Investigation of EMAT Base System for the Detection of Bolt Hole Type Cracks. Part 2. Reliability Evaluation and System Definition for Detection of Cracks Under Fasteners by EMATs

Abstract : The detection of laboratory grown fatigue cracks in aircraft wing lap joint fastener holes is demonstrated with EMATs (electromagnetic acoustic transducers). Operating at a 200 kHz central frequency, the EMATs employ acoustic waves in reflection and transmission to measure the presence of both fatigue cracks and loose fasteners. A minicomputer-based data acquisition and analysis system acquires, gates, apodizes and Fourier transforms each received time waveform. The resulting spectra are then compared to the spectra of uncracked holes with tight fasteners to determine the presence of a crack or loose fastener. A method for distinguishing the difference between a crack and a loose fastener is demonstrated. A suggested design for a fieldable instrument capable of the same abilities is presented.