Jens Meier

A material model for cellular rubber considering the most relevant inelastic effects like viscoelastic behavior and Mullins effect is presented. A substantial advantage of the model is that the identification of the foam’s behavior relies on mechanical tests of the (pore-free) matrix material only. Therefore, the model can be adapted to different pore structures by adjusting just the model’s pore parameters. These parameters are determined by a simple algorithm that operates on the basis of CT (Computer Tomography) recordings of the foam. The proposed model is validated in uniaxial tensile tests using foam samples with three different microstructures. In order to evaluate the model’s ability to predict the foam’s compressive behavior, lateral strain measurements were also obtained during the tensile tests. The validation shows a satisfying agreement between the model prediction and the experimental results.

The service lifetime of filled elastomer compounds is of high interest for manufacturers and customers. Since the conduction of lifetime experiments is expensive and time consuming, laboratory prediction concepts are necessary (Meier et al. 2020) (Robin and Alshuth 2013). Recently, an estimation concept based on the filler dispersion state analyzed with computed tomography (CT) and fatigue crack growth behavior was developed which turned out to reproduce several lifetime influencing effects qualitatively within accordance to experiment, but with several uncertainties considering the CT method. In this study, the robustness of the lifetime prediction method is investigated, with a focus on the efficiency of the CT method. CT allows for a non-destructive 3D volume analysis of density variations down to a resolution of a few micrometers. The sum of all inclusions leads to the particle size distribution of a compound. The size distribution generated from the CT data is highly dependent on the CT measurement settings and often a relative measure. Hence, this study aims to investigate the impact of the particle size distribution on the service lifetime prediction and pinpoint the critical parameter. Therefore, carbon black reinforced SBR and NR vulcanized cylinders with height and diameter both of 30 mm are measured under various CT settings and the particle size distribution is obtained for different evaluation parameters. Fatigue experiments are conducted on dumbbell specimens to test model predictions against the experimental findings. Apart from the capability of CT analysis to predict reasonable lifetime for elastomer parts under dynamic load, it seems promising to establish an efficient quality control method to investigate lifetime relevant filler dispersion states already from the compound, i.e., directly after the mixing.

An extended ultrasonic transmission method is introduced to determine the linear viscoelastic properties of elastomers. Based on increasing the frequency range of standard temperature dependent ultrasonic measurements the application of a master curve creation procedure known from classical dynamic mechanical analysis is proposed. The developed acoustic master curve technique is applied to solid and foamed SBR compounds and compared with corresponding mechanical master curves for verification. The results are discussed with respect to the influence of porosity on the material’s effective bulk modulus, with strong evidence found for viscoelasticity of the compression behavior.