Reliability Index of HDPE Pipe Based on Fracture Toughness
Keywords:HDPE Pipe, SDR, PHIMECA software, Critical toughness, Reliability index,
AbstractThis work presents a first contribution to evaluate the reliability of a polyethylene tube using the PHIMECA Software. The aim is to respond to a request from plastic pipe users who wish to have estimated durability and also to appreciate the remaining life after a given period of service in specific conditions. In this first study, we considered a HDPE pipe for transporting gas, subjected to internal pressure. The critical stress
intensity factor (KIC) was adopted as a criterion to the maximum limit the evolution of numerically calculated KI, i.e. rupture occurs when KI
reaches the limit value KIC. The reliability index β, defined as the inverse of the probability of failure, is assessed and discussed based on the crack length, the operating pressure and the dimensionless geometrical parameter (SDR: thickness to outer diameter ratio). Also, it must be noted that a thickness is limited by the minimum and maximum values set by standards and for each value of SDR. Analysis of β values allowed obtaining, for a given diameter, SDR limit value corresponding to a reliable case. The range for KIC values is determined from literature analysis and for many HDPE pipe resins and it was set in the interval [2 – 5 MPa.√m]. In regards to the variation of the β reliability index, as a function of the crack length and KIC, it has been found that increasing the size of the crack (or flaw) reduces drastically β values. In the first case of reliability analysis, i.e. KIC=2MPa.√m, the tube is safe as long as the crack length does not exceed 62μm. This size increase to 200μm at 3.5MPa.√m, and finally, if the tube is sufficiently tough, acceptable reliability is ensured at a crack length up to 370μm. This limit, as severe as it is, can serve as a criterion for tubes quality control and for estimating the recommended allowance for the
effect of scratches and surface defects caused by the storage and handling operations before burial.
MECHANICS OF SOLID BODIES