Testing of Mechanical Properties
STRM 310
Amos Madhlopa
Associate Professor
Department of Engineering
Malawi University of Science and Technology
1
Outline
❑ Introduction
❑ Tensile tests
❑ Compressive tests
❑ Shear tests
❑ Hardness tests
• Brinell
• Vickers
• Rockwell
❑ Impact tests (Izod and charpy)
❑ Fatigue tests
❑ Creep tests
2
Introduction
❑Mechanical properties: physical properties/characteristics
that are displayed by a material when in service such as
when force/load is applied or when subjected to
temperature or a combination of different external loading.
❑Examples of mechanical properties include elasticity,
plasticity, toughness, modulus of (rigidity, elasticity etc.),
tensile strength, compressive strength, elongation,
hardness, brittleness and fatigue.
❑Mechanical
properties
form
part
of
materials
characterisation.
❑Mechanical properties are found by subjecting the material
to different tests and each test is designated by the name of
the test as such hardness test, as discussed later.
3
Introduction (cont’d)
Tests are done for the following reasons (among others):
❑ To establish the suitability of a selected material in
service during the design process.
❑ To test mechanical properties like ultimate tensile
strength (UTS), yield strength (YS), % elongation
etc., used as design inputs.
❑ To make informed choices in using raw materials.
❑ To check properties of material after heat treatment.
❑ To check material property after welding/ brazing
etc.
❑ To comply to set standards.
4
Introduction (cont’d)
❑To ensure batch and production quality.
❑To understand effects of inclusions and defects
in the brittle and ductile behaviour of metals.
❑To understand why and how materials fail when
subjected to external forces.
❑As part of maintenance and to prevent failure in
usage.
❑ Factor of Safety is the ratio comparing the
actual stress on a material and the safe useable
stress.
5
Introduction (cont’d)
❑Stiffness: rigidity of a structural element. Measured from tensile
test. In general terms, this means the extent to which the element
is able to resist deformation or deflection under the action of an
applied force.
❑Strength: Yield, Ultimate, Fracture, Proof, Offset Yield. Measured
as stress (MPa) . Measured from tensile test. Proof stress is the
stress at which a material undergoes plastic deformation. It is
determined as the stress corresponding to 0.2% of strain from
the stress-strain curve of the given material.
❑Ductility: Measure of ability to deform plastically without fracture Elongation, Area Reduction, Fracture Strain - (no units or
mm/mm) . Measured from ductility test.
❑Toughness Resilience: Measure of ability to absorb energy
(J/m3) . Measured from toughness test.
6
Introduction (cont’d)
❑Hardness: Resistance to indentation/abrasion (Various scales,
e.g. Rockwell, Brinell, Vickers.) . Measured from hardness
test.
Two types of material testing
❑Destructive Testing/Mechanical Tests
• The material may be physically tested to destruction or
indentation.
• To measure the strength, hardness, toughness, etc.
• Example: Tensile testing, Impact testing, Hardness
Testing etc.
❑Non-Destructive Tests (NDT)
• Samples or finished articles are tested before being
used and as routine maintenance or quality checks.
• Example: Radiography, Dye Penetration tests etc.
7
Introduction (cont’d)
❑Strength of a material is its ability to withstand an applied load
without failure or plastic deformation.
❑ The field of strength of materials deals with forces and
deformations that result from their acting on a material.
8
Tensile tests
Test piece
Tensile
machine
Computer
9
Tensile tests (cont’d)
❑ Several standards exist for compliance and the choice depends
on design/customer requirements
• ASTM A370, ASTM B557; ASTM E8; ISO 6892-1; EN2002-1
• The most commonly used standard is the ASTM E8M.
❑ Testing is done on a tensile testing machine
• It uses a load cell and an extensometer.
• Force is applied to a test specimen, and % elongation is
obtained from the test.
• Load-elongation graph is then plotted and studied.
10
Tensile tests (cont’d)
❑Variables such as strain, stress, elasticity, tensile strength,
ductility and shear strength are measured and computed.
❑Test specimens can be round or flat.
Round sample
Rectangular/flat sample
11
Tensile tests (cont’d)
❑A stress-strain diagram for a typical steel structure in
tension is shown in figure below.
12
Tensile tests (cont’d)
❑ Some characteristics that can be obtained from tensile tests are:
13
Tensile tests (cont’d)
❑Proof stress
• Sometimes the material does not show clear yield point.
