Some improvements on Sun–Chen’s one-parameter plasticity model for fibrous composites – Part I: Constitutive modelling for tension–compression asymmetry response

Herein, a one-parameter plasticity model proposed by Sun and Chen [Sun CT and Chen JL. A simple flow rule for characterizing nonlinear behavior of fiber composites. J Compos Mater 1989; 23: 1009–1020] demonstrates features that make it highly attractive for characterizing non-linear responses of fibrous composites. However, a detailed exploitation of the model’s potential has been halted by unresolved problems that include tension–compression asymmetry in stress–strain curves, FEM implementation as well as optimal parameters determination, which are addressed in this investigation as well as proposed solutions are presented. The major focus in Part I of this three-part study was devoted to developing a simple model for predicting the tension–compression asymmetry in stress–strain curves for fibrous composites, which was based on Sun and Chen’s one-parameter plasticity model. A generalized Hill yield criterion was proposed from combinations of the Drucker–Prager yield criterion that considers the effect of hydrostatic pressure for isotropic materials and the Hill yield criterion suitable for anisotropic materials. By incorporating the yield strength-differential effect on the plastic flow rule in composite laminates, the one-parameter plasticity model was extended to a strength-differential effect-incorporated model. The improved model has been calibrated and validated by off-axis tension and compression tests on unidirectional carbon/epoxy (IM600/Q133) composite laminates. Results verified that the proposed model captured the complex tension–compression asymmetry in observed non-linear responses of stress–strain curves.