MICRO – AND MACRO-MECHANICAL APPROACHES FOR. MODELLING OF POLYETHYLENE MATERIAL FOR PIPES

J. A. Alvarado-Contreras1,3, H. Liu1, M. A. Polak1, and A. Penlidis2

1Department of Civil Engineering, University of Waterloo, Waterloo, ON, Canada
2Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada
3School of Mechanical Engineering, University of the Andes, Merida, Venezuela

Abstract

Two different approaches for modelling the mechanical behaviour of polyethylene materials are presented. In the first one, the emphasis is on the relationships between molecular features and mechanical properties. In the proposed model, the material is analyzed from a microscopic viewpoint and considered as an aggregate of crystals. The constitutive equation is expressed in a viscoplastic framework considering degradation at large deformations. For the second approach, the material response is considered to be nonlinear viscoelastic. A phenomenological approach is adopted, and attention is given on the formulation of a model that can be implemented for structural analysis of components such as pipes. In this part of the study, numerical and experimental data of creep for a medium-density polyethylene pipe material are presented. The efficacy of the micro – and macro-mechanical approaches is confirmed by experimental results.

1. Introduction

Polyethylene material has been widely used since the early 1940s. Its low cost, lightness, and chemical stability have favoured its popularity to substitute other materials in water, gas, and sewage disposal pipelines. During installation and utilization, pipes are typically subjected to complex loads and loading paths. This has motivated the interest, first, in understanding the relationships between molecular structure and end-use mechanical performance, and then, in improving the material properties. Many studies have concentrated on experimental and theoretical aspects of nonlinear history-dependent behaviour and on the deformation mechanisms at large deformations. The present work consists of two parts. The first one deals with the modelling of the mechanical response of polyethylene material from a microstructural viewpoint. The second part presents the macroscale approach to modelling the nonlinear behaviour of polyethylene based on responses obtained experimentally.