In loading process of FRP laminates, two kinds of nonlinearity are observed. One of them is concerned with damage, the other is not. The nonlinearity unrelated with damage is mostly due to the nonlinear property of lamina in shear. On the other hand, the nonlinearity related with damage is caused by matrix cracking evolution in each ply. In the previous works, we presented the theory for predicting a laminate stress-strain response in damage process on the basis of continuum damage mechanics. This paper extends the theory to include the damage unrelated nonlinearity using Hahn & Tsai nonlinear elasticity theory. Comparisons are made between the analytical predictions and the experimental results. The analytical method formulated in this paper can be used for general composite laminates under the plane stress condition.

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Numerous attempts to toughen thermosets like epoxies by incorporation of thermoplastics like PES or PEI have been made. Recently, we found that cyanate ester (CE) /soluble polyimide (PI) system can be a tough alloy with excellent heat resistance. Structure development during the curing reaction in a binary mixture of CE and soluble PI was investigated by light scattering. In the early stage of curing, the binary blend was a single-phase mixture. As the cure reaction proceeded, phase separation took place via the spinodal decomposition induced by the increase in the molecular weight of CE. This was supported by the characteristic change in light scattering profile with curing time. We also found that a ternary system, CE/epoxy/soluble PI system, render tough materials. In the ternary system, structure formation proceeded also by the reaction-induced spinodal decomposition. By incorporating epoxy, the miscibility with soluble PI was enhanced, so that the periodic distance in the two-phase structure was reduced to attain higher fracture toughness. The two-phase structure and fracture toughness in the ternary system was also affected by epoxy content and curing temperature.

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