Wavelet signal processing for enhanced Lamb-wave defect detection in composite plates using optical fiber detection

A wavelet-transform-based technique to enhance defect detection in a carbon fiber composite plate interrogated using ultrasonic Lamb waves and incorporating an optical fiber receiver is described. Fundamental symmetric (S_O) Lamb waves were introduced into the sample plates using a conventional piezoelectric transducer operating at a frequency of around 250 kHz. Coupling into the plates was achieved using a perspex phase-matching wedge. The propagating acoustic pulses were monitored using a simple embedded or surface- mounted singlemode optical fiber forming the signal arm of an optical fiber Mach-Zehnder interferometer. The direct Lamb wave reflections from delaminations in the sample plates were of low amplitude, although a degree of defect visibility enhancement was achieved by correlating the received signals with the outgoing ultrasonic pulse. A considerable improvement in the defect visibility over the latter technique was found by using a wavelet-transform-based novelty technique to identify the defective plate zones. Using an orthogonal wavelet transform to compress the data, important structurally related features were extracted by setting appropriate threshold levels on the wavelet coefficients. The reconstructed (uncompressed) data from defect-free portions of the plate were used to construct a template representing a normal condition. Defect location was achieved by analysis of the departure of signals arising from defective plate regions from the no-fault condition template.