Response of cross-ply composite to off-axis loading

Polymer composites are known to exhibit nonlinear stress-strain response due to nonlinearly elastic or plastic deformation of the matrix and damage accumulation. Mechanistic modeling of material response explicitly accounting for these interacting factors often leads to complex theories. Plasticity theory formalism provides an alternative for nonlinear deformation description of composite material. We examine the applicability of an orthotropic plasticity model, developed by Sun et al. for unidirectionally reinforced composite, to composite laminate. The response of a symmetric and balanced cross-ply glass/epoxy laminate is studied under uniaxial tensile loading at different angles to the material orthotropy axis. It is found that the associated flow rule and a quadratic approximation of the orthotropic potential function provide satisfactory description for the nonlinear strain component under monotonic loading for 15-45° off-axis angle range. The nonlinearity in on-axis loading (modulus degradation) is well described by stiffness reduction due to the cracks in transverse plies. Meanwhile the change of elastic modulus due to intralaminar cracking can be neglected in off-axis loading, but the possible intensification of nonlinear deformation in off-axis loading caused by the presence of intralaminar cracks agrees well with orthotropic potential formalism.