ABSTRACT
It is well-recognized that the nucleation of a macroscopic fatigue crack is the result of the growth of microscopic flaws initially present in the material as first shown by Gent, Lindley, & Thomas (1964) and later investigated by others (Mars & Fatemi 2002, Le Cam, Huneau, Verron, & Gornet 2004, Beurrot, Huneau, & Verron 2010). Moreover, as demonstrated experimentally by Mars (2002) and Harbour, Fatemi, & Mars (2008) among others, the macroscopic crack grows in a plane defined by loading conditions and especially multiaxiality. From an engineering point of view, it seems difficult to consider the explicit influence of flaws on mechanical fields (Aït-Bachir, Mars, & Verron 2012). Thus, practical crack nucleation approaches must focus on defect free derivations. In this context, the recent investigations have stated that the relevant mechanical quantities to predict fatigue damage in such materials are the true stress tensor (André, Cailletaud, & Piques 1999, Abraham, Alshuth, & Jerrams 2005, Saintier, Cailletaud, & Piques 2006, Brunac, Gérardin, & Leblond 2009) and energy based tensors (Mars 2002, Zine, Benseddiq, Naït-Abdelaziz, Aït Hocine, & D. Bouami
1 INTRODUCTION
In the last ten years, number of papers have proposed predictors for fatigue crack nucleation in elastomers subjected to multiaxial loading conditions. By nature, a crack nucleation approach claims to be able to capture the onset of a macroscopic crack in a yet idealized defect-free material; practically it must be able to determine the most probable locus for the growth of a fatigue crack but it can also be able to predict its direction. In this latter case, the approach is referred to as a critical plane theory. For metallic materials, most of the critical plane theories consist in combining stress components that exert on a given geometrical plane, and to determine the plane on which this combination is maximized: the use of normal and shear stress components is assessed by the microstructure that exhibits weaker mechanical resistance in some specific directions of the space due to less or more dense crystallographic planes. The case of elastomers differs because of different microstructural properties and it has received highly less attention (Mars & Fatemi 2002).
