Viscoelastic analysis of residual stress in quenched thermosetting resin beams

The residual stress generated by the molding process of thermosetting resins exerts serious influences upon their mechanical properties. This residual stress is generally classified by two groups: one produced by shrinkage in the curing reaction of monomers, the other produced by the nonuniformity of the temperature distribution in the cooling process. This paper is concerned with the theoretical and experimental analysis of the generation of residual stress of the latter type, using examples of rectangular beams of thermosetting resins quenched on both the upper and lower surfaces. First, a viscoelastic model is applied to make a qualitative prediction of the residual stress in quenched beams. Second, using linear-viscoelastic theory, fundamental equations are derived for the residual stress in a viscoelastic rectangular beam, where an unsteady and nonuniform temperature distribution is assumed in the direction of depth. The theoretical values of the residual stress in rectangular beams are calculated under various quenching conditions for two resins having different viscoelastic characteristics, i.e., epoxy and unsaturated polyester. The theoretical residual-stress distributions agree fairly well with the residual stress measured experimentally at every quenching condition for both resins. The qualitative prediction that the residual stress in quenched beams is compressive in the vicinity of the upper and lower surfaces and is tensile in the inner parts is confirmed. The relaxation modulus of epoxy resin changes more greatly with time and temperature than that of unsaturated polyester resin. The theoretical and experimental analysis shows that the residual stress for the former resin is larger than that for the latter. Therefore, it is concluded that the generation of residual stress is more significant where the relaxation modulus of resin changes greatly with time and temperature.