On the Anisotropic Damaged Behavior of Polycrystals (original) (raw)
Related papers
Damage-Induced Anisotropy with Damage Deactivation
International Journal of Damage Mechanics, 2009
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.
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.
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.
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.
Variability of the Fatigue Driving Force within Grains of Polycrystals
Icf13, 2013
Experimental studies in the last few decades have exhibited higher fatigue crack growth rates for cracks with size on the order of grains than would be predicted using growth laws based on LEFM. Small crystallographic fatigue cracks are affected by microstructure features that are not captured by traditional homogenous fracture mechanics theories (i.e., LEFM, EPFM). Since far-field driving force parameters cannot capture the intrinsic variability of the local fatigue driving force of small cracks induced by microstructure, alternative measures of the fatigue driving force are sought. This work employs finite element simulations that explicitly render the polycrystalline microstructure to compare nonlocal fatigue indicator parameters (FIPs) averaged over multiple volumes. The model employs a crystal plasticity algorithm in ABAQUS calibrated to study the effect of microstructure on early fatigue life of Ni-base RR1000 superalloy at elevated temperature under constant amplitude loading. The results indicate slight differences in the extreme values of distributions of FIPs for each element, slip plane cross-section (bands) and grain volumes. Furthermore, the grain average FIP better reflects the driving force for cracks on the cross section at the center of the grain while the extremes values of the FIPs averaged along bands tend to be located away from the grain centers.
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.
Investigation of the effect of grain clusters on fatigue crack initiation in polycrystals
International Journal of Fatigue, 2010
Fatigue crack initiation in ductile alloys like austenitic stainless steels is mainly due to the occurrence of localized deformation in persistent slip bands (PSB). The presence of PSB is classically related to the orientation of the surface grains. In fact, the local fields in a grain does not depend on the local orientation only. The aim of the present paper is to investigate the consequences of this observation, and to propose an analysis, where the neighborhood of the grain also plays a significant role. The study is made on a 316 stainless steel. Finite element computations using a crystal plasticity model are performed to simulate an aggregate submitted to a cyclic tension-compression loading. Various configurations of grain orientations ("clusters") are studied at the free surface of the aggregate. A statistical analysis of the results is carried out to extract significant information concerning the local strain and stress fields, including the most critical arrangements of grain orientations. The introduction of local fields in classical fatigue life prediction models provides an explanation of the experimental scatter.