Strain-rate dependent constitutive tensile behavior of uncured aligned discontinuous fiber composites for forming applications

This study develops and validates a strain rate dependent constitutive model for uncured highly aligned discontinuous fiber (ADF) prepregs in the fiber direction. Uniaxial tensile and stress-relaxation experiments were conducted at room temperature on 3 mm carbon fiber/epoxy prepreg specimens over three orders of magnitude in strain rate, revealing nonlinear hardening up to a rate-dependent peak stress followed by pronounced strain softening and rapid viscous relaxation. Based on these experimental observations, a phenomenological continuum model was formulated with separate hardening and softening regimes together with a time-dependent relaxation behavior. Model parameters were calibrated against normalized experimental data and shown to accurately reproduce the measured stress-strain response across all tested strain rates. An incremental formulation was derived to accommodate arbitrary strain rate histories, enabling direct implementation in finite element forming simulations. Implementation in Abaqus/Explicit reproduced the measured stress-strain response for constant and variable strain rate tensile loading with high fidelity. Experimental validation under variable strain rate histories confirms the high predictive accuracy of the proposed phenomenological continuum model. The proposed framework provides a practical methodology for converting experimental data into a constitutive model that can be used in forming simulations of ADF composites. The method can be applied to other ADF composites and establishes a foundation for future extensions to multiaxial and non-isothermal conditions.