Fatigue Damage Mechanism Characterization and Modeling of a Woven Graphite/Epoxy Composite

Abstract

A 5-harness satin woven graphite/epoxy composite was studied in both the as-fabricated and a hygrothermally aged condition. The fatigue damage mechanisms for both unaged and aged specimens were characterized using dynamic stiffness loss monitoring during fatigue loading. The major damage mechanisms of transverse yarn cracking, inter-yarn debonding, and delamination also were modeled using simple mechanistic models. It was possible to use the dynamic stiffness loss curves along with the simple damage mechanism models to characterize the evolution of transverse yarn cracking and delamination during fatigue, for both the as-fabricated and the aged specimens at room temperature (RT) and at 121°C (250°F). The present analysis predicted the trends in the matrix crack evolution and the delamination growth reasonably well. Based on the experiments and the analysis, the unaged material tested at RT and had the lowest rate of increase in the crack density with fatigue cycles. The unaged material tested at 121°C had the highest rate of crack density increase with cycles. Crack density for the 12,000 hour hygrothermally aged material increased at a rate of 95% higher than the unaged material tested at RT. Delaminations were computed to initiate at higher fatigue cycles and also reach higher delamination levels with decreasing cyclic stress levels. For the same stress level, elevated temperature and aging led to more rapidly increasing and larger delamination lengths. The hygrothermally aged material tested at 121°C had a very low threshold for delamination, which initiated below 1,000 cycles. The aged material tested at 121°C also had the largest delamination length. The models developed in this paper provide a simple means to derive information about fatigue damage mechanisms (which are often difficult to characterize), using stiffness loss measurements, which are quite easy to make.