Improvement of the matched field approach for damage imaging in plates using elastic response-based weights

A popular technique in ultrasonic imaging of defects in structural parts is the Synthetic Aperture Focus (SAF), which traditionally make use of arrays of transducers transmitting ultrasonic signals to the region to be investigated. They operate in time backpropagation mode either locating the reflector by tracking travel time between a transmission and a reception or travel time differences between two receptions. This approach can result in sidelobes and, sometimes, artifacts. We propose here a SAF image reconstruction scheme that stems from the technique of Matched Field Processing (MFP) employed in source localization in underwater acoustics and seismology. MFP includes the systematic comparison of the data recorded from an array of transducers (“data vector”) to data obtained from a model (“replica vector”). The source is then located by looking for the maximum correlation between measurements and all the envisaged trial positions of damage inside the inspected volume. MFP is in practice a method for the solution of an inverse problem and can be easily extended from the location of a source to the location of defects. Instead of including the whole time-histories in the data and replica vectors, in this work we comprise only selected features that are the time-of-flight and the amplitude of the wave reflected by the discontinuity. We consider here the case of a plate and also account for the fact that multiple wave modes can propagate, that are S0, A0 Lamb and SH0 shear modes at the frequency under study. The shape of wavefront generated by these different wave modes is exploited to define weights that are assigned to the elements of the array. In doing so, we force the array to look into a certain direction and improve the focus of the image. The focus is further improved by combining – or compounding- the information derived by the different modes, and has an effect similar to what is done in biomedical imaging, when using several frequencies. The effectiveness of the approach is shown via numerical models and experiments performed on an Aluminum plate with simulated defects, probed by piezoelectric patches used as receivers or transmitters.