The non-linear and time-dependent stress—strain response of NCF [±45]s laminates in tension is studied. Testing methodology is suggested to separate and quantify the effect of damage development, non-linear viscoelastic effects, and viscoplasticity on the inelastic response. This is achieved by decomposition of viscoelastic and viscoplastic response, both of them being affected by microdamage accumulated during the service life. Material model based on Schapery’s work on viscoelasticity and Zapas viscoplastic function with added damage terms is presented and used. Simulation is performed and validated with constant stress rate tensile tests, identifying the non-linear viscoelasticity and viscoplasticity as the major sources of the non-linear response.

Talreja and coworkers have reported brittle and tough polymer matrix composites to show different cross-ply laminate stiffness reductions due to transverse cracks, despite very similar elastic properties. Studies of crack opening displacement (COD) in [0,,,90,] laminates may help in solution of this problem. Approximate analytical models based on the variational approach pioneered by Hashin were therefore applied in addition to FEM-calculations. Approximate models were concluded not to be reliable for future analyses of this problem. Experimental COD for brittle GF/EP laminates at low crack densities was fairly well predicted by FEM-calculations. None of the models were able to predict experimental data at high crack densities. Residual plastic strains are suggested as an explanation and may also be responsible for reported matrix-related differences in stiffness reduction.