Fracture Mechanics based Lifetime Prediction of PE Pressure Pipes – The Kinetics of Macroscopic Failure and Molecular Failure Mechanisms

Session 9A
10:40 am

Reinhold W. Lang, Institute of Polymeric Materials & Testing

In previous and separate papers by the authors, a novel concept for lifetime and safety as-sessment of PE pressure pipes based on modern methods of fracture mechanics is de-scribed. At the core of this concept is the accelerated generation of so-called “synthetic” or “computational” crack growth laws for slow crack growth at service-near temperature condi-tions without making use of stress cracking liquids. Based on macroscopically determined kinetic data of crack advance, a fracture mechanics methodology is applied for pipe lifetime assessments. By using the same basic set of material laws and failure criteria combined with proper numerical (finite element) simulation techniques, this approach also allows for lifetime predictions of pipes exposed to more complex loading experienced in service such as the influence of imperfections like pre-existing defects, cracks/notches, point loads etc. Also, the approach offers great potential for a more detailed analysis of the mechanisms and kinetics of failure related to the inner material structure on a morphological and molecular scale.

Numerous studies over the past two decades have dealt with various aspects of the important role of morphology, molecular order and network structure in controlling slow crack growth in PE pipe grade materials. For example, it is now well established that the crack growth resistance in PE is highly sensitive to the molecular mass distribution (MMD) and the con-centration and length of side chain branching via the insertion of co-monomers in the high molecular mass portion. This has led to commercial pipe grade PE materials of specific MMD designs in terms of modality (bi- to multi-modal) and side-chain branching. And yet, the pre-cise nature of failure on the molecular scale as a result of the competitive interaction of crack tip plastic zone formation and breakdown is still somewhat obscure. Studying the kinetics of macroscopic failure with fracture mechanics techniques allows for some elucidation of the underlying molecular failure mechanisms when combined with local analytical investigations of the crack tip region and by making use of principal considerations of polymer physics. The paper provides a review of the current state of knowledge in this field by also addressing and outlining some of the critical open issues.

Reinhold W. Lang (1,3), G. Pinter (2) and A. Frank (3)
1. Institute of Polymeric Materials and Testing, Johannes Kepler University Linz, A-4040 Linz (Austria)
2. Institute of Materials Science and Testing of Plastics, University of Leoben, A-8700 Leoben (Austria)
3. Polymer Competence Center Leoben GmbH, A-8700 Leoben (Austria)