Strain-induced crystallization around a crack tip in natural rubber under dynamic load

The local degree of crystallinity around a crack tip in natural rubber under dynamic load was measured by time-resolved scanning wide-angle X-ray diffraction (WAXD). Using a high-flux synchrotron microbeam, WAXD patterns with less than 20 ms exposure time were acquired while the notched rubber sample was subjected to cyclic dynamic stretching at a frequency of 1 Hz, similar to the loading conditions in tear fatigue experiments. By scanning the continuously cycling sample, a complete crystallinity map at any given strain phase angle was obtained. The crystallinity at the crack tip is considerably reduced compared to static crack tip scans. Further investigations revealed the underlying structural reasons for the well-known relation between R-ratio and crack growth resistance. By performing static crack tip scans on increasingly stretched rubber samples, the mechanisms behind crack deflection and branching were studied.