Coveney Elastomers and Components

Service Life Prediction - Progress and Challenges
1. Auflage 2006
ISBN: 978-1-84569-113-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 248 Seiten

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

Autoren/Hrsg.

Coveney, V A

Leseproben

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). The purpose of the endeavour is to provide an aid for designers, using thermoset elastomers (rubbery materials) and thermoplastic elastomeric (TPE) materials, who are interested in the lifetime prediction of the material and components. Questions such as: “How long will this rubber component last?”, “Can I increase the operation temperature by 10 °C and still achieve the requirement of a service life of 20 years?” or “One of my customers has had a failure of one of our rubber components after only 2 years use, what is this due to?” can be answered with the help of the (long duration test) study. A partial goal is to explain in simple terms the concept of lifetime prediction for designers and rubber users. Another is to equip rubber technologists and designers with as complete a database as possible with regard to the lifespan of different thermoset and TPE material types. All in all, measurements were made on 48 thermoset elastomeric materials (from 21 polymer types) and 27 thermoplastic elastomeric materials (from 11 polymer types); for these materials 1-8 properties were tested over periods lasting from 1 to 5 years (43800 hours). Generally the materials were aged in an unloaded state and in three different environments: hot air, hot water and hot oil. And generally the following measures of properties were evaluated: hardness, elongation at break, tensile strength and tensile stress at 100% elongation. For some materials, compression set, tension set, relaxation in compression and relaxation in tension were also investigated. The ageing took place at several different temperatures and Arrhenius plots (lifespan diagrams) were produced for all materials. The complete material formulation and initial material property details are given by Andersson (1999). No other investigation involving accelerated ageing times as long as 5 years has, to our knowledge, previously been reported. Moreover, recently RAPRA has published a report reviewing the main theories and test methods for ageing of rubber and giving a large number of abstracts (Brown et al, 2000). Furthermore, RAPRA has also recently produced an extensive report on natural ageing (Brown & Butler 2000). The RAPRA reports complement the programme reported here. It is now possible to verify in many ways the theories and methods of accelerated ageing. The results of accelerated ageing on several materials, methods, properties and environments can be found in the Andersson (1999) handbook and without waiting 40 years they can be compared with natural ageing (Brown & Butler, 2000). Correctly used, the output of the IFP Research study (Andersson, 1999) is a powerful aid in the estimation of the lifetime of a rubber material or a thermoplastic elastomer. In the present chapter I shall summarise some of the findings of IFP Research’s 20 year study (Andersson, 1999) and also refer to other relevant studies. 2 LIFESPAN: SOME IMPORTANT FACTORS TO CONSIDER
What influences the service life of a rubbery material? A material has a number of properties such as tensile strength, elongation at break, hardness, compression set, chemical resistance, colour and glass transition temperature. Ageing can be defined as the changes in these properties, decrease or increase, which take place with time. What causes the ageing of an elastomer? There are many environmental factors that can cause degradation of the material and its performance: oxidative degradation, UV-light, moisture, temperature, fluids, ozone, micro-organisms. Mechanical factors such as changing stresses and abrasion may also contribute to or constitute ageing. In order to predict the behaviour of the material in service and the effect on the component, such factors may have to taken into account. In this chapter, though, the main focus will be on thermo-oxidative ageing, although for some test methods the behaviour may depend on other types of chemical or physical processes. In testing for ageing the most important factors are: (a) type of elastomeric material (b) temperature (c) testing environment (d) property measured and test method (e) mechanical stress (if any) The original value of the property and its dependence on time (e.g. increasing or decreasing) can vary depending on how (a)-(e) are chosen. Different materials can be compared according to any of the following or other properties or conditions: - hardness, - tensile strength, - elongation at break, - compression set, - temperature in use, - in air, oils or water. It is important to remember that service life can be a relative concept. Depending on specific requirements, one, two or several of the above-mentioned properties can be measured to evaluate the service life of the component. Because of specific requirements for different components – such as different materials, different uses and different lengths of service life – the quantitative criteria can vary greatly. It is extremely important in service life prediction to consider carefully, and at an early stage, which property or properties one considers to be critical for...