The Experts below are selected from a list of 129255 Experts worldwide ranked by ideXlab platform
R C Picu - One of the best experts on this subject based on the ideXlab platform.
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Deformation and microstructure-independent Cottrell–Stokes ratio in commercial Al alloys
International Journal of Plasticity, 2011Co-Authors: R C Picu, Gabriela Vincze, José Joaquin De Almeida GracioAbstract:Abstract Cottrell–Stokes-type experiments are performed with AA6022, a heat treatable commercial Al alloy, at different stages of precipitation. It is shown that the ratio of the flow stress at given temperature and that extrapolated to 0 K, measured at given material state, is independent of the strain and of the precipitation state. The ratio depends only on temperature and strain rate. However, when probed using strain rate jump experiments, the Cottrell–Stokes law appears not to be fulfilled in any of these materials, and the strain rate Sensitivity Parameter depends on the precipitation state. A model based on the interaction of dislocations with populations of obstacles of various types is used to provide an interpretation of the Cottrell–Stokes law. The model indicates that as the dislocation velocity increases, the effective Cottrell–Stokes ratio in systems with various obstacle compositions takes values in a narrow range close to the critical value of 1 (i.e. “microstructure” inSensitivity). Conversely, the model suggests that the Cottrell–Stokes ratio should become more sensitive to the microstructure under creep conditions.
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strain rate Sensitivity of thermally activated dislocation motion across fields of obstacles of different kind
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2009Co-Authors: R C Picu, Renge Li, Zhijie XuAbstract:Abstract The thermally activated motion of dislocations across fields of randomly distributed obstacles of two types is studied. The two types have either the same strength and different dependence of the activation energy on the applied force, or different strength but same activation behavior. The objective is to determine how the two sub-populations of obstacles contribute to defining the strain rate Sensitivity and the flow stress. Above a threshold stress, dislocation motion undergoes a transition from smooth (“unzipping”) to jerky, i.e. obstacles are bypassed in a correlated manner at high stresses. In the jerky regime, the strain rate Sensitivity Parameter depends exclusively on the ratio of the applied stress to the mechanical threshold stress of the respective array, the dynamics exhibiting near-critical behavior. This regime appears to be essential for the deformation of real crystals. When obstacles are bypassed in the unzipping mode, the strain rate Sensitivity is controlled by the strong obstacles. These results have implications for the finite temperature superposition of contributions of the two types of obstacles to the overall flow stress.
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strain rate Sensitivity of the commercial aluminum alloy aa5182 o
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005Co-Authors: R C Picu, Gabriela Vincze, José Joaquin De Almeida Gracio, Fahrettin Ozturk, Frederic Barlat, Antoinette M ManiattyAbstract:The mechanical behavior of the commercial aluminum alloy AA5182-O is investigated at temperatures ranging from −120 to 150 ◦ C and strain rates from 10 −6 to 10 −1 s −1 . The strain rate Sensitivity Parameter is determined as a function of temperature and plastic strain, and the strain rate and temperature range in which dynamic strain aging leads to negative strain rate Sensitivity is mapped. The effect of dynamic strain aging on ductility and strain hardening is investigated. The Sensitivity of the measured quantities to the experimental method employed and their dependence on grain shape are discussed. The experimental data are compared with the predictions of a model constructed based on a recently proposed mechanism for dynamic strain ageing. The mechanism is based on the effect solute clustering at forest dislocations has on the strength of dislocation junctions. The model is shown to reproduce qualitatively the experimental trends. © 2004 Elsevier B.V. All rights reserved.
R Nikoukhah - One of the best experts on this subject based on the ideXlab platform.
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robust state space filtering under incremental model perturbations subject to a relative entropy tolerance
IEEE Transactions on Automatic Control, 2013Co-Authors: Bernard C Levy, R NikoukhahAbstract:This paper considers robust filtering for a nominal Gaussian state-space model, when a relative entropy tolerance is applied to each time increment of a dynamical model. The problem is formulated as a dynamic minimax game where the maximizer adopts a myopic strategy. This game is shown to admit a saddle point whose structure is characterized by applying and extending results presented earlier in “Robust least-squares estimation with a relative entropy constraint” (B. C. Levy and R. Nikoukhah, IEEE Trans. Inf. Theory, vol. 50, no. 1, 89-104, Jan. 2004) for static least-squares estimation. The resulting minimax filter takes the form of a risk-sensitive filter with a time varying risk Sensitivity Parameter, which depends on the tolerance bound applied to the model dynamics and observations at the corresponding time index. The least-favorable model is constructed and used to evaluate the performance of alternative filters. Simulations comparing the proposed risk-sensitive filter to a standard Kalman filter show a significant performance advantage when applied to the least-favorable model, and only a small performance loss for the nominal model.
