When the incident angle is between the first and second critical angles, what ultrasonic wave mode will be observed?

When the incident angle is between the first and second critical angles, the ultrasonic wave mode that is predominantly observed is a shear wave.

In ultrasonic testing, critical angles play a significant role in determining the behavior of sound waves at interfaces. The first critical angle marks the transition point where longitudinal waves start to be refracted into the material, while the second critical angle is where shear waves can propagate through the material. When the incident angle is between these two critical angles, the conditions are met for the generation of shear waves.

Shear waves are distinguished by their ability to move perpendicular to the direction of wave propagation and are sensitive to flaws such as cracks and lamination in the material. In contrast, longitudinal waves (which account for changes in density and are typically faster) cannot propagate in the region between these angles without significant refraction. Compression waves are essentially the same as longitudinal waves and therefore share similar propagation characteristics, thus not being applicable in this specific angular range. Surface waves, on the other hand, typically travel along the surface of the material and are not part of the interaction in question between these critical angles.

This makes the observation of shear waves the correct response when the incident angle lies between the first and second critical angles.