickness, rd = plate stress / weld stress, and is called the
design ratio. Generally we design for the thinner plate.
126
An alternative is to use partial penetration fillet welds. Their advantage is that
they do not use as much filler material to gain a given throat, and they are
therefore less expensive. However their problem is that it is difficult to make
sure that the penetration is correct, and this requires testing (which increases
the expense). Therefore partial penetration fillet welds are not used as often as
might be thought.
If the penetration is increased, to the point where the two weld beads merge,
then the joint is basically a transverse butt weld.
Minimum fillet weld size
The AISC specify a minimum size of fillet weld, based on the thickness of the
thicker part to be joined. However the weld does not have to be larger than the
thickness of the thinner member. Dimensions are in inches. Note 1" = 25,4 mm
Thickness of thicker member
Minimum fillet size
to 1/4" inclusive
over 1/4" to 1/2"
over 1/2" to 3/4"
over 3/4" to 1"
over 1" to 2 1/4"
over 2 1/4" to 6"
over 6"
1/8"
3/16"
1/4"
5/16"
3/8"
2"
5/8"
Longitudinal load carrying fillet welds
These are welds that are used to make lap joints, and for cover plates. Some
strength improvement may be had by welding the end of the lap. But welds on
127
the free edges should be avoided as they severely weaken the fatigue strength
of the joint. It is true that they seal the joint from internal corrosion, but they
also weaken it.
The weld ends are common places where fatigue crack start.
Often the cover plate is much narrower than the main plate, and in this case it
is necessary to determine an effective width of the main plate for stress
calculations. A common method (but totally arbitrary), is shown in the figure.
128
6.4 Allowable stress in weld metal
The permissible stress in static loaded welds is shown in the table below. Note
that the permissible stress depends on the type of joint, and the direction of
loading.
Most codes requires that the strength of the weld material be at least that of
the parent material. Generally it is better to go for a stronger grade of weld
material, to push static failure into the base material rather than the weld.
Table: Allowable static stresses in weld material (as opposed to base material),
roughly based on AWS D1.1:2000 and also included in Standard Handbook of
Machine Design, SHIGLEY J and MISCHKE C.
Weld loading
Permissible stress in weld metal
Groove weld: complete penetration
Tension normal to throat
same as base metal
Compression normal to
throat
same as base metal
Tension or compression
parallel to weld axis
same as base metal
Shear on throat
0,30 x nominal tensile strength of weld
metal, except stress on base metal shall not
exceed 0,40x yield strength of base metal
Groove weld: partial penetration
Compression normal to
throat
same as base metal (see code)
Tension or compression
parallel to weld axis
same as base metal
129
Shear parallel to weld axis
0,30 x nominal tensile strength of weld
metal, except stress on base metal shall not
exceed 0,40x yield strength of base metal
Tension normal to throat
0,30 x nominal tensile strength of weld
metal, except stress on base metal shall not
exceed 0,60x yield strength of base metal
Fillet weld
Stress on throat, regardless of 0,30 x nominal tensile strength of weld
direction
metal, except stress on base metal shall not
exceed 0,40x yield strength of base metal
Tension or compression
parallel to weld axis
same as base metal
Plug and slot welds
Shear of joint
0,30 x nominal tensile strength of weld
metal, except stress on base metal shall not
exceed 0,40x yield strength of base metal
For Allowable stresses from AWS D1.1.200, see Public Resource
https://law.resource.org/pub/us/cfr/ibr/003/aws.d1.1.2000.pdf
Electrode filler material needs to have a tensile strength the same or greater
than the base material. This is called ‘matching’ the base metal. For ASTM steel
grades, see corresponding electrodes at Table 3.1 AWS D1.1.200
(https://law.resource.org/pub/us/cfr/ibr/003/aws.d1.1.2000.pdf)
E6010: E refers to an electrode, 60 kpi is the ultimate tensile strength, 1 refers
to all positions, 10 refers to other factors of cover and current.
130
Table: Steel consumable electrode with samples of designations.
Electrode Tensile
Example
Covering
Positions
X
refers
to
F – flat
strength
another
designation for
chemical
composition of
the metal
E60xx
E70xx
E80xx
E90xx
E100xx
E110xx
E120xx
H – horizontal
VD – vertical down
V – vertical (down
or up)
OH - overhead
60 ksi =
413 MPa
E6010
E6012
cellulose
titania
F, V, OH, H
F, V, OH, H
E7010
E7048
cellulose
Low hydrogen
F, V, OH, H
F, V, OH, H,
VD
E8010-P1
E8016-B2
cellulose
Low hydrogen
F, V, OH, H
F, V, OH, H
E9010-G
E9016-B3
cellulose
Low hydrogen
F, V, OH, H
F, V, OH, H
E10010-X
E10016-X
cellulose
Low hydrogen
F, V, OH, H
F, V, OH, H
E11016-X
Low hydrogen
F, V, OH, H
E12016-X
Low hydrogen
F, V, OH, H
70 ksi =
482 MPa
80 ksi =
550 MPa
90 ksi =
620 MPa
100 ksi =
689 MPa
110 ksi =
758 MPa
120 ksi =
827 MPa
131
Table: Designations of electrodes and weld consumables
E
R
ER
EC
EW
B
RB
F
IN
RG
6.5
Indicates an arc welding electrode, which, by
definition, carries the arc welding current.
Indicates a welding rod which is heated by means
other than by carrying the arc welding current.
Filler metal: arc welding electrode or welding rod
composite electrode
tungsten electrode (not consumed)
brazing filler metal
Dual use welding and brazing rod
flux for submerged arc welding
consumable insert
welding rod for gas welding
Groups of Welds in torsion
The weld stresses due to torsion are determined in a slightly different way. It is
necessary to take into account the distribution of the weld bead around the
axis of torsion/bending. There are two shear stresses that are generated, and
these have to be combined.
The Primary shear due to the reaction force carried by the weld is:
F
τ1 =
A
where
F
reaction force carried by weld
A
total throat area of all welds
The Primary shear stress is the same on all welds in the group (at least to a first
approximation).
132
There is also a Secondary shear due to torsion about the centroid of the weld
group:
M.r
τ2 =
J
where
M
bending moment
r
distance from the centroid of the weld group to the point of interest in
the weld
J
polar moment of area of weld group about its centroid
The Secondary shear stress varies depending on the distance between the
centroid and the portion of the weld in question. The further away the weld
material is from the centroid, the greater will be the secondary shear stress
that it carries.
133
The shear stress τ2 acts in a direction perpendicular to r. The two shear
stresses are combined by vector addition to find the resultant. It will be
necessary to perform this combination at various places along the weld, unless
the location of highest stressed is obvious by inspection (it will be at a corner
that is furthest away from the centroid).
It is usually easier to calculate the Unit polar moment of area Ju , which is the
polar moment of area divided by the throat size. The value of J u is available in
the table below for some common weld groups. For fillet welds the throat size
in turn can be readily determined from the leg length (h), as t = 0.707h.
Alternatively, the second moment of area is J = 0.707hJu
134
Weld
Throat area
Location
of
centroid
x, y
A = 0.707h(b + d)
x=0
y = dª2
A = 1.414hd
x = bª2
y = dª2
x=
A = 0.707h(b + d)
y=
b2
2(b+ d)
d2
Unit POLAR
moment of
area
Ju
Ju =
Ju =
d3
12
(3b2 +d2)
6
Ju =
(b +d)4 - 6b2d2
12(b+d)
2(b+d)
135
b2
8b3 + 6bd 2 + d 3 b4
2b+ d
Ju =
12
2b+ d
d
y=
2
x=
A = 0.707h(2b + d)
136
A = 1.414h(b + d)
A = 1.414hπr
x = bª2
y = dª2
at centre
Ju =
(b +d)3
6
J u = 2πr 3
For weld groups not shown, first determine the position of the centroid
Ai yi
Ai xi
x =
and y =
Ai
Ai
where
Ai
area of weld I, assuming unit width
xi
position of centroid of area A i
Then determine the unit polar moment of area for an area of unit width, using
conventional means.
