A Post Mortem Analysis of the Strain-Induced Crystallization Effects on Fatigue of Elastomers

Abstract

Natural rubber (NR) is the most commonly used elastomer in the automotive industry thanks to its outstanding fatigue resistance. Strain-induced crystallization (SIC) is found to play a role of paramount importance in the great crack growth resistance of NR (Lindley, Int J Fracture 9:449–462, 1973). Typically, NR exhibits a lifetime reinforcement for non-relaxing loadings (Cadwell et al., 1940; Ruellan et al., 2019). At the microscopic scale, fatigue striations were observed on the fracture surface of Diabolo samples tested in fatigue. They are the signature of SIC (Cadwell et al., 1940; Le Cam et al., 2013; Le Cam et al., 2004). In order to provide additional information on the role of SIC in the fatigue crack growth resistance of NR, striations are investigated through post-mortem analysis after fatigue experiments using loading ranging from −0.25 to 0.25. No striation was observed in the case of tests performed at 90 °C. This confirms that the formation of striation requires a certain crystallinity level in the material. At 23 °C, two striation regimes were identified: small striation patches with different orientations (Regime 1) and zones with large and well-formed striations (Regime 2). Since fatigue striations are observed for all the loading ratios applied, they are therefore not the signature of the reinforcement. Nevertheless, increasing the minimum value of the strain amplified the striation phenomenon and the occurrence of Regime 2.