Correlation of fatigue and creep slow crack growth in a medium density polyethylene pipe material

The relationship between slow crack propagation in creep and fatigue in a medium density polyethylene pipe material was studied by increasing the R-ratio (defined as the ratio of minimum to maximum stress in the fatigue loading cycle) from 0.1 to 1.0 (creep). The study included characterization of the effects of R-ratio and temperature (21 to 80°C) on the mechanism and kinetics of slow crack propagation. With increasing R-ratio and decreasing temperature, the fracture mode changed from stepwise crack propagation, i.e. crack growth by the sequential formation and breakdown of a craze zone, to a “quasi-continuous” mode of crack growth through the preexisting craze. Despite the change in fracture mode, the damage zone, as characterized by the length of the main craze, shear crazes, and crack tip opening displacement, followed the same dependence on loading parameters, and crack growth rate followed the same kinetics. Crack growth rate (da/dt) was related to the maximum stress intensity factor KI, max and R-ratio by a power law relationship (da/dt) = B′K4I, max(1 + R)−6. Alternatively, crack growth rate was expressed as (da/dt) = B′KI4(t)Tβ(∈) with a creep contribution B‹KI4(t)›T, calculated by averaging the known dependence of creep crack growth rate on stress intensity factor KI over the period T of the sinusoidal loading curve, and a fatigue acceleration factor β(∈) that depended on strain rate only. The correlation in crack growth kinetics allowed for extrapolation to creep fracture from short-term fatigue testing. The temperature dependence of crack growth rate was contained in the prefactors B and B′. A change in slope of the Arrhenius plot of B′ at 55°C indicated that at least two mechanisms contributed to crack propagation, each dominating in a different temperature region. This implied that a simple extrapolation to ambient temperature creep fracture from elevated temperature tests might not be reliable.