Effect of strain-induced crystallization on fatigue crack growth resistance of natural rubber

In tire industry, fatigue crack propagation in elastomers is usually investigated to improve the service life of products. To explain the remarkable resistance to fatigue crack growth of natural rubber as compared to synthetic rubbers, the literature often mentions the Strain-Induced Crystallization (SIC) phenomenon that takes place at the tip of fatigue cracks. This study presents an original experimental set-up that couples synchrotron radiation with a homemade fatigue machine and permits the real time investigation of the crystallized zone at crack tip during uninterrupted fatigue tests. Diffraction patterns' recording is synchronized with the cyclic loading to perform a "mapping" of Wide-Angle X-ray Diffraction (WAXD) measurements, in order to obtain the 2D spatial distribution of crystallinity in the neighbourhood of the crack tip. The influence of different parameters, such as loading conditions (level of cyclic stretching) or the carbon-black filler content, on the size of the crystallized zone, is investigated. The comparison of these results with fatigue crack propagation tests performed with the same experimental conditions leads to a first explanation of the very good macroscopic fatigue behaviour of natural rubber.