Competition between mesoplasticity and damage under HCF – Elasticity/damage shakedown concept

The aim of this paper is to present new modelling dedicated to multiaxial high cycle fatigue (HCF), and applied to polycrystalline metals. The model presented is based on the experimental characterization of damage during HCF tests, under pure tension and torsion modes. The origin of this approach is a mesoscopic model considering three plastic behaviour stages (hardening, saturation and softening) suggested by Papadopoulos [Papadopoulos I.V. Fatigue limit of metals under multiaxial stress conditions: the microscopic approach. Technical Note No. I.93.101, Commission of the European Communities, Joint Research Centre; 1993. [ISEI/IE 2495/93].], and used by Morel [Morel F. A critical plane approach for life prediction of high cycle fatigue under multiaxial variable loading. Int J Fatigue 2000;22:101–119.]. The principal evolution brought in by this study is a competition description during all the sample lifetime of the plasticity and damage effects. The plasticity mechanisms induce a hardening saturating effects (resulting from movement and accumulation of dislocations), especially significant at the beginning of the crystal life. Damage, present at the end of crystal lifetime, is considered as a degradation process inducing a strong reduction of the crystal ductility, leading to its failure (decohesion). The coexistence and the competition between these two effect (hardening and damage-induced softening) describe cyclic crystal behavior, including shakedown phase. The model is formulated in the framework of the continuum damage mechanics, according to the identified physical mechanisms during the tests.The second purpose is to compare the model predictions with experimental data, after identification of the parameter for a ferritic-pearlitic steel. The case of in-phase loading is merely studied here. In particular, the evolution of a few internal variables is discussed and correlated with the available physical features. It is shown that this model provides a complementary insight into a crystal with respect to the endurance criterion of Dang Van. The model predicts, in a particular stress amplitude range, the damage growth arrest.