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

Polytetrafluoroethylenes (PTFEs) and their composites are a special class of fluorocarbons with very high chemical resistance and wide service temperature. This makes them good candidate materials for load-bearing components exposed to harsh environments, including some space applications. In the present 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 out using single-edge notch (SEN) specimens under load control. The maximum stress 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 of cycles, crack length, and hysteresis loops were recorded in order to establish 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 FCP have been successfully determined by the modified crack layer (MCL) model. These parameters are gamma', the specific energy of damage, which reflects the FCP resistance of the PTFE materials, and the dissipative characteristic of the materials, beta'. It has been found that the MCL model describes the behavior of the PTFEs over the entire range of the energy release rate and discriminates the subtle effects introduced by changing the filler type and dosage as well as the processing conditions. The values of the specific energy of damage gamma' have been found to decrease by increasing the dosage of the fiberglass fillers. Graphite particulate filler also reduced the value of gamma' more than fiberglass filler for the same dosage. Microscopic analysis of the fracture surface in the stable crack propagation region 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 Academic Publishers.