arXiv subjects

Ioan R. Ionescu

Publications and source records attributed to Ioan R. Ionescu.

7 recordsLinked to original sources

arXiv2025

Prediction of ideal orientations in velocity gradient-driven processes for large plastic deformations of crystals

We focus on the crystal lattice ideal orientations, also referred to as preferred or attractor orientations, in crystalline materials, and how they can be used to predict the final texture of polycrystals after manufacturing processes. The simplified crystal plasticity (CP) models used here capture the main features of microstructural evolution in monocrystalline and polycrystalline materials undergoing velocity-gradient-driven processes, without considering hardening or softening effects. The evolution of the lattice orientation is described by a nonlinear ordinary differential equation, and a linear stability analysis is performed to identify the permanent orientations that act as attractors (i.e., the ideal or preferred orientations). Although our linear stability analysis is generally applicable, it is detailed using a simplified two-dimensional model with three slip systems. This approach successfully predicts lattice orientation attractors for large strains by analyzing the interplay between deformation and rotation, initial orientation, and the interaction between different slip systems under applied loads. Three fundamental problems in CP illustrate the effectiveness of the theory: polycrystal deformation under homogeneous velocity gradient loading, void evolution under radial loading, and slip band formation in a monocrystal. High-resolution CP numerical simulations, enhanced using re-meshing techniques, provide further validation of our findings concerning the impact of initial crystallographic orientations, deformation mechanisms, and loading conditions on the evolution of orientation attractors and the final crystal texture.

cond-mat.mtrl-sci↗

arXiv2025

Dislocation saturation in slip rate driven processes and initial microstructure effects for large plastic deformation of crystals

Dislocation-density-based crystal plasticity (CP) models are introduced to account for the microstructural changes throughout the deformation process, enabling more quantitative predictions of the deformation process compared to slip-system resistance-based plasticity models. In this work, we present a stability analysis of slip-rate-driven processes for some established dislocation density-based models, including the Kocks and Mecking (KM) model and its variants. Our analysis can be generalized to any type of dislocation density model, providing a broader framework for understanding the stability of such systems. We point out the existence of saturation dislocation densities and the essential role of initial dislocation density in distinguishing between hardening and softening responses. Since the initial microstructure, modeled through the dislocation density, could be related to the size or the sample preparation process, implicit size-dependent effects can also be inferred. To further explore these phenomena, we conduct numerical simulations of pillar compression using an Eulerian crystal plasticity framework. Our results show that dislocation-density-based CP models effectively capture microstructural evolution in small-scale materials, offering critical insights for the design of miniaturized mechanical devices and advanced materials in nanotechnology.

cond-mat.mtrl-sci↗

arXiv2025

Large plastic deformation of voids in crystals

The mechanisms of void growth and coalescence are key contributors to the ductile failure of crystalline materials. At the grain scale, single crystal plastic anisotropy induces large strain localization leading to complex shape evolutions. In this study, an Arbitrary Lagrangian-Eulerian (ALE) framework for 2D crystal plasticity combined with dynamic remeshing is used to study the 2D shape evolution of cylindrical voids in single crystals. The large deformation and shape evolution of the voids under two types of loading are considered: (i) radial and (ii) uni-axial loadings. In both cases, the voids undergo complex shape evolutions induced by the interactions between slip bands, lattice rotations and large strain phenomena. In case (i), the onset of the deformation revealed the formation of a complex fractal network of slip bands around the voids. Then, large deformations unearth an unexpected evolution of the slip bands network associated with significant lattice rotations, leading to a final hexagonal shape for the void. In case (ii), we obtain shear bands with very large accumulated plastic strain (> 200%) compared to the macroscopic engineering strains (< 15%). A high dependence between crystalline orientations, slip band localization and therefore shape evolution was observed, concluding in a high dependency between crystalline orientation and void shape elongation, which is of prime importance regarding coalescence of the voids, thus to the formation of macro-cracks.

cond-mat.mtrl-sci↗

arXiv2024

Accounting for localized deformation: a simple computation of true stress in micropillar compression experiments

