Michael Johlitz

CHARACTERIZING THE TIME DEPENDENCE OF FILLED EPDM

Abstract The mechanical properties of a carbon black filled rubber are investigated. The main focus lays on the theoretical modeling of the basic elasticity and the viscoelastic behavior. Therefore, uniaxial tension tests at different feedrates are performed. The occurring Mullins effect can be neglected due to adequate pretreatment of the specimens. Healing effects are also verified and investigated in the examined material. The constitutive model for the basic elasticity is based on the Yeoh model, while the theory of finite viscoelasticity with an intermediate configuration is used to describe the rate dependent behavior. The healing effects are constituted with large relaxation times and not with an additional structural parameter. As the material has a strong nonlinear behavior with respect to the deformation rate, nonlinearity in the relaxation time with respect to this behavior is introduced. The material parameters of the model are estimated using a stochastic identification algorithm.

A continuum-based model capturing size effects in polymer bonds

It is known from applications that the mechanical behaviour of polymer bonds does not only depend on the properties of the polymer itself but also on the substrate. Therefore, the mechanical behaviour, i.e. the stiffness, of a polymer joint becomes thickness dependent. In the present work we describe experiments performed on polymer joints and we develop a continuum-based model which is able to describe the experimentally observed size effects without suggesting the microstructure in detail. The continuum mechanical model is enhanced by a scalar-valued structure parameter which describes all the effects taking place in the boundary layer which arises near the substrate. It is shown that the model parameters can be determined on the basis of simple shear experiments performed on polymer layers of different thickness.

Constitutive Modelling of the Glass Transition and Related Phenomena: Relaxation of Shear Stress Under Pressure

In industrial fabrication processes as well as in many applications of polymer parts, the glass transition plays a significant role. This is due to high mechanical processing speeds, high temperatures or large cooling rates. The mechanical, the thermomechanical and the caloric properties of polymers differ below and above the glass transition which is a thermoviscoelastic phenomenon. It depends on the ratio between the intrinsic time scale of the polymer and that of the thermomechanical loading process. If both scales are comparable, the material is in the glass transition region. Otherwise it is in the equilibrium or in the glassy region. In the industry, there are increasing demands to simulate fabrication processes in order to estimate the resulting behaviour of the polymer parts before they are manufactured. To this end, constitutive models of finite thermoviscoelasticity are needed which can represent the volumetric as well as the isochoric mechanical behaviour of the polymer in combination with the caloric and the thermomechanical properties. In a recent paper of the authors, the concept of a hybrid free energy has been developed. This approach will be applied in the current essay where the pressure-dependent relaxation behaviour under shear deformations is of interest.