In situ SEM study of fatigue crack growth mechanism in carbon black‐filled natural rubber

A micro‐tensile testing machine placed in the chamber of a scanning electron microscope is used to perform in situ fatigue tests on a 43 phr carbon black‐filled cis‐1,4‐polyisoprene rubber; the crack tip is observed in real‐time during crack propagation. These observations lead to a detailed description of the crack tip morphology; the crack front is a regular pattern of diamond‐shaped zones delimited by extended straight ligaments. Fatigue crack growth is driven by the ligaments breakage, which occurs in all the zones of the crack front surface continuously in time but at different velocities. This phenomenon of nonlocalized damage explains initiation and limited propagation of branches which deviate from the main crack. All those mechanisms are sources of energy dissipation which explains the great fatigue properties of NR. Finally, from similar experiments conducted on styrene butadiene rubber, it is established that the peculiar morphology of the crack tip and mechanism of crack propagation in NR are due to strain‐induced crystallization.