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.

The Magnetic Field Distribution In Alternative Current.

The Magnetic Field Distribution In Alternative Current. 

When the conductor is carrying Alternating Current, the internal magnetic field strength rises from zero at the centre to a maximum at the surface. However, the field is concentrated in a thin layer near the surface of  the conductor. This is known as the "skin effect."  The skin effect is evident in the field strength versus distance graph for a magnetic conductor shown to the right. The external field decreases with increasing distance from the surface as it does with DC. It should be remembered that with AC the fields constantly varying in strength and direction.
In a hollow circular conductor there is no magnetic field in the void area. The magnetic field is zero at the inside wall surface and rises until it reaches a maximum at the outside wall surface. As with a solid conductor, when the conductor is a magnetic material, the field strength within the conductor is much greater than it was in the nonmagnetic conductor due to the permeability of the magnetic material. The external field strength decreases with distance from the surface of the conductor. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.

Advantages and limitation of ultrasonic testing.

 ADVANTAGES OF ULTRASONIC INSPECTION: - 

 1)   It is sensitive to both surface and subsurface discontinuities.

       
 2)   The depth of penetration for flaw detection or measurement is superior to the NDT methods.

 3)   Only single sided access is needed when the pulse echo technique is used.

 4)   It is highly accurate in determining reflector position and estimating size and shape.

                                  

 5)   Minimal part preparation is required.

                                                                                

 6)   Electronic equipment provides instantaneous results.

 7)   Detailed images can be produced with automated systems.

 8)   It has other uses,such as thickness measurement,in addition to flaw detection.                                        

LIMITATION:-

 1)  Surface must be accessible to transmit ultrasound.

 2)  skill and training is more extensive than with some other methods.

 3) It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.

 4)  Materials that are rough, irregular in shape,very small,exceptionally thin or not homogenous are difficult.

 

 5)   Cast iron and other coarse gained materials are difficult to inspect due to low sound transmission and high signal noise.                                                                                                                        

 6)  Linear defects oriented parallel to the sound beam may go undetected.

 7)  Reference standards are required for both equipment calibration and characterization of flaws.