During cure of epoxy resins, polymerization induces an increase in mechanical properties, which is accompanied by a volumetric shrinkage. When the resin is cured in a constrained mold to which it adheres, tensile stresses will hence develop, which may exceed the strength of the resin at a given curing stage. Voids will then form. The origin and governing parameters of void formation are studied using an epoxy resin cured in a three-dimensionally constrained glass mold following isothermal cure cycles. Two types of voids are shown to appear during cure, one early in the process and a second around the gelation point. A viscoelastic analysis of the material stress state over the whole range of cure is performed. Both the viscoelastic modulus obtained from a time-cure-temperature superposition and the volumetric shrinkage, which was continuously measured by density change, are taken into account. A value for the critical internal stress at void initiation is thus proposed. This criterion can be used to provide guidelines for tailoring the material properties toward an increase of the critical stress for void initiation. Also, since during processing of composite materials, cases may arise where the resin cures within the interstices left between consolidated fibers that do not move, this critical stress failure criterion can be of use in the establishment of a process window providing guidelines for the production of void free composites. lo%, depending on the type of monomer and hardener (1-3), and is generally around 5 O h or 6Yo. At the same time, the mechanical properties of the resin, in terms of shear modulus (and Young's modulus when the resin becomes a solid). increase dramaticallv as the intrinsic strength of the resin at a given time, which depends on its degree of conversion. Failure will then occur in the form of voids or cracks, which will remain in the final part.This phenomenon has extensively been reported in the literature, and was related to the occurrence of internal stresses in the curing cycle of epoxy systems. Defects appearing during cure have been experimentally observed in a three-dimensionally constrained epoxy resin by Plepys and Farris (4) who also deter-_ .the material evolves from a liquid state to a solid state. When the resin cures in a geometrically constrained environment, such as between plates separated by a fixed spacer, or within the interstices present between consolidated fibers in the case of composite material processing, tensile stresses thus mined the level of internal stresses arising from the curing process using a strain gauge method. These results were analyzed following the theory of incremental elasticity for an aging linear thermoelastic material subjected to dimensional constraints. The bulk behavior of the epoxy resin under volumetric constraint could then be calculated at temperatures above and below the glass transition temperature of the resin. The calculated internal stress levels were however expected to be overestimated, since the viscoelastic *Comsponding a...
