Advanced Analysis of Crystallization Kinetics and Tear Fatigue Behavior of Natural Rubber

There is still no viable alternative to natural rubber in truck tires when it comes to the ultimate demands for wear resistance and tear fatigue. Its outstanding mechanical properties are ascribed to strain-induced crystallization, which provides selective self-reinforcement in highly stressed regions, such as around a crack tip. Novel experiments, performed at the DESY synchrotron, gave new insight into the kinetics and the short-time behavior of the strain induced crystallization under dynamic conditions, as encountered in a rolling tire.

The kinetics of strain-induced crystallization in natural rubber was studied by synchrotron wide-angle X-ray diffraction (WAXD) in a strain jump setup over a broad temperature range. The increase of crystallinity after a strain jump was observed on a millisecond time scale, until a steady state plateau was approached. A novel model, based on chain dynamics scaling laws for diffusion in polymer networks was developed to interpret the time and temperature dependence of the strain-induced crystallization. All results demonstrate that in-situ exploration of crack tips with X-rays provides valuable information about crack propagation and tear fatigue behavior of rubbers.

In the second part of the presentation we briefly report then about a cutting-edge concept in rubber tear fatigue testing, using the Coesfeld TFA and a new designed biaxial test stand, together with implemented concepts of data evaluation. As an example we also show comparisons between an intuitive crack tip detection algorithm based on crack contour curvature changes and a new online usable crack analysis which is reminiscent of a robot seeking a path through a passage.

We believe that this combined approach of different methods looks toward the next generation of predictive tools for rubber durability under multiaxial dynamic loading conditions. The approach is especially important for crystallizing natural rubber which is still the main polymer component in truck tire tread compounds.