In this article, vibration-based monitoring of cracked shafts is addressed in the context of mechanical transmission of helicopters. The approach developed is based on the detection of cyclostationary components produced by a crack. The cyclostationary theory is first presented on a mechanical point of view. Then a signal processing indicator, the spectral coherence, is introduced in order to quantify the second order cyclostationary content of a signal. The characteristics of the vibration produced in the faulty case are explained from a mechanical model of a cracked shaft. It is then shown that small variations in the structural parameters of the crack creates cyclostationary modulations around the meshing frequencies. This justifies the spectral coherence as a relevant indicator to monitor the crack. The spectal coherence is successful applied on bench data reproducing a crack propagation on an actual helicopter shaft.
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