• As such an approximation is used by selecting a corresponding
load such as 0.1% or 0.2% of the extension (elongation).
• The stress found is called proof stress (like point C in graph
below).
• 0.2% is commonly used.
14
Tensile tests (cont’d)
Modulus of resilience: the area under the linear
part of the stress-strain curve.
❑ Fracture strength: the stress at fracture. Is
usually less than UTS.
❑ Ductility: stress-strain total elongation of the
workpiece due to plastic deformation, neglecting
the elastic stretching.
❑ Toughness: the total area under the stressstrain curve which measures the energy
absorbed by the workpiece at fracture.
❑
15
Tensile tests (cont’d)
Slope =
E
Modulus of
resilience
Ductility
Slope =
E
Toughness
16
Compressive tests
❑ Common standards used are ASTM E9, ASTM
D695 and ISO 604.
❑ A compression test is conducted in a manner
similar to the tensile test – and also using the
same machines sometimes. Gripping jaws are
replaced by anvils and the crosshead moves
toward the stationary grip.
❑ Force is compressive and the specimen
contracts along the direction of the stress.
❑ It is also a destructive tests.
17
Compressive tests (cont’d)
Specimen
Apparatus for compressive tests
18
Compressive tests (cont’d)
❑ The goal of a compression test is to determine the
behaviour or response of a material while it experiences
a compressive load by measuring fundamental
variables.
❑ Mechanical properties such as compressive strength,
yield strength, yield point, elastic modulus, and stress–
strain curve may also be determined from compressive
tests.
❑ With the understanding of these different parameters
and the values associated with a specific material it may
be determined whether or not the material is suited for
specific applications or not.
19
Compressive tests (cont’d)
❑Compression testing is usually easier to conduct than
tension test and is used more commonly at elevated
temperature in plasticity or formability studies since it
simulates compressive stress as is expected under
rolling, forging or extrusion operation.
❑This test procedure offers the possibility to test brittle
and nonductile metals that fracture at low strains and
avoids the complications arising out of necking.
20
Compressive tests (cont’d)
Limitations of compression test:
❑Friction between the machine head and the sample
affects the results causing stresses to have small
inclination.
❑Eccentricity may cause instability.
❑Buckling and barrelling complicate testing and can be
minimized by designing the samples as per
specifications and using proper lubricants, for certain
metallic materials.
❑Long samples are prone to buckling, so the length of
the specimen must be limited.
❑Using small samples results in inaccuracies in results
and using large samples requires testing machines with
21
Compressive tests (cont’d)
22
Shear tests
❑Shear stress in a rectangular block, =shear force/area
resisting shear=F/A
F


F
(a)
(b)
a) Shear force (F) and (b) shear stress (), showing a typical
form of failure by relative sliding of planes.
23
Shear tests (cont’d)
❑Shear strain in a rectangular block=angle of deformation=
24
Shear tests (cont’d)
❑ Shear strain in a rectangular block=angle of deformation=
❑ Ratio of shear stress to shear strain is modulus of rigidity (G):
G=/
25
Shear tests (cont’d)
❑ Sheets of various metals, such as aluminium,
brass, mild steel and copper, are available in the
market.
❑ The ultimate shear strength of the material of
the sheet can be obtained by punching a small
hole in sheet and noting down the force required
to punch the hole.
26
Shear tests (cont’d)
If:
d= diameter of the punch
t=thickness of the sheet
A=area under shear=dt
F=maximum force required to punch a hole
Then:
ultimate shearing strength of the material, =F/(dt)=F/A
27
Torsional tests
❑Torsion test is carried out to determine the modulus of rigidity of a
given material.
❑Torsion loads are angular distortion on a component, such as a
shaft, when a moment is applied. (twisting).
❑Torsional forces produce a rotational motion about the longitudinal
axis of one end of the member relative to the other end.
❑torsion tests twist a material or test component to a specified
degree, with a specified force, or until the material fails in torsion.
28
Torsional tests (cont’d)
❑ The purpose of a torsional test is similar to that of tensile test
and the procedure of plotting is the same.
❑ Torque is carefully applied and dots are plotted against the
angle of twist to get a curve similar to stress strain curve in
tensile test.
❑ From the plots, proportional limit and ultimate strength in
shear/torsion can be determined.
