The Influence of the Curing Process on the Response of Textile Composites.

Abstract

Textile composites are a promising engineering material that offer high specific strength and fracture toughness. To allow for rapid development of new products and to allow for virtual testing using numerical simulations, a thorough understanding of the material state and the material response to external load is necessary. In previous studies it has been observed that the use of “virgin” matrix properties leads to numerical predictions of composite behavior that is not in accord with experimental observations. It was concluded that presence of fibers during the curing process of the epoxy alters the matrix material properties. To study these effects further, different physical effects are taken into consideration. First, the evolution of temperature and degree of cure is investigated through concurrent solution of the heat conduction equation and an empirical curing law. The model is used to investigate the influence of curing parameters, geometry and thermal conductivity on the curing time and maximum temperature in the epoxy. Next, a novel curing model for epoxy to describe the phase transition from liquid resin to solid epoxy has been developed. The curing of epoxy is treated as a continuous creation of non-interacting networks. The model accounts for an evolution of elastic properties as cure progresses and predicts resulting cure stresses. It has been shown that considerable stresses develop during the curing process in the epoxy inside the composite. To account for non-linear material behavior during cure and after applying loads, the model has been augmented by a continuum damage mechanics formulation. Mesh objective results are obtained through a nonlocal formulation. Here the damage depends on the state of the material in a finite volume around the material point of interest. The combined curing and damage model has been applied to a woven composite unit cell and it is shown that cure at elevated temperatures and large chemical shrinkage can cause degradation of the epoxy material inside the composite prior to the application of external load. As a result, the onset of nonlinear effects occur at lower applied mechanical loads and the ultimate strength of the composite is significantly reduced.