Characterizing and Modelling the Strain Rate-Dependent Constitutive Response of a Unidirectional Non-Crimp Fabric Composite Material

Abstract

Liquid molded composite materials comprising heavy-tow non-crimp fabric reinforcements are increasingly being used for primary load-bearing structures in the automotive sector due to their excellent specific mechanical properties, high anisotropic tailorability, and comparatively lower cost in contrast to the other fabric reinforced composite material systems. Automotive structures may be subject to impact loads during their service life; therefore, a thorough comprehension of the constitutive behavior of non-crimp fabric composites at different strain rates is required. This study focuses on characterizing the strain rate-dependent deformation response of a unidirectional non-crimp fabric (UD-NCF) composite, comprising PX35 UD-300 carbon fiber fabric and TRAC 06150 snap cure epoxy, through physical and virtual experiments. A constitutive model is also developed and implemented for predicting the strain rate-dependent pre-peak deformation response of the same material system. The anisotropic strain rate-dependent deformation and failure response of the UD-NCF carbon fiber/snap-cure epoxy composite was characterized through physical experiments, including transverse tension, transverse compression, longitudinal compression, and in-plane shear. Hydraulic testing machines were used for performing quasi-static and intermediate strain rate tests, while tensile and compressive split Hopkinson bar apparatuses were used for the high strain rate tests. The stress-strain response for the in-plane shear and transverse compression modes was initially linear, followed by a nonlinear response at all strain rates. The yield stress and ultimate strength for the transverse compression mode were found to increase by 54% and 50%, respectively, with an increase in strain rate from 0.003 s-1 to 260 s-1. For the in-plane shear mode, the yield stress and ultimate strength were found to increase by 60% and 61%, respectively, with an increase in axial strain rate from 0.003 s-1 to 315 s-1. The stress-strain response for the longitudinal compression and transverse tension modes was approximately linear until failure. The longitudinal compression strength increased by 35% with an increase in strain rate from 0.001 s-1 to 70 s-1. Inter-tow splitting, delamination, and localized fiber kinking was observed on the corresponding failure surface, which is distinct from that typically observed in UD tape composites. The transverse tensile strength increased by 20% with an increase in strain rate from 0.1 s-1 to 126 s-1, with the fracture surface revealing pull-out of supporting glass fibers and local matrix plastic deformation. A dual-scale computational modelling framework was developed for predicting the strain rate-dependent nonlinear deformation response of the UD-NCF composite. For the micro-scale finite element model, the elastic-plastic response of the epoxy was captured using the linear Drucker-Prager model, while the carbon fibers were treated as linear elastic and transversely isotropic. For the mesoscale finite element model, Hill’s anisotropic yield function was used for modelling the effective elastic-plastic response of the impregnated tow. The Johnson-Cook model was used to capture the strain rate dependency of the epoxy and impregnated tow. The predicted and experimentally measured stress-strain response at quasi-static strain rates was found to be in good agreement. Slight discrepancies were found between the experimental and predicted stress-strain response at high strain rates at higher applied strains, which may be due to the fact that the existing material models do not consider the viscoelastic material behavior. The in-plane and out-of-plane shear stress-strain responses were found to be strongly dependent on the applied strain rate. An invariant-based constitutive model was developed and implemented as a user-defined subroutine in the commercial finite element software LS-DYNA to predict the strain rate-dependent elastic-inelastic deformation response of the UD-NCF composite. The model is calibrated using data obtained from the physical and virtual tests and verified for different cases. The model accurately captured the strain rate-dependent elastic and inelastic response. The first step validation of the model is performed for a [±45]2s laminate at quasi-static and high strain rates using a single element. A very good agreement was observed between the predicted and experimental results. The model is also validated with a single element for an IM7-8552 UD tape composite by simulating the axial compression response for 15o, 30o, 45o, 60o, 75o and 90o off-axis laminas at quasi-static and high strain rates. Overall, a good agreement between the predicted and experimental results was observed at both quasi-static and high strain rates. The main outcomes of this research work include a new dataset for the strain rate-dependent deformation response and fracture behaviour of the UD-NCF carbon fiber/snap-cure epoxy composite material system. In addition, the developed invariant-based constitutive model accurately captures the pre-peak strain rate-dependent deformation response of the UD-NCF composite and will be further developed in future work for use in impact simulations of automotive structures.