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A micromechanical model of damaged elasto-inelastic behavior is proposed to predict the plastic fatigue life for fcc metallic polycrystals under multiaxial loading paths. This model is expressed in the time-dependent plasticity for a small strain assumption. In order to generalize and then to increase the model applicability (with respect to other works of the author) in describing the cyclic stress-strain evolution during plastic fatigue, it is therefore assumed that a damage variable initiates and then evolves at the grain level where the phenomenon of the localized plastic deformation occurs. The associated thermodynamic force of the damage variable is determined as a total granular energy (elastic and inelastic). The transition of the elastic strain from the single to the polycrystal, which is classically performed by averaging procedures in this type of modeling, is modified due to the coupling of such a strain with damage. The developed model is tested under different multiaxial cyclic loading situations (tension-compression and tension-torsion with different out-of-phase angles). The effects the loading paths and the grains aggregate type on the fatigue life are appropriately investigated. It is demonstrated that the model can correctly describe the overall and local damaged behavior of polycrystals.

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In this study, a new extension of a micromechanical approach proposed recently by the authors is developed to predict the damaged behavior of polycrystals under various multiaxial cyclic loading paths. The model is expressed in the time dependent plasticity for a small strain assumption. With the framework of the continuum damage mechanics (CDM), it is assumed that a scalar damage variable (d g ) initiates and then evolves at the granular level where the phenomenon of the localized plastic deformation occurs. The driving force of this variable depends on the granular elastic and inelastic energies. This variable can globally describe the microcrack and/or microcavity. The developed aspects involve the development of a new mesodamage initiation criterion, which depends not only on the accumulated granular plastic strain but also on the applied loading path complexity; another new criterion related to macroscopic damage initiation is also developed through the probabilistic approach of Weibull. This gives finally a mixed approach (micromechanical-probabilistic). An experimental program is proposed with the purpose of studying the cyclic behavior of the aluminum alloy 2024. Hence, a series of cyclic uniaxial and biaxial tests is performed up to final fracture of the specimens. After the model parameters identification, the model is examined to demonstrate that it is powerful in reproducing the low-cycle fatigue behavior of the employed alloy. Moreover, an application of the model under various cyclic loading types is qualitatively conducted showing the model's ability in describing the principal phenomena observed, especially, in multiaxial plastic fatigue.

The low-cycle fatigue damage initiation in Waspaloy under complex cyclic loading (out-of-phase) is studied from experimental and theoretical viewpoints. Special emphasis is put on the transgranular damage development and results are compared to those reproduced in the literature. A physico-phenomenological model based on slip theory is used to predict the damage initiation lives as well as the directional aspect of the damage distribution. In this model, the micro-damage is supposed to initiate and then evolve on the activated crystallographic slip systems. The theoretical results are compared to both the experimental ones concerning the same material (Waspaloy) as well as other experimental results extracted from the literature.