Evaluation of the fatigue fracture resistance of unfilled and filled polytetrafluoroethylene materials

Polytetrafluoroethylenes (PTFEs) and their composites are a special class o f fluorocarbons with very high chemical resistance and wide service tempera ture. This makes them good candidate materials for load-bearing components exposed to harsh environments, including some space applications. In the pr esent work, fatigue crack propagation (FCP) behavior of four materials from the fluorocarbon family, including PTFE without filler (virgin PTFE), PTFE with 15% glass fiber, PTFE with '15% graphite particles, and PTFE with 25% glass fiber, were studied. Tension/tension FCP experiments were carried ou t using single-edge notch (SEN) specimens under load control. The maximum s tress was kept constant at 8 MPa for each material at a frequency of 3 Hz. The minimum to maximum stress ratio was 0.27. FCP data such as the number o f cycles, crack length, and hysteresis loops were recorded in order to esta blish the crack speed, the energy release rate, J*, and the change in work (W) over dot (i). Parameters that characterize the resistance of PTFEs to F CP have been successfully determined by the modified crack layer (MCL) mode l. These parameters are gamma', the specific energy of damage, which reflec ts the FCP resistance of the PTFE materials, and the dissipative characteri stic of the materials, beta'. It has been found that the MCL model describe s the behavior of the PTFEs over the entire range of the energy release rat e and discriminates the subtle effects introduced by changing the filler ty pe and dosage as well as the processing conditions. The values of the speci fic energy of damage gamma' have been found to decrease by increasing the d osage of the fiberglass fillers. Graphite particulate filler also reduced t he value of gamma' more than fiberglass filler for the same dosage. Microsc opic analysis of the fracture surface in the stable crack propagation regio n of each material revealed that there exists a strong correlation between the value of gamma' and the amount of damage energy manifested by different mechanisms and species during the fatigue process. (C) 1999 Kluwer Academi c Publishers.