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Modelling recrystallization textures driven by intragranular fluctuations implemented in the viscoplastic self-consistent formulation

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ACTA MATERIALIA 2019年 164卷 530-546页

作者： Zecevic, Miroslav Lebensohn, Ricardo A. McCabe, Rodney J. Knezevic, Marko Univ New Hampshire Dept Mech Engn 33 Acad WayKingsbury HallW119 Durham NH 03824 USA Los Alamos Natl Lab Theoret Div Los Alamos NM 87544 USA Los Alamos Natl Lab Mat Sci & Technol Div Los Alamos NM 87544 USA

This paper presents a recrystallization model driven by intragranular orientation gradients and strain energy fields calculated by means of the viscoplastic self-consistent (VPSC) formulation. The VPSC model is extended for calculation of the coupling between intragranular stress fluctuations with corresponding second moments of lattice spin and misorientation fields in the grains. Access to these quantities allows modelling of transition bands and nucleation kinetics. In the proposed recrystallization model, grain growth is assumed to be proportional to the difference between the stored energy of each grain and that of the effective medium. Recrystallization textures for several cubic metals are simulated, showing good agreement with corresponding experiments. The model reveals the importance of considering appropriate, microstructurally-based and orientation-dependent recrystallization nucleation mechanisms. The recrystallization texture of heavily rolled copper with a strong cube texture component is found to be a consequence of nucleation at transition bands, which is also the cause of the recrystallization textures in compressed iron and drawn copper wire. In contrast, the recrystallization texture of rolled interstitial free steel is found to be caused by grain boundary nucleation occurring in grains with the highest strain energy. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

关键词： Microstructures Texture Crystal plasticity Recrystallization numerical algorithms

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On the Mixed-Integer Linear-Quadratic Optimal Control With Switching Cost

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IEEE CONTROL SYSTEMS LETTERS 2019年第4期3卷 990-995页

作者： De Marchi, Alberto Bundeswehr Univ Munich Dept Aerosp Engn D-85577 Neubiberg Germany

This letter concerns the optimal control of a continuous-time dynamical system via continuous and discrete-valued control variables, where the objective functional also accounts for state-independent switching costs. The class of mixed-integer optimal control problems (OCPs) is interpreted as a bilevel problem, involving both switching times optimization, for a given sequence of modes, and purely continuous optimal control. Additionally, an original nonconvex formulation for the switching costs is introduced, in terms of cardinality, inspired by sparse optimization and compressed sensing techniques. We then adopt proximal algorithms to solve the resulting bilevel OCP with composite objective function. An efficient routine for evaluating the proximal operator is developed. Two examples are numerically solved via a proximal gradient method, discussed and compared with the literature. Although this letter focuses on switched linear time-varying dynamics and quadratic cost functionals with a specific formulation of the switching costs, the proposed approach may also apply to more general mixed-integer OCPs.

关键词： Optimal control numerical algorithms hybrid systems

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Experimental and theoretical study of energy transfer in a chromophore triad: What makes modeling dynamics successful?

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JOURNAL OF CHEMICAL PHYSICS 2020年第24期153卷 244114页

作者： Freixas, Victor M. Nelson, Tammie Ondarse-Alvarez, Dianelys Nijjar, Parmeet Mikhailovsky, Alexander Zhou, Cheng Fernandez-Alberti, Sebastian Bazan, Guillermo C. Tretiak, Sergei Univ Nacl Quilmes CONICET Roque Saenz Pena 352B1876BXD Bernal Argentina Los Alamos Natl Lab Theoret Div Phys & Chem Mat Los Alamos NM 87545 USA Univ Southern Calif Dept Chem Los Angeles CA 90089 USA Univ Calif Santa Barbara Dept Chem & Biochem Ctr Polymers & Organ Solids Santa Barbara CA 93106 USA Natl Univ Singapore Dept Chem Singapore 117543 Singapore Los Alamos Natl Lab Ctr Integrated Nanotechnol Los Alamos NM 87545 USA

