Figure 5-43. Example of Determining the Sound Beam Path in a Test Part with a Curved Surface

Figure 5-42. Generation of Unwanted Longitudinal and Surface Waves on Curved Surface

Figure 5-44. Straight Beam Inspection of Test Part with Curved Surface

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 5-39 NOTE Unwanted surface waves can be detected by noting additional unexpected signals on the waveform display. If these signals can be damped and traced to their source using an oil-wetted finger as explained in 5.3.3.3, unwanted surface waves are being generated. j. When designing shoes for curved surfaces, the sound beam path in the shoe and the test part must be considered in order to assure coverage of the area of interest within the test part. Generally the sound beam path in the shoe can be considered to be a straight projection of the face of the search unit; in almost all cases the sound travel in the shoes will be in the near field, characterized by no beam spread (see paragraph 5.1.6.2.2 and Figure 5-12). The beam path in the part can be obtained by using Snell's Law to determine the refracted angle at various points across the sound beam where it enters the test part surface (see paragraph 5.1.5.1 and Figure 5-43). Figure 5-43. Example of Determining the Sound Beam Path in a Test Part with a Curved Surface. k. With certain inspection setups, particularly when using shoes to generate straight beams in parts with curved surfaces, multiple reflections from the shoe-to-test part interface can interfere with the inspection. To avoid this, the shoe shall be made thick enough to avoid interference with the intended inspection application. Consider the inspection setup shown in Figure 5-44. It is important only that the inspector be able to recognize and identify indications on the waveform display. Reflections caused by the shoe are easily recognized simply by raising the shoe off the surface of the material. If the indications remain on the screen, the plastic shoe is the cause. Slotting the shoe as shown in Figure 5- 41 may reduce or eliminate such interference signals. It is not necessary for the operator to calculate the sound paths to and from various reflectors. However, it is important the operator know how to recognize nonrelevant indications from the reflectors and minimize their cause.

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