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robust state space filtering under incremental model perturbations subject to a relative entropy tolerance
arXiv: Optimization and Control, 2010Co-Authors: Bernard C Levy, R NikoukhahAbstract:This paper considers robust filtering for a nominal Gaussian state-space model, when a relative entropy tolerance is applied to each time increment of a dynamical model. The problem is formulated as a dynamic minimax game where the maximizer adopts a myopic strategy. This game is shown to admit a saddle point whose structure is characterized by applying and extending results presented earlier in [1] for static least-squares estimation. The resulting minimax filter takes the form of a risk-sensitive filter with a time varying risk Sensitivity Parameter, which depends on the tolerance bound applied to the model dynamics and observations at the corresponding time index. The least-favorable model is constructed and used to evaluate the performance of alternative filters. Simulations comparing the proposed risk-sensitive filter to a standard Kalman filter show a significant performance advantage when applied to the least-favorable model, and only a small performance loss for the nominal model.
Valentin L Popov - One of the best experts on this subject based on the ideXlab platform.
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role of adhesion stress in controlling transition between plastic grinding and breakaway regimes of adhesive wear
Scientific Reports, 2020Co-Authors: A V Dimaki, Evgeny V Shilko, Ivan V Dudkin, S G Psakhie, Valentin L PopovAbstract:: A discrete-element based model of elastic-plastic materials with non-ideal plasticity and with an account of both cohesive and adhesive interactions inside the material is developed and verified. Based on this model, a detailed study of factors controlling the modes of adhesive wear is performed. Depending on the material and loading Parameters, we observed three main modes of wear: slipping, plastic grinding, cleavage, and breakaway. We find that occurrence of a particular mode is determined by the combination of two dimensionless material Parameters: (1) the ratio of the adhesive stress to the pure shear strength of the material, and (2) Sensitivity Parameter of material shear strength to local pressure. The case study map of asperity wear modes in the space of these Parameters has been constructed. Results of this study further develop the findings of the widely discussed studies by the groups of J.-F. Molinari and L. Pastewka.
Ferdinand Dobes - One of the best experts on this subject based on the ideXlab platform.
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creep behaviour of a magnesium alloy as21 and its fibre strengthened composite
Journal of Alloys and Compounds, 2004Co-Authors: K Milicka, Ferdinand DobesAbstract:Compressive creep tests of a magnesium alloy and its fibre composite (with 27 vol.% of Saffil fibres) were performed in temperature interval from 423 to 523 K in order to judge the strengthening influence of fibres and to analyse possibilities of an explanation of physical bases of the creep strain. Both materials were prepared by squeeze casting. From stress and temperature dependence of the minimum creep rates, activation Parameters of creep, i.e., the apparent activation energy and the stress Sensitivity Parameter of creep rate, were evaluated and compared with the purpose to suggest possible mechanisms of creep deformation. In both materials investigated, the stress Sensitivity Parameters and the activations energies are relatively high and cannot be directly attributed to any creep mechanism. No threshold behaviour was observed in the stress dependence of the creep rates in both materials. A dependence of the creep behaviour on the orientation of composite specimens is presented.
Antoinette M Maniatty - One of the best experts on this subject based on the ideXlab platform.
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strain rate Sensitivity of the commercial aluminum alloy aa5182 o
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005Co-Authors: R C Picu, Gabriela Vincze, José Joaquin De Almeida Gracio, Fahrettin Ozturk, Frederic Barlat, Antoinette M ManiattyAbstract:The mechanical behavior of the commercial aluminum alloy AA5182-O is investigated at temperatures ranging from −120 to 150 ◦ C and strain rates from 10 −6 to 10 −1 s −1 . The strain rate Sensitivity Parameter is determined as a function of temperature and plastic strain, and the strain rate and temperature range in which dynamic strain aging leads to negative strain rate Sensitivity is mapped. The effect of dynamic strain aging on ductility and strain hardening is investigated. The Sensitivity of the measured quantities to the experimental method employed and their dependence on grain shape are discussed. The experimental data are compared with the predictions of a model constructed based on a recently proposed mechanism for dynamic strain ageing. The mechanism is based on the effect solute clustering at forest dislocations has on the strength of dislocation junctions. The model is shown to reproduce qualitatively the experimental trends. © 2004 Elsevier B.V. All rights reserved.
