Non-destructive testing methods for detection of creep-induced defects by means of ultrasonic surface waves

A method is presented for measuring the propagation velocity of surface waves in the ultrasonic frequency range. The creep-induced defect occurs predominantly at the topmost surface and leads to a minor deceleration of the ultrasonic waves in the material. With increasing penetration depth, the waves increasingly scan the material layers deeper down from the surface, which are increasingly less damaged. This indicates a smooth increase in the propagation velocity. Initial measurements at two damaged pipe bends, both after service times of more than 200.000 hours, exhibit the predicted behaviour. In both cases, the wave velocity increases at damaged sites with increasing penetration depth. The results obtained are in good agreement with metallographic examinations of the detected defect sites at the pipe bends. (orig./MM)