Test Part Variables.
Rough surfaces and surfaces with loose or pitted paint, scale, or corrosion distort ultrasonic inspection results, and can
prevent a meaningful inspection due to scattering of the sound beam and/or poor coupling. This can cause:
a. Insufficient ultrasonic energy reaching discontinuities within the part.
b. Loss of resolving power due to an increase in the length of the dead zone caused by a lengthening of the
front surface echo. This is caused by reflections of side lobe energy. On smooth surfaces, the side lobe
energy is not normally reflected back to the search unit, and, therefore, does not interfere with
c. Beam divergence, or widening of the sound beam within the test part.
To minimize these effects, the sound entry surface and the back surface of a test part shall be free from loose, heavy or
uneven scale, machining or grinding particles, or other loose foreign matter. As required, clean parts before ultrasonic
inspection. Current specifications require surface finish of 250 microinches or better; a finish of 125 micro inches is
Geometry of the Part.
The position and shape of the sides and walls of the part can affect the test. A back surface not parallel to the front
surface can cause reflections at other than normal angles, and thus mode conversion in the part; this can cause
confusing indications or complete loss of back reflection.
Flat Sound-Entry Surfaces.
In the case of test parts with parallel front and back surfaces, it is often required to monitor the back reflection signal in
order to evaluate the material and/or assure ultrasonic energy is passing through the part. Any loss of back reflection
may be cause for rejection unless it can be shown that the loss of back reflection is due to a non-parallel back surface or
back surface roughness. If back surface roughness is found to be the cause of the back reflection loss and cannot be
eliminated, the entire test item shall be inspected with another technique to assure conformance to the applicable
specification or test procedure.
Curved Sound-Entry Surfaces.
If the test specimen surface is curved beyond certain limits, a plastic shoe is required to match the search unit face to
the curved surface (see paragraph 18.104.22.168).
For a concave surface, the sound beam tends to be focused as it passes into the test part (see Figure 5-17). Depending
on the depth in the part, discontinuity signals can be increased in amplitude over signals received from an equivalent
discontinuity in a part with a flat sound entry surface.