In CHAPTER 1 slow crack propagation in MDPE pipe was studied in air and Igepals at 50 °C to determine the possibility for fatigue to creep correlation in environmental liquids. The stepwise fatigue crack growth in air was preserved in Igepal solutions. Lifetime in Igepal was affected to a much smaller extent as compared to air. The correlation in air was previously established primarily for tests at 21 °C. The stepwise mechanism was verified in air at 50 °C. The crack growth rate under various loading conditions was related to the maximum stress and R ratio by a power law relationship. Alternatively a strain rate approach reliably correlated fatigue and creep in air at 50 °C except at R = 0.1 and frequency less than 1 Hz.
In CHAPTER 2 the effect of concentration of Igepal CO 630 on slow crack propagation in MDPE pipe was investigated to determine whether the mechanism was conserved in creep and fatigue as required for the fatigue-to-creep correlation. The mechanism of crack propagation and lifetimes in creep and fatigue at R=0.1 at 50 °C were compared to those in air and water. The fatigue and creep behavior followed the same stepwise crack growth mechanism as in air at all the concentrations used. As the concentration increased to 0.01 vol. %, the creep lifetime decreased significantly whereas the lifetime in fatigue gradually increased. At higher concentrations the lifetime was similar in creep and fatigue.
In CHAPTER 3 effect of R-ratio on kinetics and mechanism of environmental fatigue and creep crack growth was analyzed in an attempt to predict the environmental stress crack resistance at 50 °C. Same methodology was used as previously established for fatigue to creep formulation in air at 50 °C. The stepwise mechanism of crack growth in air was conserved in Igepal solutions as R-ratio approached to unity (creep) with few exceptions. At higher R-ratio, the lifetime decreased systematically in Igepal solutions relative to air and was defined as ‘Igepal transition time (ITT)’. It depended on concentration and molecular weight of Igepal. Both fatigue and creep crack growth rate in Igepal showed significantly higher crack growth rate after ‘ITT’ relative to air. To probe the Igepal effect on kinetics, fracture processes involved in first craze failure were compared to that in air.