Preliminary Study On The General Strength Criterion Of Advanced Composite Laminates

Advanced composites mainly referring to the fibre reinforced polymer composites have been widely used in the field of aeronautics and astronautics. However, there are some difficulties with the strenghth analysis and safety evaluating of advanced composites. Firstly, it's difficult to find an accurate strength criterion applicable to all kinds of composites; secondly, there is still not a perfect solution to the analysis method of ultimate strength of composites.In this paper, the failure mechanisms and characteristics of laminae under off-axis load are studied, and the failure of laminae is divided into five modes. The coupling effect among the stress components and the effect of off-axis angle on the coupling effects are studied, and then a modified strength criterion is put forward, whose main idea is the involvement of coupling effect, effect of off-axis angle and difference between the modulus of fiber and matrix by introducing stress ratio, coupling parameter and additional stress. The coupling parameter closely related to the applicability of the modified strength criterion is determined by off-axis test. Comparison among the modified strength criterion, three other criteria and experimental data is made and shows that the modified strength criterion is feasible.The analysis method of ultimate strength of composites is also studied in this paper. Based on linear regression method, a simple method considering the nonlinear stress-strain behaviors of composites is given. Based on the yielding model of matrix and ability function of fiber, a degradation model, in which the interactions between failure mode and elastic modulus are considered, is presented and applicable to composites under tensile load. The presented model is tested against experimental data and satisfactory results are obtained. Based on the presented model, influences of biaxial tensile stress ratio and laminate lay-up angle on the stress-strain behavior and ultimate strength of angle-ply laminates are studied.To facilitate the use of the strength analysis method presented in this paper, the strength analysis system of composite laminates is developed based on MSC.Patran/Nastran software, and some widely used strength criteria, constitutive models and degradation models are integrated into the analysis system through PCL language and FORTRAN language. Meanwhile, some examples are done to verify the strength analysis system.