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Piezo crystal elements or composite elements are commonly used in conventional ultrasonic testing. The Piezo effect describes the conversion of electric voltage into mechanical deformations or reverse. UT electronics are suitable to generate electric pulses which will excitate a probe's crystal in the frequency range of ultrasonic waves (typically 1-20 MHz for metal testing). The transducers emitt longitudinal waves (pressure waves / compression waves), which are coupled into the product under test by means of an immersion agent or couplant (water, gel, oil). In solid bodies transverse waves or shear waves (oscillations perpendicular to the wave propagation direction) can be generated as well as longitudinal waves (oscillation direction along the wave propagation direction) and further kinds of waves like surface waves, Rayleigh waves or lamb waves.

For the evaluation the ultrasonic waves arriving at the probe are converted into electric pulses. The evaluation is based on the time of flight and intensity (sound pressure) of the ultrasonic wave; it is possible to draw conclusions on imperfections, material consistence and geometry from Ultrasonic signals. This requires a detailed knowledge about influencing factors to the ultrasonic wave and its way through the material under test. The final evaluation whether a product can be used for it's intended purpose is done by comparing the signals with signals from artificial reflectors in the same material under controlled conditions. The details are rules in technical standards and testing instructions. Two kinds of evaluation of the electrical pulses are possible:

  • Time-of-flight measurement
  • Amplitude measurement and threshold evaluation

Sound propagation / Behaviour at interfaces

When an ultrasonic wave hits a surface with different sound properties at an angle, the acoustic impedance ratio of the two materials involved determines the reflected as well as the transmitted parts (the amplitudes) of the acoustic pressure. The conditions at a material transition are determined by the reflection and transmission coefficient (see equations 1 a, b).

Reflection coeffczint eq1a

(equation 1 a)

Transmission coefficient eq1b
(equation 1 b)

With Z1/2: acoustic impedance in the material 1/2

R: reflection coefficient for the transition of ultrasonic sound of material 1 into material 2

T: Transmission coefficient for the transition of ultrasonic sound of material 1 into material 2

The direction of transmitted and reflected waves is determined by the incidence angle and the ratio of the sound velocities in the materials involved. It is standard to illustrate the propagation direction of an ultrasonic wave by its sound beam. The main sound beam connects wave fronts of the same phase and maximum amplitude. It is at an right angle to the wave fronts and illustrates the main propagation direction of a wave. In general it is valid that the sound velocity of a pressure wave in a material is always higher than the sound velocity of a shear wave in the same material. For the angle ratio for sound beams of ultrasonic waves the law of Snellius (equation 2) applies:

Snellius law eq2
(equation 2)

Grundlagen klBild1