The mechanical properties of magnetorheological materials can be changed and controlled by an applied magnetic field. This makes the group of materials interesting for damping applications and vibration control. Most of the work done so far has been performed on magnetorheological fluids, which are suspensions of magnetically polarizable particles in a carrier liquid. For some applications it would be preferable to use a rubber or a gel instead of the liquid. Such magnetorheological solids would not have any problems of sedimentation of particles, the devices could be made smaller and cheaper since there would be no need of a container for the fluid, and the changes in the mechanical properties could be expected to be faster than in a liquid. However, magnetorheological solids work only in the pre-yield region while the fluids typically work in the post-yield region, so the two groups of materials are therefore more complementary than competitive. Examples of rubbers that are interesting for magnetorheological applications are silicone, natural rubber and nitrile rubber. The addition of iron particles to the rubber will influence the service life of these rubber devices. For optimal function of the devices, prediction of service life is necessary. Such predictions are also necessary for magnetorheological devices to become commercially successful.

SYNOPSIS Elastomeric O-ring seals perform essential functions in domestic equipment, transport and the energy industries, often over many years and in environments which are demanding in terms of fluid exposure, temperature or both. In this chapter a step towards improved service life prediction of these components is described. The literature and work performed by the authors on pertinent aspects of the long-term behaviour of elastomers including stress relaxation and set are discussed, as are experiments to study seal failure directly. It is concluded that current methods for life prediction may not be appropriate in all cases. Alternative approaches are suggested.

The advent of the polymeric synthetic heart valve will hopefully allow the limitations of other commonly used artificial heart valves to be overcome. Currently the valves adopted in practice include mechanical heart valves and biological heart valves. The mechanical valves are prone to thrombosis formation in the blood and require long-term anticoagulant therapy. The biological valves lose their tissue replacement function as a result of their chemical treatment which results in tissue fatigue damage in service. Polymeric synthetic heart valves that mimic the function of a real valve will hopefully overcome these difficulties. In this chapter, various elastomeric materials (polyurethane, silicone rubber and EPDM), that can be used to make polymeric synthetic heart valves, have been assessed by measuring their strain energy release rate for a wide range of crack growth rates. This was measured using static loading under a variety of conditions in order to evaluate their potential performance as target materials. Polyurethane is the most suitable matrix material of the three candidates from a consideration of the tear resistance. The behaviour of the polyurethane is similar to that of a strain crystallising material and as a result it would appear that some comparable form of reinforcement takes place. At the temperatures encountered in the body there is a significant decrease in the tearing energy for a given crack growth rate due to a reduction in the viscoelasticity. Despite this, it would still appear that polyurethane is the most suitable material for use in a synthetic heart valve.

In this chapter, a parameter is presented that attempts approximately to describe the energy release rate of a small crack of arbitrary orientation under multiaxial loading. The parameter, herein called the cracking energy density, specifically attempts to address several aspects common to the analysis of fatigue in rubber: finite straining, nonlinear elasticity, and the possibility of crack closure under compressive loading. As motivation, the connection between the strain energy density and the energy release rate for small cracks under uniaxial loading is first discussed. Small strain and finite strain definitions of the cracking energy density are then presented. The accuracy of the cracking energy density as an approximation of the energy release rate at small strains is assessed via comparison with results from linear elastic fracture mechanics (LEFM). The cracking energy density is shown to exhibit dependence on crack orientation and stretch biaxiality that resembles that predicted via LEFM. Since the cracking energy density is evaluated at a material point in the uncracked material, this parameter is particularly useful for the analysis of crack nucleation from initially unobserved flaws, a common task in the design of rubber components. In such applications, it is possible to predict not only a fatigue crack nucleation life, but also specific planes on which cracks would be expected to appear.

SYNOPSIS Rubber-to-steel bonded automotive components in temperate climates often suffer separation of the rubber from the steel, with corrosion evident on all exposed surfaces of the steel – including that originally bonded. The objective of this chapter is to clarify whether corrosion causes the bond to fail, or whether it merely attacks the metal exposed by the bond failure. This chapter shows that the phenomenon may be simulated in the laboratory, either by salt spray tests or simply by immersion in aerated salt solution. Since bond failure does not occur for rubber bonded to electrochemically passive substrates, such as nylon, and is greatly reduced in deionised water, it is concluded that it is electrochemical in origin. Possible mechanisms are discussed in the light of a review of similar phenomena observed with coatings for metal.

