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A plasticity model for metal powder forming processes

INTERNATIONAL JOURNAL OF PLASTICITY 2001年第12期17卷 1659-1692页

作者： Lewis, RW Khoei, AR Univ Coll Swansea Dept Mech Engn Swansea SA2 8PP W Glam Wales Sharif Univ Technol Dept Civil Engn Tehran Iran

In this paper, a double-surface plasticity model, based on a combination of a convex yield surface consisting of a failure envelope, such as a Mohr-Coulomb yield surface and, a hardening cap model, is developed for the nonlinear behaviour of powder materials in the concept of a generalized plasticity formulation for the description of cyclic loading. This model reflects the yielding, frictional and densification characteristics of powder along with strain and geometrical hardening which occur during the compaction process. The solution yields details on the powder displacement from which it is possible to establish the stress state in the powder and the densification is derived from consideration of the elemental volumetric strain. A hardening rule is used to define the dependence of the yield surface on the degree of plastic straining. Finally, an adaptive finite element model (FEM) analysis is employed by the updated Lagrangian formulation to simulate the compaction of a set of complex powder forming processes. (C) 2001 Elsevier Science Ltd. All rights reserved.

关键词： powder compaction elastic-plastic material constitutive behaviour numerical algorithms finite elements

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THE FLEXIBLE GABOR-WAVELET TRANSFORM FOR CAR CRASH SIGNAL ANALYSIS

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INTERNATIONAL JOURNAL OF WAVELETS MULTIRESOLUTION AND INFORMATION PROCESSING 2009年第4期7卷 481-490页

作者： Onchis, Darian M. Suarez Sanchez, Esperanza M. Univ Vienna Inst Math Numer Harmon Anal Grp A-1090 Vienna Austria Univ Seville Dept Appl Math 1 E-41012 Seville Spain

This paper is concerned with the spectral decomposition and the adaptive analysis of data coming from car crash simulations. The mathematical ingredient of the proposed signal processing technique is the flexible Gabor-wavelet transform or the alpha-transform that reliably detects both high and low frequency components of such complicated short-time signals. We go from the functional treatment of this wavelet-type transform to its numerical implementation and we show how it can be used as an improved tool for spectral investigations compared to the short-time Fourier transform or the classical wavelet transform.

关键词： Adaptive time-frequency transform alpha-modulation spaces signal processing numerical algorithms short-time Fourier transform

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A hierarchical model for rate-dependent polycrystals

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INTERNATIONAL JOURNAL OF PLASTICITY 2009年第5期25卷 752-767页

作者： Mahesh, Sivasambu Indian Inst Technol Dept Mech Kanpur 208016 Uttar Pradesh India Indian Inst Technol Dept Aerosp Engn Kanpur 208016 Uttar Pradesh India

A hierarchical model of a polycrystalline aggregate of rigid viscoplastic grains is formulated, and a robust and efficient computational algorithm for its solution is proposed. The polycrystalline aggregate is modeled as a binary tree. The leaves of the binary tree represent grains, and higher tree nodes represent increasingly larger sub-aggregates of grains. The root of the tree represents the entire polycrystalline aggregate. Velocity and traction continuity are enforced across the interface between the children of each non-leaf node in the binary tree. The hierarchical model explicitly models intergranular interactions but is nevertheless comparable in computational effort to the mean field models of polycrystal plasticity. Simulations of tensile, compressive, torsional, and plane strain deformation of copper lead to predictions in good agreement with experiments, and highlight the interconnection between grain deformations and intergranular constraints. It is inferred from the results that a hybrid mean field/hierarchical model represents a computationally efficient methodology to simulate polycrystal deformation while accounting for intergranular interactions. (C) 2008 Elsevier Ltd. All rights reserved.

关键词： Crystal plasticity Hierarchical model Texture modeling numerical algorithms

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Direct nonlinear Fourier transform algorithms for the computation of solitonic spectra in focusing nonlinear Schrodinger equation

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COMMUNICATIONS IN NONLINEAR SCIENCE AND numerical SIMULATION 2019年 68卷 347-371页

作者： Vasylchenkova, A. Prilepsky, J. E. Shepelsky, D. Chattopadhyay, A. Aston Univ Aston Inst Photon Technol Birmingham B4 7ET W Midlands England Aston Univ Syst Analyt Res Inst Birmingham B4 7ET W Midlands England B Verkin Inst Low Temp Phys & Engn UA-61103 Kharkov Ukraine Kharkov Natl Univ UA-61022 Kharkov Ukraine Aston Univ Math Birmingham B4 7ET W Midlands England

