T.O. 33B-1-15-18Because of the variation in a signal due to curved surfaces, it is best to have a curved surface reference standard for setup of the test. The curved surface of the reference standard should be similar to the curved surface of the test part.Specifically, when performing straight beam inspection on curved surfaces of cylindrical or irregularly shaped products,special ultrasonic test blocks, containing specified radii of curvature and flat-bottom holes of standard diameter may berequired. For inspecting parts with convex surfaces or radii up to 4 inches (8-inch diameter), blocks conforming to theapplicable specification or procedure shall be used. For parts with convex radii over 4 inches, use standard flat faceblocks. For more information see ASTM standout E1315 for steel blocks (Ultrasonic examination of steel with convexcylindrically curved entry surfaces.)5.1.7.3 InternalModeConversion.A frequently misinterpreted form of mode conversion found in the field is shear wave converted to longitudinal. Forexample, on an H-3 sleeve and spindle inspection using a 45 transducer to inspect a large radius or bore, anonrelevant indication occurs in the area of interest as a result of this conversion. At a certain transducer position, partof the shear wave will convert to longitudinal as it reflects from the bore. This longitudinal wave will travel at doublethe velocity of the shear wave and will be reflected to the surface, then back to the bore. It then returns to thetransducer to cause a nonrelevant indication that is similar to a crack indication. In this case, finger damping the partsurface where the longitudinal wave reflects off of the part surface in front of the transducer will identify the indicationas nonrelevant.Figure 5-19. Example of Mode Conversion
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