ANALYSIS OF FATIGUE-CRACK GROWTH IN A RUBBER-TOUGHENED EPOXY-RESIN - EFFECT OF TEMPERATURE AND STRESS RATIO

Fatigue crack propagation in a rubber-toughened epoxy resin was studied at different test temperatures (-40 to 60-degrees-C) and stress ratios (0.05 to 0.70) using single edge-notched specimens at a frequency of 5 Hz. Fatigue crack propagation rates (da/dN) were plotted against the stress-intensity factor amplitude (DELTAK) in accordance with the Paris power-law equation. For a given stress ratio da/dN was not sensitive to variations in test temperature. But for a given temperature da/dN increased with stress ratio. Using the Williams' two-stage line zone model to analyse these experimental data, it was shown that the main failure process was due to shear plastic flow at the crack tip. The fatigue stress and the closure stress-intensity factor both decreased with increasing temperature, implying that more severe damage had occurred at higher testing temperatures. The experimental data were also analysed in terms of a new fatigue model, which considers the accumulation of damage due to cyclic plastic strain in the reversed plastic zone similar to the Coffin-Manson law and crack closure due to residual plastic stretch at the crack wake. There was good agreement between theory and experiment, suggesting a simpler way to correlate fatigue crack growth rates in this and other polymeric materials.