Prof. Mikhail ItskovDepartment of Continuum Mechanics, RWTH Aachen University, Germany
Speech Title: Mechanics and Thermodynamics of Strain-Induced Crystallization
Abstract: Strain-induced crystallization is a very interesting and important phenomenon appearing in elastomers as for example natural rubbers subjected to large stretches. In this case, polymer molecules come very close to each other and begin to form crystallites oriented mostly in the direction of stretch. These crystallites can retard or even stop crack propagation in highly loaded elastomeric structures as for instance truck tires and thus improve their durability and lifetime. So far, the strain-induced crystallization has mostly been investigated by using X-ray diffraction, which is a very expensive and hardly accessible procedure for a classical mechanical lab. Moreover, the X-ray diffraction is not able to measure calorimetric effects always accompanying the strain-induced crystallization. These effects results from the phase transition during crystallite nucleation, growth and melting. In this regard, the infrared thermography combined with mechanical measurements appears to be very suitable. Indeed, by means of such a procedure the heat production and absorption as well as the crystallization degree during cyclic loading can be detected and characterized in natural rubbers. Based on the energy balance the intrinsic dissipation due to viscosity and stress softening can thus be evaluated under cyclic loading. Energy contributions to the hysteresis loop converted into heat and stored in the material can further be separated. In the present contribution, we also present a physically based constitutive model for filled natural rubbers coupled with infrared thermography based calorimetry to study the strain-induced crystallization. The kinetics of phase transition outside thermodynamic equilibrium is also discussed and underlying mechanisms of nonequilibrium strain-induced crystallization are interpreted. To capture multiaxiality of strain-induced crystallinity, we apply the analytical network-averaging concept proposed in our earlier papers. Predictions of the so-developed model of the strain-induced crystallization demonstrate good agreement with various experimental data.
Keywords: Strain-induced crystallization, calorimetric effects, physically based modeling, natural rubbers.
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