A nondestructive method to detect delaminations and defects in plates

Abstract

Double exposure holographic interferometry, with a ruby laser as the light source, is used to study the influence of local changes in thickness, of delaminations and variations in damping upon propagating transient bending waves in plates and shells. The bending waves are created by the impact of a pendulum. Defects cause changes in the wave impedance of the plate, which will give rise to changes relative to the undisturbed fringe pattern, by reflected and transmitted waves. The size of a defect has to be of the same size or larger than the wavelength of the bending wave to be visible. Since bending waves are dispersive, high frequency components travelling faster than low frequency ones, a small defect is most likely to be observed shortly after impact. The influence of local changes in thickness and delaminations upon the fringe pattern is presented, together with a method to interpret the fringe pattern when several defects are present.