Non-local gradient plasticity effects on curing-induced stresses in fibre-reinforced polymers

Curing-induced residual stresses affect the mechanical performance and failure of fibre-reinforced thermoset composites. Yet, their prediction remains challenging due to heterogeneous microstructures, non-uniform thermal and chemical fields, and pronounced strain localisation in the polymer matrix. This work presents a finite element–based microscale framework to simulate the coupled evolution of temperature, degree of cure, chemical shrinkage, and residual stresses during composite curing. A fully implicit integration scheme for the curing kinetics model is derived. As a key element of novelty, a non-local, strain-gradient plasticity model is incorporated and consistently coupled with the curing simulation to properly capture size effects and strain localisation in the matrix. Numerical results highlight the influence of curing parameters and thermal cycles on microscale residual stress development and premature failure of composites under transverse tension. Accounting for non-local effects is essential to prevent highly localised regions of plastic deformation. The numerical results show that, while the presence of a residual stress field does not justify the differences between the resin properties measured by in-situ nanoindentation and in the bulk, the data reduction methods for the fibre push-in tests need to be revisited. The differences in the residual stress field in under-cured resins explain the experimentally found larger strain associated to the onset of transverse matrix cracking in such resins.