One of the major problems limiting the life of critical aircraft engine components, such as compressor discs and spacers, is the formation of low cycle fatigue (LCF) cracks in the fastener bolt holes. Such cracks are often initiated from corners and their surfaces are oxidized during the engine operation. Eddy current techniques using rotating probes are considered to be the most appropriate for detecting bolt hole cracks. Inspection according to damage tolerance criteria requires repeatable detection (90% probability of detection with 95% confidence) of cracks of the order of 0.125 mm (0.005″). If only threshold setting methods are used by a human analyst or implemented by means of electronic instrumentation, detectability can be low since it is difficult to distinguish between the actual flaw signal and noise in the eddy current signal as both are of similar amplitude. However, in certain cases, searching for structure in the noisy waveform can provide indications of defects that escape detection by threshold setting techniques. One way of achieving this is by using multi-parameter signal analysis and pattern recognition methods.
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