ABSTRACT
Natural rubber (NR) exhibits extraordinary physical properties. Among them, its remarkable fatigue resistance was reported by Cadwell et al. as soon as 1940 (Cadwell et al. 1940). In particular, they found that NR exhibits a strong lifetime reinforcement for non-relaxing loadings (i.e. for R > 0). Since it was not observed in the case of non-crystallizable rubbers, such reinforcement is generally attributed to strain-induced crystallization (SIC). In automotive applications, NR is used in anti-vibratory systems subjected to high temperatures. Surprisingly, few studies investigated the effect of temperature on the fatigue properties of NR, and more particularly on the lifetime reinforcement (Bathias et al. 1998), while SIC is a highly thermosensitive phenomenon (Trabelsi et al. 2002). The present study aims therefore at investigating how temperature affects the fatigue life reinforcement due to SIC under non-relaxing loading conditions. Fatigue experiments are first carried out at 23°C for loading ratios ranging from -0.25 to 0.35, before being compared to results obtained at 90°C and 110°C where the ability of NR to crystallize is reduced or cancelled. Fatigue damage has been analyzed at both the macro and the microscopic scales. As expected, the material exhibits a strong lifetime reinforcement at 23°C and the fracture surfaces are peopled with SIC markers (wrenchings (Le Cam et al. 2004), striations (Le Cam and Toussaint 2010, Ruellan et al. 2018) and cones (Ruellan et al. 2018)). Their distribution is different between relaxing and non-relaxing loadings. At 90°C, the reinforcement still occurs, even though SIC markers disappear from the fracture surfaces. Finally, the lifetime reinforcement totally vanishes at 110°C.
