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 studie d at different test temperatures (-40 to 60-degrees-C) and stress rati os (0.05 to 0.70) using single edge-notched specimens at a frequency o f 5 Hz. Fatigue crack propagation rates (da/dN) were plotted against t he stress-intensity factor amplitude (DELTAK) in accordance with the P aris power-law equation. For a given stress ratio da/dN was not sensit ive to variations in test temperature. But for a given temperature da/ dN increased with stress ratio. Using the Williams' two-stage line zon e 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 f atigue stress and the closure stress-intensity factor both decreased w ith increasing temperature, implying that more severe damage had occur red at higher testing temperatures. The experimental data were also an alysed in terms of a new fatigue model, which considers the accumulati on of damage due to cyclic plastic strain in the reversed plastic zone similar to the Coffin-Manson law and crack closure due to residual pl astic stretch at the crack wake. There was good agreement between theo ry and experiment, suggesting a simpler way to correlate fatigue crack growth rates in this and other polymeric materials.