Fatigue damage mechanism characterization and modeling of a woven graphite/epoxy composite

A 5-harness satin woven graphite/epoxy composite was studied in both the as -fabricated and a hydrothermally aged condition. The fatigue damage mechani sms for both unaged and aged specimens were characterized using dynamic sti ffness loss monitoring during fatigue loading,. The major damage mechanism, , of transverse yarn cracking, inter-yarn debonding, and delamination also were modeled using simple mechanistic models. It as possible to use the dyn amic stiffness loss curves along with the simple damage mechanism models to characterize the evolution of transverse yarn cracking and delamination du ring fatigue, for both the as-fabricated and the aged specimens at room tem perature (RT) and at 121 degreesC (250 degreesF). The present analysis pred icted the trends in the matrix crack evolution and the delamination growth reasonably well. Based on the experiments and the analysis, the unaged mate rial tested at RT and had the lowest rate of increase in the crack density with fatigue cycles. The unaged material tested at 121 degreesC had the hig hest rate of crack density increase with cycles. Crack density for the 12,0 00 hour hygrothermally aged material increased at a rate of 95% high er tha n the unaged material tested at RT. Delaminations were computed to initiate at higher fatigue cycles and also reach higher delamination levels with de creasing cyclic stress levels. For the same stress level, elevated temperat ure and aping led to more rapidly increasing and larger delamination length s. The hydrothermally aged material tested at 121 degreesC had a very low t hreshold for delamination, which initiated below 1.000 cycles. The aged mat erial tested at 121 degreesC 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.