Fatigue crack growth experiments on different carbon black–filled rubber compounds have been carried out to evaluate the influence of pure-shear and strip tensile testing mode by using sine and pulse as waveforms. In a previous set of experimental investigations regarding the influence of both waveform and tested material, it was found that the mode I of crack opening sometimes propagates too quickly to be properly monitored in tests involving strip-tensile specimens. An alternative test methodology based on pure-shear test mode has been investigated, optimizing both the shape of the specimen and the test equipment. Data obtained from the different compound formulations were consistent with the theoretical background and resulted in similar ranking of compound crack growth resistance for the two testing modes; in addition, pure-shear mode showed a higher sensitivity to formula variations.
The interest towards natural rubber (NR) is progressively increasing due to its sustainable production and remarkable mechanical properties, presenting a wide application range in the automotive industry and civil engineering. In this paper we report, for the first time, the use of electrospinning technique to produce neat and graphene nanoplatelets (GNPs, 1 wt.%) loaded natural rubber fibers. Both randomly distributed and aligned fibers (average diameter size ~ 1 μm) mats were obtained, resulting uniform and defect-free. A detailed characterization of these fibers is reported, including field emission-scanning electron microscopy (FEG-SEM), X-Ray diffraction (XRD) and infrared spectroscopy (FTIR-ATR) techniques. It has been demonstrated that the electrospinning process is able to induce a strong orientation of the polymeric chains in the case of aligned fibers, with respect to the randomly oriented fibers and solvent cast films
Fatigue crack growth experiments on carbon black-filled rubber compounds have been carried out to evaluate the influence of testing conditions over different compound formulations. Investigations on the influence of waveform, data acquisition, and compound formulation have been performed on strip-tensile specimens reproducing the model of crack opening. The response of three different compound formulations (based on either natural rubber, butadiene rubber, or styrene-butadiene rubber) to the application of two different waveforms, pulse and sine, has been analyzed, showing significant differences in fatigue behavior and ranking of the various compounds. Compared to the sinusoidal waveform, the use of a pulse waveform provided an improved correlation of the tearing energy with the crack propagation speed. This difference was particularly evident in the case of natural rubber and butadiene rubber, while it resulted negligible in the case of styrene-butadiene rubber. Such a different behavior could be attributed to differences in macromolecular chains orientation. Fine-tuning of video acquisition parameters provided an accurate observation of the crack growth process, as confirmed by the low standard deviation of the estimated tearing energy and crack growth rate
A refinement of the Kummer friction model is considered to determine the friction coefficient through analysis of the mechanical properties of rubber compound materials. An experimental sets of trials was carried out with a linear friction tester evaluating the friction coefficient for CB filled SBR on an ideal sinusoidal substrate under lubricated conditions. Lubricated conditions were chosen to obtain the pure hysteresis contribution of the friction coefficient, eliminating the effect of adhesion. For finite element analysis (FEA) simulations a visco-hyperelastic material model is considered to estimate the actual strain level close to the contact area. Based on the proper analysis of the temperature and frequency dependence behavior of experimental model rubber compounds, master curves for the viscoelastic moduli are constructed. Dynamic data were selected based on the strain level evaluated by first FEA simulations, actual temperature, and excitation frequency of each experiment. The proposed refined friction model and the simulated friction coefficient from FEA based on the precise evaluation of the material property show good prediction within the ideal experimental conditions.
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