Chapter 29 Shielded Metal Arc Welding 799
Copyright Goodheart-Willcox Co., Inc.
Nondestructive Weld Testing
Nondestructive testing evaluates the integrity of
welds without causing damage. With these methods,
a welding inspector is able to examine the internal
structure of the weld to establish the weld’s integrity
without destroying the welded component. Several of
these tests are described in the following sections.
Surface Crack Detection
A welding inspector can test for surface cracks
that are not visible using simple visual inspection.
Testing methods, such as liquid penetrant dye and
magnetic particle inspection, are used to indicate the
presence and location of invisible cracks. Penetrating
dyes, sometimes viewed under special lighting, fl ow
into cracked surfaces and leave a visible residue at the
crack’s location. See Figure 29-39. The sharp edges of
a crack, when subjected to a magnetic fi eld, cause a
sprinkling of iron particles to stand up along the line of
the cracked surface. These testing methods can be used
without destroying the weld. However, if cracks are
found, the weld is unacceptable.
Radiographic Weld Inspection
Radiographic testing (X-ray testing) makes use of
X-rays to penetrate through a weld onto a photographic
fi lm, resulting in an image of the welded joint’s internal
structure. See Figure 29-40. The amount of energy
absorbed by the object depends on its thickness and
density. Energy not absorbed by the metal causes
exposure of the X-ray fi lm. These areas are dark when
the fi lm is developed, while areas of the fi lm exposed
to less energy remain lighter. Discontinuities, such as
porosity or cracks, appear as dark outlines on the fi lm.
Inclusions of low density, such as slag, also appear as
dark spots. All discontinuities are detected by viewing
shape and variation in density of the processed fi lm.
Ultrasonic Weld Inspection
In ultrasonic testing, a stream of ultrasonic energy
is directed into the weld to be tested. These ultrasonic
waves travel through the weld with insignifi cant
loss, except when they are intercepted and refl ected
by the sharp edges of a discontinuity. This system
uses a transducer, which produces a high-frequency
vibration. When the pulse of ultrasonic waves strikes
a discontinuity in the test piece, it is refl ected back
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Figure 29-37. The change in color around this bead
shows the heat-affected zone.
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Figure 29-38. Penetration of a fillet weld can be
destructively tested by exerting pressure back
toward the welded side.
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Figure 29-39. Dye penetrant testing illuminates
surface discontinuities of a weld.