Many elastomer products come to the end of their useful life through abrasive wear (wear brought about through local or general sliding); however the processes of abrasive wear are complex and challenging. Approaches towards improved understanding and prediction of abrasive wear under various conditions are outlined. The most fully developed class of model is that originated by Southern & Thomas (1978) who consider abrasion under conditions of moderate-to-high tangential contact stress; they envisage abrasion progressing by fatigue crack growth at the roots of macroscopic flaps or “tongues” of rubber. The Southern & Thomas model fits some experimental observations but not others. The roles of temperature, oxygen and mechano-chemical effects are discussed (Gent & Pulford, 1983). Experiments are described in which five materials [based on natural rubber (NR), acrylonitrile butadiene (NBR) and ethylenepropylenediene (EPDM)] were abraded with a (blunt) “blade”. Evolving visual patterns and patterns of blade abrader force were monitored as was material loss. Similarities between the visual wavy (Schallamach) patterns, often associated with rapid wear, and the force patterns suggest a close relationship between the two. The fact that force patterns, of a wavelength which persists, are discernable from the start of scraping suggests that aspects of the Schallamach pattern have their origin in non-destructive abrader-elastomer interactions. For unidirectional abrasion the wear rates observed under similar conditions and for elastomeric materials of similar hardness varied widely – NR at 55 International Rubber Hardness Degress (IRHD) giving 60 times the wear rate of 55 IRHD EPDM – for which no Schallamach pattern was observed. In bidirectional abrasion, wear rates were generally a little lower than under unidirectional conditions but the reduction was rather minor. Although there has been considerable progress made, a comprehensive quantitative theory of abrasion of rubbery materials is still to be developed.

SYNOPSIS Elastomer components frequently fail in service through cumulative cyclic crack growth (fatigue) behaviour. Techniques are described that can be used to characterise this behaviour for elastomers using a fracture mechanics approach where a measure of the crack growth rate is made at specific levels of stored energy release rate. These derived constitutive relationships can be applied using finite element analysis (FEA) techniques to complicated real elastomer component geometries. The energy balance technique, that is available within a typical FEA package to characterise the stored energy release rate for a wide range of components, is described. It is proposed that a realistic measure of the crack growth rate can be made from a finite element analysis that therefore allows a realistic calculation of fatigue life to be made.

A case study is reported on the design of a Metacone mounting used as a primary spring fitted at the wheel axle box on a railway vehicle. In use the rubber layers are loaded in shear and compression and the Metacone mounting provides controlled complicance along three axes. Requirements for increased service life and dimensional limitations (implying high strain levels) have led to an ever closer focus on suspension spring design. (In the current example the general space envelope was 250 mm in diameter by 250 mm high.) Furthermore, as was the case here, end of life is often defined in terms of clearly noticeable cracks or changes in stiffness rather than final failure. The location, formation and early stage development of fatigue cracks are therefore key issues in such cases. Design methods involving finite element (FE) and fatigue life analysis (FLA) were used to redesign the Metacone. As a result the life of the Metacone undergoing customer-specified (single axis) fatigue testing was increased more than ten-fold.

SYNOPSIS Strain energy release rate or tearing energy has proven to be a valid failure criterion for a variety of rubber failure phenomena. However, apart from a small number of classical test pieces, it is difficult to calculate the tearing energy. This hampers the wider use of this criterion in failure analysis. As a benchmark exercise, finite element analysis is used here to compute the tearing energy for some classical rubber test pieces consisting of cracked strips and cylinders in tension. Results are used to reconcile some conflicts in the literature. Some guidance is given on choice of element types for such analyses.

SYNOPSIS The prediction of the life of a sealing element within a system is required to allow planned maintenance to be carried out at the most efficient time. In the automotive industry there is a trend for seals to last for ever increasing amounts of time, with the latest goal being 15 years (150 000 miles). With greater life times required, methods have to be adopted to predict the life of the seal in application, and accelerate testing to allow this to be completed in an acceptable time period. In this chapter several common methods of prediction for static seals are reviewed and their applicability to the actual sealing application environment assessed.