Experimental evaluation of delaminations in composite plates by the use of Lamb waves

The fundamental S₀ mode of Lamb waves can be used in testing for delaminations in plate material. By scanning over the surface beneath which a delamination lies, it is possible to measure the size of the delamination. The amplitude of the wave decreases over a delamination. It has been found that the phase velocity does not change significantly with the depth of delamination. The rate of decrease in amplitude of an individual pulse cycle was detected to vary with the depth of the delamination, being most sensitive to delaminations near the surface of the plate. This is particularly useful when sizing for defects close to the surface, where a normal-incidence pulse-echo ultrasonic method has problems, particularly when the depth of the defect or the back-wall echoes lie within the length of the transmitted ultrasonic pulse. The rate of change in signal amplitude over the delamination also depends on the surface area of the probe in contact with the specimen surface, i.e. a small probe is more receptive to the presence of a delamination. The technique has potential for faster c-scanning of a complete plate than the usual normal-incidence pulse-echo method.