Fracture Mechanics of Creeping Solids Applied to Pre-cracked NOL Ring Specimens to Predict Residual Lifetime of High Density Polyethylene Pipes

High Density Polyethylene (HDPE) is widely used for the distribution of drinking water and are exposed to an internal pressure due to water flow. Furthermore, when they are in contact with disinfectants, oxidation of HDPE occurs at the immediate surface of the inner wall. This leads to a decrease in the HDPE molar mass and consequently, to a hardening as well as an embrittlement of the material. The oxidised layer thickness seems to stabilize at 200 μm whatever the initial pipe thickness due to the diffusion of reactive species in the inner wall. Inspections with a scanning electron microscope (SEM) of the inner wall of pipes collected on site, after several years of service, showed a network of cracks. The most noxious (deepest) longitudinal crack propagates under the steady internal pressure until the complete failure of the pipe. The experimental investigations consisted of creep crack growth tests, carried out on an original geometry for this type of test using NOL ring. These specimens were cut from the pipes and an internal longitudinal crack implanted. Creep crack growth tests were performed at various net stresses and at various crack depth ratios. At the end of each test, the time to failure was recorded. Before applying the theory of fracture mechanics of creeping solids, 3D finite element simulations were carried out to assess the suitability of assuming plane strain conditions. To this end, a porous viscoplastic model was implemented into an in-house finite element code. A fracture criterion based on critical porosity allowed for the simulation of creep crack growth. The localization of the maximum damage at mid-thickness and the thickness reduction were well captured. In these conditions, the crack front curvature is located near the surface so that a 2D calculation with plane strain conditions can then be justified to determine the C* load parameter. The residual lifetime of a real pipe containing a longitudinal defect, under in-service loading was estimated by using the correlation between the time to failure and the C*-integral.