R
Ju =  r2 dA
0
Example: Welded attachment in torsion
The diagram shows a plate that is welded onto a base member. The stresses in
the welds are determined as follows.
137
The centroid of the weld group is found, using the previous tables, as
x = b2/(2b+d) = 602/(2x60 + 100) = 16.36 mm
y = d/2 = 100/2 = 50 mm
The bending moment about the centroid is
M = F.r = 50 x 103 x (0.060 + 0.200 - 0.016) = 12.2 k Nm
The unit polar moment of area is
Ju = (8b3 + 6bd2 + d3)/12 - b4/(2b+d) = (8x0.0603 + 6x0.060x0.1002 + 0.1003)/12
- 0.0604/(2x0.060+0.100) = 4.68x 10-4 m3
The polar moment of area is
J = 0.707hJu = 0.707x0.006x 4.68 x 10-4 = 1.99x10-6m4
The area of the welds is
A = 0.707h(2b+d) = 0.707x0.006x(2x0.060+0.100) = 9.332x10-4m
The primary shear stress is
τ1 = F/A = 50x103/9.332x10-4 = 5.358x107Pa
The secondary shear stress depends on where we consider. It is largest at weld
material furthest away from the centroid.
138
Also, as shown in the diagram above, the direction of the secondary shear
stress is perpendicular to the radius from the centroid. Therefore it is possible
to say beforehand that the greatest final stress (resultant) will arise at point A.
It is here that the secondary stress is (1) large and (2) closest in direction to the
primary stress. It will not always be as clear as this.
We could now determine the radius from the centroid to A, but it is easier to
work in co-ordinates as follows
xA = 60-x = 60-16.36 = 43.64 mm
yA = d = 100mm
Then the secondary shear stresses at A are
τ2Ax = M.xA/J = 12.2 x 103 x 0.0436/1.99x10-6 = 267.3 x 106Pa
τ2Ay = M.yA/J = 12.2 x 103 x 0.1/1.99x10-6 = 613.1 x 106Pa
In this case the primary shear stress is in the y direction, and therefore the
resultant stress is
τA = [τ2Ax2 + {τ2Ay + τ1}2]0.5 = [267.32 + {613.1+ 53.58}2]0.5 = 718.3 MPa
This stress would be well over the permissible shear stress for most materials
that are likely to be welded. There are materials that can take this type of
stress, but generally they have too much carbon to be easily welded.
139
6.6
Groups of Welds in bending
This section applies to beams made of welded fabrications. The method is very
similar to that for torsion, except that the second moment of area must be
used, rather than the polar moment of area. As for torsion, there are two
stresses that are generated, as follows.
Shear due to the reaction force carried by the weld:
F
τ1 =
A
where
F
reaction force carried by weld
A
total throat area of all welds
Normal stress due to bending about the centroid of the weld group:
M.d
σ=
2.I
where
M
bending moment
d
depth of beam
I
second moment of area of weld group about a section through the
neutral axis
140
It is usually easier to calculate the Unit second moment of area Iu , which is
available in tables for many common weld types. The value of I is then
I = Iu .t
where t is the weld throat size. For a fillet weld t = 0.707h where h is the leg
length.
The normal stress will vary depending on the distance of the weld material
being considered. The further the weld material is from the neutral axis (not
the centroid), the greater the normal stress.
Once the shear stress and bending stress have been determined, they need to
be combined. In the case of bending it is necessary to resort to structural
mechanics to find the principal stress or the maximum shear stress. This may
be done using a Mohr's circle. Alternatively, the following equations can be
used.
The maximum and minimum principal stresses are:
1
2
σ1,2 = [ (σx+σy) 
(σx-σy)2+4τ ]
2
xy
and maximum shear stress is given by:
σ1-σ2 1
2
τ3 = 
=(σx-σy)2 + 4.τ
2
2
xy
In most cases only one normal stress exists, say σx and therefore σy = 0.
141
Weld
A = 0.707h(b+d)
A = 1.414hd
Throat area
x=0
y = d2
Location of
centroid
Unit SECOND
moment of
area, about a
horizontal axis
through the
centroid
d3
Iu =
12
x = b2
y = d2
Iu =
d3
6
142
b
2
d
y=
2
x=
A = 1.414hb
bd2
Iu =
2
b2
2b+d
d2
Iu = .(6b+d)
d
12
y=
2
x=
A = 0.707h(2b+d)
Weld
Throat area
A = 0.707h(b+2d)
Location of
centroid
Unit SECOND
moment of area,
about a
horizontal axis
through the
centroid
x = b2
2d3
d2
Iu =
- 2d2y + (b+2d)y2
3
y=
b+2d
143
as above for U section
A = 1.414h(b+d)
x = b2
y = d2
Iu =
d2
.(3b+d)
6
as above for closed rectangle
144
Weld
A = 1.414hπr
Throat area
at centre
Location of
centroid
Iu =
Unit SECOND
moment of area,
about a
horizontal axis
through the
centroid
πr3
2
For weld groups not shown, first determine the position of the centroid
Ai yi
Ai xi
x =
and y =
Ai
Ai
where
Ai
area of weld I, assuming unit width
xi
position of centroid of area A i
Then determine the unit second moment of area for an area of unit width,
using conventional means.
y
Iu =  y2 dA
0
145
6.7 Stresses due to misalignment
Misalignment
Misalignment can occur in two ways:
Angular: plates that are joined at an angle other than 180 degrees. The effect is
also called peaking because of the visual effect created.
Lateral: where plates are parallel, but their axes are not co-linear. This in turn
can occur in two ways: either in plates of the same thickness that are offset, or
when joining plates of unequal thickness.
Misalignment increases the local stresses because it introduces secondary
bending stress on top of the existing axial stress. Consequently the fatigue
strength for a misaligned joint is lower than for a flat joint. However this only
applies to joints which are transverse to the loading.
Longitudinal welds do not feel the effect of misalignment, even if it is present.
Also, the effect of misalignment must be only taken into account, where it
causes secondary bending. For example when there is lateral support, then
secondary bending does not occur.
Misalignment can be reduced by careful attention to joint preparation, and the
fitting of the parts before welding. It is often impractical to avoid misalignment
totally.
The effect of misalignment in design is accounted for either by a stress
concentration factor, or by calculating the secondary bending stress and adding
it to the primary stress.
146
Secondary bending stress due to lateral misalignment
The stress depends on the geometry of the joint, as follows. The following
parameters are used:
H
weld leg length
Sw
stress on the weld throat without secondary bending
Sb
secondary bending stress
Sa
primary stress
B
plate thickness (subscript 1 or 2 are appropriate)
v
Poisson=s Ratio (0,3 for steel)
A1 Lateral misalignment in butt or cruciform joints between flat plates of
equal thickness:
3e
Sb = .Sa
B
A2 Lateral misalignment in butt or cruciform joints between flat plates of
unequal plate thicknesses with B1 < B2 the equation is
147
B
1
1,5
6e
.[
].S
B1 1,5 1,5 a
B +B
1
2
A3 Lateral misalignment in fillet welded cruciform joints (root failure):
e
Sb =
.S
B+H w
where H is the weld leg length, and Sw is the stress on the weld throat without
secondary bending.