❑ Modulus of rigidity (G) can be determined the same was as
modulus of elasticity from the graph.
❑ Torsion test is a test which places a workpiece in pure shear.
29
Torsional tests (cont’d)
30
Torsional tests (cont’d)
31
Hardness tests
Hardness is the resistance of a material to localized plastic deformation
(e.g. a small dent or a scratch).
Brinell hardness test
❑ Spherical indenter is forced into the surface of the metal to be tested.
❑ Diameter of the indenter (hardened steel or tungsten carbide) is 10.00 mm.
❑ Range of standard loads is 5-30 kN in 5 kN increments.
❑ The load is maintained constant for a specified time during a test.
❑ Diameter (d) of resulting indentation is measured and then converted to a
suitable Brinell hardness number (HB) using a chart or table.
❑ HB can also be calculated from:
Indenter
HB =F/(dh)F/(r2)
Where h is indentation depth, r=d/2 and F is the applied load.
32
Hardness tests (cont’d)
Indenter
33
Hardness tests (cont’d)
Vickers hardness test
❑ In the Vickers hardness test, a pyramid shaped indenter is pressed into the surface
of test piece,
❑ Two axes of the diamond shaped indentation are measured (in mm) and averaged to
give the dimension d from which the Vickers hardness (HV) is determined, based on a
HB =
calibration for different kilogram loads (P).
❑ HV is expressed as for example, HV 10, for a 10 kg load or HV 5 for a 5 kg load.
❑ Modern microindentation hardness-testing equipment
is automated and computerised.
Indenter
34
Hardness tests (cont’d)
Rockwell hardness test
❑ Rockwell tests form the commonest method for measuring hardness because they
are simple to perform and require no special skills.
❑ Different scales may be used from possible combinations of various indenters and
loads, a process that permits the testing of virtually all metal alloys.
❑ Indenters include spherical and hardened steel balls having diameters of 1/16,
HB =
1/8,1/4 and 1/2 inch (1.588, 3.175, 6.350,and
12.70 mm, respectively), as well as a
conical diamond (Brale) indenter, which is used or the hardest materials.
❑ A Rockwel hardness number (HR) is determined by the dfference in depth of
penetration resulting from the application of an initial minor load followed by a larger
major load;utilization of a minor load enhances test accuracy.
❑ The Rockwell hardness values are expressed as a combination of hardness
number
Indenter
and a scale symbol representing the indenter and the minor and major loads. For
example, 64 HRC represents the Rockwell hardness number of 64 on the Rockwell
hardness scale of C (see table).
35
Hardness tests (cont’d)
Rockwell hardness test
HB =
Indenter
36
Hardness tests (cont’d)
Rockwell hardness scales according to the ASTM standard E18 (1984),
Scale
Indenter
symbol
Major load
Typical applications
(kgf)
A
Diamond (two scales—carbide and steels)
60
Cemented carbides, thin steel, shallow case-hardened steel
B
1.588 mm ball
100
Copper alloys, soft steels, aluminum alloys, malleable iron
C
Diamond
150
Steel, hard cast irons, pearlitic malleable iron, titanium, deep case-hardened steel, other materials
harder than HRB 100
D
Diamond
100
Thin steel and medium case-hardened steel and pearlitic malleable iron
E
3.175 mm ball
100
Cast iron, aluminum and magnesium alloys, bearing metals
F
1.588 mm ball
60
G
1.588 mm ball
150
H
3.175 mm ball
60
K
3.175 mm ball
150
Annealed copper alloy, thin soft sheet metals
Phosphor bronze, beryllium copper, malleable irons. Upper limit HRG 92 to avoid possible flattening
HB =
of ball
Aluminum, zinc, lead
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not
produce anvil effect.
L
6.35 mm ball
60
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not
produce anvil effect.
M
6.35 mm ball
100
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not
produce anvil effect.
P
6.35 mm ball
150
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not
produce anvil effect.
Indenter
R
12.70 mm ball
60
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not
produce anvil effect.
S
12.70 mm ball
100
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not
produce anvil effect.
V
12.70 mm ball
150
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that do not
produce anvil effect
37
Impact tests
❑ Impact
test conditions are chosen to represent those
most severe relative to the
potential for fracture:
(a) deformation at a relatively low temperature,
(b) a high strain rate (i.e., rate of deformation), and
(c) a triaxial stress state (which may be introduced by
the presence of a notch).
