Fatigue Fracture Mechanisms of Particle and Fiber Filled PTFE Composites

The effect of filler type on the fatigue fracture mechanisms of polytetrafluoroethylene (PTFE) composites was studied. The two composites were a silica particle filled PTFE (Garlock 3502') and a glass fiber filled PTFE (Garlock 8573'). Tension-tension fatigue crack propagation tests were conducted on both materials at room temperature at a frequency of 3 Hz. The maximum stress was 6 MPa and the ratio of minimum load to maximum load was 0.1. It was found that the fatigue lifetime of the particle filled PTFE is approximately four times higher than that of the fiber filled PTFE. The fatigue data also revealed that the crack speed of the particle filled composite is always lower than that of the short fiber filled composite. Microscopic analysis on representative fracture surface of each material was performed to identify different fracture surface features. The three fracture regions, crack initiation, stable crack growth and unstable crack growth were examined. In the first region, both composites displayed extensive plastic deformation and severe debonding at the filler/matrix interface. In the second region, stable crack propagation, torn ligament bundles, fibrillation and debonded fillers are the main fracture surface features. The fracture surface of the fiber filled PTFE in the unstable crack propagation region has a more smooth appearance with extensive fiber pull-out. This indicates a fast fracture process and a brittle fracture mechanism dominating this region. The third region of the fracture surface for the particle filled PTFE displayed more severe matrix deformation. The particle filled PTFE displayed more intensive fibrillation in the second region than the fiber filled PTFE, indicating more damage formation and thus higher energy consumption in the stable crack propagation stage. Consequently, the crack speed of the particle filled PTFE is lower than that of the fiber filled PTFE composite.