Sb =
A4
Lateral misalignment in butt welded seam of vessel or pipe:
n
B
1
6e
Sb =
.[
].Sa
B1(1-v2) Bn+Bn
1 2
where for unequal plate thicknesses with B1 < B2, and n = 1,5 for
circumferential welds and welds in spheres, and n = 0,6 for longitudinal seams.
Secondary bending stress due to angular misalignment
Angular misalignment of vessel or pipe
6d
Sb = .Sa
B
where
d
deviation from true circular shape
Total stress due to misalignment
The total stress due to the nominal applied stress and the secondary stress due
to misalignment is
S = Sa+Sb
Stress magnification factor due to misalignment
If it is necessary to determine a stress magnification factor for misalignment, it
is
Sb
Km = 1 +
Sa
148
7 Weld fatigue
There are generally accepted design rules for welded structures, and these
have been incorporated into the many welding codes that exist. This section
describes the underlying principles on which such codes are based.
Welds are intrinsically full of cracks waiting to grow. Consequently welded
structures are vulnerable to failure by fatigue at stresses that appear to be very
low.
Fatigue is the dominant mode of failure in welded joints. It shows with cracks in
the weld and base material. Fatigue only occurs when there are loads that
change with time. In practice this includes practically every structure, since
even nominally statically loaded parts are subject to some changing load. Some
of the more subtle changing loads include wind forces, vehicle and pedestrian
traffic, wave action, and machine induced vibration. There are very few
structures that are truly statically loaded, and therefore the consideration of
fatigue is usually necessary in safety-critical structures.
Welds contain pre-existing crack-like features. Since crack initiation need not
take place, only crack growth, the fatigue life is relatively low. Fatigue
strengths of welds are primarily determined by geometry, and there is not
much value in using stronger materials. The effects of welding flaws and lack of
penetration are to provide stress raisers. Misalignment is another important
effect.
Fatigue loading
Fatigue occurs where the stress changes with time. There will exist a maximum
stress, and also a minimum stress. The minimum stress can be positive, zero, or
negative, where the sign shows the direction relative to the maximum stress.
Stress reversal occurs when the minimum stress has the same magnitude as
the maximum stress, but opposite direction (-sign). In most of the codes the
allowable fatigue strength refers to the range of stress (maximum – minimum)
but sometimes it is the amplitude (half the range). If the code is ambiguous
about this, as is the case surprisingly often, then make a conservative
assumption.
149
In the case of welds, the crack growth rate is relatively constant for the various
mild steels, irrespective of tensile strength. Hence fatigue life of welds is not
dependent on material properties such as tensile strength, but rather it
depends on the type of microscopic weld defects. In turn this depends on the
type of weld (butt vs. fillet) and orientation (parallel or transverse to the
loading). Thickness of the plate is also a variable.
This results in the fatigue strength, more accurately the permissible stress
range, being defined by weld categories A-D, irrespective of the tensile
strength or grade of the material. Many definitions of weld categories exist, all
slightly different. The familiar SN curve is still used, where life less than infinite
life is acceptable.
For infinite life, the stress value at 107 cycles is taken as the endurance limit for
welded steels in air. There are variants on this, with some codes using 2x106
and some having no endurance limit at all.
7.1
Design life
Design life is typically 25 years. It is not always necessary to design for infinite
life. Some machines like cranes experience only a few load cycles each day,
depending on their purpose. If you are uncertain, then assume 𝑁 = 107load
cycles (conservative), and determine endurance limit from the table below, for
different classes of welds.
Before using the design rules, you need to decide whether or not the structure
warrants it. For non-critical structures where the consequences of failure are
relatively minor, then it may be acceptable to use a rule-of-thumb (such as fillet
leg length = 0.5 x plate thickness) rather than any serious analysis.
Design procedure
The procedure that would typically be followed is:
1 Decide which code to use, and get a copy of it. For welds without
considerations of beam stresses, probably the best is AWSD1.1:2000
150
For definitive reference please see Table 2.4 Fatigue stress provisions, AWS
D1.1.200, source Public Resource
https://law.resource.org/pub/us/cfr/ibr/003/aws.d1.1.2000.pdf
For simple mechanical structures and a non complicated weld code see (AS
3990, 1993). For cranes design and weld conditions see (NZS/BS 2573.1:1983,
1983).
2 Determine all loads are accurately as possible (directions, magnitudes,
frequency). If necessary, use one of the fatigue techniques to determine the
effect of random or cumulative loading (e.g. Miners rule, Reservoir method
etc.). For cranes, use a load impact factor, typically 1.3 for medium to heavy
workshop duty per Table 4 (NZS/BS 2573.1:1983, 1983). The impact factor
accounts for the abruptness of the load pickup.
3 Perform preliminary design of structure: positions of welds, and types of
welds (‘condition’). Use the better class weld details where possible. Try move
welds out of known areas of high stresses, e.g. use bolts rather than welding at
root of cantilever (not always possible).
4 Allocate a class to each weld, from tables. For example, if you expect a fillet
weld across the load path, then select weld class F. See classes of joints below.
5 Decide on the level of confidence required. Normally used M-2SD, except
where reasons exist to go to more or less standard deviations.
6 Decide on the life required. This is usually 25 years life, although lower life
can be tolerated if regular and adequate inspection is performed. Determine
utilisation – how frequently the load cycles arise.
7 From the desired life, determine the permissible fatigue stress range for each
of the weld classes. This is found in tables. The stress range refers to the
maximum less the minimum stress, e.g. one load pickup and put down cycle of
a crane. The standards are not concerned about mean and amplitude – it is
only the stress range that matters.
8 Select structural steel members. These might be plates or beam sections.
Calculate stresses when the loads are applied. Use the most severe
combination of loads. Determine the thickness or size of the structural
members to give a stress less than the appropriate fatigue strength. No safety
151
factors are required. Make sure that the static tensile yield strength of the base
materials has not been exceeded.
9 Use the fillet weld equations to determine the required throat and leg lengths
of the weld(s), for a given strength of electrode. Check that weld stress is less
than the allowable weld stress for the code. Check that the member has
sufficient thickness, e.g. in flange, to support the desired weld. Butt welds are
simpler - determine appropriate electrode.
Corrosive Environment
The allowable stresses are based on steel in air. If corrosion is present then the
design rules do not generally apply. Unfortunately there are no general design
rules for corrosion fatigue, and it is necessary to use test results (and test
results take time, since corrosion cannot be hurried up like fatigue!). Corrosion
plus fatigue is worse than either of the two acting on their own.
Tests on corroded North Sea structures show that fatigue life is reduced by a
factor of about two, and the endurance limit S o is so small that it is practically
negligible.
High temperature Environment
Generally the design codes are applied up to about 375 oC for steels, 430oC for
austenitic stainless steels, and 100oC for aluminiums. Higher temperature
service requires special attention. Also high temperature plus corrosion can be
a problem.
Low temperature Environment
Fatigue lives increase at lower temperatures, as the crack growth is slowed
down. However the fracture toughness also decreases. Brittle fracture
therefore starts to become a problem.
Reference: MADDOX SJ, 1991, Fatigue strength of welded structures,
Woodhead.
152
7.2
Classes of Joint
There are nine classes of joint, depending on the orientation, type of weld etc.
These are named A B C D E F F2 G and W (this designation varies across the
codes). For each class there is a given permissible stress based on fatigue. Class
A is basically unwelded material, with the highest fatigue strength, and class W
has the lowest fatigue strength. Several types of weld detail are put into each
class.
Each weld in a structure has to be assessed, and put into one of the nine
classes. Then the permissible stress may be found (for the required life), and
the necessary throat thickness determined. It is quite possible that a joint can
fit into more than one class, depending on the ways that it could fail.
There are multiple welding codes, each of which is slightly different and has
many fussy little rules. This is frustrating divergence arises because the expert
committees in each country feel they know best, and the standards
organisations want both control of the process (without perceived interference
from other countries) and their own copyright code to sell for income
purposes.
