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 m edium density polyethylene pipe material was studied by increasing the R-ra tio (defined as the ratio of minimum to maximum stress in the fatigue loadi ng cycle) from 0.1 to 1.0 (creep). The study included characterization of t he effects of R-ratio and temperature (21 to 80 degrees C) on the mechanism and kinetics of slow crack propagation. With increasing R-ratio and decrea sing temperature, the fracture mode changed from stepwise crack propagation , i.e. crack growth by the sequential formation and breakdown of a craze zo ne, to a "quasi-continuous" mode of crack growth through the preexisting cr aze. Despite the change in fracture mode, the damage zone, as characterized by the length of the main craze, shear crazes, and crack tip opening displ acement, followed the same dependence on loading parameters, and crack grow th rate followed the same kinetics. Crack growth rate (da/dt) was related t o the maximum stress intensity factor K-I, max and R-ratio by a power law r elationship (da/dt) = B'K-I,(4)(max) (1 + R)(-6)(.) Alternatively, crack gr owth rate was expressed as (da/dt) = B < K-I(4)(t)>(T)beta(.epsilon) with a creep contribution B < K-I(4)(t)>(T), calculated by averaging the known de pendence of creep crack growth rate on stress intensity factor K-I over the period T of the sinusoidal loading curve, and a fatigue acceleration facto r beta(.epsilon) that depended on strain rate only. The correlation in crac k growth kinetics allowed for extrapolation to creep fracture from short-te rm fatigue testing. The temperature dependence of crack growth rate was con tained in the prefactors B and B'. A change in slope of the Arrhenius plot of B' at 55 degrees 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. (C) 2000 Kluwer Acad emic Publishers.