Near surface defects detectable by ultrasonics

Abstract

It is a mistake to believe that sub-surface flaws cannot be detected using pulse echo due to the presence of a dead zone associated with the surface or interface echo. The dead zone is usually caused by the electronic instrumentation either by the envelope detector or by the overload characteristic of the amplifying chain. By using an accurately controlled acceptance gate situated in the tail of the interface echo, where interference of the flaw echo is most pronounced, this dead zone is avoided and the sub-surface flaws revealed. It requires a narrow or focused ultrasonic beam at the surface and it suffers from the limitations that best results are obtained with smooth surfaces and that the response of flaws tends to be preferentially weighted as they lie further from the surface. However, within these limitations it is an extremely useful technique especially if combined with a suitable scan as is shown by the illustrative results.