Effect of an environmental stress cracking agent on the mechanism of fatigue and creep in polyethylene
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It is of interest to determine whether the prediction of long-term creep failure from short-term fatigue experiments, as established for polyethylene in air, can be extended to environmental liquids. This article was undertaken to characterize the mechanism of creep crack growth in an environmental liquid at 50 °C and to determine whether the mechanism was conserved in fatigue as required for the fatigue-to-creep correlation. For this purpose, creep and fatigue tests at R-ratio (the ratio of minimum to maximum load in the fatigue cycle) of 1.0 (creep) and 0.1 were performed in air, water, and aqueous Igepal CO-630 (Igepal-630) solutions at various concentrations. It was found that fatigue and creep followed the same stepwise crack growth mechanism as in air in all the Igepal-630 concentrations studied. In air and water, fatigue substantially accelerated the crack growth kinetics compared to creep. A fatigue acceleration effect was also seen with the lower Igepal-630 concentrations. However, the acceleration effect lessened as the concentration increased to 0.05 vol.% due to the combined effects of the gradually decreasing creep lifetime and the gradually increasing fatigue lifetime. Above 0.05%, the lifetimes in creep and fatigue decreased in parallel with the fatigue lifetime only slightly lower than the creep lifetime. It appeared that Igepal-630 reduced the frictional resistance to chain slippage to the extent that any significant strain rate sensitivity was lost. Increasing the molecular weight had the equivalent effect of decreasing the Igepal-630 concentration. This was probably a kinetic effect related to the diffusion of the stress cracking liquid.