Pons design code
This is my code, for illustration & teaching purposes. It includes elements from
several other codes. If you use this for design purposes it is your own
responsibility as I accept no liability for the outcomes. If you need to cite it, use
the title of this book.
153
Figure. Generally accepted weld fatigue classes. Adapted from AWS.
154
Figure. Generally accepted weld fatigue classes. Adapted from AWS.
Weld groups
Group 1 Plain material free from welding
Fatigue cracks generally do not start in the base material as the welds have
much lower fatigue strength. If there is fatigue in base material, the cracks start
155
at stress concentrations, such as holes and re-entrant (internal) corners. When
plain material is repaired by welding, then it is reclassified as a transverse butt
weld.
Requirements
CLASS
Illustration
Plate and structural sections with no holes. No subsequent corrosion or surface damage permitted.
1.1 All surfaces and edges
machined (not flame cut)
and polished.
A
1.2 All surfaces and edges
B
machined or rolled, or
extruded. Flame cut
surfaces machined in the
direction of stressing.
Plate and structural sections with holes.
1.3 Flame cut surfaces
machined in the direction
of stressing. Designers
must make allowance for
the appropriate stress
concentration factor for
geometry.
B
1.4 Planed or Flame cut
surfaces, free from cracks.
Designers must make
allowance for the
appropriate stress
concentration factor for
geometry.
C
1.5 Small holes (drilled or
reamed) with Diameter
less than plate thickness.
Class already includes
stress concentration
factor.
D
Crotch corner at branch connection. SCF must be applied by designer.
1.6 Free from welding
C
1.7 Repair welds dressed
flush and proved free from
flaws
D
156
Group 2 Butt welds with loading parallel to the weld
Fatigue cracks generally start at weld ripples, start-stop positions, and weld
flaws. NDT is required to ensure freedom from significant flaws. Permanent
backing strips must be continuous, and any joints between pieces must be full
penetration butt welds. To avoid yielding of the Backing strip, the material
should be of similar strength to the main member. Welds attaching backing
strip to main member must be of the same CLASS as those used in the main
member. In some codes the presence of a fillet weld within 10 mm of an edge
causes the weld to be downgraded to Class G.
No undercutting permitted on corners and returns, if it occurs it must be
ground flush.
Requirements
CLASS
Illustration
Full penetration butt weld. From both sides, or one side onto consumable insert or temporary non-fusible backing.
2.1 Welds dressed flush. Weld
proved free of flaws by NDT.
B
Full penetration butt weld. From both sides, or one side. Continuous Permanent backing (integral or attached by
welding) is permitted.
2.2 Automatic process. No
stop-starts, and proved free
of significant flaws by NDT.
Any tack welds on backing
must be ground out or buried
in weld.
C
2.3 With stop-starts. Backing
may be attached with
continuous fillet weld.
D
E
157
2.4 Backing may be attached
with intermittent fillet weld.
Group 3A Full penetration Butt welds in plates with loading across the weld
(transverse butt welds)
Fatigue cracks generally start at weld toe. If the overfill is dressed flush, then
weld flaws become significant as crack initiators. NDT is required to ensure
freedom from significant flaws. Effect of misalignment must be taken into
account, where it causes secondary bending. There is a plate thickness penalty
for thicker plates, which must be used to decrease the permissible stress.
Requirements
CLASS
Illustration
Full penetration butt weld. Plates of equal width and thickness. Weld from both sides, or one side
onto consumable insert or temporary non-fusible backing.
4.1 Weld overfill dressed
flush. Weld proved free of
flaws by NDT. Any
misalignment blended with
slope of 51:4
C
(Fatigue cracks usually start at
weld flaws.)
Full penetration butt weld. Plates of any width and thickness, but changes tapered to slope 1:4. Up to
15% thickness change can be accommodated in the weld without taper.
4.2 Shop welds in flat
position. Manual or
Automatic process, but not
submerged arc.
D
Field welds, or position welds,
or submerged arc welds
E
158
4.3 Welds by any process.
Weld from both sides, or one
side onto consumable insert
or temporary non-fusible
backing. Overfill profile θ 
150o
D
4.4 Overfill profile θ < 150o
E
4.5 Weld from one side, full
penetration, and proved free
of significant flaws by NDT.
Not recommended for fatigue
loaded joints, as life critically
dependent on root condition.
E
4.6 Weld from one side, onto
Permanent backing (integral
or attached by tack or fillet
welds).
F
Full penetration butt weld. Plates of unequal width, which are not tapered. SCF is already included in
the classification.
4.7 Weld ends ground to a
radius 1,25 times plate
thickness.
NB This class can be avoided
by using shaped transition
pieces instead.
F2
Group 3B Full penetration Butt welds in Sections with loading across the weld
(transverse butt welds)
Fatigue cracks generally start at weld flaws. Failure from toe or root is possible
in some cases. NDT is required to ensure freedom from significant flaws. There
is a plate thickness penalty for thicker materials, which must be used to
decrease the permissible stress.
Requirements
CLASS
Illustration
Full penetration butt weld between rolled, extruded or built-up sections.
5.1 Advised that Weld overfill
be dressed flush.
F2
159
If special precautions are
taken, it may be possible to
assume group 4 welds.
5.2 Weld in web (not flange)
at Semi-circular cope hole.
Weld end and overfill dressed
flush within distance R from
edge of cope hole. Class
includes a SCF of 2,4, Mitred
(450) cope holes are not
recommended.
D
Full penetration butt weld, between hollow sections. Includes cylindrical and conical shapes.
5.3 Weld from both sides.
Overfill dressed flush. Proved
free of significant flaws by
NDT. (Not appropriate for
structural work).
C
5.4 Weld from both sides, or
one side onto consumable
insert or temporary nonfusible backing.
E
5.5 Weld from one side onto
Permanent backing (integral
or attached by tack or fillet
welds).
F
5.6 Weld from one side
without backing, but full
penetration is assured.
F2
160
Group 4 Fillet and T butt welds
Fatigue cracks generally start at the toe, and sometimes the root. Weld ends
are typical sources for loading parallel to the weld line. In some codes the
presence of a fillet weld within 10 mm of an edge causes the weld to be
downgraded to Class G. There is a plate thickness penalty for thicker plates,
which must be used to decrease the permissible stress.
Requirements
CLASS
Illustration
Cruciform or T joints between plates, or sections or built-up members.
6.1 Full penetration butt weld
F
6.2 Partial penetration butt or
fillet weld. Weld sufficiently
large to prevent failure in
throat.
F2
6.3 Partial penetration butt or
fillet weld, failure in throat.
Assumes no load is taken in
bearing between plates.
W
Fillet welded lap joints
6.4 Lap joint symmetrically
arranged on both surfaces.
Determine effective width.
F2
6.5 Lap joint symmetrically
arranged on one surface.
Determine effective width.
G
6.6 Any Lap joint
W
6.7 Any Lap joint, with
undercut ground, and no
returns around the laps.
G
Group 8 Welded attachments
Fatigue cracks generally start at the weld ends (for loading parallel to weld),
and at the toe (for loading across the weld). One sided welds sometimes fail at
the root. In some codes the presence of a fillet weld within 10 mm of an edge
161
causes the weld to be downgraded to Class G. There is a plate thickness penalty
for thicker plates, which must be used to decrease the permissible stress.
Description
Requirements
CLASS
Attachment of any
shape. In contact
with stressed
member. Weld lies
across the
direction of
loading.
8.1 Fillet or butt + fillet weld.
Welds continuous around
ends or not. Attachment
thickness t  55 mm
F
8.2 Attachment thickness t >
55 mm
F2
Attachment to
web, in region of
combined bending
and shear
8.3 Use principal stress range
in vicinity of weld.