38
Impact tests
❑Two standardized tests for measuring the impact energy
(sometimes also termed notch toughness) are the Charpy and the
Izod,.
❑ The Charpy V-notch (CVN) technique is most commonly used in
the United States.
❑ For both the Charpy and the Izod, the specimen is in the shape
of a bar of square cross section, into which a V-notch is machined.
❑ load is applied as an impact blow from a weighed pendulum
hammer released from
a cocked position at a fixed height h.
❑ Upon release, a knife edge mounted on the pendulum strikes
and fractures the specimen at the notch, which acts as a point of
stress concentration for this high-velocity impact blow.
❑ The pendulum continues its swing, rising to a maximum height
h‘, that is lower than h
❑ The energy absorption, computed from the dierence between h
and h', is a measure of the impact energy.
39
Fatigue tests
❑ Fatigue is a form of failure that happens in structures (such as bridges,
aircraft, machine components) subjected to dynamic and fluctuating
stresses.
❑ Under these circumstances, it is possible for failure to occur at a stress
level considerably lower than the tensile or yield strength for a static load.
❑ Fatigue is important inasmuch as it is the single largest cause of failure
in metals, estimated to be involved in approximately 90% of all metallic
failures; polymers and ceramics (except for glasses) are also susceptible
to this type of failure.
❑ Fatigue is catastrophic and insidious, occurring very suddenly
and without warning.
❑ Fatigue failure is brittle-like in nature even in normally ductile metals in
that there is very little, if any, gross plastic deformation associated with
failure.
40
Fatigue tests (cont’d)
Cyclic stresses
❑ Cyclic stresses induce cracks in the connections of the structural
components.
❑ Applied stress may be axial (tension, compression), flexural (bending),
or torsional (twisting) in nature.
Three different fluctuating stress-time modes are possible: a) reversed
stress cycle: the stress alternates from a maximum tensile stress (+) to a
maximum compressive stress (-) of equal magnitude, b) repeated stress
cycle: maximum and minimum stresses are asymmetrical relative to the
zero-stress level and c) Random stress cycle.
41
Fatigue tests (cont’d)
Cyclic stresses
42
Fatigue tests (cont’d)
The S-N Curve
❑ Fatigue properties of materials can be determined from laboratory simulation
tests.
❑ Commonest type of test conducted in a laboratory setting employs a
Rotating-bending beam: alternating tension and compression stresses of equal
magnitude are imposed on the specimen as it is simultaneously bent and rotated
(reversed stress cycle).
43
Fatigue tests (cont’d)
The S-N Curve
❑ A series of tests is conducted by subjecting a specimen to stress cycling at a
relatively large maximum stress.
❑ This procedure is repeated on other specimens at progressively decreasing
maximum stress levels.
❑ Data is plotted as stress S versus the logarithm of the number N of cycles to
failure for each of the specimens.
❑ For some ferrous (iron-base) and titanium alloys, the S-N curve becomes
horizontal at higher N values; at a limiting stress level called the fatigue limit.
❑ Fatigue failure will not occur below fatigue limit.
❑ Fatigue limit represents the largest value of fluctuating stress that will not
cause failure for essentially an infinite number of cycles.
❑ Most nonferrous alloys (e.g., aluminum, copper) do not have a fatigue limit, in
that the S-N curve continues its downward trend at increasingly greater N values.
44
Fatigue tests (cont’d)
The S-N Curve
❑ For nonferrous materials, the fatigue
response is specified as fatigue strength,
which is defined as the stress level at which
failure will occur for some specified number
of cycles (e.g., 107 cycles)
45
Creep tests
❑Usually, materials are often placed in service at elevated temperatures and exposed
to static mechanical stresses (e.g., turbine rotors in jet engines and steam generators
that experience
centrifugal stresses; high-pressure steam lines).
❑ Deformation under such circumstances is known as creep.
❑ Defined as the time-dependent and permanent deformation of materials when
subjected to a constant load or stress, creep is normally an undesirable phenomenon
and is often the limiting factor in the lifetime of a part.
❑ It is observed in all material types; for metals, it becomes important only for
temperatures greater than about 0.4Tm, where
Tm is the absolute melting temperature.
❑ Amorphous polymers (such as plastics and rubbers) are especially sensitive to
creep deformation.
