Search units that include a plastic wedge or standoff between the transducer and the test piece are used for which types of testing?

Search units that incorporate a plastic wedge or standoff between the transducer and the test piece are utilized in various types of ultrasonic testing methods. The function of the wedge or standoff is to modify the angle of incidence of the ultrasonic waves, facilitating better coupling between the transducer and the test material.

In dual transducer straight beam testing, these wedges help to generate and receive longitudinal waves effectively by ensuring proper alignment and minimal signal loss at the material's surface. In angle beam contact testing, the wedge is essential as it allows the ultrasonic waves to enter the material at a predetermined angle, which is crucial for detecting flaws at certain orientations. Similarly, in surface wave contact testing, the standoff or wedge aids in optimizing the interaction of ultrasonic waves with the surface of the test piece, enhancing the ability to detect surface and near-surface defects.

Thus, since plastic wedges or standoffs play a vital role across all these testing methods, the correct answer acknowledges that they are indeed applicable to dual transducer straight beam testing, angle beam contact testing, and surface wave contact testing altogether.