In contact testing, shear waves can be induced in the test material by

• A. Placing a spherical acoustic lens on the face of the transducer
• B. Using two transducers on opposite sides of the test specimen
• C. Placing an X cut crystal directly on the surface of the material, and coupling through a film of oil
• D. Using a transducer mounted on a plastic wedge so that sound enters the part at an angle

Answer: (D)

In contact testing, shear waves are generated by mode conversion that occurs when a longitudinal wave hits the test surface at an oblique angle. Mounting the transducer on a plastic wedge sets up that controlled oblique incidence, so part of the energy converts to a shear wave in the material as the wave enters. This angle-beam arrangement is specifically designed to produce the shear mode, which travels with a different velocity and can reveal flaws that longitudinal waves might miss.

Other approaches don’t reliably create the required angle and boundary conditions for this mode conversion. A spherical lens changes beam shape but not the necessary oblique incidence to trigger shear generation. Using two transducers on opposite sides mainly transmits energy straight through and doesn’t rely on conversion to shear waves. Directly placing an X-cut crystal on the surface with an oil film would still favor the transducer’s primary mode rather than consistently producing a shear wave in the material.