Relationship between Short-Term and Long-Term Creep, and the Molecular Structure of Polyethylene

Abstract

Polyethylene has been studied from many different perspectives; a final application property perspective, in which the response of the material to loads is the topic; a micromechanical point of view, in which the macroscopic state of the material is related to its microstructure, e.g., Alvarado (2007), and a chemical point of view in which the molecular structure and the processes that create polyethylene are investigated. This thesis focuses on the mechanical behavior of polyethylene observed from testing and relates the mechanical behavior to the molecular structure of the material.

High density polyethylene is a material used in civil engineering applications such as pipes and containers. There are two general modes of failure for polyethylene: ductile failure that happens at relatively large stresses (up to 200MPa) and in short amount of time, and brittle failure that occurs when a much lower stress is sustained over a long period of time (Cheng 2008). Other than these two modes of failure, excessive deformation of the material that is usually caused by creep is also to be avoided. This thesis studies the relationship between short-term and long-term creep of polyethylene and its molecular structure.

In this work three types of mechanical tests were performed on six samples of polyethylene. The existing models that prescribe the constitutive behavior of the material were then critically evaluated against the observed data. Furthermore the molecular properties of the samples that had been obtained from previous research by Cheng (2008) were compared against the mechanical behavior observed from testing in order to assess what molecular properties are important in determining the mechanical behavior of polyethylene. This information can also help polyethylene designers to produce longer lasting material, or a material that has high stiffness, by knowing what molecular properties to control and optimize.