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 c arbon fiber composite plate interrogated using ultrasonic Lamb waves a nd incorporating an optical fiber receiver is described. Fundamental s ymmetric (S-0) Lamb waves were introduced into the sample plates using a conventional piezoelectric transducer operating at a frequency of a round 250 kHz. Coupling into the plates was achieved using a perspex p hase-matching wedge. The propagating acoustic pulses were monitored us ing a simple embedded or surface-mounted singlemode optical fiber form ing the signal arm of an optical fiber Mach-Zehnder interferometer. Th e direct Lamb wave reflections from delaminations in the sample plates were of low amplitude, although a degree of defect visibility enhance ment was achieved by correlating the received signals with the outgoin g ultrasonic pulse. A considerable improvement in the defect visibilit y over the latter technique was found by using a wavelet-transform-bas ed novelty technique to identify the defective plate zones. Using an o rthogonal wavelet transform to compress the data, important structural ly related features were extracted by setting appropriate threshold le vers on the wavelet coefficients. The reconstructed (uncompressed) dat a from defect-free portions of the plate were used to construct a temp late representing a normal condition. Defect location was achieved by analysis of the departure of signals arising from defective plate regi ons from the no-fault condition template. (C) 1997 Society of Photo-Op tical Instrumentation Engineers.