Modelling the cure-dependent viscoelastic behaviour of a thermosetting resin and residual curing stresses

Thermosetting polymers are used as the matrix in structural composite materials, and also as underfill or encapsulating materials in electronic packages. The performance of the thermosetting parts is severely affected by the processing induced residual stresses. Having an appropriate estimation of the magnitude and development of residual stresses, for design purposes, requires a good understanding of the evolution of the viscoelastic mechanical behaviour and the shrinkage of a thermosetting resin during curing. In the present work, a cure-dependent viscoelastic model is proposed for an unsaturated polyester resin, which is based on the relaxation behaviour of the resin at any state of the cure. The curing resin is characterized for its mechanical properties through theological tests, i.e. normal and torsional dynamic mechanical analysis. An incremental constitutive equation is developed for the proposed viscoelastic model, and implemented in a finite element code in order to analyse any three-dimensional (3D) stress field. The model is verified by the results of the newly designed experiments in which the resin is geometrically constrained and the curing induced stresses are monitored.