Fatigue crack growth in PMMA and rigid PVC under biaxial stress

Fatigue crack growth rate results for PMMA and rigid PVC are presented on a fracture mechanics basis. Centre-notched specimens were used to measure the effects of the stress biaxiality factor $\text{B}$, defined as the ratio of load acting in the crack line to that acting on a line normal to it. Variations in $\text{B}$ from 0 (uniaxial) to 2 have little effect on the fatigue crack growth rate in PVC, but reduce that in PMMA by a factor of 2 or 3. If the variation in $\text{B}$ is a stepwise one, this growth rate reduction may be accompanied by a temporary crack arrest period. The observations can be explained for both materials by postulating a direct effect of $\text{B}$ on the stress intensity factor, for which a possible mechanism-using crack closure concepts is suggested. It is shown that the regime of true fatigue crack growth in PMMA, which is investigated here, is limited at its upper end by a sharp transition to a slow-growth dominated crack extension mode.