Measurement of Defect Sizes and Physical Limitations of Digital Data Processing in Ultrasonic Flaw Detection by Echo and Diffraction Methods

It is noted that manufacturers and suppliers of ultrasonic flaw detectors usually focus on the ultimate sensitivity of the equipment but pay little attention to the error that the linear dimensions of defects can be measured with. It is also noted that the physical principles underlying ultrasonic flaw detection and the accumulated practical experience of its application show that currently, defect sizes can be measured no more accurately than with an error of 1 mm. Therefore, it would be more accurate to talk not about measuring the height of defects in the cross section of welded seams but about estimating this height using ultrasonic flaw detection methods. Examples of representing sections with defects in the form of acoustic B-scans are provided, and it is shown that these images have changed little over the past 60 years, despite the fact that signal processing techniques and technology have developed significantly during this time. At the same time, the article shows that it is possible to qualitatively assess the change in defect height by fractions of a millimeter. In this regard, the results of calculation and experiment on assessing the influence of weak material anisotropy on the shape of the phase spectra of bottom pulses are presented, and it is shown that, in addition to the traditionally used time sweeps of signals (A-scans) and amplitude spectra, it is advisable to pay more attention to the analysis of phase spectra of signals received from products. Phase spectra change significantly more than other characteristics of the recorded pulses also when the height of defects in the cross section of a welded seam increases. This conclusion also applies to cases where the defect height does not exceed the length of the used ultrasonic waves.