E
Attachment of any
shape, with
surface in contact
with stressed
member.
Fillet weld, continuous
around ends or not.
Attachment length L and
width W:
8.4
L  160 mm
W  55 mm
8.5
L > 160 mm
W  55 mm
8.6
L > 160 mm
W > 55 mm
Cover plate of any length, and
wider than girder flange.
Ensure flange edge is not
undercut. Avoid weld returns
around corners.
Illustration
F
F2
G
G
162
Description
Requirements
CLASS
Attachment of any
shape, on or close
to the edge of a
stressed member.
8.8 Any size fillet or butt weld,
continuous around ends or
not. Avoid undercut on
corners, or remove by
grinding. Avoid weld returns
around corners.
G
Attachment of any
shape, with edge
on contact to a
stressed member,
with weld parallel
to direction of
loading.
Fillet or butt weld, continuous
around ends or not.
Attachment length L and
width W:
8.9
L  160 mm
W  55 mm
8.10
L > 160 mm
W  55 mm
8.11
L > 160 mm
W > 55 mm
Attachment with
bending stress
8.12 Local bending stress to
be included by designer.
Illustration
F
F2
G
F
Group 4 Continuous welded attachments parallel to the applied stress
Fatigue cracks generally start at weld ripples, start-stop positions, weld ends,
and weld flaws. Root failure in fillet welds is also possible, especially for single163
sided welds. In some codes the presence of a fillet weld within 10 mm of an
edge causes the weld to be downgraded to Class G.
Requirements
CLASS
Illustration
Fillet and butt welds from one or both sides.
3.1 Automatic process. No
stop-starts.
C
3.2 With stop-starts.
D
3.3 Intermittent fillet weld
with g/h  2,5
E
3.4 Intermittent fillet weld
with g/h > 2,5
F
3.5 Fillet at cope hole. Weld
may or may not continue
around plate end
F
Group 7 Penetrations through stressed members
164
Description
Requirements
CLASS
Slotted throughmember
7.1 Full penetration butt
weld. Length of through
member  160 mm. Apply
plate thickness penalty.
F
7.2 Length of through
member > 160 mm. Apply
plate thickness penalty.
F2
7.3 Fillet or butt welded.
Apply SCF and plate thickness
penalty.
F
7.4 Full penetration butt weld
D
7.5 Partial penetration butt or
fillet weld
F
7.6 Partial penetration butt or
fillet weld, with throat failure.
W
Branch and tube
connections and
penetrations
Illustration
7.3 Allowable stress range
Separately the designer has to decide what kind of fatigue life is required, in
number of load cycles. Having identified the joint, and found its alphabetic
rating, with the fatigue life, the designer can now determine the allowable
stress range 𝑅𝑓 . This is shown in the tables below. The various codes are
slightly different.
165
Pons allowable stress range in fatigue
20 000
to
Category
100 000
cycles
100 000
to
500 000
cycles
500 000
over
to
2 000 000
2 000 000
cycles
cycles
A
(material
A514)
310
241
172
172
A
276
221
165
165
B
228
172
117
103
C
193
145
97
83
D
165
117
69
62
E
117
83
48
41
F
117
97
76
62
G
103
83
62
55
Table: Allowable stress range 𝑅𝑓 [MPa] as a function of weld category and
desired fatigue life. Adapted from AWS.
AS3990 permissible stress range
Table. Permissible stress range [MPa] (max-min) adapted from (AS 3990, 1993).
20 000 to 100 000
Category
100 000 to
or
cycles
500 000
condition
cycles
500 000
over
to
2 000 000
2 000 000
cycles
cycles
Load
Load
Load
Load
condition condition condition condition
1
2
3
4
A
B
410
310
245
185
165
120
165
110
166
C
D
E
F
G
220
185
140
100
130
110
85
80
85
65
55
60
65
45
30
55
See also https://www.imorules.com/GUID-08E7C2C0-C82E-4511-8676232C07928B1A.html#GUID-08E7C2C0-C82E-4511-8676-232C07928B1A
For the FAT system see https://www.efatigue.com/welds/background/iiw.html
Fatigue categories per AWSD1.1:2000
For definitive reference please see Table 2.4 Fatigue stress provisions, AWS
D1.1.200, source Public Resource
https://law.resource.org/pub/us/cfr/ibr/003/aws.d1.1.2000.pdf
Table: A simplified set of fatigue strengths adapted from AWS D1.1:2000 Table
2.4. See full table for further details and other conditions. This is a conservative
table as the life is given at 10^7 cycles.
Base metal
Base metal
Base metal
Base metal
Weld
Allowable stress
category range at 10^7
cycles
With rolled or cleaned surface. A
138 MPa
At Welded cover plates with
E
13.8 MPa
or without welds across the
ends
Next to complete penetration B
103.5 MPa
groove welds that are ground
and NDT
Next to details with groove
E
13.8 MPa
welds under transverse or
longitudinal loading or both,
with NDT
167
Base metal
Welds
Groove welds
Fillet welds
Next to Longitudinal stiffeners
with intermittent fillet welds
Fillet or groove welds parallel
to applied load. Without
attachments. Welds must be
continuous
complete penetration groove
welds that are ground and
NDT
Shear stress on throat
E
13.8 MPa
B
103.5 MPa
B
103.5 MPa
F
48.3 MPa
168
7.4 Theoretical approach to weld fatigue
The following is the basic theory on which all the codes are constructed. This
theory also allows determination of S-N curves and permissible fatigue stress
for shorter lives.
The S-N curve shows a linear relationship between log(𝑁) and log(𝑆) where 𝑁
is number of cycles of life before failure, and 𝑆 is the fluctuating stress range.
Figure: Experimental SN curve of 4130 tensile specimens with R_a=0.1 surface
finish, Kushagrs,
https://commons.wikimedia.org/wiki/File:Experimental_SN_curve_of_4130_te
nsile_specimens_with_smooth_surface_finish.png Creative Commons
Attribution-Share Alike 4.0 International
Hence also
log(𝑁) = log(𝐾) − 𝑚. log(𝑆)
169
A generalised approach
The permissible fatigue stress (strength) 𝑆 is described by the relationship
𝑆 𝑚 𝑁 = 𝑎∆𝑑 = 𝐴
where
S
stress
m
stress exponent
N
number of cycles of life before failure
a
constant
Δ
standard deviation factor
d
number of standard deviations (d = 0 for mean, d = 2 for two standard
deviations below mean line M-2SD).
A
parameter for M-2SD. Values of some of the parameters are given in the
table below, for M-2SD (d=2). Two standard deviations below the mean M-2SD
is 98% confidence.
This is the general equation. However this is not commonly used in design.
Instead the equation is adapted to give the endurance limit strength 𝑅𝑓 and
this is used for design purposes.
Permitted stress for finite life
If a finite life is acceptable and desirable, then the permitted stress amplitude
(range/2) 𝑅𝑓𝑎 or 𝑆𝑜 for life N may be determined using
1
𝐴 𝑚
𝑅𝑓𝑎 = ( )
𝑁
and the tabular values for A and m given below.
Table. Endurance limits (amplitude based) for structural steels. M-2SD refers to
mean less two standard deviations, i.e. is a conservative value.
m
Δ
A
[for d=2 i.e.
M-2SD]
Endurance limit So for 107 cycles for M-2SD
[ MPa = N/mm2]
B
4,0
0,657
1,10 x 1015
100
C
3,5
0,625
4,22 x 1013
78
Class
A
Base
material
(unwelded)
170
Class
m
Δ
A
[for d=2 i.e.
