What type of ultrasonic waves have elliptical particle motion and travel along a material's surface?

Rayleigh waves are a type of surface wave that exhibit elliptical particle motion and travel along the surface of a material. This unique particle motion occurs because, as the wave travels, it creates movements that are not only in the direction of wave propagation (as seen in longitudinal waves) but also involve vertical oscillations. This results in a rolling motion akin to ocean waves, giving rise to the elliptical trajectories of particles on the surface.

Rayleigh waves are particularly important in the context of nondestructive testing because they are sensitive to surface defects and can provide valuable information about the integrity of a material. Their ability to penetrate only a short distance into the material while retaining sensitivity to surface conditions makes them useful for certain types of inspections.

Longitudinal and shear waves, while also important in ultrasonic testing, do not exhibit the same elliptical motion. Longitudinal waves involve particle motion parallel to the wave direction, while shear waves, or transverse waves, involve particle motion perpendicular to the wave direction. Thus, the distinct characteristics of Rayleigh waves set them apart as the correct choice for the type of ultrasonic wave described in the question.