Fatigue Lifetime Reinforcement of Natural Rubber: Loading Multiaxiality Effects at Ambient Temperature

Carbon black-filled natural rubber is the most commonly used elastomer for anti-vibratory applications. For decades, uniaxial fatigue tests were carried out to construct models for fatigue lifetime prediction. It is now well established that NR exhibits a lifetime reinforcement when subjected to non-relaxing loadings (Cadwell, Rubber Chem Technol 13:304-315, 1940; Ruellan et al, Int. J. Fatigue 124: 544-557, 2019). This reinforcement observed for uniaxial fatigue is classically attributed to strain-induced crystallization, even though this has never been demonstrated. In the late 2000s, special interest was dedicated to multiaxial fatigue of elastomers to enrich the existing models (Andre, Critere local d'amorcage de fissure en fatigue dans un elastomere de type NR, 1998; Saintier, Fatigue multiaxiale dans un elastomere de type NR charge : mecanismes d'endommagement et critere local d'amorcage de fissure, 2001). Nevertheless, the effect of non-relaxing loading conditions on the multiaxial fatigue life has not been addressed and is still an opened question. In the present study, both uniaxial and multiaxial fatigue tests have been carried out with an axisymmetric specimen geometry to investigate the effect of non-relaxing loading conditions on the fatigue life reinforcement. It was shown that a lifetime reinforcement is observed in the case of pure torsion, similarly to the case of uniaxial tension (Ruellan et al, Int. J. Fatigue 124: 544-557, 2019). Post-mortem analysis of the fracture surface has been performed to better understand the effect of SIC on the damage mechanisms, especially through the observation of fatigue striations. This work provides new experimental results to enrich and benchmark existing fatigue lifetime prediction models.