ULTRASONIC TESTING
NATURE OF SOUND WAVES
Sound waves are elastic waves
It can be transmitted through both fluid and solid media
Elastic waves with frequency higher than audio range is termed as ULTRASONIC WAVES
Ultrasonic waves used for non destructive inspection of materials are usually within frequency range 0.5 MHz to 20MHz
In fluids sound waves are of longitudinal compression type
Particle displacement is in the direction of wave propagation
In solids they are shear waves and elastic surface waves can also occur(RAYLEIGH WAVES)
Particle displacement is normal to the direction of wave travel
CHARACTERISTICS OF SOUND WAVES
)WAVE VELOCITY
velocity of longitudinal compression waves in a fluid
Vc =√ (Ka/ρ)
Where
Ka is adiabatic volume elasticity
ρ is density
In solids the velocity of compression waves is given by
Vc =√E(1-ν)/(1+ ν)(1-2ν)ρ
where
E is modulus of elasticity
ν is Poisson’s ratio
ρ is density

The velocity of shear waves is given by
Vc = √G/ρ
G is modulus of rigidity
2) WAVELENGTH :-
Related to frequency and wave velocity as
V=λf
Where f is frequency
Sound will only be reflected effectively by objects which have dimensions equal to or greater than the wavelengths of the radiation
ULTRASONIC WAVES
Formed by crystals showing piezoelectric effect
PIEZOELECTRIC EFFECT:- It is the phenomenon in which certain crystal will dilate or strain if a voltage is applied across the crystal faces and conversely an electric field will be created in such a crystal if it is subjected to a mechanical strain, and the voltage produced will be proportional to the amount of strain.
The crystals showing this phenomenon is called piezoelectric crystals
For example QUARTZ, barium titanate, lead metabionate and lead zirconate
When an alternating voltage is applied across the thickness of a disc of a piezoelectric material, the disc will contract and expand and in doing so it will generate a compression wave normal to the discin the surrounding medium