What is wave propagation in ultrasonic testing.

What is wave propagation in ultrasonic testing. 

Ultrasonic testing is based on time-varying deformation or vibrations in materials, which is generally refered to as accustics.All material substances are comprised atoms,which may be forced into vibrational motion about their equilibrium positions.

 Many different patterns of vibrational motion exist at the atomic level,however,most are irrelevant to acoustics and ultrasonic testing.Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave.when a material is not stressed in tension or compression beyond its elastic limit.its individual particles perform elastic oscillation.When the particles of a medium are displaced from their equilibrium position, internal(electrostatic)restoration forces arise.It is these elastic restoring forces between particles,combined with inertia of the particles, that lead to the oscillatory mation of the medium.In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate.Sound can propagate as longitudinal waves, shear waves, surface waves,and in thin material as plate waves.Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.                                                      

      In longitudinal waves,the oscillations occur in the longitudinal direction or the direction of wave propagation since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves.They are also sometimes called density waves because their particle density fluctuates as they move.compression waves can be generated in liquids,as well as solids because the energy travels through the atomic structure by a sereies of comparison and expansion (rarefaction) movements.