Impact of High Strain Rate Cycles On the Strain Induced Crystalline Properties of Natural Rubber

Strain Induced crystallization (SIC) of Natural Rubber (NR) is currently of great interest to both industrial and academic communities (1-3). The study of rubber crystallization is essential in order to understand the properties of NR compared to those of synthetic elastomers. In particular, there are ongoing works in different countries to understand the crack growth behavior of NR (4-7), and more generally its wear resistance. The excellent properties of NR are thought to be the consequences of its ability to crystallize under strain. The SIC kinetics is known to be fast; however, the characteristic times of this process are in the range of the characteristic times of the loading of the material in usual applications (the frequency domain of mechanical loading for tires spreads over several decades). The problem is that such rapid SIC kinetics remains difficult to characterize. In the literature, crystallization rate were measured by thermal measurements (8) and WAXS (1). However, the last technique, more straightforward, is still too limited to capture a characteristic time below several milliseconds. For this reason, we have built a specifically designed homemade machine (strain rate from 8s^-1 to 280 s^-1), which enables to measure the sample crystallinity at a given elongation during a cyclic tensile test. The advantages of such a machine are first to avoid any averaging over an elongation domain (since a stroboscopic device selects the desired elongation level), secondly to enable in situ WAXS measurements in a frequency range never met before. Results evidenced a rapid increase of the strain induced crystallization kinetic with the elongation. It should be explained by the enhancement of the crystallization process with the presence of oriented nucleus above a critical elongation. With the aim of better understanding the microstructural process leading to the fast strain induced crystallization kinetic, pertinent parameters such as crystallinity, morphological features and crystalline orientation have been analysed for different positions of the cycle and for frequencies varying from 2Hz to 80Hz will be discuss.

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