TM 1-1500-204-23-11
(1) Process. The acoustic response of the
(1) Ultrasonics. This method uses high fre-
damaged area is compared with that of a known
quency sound waves, called ultrasound, transmitted
good area. An acoustically lat or dull response is
into the part by a transducer placed in contact with the
considered unacceptable. A dull sound is a good
part. A liquid couplant is used to eliminate air gaps
indication that some delamination or disbond exists,
between the transducer and the part as the air gaps
although a clear, sharp tapping sound does not nec-
interrupt sound being induced into the part. An internal
essarily ensure the absence of damage, especially in
defect, such as a delamination, interrupts the sound
thick panels. The acoustic response of a good part
traveling through the laminate and an indication is
can also vary dramatically with changes in geometry,
received on the ultrasonic unit indicating the presence
therefore, the geometry and interior construction of
of a defect. Interpretation of ultrasonic inspection indi-
the part must be known before performing the tap
cations requires a reference standard made from the
test. If results are questionable, the technician can
same material as the part being tested and contain-
compare the results to a tap test of other like parts
ing laws of known size and location for comparison
which are known to be good or use another inspec-
purposes. Two techniques are in use in the ield,
tion method. The entire area of concern must be
pulse-echo ultrasonics and through transmission ultra-
tapped in order to map the damage. Where multiple
sonics. Ultrasonics is effective in detecting internal
bondlines exist over the core, the core bond cannot
laws in a variety of materials, including parts with very
be evaluated. In a honeycomb structure, the farside
thick sections. However, the laws must be relatively
bondline cannot be evaluated. Thus, two side access
perpendicular to the ultrasonic beam to be detected.
is required for a complete inspection of honeycomb
A disadvantage of ultrasonics for the detection of
structures. Experience has shown that areas that
laws in composite materials is the high attenuation
provide a defect indication do in fact contain defects.
caused by absorption in porous resin and scattering
However, areas that sound good by coin tap may
by the ibers. Hybrid conigurations pose particular
still contain disbonds and delaminations and must be
problems because the abrupt variations in acoustic
inspected using ultrasonic techniques. Surfaces to
properties from one material to another tend to mask
be inspected should have all oil or grease removed
internal laws. Complex part geometry (variable sur-
and should be dry. No peculiar safety precautions
face contours, section thicknesses, wall thicknesses,
are required.
etc.) make ultrasonic inspection dificult. Interpreta-
tion of ultrasonic test responses requires a reference
standard of the same material containing laws of
NOTE
known size and location for comparison purposes.
The following summary provides an overview
For some types of tests, signal gates and sensitivi-
only and is not suficiently detailed for per-
ties must be varied in the course of the test, placing
forming ultrasonic or radiographic inspection
a high demand on operator skill.
of ACM.
(a) Pulse-Echo Longitudinal Wave Ultra-
c. Nondestructive Inspection Method (NDI) The
sonics. This is the most common technique used in
following paragraphs describe the various nondestruc-
the ield. It makes use of a single transducer that
tive test methods that can be used to inspect composite
sends and receives sound energy perpendicular to the
parts and repairs for defects and damage. The actual
entry surface. The sound energy is relected back to
damage detection capabilities vary with defect size,
the transducer by the initial surface of the part, by the
depth, and location; part geometry; type and setup
backside surface of the part, by interfaces between
of inspection equipment; and the skill of inspection
different materials (such as composite and adhesive
personnel. Conditions favorable to accurate damage
interfaces) and by locations of internal defects. The
detection include uniform, thin outer skins, lack of
amplitude of the relected sound wave can be used
obstructions, shallow defect location, and small total
to measure the approximate size of an internal defect
part thickness. The primary NDI method to detect
and the travel time of the relected wave to measure
defects is ultrasonic testing. This method provides
its location from the surface of the article. When
an accurate deinition of the defect and allows evalu-
this method is used by a skilled technician, a skin
ation by comparison to a reference standard. These
delamination can usually be distinguished from a skin
methods, along with the equipment they require, are
to core or skin to substructure disbond.
outlined in TM 1-1500-335-23, Nondestructive Inspec-
tion Methods. Regardless of the speciic equipment
1 Equipment Required. The equipment
available at a particular facility, an assessment of real
required for pulse-echo ultrasonic testing includes a
or suspected damage must be made and disposition
pulsed ultrasonic generator, a transducer which acts
of the structure determined. System speciic tech-
alternately as a transmitter and receiver, and a signal
nical manuals will explain the use of peculiar NDI
processing and display device.
equipment on a particular job.
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