NONDESTRUCTIVE EVALUATION OF THE ADHESIVE FILLET SIZE IN A T-PEEL JOINT USING ULTRASONIC LAMB WAVES AND A LINEAR-NETWORK FOR DATA DISCRIMINATION

This paper describes a novel application of ultrasonic Lamb waves comb ined with network methods for data analysis for a non-destructive eval uation of the adhesive fillet size in the cusp of an aluminium T-peel joint. The ultrasonic signals were transmitted and received using a pu rpose-built cross correlator system, which generated filtered pseudo-r andom binary sequences tailored to excite s0 and a1 wave modes in the specimens. Signals received after propagation across the joint area we re analysed in the modulus frequency domain by means of a standard fas t Fourier transform. Simple statistical measures (such as mean and sta ndard deviation) applied to peaks in the frequency spectra did not pro vide a robust basis for automatic discrimination between classes of bo nd fillet size, with 'recognition' success being of the order of chanc e alone. The most basic form of artificial neural network, the linear network, was then trained to recognize bond fillet radius as belonging to one of three categories of size. When presented with regions of bo nd fillet that were not included in its training data, it was able to 'recognize' fillet sizes with a success rate of 95%. The sensitivity o f the method to experimental arrangements was examined by comparing th e results obtained with well-collimated water-coupled transducers with those obtained by using mode-converting contact probes, which exhibit ed greater angular dispersion in the excited waves. Comparisons were m ade between different transducer excitations designed to excite s0 alo ne, a1 alone, s0 and a1 together and also a broadband excitation. Vari ous training protocols for the network were also compared as were the results of output thresholding to minimize the number of wrong decisio ns made by the network. Overall we find that the best fillet size reco gnition performance was obtained with the broadband excitation applied to mode-converting wedge transducers set on either side of the bond.