Multiscale Modeling of Residual Stress Development in Continuous Fiber-Reinforced Unidirectional Thick Thermoset Composites

The primary objective of this research is to develop a multiscale simulation framework to arrive at more realistic estimates of cure-induced residual stresses in the vicinity of the fiber-matrix interface in thick thermoset composites. The methodology involves simulations at the part level—employing homogenized rendering of the composite using micromechanics approach—within a finite element framework to obtain part-level temperature and degree-of-cure gradients and strains, and imposition of this information as boundary conditions at the mesoscale simulations, employing microstructural representative volume elements (RVE). A simple implementation of the multiscale framework, involving simulations at the part as well as the RVE levels, is demonstrated in the context of a thick, unidirectional continuous-glass-fiber-reinforced thermoset composite. The trends in the mesoscale residual stresses estimated by employing different RVE-level thermal and thermomechanical boundary conditions—displaying different degrees of coupling between the global and part-level simulations—are then examined. Significant differences are observed in the estimates of mesolevel cure-induced residual stress evolution obtained from simulations with a conventional symmetric RVE and those obtained by employing the multiscale approach involving detailed boundary conditions that realistically account for global thermal and mechanical strain histories. 1. Introduction and Background During fabrication and cure of fiber-reinforced thermoset-matrix composites, chemical shrinkage of the thermoset matrix, the differential thermal shrinkage of the fiber and the matrix, and the constraints arising due to mold-composite interaction result in residual stresses in the resulting parts. In thick composites, additional large cross-thickness gradients in temperature and extent of cure (and the resulting resin property evolution) develop due to the exothermic nature of the cure reaction and the low thermal conductivity of the resin. Combined with microstructural defects and inhomogeneities, such large gradients in temperature and cure can result in residual stresses that can be a significant fraction of the matrix strength and/or the interfacial shear strength [1, 2] and may be comparable to or higher than the stresses encountered during service conditions [3–6]. The phenomenological aspects of residual stress development during thermoset composite cure have been enunciated in detail in several earlier reviews (e.g., [7]). At the length scales of the part, residual stresses result in shape distortion