T.O. 33B-1-1
5-57
Figure 5-61. Irrelevant Surface Wave Signals
5.3.6.5.4
Parallel Boundaries.
When using straight beam inspection near a boundary parallel to the sound beam axis, the spreading sound beam
results in reflections and mode conversion at the boundary (see Figure 5-14). These reflections from the boundary
interfere with the main sound beam and can greatly reduce the sensitivity for detecting discontinuities close to or
coming from the boundary. Such a case could occur when inspecting a bolt. As the search unit is moved closer to the
boundary, the sensitivity is further reduced. When inspecting close to a boundary, it is therefore necessary to use a
reference standard with the reference discontinuity located at the boundary. An example of such a discontinuity is a
lateral saw cut (see Figure 5-62). Flaws close to boundaries are better located by using, when possible, angle beam
techniques (see Figure 5-63).
