Fatigue Fracture Resistance Analysis of Polymer Composites Based on the Energy Expended on Damage Formation

Abstract

Analysis of the fatigue crack propagation behavior of filled polytetrafluoroethylene (PTFE) composites using the energy expended on damage formation was performed. A 25% chopped glass fiber filled PTFE with a commercial name of Rulon LR and a 25% polymer particle filled PTFE (Rulon J) were chosen as candidate materials for the study. Fatigue crack propagation (FCP) tests were performed using both unnotched and single edge notched (SEN) specimens under tension-tension load control condition. The notched specimens were prepared with the same notch length to sample width ratio (a/w) of 0.09. The applied frequency was 3 Hz. The maximum stress was 6 MPa, and the ratio of minimum stress to maximum stress was kept at 0.10. FCP data such as the crack length and the number of cycles were used to calculate the crack speed. The hysteresis energies for both notched and unnotched specimens during cyclic loading were obtained. The energy release rate was determined as the first derivative of the relationship between the potential energy and the crack length. The potential energy was calculated as the area above the unloading curve from the hysteresis loops at various crack lengths. The cyclic rate of energy dissipation on damage formation was calculated based on the hysteresis energies for both notched and unnotched specimens. It was found that the fatigue kinetics are related to the cyclic rate of energy dissipation into the active zone evolution. The specific energy of damage (γ) a material parameter characteristic of the fatigue fracture resistance was evaluated using the Modified Crack Layer (MCL) theory. It was found that the value of γ for the particle filled PTFE is 3200 kJ/m3, which is about three times higher than that for the fiber filled PTFE (750 kJ/m3). This indicates that the particle filled PTFE has higher fracture resistance than the fiber filled PTFE. The fracture surface of the second region for both materials was examined using scanning electron microscopy (SEM). Interfacial debonding, drawn ligaments and large voids were associated with the fatigue fracture of the fiber filled PTFE. The particle filled PTFE displayed more fibrillation than that of the fiber filled PTFE, which indicates a higher energy consuming process related to the fatigue crack growth in the particle filled PTFE.