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Gas Tungsten Arc Welding Handbook
The test can be modifi ed by adding fl uorescent
dye to the particles. In this method, an ultraviolet light
is used to illuminate fl uorescent dye on the iron parti-
cles, allowing the inspector to clearly see and interpret
the formation of the particles at the defect. As with
fl uorescent penetrant testing, it may be necessary to
examine the weldment in a darkened area.
Accurate interpretation of magnetic particle tests
requires training. Discontinuities revealed by the test
pattern can be misleading to the untrained eye and
may have no consequence on the weld’s acceptability.
If the size of the discontinuity falls within allowable
limits, the weld is still acceptable. If the size of the
discontinuity is larger than the allowable limit, the
weld is rejectable.
Ultrasonic Test
Ultrasonic testing (UT) is a nondestructive
method of detecting the presence of internal cracks,
inclusions, segregations, porosity, lack of fusion, and
similar discontinuities in all types of metals. It can be
used as the sole type of inspection, or it can be used
with other types of testing. UT is often used in conjunc-
tion with radiographic testing because it determines
the depth of the defect from the test surface.
In ultrasonic testing, very-high-frequency sound
waves are transmitted through the part to be tested. The
sound waves then return to the sender and are displayed
as a graph on a monitoring screen for interpretation.
Since very-high-frequency sound waves travel
only short distances in air, the test must be done with
the part (signal sender) and the transducer (receiver)
immersed in water or with the transducer coupled to
the workpiece by a thin liquid fi lm. These two methods
are shown in Figure 18-8. UT inspection techniques
include several different patterns and techniques. The
technique used depends on the material, weld thick-
ness, welding process, and inspection criteria being
used. Where tests are required out-of-perpendicular
with the transducer, a wedge or angle block is placed
under the transducer at the desired angle to properly
scan the material, as shown in Figure 18-9.
Ultrasonic testing is portable and nonhazardous. In
addition, UT inspection has the following advantages:
• Great penetration power allows the testing
of thick materials.
• High sensitivity allows detection of small
discontinuities in a short period of time.
• Inspection can be done from one surface.
The major disadvantage of ultrasonic testing is
the advanced skill required to properly interpret the
results. Weld design, location of the defect, internal
structure, and complexity of the weldment affect the
interpretation of the ultrasonic signal.
In order to achieve the desired results, calibration
blocks and reference weld samples are used to cali-
brate the equipment prior to making the test. With the
proper calibration, the operator can then interpret the
results to the inspection specifi cation.
Radiographic Test
A radiographic test (RT) is a nondestructive
method that reveals the presence and nature of discon-
tinuities in the interior of welds. This test makes use
Transducer
Water
Immersion Test Contact Test
Coupling
fluid
Transducer
Cathode ray
tube (CRT)
Figure 18-8. Ultrasonic tests are made with the part and
the transducer submerged in water. If this is not practical,
the transducer is coupled (connected) to the test area by a
thin layer of liquid.
Cathode ray tube (CRT)
Transducer
Angle Test
Plastic mount block machined to fit
transducer at desired angle.
Figure 18-9. Wedges or angle blocks are used to send
and receive ultrasonic signals in areas where signal
transmission could be blocked in a straight-line plane.