T.O. 33B-1-15-57Figure 5-61. Irrelevant Surface Wave Signals5.3.6.5.4 ParallelBoundaries.When using straight beam inspection near a boundary parallel to the sound beam axis, the spreading sound beamresults in reflections and mode conversion at the boundary (see Figure 5-14). These reflections from the boundaryinterfere with the main sound beam and can greatly reduce the sensitivity for detecting discontinuities close to orcoming from the boundary. Such a case could occur when inspecting a bolt. As the search unit is moved closer to theboundary, the sensitivity is further reduced. When inspecting close to a boundary, it is therefore necessary to use areference standard with the reference discontinuity located at the boundary. An example of such a discontinuity is alateral saw cut (see Figure 5-62). Flaws close to boundaries are better located by using, when possible, angle beamtechniques (see Figure 5-63).
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