Reliability Index of HDPE Pipe Based on Fracture Toughness

Abstract

This 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.

DOI: http://dx.doi.org/10.5755/j01.mech.23.6.17265