: Do anybody know why there are different velocities of ultrasonic waves when applying the probe in different directions of the sample?
reply by Udo Schlengermann
Supposing you are talking about the same wave mode, e.g. transverse waves, velocity will depend on direction, if the elastic constants of the material - here steel - are different in different directions, this is called anisotropy.
Ultrasonic waves are elastic waves. So their velocity depends on the elastic constants of the material and the density. If these parameters change for different directions, e.g. parallel and perpendicular to rolling direction of a steel plate, then the velocities will change with direction or angle of incidence (acoustic anisotropy). Anisotropic steels will even show birefringence, i.e. two reflected echos caused by a single transmitted pulse. Only with isotropic steels the velocities are independent of direction.