A. G. Thomas

Effect of materials variables on the tear behaviour of a non-crystallising elastomer

Crack growth rates (r) were measured in pure shear test specimens as a function of strain energy release rate (G) for a non-crystallising SBR elastomer. Measurements were made as a function of: extent of swelling with Dibutyl Adipate; carbon black content; and crosslink density. In some cases experiments were carried out over a range of temperatures. In most cases the resulting G versus r plots showed a clear transition from rough to smooth crack surface behaviour with increasing crack growth rate, with an intervening slip/stick region. In the high speed/steady tear/smooth region the value of G necessary to drive a crack at a given rate was determined largely by the magnitude of the visco-elastic losses in the crack tip region, increasing with: decreasing temperature; increasing molar mass between crosslinks; decreasing extent of swelling; and increasing carbon black content. However G was independent of specimen thickness in this region suggesting that crack tip effects were minimal. In the low speed/rough region changes in the magnitude of G with materials and temperature/rate variables could not be explained by changes in visco-elastic loss alone. Furthermore the magnitude of G increased significantly with increasing specimen thickness. This suggested that in this region cavitation ahead of the growing crack tip resulting from dilatational stresses determined the crack tip diameter, and hence the magnitude of G.

The effect of liquids on the dynamic properties of carbon black filled natural rubber as a function of pre-strain

A free oscillation technique has been adopted to measure the dynamic storage and loss moduli of carbon black filled natural rubber materials. These tests are conducted with small oscillations that are superimposed on a range of tensile pre-strains. In addition, the effect of temperature on the dynamic moduli is measured as well as the effect of swelling the materials to various extents by liquids with a range of viscosity. It is observed that the dynamic storage and loss moduli do not depend strongly on the pre-strain at small pre-strains. At higher pre-strains there is a marked increase in both the storage and the loss moduli. An increase in temperature causes a dramatic reduction in both the storage and loss moduli. The dynamic behaviour of the filled rubbers when swollen can be approximately ascribed to the combined effects of a reduction in the modulus of the rubber matrix (caused by the swelling action) and a reduction in the effective volume fraction of the filler. The liquids used had a range of viscosity of more than a factor of a thousand. Despite this, the loss moduli of the swollen rubbers varied by only about a factor of two. This insensitivity could be understood in terms of a previously developed theory, based on free volume considerations.

Prediction of fatigue crack growth using finite element analysis techniques applied to three-dimensional elastomeric components

Abstract Elastomer components fail at cyclic strain amplitudes much lower than their catastrophic tear strength as a result of cumulative cyclic fatigue crack growth. Cracks typically develop in regions of high stress concentrations. In general, the rate of growth is determined by the shape of the component, and the nature and magnitude of the deformation imposed. Extensive earlier work has been done on the prediction of fatigue life of components. However, the reproducibility of the results was poor and, in addition, there was a low degree of accuracy. A fracture mechanics approach, which uses finite element analysis techniques to calculate strain energy release rates for cracks located in three-dimensional components, was used in combination with experimental measurements of cyclic crack growth rates of specific strain energy release rate to predict the cyclic crack growth propagation rate and the eventual fatigue failure of an elastomeric engineering component in three modes of deformation, namely: tension, simple shear and combined shear and tensile (45° angle) deformations. The fatigue crack growth for the gearbox mount under investigation was predicted within a factor of 2 at different displacements for all three modes of deformation.

Indentation Tests on Elastomer Blocks

Abstract The problem of indentation hardness for elastomer blocks has been examined at two levels. Initially an examination of the geometric non-linearity was undertaken. It was observed that the empirical equations adopted by the various standards organizations to predict the stiffness relationships were not always applicable. It appears that the classical Hertz solution to the problem gives a better representation of the general behavior. A finite element approach was also adopted here to tackle the large displacement problem and the limitations of this approach have been discussed. This geometric problem is further complicated in practice by the effect of the finite thickness of the elastomer sheet. This problem has also been analyzed and a suitable general relationship proposed to account for the finite thickness effects. The second problem examined is how the effects of the non-linear elasticity of the material can be tackled. It is shown that the form of the elastic stored energy function at small s...

