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This paper presents a simplified method of predicting the nonlinear stress-strain curves upt to the tensile and compressive failures for an unidirectionally orthotropic lamina, symmetric biaxial and triaxial laminates. The analytical procedure is based on the classical Laminated Plate Theory (L.P.T.). With applying L.P.T. to the small sress or strain increments of the stress-strain curve, nonlinear stress-strain curve is continuously predicted for various laminates. Comparisons are made between analytical predictions and experimental results. As the results, the nonlinear stress-strain curves for various cases could be estimated and the results have considerably good agreement with experimental ones.

Strength and stress-strain properties are investigated for plain woven glass fiber laminates using thin tubular specimens under biaxial static and cyclic loadings. Some composites show large distortion and fiber reorientation close to failure under biaxial load ing. For such composites the usage of the 2nd Piola-Kirchhoff stress instead of the nominal stress is proposed for strength evaluation. The experimental result reveals that the Tsai-Wu criterion fits the biaxial strength expressed by the 2nd Piola-Kirchhoff stress much better than the strength expressed by the nominal stress. Strong interaction between axial and shear deformations is found beyond the knee/yield points while, within the elastic range no interaction is found between the axial and shear stiffness. Fiber misalignment with the loading axis affects the observed stress-strain behavior under cyclic loading. More than two S-S curves are distinguishable when the fiber misalignment is higher than 2°. Such stress-strain relations are successfully simulated using an incremental method in conjunc tion with updating fiber orientation. Good agreement between the calculated result and ex perimental one confirms that the proposed model which considers fiber misalignment can explain the observed stress-strain response under biaxial fatigue loading.

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