Simulation of electronic dynamics in realistically large molecular systems is a demanding task that has not yet achieved the same level of quantitative prediction already realized for its static counterpart. This is particularly true for processes occurring beyond the Born-Oppenheimer regime. Non-adiabatic molecular dynamics (NAMD) simulations suffer from two convoluted sources of error: numerical algorithms for dynamics and electronic structure calculations. While the former has gained increasing attention, particularly addressing the validity of ad hoc methodologies, the effect of the latter remains relatively unexplored. Indeed, the required accuracy for electronic structure calculations to reach quantitative agreement with experiment in dynamics may be even more strict than that required for static simulations. Here, we address this issue by modeling the electronic energy transfer in a donor-acceptor-donor (D-A-D) molecular light harvesting system using fewest switches surface hopping NAMD simulations. In the studied system, time-resolved experimental measurements deliver complete information on spectra and energy transfer rates. Subsequent modeling shows that the calculated electronic transition energies are "sufficiently good" to reproduce experimental spectra but produce over an order of magnitude error in simulated dynamical rates. We further perform simulations using artificially shifted energy gaps to investigate the complex relationship between transition energies and modeled dynamics to understand factors affecting non-radiative relaxation and energy transfer rates.

关键词： Excitons: Energetics and Spatio-temporal Dynamics Topics Electronic structure methods Absorption spectroscopy Tully's fewest switches surface hopping algorithm Photoluminescence spectroscopy Transient-absorption spectroscopy Non-adiabatic molecular dynamics numerical algorithms Fluorescence resonance energy transfer Localized states

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A comparative study between elasto-plastic self-consistent crystal plasticity and anisotropic yield function with distortional hardening formulations for sheet metal forming

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MECHANICS OF MATERIALS 2020年 148卷 103422-103422页

作者： Feng, Zhangxi Yoon, Seong-Yong Choi, Jae-Hyun Barrett, Timothy J. Zecevic, Milovan Barlat, Frederic Knezevic, Marko Univ New Hampshire Dept Mech Engn 33 Acad WayKingsbury HallW119 Durham NH 03824 USA Pohang Univ Sci & Technol Grad Inst Ferrous Technol 77 Cheongam Ro Pohang 37673 Gyeongbuk South Korea

A comparative study between micro- and macro-mechanical constitutive models is carried out while predicting deformation behavior of an aluminum alloy (AA) 6022-T4 during several loading scenarios of increasing complexity including monotonic tension, large strain cyclic deformation, and drawing of a cylindrical cup. The micro-model is a recently developed implicit formulation of the elasto-plastic self-consistent (EPSC) crystal plasticity, which is coupled with the implicit finite element method (FEM) through a user material subroutine in Abaqus. In the coupled formulation, every finite element integration point embeds the implicit EPSC constitutive law that accounts for the directionality of deformation mechanisms and microstructural evolution. The crystallography based EPSC model integrates a dislocation-based hardening law and accounts for inter-granular and slip system level back-stresses, which make it capable of capturing non-linear unloading and the Bauschinger effect. The macro-model is a recently developed anisotropic yield function incorporating distortional hardening using the homogeneous anisotropic hardening (HAH) approach. The model is also implemented as a user material subroutine in Abaqus. Parameters pertaining to the micro and macro models are identified using experimental data from a set of monotonic and cyclic tests performed for AA6022-T4. Additional experimental data for the alloy in terms of flow stress curves, R-value, and anisotropic yield surface evolution are used to verify the models. Finally, the cup drawing simulations are carried out in the FEM using the two constitutive formulations and geometrical changes including the earing profile and sheet thinning/thickening are compared against each other and with experiments to further verify the predictive characteristics of the models. The two formulations and results are discussed in terms of accuracy and computational efficiency.

关键词： Polycrystalline material numerical algorithms Finite elements Deep drawing AA6022-T4

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An optimal planning for a laundromat system with a multiple linen type in a hospital supply chain under subcontractor consideration

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IFAC-PapersOnLine 2019年第13期52卷 2213-2218页

作者： Ndhaief, Nadia Hajej, Zied Rezg, Nidhal LGIPM Université de Lorraine Metz France

This paper addresses the laundry management problem of multiple type of linen in a hospital supply chain using subcontractor strategy. In order to satisfy several random demands corresponding to every laundry type under a given service level, the principal washing provider can make a call another laundromat subcontractor. We aim to establish the usage economic plan of the principal and subcontractor laundromats for multiple laundry type. This is done by analyzing the influence of the service delay (transport, preparation, etc.) of each product line returned. The main goal is to ensure satisfaction of each type of cleaned laundry demand and minimize purchases of new products. Finally, we use a numerical algorithm for optimization and illustration purposes. © 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.

