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Materials Science and Engineering A
journal homepage: www.elsevier.com/locate/msea
Cyclic loadings and crystallization of natural rubber: An explanation of fatigue crack propagation reinforcement under a positive loading ratio N. Saintier a,∗ , G. Cailletaud b , R. Piques b
LAMEFIP, Arts et Métiers ParisTech, Esplanade des Arts et Métiers, 33405 Talence Cedex, France Centre des matˇeriaux P.M. FOURT, Ecole Nationale Supˇerieure des Mines de Paris, UMR CNRS 7633, Evry Cedex 91003, France ı ı
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Natural rubber is known to have excellent fatigue properties. Fatigue crack propagation studies show that, under uniaxial tension loading, fatigue crack growth resistance increases with the loading ratio, even if the peak stress increases. Studies dealing with crack initiation conﬁrm this trend. If strain induced crystallization is believed to play a major role in this reinforcement process, it is not clear yet by which mechanism this reinforcement takes place. Using SEM investigation, it is shown here that the reinforcement process is associated with strong crack branching in the crack tip region. From experimental results it is shown that under particular reinforcing loading condition a cyclic strain hardening process can be observed on the natural rubber which is able to overcome classically observed softening effects. A cumulative strain induced crystallization process is proposed to explain the stress ratio effect on fatigue crack initiation and propagation properties of natural rubber. © 2010 Elsevier B.V. All rights reserved.
Article history: Received 30 June 2010 Received in revised form 22 September 2010 Accepted 27 September 2010
Keywords: Natural rubber Fatigue Crystallization Crack branching
1. Introduction Natural rubber’s resistance to crack growth and its ability to withstand large strains without permanent deformation are two of the main reasons for its extensive use in many industrial applications. Most of them involve signiﬁcant static and cyclic loading. The need for appropriate multiaxial fatigue life criteria has become crucial over the past 10 years. A good understanding of the micromechanisms involved in the fatigue crack initiation process are essential for the establishement of physically motivated criteria. In the case of natural rubber, in addition of damage mechanisms, speciﬁc reinforcing mechanisms still have to be understood. Indeed, under particular loading conditions (under uniaxial loading these particular conditions correspond to positive stress ratio tests), natural rubber shows an exceptional fatigue resistance so that damage mechanisms only cannot fully describe the fatigue behavior of natural rubber. A reinforcing mechanism must be taken into account in order to propose a fatigue life criterion suitable for all types of loading. The aim of this paper is not to propose such a criterion but rather to understand the origin of this mechanism so that a physically based criterion can be proposed (see [1,2] for a multiaxial fatigue life prediction method taking into account reinforcing mechanisms on natural rubber). The particular strength of natural rubber, as opposed to synthetic rubber on
∗ Corresponding author. Fax: +33 (0) 556845366. E-mail address: firstname.lastname@example.org (N. Saintier). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.09.079
which such reinforcing mechanisms are not observed, has been attributed to its ability to crystallize upon stretching. Strain induced crystallization is believed to play a major role in fracture properties of rubber by inducing strong microstructural changes in the crack tip region. Strain induced crystallization of natural rubber has been largely investigated using numbers of techniques such as X-ray diffraction [3–6], infra-red...