What's ultrasonic test?    
  The application of ultrasonic techniques to determine the elastic properties of materials involves the measurements of velocities of ultrasonic waves. These include longitudinal waves (wherein the particles in the solid material vibrate in the direction of wave propagation) and transverse waves (wherein the particles in the solid material vibrate perpendicular to the direction of wave propagation).
  
  Application:    
  These measurements can be used to determine the elastic stiffness constants of anisotropic material. For this aim, Christoffel’s equation, which relates wave propagation direction and particle displacement directions with the elastic stiffness constants, can be used to characterize the type of anisotropy  
 
 
 

where c is the wave velocity, is the material volume density, U is the particle displacement vector and second-rank Cristoffel’s tensor L is related to the fourth rank elastic stiffness tensor C by

  
 
 
  Thus based on measurements of the velocities of ultrasonic waves propagating along different directions, one can evaluate the elastic constants of a material. Ultrasonic excitation of a surface of an infinite anisotropic solid propagates three types of waves into the material. The faster longitudinal wave is followed by two transverse waves. The directions of particle displacements due to three waves are mutually perpendicular. For longitudinal wave propagation in a perpendicular direction, if the particle displacements of the wave are in the direction of propagation of the wave, the mode is called a pure mode, and the wave is referred to as a pure longitudinal wave. Otherwise the wave propagation mode is called a quasi-mode, and the wave is referred as a quasi-longitudinal wave. Similarly, for transverse wave propagation, if the particle displacements are perpendicular to the propagation direction, the wave is called a pure transverse wave. Otherwise the transverse wave is referred to as a quasi-transverse wave.
In the case os isotropy, material is characterized by two independent elastic constants:
  
   
 


 
   
  where G is the shear modulus, VS and VL are shear and longitudinal wave velocities, E is the Young’s modulus, r is the volume density, and v is the Poisson’s ratio.

  
  Main Advantage:  
  There is no need to destroy a sample in order to measure the mechanical properties.