BASICS OF ACOUSTICS: Measurable flaw parameters and flaw coordinates

5 MEASURABLE FLAW CHARACTERISTICS

After the flaw is detected, the operator must make decisions about the rejection or the suitability of the product for further use. For a reasonable decision it is necessary to determine the measured characteristics of the flaw and to compare them with the values specified in the normative documentation. In most cases, one or more measured characteristics are used as the rejection criterion, which in this case can be called the main measurable characteristics.

The aggregate of all measured characteristics of the defect is summarized in Table 6.1
Table 6.1 – Measured characteristics of the defect

Defect coordinates

Depth of occurrence of the flaw, Y, mm
Distance to the flaw on the input surface, X, mm
Distance to the flaw along the beam, R, mm

 

Amplitude characteristics

Amplitude, N, dB
detectability factor, KD, dB
Equivalent area, SE, mm2

 

Defect coordinates	Depth of occurrence of the flaw, Y, mm Distance to the flaw on the input surface, X, mm Distance to the flaw along the beam, R, mm
Amplitude characteristics	Amplitude, N, dB detectability factor, KD, dB Equivalent area, SE, mm2
Conditional sizes	The conditional boundary of the flaw Conditional width, ΔX, mm Conditional length, ΔL, mm Nominal height, ΔY, mm
Flaw shape	Flaw form factor, KD Compact and extended Round and planar

 

5.1 Flaw Coordinates

To determine the coordinates of the flaw, the flaw detector measures time between the moment of generation of the probe pulse and the moment of arrival of the echo pulse (TEC), as a consequence of reflection from the flaw. The distance to the flaw along the ray (R) is calculated as a segment of the line coinciding with the acoustic axis and connecting the center of the reflector to the exit point of the beam. The time spent by the wave for traveling through the prism of the PET in the forward and backward directions, the so-called the prism delay time (2TEP) is to be excluded.

The distance to the defect along the ray can be calculated by the formula:

Where C – sound velocity in the controlled object.

To find the depth and the distance to the defect on the input surface, you need to use the value of the angle of entry

 

Y – the cause of the negligence of the object; X – the structure of the object to the accuracy of the word; R – the process for the object to go.

Fig.6.1 – Controlling the coding of the product

When calculating X and Y, some errors may arise, primarily related to the determination of the wave velocity in the product, the angle of input (α) and the delay time in the prism, since the values of these parameters are determined by the operator when tuning the flaw detector. The calculation of the coordinates of the defect can also be unreliable if we use an erroneous determination of the exit point of the beam or for the maximum echo signal to receive the signal received from the defect by a lateral ray (Fig. 6.2).

Fig.6.2 – Illustration of one of the reasons for the appearance of the error in determining the coordinates