An ultrasonic solution for second-layer crack detection

Eddy current methods often provide adequate crack detection in thin non-ferrous multi-layer structures such as aircraft fuselage skins. However, as the first layer becomes thicker, the second-layer crack detectability quickly becomes inadequate using eddy currents. This is the case in many aircraft wing structures where the structurally-critical spar flange is often hidden beneath a thick skin layer. For many years, ultrasonic methods have been ruled out due to the difficulty and unreliability of coupling the ultrasound into the second layer. Whilst this situation has not fundamentally changed, the improved adhesion and widespread use of sealants for corrosion protection. and to seal wing fuel tanks, has resulted in some opportunities for the use of ultrasound in the second layer of wing skins. Groups in the USA and Australia have developed techniques for second-layer crack detection using single-element angle probes and have evaluated the resulting probability of detection for their techniques. The NDE Group at QinetiQ has recently developed a method using an angled phased-array probe that can acquire full-waveform data in a fraction of the time. However, regardless of the acquisition method, the scans are somewhat complex to understand, so an automated analysis program has been developed to enhance the distinction between crack indications and potential false calls. This program also sentences each fastener hole using pass/maybe/fail criteria and attempts to size any crack that is found - based on calibration data from an EDM-notched specimen. This paper describes the difficulties of the complex analysis required and explains the methodology behind the automated analysis technique. Examples are given on EDM-notched real structures.