SELECTION OF LAMB MODES FOR DETECTING INTERNAL DEFECTS IN COMPOSITE LAMINATES

In this paper an investigation is carried out to predict the most effi cient leaky Lamb mode and associated frequency to detect defects in a specific layer of a multilayered composite plate. To this end the stre ss and displacement fields inside a plate are theoretically calculated for different propagating leaky Lamb modes. It is observed that for e very leaky Lamb mode the stress held inside the plate varies significa ntly with depth. A five-layer fiber reinforced composite plate with pr e-existing defects, such as missing or broken fibers, and delamination is ultrasonically scanned with different leaky Lamb modes for which i nternal stress and displacement fields have been theoretically compute d in absence of any defect. In presence of defects such as broken fibe rs and delamination the stresses on the defective surface are signific antly reduced. Hence, if these defects are located in a region where t he 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 pro ducing the image of the defect. Theoretical predictions are then exper imentally verified. (C) 1997 Elsevier Science B.V.