A study of structural damage detection in plates by wavelet transform

In this study, based on the wavelet transform, a method for detecting the location of a flaw in a rectangular plate is proposed. In the experiment, the strain histories caused by impact loads are measured by strain gauges which are mounted on the rectangular plate with two longitudinal-side clamps while leaving the other two sides free. The wavelet transforms are applied to identify the group velocity of flexural wave which were measured and recorded by the strain gauges and a computer. Since the flexural waves in the plate will produce scattered waves when they propagate through the flaw of a plate, the measured signals of a strain gauge content the direct waves propagated from the impact source and the scattered waves propagated from the flaw. The time lag between the direct wave and the scattered wave can be computed by using the simplex algorithm method. By comparing the group velocities of the flexural waves and the time lag between the direct wave and the scattered wave, the exact location of the flaw can be found by the contour map containing the global minimum. The advantage of the method is that less expensive equipments may be used to detect the crack or flaw on a structural plate. However, it cannot detect the shape and the depth of flaws at the present stage. The proposed method may become a feasible alternative among non-destructive inspection methods for damage detection of structural members. It is believed that by utilizing multiple-array measurement and certain numerical analysis to evaluate the scatter waves' spectrum, the flawed shape and depth in a plate may be found in the future.