Multilayer polymer pipes failure assessment based on a fracture mechanics approach

It is widely recognised that quasi-brittle fracture (through initiation and subsequent crack propagation mechanism) at low stresses is the most common mode of failure for high-density polyethylene pressure pipes. Slow crack growth in such pipes usually starts at a small defect at or near the inner pipe surface. Knowledge of a stress intensity factor is a key point for establishing the maximum load that a cracked pipe can withstand without failure, for description of the crack kinetic, and consequently for assessment of the pipe lifetime. To this aim a finite element stress analysis is used to calculate the stress intensity factor for internal and external cracks in a three layer composite plastic pipe consisting of two protective layers and the main pipe. The polyethylene pressure pipe is loaded by internal pressure. In contrast to homogeneous pipes the estimations of KI for multilayer (composite) pipes are numerically more elaborated and the fracture mechanics approach is complicated by the existence of interfaces between single layers, where material parameters are changed by a step. Special attention is paid to the configuration of a crack growing close to the interface and the effective values of stress intensity factor are estimated for a crack with its tip at the interface. It is shown that under special conditions (depending mainly on the elastic mismatch of materials) the crack can be arrested at the interface and significantly influence the lifetime of the pipe.