An invariant-based strain rate-dependent inelastic deformation model for unidirectional fiber-reinforced composite materials: Model development and finite element implementation

• An invariant-based model is proposed to analyze composites under high strain rate. • Pre-peak non-linearity and strain rate-dependency are included. • The model requires simplified calibration using only uniaxial tests. • The model can predict linearity, non-linearity and rate-dependency with high accuracy. A new invariant-based anisotropic constitutive model is developed and implemented in finite element software via a user-defined material subroutine to predict the strain rate-dependent inelastic deformation response of unidirectional fiber-reinforced composites. A non-associated flow rule captures the evolution of plastic strains, while strain rate-dependency is incorporated using logarithmic scaling functions. The proposed yield function allows for a simplified calibration process that relies solely on standard uniaxial tests, eliminating the need for more complex biaxial or off-axis tests. The ability of the model to predict the pre-peak inelastic response is validated with experimental results for an off-axis unidirectional tape composite and multidirectional laminates comprising unidirectional non-crimp fabric composite plies under quasi-static and high strain rates. The model accurately captures the elastic and inelastic responses of the off-axis unidirectional laminae, with deviations remaining below 10% across most evaluated cases. For multidirectional laminates, the strain rate-dependent effective response is predicted with good accuracy for the symmetric angle-ply laminates, with percentage differences of approximately 1% under quasi-static loading and 4 – 5% under high strain-rate conditions. For multidirectional laminates where local delamination initiates, the model predictions begin to deviate from the experimental data given that the current formulation does not yet consider such damage mechanisms. Overall, these results demonstrate that the constitutive model is well-suited for predicting the pre-peak anisotropic, strain rate-dependent responses of composite laminates.