Influence of texture and test velocity on the dynamic,high-strain,tensile behavior of zirconium

Experiments were conducted to characterize the influence of texture and impact velocity on the dynamic, high-strain, tensile extrusion of zirconium. Bullet-shaped samples were machined from a clock-rolled, highly textured Zr plate. Specimens in two orthogonal directions were tested: the extrusion direction aligned with either the in-plane (IP) rolling or the through-thickness (TT) direction of the plate. The post-extrusion microstructure and texture evolution were examined using electron backscatter diffraction microscopy and modeled using the viscoplastic self-consistent model. It was found that extrusion deformation was accomplished through a combination of twinning and slip with their relative activity greatly depending on the initial texture. In this regard, higher elongations in the IP samples as compared to the TT samples were observed at similar test velocities. This difference in ductility is discussed in terms of the material’s ability to accommodate plastic deformation. Due to the availability of a larger number of slip systems with relatively high Schmid factors in the IP samples under this configuration, plastic deformation by prismatic slip can be easily achieved, resulting in larger elongations. On the contrary, for TT samples, twinning preceded deformation by slip. This sequential deformation process, driven by the need to reorient the microstructure favorably to slip, led to diminished elongations to failure.     

Influence of Temperature on the Dynamic Tensile Behavior of Zirconium

The influence of temperature on the dynamic tensile behavior of Zr has been investigated. Bullet-shaped Zr samples with two different textures were dynamically extruded at room temperature and 523 K (250 °C). A higher ductility was measured for samples deformed at elevated temperature as compared to those extruded at room temperature. This difference in ductility is discussed in terms of zirconium’s ability to accommodate plastic deformation via thermally enhanced slip activity, as evidenced by examination of the deformed microstructures.     

Modeling void growth in polycrystalline materials

Most structural materials are polycrystalline aggregates whose constituent crystals are irregular in shape, have anisotropic mechanical properties and contain a variety of defects, resulting in very complicated damage evolution. Failure models of these materials remain empirically calibrated due to the lack of a thorough understanding of the controlling processes at the scale of the materials’ heterogeneity, i.e. the mesoscale. This paper describes a novel formulation for a quantitative, microstructure-sensitive three-dimensional mesoscale prediction of ductile damage of polycrystalline materials, in the important void growth phase of the process. Specifically, we have extended a formulation based on fast Fourier transforms to compute growth of intergranular voids in porous polycrystalline materials. In this way, two widely used micromechanical formulations, i.e. polycrystal plasticity and dilatational plasticity, have been efficiently combined, with crystals and voids represented explicitly, to predict porosity evolution. The proposed void growth algorithm is first validated by comparison with corresponding finite-element unit cell results. Next, in order to isolate the influence of microstructure on void growth, the extended formulation is applied to a face-centered cubic polycrystal with uniform texture and intergranular cavities, and to a porous material with homogenous isotropic matrix and identical initial porosity distribution. These simulations allow us to assess the effect of the matrix’s polycrystallinity on porosity evolution. Microstructural effects, such as the influence of the Taylor factor of the crystalline ligaments linking interacting voids, were predicted and qualitatively confirmed by post-shocked microstrostructural characterization of polycrystalline copper.     

Experimental and finite-element analysis of the anisotropic response of high-purity α-titanium in bending

In this paper, we present results of four-point bending tests performed on beams of high-purity α-titanium material. These tests have been performed at room temperature for different beam configurations and loading orientations with respect to the orthotropy axes of the material. Digital image correlation was used to determine local strains in the deformed beams. Experimental results compare very well with the predictions of finite-element simulations obtained using the elastic/plastic model developed by Nixon et al. (2010) [12]. Specifically, we compare local deformations and the cross-sections of each beam for all loading configurations. We show that the model predicts with great accuracy the tension–compression asymmetry and the evolving anisotropy of the material. The experimentally observed upward shift of the neutral axis, as well as the rigidity of the response along the hard to deform c-axes are very well described by the proposed model.     

