Detectability of Known Cracks Under Fasteners Using Ultrasonic Imaging.

Abstract : The detection of fatigue cracks in fastener holes in aircraft structures has been an area of concern for many years. These cracks commonly initiate at the bottom of the countersink, but may be found at the faying surface or at the mid-bore of the hole. Their early and reliable detection is important. There has been considerable research devoted to increasing the sensitivity of nondestructive inspection techniques in order to reduce the minimum detectable crack size, and in particular, detecting those occurring under installed fasteners. This paper describes results obtained using one of these methods, an ultrasonic shear wave technique, to systematically inspect aluminum coupons with a variety of simulated fatigue cracks of known dimensions located at each of the three possible sites. These test specimens were manufactured by the Canadian Forces' Quality Engineering Test Establishment (QETE) as part of an international probability of detection (POD) study. They reproduce the geometry of the CFl16 upper wing skin golden triangle area and are described in greater detail below. Details of the ultrasonic technique are also described elsewhere.