A comprehensive study on the effects of the macromolecular network structure on the fatigue properties and damage modes of natural rubber

The present study investigates the effects of the macromolecular network structure, i.e. , the active chain density and cross-link length, on the fatigue resistance of carbon-black filled natural rubber. A wide range of vulcanization systems (conventional, semi-efficient and efficient) and conditions (time and temperature) have been considered. The active chain density was evaluated by swelling tests and using the Flory-Rehner relationship. Uniaxial fatigue tests under relaxing and non-relaxing tension loading conditions were performed, in order to characterize the fatigue resistance and the fatigue lifetime reinforcement. Moreover, a complementary post-mortem analysis at both the macro- and the microscopic scales was systematically carried out to understand the link between the damage mechanisms and the macromolecular network structure. The active chain density was found to drive the fatigue properties and damage mechanisms. Furthermore, the fatigue lifetime reinforcement and the markers of the strain-induced crystallization have been correlated. They both depend on the active chain density and the strain level. Long cross-link length enhanced the fatigue properties at low active chain density, but this effect tends to decrease as the active chain density increases for relaxing tension loadings. A contrario , the cross-link length was found to have negligible effect on the fatigue lifetime reinforcement and damage mechanisms. The vulcanization time showed a strong effect for long cross-link length, while no effect was observed for short cross-links. The vulcanization temperature had only slight effect on fatigue resistance for each vulcanization system studied. Finally, several diagrams were proposed to fully describe and characterize the relationship between active chain density, fatigue properties and damage mechanisms. • The link between macromolecular structure and fatigue resistance has been established. • A wide range of vulcanization systems and conditions have been considered. • Under relaxing loading conditions, ν ac drives the fatigue properties and damage mechanisms • Under non-relaxing loading conditions, the highest reinforcement was obtained for ν ac ranging from 1.20 to 1.50×10 4 mol/cm 3 . • Fractographic analyses were carried out at both the macro- and the microscopic scales. • Damage mechanisms have been linked to the macromolecular structure.