Ultrasonic defect-sizing using decibel drop methods. II

Results are reported of a study performed to investigate the accuracy and repeatability of various ultrasonic decibel (dB) drop sizing methods in determining the length, vertical extent and orientation of artificial and real weld flaws in thin steel section. Seven artificial flaws and nine real weld flaws were examined; over 200 data plots were produced. The general findings are: a) length and vertical extent are assessed most accurately when using a 14 dB drop from the maximum indication amplitude, b) decibel drops less than 14 dB generally undersize flaws while decibel drops greater than 14 dB generally oversize flaws, c) flaws which are smaller than the width of the sound beam cannot be assessed accurately using dB drop methods, d) large flaws are assessed most accurately when the sound beam strikes the flaws at near normal incidence, e) the vertical extent and orientation of large flaws are plotted most accurately using the beam centre line method as opposed to the beam profile method, and f) the limitations of dB-drop-sizing methods have considerable ramifications for CAN3-N285.4-M83 and ASME XI evaluation criteria