An improved non-associative plastic flow rule for CF/PEEK thermoplastic composites under low-velocity impact

Carbon fiber-reinforced thermoplastic composites such as CF/PEEK are increasingly used in aerospace structures due to their higher strength and damage tolerance compared to thermoset composites. Existing plasticity models for thermoplastic composites often neglect two critical factors: the hydrostatic pressure effect in the yield function and the dilatant effect in the plastic potential function, leading to inaccurate predictions of impact mechanical behaviors. This study proposes an improved non-associative plastic flow rule that addresses these limitations in the Sun-Chen model for thermoplastic composites under low-velocity impact (LVI). Theoretically, we derive: 1. a hardening law linking the equivalent stress-plastic strain to the uniaxial responses, showing dependence on the off-axis angle, shear stress, and hydrostatic coefficient, independent of the potential function; 2. a transverse plastic Poisson's ratio-based method to calibrate the hydrostatic coefficient in the potential function via the transverse compressive experimental data of thermoplastic composites. The hydrostatic coefficient and shear stress coefficient in the yield function are identified by using the off-axis tensile experimental data of thermoplastic composites. Combining Puck failure criteria and cohesive zone model, the developed model is implemented in ABAQUS-VUMAT to analyze the 150 mm × 100 mm × 2 mm CF/PEEK composite laminates with two stacking sequences under 10J/20J impact energy. Key findings demonstrate that numerical models ignoring hydrostatic pressure overestimate central displacement and plastic dissipation, while underestimating damage dissipation. The incorporation of hydrostatic pressure significantly improves agreement with experimental load-displacement curves and enables precise quantification of deformation mechanisms.