Global crack detection for aircraft monitoring using bispectral analysis

A global non-destructive testing technique for detecting cracks has been developed aimed at the structural health monitoring of aircraft. The technique uses the bispectrum to analyse the mixing of two ultrasonic sinusoidal waves in a sample. The bispectrum's insensitivity to noise allows more sensitive detection of mixing signals than the power spectrum. Two sinusoidal signals are generated by signal generators and amplified separately, to reduce the amount of mixing outside the sample, and then applied to piezoceramic disks attached to a sample. The sample is excited at very-high-order modes of vibration. The response of the sample is measured by a third piezoceramic disk and the received signal analysed using the bispectrum signal-processing technique. Frequency mixing occurs as a result of the nonlinear behaviour of cracks in the structure. Experiments with fatigue-cracked steel beams demonstrate that fatigue cracks produce a strong mixing effect and that the bispectrum signal relating to the frequency mixing is sensitive to the length of the crack; allowing the quantification of the crack length. Progress is being made in applying the technique to more practical situations: the behaviour of the technique with regards varying the transducer positions and the support conditions has been investigated, the input signal parameters (frequency and amplitude) required for effective detection quantified experimentally and the technique has been tested on thin aluminium and carbon-fibre-reinforced plastic samples. The experimental development of the technique has been supported by a finite-elements model designed to identify the relevant modes of the sample and their behaviour in the crack region.