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Description |
Explanation |
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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. |
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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. |
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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”. |
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Acoustic intensity |
I=P2/2Z=Pa/2 |
The amount of energy per unit area in unit time. |
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Types of ultrasonic waves |
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In longitudinal waves particles vibrates along the direction of travel of the wave. Such waves can propagate in solids, liquids and gasses. |
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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. |
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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. |
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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. |
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Wave parameters |
λ= c/f = cT |
λ – Wavelength [mm]: Distance traveled during the time period. |
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f – Frequency [MHz]: Number of cycles per second |
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c – Velocity [mm/us]: Speed at which energy is transported between two points in a medium. |
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T – Period [1/f]: oscillation time. |
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Velocity of ultrasonic waves |
Longitudinal |
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E = Young’s modulus of elasticity [N/m2]. ρ = material density [Kg/m3]. μ = Poisson’s coefficient = (E-2G)G G = modulus of rigidity. |
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Transverse |
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Surface |
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