Page 281 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

266 Chapter Three

detecting all discontinuities in a weld. The advantages and limita-
tions of each method must be clearly understood in order to ensure an
appropriate level of confidence is placed in the results obtained.
Dye penetrant inspection (PT) involves the application of a liquid
which is drawn into a surface-breaking discontinuity, such as a crack
or porosity, by capillary action. When the excess residual dye is
removed from the surface, a developer is applied which will absorb
the penetrant that is contained within the discontinuity. The result is
a stain in the developer that shows that a discontinuity is present.
Dye penetrant testing is limited to surface-breaking discontinuities.
It has no ability to read subsurface discontinuities, but it is highly
effective in accenting the discontinuities that may be overlooked, or
may be too small to detect, by visual inspection.
Magnetic particle inspection (MT) utilizes the change in magnetic
flux that occurs when a magnetic field is present in the vicinity of a
discontinuity. The change in magnetic flux density will show up as a
different pattern when magnetic dustlike particles are applied to the
surface of the part. The process is highly effective in locating disconti-
nuities that are surface-breaking or slightly subsurface. The magnetic
field can be created in the material in one of two ways: the current is
either directly passed through the material, or the magnetic field is
induced through a coil on a yoke. The process is most sensitive to dis-
continuities that lie perpendicular to the magnetic flux path, so it is
necessary to energize the part being inspected in two directions in
order to fully inspect the component.
Ultrasonic inspection (UT) relies on the transmission of high-
frequency sound waves through materials. Solid discontinuity-free
materials will transmit the sound throughout the part in an uninter-
rupted manner. A receiver “hears” the sound reflected off of the back
surface of the part being inspected. If a discontinuity is contained
between the transmitter and the back of the part, an intermediate signal
will be sent to the receiver, indicating the presence of a discontinuity.
The pulses are read on a CRT screen. The magnitude of the signal
received from the discontinuity indicates its size. The relationship of
the signal with respect to the back wall is indicative of its location. UT
is most sensitive to planar discontinuities, that is, cracks. UT effec-
tiveness is dependent upon the operator’s skill, so UT technician train-
ing and certification is essential. With the available technology today,
UT is capable of reading a variety of discontinuities that would be
acceptable for many applications, it is important that the acceptance
criteria be clearly communicated to the inspection technicians so
unnecessary repairs are avoided.
Radiographic inspection (RT) uses x-rays or gamma rays that are
passed through the weld to expose a photographic film on the opposite

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