Fracture Mechanics Lifetime Prediction of Polyethylene Pipes

Abstract

Polyethylene pressure pipes are designed for service lives of at least 50 years. While so far lifetimes have been assessed by pipe pressure tests, due to improved slow crack growth (SCG) resistance and increased testing times such tests are not suitable anymore for modern pipe grades. The paper presents an alternative methodology for lifetime prediction, combining practical advantages of the cyclic cracked round bar (CRB) test and linear elastic fracture mechanics. A major advantage of this procedure is that material characterization is done at ambient temperatures. The key challenge in this context is to measure SCG rates in CRB specimens. After determination of crack kinetics at different dynamic loading ratios, material laws for SCG at static loading are extrapolated. Based on the stress intensity factor concept and realistic considerations of initial defect size and changing crack front geometry, pipe lifetimes of four different PE pipe grades are predicted. The results show that for all materials the required minimum lifetime of 50 years will be reached and under practical assumptions even 100 years will be exceeded.