Finite Element Study of Lamb Wave Interactions with Holes and Through Thickness Defects in Thin Metal Plates

TAP/NDE (Thermo-Acousto-Photonic nondestructive evaluation) has been developed to the point where it is possible to confidently explore many different application areas. One area of interest is the inspection of defects around rivet holes and under rivet heads. This problem is of concern not only from a safety point of view in aging aircraft, but also has major economic implications. For example, a heavy “C” check for a Boeing 707 requires the physical removal, inspection and replacement of many thousands of structural fasteners. Of the various types of rivets the round head rivet hole was chosen for this preliminary analysis (Fig. 1). Since it is not known how Lamb waves interact with an in situ rivet, we have chosen to restrict this analysis to the simplest possible case where there is no rivet in the hole. This corresponds to the current practice of removing the rivet and then using eddy current probes to inspect the hole. Our goal is to develop an inspection technique that does not require the physical removal of the rivet. The buried in-plane defect study was undertaken to provide real time, on-line detection of flaws during the processing of the material to prevent these flaws from causing in-service, premature rivet hole degradation. This inspection would prevent flawed materials from entering service.