Experimental Determination of Fiber Mat Permeability of Chopped Fiberglass Mats

Abstract

This paper presents the results of experimental investigation on fiber bed permeability variation with porosity and injection pressure. Flow measurement experiments were designed to measure fiber mat permeability for chopped fiberglass preforms with various fiber volume fractions. These experiments were performed using a rectangular mold cavity. Several layers of chopped fiberglass mats were used as reinforcement and RL 440 epoxy resin as the working fluid in the experiments. The effects of porosity and injection pressure on fiber bed permeability are investigated. Resin flow in the chopped fiberglass mats was very closely circular, suggesting an isotropic permeability tensor. Still, the analysis was performed based on an anisotropic preform permeability. The results indicate a nonlinear permeability increase with fiber bed porosity (Fig. 1). In Fig. 1 the permeability values in the maximum and minimum flow directions are plotted versus preform porosity. Close values of K1 and K2 suggests isotropic preform permeability. Note also that permeability increases sharply as porosity increases from 0.8 to 0.84. This suggests that, at high porosity values, resin flows more easily into the preform since more voids are present within the mats inside the mold cavity. The effects of injection pressure on permeability indicate a lower limit where the flow becomes too slow, thus resulting in an inaccurate permeability determination. On the other hand, if the injection pressure is set too high, resin rushes into the fiber preform. This results in an unreal high permeability determination for the fiber preform. The results of this investigation could be employed in the process/product optimization in an RTM process.