Compression experiments are widely used to study the mechanical properties of materials at micro- and nanoscale. However, the conventional engineering stress measurement method used in these experiments neglects to account for the alterations in the material's shape during loading. This can lead to inaccurate stress values and potentially misleading conclusions about the material's mechanical behavior especially in the case of localized deformation. To address this issue, we present a method for calculating true stress in cases of localized plastic deformation commonly encountered in experimental settings: (i) a single band and (ii) two bands oriented in arbitrary directions with respect to the vertical axis of the pillar (either in the same or opposite directions). Our simple analytic formulas can be applied to homogeneous and isotropic materials and crystals, requiring only standard data (displacement-force curve, aspect ratio, shear band angle and elastic strain limit) obtained from experimental results and eliminating the need for finite element computations. Our approach provides a more precise interpretation of experimental results and can serve as a valuable and simple tool in material design and characterization.

cond-mat.mtrl-sci↗

arXiv2022

Design of pre-stressed plate-strips to cover non-developable shells

In this paper we address the following design problem: what is the shape of a plate and the associated pre-stress that relaxes toward a given three-dimensional shell? As isometric transformations conserve the gaussian curvature, three-dimensional non-developable shells cannot be obtained from the relaxation of pre-strained plates by using isometric transformations only. Overcoming this geometric restriction, including small-strains and large rotations, solves the problem for small areas only. This paper dispenses with the small-area restriction to cover three-dimensional shells fully by using shell-strips. Since shell-strips have an additional geometric parameter, we show that under suitable assumptions that relate the width of the strip to the curvature of the shell, we are able to design arbitrary shell surfaces by covering them with shell-strips. As an illustration, we provide optimized covers of the sphere in a variety of different surface-strips relaxed from plate-strips with homogeneous and isotropic pre-stress. Moveover, we propose the design of the torus, of the helicoid and of the non-developable Möbius band, which requires inhomogeneous and anisotropic pre-stress.

physics.class-ph↗

arXiv2021

Covering a surface with pre-stressed ribbons : from theory to nano-structures fabrication

The paper deals with the fabrication of nano-shells from pre-stressed nano-plates release. Due to geometrical and technological restrictions we have to cover a given surface with three-dimensional thin ribbons. We discuss the key role of the geodesic curvature in the design of such shell-ribbons. We show that including small-strains but large rotations we are able to control the metric tensor of both Lagrangian and Eulerian ribbons by an appropriate choice of the width and thickness of the ribbons. Moreover, the Green-Lagrange strain tensor is controlled by the distance between the curvature of the planar ribbon and the geodesic curvature of the supporting curve of the shell-ribbon. Under suitable constitutive assumptions, we deduce the field equations, the boundary conditions and the design equations. The former relate the pre-stress in the planar layer to the final geometry of the desired shell-ribbon. A fine tuning of the composition, geometry and of the pre-stress of the plate-ribon is necessary to design and fabricate the shell-ribbon. We design and fabricate a partial cover of the sphere with constant latitude ribbons starting from planar multi-layer semiconductor materials grown by molecular beam epitaxy. The details of fabrication method and its limitations are discussed.

cond-mat.mtrl-sci↗

arXiv2020

Tempering the mechanical response of FCC micro-pillars: an Eulerian plasticity approach

The mechanical response of almost pure single-crystal micro-pillars under compression exhibits a highly localized behavior that can endanger the structural stability of a sample. Recent experiments revealed that the mechanical response of a crystal is very sensitive to both the presence of a quenched disorder in the sample and the orientation of the crystal. In this work, we study the influence of disorder and crystal orientation on the large strain response of a 2D FCC crystal with three active glide planes using a very simple Eulerian plasticity model in the FE framework. Our numerical and theoretical results on clean crystal pillars suggest that a single plane or many gliding planes can be activated depending on the crystal orientation. While in the former case, the deformation is localized, leading to ductile rupture, in the latter, a complex interplay between active planes takes place, resulting in a more uniform deformation. The strain-localization can be avoided when inhomogeneities are engineered inside the crystal, or the crystal orientation is altered because of the activation of multiple slip systems, resulting in a "patchwork" of the distribution of the slip systems.

cond-mat.mtrl-sci↗