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Fatigue Cracks Detection and Their Growth Monitoring During Fatigue Test of a Helicopter Tail Boom



In the paper a technique for qualitative assessment of fatigue crack growth monitoring is presented, utilizing guided elastic waves generated by sparse PZT piezoelectric transducers network in the pitch – catch configuration. Two Damage Indices (DI’s) correlated with the total energy received by a given sensor are used to detect fatigue cracks and monitor their growth. The indices proposed carries marginal signal information content in order to decrease their sensitivity with respect to other undesired non-controllable factors which may distort the received signal. The reason for that is to limit the false calls ratio which besides the damage detection capability of a system, plays a crucial role in applications. However even such simplified damage indices can be altered over a long term, leading to the misclassification problem. Considering single sensing path, it is very difficult to distinguish whether the resultant change of DI’s is caused by a damage or due to such DI’s decoherence. Therefore assessment approaches based on threshold levels fixed separately for DI’s obtained on each of the sensing paths, would eventually lead to a false call. An alternative approach is to compare changes of DI’s for all of the sensing paths. A developing damage distort the signal only for sensing paths in its proximity. In order to decrease the misclassification risk a method to compensate such DI’s drift is proposed. The main features and damage detection capabilities of the method will be illustrated on a laboratory fatigue test of an aircraft panel. The proposed approach has been verified on a real structure during fatigue test of a helicopter tail boom.

doi: 10.12783/SHM2015/11

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