Modelling of elastomeric materials and products

The Rubber in Engineering Group (RIEG) was formed as part of the Institute of Rubber Industries more than half a century ago. Today it is a committee of the Polymer Society at the Institute of Materials, Minerals and Mining. The group arranges meetings, conferences and publications, with the principal aim of promoting understanding of the behaviour of rubber from an engineering perspective.

There is a growing need for this; the performance requirements for engineered rubber products such as tyres, anti-vibration mountings, hoses and seals have increased over the last few decades and are likely to become even more demanding in future. The trends of weight reduction coupled with an increase in operating temperature for some components have made it increasingly difficult to meet service life requirements, while elastomeric components are required to last longer because of an increase in warranty periods offered for vehicles and engineered appliances. In the tyre industry, the conflicting demands of weight reduction and reduced rolling resistance coupled with increases in abrasion resistance and wet and dry friction performance also make the tyre designer’s role increasingly difficult. Therefore, all rubber components will have to be designed and manufactured using robust engineering principles to ensure that they comply with expected performance and longevity requirements.

In light of these requirements, the RIEG organised a one day conference on the 14 October 2010 on the topic of ‘Modelling of Elastomeric Materials and Products’. This event was championed by Alan Muhr of TARRC and James Busfield of Queen Mary, University of London. This special edition is made up from selected papers presented at this conference. This is the second consecutive RIEG conference that has been made into a special edition. The previous event being a conference on the topic of ‘Time Dependent Behaviour of Rubber’ which was also published as a special edition in Plastics, Rubber and Composites (Vol. 38 No. 8) in October 2009.1

The themes of rubber research that are being examined globally are reflected in the topics that are explored by the RIEG for their conferences and afternoon technical discussion meetings as well as in the scientific literature for elastomer and rubber materials. These same themes are reflected in the various papers that have been published recently in this Journal. Throughout the various discussion meetings held by the RIEG in 2010–2011, it is apparent that the most important topics that require ongoing research in the rubber community include studies into: how and why do products fail; what are the precise mechanisms that make fillers effective and can more effective fillers be developed; what controls the visco-elastic behaviour in rubbers and how can it be effectively modelled; what additional functionality can we get from our materials, for example, by exploiting smart materials technologies and how to model frictional sliding reliably in rubber products?

If we examine each of these topics then the largest subject of interest in this special edition concerns predicting the fatigue life of rubber components. There are several different ways in which a part can be considered to have failed in service. These can include a loss in performance due to chemical ageing, or it can result from crack growth fatigue type behaviour. Mike Roland2 explored both in his 2009 Rubber Foundation Lecture talk and in this special edition Steinke et al.3 have continued to develop a model that can describe the heterogeneous ageing of rubber. Busfield et al.4 previously described a fracture mechanics model to examine fatigue life prediction using a finite element analysis method5 to calculate strain energy release rates in real components. This method is developed further in the work by Mirza et al.6 described in this special edition who have examined rubber bonded components used in the rail industry as well as by Asare and Busfield7 who have validated the approach for use in rubber components at elevated temperatures. Juhre et al.8 have highlighted potential difficulties with selecting an appropriate material model when trying to predict the fatigue life behaviour as effects such as cyclic stress softening can significantly alter the amount of energy that is available to drive a crack.

Previous papers9^–11 have examined the elastic behaviour of rubber components and these have also identified the limitation when trying to predict the behaviour of real components. In this special edition Luo et al.12 have adopted these various approaches to give a practical guide on how to use a finite element design methodology to not only predict the behaviour of a component but also how to optimise the design.

There are a wide range of different viscoelastic effects that complicate the modelling of rubber behaviour. One very common problem is encountered when modelling the cyclic stress softening behaviour,13 whereby the behaviour becomes less stiff from cycle to cycle due to changes in the macromolecular network as a result of the mechanical strain. Sometimes rubber is used to damp noise and so it is useful to model the behaviour in the acoustic range14 on other occasions it is necessary to try and predict the viscoelastic behaviour outside of the frequency range15 in which it is possible to characterise the behaviour. Viscoelastic phenomenon arise over a very wide range of frequencies from at one extreme, the MHz range in acoustic behaviour down to single cycle behaviour over many years as is often the case in stress relaxation16 or creep. In this special edition Kingston and Muhr17 have developed an empirical visco-plastic model that seeks to model the viscoelastic behaviour after a series of stress softening cycles in rubber materials.