M-2SD]
Endurance limit So for 107 cycles for M-2SD
[ MPa = N/mm2]
D
3,0
0,617
1,52 x 1012
53
E
3,0
0,561
1,04 x 1012
47
F
3,0
0,605
6,33 x 1011
40
F2
3,0
0,592
4,31 x 1011
35
G
3,0
0,662
2,50 x 1011
29
W
3,0
0,654
1,58 x 1011
25
For example, for a fillet weld class E, m=3.0, A= 1.04 x 1012, and if the required
life is N=100,000 cycles, then the permissible stress range (endurance limit) is
1/3
𝐴 1/𝑚
1.04𝑥1012
5
𝑆(𝑁 = 10 ) = ( )
=(
)
𝑁
105
= 218 [𝑀𝑃𝑎]
Don’t use this in practice. The codes apply additional safety factors to this.
171
The categories or classes of joint are determined by the orientation of the weld
to the load path.
Plate thickness effect
The effect of plate thickness is taken into account (where required by the
tables), by the following factor:
(22/B)0.25
where B is the plate thickness in mm. This factor is used to decrease the
permissible stress.
Stress ratio
Generally, tensile stresses produce more fatigue damage than compressive
stresses, and therefore it is the type of loading and not only the number of
cycles which is important. The type of loading is denoted by the stress ratio R,
which is the minimum stress divided by the maximum stress. For reversed
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bending R=-1. The more positive the stress ratio the more damaging the
loading.
Fatigue design does not take into account the shape of the stress waveform or
the periods of rest in between, or the frequency of loading. For example is high
frequency any worse or better than low frequency? There is currently no
answer to this, at least not in the form that can reliably be used in design.
Instead the values which are used in fatigue analysis are simply the number of
load cycles, the peak stress, and the stress ratio.
Fatigue strengths of welded structures are much lower than those of unwelded
parts. The fundamental reason is that welds introduce a large number of stress
raisers and ready-made crack-like features. For an unwelded part, the fatigue
life consists of a relatively long period in which the crack is initiated, and then
another period when the crack grows. For welded parts the crack is already
provided, and the only substantial life is that taken to grow the crack.
Fatigue strength
Fatigue strength refers to the maximum stress that may be permitted in the
part if fatigue cracking is to be avoided. The fatigue strength depends on the
number of load cycles. A load cycle is caused each time the load is applied and
removed. In the case of structures like rotating shafts subject to bending, then
the load cycles come very quickly. For example at 3000 rpm, there are 3000
load cycles per minute. Other structures like pressure vessels may be subject to
much less frequent load cycling. In determining the load cycle the designer
must consider not only these primary loads, but also vibration loads introduced
by rotating or moving machines, wind forces, wave action etc.
The greater the frequency of the loading, the shorter will be the time that
passes before a certain total number of cycles is past. For example, a million
cycles at 3000 rpm takes 5.6 hours, but at 1 Hz it takes 278 hrs. Fatigue
strengths are presented on graphs showing the fatigue strength as a function of
the total number of cycles.
Large number of specimens are required to generate these curves, and each
test takes a significant length of time. Results are applied stress [S], plotted
against number of stress cycles [N]. Usually log-log axes are used rather than
linear. There is scatter in the results, more so than in static tensile tests, which
is to be expected given the nature of the fatigue mechanism. For most
173
materials, especially ferrous metals, there is a certain stress below which
fatigue failure will not occur however long the alternating stress is applied. This
stress is called the endurance limit Rn, and it usually occurs at about 10 6 load
cycles. The essence of preventing fatigue is to keep the stresses below the
endurance limit so low that no crack growth occurs at all. Alternatively the part
can be deliberately designed for a finite life, if this is acceptable.
In welding, it is common to take the fatigue strength for infinite life at 2 x 10 6
cycles (or sometimes 107 cycles), and these values are quoted in the various
codes. If a finite life is acceptable, then a higher fatigue strength may be used.
174
Not sure about this section – ignore for now
Next the allowable maximum stress may be determined from the expression
σallow
σmax =
1-K
where K is the ratio of maximum to minimum loading:
σmax Mmax Fmax τmax Vmax
K =
=
=
=
=
σmin Mmin Fmin τmin Vmin
where M is moment, F is force, V is shear force.
For those categories marked with an asterisk (*), in the case of a stress
reversal, use the equation
σallow
σmax =
1-0,6K
The allowable stress calculated for fatigue may not exceed the stress calculated
for static loading. Note that the allowable stress values given in the table are in
ksi. 1 ksi = 6,89 MPa.
175
8 Deeper physics of weld fatigue cracks
8.1 Fatigue Crack growth
It is generally accepted that the process of crack growth under cyclic loading is
divided into three phases [36]: crack initiation, crack propagation, and
structural fracture, see Figure 8.1.
Figure 8.1. Three stages of crack growth. Image [14] used by permission.
In wrought steels as used for machine parts, many loading cycles are necessary
to get the fatigue crack started and grown to a small-crack size. Even in
materials without microscopic defects, it is possible for cracks to be nucleated
as follows: dislocations coalesce into thicker slip planes and eventually
macroscopic slip bands. These localised hardening effects are irreversible, i.e.
not undone by the next loading cycle. Eventually the slip bands protrude to the
surface, creating opportunities for stress concentration and crack nucleation.
176
Figure 8.2. Crack initiation due to dislocation pile up at the surface of the part.
Image [14] used by permission.
Below a certain threshold of stress, cracks do not nucleate at all. This
corresponds to the endurance strength. Thus the fatigue design for machine
parts is primarily aimed at retarding the Crack Initiation stage.
In welds by contrast, there are many established small cracks and adverse
residual stresses immediately after the weld cools. Hence the Crack Initiation
phase is skipped entirely, and the loading cycles are immediately applied to
Crack Propagation.
In the Propagation stage the crack grows by two mechanisms. One is plastic
deformation and damage around the crack tip at each opening & closing load
cycle, hence plastic blunting. The other is shear stress ahead of the crack, on
planes at 45° to the loading direction. Cracks initially propagating through the
grains, and are somewhat delayed at grain boundaries. Hence finer
microstructure and internal precipitates are usually beneficial for fatigue
resistance.
177
Figure 8.3. Plastic forward destruction, or plastic blunting process. Image [14]
used by permission.
Around the main crack a set of micro-cracks arise, creating a crack net. At the
atomic level the shear stress is not reduced by having more micro-cracks. Thus
the crack net probes a larger volume of material for weaknesses than a single
crack could do on its own. This promotes the main crack to automatically grow
in a direction most favourable to increasing the total strain (hence most
injurious to the integrity of the part). This is consistent with the general
understanding that cracks always propagate towards the direction which
requires the minimum energy (stress).
If there is also high temperature then creep occurs by diffusion of vacancies
(dislocations) from the bulk of the grain to the grain boundaries, hence creep
cracks tend to propagate along the grain boundaries.
The Propagation stage shows a relatively steady growth process. This is
modelled by Paris’ Law [37], which shows a power-law relationship between
the crack growth rate and the range of the stress intensity factor during the
fatigue cycle:
𝑑𝑎
(1)
= 𝐶(∆𝐾)𝑚
𝑑𝑛
𝑑𝑎
where is the crack growth rate; ∆𝐾 is the effective stress intensify factor,
𝑑𝑛
which is identified as the difference between maximum and minimum stress
intensify factors for one cycle; 𝑎 is the crack growth length; 𝑛 is the number of
cycles; and C and m are constants. None of these factors is available at early
design, except for specific materials and cases. The rate of crack growth is
merely a curiosity to an engineering designer, who rather wants to prevent any
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crack from ever starting in the first place. Hence keeping the stresses below the
endurance limit is usually of greater importance than predicting crack length.
