Abstract
Fatigue striations are investigated in order to provide additional information on the role of SIC in the fatigue crack growth resistance in natural rubber (NR). Uniaxial fatigue tests were carried out with Diabolo samples under a wide range of loading ratios and loading levels. Fracture surfaces were analyzed by using SEM. Two striation regimes were identified: small striation patches with different orientations (Regime 1) and zones with large and well formed striations, where wrenchings are no longer observed (Regime 2). As fatigue striations were observed for all the loading ratios applied, they are therefore not the signature of SIC in the lifetime reinforcement. Nevertheless, increasing the minimum value of the strain amplified the striation phenomenon and the occurrence of Regime 2. Results obtained have been qualitatively put into perspective with crack propagation curves provided in Lindley (1973). The analysis carried out unifies the results obtained in the literature for relaxing and fully relaxing loadings (Rε⩽0) in the sense that increasing the loading, i.e. the tearing energy, leads to an increase in the crack growth rate and to a striation typology evolution, especially the striation size. For non-relaxing loadings, our results suggest that the striation typology may be governed by the loading ratio rather than the crack growth rate. Finally, as striations did not appear when the tests were performed at temperature superior or equal to 90 °C, the formation of striation requires therefore that the material is crystallizing.
Highlights
Since the beginning of the 20th century, rubber-like materials have been widely used in many applications for their damping properties and their resistance to fatigue
Fatigue striations mainly appeared along the surface of the Diabolo sample and peopled the bulk when the loading applied at the microscopic scale increased
strain-induced crystallization (SIC) is assumed to be the main phenomenon involved in crack growth resistance of natural rubber
Summary
Since the beginning of the 20th century, rubber-like materials have been widely used in many applications for their damping properties and their resistance to fatigue. Fatigue damage and resistance mechanisms, especially in crystallizable rubbers, remained not well understood, which is a real obstacle to fatigue lifetime prediction and improvement of rubber part design This explains the upsurge of experimental studies since the last two decades. In case of crystallizable natural rubber (NR), the material submitted to cyclic loadings exhibits a high duration life and a strong lifetime reinforcement for non relaxing loadings (typically for uni-axial tension-tension loadings, when the minimum force Fmin is superior to zero). This was observed as soon as 1940 by Cadwell and co-workers in NR, but this was not observed in non-crystallizable rubbers.
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