Damage states in laminated composites were studied by considering the model problem of a laminated beam subjected to three-point bending. A combination of experimental and theoretical research techniques was used to correlate the experimental results with the analytical stress distributions. The analytical solution procedure was based on the stress formulation approach of the mathematical theory of elasticity. The solution procedure is capable of calculating the ply-level stresses and beam displacements for any laminated beam of finite length using the generalized plane deformation or plane stress state assumption. The beam lamination can be any arbitrary combination of monoclinic, orthotropic, transversely-isotropic, and isotropic layers. Prior to conducting the experimental phase of the study, the results from preliminary analyses were examined. Significant effects in the ply-level stress distributions were seen depending on the fiber orientation, aspect ratio, and whether or not a grouped or interspersed stacking sequence was used.

The experimental investigation was conducted to determine the different damage modes in laminated three-point bend specimens. The test matrix consisted of three-point bend specimens of 0° unidirectional, cross-ply, and quasi-isotropic stacking sequences. The dependence of the damage initiation loads and ultimate failure loads were studied, and their relation to damage susceptibility and damage tolerance of the beam configuration was discussed. Damage modes were identified by visual inspection of the damaged specimens using an optical microscope. The four fundamental damage mechanisms identified were delaminations, matrix cracking, fiber breakage, and crushing. The correlation study between the experimental results and the analytical results was performed for the midspan deflection, indentation, damage modes, and damage susceptibility. The correlation was primarily based on the distributions of the in-plane component of shear stress, t_xz. The exceptions were for the case of a very small aspect ratio (less than 1.0) where the crushing mode of damage was predicted based on the maximum contact pressure, and for very large aspect ratios (greater than 12.0) where a maximum tensile bending stress criterion was used for predicting the damage initiation loads.