Fracture Mechanics Lifetime Prediction of PE 80 and PE 100 Pipes under Complex Loading Conditions

Session 4B
9:30

Andreas Frank, Polymer Competence Center Leoben GmbH

For a fracture-mechanics lifetime assessment of pressurized polyethylene (PE) pipes the knowledge of the material specific creep crack growth (CCG) kinetics at application relevant temperatures is of essential significance. Modern pipe grade materials were systematically improved in their resistance against crack initiation and CCG which, however, makes a characterization of the relevant quasi-brittle failure mechanisms difficult and time-consuming.

The geometry-specific advantages of cracked round bar (CRB) specimens in cyclic fatigue tests open new options for a quick characterization of crack resistant PE pipe grades, even at application near temperatures of 23 °C and without any additional stress cracking liquid. In addition to a quick ranking procedure based on simple failure times also an accelerated determination of CCG is possible by an extrapolation procedure based on fatigue tests at different loading amplitudes.

Within the current work the developed test procedure with CRB specimens was applied to different commercially available PE 80 and PE 100 pipe grade materials for which the relevant fracture mechanics parameters for CCG at 23 °C were derived within a couple of weeks.

Combined with available equations for the stress intensity factor (SIF) of pressurized pipes these parameters were used for a fracture-mechanics lifetime prediction, which shows a good correlation to the failure times of real internal pressure tests according to EN ISO 9080. For an advanced lifetime prediction a simulation model of the pressurized pipe was developed with finite element methods (FEM). To consider the effect of additional loads, the SIF for a pipe under bending loads, soil loads and point loads with different indenter size was simulated with FEM. Then, using the experimentally measured fracture mechanics material parameters,
it was possible to calculate pipe lifetimes for different complex loading conditions.

A. Frank (1), G. Pinter (2), R.W. Lang (3)
1) Polymer Competence Center Leoben GmbH, Austria
2) Institute of Materials Science and Testing of Plastics, University of Leoben, Austria
3) Institute of Polymeric Materials and Testing, University of Linz, Austria