Investigating strain-induced crystallization through fatigue striations in filled NR

Abstract

: Natural Rubber (NR) exhibits a remarkable fatigue resistance, especially for non-relaxing loadings under which a strong lifetime reinforcement is observed (Cadwell et al. 1940). Such a resistance is classically attributed to strain-induced crystallization (SIC). At the microscopic scale, it has been shown that SIC induces striations on the fracture surface of NR samples tested under fatigue loadings (Le Cam and Toussaint 2010, Muñoz-Mejia 2011, Le Cam et al. 2013, Ruellan et al. 2018). In order to provide additional infor- mation on the role of SIC in the fatigue crack growth resistance of NR, striations are investigated through post-mortem analysis after fatigue experiments carried out under both relaxing and non-relaxing loadings. Results show that two striation regimes take place. Regime 1 corresponds to small striation patches with different orientations and Regime 2 induces zones with large and well-formed striations. As fatigue striations are observed for all the loading ratios applied, they are therefore not the signature of the reinforcement. Neverthe-less, increasing the minimum value of the strain ampli fi ed the striation phenomenon and the occurrence of Regime 2. The analysis carried out uni fi es the results obtained in the literature for relaxing and fully relaxing loadings in the sense that increasing the loading, i.e. the tearing energy, leads to an increase in the crack growth rate Lindley (1973) and to a striation typology evolution, especially the striation size (Ruellan et al. 2018).