Comparison of Damage Development in Random Fiber-reinforced Polymers (FRPs) under Cyclic Loading

It has been well known that suppression of debonding and matrix cracking could improve fatigue resistance of fiber-reinforced polymers (FRPs). In this study, the roles of these two mechanisms on the damage development in FRPs with in-plane random glass fiber reinforcement have been investigated. Two polymers are used as the matrix - isophthalic polyester and polyurethane. Polyurethane-based FRP shows higher ultimate tensile strength (UTS) and strain to failure, but lower elastic modulus. Under zero-tension fatigue loading (with the maximum stress level equivalent to 50% of their respective UTS), the change in modulus, energy dissipation rate, and the corresponding damage development process are investigated. The damage development is analyzed at the macroscopic and microscopic levels, and found to be closely related to the modulus degradation and change in energy dissipation rate. The study concludes that the two FRPs show significantly different behavior under fatigue loading. The polyurethane-based FRP had better fatigue resistance, in view of the mild modulus change and the capability of absorbing energy through plastic deformation. Results from the study suggest that the excellent fatigue resistance of the polyurethane-based FRP is due to good toughness of the matrix.