a / â are symbols attached to data used to calculate probability of detection (POD) curves. a represents a crack size dimension, which could be the depth, through-wall extent, length or area.
â represents the signal amplitude or other relevant characteristic depending on the non-destructive testing technique applied. Test-pieces containing defects with known dimensions are inspected a number of times with the same technique and the different signal amplitudes, â, are recorded for each crack size, a.

A plot of â on the y-axis and a on the x-axis is produced similar to that shown. Alternatively, this could be log(â) versus log(a) or any combination that is applicable. Due to random errors in the measurement, any particular value of a will have a distribution of â values associated with it. The inspection will have a detection threshold, â0 , above which signals will be detected. The analysis of the proportion of â signals that are above the â0 threshold for a particular value of a allows the POD for that a value to be derived. Hence, a POD versus a curve can be established.

For more information on a / â see: https://bit.ly/2HLTusg

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org