Ultrasonic flaw sizing methods are as follows
This is the simplest method in which the signal amplitude is correlated to flaw size. Typically, a calibration curve relating the area/size of known flat bottom holes and signal amplitude is created. And this curve is used to estimate flaw size. The method is only applicable to flaws that are smaller is size than the beam diameter.
This is a common method used in ultrasonic testing. In this method, the UT probe is moved across the flaw and the 6 dB drop points measured. This measurement is related to the size of the flaws. The method is generally applicable for flaws larger than the beam size. When used for flaws smaller than the beam size, the method measures beam width and oversizes flaws
This method works well when the flaw is within the near field and the phased array beam is focused. Again sizing is done by comparing to known reflector sizes and making corrections. The method oversizes flaws that are past the near field which is also the unfocused zone.
Time of flight Diffraction (TOFD)
This is time measurement based method and is used when inspecting welds. The method requires two probes placed on opposite sides of the weld. The method has high degree of accuracy.
1. Anmol Birring, Sizing Discontinuities by Ultrasonics, Materials Evaluation, November 2010, pp 1208-1215