DSpace Collection:
https://hdl.handle.net/2440/306
2024-03-18T16:03:32Z
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Refined MDP-based branch-and-fix algorithm for the Hamiltonian cycle problem
https://hdl.handle.net/2440/86243
Title: Refined MDP-based branch-and-fix algorithm for the Hamiltonian cycle problem
Author: Ejov, V.; Filar, J.; Haythorpe, M.; Nguyen, G.
Abstract: We consider the famous Hamiltonian cycle problem (HCP) embedded in a Markov decision process (MDP). More specifically, we consider the HCP as an optimisation problem over the space of occupation measures induced by the MDP's stationary policies. In recent years, this approach to the HCP has led to a number of alternative formulations and algorithmic approaches. In this paper, we focus on a specific embedding, because of the work of Feinberg. We present a “branch-and-fix” type algorithm that solves the HCP. At each branch of the algorithm, only a linear program needs to be solved and the dimensions of the successive linear programs are shrinking rather than expanding. Because the nodes of the branch-and-fix tree correspond to specially structured 1-randomised policies, we characterise the latter. This characterisation indicates that the total number of such policies is significantly smaller than the subset of all 1-randomised policies. Finally, we present some numerical results.
2009-01-01T00:00:00Z
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A coupled damage-plasticity model for concrete based on thermodynamic principles: Part II: non-local regularization and numerical implementation
https://hdl.handle.net/2440/85371
Title: A coupled damage-plasticity model for concrete based on thermodynamic principles: Part II: non-local regularization and numerical implementation
Author: Nguyen, G.; Houlsby, G.
Abstract: <jats:title>Abstract</jats:title><jats:p>Non‐local regularization is applied to a new coupled damage–plasticity model (<jats:italic>Int. J. Numer. Anal. Meth. Geomech.</jats:italic> 2007; DOI: 10.1002/nag.627), turning it into a non‐local model. This procedure resolves softening‐related problems encountered in local constitutive models when dealing with softening materials. The parameter identification of the new non‐local coupled damage–plasticity model is addressed, with all parameters being shown to be obtainable from the experimental data on concrete. Because of the appearance of non‐local spatial integrals in the constitutive equations, a new implementation scheme is developed for the integration of the non‐local incremental constitutive equations in nonlinear finite element analysis. The performance of the non‐local model is assessed against a range of two‐dimensional structural tests on concrete, illustrating the stability of the stress update procedure and the lack of mesh dependency of the model. Copyright © 2007 John Wiley & Sons, Ltd.</jats:p>
2008-01-01T00:00:00Z
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A coupled damage-plasticity model for concrete based on thermodynamic principles: Part 1: model formulation and parameter identification
https://hdl.handle.net/2440/85370
Title: A coupled damage-plasticity model for concrete based on thermodynamic principles: Part 1: model formulation and parameter identification
Author: Nguyen, G.; Houlsby, G.
Abstract: <jats:title>Abstract</jats:title><jats:p>The development of a coupled damage‐plasticity constitutive model for concrete is presented. Emphasis is put on thermodynamic admissibility, rigour and consistency both in the formulation of the model, and in the identification of model parameters based on experimental tests. The key feature of the thermodynamic framework used in this study is that all behaviour of the model can be derived from two specified energy potentials, following
procedures established beforehand. Based on this framework, a constitutive model featuring full coupling between damage and plasticity in both tension and compression is developed. Tensile and compressive responses of the material are captured using two separate damage criteria, and a yield criterion with a multiple hardening rule. A crucial part of this study is the identification of model parameters, with these all being shown to be identifiable and computable based on standard tests on concrete. Behaviour of the model is assessed against experimental data on concrete. Copyright © 2007 John Wiley & Sons, Ltd.</jats:p>
2008-01-01T00:00:00Z
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Development of an approach to constitutive modelling of concrete: isotropic damage coupled with plasticity
https://hdl.handle.net/2440/85359
Title: Development of an approach to constitutive modelling of concrete: isotropic damage coupled with plasticity
Author: Nguyen, G.; Korsunsky, A.
Abstract: The paper presents an approach to constitutive modelling of concrete using damage mechanics and plasticity theory. The thermodynamic formulation, and parameter identification of a non-local coupled damage-plasticity model are presented in this study. The particular focus is the calibration of model parameters. It is shown that both the local parameters and the parameters governing the non-local interaction can be determined from experimental data reliably and consistently. A novel procedure is developed for parameter identification, using the separation of total dissipation energy into additive parts corresponding to different dissipation mechanisms. The relationship between the local and non-local parameters is also addressed, helping to obtain model responses consistent with the fracture energy of the material. The application of the model and the calibration procedure proposed in this study to the numerical failure analysis of concrete structures is illustrated through a series of real structural tests, showing both the performance of the model and the consistency of the proposed calibration procedure. © 2008 Elsevier Ltd. All rights reserved.
2008-01-01T00:00:00Z