Rubber materials are often used in contact problems. Frequently their purpose is to maximise friction at the interface to ensure that a tyre, for example, is able to interact well with a road surface. On other occasions the friction at the interface is minimised, in the case of windscreen wiper blades or shaft seals, for instance. Modelling friction behaviour in the case of rubber contact is a significant challenge.18,19 The modelling of abrasion has also been tackled using a fracture mechanics approach.20 In this special edition Moldenhauer et al.21 examines the friction and abrasion that results from the contact between a tyre tread block and a rigid surface. Their approach produces a good correlation with experimentally observed abrasion rates.

Rubber materials are rarely used as pure polymeric materials. Typically they are used as composites with a significant volume fraction taken up by different types of fillers. Even though carbon black has been in use for more than a century it is still the filler that is most widely used and it still generates a large amount of scientific interest.22^–26 The optimum behaviour of the rubber composite is determined by the morphology as well as the dispersion of the filler and by accurate control of the filler aggregation during mixing and moulding. More recently the use of carbon nanotubes27 as a composite filler has aroused much interest. Their unusual behaviour arises from their large shape factor (broadly speaking a measure of the largest dimension of a filler particle divided by the shortest), which results in the formation of a percolating network at very small filler volume fractions. Other filler materials have also become commonplace. In particular silica fillers have been promoted for use in the tyre industry as an alternative to carbon black due to an apparent reduction in rolling resistance coupled with an increase in wet friction. As the filler is essentially incompatible with the rubber, it requires the use of a coupling agent28 to develop any significant polymer-filler interaction and the interface is therefore easy to tailor29 for a required application. More recently organoclay fillers30 have been used as fillers in the rubber industry as they may offer some advantages over the or carbon black materials due to their very high shape factor once they have been intercalated and they have been carefully integrated into the composite.

The development of fillers has often been done in parallel with a desire to develop smart elastomer components. An example of which might be a device developed to exploit reorientation of the filler structure under strain. As most rubber materials are essentially insulators and some fillers such as carbon black are good electrical conductors it is possible once the filler is near or above the percolation threshold to examine how the conductivity31,32 varies with composition and processing in the filled composite. One possible application would be to develop components whereby monitoring how the electrical resistance changes with strain33 would allow a load cell to be created. This effect is not restricted to carbon black fillers and other conducting fillers have been explored including metallic type fillers.34,35 By choosing suitable metallic powders it is also possible to develop magneto-elastomers36^–38 that can be orientated and therefore have their properties altered by changing the magnetic field in which they operate. This approach has the potential to allow tuneable rubber components to be developed. The most interesting smart materials involve visual changes in an elastomer with strain. This has been demonstrated recently by Anuchai et al.,39 who are one of a number of groups who have been developing a sensor that combines the properties of a photoluminescent polymer, which emits light at different wavelengths dependent upon the amount of strain when excited by photons. Clearly the opportunities for smart rubber materials are vast and have only just started to be developed.

The papers presented in this special edition give just a snap shot of a subset of these recent developments in rubber materials. It is hoped that the leading researchers in the rubber materials field will continue to choose Plastics, Rubber and Composites: Macromolecular Engineering as their Journal of choice for publishing their findings. Further information about the journal can be found at www.maney.co.uk/journals/prc. Members of the Institute of Materials, Minerals and Mining have free online access to Plastics, Rubber and Composites articles and to other Institute journals through the website at www.iom3.org.

The Rubber in Engineering Group continues to host regular meetings themed on all aspects of engineering with rubber. They typically run three or four afternoon technical discussion meetings a year that are free to members of the Institute of Materials, Minerals and Mining and run a major conference almost every year as well. There are discussion meetings planned for the 24th June 2011 on the theme of ‘Elastomers in Defence’, 9th September 2011 on ‘Prototyping using Elastomers’ and 9th December 2011 on ‘Textile Reinforced Rubber’. If you are interested in finding out about these or other future events organised by the RIEG then you should check the website: http://www.rieg.org.uk for further details and for information on how to register.