关键词： Supply chain management Cleaning Contractors Flax Hospitals Laundries Linen Optimization Purchasing Supply chains Economic plan Management problems Multiple products numerical algorithms Optimal planning Product lines Service delays Service levels

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On the Convergence of the Inexact Running Krasnosel'skii-Mann Method

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IEEE CONTROL SYSTEMS LETTERS 2019年第3期3卷 613-618页

作者： Dall'Anese, Emiliano Simonetto, Andrea Bernstein, Andrey Univ Colorado Dept Elect Comp & Energy Engn Boulder CO 80309 USA IBM Res Ireland Optimizat & Control Grp Dublin D15 HN66 15 Ireland Natl Renewable Energy Lab Golden CO 80401 USA

This letter leverages a framework based on averaged operators to tackle the problem of tracking fixed points associated with maps that evolve over time. In particular, this letter considers the Krasnosel'skii-Mann (KM) method in a settings where: 1) the underlying map may change at each step of the algorithm, thus leading to a "running" implementation of the KM method, and 2) an imperfect information of the map may be available. An imperfect knowledge of the maps can capture cases where processors feature a finite precision or quantization errors, or the case where (part of) the map is obtained from measurements. The analytical results are applicable to inexact running algorithms for solving optimization problems, whenever the algorithmic steps can be written in the form of (a composition of) averaged operators;examples are provided for inexact running gradient methods and the forward-backward splitting method. Convergence of the average fixed-point residual is investigated for the non-expansive case;linear convergence to a unique fixed-point trajectory is showen in the case of inexact running algorithms emerging from contractive operators.

关键词： numerical algorithms optimization

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Novel algorithms for analyzing the robustness of difference coarrays to sensor failures

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SIGNAL PROCESSING 2020年 171卷 107517-107517页

作者： Liu, Chun-Lin Vaidyanathan, P. P. Natl Taiwan Univ Dept Elect Engn Taipei 10617 Taiwan Natl Taiwan Univ Grad Inst Commun Engn Taipei 10617 Taiwan CALTECH Dept Elect Engn MC 136-93 Pasadena CA 91125 USA

Sparse arrays have drawn attention because they can identify O(N-2) uncorrelated source directions using N physical sensors, whereas uniform linear arrays (ULA) find at most N - 1 sources. The main reason is that the difference coarray, defined as the set of differences between sensor locations, has size of O(N-2) for some sparse arrays. However, the performance of sparse arrays may degrade significantly under sensor failures. In the literature, the k-essentialness property characterizes the patterns of k sensor failures that change the difference coarray. Based on this concept, the k-essential family, the k-fragility, and the k-essential Sperner family provide insights into the robustness of arrays. This paper proposes novel algorithms for computing these attributes. The first algorithm computes the k-essential Sperner family without enumerating all possible k-essential subarrays. With this information, the second algorithm finds the k-essential family first and the k-fragility next. These algorithms are applicable to any 1-D array. However, for robust array design, fast computation for the k-fragility is preferred. For this reason, a simple expression associated with the k-essential Sperner family is proposed to be a tighter lower bound for the k-fragility than the previous result. numerical examples validate the proposed algorithms and the presented lower bound. (C) 2020 Elsevier B.V. All rights reserved.

关键词： Sparse arrays Difference coarrays Essentialness Fragility numerical algorithms

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Optimal dimensionality reduction of Markov chains using graph transformation

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JOURNAL OF CHEMICAL PHYSICS 2020年第24期153卷 244108页

作者： Kannan, Deepti Sharpe, Daniel J. Swinburne, Thomas D. Wales, David J. Univ Cambridge Dept Chem Lensfield Rd Cambridge CB2 1EW England Aix Marseille Univ CNRS CINaM UMR 7325 Campus Luminy F-13288 Marseille France

