Non-destructive testing of hollowing defects in external wall tiles

A non-contact inspection method incorporating acoustic-optical fusion is proposed in this paper to detect hollowing defects in external wall tiles in order to overcome the practical problems of traditional contact detection methods. Based on laser speckle interferometry (LSI) technology, this approach employs audio loading to detect hollowing in the wall structure. In this paper, the ceramic tile above the hollowing is equivalent to a circular thin plate with peripheral fixed support. Following Kirchhoff's classical theory of the circular plate, the circular plate displacement function based on the improved Fourier series is employed for theoretical modelling. The theoretical natural frequencies of the circular plate under peripheral fixed support boundaries are derived and compared with finite element simulation results, a theoretical mapping model of the structure vibration signals and the laser speckle signal is established and the phase difference change diagram and Bessel interference fringe pattern are derived. The result indicates the accuracy of the vibration theoretical model based on the classical Kirchhoff circular plate theory. Subsequently, an experimental system is established and non-destructive testing experiments for hollowing defect inspection are conducted based on theoretical predictions. Different acoustic parameters are used to excite the wall sample and interference fringe patterns are obtained through use of a laser speckle interferometer. The results indicate that the experiment result is consistent with the theoretical result and the fringe pattern near the theoretical solution value of the natural frequency is more pronounced, confirming the effectiveness and feasibility of the non-destructive testing technique for characterising hollowing defects based on laser speckle interferometry.