Influence of Matrix on Stress Relaxation Behavior of Fiberglass Reinforced Plastics

Abstract

Fiber glass reinforced plastics (FRP) has now become to be used as structural materials in many fields. The mechanical properties of FRP vary widely depending upon the configuration and volume content of glass reinforcement. Glass fabrics woven by roving and mats of chopped strand are often used in FRP for large-sized structures such as ships and boats. This type of FRP contains a medium amount of fiber among various kinds of FRP. The mechanical properties of such FRP show often time-dependence even at near room temperature due to viscoelasticity of the matrix.
The objective of this study is to clarify the rule of mixture governing the viscoelastic behavior of composite materials such as FRP from macroscopic and microscopic viewpoints. In this paper, as the first report of this study, the relaxation moduli of two types of FRP, consisting of unsaturated polyester reinforced with woven rovings and chopped strand mats have been analyzed by means of a simplified model, in which the effect of the constitution of reinforcement and the time-dependent Young's modulus of matrix was taken into consideration, and compared with the experimental results. At first, the simplified elastic model including the effect of the configuration of reinforcement was devised to express Young's modulus of FRP with woven roving or chopped strand mat in terms of elastic moduli of the constituents. The accuracy of the model was confirmed by tensile tests of FRP with various glass contents. Then the viscoelastic behavior of unsaturated polyester resin at room temperature was examined, and the time-dependence of Young's modulus of matrix was obtained based on a four-element mechanical model. The relaxation modulus of FRP was then calculated by substituting the time-dependent Young's modulus of the matrix into the above model. The calculations have been shown to agree fairly well with the results obtained by stress relaxation testing of FRP at room temperature.