In the absence of mean stress, the fatigue life of a material which is
experiencing pure fatigue may be expressed in the form of stress-life [82, 83]:
𝜎𝑟𝑒𝑓 = 𝐶𝜎 𝑁 −𝛽𝜎
(2)
where ref is the alternating amplitudes of stress strain, C, is the fatigue
capacity for one cycle of fatigue life; and , is the fatigue exponent which
describes the sensitivity of the fatigue capacities to increasing fatigue life. The
subscript “ref” emphasizes the condition of pure fatigue in the absence of any
other damage driving forces such as creep. The equation may be rearranged to
log(𝜎) = −𝑘 log(𝑁) which is the basic shape of the SN life relationship which
is well known to design engineers.
In the case of welds, the crack growth rate is relatively constant for the various
mild steels, irrespective of tensile strength. Hence fatigue life of welds is not
dependent on material properties such as tensile strength, but rather it
depends on the type of microscopic weld defects. In turn this depends on the
type of weld (butt vs. fillet) and orientation (parallel or transverse to the
loading). Thickness of the plate is also a variable.
8.2
Probability of failure
Extensive tests have been done on steels in various types of joint configuration
and loading, and the fatigue strengths determined. The results are plotted as
log Stress vs log life. There is always some scatter, as would be expected. A
straight line is fitted to the log-log data, at two standard deviations below the
mean (M-2SD). This line represents the boundary for about 98% survival, and
this is commonly used for design purposes. The same line is used regardless of
the stress ratio. It gives the permissible stress range for various lives, regardless
of the stress magnitude.
Fatigue strengths of other welded materials
Fatigue strengths of welded materials are well researched for steels, and to a
lesser extent for aluminiums. For other materials there are very much less data
available. However it appears that (at least for aluminium and steel), that the
fatigue strength is proportional to the elastic modulus: e.g. if a material has an
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elastic modulus 1/4 that of steel, then the fatigue strength for the weld is likely
to be 1/4 of that for steel.
Aluminium
The fatigue mechanisms are the same as for steels. Allowable stresses are
approximately one third those of steel (i.e. proportional to the ratio of the
elastic moduli). For aluminium, the crack growth rate is an order of magnitude
faster, so these materials have a lower fatigue limit.
Stress concentration factors
Note that the joint classes take into account the Stress concentration factor
due to the weld itself. Some of the classes also take into account the SCF due to
holes or other features. However on the whole, the designer has to apply any
SCF that is due to the large scale geometry of the structure.
Principal stress
The stresses that are used for fatigue assessment are traditionally the principal
stresses. It is important to take into account the direction of the principal
stresses, and therefore a joint could have different classifications in the
different directions of stress.
9 A qualitative understanding of fatigue
mechanism in welds
What happens to cause the fracture of a weld in service?
Fracture surface
In fatigue failure the fractured surface is often smooth, possibly with visible
rings (called beach marks) spreading out from the origin of the crack. The origin
is usually a small defect or stress raiser.
9.1
Location of fatigue cracks
Is it possible to predict which part of a weld is most likely to suffer a fatigue
crack?
Fatigue failure at weld toe
In most cases the fatigue crack starts at the toe of the weld (where the weld
material joins the outside surface of the part), when the loading is transverse to
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the weld (i.e. loading pulls the weld apart). Failure at the toe is caused by the
Stress concentration factor due to:
*
undercutting of the surface
*
convex profile
*
intrusions at the toe: these are ready made cracks which are an
inevitable consequence of the welding process
The intrusions cause the fatigue strength of a weld to be less than that due to
just the undercutting or convex profile. Therefore the use of theoretical Stress
concentration factors (Kt) for welds is not appropriate. As there are pre-existing
crack like features in any weld, it takes less stress cycles to initiate a crack, and
therefore the life is shorter. The three factors listed above, exist in all welds to
some extent. Naturally, if one of the factors is particularly bad, or there are
additional flaws in a weld (such as hydrogen cracking), then the fatigue
situation is made even worse.
Fatigue failure at weld roots
Failure can also occur from the root of the weld, that is the inside part of the
weld cross section, for fillet welds and partial penetration welds. The unpenetrated region of the joint provides a crack-like feature which relatively
easily initiates fatigue.
181
Sometimes partial penetration welds (fillet and butt) are deliberate, and at
other times they are flaws, depending on the intention.
Fatigue failure at weld ripples
When the loading is parallel to the weld, then there is no stress concentration
effect at the toe, and weld irregularities such as surface ripples become the
sources of failure.
182
9.2
Factors affecting fatigue strength
Welded structures behave differently to other parts, since there are different
principles at work.
Residual stresses
When a weld bead solidifies, it shrinks. This applies forces to the members on
each side, pulling them inwards on the sides, and along the long axis of the
weld. However the members are stiff, and cannot accommodate all the strain
that the weld requires. Consequently a compensated situation arises where the
weld remains partly in tension, and the surrounding members partly in
compression.
The structure may warp as a result of these forces. The warping forces are
especially strong if there is more weld material on one side of the joint that the
other. There can also be warping when welding one side, and then coming
back afterwards for the other side.
Normally an unwelded material is not too much troubled by compressive
stresses. These do not contribute much to fatigue, since existing cracks tend to
be closed under compressive forces. However in the case of welded material,
the fatigue behaviour is very different. The residual stress is so highly tensile
that only exceptionally large compressive stress could have any beneficial
effect.
Often the weld material has such high residual stresses that it is at the point of
yielding. Even if an external compressive stress is now applied, it only
temporarily relieves the tensile residual stress in the weld, and when the
external force is removed the stresses return to the former tensile value. The
important consequence of this is that fatigue failure can still occur with
compressive loading, since the stress fluctuations are still in the tensile region.
Almost all welded joints are assumed to have significant tensile residual
stresses in the weld, regardless of any stress relief. Consequently there are two
important laws of fatigue in welding.
A
In welding, it is not the direction of loading (tensile or compressive) that
is critical in fatigue of welds, but the stress range. Regardless of the stress ratio,
the range (alternating stress) is more important than the mean stress.
183
B
Fatigue strength of welded structures does not depend significantly on
material strength. Instead it depends largely on weld quality.
In unwelded parts the time spent just getting a crack started is a large part of
the total fatigue life. Stronger materials help retard crack formation. However
in welds the pre-existing crack-like structures in all welds mean that fewer
cycles have to be spent developing a crack. Therefore the fatigue of a weld is
mostly spent in propagating the crack rather than starting it. Once a crack is
started, its subsequent propagation speed does not depend heavily on the
material strength, but on the loading. The better the weld quality, the fewer
the flaws, and the longer the time taken to get a fatigue crack started.
All welds have high residual stresses in the weld material. These are caused by
the weld bead cooling and shrinking, which is resisted by the base material. The
stresses are tensile in direction, and are the major contribution to the poor
fatigue life of welds. Stress relief is a heat treatment process that attempts to
alleviate the residual stresses. It involves heating the structure and allowing it
to cool slowly. There is a cost to the process, and it is sometimes impractical to
perform, particularly on large structures. Anyway, it has been found that stress
relief is only beneficial if the loading is at least partially compressive. There is a
smaller benefit (15% improvement in fatigue strength) in using stress relief in
structures subject to tensile loading. Furthermore, it appears that the stress
relief process does not always reliably achieve the purpose. For these reasons,
the design codes do not generally acknowledge stress relief.
Strength of filler material
Weld filler material is usually a better grade than the base material, and
therefore has a better static tensile strength. Therefore a full strength joint, like
a full penetration butt weld (transverse) will be at least as strong in a static test
as the base material. In a static tensile test, failure would be expected to occur
in the parent material rather than in the weld. Note that static refers to
stresses that do not change fast with time, i.e. this does not include fatigue.
