ANALYSES OF QUASI-ISOTROPIC COMPOSITE PLATES UNDER QUASI-STATIC POINT LOADS SIMULATING LOW-VELOCITY IMPACT PHENOMENA (FINITE-ELEMENT, FRACTURE MECHANICS)

Composite laminates have high strength to density ratios that make them attractive for use in aircraft structures. However, the damage tolerance of these materials is limited because they have very low ultimate strains, no plastic deformation range, and no usable strength in the thickness direction. These limitations are very obvious when laminates are subjected to impact loads. Due to these impact loads, laminates suffer visible and invisible damage. To improve the material performance in impact requires a better understanding of the deformation and damage mechanics under impact type loads.; In thin composite laminates, the first level of visible damage occurs on the back face and is called "back face spalling." A plate-membrane coupling model, and a finite element model to analyze the large deformation behavior of eight-ply quasi-isotropic circular composite plates under impact type point loads are developed. The back face spalling phenomenon in thin composite plates is explained by using the plate-embrane coupling model and the finite element model in conjunction with the fracture mechanics principles. The experimental results verifying these models are presented. The study resulted in the following conclusions: (1) The large deformation behavior of circular isotropic membranes subjected to arbitrary axisymmetric loading can be obtained by solving a single nonlinear governing equation in terms of a radial stress. (2) Accurate large deflection behavior of circular quasi-isotropic T300/5208 laminates can be obtained by using a simple plate-membrane coupling model. (3) The functional form of deformed shape of the plate undergoing large deformations is different from the small deflection plate solution. (4) The back face spalling action in thin composite laminates is a spontaneous action and can be predicted by using the fracture mechanics principles. (5) Mixed mode (I and II) type deformations probably occur during back face spalling, however, mode I appears to govern the delamination growth during the spalling action.