46
Creep tests (cont’d)
Generalised creep behaviour
❑ A typical creep test consists of subjecting a specimen to a constant load or stress
while maintaining the temperature constant; deformation or strain is measured and
plotted as a function of elapsed time.
❑ Most tests are the constant-load type that yield information of
an engineering nature; constant-stress tests are employed to provide a better
understanding of the mechanisms of creep.
47
Creep tests (cont’d)
Generalised creep behaviour
(a)
(b)
a) Typical creep curve of strain versus time at constant load and constant elevated temperature and b)
Influence of stress  and temperature T on creep behaviour.
48
Creep tests (cont’d)
Generalised creep behaviour
❑ In a typical creep curve, there is an instantaneous deformation that is totally elastic.
The typical creep curve consists of three regions, each of which has its own distinct strain-time feature:
a) Primary region: creep occurs first, typified by a continuously decreasing creep rate that is, the slope of the
curve decreases with time, suggesting that the material is experiencing an increase in creep resistance
or strain hardening deformation becomes more difficult as the material is strained.
b)secondary region: the rate is constant- that is, the plot becomes linear. Usually, this stage of creep has the
longest duration. The constancy of creep rate is attributed to a balance between the competing processes of
strain hardening and recovery, recovery being the process by which a material becomes softer and retains its
ability to experience deformation.
c) Tertiary region: there is an acceleration of the creep rate and ultimate failure. This failure is frequently
termed rupture and results from microstructural and/or metallurgical changes, for example, grain boundary
separation, and the formation of internal cracks, cavities, and voids. In addition, for tensile loads. a neck may
form at some point within the deformation region. These all lead to a decrease in the effective cross-sectional
area and an increase in strain rate.
❑ For metallic materials, most creep tests are conducted in uniaxial tension using a specimen
having the same geometry as for tensile tests.
49
Creep tests (cont’d)
Generalised creep behaviour
❑ Uniaxial compression tests are more appropriate for brittle materials; these provide a better
measure of the intrinsic creep properties because there is no stress amplification and crack
propagation, as with tensile
Loads.
❑ Compressive test specimens are usually right cylinders or parallelepipeds having
length-to-diameter ratios ranging from about 2 to 4.
❑ Creep properties are virtually independent of loading direction for most materials.
❑ The most important parameter from a creep test is the slope (/t) of the secondary portion
of the creep curve, often called the minimum or steady-state creep rate.
❑ The steady-state creep rate is the engineering design parameter considered for long-life
applications,
such as a nuclear power plant component that is scheduled to operate for several decades,
and when failure or too much strain is not an option.
❑ For many relatively short-life creep situations (e.g., turbine blades in military aircraft and rocket
motor nozzles), time to rupture or the rupture lifetime tr is the major design consideration.
50
Creep tests (cont’d)
Stress and temperature effects
❑ Both temperature and the level of the applied stress influence
creep characteristics.
❑ At a temperature substantially below 0.4Tm, and after the initial
deformation, the strain is virtually independent of time.
❑ Increasing stress or temperature will lead to:
(1) increase in the instantaneous strain at the time of stress
application,
(2) increase in steady-state creep rate, and
(3) decrease in rupture lifetime.
51
Exercise 1
1) A 25mm square cross-section bar of length 300mm carries an axial
compressive load of 50kN. Determine the stress set up in the bar and its
change of length when the load is applied. For the bar material E = 200
GN/m2. (Solution: 80 MN/m2; 0.12mm).
2) A simple turnbuckle arrangement is constructed from a 40 mm
outside diameter tube threaded internally at each end to take two
rods of 25 mm outside diameter with threaded ends. What will be the
nominal stresses set up in the tube and the rods, ignoring thread
depth, when the turnbuckle carries an axial load of 30 kN? Assuming
a sufficient strength of thread, what maximum load can be transmitted
by the turnbuckle if the maximum stress is limited to 180 MN/m2?
(Solution: 39.2, 61.1 MN/m2, 88.4 kN).
52
Exercise (cont’d)
3) A steel bar ABCD consists of three sections: AB is of 20mm
diameter and 200 mm long, BC is 25 mm
square and 400 mm long, and CD is of 12 mm diameter and 200mm
long. The bar is subjected to an axial compressive
load which induces a stress of 30 MN/m2 on the largest cross-section.
Determine the total decrease in the length of
the bar when the load is applied. For steel E = 210GN/m2. (Solution:
0.272 mm).
53