Contributions of time dependent and cyclic crack growth to the crack growth behavior of non strain-crystallizing elastomers

Engineering components are observed to fail more rapidly under cyclic loading than under static loading. This reflects features of the underlying crack growth behavior. This behavior is characterized by the relation between the tearing energy. T, and the crack growth per cycle, dc/dn. The increment of crack growth during each cycle is shown here to result from the sum of time dependent and cyclic crack growth components. The time dependent component represents the crack growth behavior that would be present in a conventional constant T erack growth test. Under repeated stressing additional crack growth, termed the cyclic crack growth component, occurs. For a non-crystallizing elastomer, significant effects of frequency have been found on the cyclic crack growth behavior, reflecting the presence of this cyclic element of crack growth. The cyclic crack growth behavior over a wide range of frequencies was investigated for unfilled and swollen SBR materials. The lime dependent crack growth component was calculated from constant T crack growth tests and the cyclic contribution derived from comparison with the observed cyclic growth. It is shown that decreasing the frequency or increasing the maximum tearing energy during a cycle results in the cyclic crack growth behavior being dominated by time dependent crack growth. Conversely at high frequency and at low tearing energy, cyclic crack growth is dominated by the cyclic crack growth component. A large effect of frequency on cyclic crack growth behavior was observed for highly swollen SBR. The cyclic crack growth behavior was dominated by the time dependent crack growth component over the entire range of tearing energy and/or crack growth rate. The origin of the cyclic component may be the formation/melting of quasi crystals at the crack tip, which is absent at fast crack growth rates in the unswollen SBR and is absent at all rates in the swollen SBR.

Recycled Rubber: The Rubber Granulate - Virgin Rubber Interface

Abstract Reusing granulates derived from old tire stock and other sources in high tech engineering applications is still considered a high risk option. In addition to ensuring that the granulates are correctly identified, it is important to know how the incorporation of these materials alters the intrinsic flaw size of a finished product and to see how much the strength of the interface between these materials and the virgin materials compares to the basic strength of the virgin stock and the granulates. This paper explores possible techniques that can examine both properties so that an informed evaluation of the effect of reincorporating granulates can be established in practice.

CYCLIC STRESS RELAXATION (CSR) OF FILLED RUBBER AND RUBBER COMPONENTS

Under repeated stressing it is well known that rubber materials exhibit cyclic stress relaxation (CSR). Previous work has shown that the amount of relaxation observed from cycle to cycle is significantly greater than that expected from static relaxation measurements. The reduction in the stress attained on the second and successive loading cycles as compared to the stress attained on the first cycle in a stress strain cyclic test of fixed amplitude has been measured for elastomer test pieces and engineering components. It is seen that the peak force, under cyclic testing to a specific maximum displacement, plotted against the number of cycles on logarithmic scales produces a straight line graph, whose slope correlates to the rate of cyclic stress relaxation per decade. The rate of cyclic stress relaxation was found to increase with displacement amplitude in all modes of deformation. Plotting the rate of stress relaxation per decade against the maximum average strain energy attained in the cycle reduces the data measured in different deformation modes for both simple test pieces and components to a single curve. This approach allows the cyclic stress relaxation in a real component to be predicted from simple laboratory tests.

Silica–rubber microstructure visualised in three dimensions by focused ion beam–scanning electron microscopy

A focused ion beam–scanning electron microscope (FIB–SEM) technique for three‐dimensional reconstruction and representation of material microstructures was applied to a silica‐filled synthetic rubber for the first time. Backscattered electron imaging allowed differentiation between rubber matrix, silica filler and zinc oxide (used as an activator for the sulphur vulcanisation reaction). Subsequent image processing allowed three‐dimensional isosurface model generation of the particulate structure within the rubber composite and separation of zinc oxide from the silica filler. The potential for development and application of this technique using finite element analysis modelling is also highlighted.

FEA MODELING OF SCHALLAMACH WAVES

Abstract The term Schallamach wave is used to describe a wave of detachment that sometimes arises during the frictional sliding of a smooth rubber surface against a smooth rigid surface. Extensive experimental investigations have been made since Schallamach first observed the waves in 1971; however, no successful finite element analysis (FEA) modeling of the behavior has been reported yet. This work uses an explicit dynamics FEA approach to model both the initiation and progression of waves of detachment for the first time. The use of the finite element method allows for the detailed stress and strain analysis at the interface to be examined. Here, the limitations of using a purely elastic solution in plane-strain are explored. To validate the explicit dynamics approach for modeling the Schallamach waves, another biaxial compressive buckling mode has also been modeled.

Diffusion and Reactions of Oxygen During Ageing for Conventionally Cured Natural Rubber Vulcanisate

At high temperatures, oxidative ageing affects only the rubber near the surface, as essentially all the oxygen is consumed by reaction with antioxidant or rubber before it can diffuse into the bulk. A resinous oxidized layer is formed and in the center of large components is essentially anaerobic. The inhomogeneous natures of ageing for a large rubber component were carried out utilizing thickness effect measurements. The effect of ageing at different distances from the surface was studied using volume swelling measurements. Based on diffusion and chemical reactions theory, an estimation of the oxidized layer formation during ageing can be calculated provided the reaction rate is constant and the diffusion coefficient for given ageing temperature is known. For this purpose, oxygen uptake measurements were carried out in pure oxygen at 100°C.