The ultimate tensile strengths (UTS) of the parent and the weld materials are
much the same, with the weld material being somewhat higher.
However, under fatigue loading (dynamic loading) the picture changes very
radically. The failure is almost certainly going to occur in or near the weld now.
A plain unwelded piece of mild steel plate might have a fatigue strength of say
250 MPa. The same type of plate with a transverse butt weld on it would have
184
a fatigue strength of say 150 MPa. The fatigue strength of a weld will always be
less than that of the parent material.
There are few structures that are only loaded statically. Most structures have at
least some dynamic load, for example from vibration. The dynamic loading can
sometimes be deceptive, appearing trivial when in fact it may not be. The
steady vibration of an engine through its mountings can sometimes be enough
to cause fatigue failure. In such cases the failure of the joint is better predicted
from the fatigue strength of the weld rather than the ultimate tensile strength.
The question often arises about whether it is always necessary to take fatigue
into account, or whether the weld can be designed by quicker and simpler
methods. This is not a simple question, and there are several answers. For
structures that are covered by codes (such as pressure vessels), then there are
specific codes that legally need to be followed, and these take fatigue into
account. For safety critical structures for which there are no codes, then of
course if in doubt then it is better to take fatigue into account. For structures
that are not dangerous if they fail, then sometimes designers just use a rule of
thumb. There are many engineering structures where it is difficult if not
impossible to predict the loading that the part will be subject to, and without
this information many of the weld calculations grind to a halt.
Type of joint
The fatigue strength largely depends on the type of joint, some joints being
intrinsically weaker in this regard than other. It is possible to improve the
fatigue strength by grinding welds down to flush with the parent material.
However this is not usually practical to do. Even if welds are ground flush, there
is still the trouble of the internal flaws which will exist.
Stress concentration effects
Weld joints have the worst possible combination of stress concentration
effects:
*
severe geometric Stress concentration factors
*
abundant crack-like discontinuities externally
*
abundant crack-like discontinuities internally
Size effects
The basic principle here is that the bigger the part the lower the fatigue
strength. This is not an intuitive conclusion, but the mechanism is nonetheless
185
simple. Larger parts have the potential to contain more flaws (and more severe
flaws), and so there is a greater chance that one of the flaws may be suitably
lined up to start a crack. Also, the larger the feature, the more the temptation
for the stress lines to deviate, that is a larger Stress concentration factor. In this
regard a small attachment is better than a large one. The current design
philosophies for welded joints do not take size into account: they mainly refer
to joints of a size common in industrial practice.
Weld overfill-reinforcement
The reinforcement of a weld is the part that protrudes above the surface of the
parent material. This is a bad name, since the feature does not add to the
strength of the weld, but actually takes away from it. The better name for the
material is overfill. The smoother the transition between the parent- and the
weld materials, the better the fatigue strength.
In practice it is impossible to create a weld that is completely flat, with no
underfill or overfill, by welding alone. As grinding is not always practical, it is
assumed that a small overfill will usually result. Design is normally based on
such an assumption, and any welds that do not meet the standard may need to
be reworked.
One of the chief tasks of the designer is to try to give the welder the best
practical opportunities of making a good weld. Welding position is important in
this, as is access and the type of weld.
186
Weld root conditions
The same considerations about reinforcement apply to the shape of the root.
The root needs to have a smooth shape, and not have excessive penetration.
Crevices (undercut) are also bad for fatigue strength. It is bad practice, but
relatively common nonetheless, to take care over the reinforcement, but
ignore the root. A fatigue failure can start just as easily in the root as in the top
surface of the weld. In practice the root is difficult to control, because access is
often limited, and the welding might have to be done from one side only. This
situation typically arises in pipe welds.
The welding procedure has a significant effect on the shape of the root bead.
Certain procedures (eg TIG or oxyacetylene) tend to produce a better shape
than others. Furthermore, it is possible to use fusible root inserts, or removable
backing strips. More discussion on backing bars follows below.
Backing bars
Another option to control the root and prevent burn through is to use
permanent backing bars. These are a relatively common approach, but they can
easily be misused. The main requirement is that backing bars must be fully
fused with the parent material. It is in failing to achieve this that a lot of abuse
occurs. There is a temptation to attach the backing bar with poor quality
welds, in the mistaken belief that it is a strip that does not carry any load. In
fact the backing bar becomes an integral part of the joint, and attracts loading
to itself (the Stress concentration factor effect). A poorly welded backing bar
can severely reduce the fatigue strength of the whole joint.
187
Even a properly attached backing bar, as in the diagram, will be susceptible to
failure that originates in the root. The crack will initiate from the ready-made
crack provided by the gap between the bar and the parent materials.
The fatigue strength of a weld with a backing bar is relatively low, even with
good weld practice. The reasons for using backing bars are NOT for strength,
but to provide full penetration joints, allow easier welding, and provide
location for setting up the joint.
Important practices in backing bars are:
 bars must be continuous in length, or properly welded together (with full
penetration joints) if made of joined lengths. There may not be any air
gaps between backing bar segments, since such gaps provide instant
cracks
 any tack welds used to locate the backing strip must be buried by
subsequent welding
 attaching a backing bar with intermittent fillet welds causes a significant
loss in fatigue strength compared to a continuous fillet weld
Other alternatives to backing bars are
 Consumable inserts, which are thin strips put into the root, and which
are completely fused during welding.
 Temporary non-fusible inserts, made of glass, ceramic or copper. These
may be removed after welding.
 Root face. However use one or the other, not both backup strip and root
face, otherwise there will be inadequate penetration. Backup strips are
held in position by short tack welds, preferably staggered. The welder
should make sure that the strip seals the root properly.
Weld orientation
Longitudinal welds are ones that run in the same direction as the applied
loading. The geometry does not create a stress concentration, so the fatigue
188
strength is higher than welds which are transverse to the applied load. All gaps,
lack of penetration, undercut, backing strips, etc, are much less severe in
longitudinally loaded welds. Naturally the internal and external defects
(especially weld ripples and start/stop positions) still exist, and so the fatigue
strength is not as high as an unwelded plate. If a designer can arrange for the
welds to be longitudinal, then they will be stronger. However the geometry
does not always permit this, and furthermore there is often a fair amount of
uncertainty in the direction of the loading. Note that a weld on the long axis of
a structure (eg a longitudinal weld on a pipe), is not necessarily loaded in the
same axis. In the case of a pipe the hoop (circumferential) stress means that
the welds along the axis are actually transverse to the loading, and therefore
the critical ones. The circumferential welds will be less critical (providing that
there is no axial load to stress them).
Weld ends
The end of a weld is a problem area for fatigue. The profile of the end creates a
stress concentration, which reduces the fatigue strength. Many weld cracks
originate from such regions. It is often difficult to eliminate the feature
altogether. However it is also possible to avoid intermittent welds which have
multiple weld ends. Intermittent welds are suitable for saving fabrication cost,
but they are not really compatible with heavily stressed structures. Weld ends
are sometimes wrapped round a structure, mainly to seal the joint from
corrosion. While this is an advantage for corrosion resistance, it does weaken
the joint for fatigue. This is because some part of the weld will now definitely
be across the load path.
189
Welding on edges
Edge welds are not good for fatigue strength. Attachments on the edges of load
carrying members cause low fatigue strength.
Tubular frames
Structural hollow sections have very high stress concentration due to the
bending of the side wall where another tube joins.
Weld quality
Weld quality is important, but only if the defects introduce more severe Stress
concentration factors than those already present in any weld. Typical problems
in welds include:
O
welding flaws: porosity, slag inclusions, lack of fusion (weld bead
detaches from base material)
O
lack of penetration at the root
O
misalignment
190