What is the term for the phenomenon that occurs when an ultrasonic wave changes direction at a boundary between materials with different velocities?

The phenomenon that occurs when an ultrasonic wave changes direction at a boundary between materials with different velocities is known as refraction. This occurs because the speed of sound waves varies in different materials due to differences in density, elasticity, and other physical properties. When an ultrasonic wave crosses the interface of two materials with differing acoustic velocities, part of the wave is transmitted into the second material, and its direction changes based on Snell's Law.

Refraction is crucial in nondestructive testing as it affects the propagation path of ultrasonic waves and must be considered when evaluating signals received back from the test specimen. It plays a significant role in imaging and thickness measurements, allowing inspectors to interpret the data accurately.

Other options, while related to wave behavior, do not correspond specifically to the change of direction resulting from differing velocities. Reflection refers to the bouncing back of waves from surfaces, absorption is the conversion of wave energy into other forms of energy within the material, and diffraction involves the spreading of waves as they pass through an aperture or around obstacles. Each of these phenomena serves a different function in the context of ultrasonic testing but does not define the specific process of a wave bending at material boundaries.