Selection of Lamb modes for detecting internal defects in composite laminates

In this paper an investigation is carried out to predict the most efficient leaky Lamb mode and associated frequency to detect defects in a specific layer of a multilayered composite plate. To this end the stress and displacement fields inside a plate are theoretically calculated for different propagating leaky Lamb modes. It is observed that for every leaky Lamb mode the stress field inside the plate varies significantly with depth. A five-layer fiber reinforced composite plate with pre-existing defects, such as missing or broken fibers, and delamination is ultrasonically scanned with different leaky Lamb modes for which internal stress and displacement fields have been theoretically computed in absence of any defect. In presence of defects such as broken fibers and delamination the stresses on the defective surface are significantly reduced. Hence, if these defects are located in a region where the stress level is high for certain leaky Lamb modes then the presence of the defects will affect these modes significantly by releasing the high stress. These specific modes would then be most effective in producing the image of the defect. Theoretical predictions are then experimentally verified.