The effect of matrix and fibre cracks on the static strength and stiffness of uni-directional plies and cross-ply laminates under tensile loads was examined with the goal of understanding the main damage mechanisms. The size of matrix cracks emanating from cracked fibres was determined and found to be in excellent agreement with test results. This was used to predict static tensile failure of uni-directional laminates and was shown to be in good to excellent agreement with test results for six different materials. The approach was extended to fatigue loading by determining the number of cycles for the plastic zone ahead of a matrix crack tip to fail. It was combined with an approach to quantify the effect of matrix cracks under any in-plane loading previously developed by the authors to predict changes in laminate stiffness as a function of cycles. Comparisons with test results showed good to excellent agreement between predicted and measured laminate stiffness as a function of fatigue cycles.

A model is described which allows the exact calculation of stresses in a cracked ply under combined state of strain. Closed form expressions for the stresses in each ply are combined in an energy density-based criterion to predict crack spacing and the resulting transverse modulus and shear modulus. Inelastic effects due to non-linearities of the ply-level shear stress-strain curve are accounted for through computation of the permanent shear strain in the ply. The model accounts for ply thickness, stacking sequence and load redistribution effects of a relatively broad class of laminates. Comparisons with test results for a variety of laminates and materials show very good to excellent agreement. The approach developed is extremely efficient and can easily be incorporated in numerical progressive failure analysis, where the stiffness properties of each element can be updated every time the crack pattern changes and in fatigue analysis where the stiffness of a ply or a laminate can be determined during every load cycle.