252
Welding Engineering
Table 13.1
Technique
Visual
Liquid
penetrant
Magnetic
particle
Radiography
Ultrasonic
Copyright © 2016. John Wiley & Sons, Incorporated. All rights reserved.
13.4
A summary of common NDT techniques
Applications
Welds with surface
discontinuities
Welds with
discontinuities
open to the surface
Ferrous materials
with cracks open to
the surface
Surface and
subsurface defects,
works with
virtually all
materials
Can detect virtually
all weld defects in
most materials,
thickness
measurement
Advantages
Economical, little
training required
Use with all materials,
inexpensive, portable,
expedient, results easy
to interpret
Relatively economical,
interpretation is easy,
equipment is portable
Provides permanent
record, can be used in
the field (gamma rays),
detects buried flaws
Most sensitive of all
techniques, provides
info on size and location,
portable, provides
permanent record
Limitations
Limited to gross surface defects,
very subjective
Proper surface prep required,
only detects surface flaws, false
positives possible
Only ferromagnetic materials,
surface or near‐surface flaws
only, parts may be magnetized
Sensitivity is a function of
material type and thickness,
must have access to both sides
of weld, interpretation can be
difficult, safety issues, expensive
Need good surface conditions
for ultrasonic “coupling”, skilled
operator/interpreter required,
reference standards needed,
equipment relatively expensive
Introduction to Fractography
Fractography refers to a method for determining the cause of a failure by examining the
­microscopic features of the failed fracture surface. This technique can be quite effective for
determining why welds fail. It typically requires the use of a powerful Scanning Electron
Microscope (SEM) as well as a knowledge of characteristic fracture surface features.
A significant amount of information about the material and failure mechanism can be
revealed on a fracture surface. For example, Figure 13.25 shows fracture surface features
that are characteristic of a ductile failure, and are known as ductile dimples or shear
­dimples. This type of fracture surface indicates that the failure occurred due to overload,
such as the case when a part is under‐designed or the weld is too small for the load it is
expected to support.
Fatigue cracks typically create very distinctive fracture surfaces. Figure 13.26 shows the
fracture surface of a Ti‐8Al‐1Mo‐1V alloy. Features known as beach marks and striations
provide evidence of the cyclic progression of the crack tip during its growth. The very fine
striations (Figure 13.26a) represent individual stress cycles, and generally are aligned
­perpendicular to the fatigue crack direction. The schematic on the right (Figure 13.26b)
­represents a typical fatigue fracture surface, which often also reveals beach marks that point
to the crack origin, as well as a region of static or overload failure. The overload failure
occurs after the fatigue crack gets so large that the remaining material can no longer support
the load. This portion of the fracture surface would be expected to reveal the ductile dimple
features shown in Figure 13.25.
A brittle fracture surface will exhibit what is known as chevron markings (Figure 13.27).
A weld that fractures this way is known to have poor impact properties or notch toughness. For
Phillips, D. H. (2016). Welding engineering : An introduction. John Wiley & Sons, Incorporated.
Created from biblioucv on 2023-05-30 02:53:17.
253
Weld Quality
10 μ
3792
Figure 13.25
Ductile dimple fracture surfaces indicate a ductile or a simple overload failure
(a)
(b)
Area of fatigue
Origin
Copyright © 2016. John Wiley & Sons, Incorporated. All rights reserved.
Area of
“static”
failure
Figure 13.26 Fatigue failures produce very distinctive fracture surfaces. (a) Electron microscopic fractograph of a Ti‐8Al‐1Mo‐1V alloy and (b) Schematic representation of a typical fatigue fracture surface
(Source: Unpublished Work at Battelle)
example, steels that are Charpy V‐Notch tested at temperatures below their ductile‐to‐brittle
transition temperature would be expected to exhibit this type of fracture surface. This example,
and those just reviewed represent a small sampling of the many characteristic fracture surface
features that can be evaluated in order to determine the cause of a failure.
Phillips, D. H. (2016). Welding engineering : An introduction. John Wiley & Sons, Incorporated.
Created from biblioucv on 2023-05-30 02:53:17.