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The fracture and fatigue behavior of 15% chopped glass fiber reinforced PTFE was studied. Two fracture regions were observed with the static tensile failure of this material. Torn ligament bundles, pulled out fibers and microfibrillation are the fracture surface features associated with the first region. These features were responsible for the initial slow crack observed during tensile loading. The second region was characterized by a smoother surface with little damage associated features resulting in an unstable crack propagation. The Modified Crack Layer Model was used to analyze the fatigue crack propagation behavior of the material. The fatigue fracture surface of the 15% glass fiber reinforced PTFE showed three distinct regions. These regions were comparable in location with the three stages of fatigue crack propagation (FCP) kinetics of this material. A large number of torn fracture ligaments and long fibrils were observed in the first region, which is associated with the threshold state of FCP. Extensive random fibrillation was found in the second fracture region which was associated with the second stage of FCP kinetics (stage of reduced acceleration). Undrawn matrix, very little fibrillation and a smooth surface were the fracture surface features of the third region. This region was associated with the third stage, unstable crack propagation, of FCP kinetics.

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.

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