J. J. C. Busfield

Microstructure of ceramic foams

This paper describes the preparation of ceramic foams by expansion of a ceramic suspension based on a polyurethane system. The microstructure and degree of reticulation of the foamed ceramic were examined and analysed with the help of a simple geometrical model. Like the porous ceramics prepared by the replica processing method, these foamed ceramics possess open cells in a nearly equiaxed shape but the cell size is much finer. The ratio of the window size to the cell size is a useful parameter for characterising the geometry of the foam and is related to the qualitative concept of degree of reticulation. For a face centred cubic array of cells it is related geometrically to the volume fraction of porosity and this relationship is tested using microstructural measurements for a range of ceramic foams.

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.

Piezoresistive polymer composites based on EPDM and MWNTs for strain sensing applications

Abstract Elastomeric composites based on Ethylene-Propylene-Diene-Monomer (EPDM) filled with multi-wall carbon nanotubes (MWNTs) have been prepared, showing improved mechanical properties as compared to the pure EPDM matrix. The results have been discussed using the Guth model. The main focus of the study was on the electrical behavior of these conductive polymer composites (CPCs), in view of possible sensor applications. A linear relation has been found between conductivity and deformations up to 10% strain, which means that such materials could be used for applications such as strain or pressure sensors. Cyclic experiments were conducted to establish whether the linear relation was reversible, which is an important requirement for sensor materials.

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...

Effect of processing methods and functional groups on the properties of multi-walled carbon nanotube filled poly(dimethyl siloxane) composites

Pristine and functionalized multi-walled carbon nanotubes (MWCNTs) filled poly(dimethyl siloxane) (PDMS) composites were produced by two different methods, namely the solution mixing method and the mini-extruder method. The composites produced using the mini-extruder exhibit relatively higher tensile strength and higher thermal conductivity due to better nanotubes dispersion. On the other hand, the composites prepared via solution mixing have higher electrical conductivity and better thermal stability due to the high aspect ratio of nanotubes. Scanning electron micrographs of composites fracture surface revealed that composites produced by mini-extruder resulted shorter nanotube length, thus lowering the aspect ratio of MWCNTs. In general, functionalization of nanotubes increases the tensile strength, thermal conductivity, and thermal stability of the PDMS composites due to the improved interfacial adhesion and nanotubes dispersion.

Effects of types of fillers and filler loading on the properties of silicone rubber composites

In this study, the effects of loading levels of nano BN, nano SN and synthetic ND particles on the thermal and mechanical properties of silicone rubber were investigated. Nanoparticle-filled silicone rubber composites were prepared by cast molding process. In general, incorporation of thermally conductive nanoparticles to the silicone rubber matrix improved the thermal conductivity of the composites. BN particles have the most pronounced effect on the thermal conductivity in comparison to SN and ND particles at any given loading level. TGA studies revealed that the thermal stability of the SN/silicone rubber system increased as the filler loading level increased; however, BN and ND facilitated the thermal degradation of the silicone rubber nanocomposites. For tensile test, it is observed that the addition of nanoparticles generally increased the tensile strength of silicone rubber composites compared to the neat silicone rubber. However, for BN system, the tensile strength of the composites decreased when more than 1.5 vol.% of BN is loaded in the system. The reduction was caused by the existence of voids and agglomerations in the BN/silicone rubber system. In general, tensile modulus and strain at break of nanocomposites increased with the incorporation of nanoparticles into silicone rubber matrix.

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.

Stiffness of simple bonded elastomer bushes. Part 1 – Initial behaviour

Abstract This work examines the stiffness of cylindrical elastomer bush mountings in different modes of deformation. Stiffness predictions in different deformation modes obtained using published analytical relationships are compared with finite element analysis (FEA) and with experimental measurements made on a range of elastomer bush geometries. The results for the torsional and axial stiffness from all three methodologies agree reasonably well. However, existing analytical approaches predict values that are clearly incorrect for both the conical and the radial stiffness. Both a revised analytical and a graphical approach are proposed, which predict the radial stiffness more reliably. The work also demonstrates that to predict the initial stiffness of bushes manufactured from filled elastomers, a neo-Hookean stored energy function implemented in a FEA package is sufficient. Hence a single measure of shear modulus taken over the correct strain range is all that is required to characterise the elastomer behaviour for this purpose.