Accurate prediction of the lifetime of unidirectional fibre composites requires a model that captures matrix viscoplasticity across generic loading histories. We propose a compact, rate-dependent hardening law for highly crosslinked epoxies that unifies constant-strain-rate and creep behaviour. The yield stress depends exponentially on accumulated plastic strain and logarithmically on plastic strain rate, and the relationship is analytically invertible for direct use in a finite element code. Parameters are calibrated from compression tests at multiple strain rates and from hold-at-load creep tests; validation is performed on RTM-6 and new 736LT epoxy data. The model reproduces (i) the near-linear σ _y – logε̇ trend from pre-yield through softening and hardening, (ii) the time-dependent transition from pre- to post-yield during creep, including the rate surge near softening, (iii) long-term (14.5 h) creep more faithfully than stress–time power laws, and (iv) trends in cyclic, variable-rate, and tensile tests. The resulting, easily calibrated formulation enables robust simulation of matrix viscoplasticity in composite-scale models, improving durability predictions for load-bearing structures such as pressure vessels and wind-turbine blades.
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