Starting from a comparison of some established numerical algorithms for the computation of the eigenvalues (discrete or solitonic spectrum) of the non-Hermitian version of the Zakharov-Shabat spectral problem, this article delivers new algorithms that combine the best features of the existing ones and thereby allays their relative weaknesses. Our algorithm is modelled within the remit of the so-called direct nonlinear Fourier transform (NFT) associated with the focusing nonlinear Schrodinger equation. First, we present the data for the calibration of existing methods comparing the relative errors associated with the computation of the continuous NF spectrum. Then each method is paired with different numerical algorithms for finding zeros of a complex-valued function to obtain the eigenvalues. Next we describe a new class of methods based on the contour integrals evaluation for the efficient search of eigenvalues. After that we introduce a new hybrid method, one of our main results: the method combines the advances of contour integral approach and makes use of the iterative algorithms at its second stage for the refined eigenvalues search. The veracity of our new hybrid algorithm is established by estimating the convergence speed and accuracy across three independent test profiles. Along with the development of a new approach for the computation of the eigenvalues, our study also addresses the problem of computation of the so-called norming constants associated with the eigenvalues. We show that our formalism effectively amounts to accurate and fast enough computation of residues of the reflection coefficient in the upper complex half-plane of the spectral parameter. (C) 2018 Elsevier B.V. All rights reserved.

关键词： Nonlinear Schrodinger equation Inverse scattering method numerical algorithms Signal processing

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LOWER UPPER IMPLICIT TOTAL VARIATION DIMINISHING SOLUTION OF VISCOUS HYPERSONIC FLOWS

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AIAA JOURNAL 1992年第9期30卷 2184-2185页

作者： GRASSO, F MARINI, M University of Rome "La Sapienza" Rome Italy

In this paper an efficient implicit LU-TVD scheme is developed to solve the two-dimensional Navier-Stokes equations. The scheme uses a lower and upper approximate factorization of the implicit operator and has been applied to a viscous high-speed flow problem for validation purposes. Extensive testing indicates that the efficiency of a TVD method can be greatly increased without losing accuracy.

关键词： Total Variation Diminishing Navier Stokes Equations Hypersonic Flows Freestream Mach Number Pressure Coefficient Aspect Ratio Finite Volume Formulation Hypersonic Boundary Layer Skin Friction Coefficient numerical algorithms

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Trajectory Optimization for High-Dimensional Nonlinear Systems Under STL Specifications

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IEEE CONTROL SYSTEMS LETTERS 2021年第4期5卷 1429-1434页

作者： Kurtz, Vince Lin, Hai Univ Notre Dame Dept Elect Engn Notre Dame IN 46556 USA

Signal Temporal Logic (STL) has gained popularity in recent years as a specification language for cyber-physical systems, especially in robotics. Beyond being expressive and easy to understand, STL is appealing because the synthesis problem-generating a trajectory that satisfies a given specification-can be formulated as a trajectory optimization problem. Unfortunately, the associated cost function is nonsmooth and non-convex. As a result, existing synthesis methods scale poorly to high-dimensional nonlinear systems. In this letter, we present a new trajectory optimization approach for STL synthesis based on Differential Dynamic Programming (DDP). It is well known that DDP scales well to extremely high-dimensional nonlinear systems like robotic quadrupeds and humanoids: we show that these advantages can be harnessed for STL synthesis. We prove the soundness of our proposed approach, demonstrate order-of-magnitude speed improvements over the state-of-the-art on several benchmark problems, and demonstrate the scalability of our approach to the full nonlinear dynamics of a 7 degree-of-freedom robot arm.

关键词： Autonomous systems predictive control for nonlinear systems numerical algorithms

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CALCULATION OF ANNULAR NOZZLE TRISONIC FLOWFIELDS BY THE METHOD OF CHARACTERISTICS

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JOURNAL OF PROPULSION AND POWER 1988年第3期4卷 228-235页

作者： WANG, BN HOFFMAN, JD PURDUE UNIV CTR THERMAL SCI & PROPW LAFAYETTEIN 47907

A numerical algorithm based on the unsteady two-dimensional method of characteristics is presented for calculating axisymmetric inviscid trisonic (i.e., subsonic-transonic-supersonic) flowfields in annular nozzles. The algorithm is an explicit second-order accurate predictor-corrector procedure. An inverse marching method based on a local network of wavelines and the pathline is employed wherein the solution grid in physical space is prespecified. An orthogonal uniform transformed space, obtained by an algebraic grid generation method, is used for flow property interpolation in the initial-data surface. Steady flow solutions are obtained as the asymptotic solution in time. Results for four annular nozzles are presented. Comparison of the predicted and measured wall pressure distributions and nozzle efficiencies shows good agreement. © 1987 American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

