6.2 The second group of parameters: the dimensions of the PET – the radiation pattern
The beam exit point is the point on the transducer through which the acoustic axis passes. It is determined by the standard CO-3 or CO-3P sample, using a fillet, in a state of a PET with a maximum amplitude. In this case, the center of the fillet on which the rays emerge.
Wave time in the PET (delay time) – this parameter adjustment allows to adjust the depth gauge of the flaw detector, set the true values: X, Y, R.
The angle of the prism (β) directly affects the angle of the input, if it is known for which material the PET is used, then the prism angle is performed in such a way as to achieve one of the standard values of the input angles: 40 °, 45 °, 50 °, 60 °, 65 ° , 70 °.
The radius of the piezoceramic plate (a) determines the parameters of the radiation pattern, such as: the angle of the opening, the boundary of the near zone.
The above parameters of the PET do not depend on the properties of the material and relate to the parameters of the equipment. But in turn, along with the speed in the material, they determine the characteristics of the radiation pattern.
The angle of exposure of the radiation pattern is:
Where n is a coefficient taking into account the shape of the piezoceramic plate
(0.61 for round, 0.5 for rectangular ones); C is the velocity of the wave in the medium; F is the frequency; A is the radius of the piezoceramic plate.
The boundary of the near zone of the radiation pattern is:
Where λ is the wavelength.
The input angle (α) is the angle between the normal to the input surface and the line connecting the center of the omnidirectional reflector (6 mm hole in the CO) to the exit point of the beam at the peak position with the maximum amplitude. The angle of input and the angle of inclination of the acoustic axis differ in their values, and the angle of input is always smaller. This is explained by the fact that in a divergent beam of rays, a beam with a smaller angle than the acoustic axis passes a smaller path, its signal is less attenuated and it is at this position of the PES that the amplitude of the signal is maximal. The greater the depth of the defect and the higher the attenuation coefficient in the material, the greater the angle of incidence with respect to the angle of inclination of the acoustic axis.
Fig.7.1 – Parameters of the PET and the radiation pattern
6.3 The third group of parameters: the error of the depth gauge – the error in determining the coordinates of defects
The error of the depth gauge of the flaw detector consists of inaccurate determination of the time intervals between the pulses, the speed of the wave, the angle of entry, and the delay time in the PET.
The error in determining the coordinates of defects is related to the error of the depth gauge, the inaccuracy of the installation of the PET in the position of the maximum amplitude of the echo signal.