Fast fourier transform-based modeling for the determination of micromechanical fields in polycrystals

Emerging characterization methods in experimental mechanics pose a challenge to modelers to devise efficient formulations that permit interpretation and exploitation of the massive amount of data generated by these novel methods. In this overview we report on a numerical formulation based on fast Fourier transforms, developed over the last 15 years, which can use the voxelized microstructural images of heterogeneous materials as input to predict their micromechanical and effective response. The focus of this presentation is on applications of the method to plastically-deforming polycrystalline materials.     

Interplay of martensitic phase transformation and plastic slip in polycrystals

We study the interplay between martensitic phase transformation and plastic slip in polycrystalline media. The work is motivated by the phenomenon of superelasticity – the ability of the material to recover strains beyond their apparent elastic limit – observed in shape-memory alloys. Often the recovery is not perfect with residual strain after a deformation and recovery cycle, and the stress–strain curve changes with cycling. We develop a mesoscale model at the single crystal level, and use it to study polycrystals. The model is able to reproduce various observations and provide important insight into the interplay. In particular, we show that transformation and plasticity can occur synergistically, with plasticity providing a mechanism for bridging across poorly oriented and thus non-transforming grains.     

Modeling viscoplastic behavior and heterogeneous intracrystalline deformation of columnar ice polycrystals

A full-field formulation based on fast Fourier transforms (FFT) has been adapted and used to predict the micromechanical fields that develop in two-dimensional columnar Ih ice polycrystals deforming in compression by dislocation creep. The predicted intragranular mechanical fields are in qualitative good agreement with experimental observations, in particular those involving the formation of shear and kink bands. These localized bands are associated with the large internal stresses that develop during creep in such anisotropic material, and their location, intensity, morphology and extension are found to depend strongly on the crystallographic orientation of the grains and on their interaction with neighboring crystals. The predictions of the model are also discussed in relation to the deformation of columnar sea and lake ice, as well as with the mechanical behavior of granular ice of glaciers and polar ice sheets.     

Study of internal lattice strain distributions in stainless steel using a full-field elasto-viscoplastic formulation based on fast Fourier transforms

In this work, the evolution of internal lattice strains in face-centered cubic stainless steel under uniaxial tension is studied using a recently developed full-field elasto-viscoplastic formulation based on fast Fourier transforms. The shape of the diffraction peaks is simulated, and the predicted lattice strains (peak shift and broadening) are compared with the experimental measurements obtained by in situ tensile neutron diffraction. Detailed analysis of the lattice strain distributions reveal that {1 0 0} and {1 1 0} transverse families exhibit a bimodal nature, and that transverse lattice strains are more sensitive to local grain interactions compared with longitudinal lattice strains. A comparison with the results of a mean-field formulation indicates that type III (intragranular) stresses play a much larger role than type II (intergranular) stresses in diffraction peak broadening.     

Parameter estimation in a thermoelastic composite problem via adjoint formulation and model reduction

Advances in nondestructive material characterization are providing a wealth of information that could be exploited to gain insight into general aspects of material performance and, in particular, discover relationships between microstructure and thermo‐mechanical properties in polycrystalline and other complex composite materials. In order to facilitate the integration of such measurements into existing models, as well as inform new physics‐based predictions, we developed a C++/MPI computational framework for sensitivity analysis and parameter estimation. The framework utilizes a micro‐mechanical modeling based on fast Fourier transforms, direct and adjoint formulations, and Markov chain Monte Carlo sampling techniques. We illustrate the characteristics of this framework and demonstrate its utility by computing the residual stresses arising from thermal expansion of an elastic composite and using data from simulated experiments. We show that the availability of nondestructive 3‐D measurements is crucial to reduce the uncertainty in predictions, emphasizing the importance of an integrated experimental/modeling/data analysis approach for improved material characterization and design. Copyright © 2017 John Wiley & Sons, Ltd.