Damage-Induced Anisotropy with Damage Deactivation (original) (raw)
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On the Anisotropic Damaged Behavior of Polycrystals
Based on a well-established micromechanical model of damage initiation in low-cycle fatigue (LCF) already developed in , a new extension is proposed for describing the damage deactivation effect. With a small strain assumption, it is assumed that the local damage variables initiate at the crystallographic slip system level. It is considered that the damage is active only if micro-cracks (MC) are open, while damage affects differently the mechanical properties of polycrystals during its closure (inactive phase). The anisotropic damaged (activation and deactivation) behavior concept is adopted only at the macroscopic level. With a fourth-order damage tensor, the deactivation damage effect under multiaxial cyclic loadings is modeled describing the related phenomenon of the induced-oriented anisotropy. Several numerical simulations are conducted describing the overall damaged behavior of polycrystals in biaxial LCF. The responses of a given grains aggregate are recorded and then discussed. As a conclusion, the model describes fairly well the damage activation and deactivation effect in plastic fatigue, notably under multiaxial complex loading paths.
Coupling Between Mesoplasticity and Damage in High-cycle Fatigue
International Journal of Damage Mechanics, 2007
The multiaxial fatigue loading in the high-cycle regime leads to localized mesoscopic plastic strain that occurs in some preferential directions of individual grains for most metallic materials. Crack initiation modeling is difficult in this fatigue regime because the scale where the mechanisms operate is not the engineering scale (macroscopic scale), and local plasticity and damage act simultaneously. This article describes a damage model based on the interaction between mesoplasticity and local damage for the infinite and the finite fatigue life regimes. Several salient effects are accounted for via a simple localization rule, which connects the macroscopic scale with the mesoscopic one, and by the model presented here, which describes the coupled effects of mesoplasticity and damage growth. Irreversible thermodynamics concepts with internal state variables are used to maintain a balance between extensive descriptions of plastic flow and damage events. Cyclic hardening behavior is...
Fatigue & Fracture of Engineering Materials & Structures, 1998
Simulations of the nucleation of dislocations, glide and annihilation ahead of a fatigue crack growing along a localized slip band (a 'long' Stage I crack or a Stage II crack with a K value close to the threshold) are performed for the case of push-pull or reversed torsion loadings, ignoring, in a first approach, the effect of grain boundaries. The crack growth rates are deduced from the dislocation flux at the crack tip. An influence of the normal stress on the friction between the crack flanks as well as on the condition for dislocation emission is introduced. A slower Stage I growth rate is then predicted for reversed torsion, consistent with experimental data.
International Journal of Fatigue, 2014
A new finite element-based mesoscale model is developed to simulate the localization of deformation and the growth of microstructurally short fatigue cracks in crystalline materials by considering the anisotropic behavior of the individual grains. The inelastic hysteresis energy is used as a criterion to predict the fatigue crack initiation and propagation. This criterion in conjunction with continuum damage modeling provides a strong tool for studying the behavior of materials under cyclic loading at the level of the microstructure. The model predictions are validated against an austenitic stainless steel alloy experimental data. The results show that a combined microstructural and continuum damage modeling approach is able to express the overall fatigue behavior of crystalline materials at the macroscale based on the microstructural features. It correctly predicts the crack initiation on slip bands and at inclusions in low-cycle and high-cycle fatigue, respectively, in agreement with experimental observations reported in the literature.
International Journal of Damage Mechanics
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.
Fatigue damage in polycrystals – Part 1: The numbers two and three
Theoretical and Applied Fracture Mechanics, 2008
The fundamental difference between the cyclic yield stress and the fatigue limit is scale. The basic rate of slip mismatch towards a saturated condition, representing polycrystalline behaviour, is a material property embedded into the grain size distribution. Deviations from its basic rate will depend on the shape of the crack and the selected crack path. This work concludes that during the first loading cycle, and for stresses equal and above the fatigue limit, the surface will always been deformed plastically.
Plasticity-damage based micromechanical modelling in high cycle fatigue
Comptes Rendus Mécanique, 2006
A micro-macro approach of multiaxial fatigue in unlimited endurance is proposed. It allows one to take into account plasticity and damage mechanisms which occur at the scale of Persistent Slip Bands (PSB). The proposed macroscopic fatigue criterion, which corresponds to microcracks nucleation at the PSB-matrix interface, is derived for different homogenization schemes (Sachs, Lin-Taylor and Kröner). The role of a mean stress and of the hydrostatic pressure in high cycle fatigue is shown; in particular, in the case of Lin-Taylor scheme and linear isotropic hardening rule at microscale, one recovers the linear dependance in pressure postulated by K. Dang Van for the macroscopic fatigue criterion. This dependence is related here to the damage micro-mechanism. Finally, the particular case of affine loading is presented as an illustration. To cite this article: V. Monchiet et al., C. R. Mecanique 334 (2006). Résumé Une modélisation micromécanique couplant plasticité et endommagement en fatigue polycyclique. On propose une approche micro-macro de fatigue multiaxiale en endurance illimitée. Elle permet de prendre en compte des mécanismes de plasticité et d'endommagement apparaissant à l'échelle des Bandes de Glissement Persistantes (BGP). Le critère macroscopique proposé, qui correspond à la nucléation de microfissures à l'interface BGP-matrice, est explicité pour différents schémas d'homogénéisation (Sachs, Lin-Taylor, Kröner). On montre qu'il rend compte du rôle de la contrainte moyenne et d'une pression hydrostatique en fatigue polycyclique ; en particulier, on retrouve, dans le cas du schéma de Lin-Taylor et d'une hypothèse d'écrouissage isotrope linéaire à l'échelle microscopique, la dépendance linéaire en pression postulée par K. Dang Van pour le critère macroscopique de fatigue. Cette dépendance est liée ici au micromécanisme d'endommagement. Enfin, on présente, à titre d'illustration, le cas particulier des chargements affines. Pour citer cet article : V. Monchiet et al., C. R. Mecanique 334 (2006).
Micromechanical modeling of low cycle fatigue under complex loadings — Part II. Applications
International Journal of Plasticity, 1996
A micromechanical model of the early fatigue damage initiation is proposed based on the slip theor¢. For each slip system, a local micro-damage variable is introduced to describe globally all phenomena related to the level lower than the crystallographic slip system, such as dislocations, atoms, molecules, lattice defects, etc., of FCC polycrystalline materials. This transgranular damage variable is fully coupled with micro inelastic constitutive equations. It is supposed that the local damage appears when the dislocation density reaches some critical values. The obtained model is devoted to describing the cyclic behavior of metallic materials under proportional and non-proportional loading paths neglecting the quasi-unilateral effect as well as the localization of the fatigue damage on the free surface of the specimen.
Cette étude propose une analyse des modes d'amorçage de fissures en fatigue multiaxiale à grande durée de vie en se basant principalement sur des observations MEB. L'étude statistique des sites préférentiels d'amorçage de fissures montre que les grains à glissement multiple présentent une forte probabilité d'amorçage de fissures. L'application du critère de Dang Van à l'échelle des grains grâce à des calculs par éléments finis (élasticité cubique avec/sans plasticité cristalline) sur des microstructures synthétiques 3D semi-périodiques montre une forte hétérogénéité de la contrainte hydrostatique et du cisaillement. L'évolution de cette hétérogénéité en fonction du type de comportement introduit dans le calcul est discutée. Enfin, une méthode basée sur la statistique des valeurs extrêmes est proposée pour dépouiller les calculs sur agrégats et est appliquée à la contrainte équivalente associée au critère de fatigue de Dang Van. Les effets de surface libre et du modèle de comportement ont été analysés.