Markov chains can accurately model the state-to-state dynamics of a wide range of complex systems, but the underlying transition matrix is ill-conditioned when the dynamics feature a separation of timescales. Graph transformation (GT) provides a numerically stable method to compute exact mean first passage times (MFPTs) between states, which are the usual dynamical observables in continuous-time Markov chains (CTMCs). Here, we generalize the GT algorithm to discrete-time Markov chains (DTMCs), which are commonly estimated from simulation data, for example, in the Markov state model approach. We then consider the dimensionality reduction of CTMCs and DTMCs, which aids model interpretation and facilitates more expensive computations, including sampling of pathways. We perform a detailed numerical analysis of existing methods to compute the optimal reduced CTMC, given a partitioning of the network into metastable communities (macrostates) of nodes (microstates). We show that approaches based on linear algebra encounter numerical problems that arise from the requisite metastability. We propose an alternative approach using GT to compute the matrix of intermicrostate MFPTs in the original Markov chain, from which a matrix of weighted intermacrostate MFPTs can be obtained. We also propose an approximation to the weighted-MFPT matrix in the strongly metastable limit. Inversion of the weighted-MFPT matrix, which is better conditioned than the matrices that must be inverted in alternative dimensionality reduction schemes, then yields the optimal reduced Markov chain. The superior numerical stability of the GT approach therefore enables us to realize optimal Markovian coarse-graining of systems with rare event dynamics.

关键词： Renormalization and regularization Markov processes Probability theory algorithms and data structure numerical algorithms Programming languages Computer simulation Stochastic processes Monte Carlo methods Statistical analysis

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An enhanced distortional-hardening-based constitutive model for hexagonal close-packed metals: Application to AZ31B magnesium alloy sheets at elevated temperatures

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INTERNATIONAL JOURNAL OF PLASTICITY 2020年 126卷 102618-000页

作者： Lee, Jinwoo Bong, Hyuk Jong Kim, Se-Jong Lee, Myoug-Gyu Kim, Daeyong Korea Inst Mat Sci Mat Deformat Dept Chang Won 51508 South Korea Korea Inst Mat Sci Virtual Mat Proc Dept Chang Won 51508 South Korea Seoul Natl Univ Dept Mat Sci & Engn Seoul 08826 South Korea Seoul Natl Univ RIAM Seoul 08826 South Korea

In this study, a new material model for hexagonal close-packed (HCP) metals for predicting their flow responses under monotonic/cyclic loadings at various temperatures is proposed. The temperature-dependent constitutive model is re-formulated using a continuum-based distortional hardening law, which affects the shape of the yield surface depending on the loading direction, and also uses the concept of the dominant deformation modes in magnesium alloys: twin, untwin, and slip-dominate modes, considering the role of the twins in plastic deformation. In terms of the material asymmetry in yield strength, the Cazacu-Barlat-Plunkett (CPB) '06 yield criterion is extended to include thermal effects. A numerical formulation of the material model has also been developed to allow its implementation into finite element analysis software via a user-defined material subroutine (UMAT). The predicted results of stress-strain response for monotonic and cyclic loadings are excellent agreement with the experimental data. Moreover, the simulated results show the strength differential (SD) effect and unusual flow responses of AZ31B magnesium alloy sheets at room temperature, as well as the variation in the SD effect and anisotropic hardening behavior at elevated temperatures.

关键词： Twinning Yield condition Constitutive behavior Finite elements numerical algorithms

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Enhanced directed search: a continuation method for mixed-integer multi-objective optimization problems

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ANNALS OF OPERATIONS RESEARCH 2019年第1-2期279卷 343-365页

作者： Wang, Honggang Laredo, David Cuate, Oliver Schutze, Oliver Univ La Verne Dept Appl Business Sci & Econ Coll Business & Publ Management La Verne CA 91750 USA IPN CINVESTAV Dept Comp Sci Mexico City DF Mexico

Multi-objective optimization problems (MOPs) commonly arise in various applications of engineering and management fields. Many real-world MOPs are mixed-integer multiobjective optimization problems (MMOP), where the solution space consists of real and integer decision variables. The research regarding MMOPs is still scarce due to the mixture nature of the solution space and difficulty of finding the set of trade-off solutions. In this work we propose a continuation based method that efficiently solves MMOP problems. Our method, called Enhanced Directed Search (EDS), is capable of steering the search along a predefined direction along the Pareto front in the objective function space. EDS traces the Pareto front by following closest predictor and corrector solutions in the course of optimization. By searching around the objective function boundary, EDS can solve problems with k > 2 objectives. With five example problems widely studied in the literature, we demonstrate that EDS outperforms the recently developed Direct Zig Zag algorithm and the popular NSGA-II method.

关键词： Multiple criteria decision Multi-objective optimization Pareto optimum Continuation based heuristics numerical algorithms

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