5.3 Relative dimensions
Measurement of the defect with the help of ultrasonic inspection methods can be performed with limited accuracy, so the determination of the height, width and extent of the defect is conditional. The boundaries of the conditional defect size are determined from the threshold crossing by the amplitude of the echo signal (Fig.6.7).
Fig.6.7 – Determination of conventional size
The relative width (ΔX) is the distance between the conditional boundaries in the plane of incidence of the wave.
The conditional length (ΔL) is the distance between the conditional boundaries perpendicular to the plane of incidence of the wave.
The nominal height (ΔH) is the difference in the readings of the depth gauge at the conditional boundaries.
Fig.6.8 – Measurement of the nominal width and extent of the defect
The conditional size of the flaw can be represented as the sum of the real size and width of the beam at the height where the flaw is located. When measuring defects of the same real size, but located at different heights, the relative dimensions of the flaws will differ.
Fig.6.9 – Relation of the real and the relative flaw size
Determination of the conditional size of the defect can be carried out in two ways: absolute and relative. Each method has a certain order of actions.
For the absolute:
1. – Find the position of the maximum amplitude of the echo pulse;
2. – Set the sensitivity of the Evaluation on the defectoscope;
3. – Determine the conditional dimensions for the boundary positions of the PET;
For the relative:
1. – Find the location of the maximum amplitude of the echo pulse;
2. – Strengthen the amplitude to the threshold;
3. – Increase the gain by the preset number of dB (6 … 12);
4. – Determine the conditional dimensions for the boundary positions of the PET;
Fig.6.10 – Illustration of the relative measurement method
5.4 Form of the defect
Defects in shape can vary significantly from one to another. For ultrasonic inspection problems, the entire variety of defect forms can be classified into several groups. Defects can be compact and extended, rounded and flat.
To assign a defect to rounded or flat ones, the defect shape factor (SF) is used. It is defined as the ratio of two signals directed from different sides to the defect. In the case of a rounded defect, these signals are approximately equal, so the form factor tends to unity. For planar defect, one of the signals will always be much smaller than the other.
Figure 6.11 – Determining the defect shape factor
A defect is compact if any of its dimensions is less than the width of the radiation pattern. An extended defect, one that has at least one dimension larger than the width of the radiation pattern.