Fatigue Crack Propagation in Weld Zone of CPVC Pipe Fittings at Different Temperatures

Abstract

CPVC pipe fittings are tested under fatigue loading at 24, 50, and 70°C. Special fixtures are designed to simulate internal pressure in the couplings. Fatigue crack initiation and propagation in the weld zone are studied. Results show that the initiation period is limited to less than 10% of the fatigue life. This is mainly due to the knit-line V-notch effect. Following initiation, the crack is observed to propagate in a discontinuous manner; growing faster in craze zones forming ahead of crack tip. Crack propagation rate, da/dN, is characterized with the stress intensity factor range, AK, assuming long through width crack and linear elastic fracture mechanics behavior. Fatigue crack growth (FCG) in the weld zone is found to exhibit inferior resistance as compared with that of the region away from weldline. In low AK region, crack growth rate da/dN shows complex short crack growth behavior. Increasing the temperature reduced FCG resistance of weld material and different da/dN-AK curves were obtained at different temperatures. These curves were brought closer using the variation in mechanical properties of CPVC with temperature.