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Persistent URL http://purl.org/net/epubs/work/51121
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Record Id 51121
Title Formulation and calibration of a crystal plasticity finite element model for HCP polycrystals
Abstract Conventional models based on continuum mechanics do not allow accounting for grain inhomogeneity at the micro-scale. However, in many practical situations the monotonic and cyclic strength of polycrystalline aggregates is determined precisely by this inhomogeneity in combination with the anisotropy of elastic stiffness and plastic strength of individual grains. We are particularly interested here in the phenomenon of fatigue crack initiation due to accumulated plastic slip at the microscopic level. To achieve an improved formulation of predictive initiation criteria, a computational polycrystal plasticity model is required capable of taking into account the details of grain morphology and crystallographic orientation. Crystal plasticity finite element (CPFE) modelling offers an excellent tool for simulating the mechanics of polycrystalline metals. However, a critical requirement to any such model is the possibility of validating this model against suitably fine-grained experimental data. In this study a microstructurally flexible, three-dimensional, elastically anisotropic CPFE formulation is developed for a model HCP polycrystal. The model allows the prediction of the macroscopic mechanical response to both monotonic and cyclic loading. Furthermore, via the use of the so-called "diffraction post-processor", the model allows the evaluation of the stress and strain field history at the mesoscopic scale (grain-orientation average), thus providing for the characterisation of deformation heterogeneity, and for comparison against diffraction measurements of orientation specific elastic lattice strains. Time-independent crystal plasticity constitutive equations were implemented in the form of a UMAT within the ABAQUS commercial FE package. Basal, prismatic and pyramidal slip systems were considered in the model. 3D microstructure implementations were generated by introducing a spatial distribution of "seeds", ascribing certain crystal orientations to them, and performing Vorono├» tessellation. Although a "static" FE mesh was used, each integration point was given initial properties (lattice orientation) associated with the closest "seed". Elasto-plastic deformation simulation used critical resolved shear stress varying as a function of accumulated slip. Model predictions were validated against experimentally obtained monotonic and cyclic macroscopic stress-strain curve. In the pre- and post-processing analysis of the simulation, elastic strains within the RVE were interpreted so as to predict peak centre positions in a diffraction experiment. In situ loading and diffraction experiments on near- Ti-6Al-4V alloy samples were carried out on station ID11 at the ESRF. From the experiment, intergranular strain responses for differently oriented crystallites were determined and compared to the model predictions.
Organisation ISIS , STFC
Keywords crystal plasticity finite element , engineering , diffraction
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Language English (EN)
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Paper In Conference Proceedings In 9th International Conference on Computational Plasticity Fundamentals and Applications (COMPLAS 2007), Barcelona, Spain, 5-7 Sep 2007, (2007). 2007