Application of the endochronic theory to model the mechanical behavior of short-fiber thermoplastics

• Endochronic theory is used to model the mechanical behavior of short-fiber thermoplastics. • The model is implemented as a UMAT user subroutine in the ABAQUS solver. • The model is validated in three-point bending and low-cycle fatigue tests. • Results reproduce nonlinear response and hysteresis loops observed in experiments. Constitutive models based on flow plasticity theory are widely used to describe the mechanical behavior of thermoplastic composites under static and cyclic loading. These models introduce a yield surface that separates elastic and inelastic deformation regions. However, experimental studies show that plastic behavior appears at the initial stages of loading, which makes the identification of the yield surface position challenging. In the present work, the endochronic plasticity theory was employed to model the plastic behavior of composites reinforced with short glass fibers and based on polyphenylene sulfide and polypropylene matrices. The constitutive relations were presented in a general form with an extended set of parameters, enabling accurate modeling of composite behavior without invoking a yield surface. The model parameters were identified by minimizing the deviation function between calculated and experimental curves under uniaxial, off-axis, and shear loading. For the numerical implementation of the proposed approach, the method of successive approximations was used. The model was implemented into the ABAQUS solver using a user-defined UMAT subroutine. To validate the model, three-point bending simulations and fatigue modelling were performed. The overall simulation results demonstrated good agreement with the experimental data. However, several discrepancies remain, which may be attributed to the scatter in the mechanical properties of the thermoplastics and to the parameter identification process.