关键词： Expansion Deflection Nozzle Two Dimensional Flow numerical algorithms numerical Integration Continuity Equation Mach Number Courant Friedrichs Lewy Flow Separation Computer Programs Isentropic Expansion

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OPTIMAL PLACEMENT OF ACTIVE PASSIVE MEMBERS IN TRUSS STRUCTURES USING SIMULATED ANNEALING

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AIAA JOURNAL 1991年第8期29卷 1327-1334页

作者： CHEN, GS BRUNO, RJ SALAMA, M Jet Propulsion Laboratory California Institute of Technology Pasadena California 91109

Active structural members with built-in sensing, feedback control, and actuation functions are used herein, along with passively damped members, to augment the inherent damping in truss structures. The effective use of such members makes it desirable to distribute them optimally throughout the structure. For simple structural systems, it is possible to place these members with some degree of optimality on the basis of engineering judgment. However, for more complex systems, the number of possible choices is so large that one may have to rely on a more formal optimization technique. This paper deals with the optimal placement of active and passive members in complex truss structures. The problem falls in the class of combinatorial optimization, for which the solution becomes exceedingly intractable as the problem size increases. This difficulty is overcome herein by use of the simulated annealing technique. We adopt the maximization of the cumulative energy dissipated over a finite time interval as the measure of optimality. The selection of nearly optimal locations for both passive and active members is consistently treated through the use of the finite-time energy dissipation criterion within the framework of the simulated annealing algorithm. numerical examples are used to illustrate the effectiveness of this methodology.

关键词： Annealing Temperature Trusses Structural Members Feedback Control Hankel Singular Value Energy Distribution numerical algorithms Quenching Process Piezoelectric Actuators Strain Energy Method

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ESTIMATION OF DELAYS AND OTHER PARAMETERS IN NON-LINEAR FUNCTIONAL-DIFFERENTIAL EQUATIONS

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SIAM JOURNAL ON CONTROL AND OPTIMIZATION 1983年第6期21卷 895-915页

作者： BANKS, HT LAMM, PKD SO METHODIST UNIV DEPT MATHDALLASTX 75275 NASA LANGLEY RES CTRINST COMP APPLICAT SCI & ENGNHAMPTONVA 23665

We discuss a spline-based approximation scheme for nonlinear nonautonomous delay differential equations. Convergence results (using dissipative type estimates on the underlying nonlinear operators) are given in the context of parameter estimation problems which include estimation of multiple delays and initial data as well as the usual coefficient-type parameters. A brief summary of some of our related numerical findings is also given.

关键词： delay equations parameter estimation splines numerical algorithms

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DAMAGE EVOLUTION MODELING IN ORTHOTROPIC LAMINATED COMPOSITES

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AIAA JOURNAL 1994年第2期32卷 357-364页

作者： DZENIS, YA JOSHI, SP BOGDANOVICH, AE LATVIAN ACAD SCI CTR ENGN & TECHNOLRIGALATVIA

Composites have inherent scatter in elastic and strength properties. A probabilistic model utilizing random material characteristics to predict damage evolution in orthotropic laminated composites is presented in this paper. The proposed model is based on the division of laminated composites into a statistically large number of mesoelements (mesovolumes). The mesovolume is assumed to be large enough to be structurally homogeneous, and at the same time it has to be comparatively small to satisfy the condition of stochastic homogeneity of stress and strain fields. Three modes of mesovolume failure, i.e., fibre breakage and matrix failure in the transverse direction as well as matrix shear cracking, are taken into account. Damage formation in a ply and in a laminate as a whole for a given plane stress state is calculated from the probabilities of mesovolume failure. These probabilities are directly utilized in reducing ply material constants. A numerical algorithm for damage accumulation and deformation history predictions for orthotropic laminated composites are developed. The behavior of a laminated orthotropic composite is presented as an illustrative example. Analysis of angle-ply Kevlar/epoxy laminates subjected to tension, compression, and shear loading is performed. The effect of scatters in elastic and strength characteristics on damage evolution is shown.

关键词： Fiber Reinforced Composites Elastic Properties Plane Stress numerical algorithms Material Properties Cumulative Damage Quasistatic Loading Statistical Distributions Monte Carlo Simulation Quasistatic Process

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