Elastomers and Components

Service Life Prediction - Progress and Challenges

V A Coveney

• Publisher: Woodhead Publishing
• Year: 2006
• Print ISBN: 9781845691004
• eBook ISBN: 9781845691134
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Elastomeric components are widely used in engineering. Increasing demands are placed on them to withstand hostile conditions such as high temperature and corrosive environments. These demands make it harder to predict likely service life or improve design to ensure their longer-term performance. This important book reviews the wealth of research on understanding fatigue and failure in elastomers, and how this understanding can be used to predict and extend their service life.The first part of the book reviews factors determining ageing behaviour such as heat, corrosive environments, wear and cracking. It also discusses the strengths and weaknesses of current service prediction models. The second part of the book focuses on analysing and improving the design and service life of particular applications such as O-rings, bearings, springs and valves.With its distinguished editor and team of contributors, Elastomers and components: service life prediction; progress and challenges is an invaluable reference for engineers involved in the design and use of elastomers. - Looks at the wealth of research on understanding fatigue and failure in elastomers - Discusses the strengths and weaknesses of current service prediction models - An invaluable reference for engineers

Theme I

Ageing Behaviour and Life Prediction Methods for Elastomeric Materials

Chapter 1

The 5-year Accelerated Ageing Project for Thermoset and Thermoplastic Elastomeric Materials: A Service Life Prediction Tool

P. Albihn The Swedish Institute for Fibre & Polymer Research (IFP Research AB) Mölndal, Sweden

SYNOPSIS

A large long-term accelerated test programme for elastomers (rubbery materials) is reported. In the programme the effects, on a range of physical properties, of accelerated ageing in a variety of environments for periods up to five years were studied for 48 thermoset rubbery materials and 27 thermoplastic materials. Example results for thermoset rubbery materials are given here.

The main focus is on thermo-oxidative ageing although the, sometimes very significant, effects of other factors is also considered. Key findings include the following [(a)-(c)].

(a) The wide validity of the (log time to failure against reciprocal absolute temperature) Arrhenius extrapolation method is confirmed.

(b) The results re-emphasise the importance of carefully specifying ageing conditions (e.g. which fluids there will be exposure to). The suitability ranking of two different elastomeric materials is often reversed if the conditions are changed.

(c) Ageing causes some measures of material properties to decrease more rapidly than others – indeed, some increase rather than decrease. Moreover, the slopes of the Arrhenius plots for (values of) different material properties can differ markedly. Selection of failure criteria is therefore crucial for lifespan prediction; thus it needs to be known which physical properties are important and what range of values of those properties is acceptable for a given application.

1 INTRODUCTION

Rubbery materials, or elastomers, are often used in demanding environments and in components with high reliability requirements even over long periods in use. Operational lives of 10 years and more are not unusual. This has increased the importance of having access to adequate material data and of having methods for service life prediction.

In practice, it is generally impossible (from a time or cost viewpoint) to perform laboratory testing of the ageing properties of a material for times approaching the lives of rubbery materials in service. Manufacturers and/or users of elastomers usually have to do short-term accelerated tests and thereafter, on more or less well justified grounds, have to make a judgement on the suitability of the material. Correspondingly, most specification sheets and data in the literature relating to the ageing of rubbery materials refer to short-term (accelerated) tests of a few days or weeks. As a basis for the choice of materials for components subjected to long-term use, these data are not sufficient; and more time-consuming testing is usually necessary. The reason for this is that a non-negligible error arises when results are extrapolated from inappropriately high testing temperatures to normal operating temperatures. A rough guide is that a temperature increase of ten degrees increases the reaction rate by a factor of 2-3. However, extrapolation is not recommended in cases where the test temperature is more than 20-30 °C above the operational temperature range, i.e. a four to eight fold acceleration.

During the last 20 years, IFP Research (The Swedish Institute for Fibre & Polymer Research) has, within the Rubber sub-programme, worked on testing different rubbery materials – both standard materials and special materials – for test periods that greatly exceed what is normal. The work has enabled the Arrhenius plot method (see below) to be comprehensively checked for a wide range of materials and has, moreover, resulted in extensive test data which is only modestly accelerated. All data from the series of long duration tests performed at IFP Research have been compiled into a handbook (Andersson, 1999)....

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• The Rubber in Engineering Series IOM
• Editor’s Foreword
• Theme I: Ageing Behaviour and Life Prediction Methods for Elastomeric Materials
• Theme II: Specific Geometries and Application Issues
• Appendix: Contact Details for Authors
• Index