The role of crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE)

The swelling-driven fatigue behavior of polymer fuel cell membranes during relative humidity (RH) cycling is investigated. In particular, swelling-induced membrane stresses are obtained from a numerical model simulating fuel cell RH cycle tests, and compared to the lifetimes obtained experimentally from tests conducted in the absence of electrochemical effects. A strong correlation between the lifetimes of the membranes in the actual tests and model results is obtained. In general, higher RH (or swelling) amplitude results in larger stress amplitudes and shorter lifetime, that is, fewer cycles to failure. Tensile stresses are needed for forming local cavities in the membrane, which may eventually lead to craze formation. Cavitation is less likely to occur in compressed membrane at high humidities. The stress–lifetime plots for polymer fuel cell membranes exhibit similar features to those observed for other polymers. The crazing criterion for polymers suggests that craze initiation during RH cycling is more likely to occur in the low compression regions, such as under the channels, which is in agreement with experimental observations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1506–1517, 2011

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Aspects of Fatigue Failure Mechanisms in Polymer Fuel Cell Membranes

Tribological investigation of PTFE composite filled with lead and rare earths-modified glass fiber

High-strain-rate tensile mechanical response of a polyurethane elastomeric material

In this research, we have investigated the effects of addition of different percentages of nanoclay to the ethylene propylene diene monomer (EPDM) and nitrile butadiene rubber (NBR) on the characteristics of these rubbers as seal material. Properties such as tensile strength, modulus at different extensions, elongation at break, compressive set, hardness, and permeability and abrasion resistance are tested to assess the effect of addition of the nanoclay. Results indicate that addition of nanoclay at certain compositions could slightly reduce the strength of the rubber. However more stable modulus at different strains are provided, the hardness of the rubber is preserved and slightly enhanced, the permeability is reduced in both rubbers especially considerable decrease in EPDM is observed which is desirable in diminishing the effect of explosive decompression. At the same time the compression test shows that the nanoclay improves the performance of the rubbers under compression which is essential in seal application. The X-ray diffraction tests clarify that the dispersion of the nanoclay in the NBR samples is of high quality. In the EPDM samples, the dispersion is in need of improvement. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers

Enhancement of the properties of EPDM/NBR elastomers using nanoclay for seal applications

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 Wɛi. Parameters that characterize the resistance of PTFEs to FCP have been successfully determined by the modified crack layer (MCL) model. These parameters are γ′, the specific energy of damage, which reflects the FCP resistance of the PTFE materials, and the dissipative characteristic of the materials, β′. 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 γ′ have been found to decrease by increasing the dosage of the fiberglass fillers. Graphite particulate filler also reduced the value of γ′ 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 γ′ and the amount of damage energy manifested by different mechanisms and species during the fatigue process.

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

Fatigue crack growth behavior of a polymonochlorotrifluoroethylene (CTFE) was investigated Tension-tension fatigue propagation tests on geometrically identical specimens were performed at different frequencies Specimens with different notch to width ratios (a/w) were also tested The applied stress level in all tests was kept the same The maximum stress was 160 MPa and the ratio of minimum stress to maximum stress was kept at 25% Fatigue test data such as the number of loading cycles, the crack length, and the hysteresis loops were recorded The Modified Crack Layer (MCL) theory was employed to analyze the FCP behavior of this CTFE material Also the results were analyzed using the Paris equation for comparison It has been found that the fatigue crack growth behavior of this material cannot be described by the Paris power law due to the three distinct stages of the FCP kinetics The MCL model was successfully used for extracting material parameters; γ', the specific energy of damage, and β', the coe

M. El-Hadik

Papers

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

Fatigue crack growth analysis of polymonochlorotrifluoroethylene (CTFE)