Reference-free damage localization based on pump-probe measurements
Résumé
Nondestructive testing is often based on comparison between experimental data measured on damaged and intact samples. However, properties of intact samples can change with time, temperature or other external factors. An alternative approach discussed here solely uses data obtained on a damaged sample under varying loading conditions. In our experiments, a series of ultrasonic probe pulses excite a known Lamb mode in a thin aluminum plate subject to vibrations, while damaged region is modeled with a steel ball pressed against the plate. LF vibrations produce changes in contact stresses and displacements which, in turn, slightly modulate each HF reverberation record. By subtracting recorded HF signals from each other we get an array of data that finally allow us to localize the contact. To do so, we apply a backpropagation algorithm in which the differential data are multiplied by phase terms accounting for the Lamb wave propagation from the source to the receiver via a certain point on the plate (pixel). All different contributions from all the receivers are averaged producing a function having the dimension of energy. It was shown that if a pixel is located on an intact zone of the plate, particular contributions are summed up in an incoherent way which results in zero average. An essential peak is obtained only when the pixel is located at the contact. The eventual image was additionally enhanced by application of a special technique that includes random signs for each back-propagated contribution. Without changing the fact that the peak corresponds to a pixel located on damage, each set of random signs change contrast of the obtained image. The final image corresponds then to maximum contrast. The experimental procedure together with data processing are also applied in a passive mode in which ultrasound generation is replaced with an ambient noise.
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