Effect of ply orientation and through-thickness position of delamination on the reflection of fundamental symmetric S0 Lamb mode in GFRP composite plate structures

This paper studies the interaction of the fundamental symmetric Lamb wave mode S0 with delaminations in 8-layered glass fibre reinforced polymer (GFRP) laminated composites, using numerical models validated against experiments. The validation is performed using experiments on the eight layer quasi-isotropic GFRP composite laminate [0/90/45/ - 45](s). Further numerical studies were performed on the laminates [0/45/ - 45/90](s), [0/ 45/90/ - 45](s), [90/45/ - 45/0](s), [90/ - 45/0/45](s), [45/ - 45/0/90](s), and [45/90/ - 45/0](s) to study the effect of ply layups on the reflection of S0 Lamb waves. The reflection coefficients calculated for these laminates followed a similar trend as the laminate [0/90/45/ - 45](s). Physical insight was obtained into the reflection behaviour, based on an analysis of wave dispersion in the sub-laminates. It was difficult to detect the delamination situated at the mid plane of the laminate, where the shear stress was found to be zero for all ply layups. The delaminations present at a depth greater than DL12 ( > 0.42 mm) from the source of excitation, were found to be accessible for the laminates [0/90/45/ - 45](s), [0/45/ - 45/90](s) and [0/45/90/ - 45](s). The sensitivity of the S0 mode reflection depends upon the position of the delamination along the thickness direction and ply layup orientation of the composite laminate. The SO mode showed higher sensitivity to DL23 and DL34 (0.42 mm to 1.26 mm) in all the laminates that were inspected.