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184 Part III: Inspection and Test
Pulser/receiver
Oscilloscope or flaw
detector screen
Transducer
Initial pulse
Back surface
echo
Crack
echo
Crack
0 2 4 6 8 10 12
Plate
Figure 17.5 The general inspection principle for ultrasonic testing.
Part III.D.1
Ultrasonic. Ultrasonic testing (UT) uses high frequency sound waves to conduct
examinations and make measurements. The pulse echo technique is accomplished
by introducing sound into the test object so that reflections (echoes) are
returned to a receiver from internal imperfections or from geometrical surfaces
of the part. It is typically used to detect subsurface defects, or defects originating
from surfaces not accessible without disassembly or removal. It can also be used
to detect laminations, lack of fusion, and corrosion of various materials (Aircraft
NDT) (NDT-ED 2008a). Figure 17.5 shows the general inspection principle.
Some of the advantages of ultrasonic inspection that are often cited include
(NDT-ED 2008a):
• It is sensitive to both surface and subsurface discontinuities.
• The depth of penetration for flaw detection or measurement is
superior to other NDT methods.
• Only single-sided access is needed when the pulse-echo technique
is used.
• It is highly accurate in determining reflector position and estimating
size and shape.
• Minimal part preparation is required.
• Electronic equipment provides instantaneous results.
• Detailed images can be produced with automated systems.
• It has other uses, such as thickness measurement, in addition to
flaw detection.
As with all NDT methods, ultrasonic inspection also has its limitations, which
include (NDT-ED 2008):
• Surface must be accessible in order to transmit ultrasound.