Mechanical behaviour of particlefilled elastomers at various temperatures: An experimental and numerical study

Abstract

Numerical tools that facilitate predictions of the mechanical behaviour of elastomeric products exposed to various temperatures have great cost saving potentials in multiple industries. To unleash these potentials, precise experimental data and improved constitutive models that capture the essential physics of the materials must be obtained. This work consist of an experimental and numerical study of the mechanical behaviour of particle-filled HNBR and FKM elastomers subjected to various loading modes at a range of temperatures. The thesis is organized as a synopsis, providing the motivation, the objectives, the theoretical background, and a summary of the work, while the main body is a collection of three journal articles providing the scientific contribution.

Paper I deals with the tension behaviour of three different commercial elastomeric materials, being two HNBR and one FKM compound, at temperatures ranging from −20 to 150 oC. A cyclic deformation history was applied to quantify viscoelastic and history dependent effects, and optical techniques were used to measure the local deformations of the samples. The level of viscous behaviour was seen to increase as the temperature was lowered. A dip of the stress-strain behaviour was seen for one of the HNBR compounds tested at low temperatures, and a theory of matrix-particle debonding was proposed to explain this dip. Unit cell simulations suggested that the observed stress dip might be caused by a reduction of the fracture energy of matrix-particle cohesive zones at low temperatures. For the test at higher temperatures, the samples were found to display premature material failure during the deformation cycles.

Paper II illuminates the difference in volumetric behaviour obtained in uniaxial tension and confined axial compression experiments on HNBR and FKM elastomers. While a stiff and fully elastic volumetric response was obtained for both materials in confined axial compression, with a volume reduction of about 6 % for a hydrostatic stress level of 140 MPa, a relatively compliant and viscous volume behaviour was found in uniaxial tension, with nearly 20 % volume increase at a hydrostatic stress of about 6 MPa found for the FKM compound. This difference has severe implications for the constitutive modelling of elastomeric materials. To increase the understanding of the micromechanical mechanisms causing the large difference in volume behaviour between the loading modes, an in situ scanning electron microscopy study was performed to examine if matrix-particle decohesion can occur during uniaxial tension. It was found that ZnO particles tend to debond from the matrix material during tension. Using finite element simulations of a unit cell configuration it was shown that such debonding could explain the volume response in uniaxial tension and confined axial compression in a qualitative manner.

Paper III pays attention to the volume growth accompanying uniaxial tension at temperatures from 23 to −18 oC. A new experimental set-up, building on the experience made in Paper I, is outlined to measure volume growth accompanying uniaxial tension at low temperatures. A significant increase in the volume growth accompanying tension was found for all materials as the testing temperature was reduced. To incorporate the large volume changes in finite element simulations, a new constitutive model outlined in the framework of finite strains continuum mechanics is presented. The model alters the well-known visco-hyperelastic Bergström-Boyce model by use of a Gurson viscous flow potential to account for the matrix-particle decohesion observed in Paper II. The temperature dependence of the material parameters is included thorough the use of a simple mathematical relation and the model is shown to yield a good prediction of the experimental results.