Conventional UT

Fundamental of ultrasound

 

Description

Explanation

Ultrasonic waves

Ultrasound can be defined as high frequency mechanical waves (>20Khz). In solids, ultrasound waves have different propagation modes, depending on of the way of vibration of the material particles.

Acoustic Impedance

Z=ρV [Kg/m2s]

Resistance offered to the propagation of an ultrasonic wave by a material. It is obtained by multiplying the density ρ of the material and the velocity V of the ultrasonic wave in the material.

Acoustic pressure

P= Za

Denote the amplitude of alternating stresses on a material by a propagating ultrasonic wave. It is related to the acoustic impedance “Z”  and the amplitude of the particle vibration “a”.

Acoustic intensity

I=P2/2Z=Pa/2

The amount of energy per unit area in unit time.

Types of ultrasonic waves

In longitudinal waves particles vibrates along the direction of travel of the wave. Such waves can propagate in solids, liquids and gasses.

Shear Waves or Transverse waves: the particle movement is at right angle or transverse to the propagation direction. Sound velocity in a material is usually different for shear and longitudinal waves.

Surface waves or Rayleigh waves: are produced in a semi-infinite material. They can propagate in a region no thicker than about one wavelength below the surface material. Particles vibrate following an elliptical orbit.

Lamb waves are generated when a second Boundary surface is introduced, i.e. a plate. They can produce symmetric or antisymmetric vibrations in plates with a thickness of several wavelengths. The particles follow an elliptical orbit.

Wave parameters

λ= c/f = cT

λ – Wavelength [mm]: Distance traveled during the time period.

f  – Frequency [MHz]: Number of cycles per second

c – Velocity [mm/us]: Speed at  which energy is transported between two points in a medium.

T – Period [1/f]: oscillation time.

Velocity of ultrasonic waves

Longitudinal

E = Young’s modulus of elasticity   [N/m2].

ρ = material density [Kg/m3].

μ = Poisson’s coefficient = (E-2G)G

G = modulus of rigidity.

Transverse

Surface

 

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