Defect Assessment with <I>DAC</I> and <I>DAM</I> Methods; Measurement Accuracy Problems

Conventional pulse-echo ultrasonic inspection uses the ratio of the signal from a crack-like defect to the signal from a reference reflector as one factor which determines whether the flaw merits reporting, further sizing, and, possibly, removal. As these defects are smooth, on the scale of an ultrasonic wavelength, and generally flat, and also large relative to the wavelength, they can be successfully modelled using the geometrical theory of diffraction (GTD). GTD is a rapid method for evaluating the ultrasonic signal from a defect. The signal from the reference reflector is easy to calculate if the reflector is a side-drilled hole whose axis is normal to the ultrasonic beam axis and provided it is in the far field of the transducer. If the reference reflector is a flat-bottomed hole then prediction of the signal for non-normal angles of incidence is more difficult since the signal arises from the curved edge at the intersection of the flat bottom of the hole and its cylindrical side face.