Microfocused Beam SAXS and WAXS Mapping at the Crack Tip and Fatigue Crack Propagation in Natural Rubber

We investigated crack propagation in cyclic fatigue in filled (50 phR of N347 carbon black), highly crosslinked, natural rubber pure shear samples and, in parallel, mapped in detail the microstructure present at the crack tip with a microfocused X-ray synchrotron beam. We acquired data by wide-angle X-ray scattering (WAXS) to characterize strain-induced crystallization (SIC) and by small-angle X-ray scattering (SAXS) to detect nano-cavitation. Crack propagation experiments were carried out for a range of maximum energy release rates during the cycles varying from 1 to 4 kJ/m(2). Each material was tested at two temperatures: room temperature and 100 degrees C and in new and aged conditions (10 days at 100 degrees C in an inert atmosphere). We found that although significantly less SIC at the crack tip was present at the crack tip at 100 degrees C, the resistance to crack propagation under cyclic loading was barely affected. In aged samples SIC at the crack tip was significantly lower than in new samples at room temperature, and was not detectable at all in aged samples at 100 degrees C. The crack propagation rate, however, only increased with aging for the more crosslinked sample at 100 degrees C and never increased catastrophically. Finally, a few percent of nano-cavities were detected at the crack tip and the comparison of X-ray transmission and digital image correlation suggested the presence of a significant fraction of large cavities at the very crack tip. In conclusion while strain-induced crystallization in NR may be affecting fatigue resistance we could not establish a quantitative correlation between the volume crystallized at the crack tip and the crack propagation rate in cyclic fatigue.