The method of characterizing a flaw depends on whether the beam diameter of the ultrasonic transducer is smaller or larger than the diameter of the flaw. If the ultrasonic beam diameter is smaller than the flaw size, the beam is used to characterize the flaw by scanning the flawed area at a resolution that is finer than the beam size. If the beam diameter is larger than the flaw, it does not make sense to attempt to image the flaw in this way. Characteristics of the reflected waveform must be used to classify the flaw, which may not be possible. It is generally preferable to image the flaw directly, which means that the transducer beam diameter must be smaller than the smallest flaws to be characterized.
Common transducer focussing techniques permit beam diameters of a small fraction of a millimeter.
In the remainder of this paper, it is assumed that flaws will be characterized by imaging rather than waveform classification. The time required to perform a flaw characterization scan depends upon the resolution. For example, if the flaw was detected using a paintbrush transducer with a 50 mm width, then the area to be scanned for characterization would be approximately 50 mm by 50 mm. Since such a small area is being scanned, the scanner cannot accelerate to its full speed. Assuming that an average scan speed of 50 mm per second can be achieved, a 50 mm x 50 mm region can be scanned with a resolution of 6.25 mm in about eight seconds.