The Experts below are selected from a list of 255 Experts worldwide ranked by ideXlab platform
J E Hirsch - One of the best experts on this subject based on the ideXlab platform.
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Thermodynamic inconsistency of the Conventional Theory of superconductivity
International Journal of Modern Physics B, 2020Co-Authors: J E HirschAbstract:A type I superconductor expels a magnetic field from its interior to a surface layer of thickness λL, the London penetration depth. λL is a function of temperature, becoming smaller as the temperat...
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Inconsistency of the Conventional Theory of superconductivity
EPL (Europhysics Letters), 2020Co-Authors: J E HirschAbstract:In a process where the temperature of a type I superconductor in a magnetic field changes, the Conventional Theory of superconductivity predicts that Joule heat is generated and that the final state is independent of the speed of the process. I show that these two predictions cannot be simultaneously reconciled with the laws of thermodynamics. I propose a resolution of this paradox.
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Thermodynamic inconsistency of the Conventional Theory of superconductivity
arXiv: Superconductivity, 2019Co-Authors: J E HirschAbstract:A type I superconductor expels a magnetic field from its interior to a surface layer of thickness $\lambda_L$, the London penetration depth. $\lambda_L$ is a function of temperature, becoming smaller as the temperature decreases. Here we analyze the process of cooling (or heating) a type I superconductor in a magnetic field, with the system remaining always in the superconducting state. The Conventional Theory predicts that Joule heat is generated in this process, the amount of which depends on the rate at which the temperature changes. Assuming the final state of the superconductor is independent of history, as the Conventional Theory assumes, we show that this process violates the first and second laws of thermodynamics. We conclude that the Conventional Theory of superconductivity is internally inconsistent. Instead, we suggest that the alternative Theory of hole superconductivity may be able to resolve this problem.
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Entropy generation and momentum transfer in the superconductor-normal and normal-superconductor phase transformations and the consistency of the Conventional Theory of superconductivity
International Journal of Modern Physics B, 2018Co-Authors: J E HirschAbstract:Since the discovery of the Meissner effect, the superconductor to normal (S–N) phase transition in the presence of a magnetic field is understood to be a first-order phase transformation that is re...
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entropy generation and momentum transfer in the superconductor normal and normal superconductor phase transformations and the consistency of the Conventional Theory of superconductivity
arXiv: Superconductivity, 2017Co-Authors: J E HirschAbstract:Since the discovery of the Meissner effect the superconductor to normal (S-N) phase transition in the presence of a magnetic field is understood to be a first order phase transformation that is reversible under ideal conditions and obeys the laws of thermodynamics. The reverse (N-S) transition is the Meissner effect. This implies in particular that the kinetic energy of the supercurrent is not dissipated as Joule heat in the process where the superconductor becomes normal and the supercurrent stops. In this paper we analyze the entropy generation and the momentum transfer between the supercurrent and the body in the S-N transition and the N-S transition as described by the Conventional Theory of superconductivity. We find that it is impossible to explain the transition in a way that is consistent with the laws of thermodynamics unless the momentum transfer between the supercurrent and the body occurs with zero entropy generation, for which the Conventional Theory of superconductivity provides no mechanism. Instead, we point out that the alternative Theory of hole superconductivity does not encounter such difficulties.
Yonggang Huang - One of the best experts on this subject based on the ideXlab platform.
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A study on the Conventional Theory of mechanism-based strain gradient plasticity for mixed hardening by the method of characteristics
Engineering mechanics, 2009Co-Authors: Keh-chih Hwang, Yonggang HuangAbstract:The Conventional Theory of Mechanism-based Strain Gradient Plasticity (CMSG) is of lower-order strain gradient that retains the essential structure of classical plasticity Theory. It does not require additional non-classical boundary conditions, thus it can be easily applied in numerical analysis. The constitutive relations of CMSG Theory for mixed hardening are established, and its well-posedness is studied by the method of characteristics. For an infinite layer in shear, the "domain of determinacy" for CMSG Theory at different mixed hardening states is determined. Within the "domain of determinacy", the presented results agree well with the numerical solution obtained by the finite difference method. Outside the "domain of determinacy", the solution may not be unique, in that case, the additional, non-classical boundary conditions are needed for the well-posedness of CMSG Theory. As the applied shear stress increases, the "domain of determinacy" shrinks and eventually vanishes.
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A Conventional Theory of strain gradient crystal plasticity based on the Taylor dislocation model
International Journal of Plasticity, 2007Co-Authors: Huamiao Wang, Yonggang Huang, Chung-souk Han, Kuo Chu Hwang, B. Liu, G. Ravichandran, Huajian GaoAbstract:Single crystal metallic materials display strong size effects when the characteristic length of plastic deformation is on the order of microns. The classical crystal plasticity Theory cannot explain the size effects since its constitutive model possesses no intrinsic material length. The strain gradient crystal plasticity Theory [Han, C.S., Gao, H.J., Huang, Y., Nix, W.D., 2005a. Mechanism-based strain gradient crystal plasticity – I. Theory. Journal of the Mechanics and Physics of Solids 53, 1188–1203; Han, C.S., Gao, H.J., Huang, Y., Nix, W.D., 2005b. Mechanism-based strain gradient crystal plasticity – II. Analysis. Journal of the Mechanics and Physics of Solids 53, 1204–1222] has been modified to incorporate a new quasi rate-independent formulation for the slip rate. Its major advantage is that it is not necessary to distinguish plastic loading and unloading in a rate-independent formulation, and therefore avoids the complexity of determining the set of active slip systems in single crystals. The intrinsic material length is identified from the Taylor dislocation model as l=α2(μτ0)2b, where μ is the shear modulus, τ0 is the initial yield stress (critical resolved shear stress) in slip systems, b is the magnitude of Burgers vector, and α is an empirical coefficient between 0.3 and 0.5. For non-uniform plastic deformation with the characteristic length of deformation comparable to the intrinsic material length l, the present Theory gives higher plastic work hardening than the classical crystal plasticity Theory due to geometrically necessary dislocations.
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FRICTION EFFECT ON INDENTATION
Engineering mechanics, 2006Co-Authors: Fan Zhang, Keh-chih Hwang, Yonggang Huang, Jiang QinAbstract:Microindentation is a classical experiment which shows a size-dependent material behavior under nonuniform deformation at micron and submicron scales. Many investigations on microindentation have been carried out under frictionless assumption while less discussion has been undertaken on the friction effect. Based on a Conventional Theory of mechanism based strain gradient plasticity (CMSG), a FEM analysis was developed to investigate the friction effect on micro-indentation. For the widely used Berkovich indenter, which can be simplified to a conical indenter with 140.6 cone angle, the result shows that the friction effect was negligible and the problem could be simplified to be frictionless.
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The indentation size effect in the spherical indentation of iridium: A study via the Conventional Theory of mechanism-based strain gradient plasticity
International Journal of Plasticity, 2006Co-Authors: Yonggang Huang, George M. Pharr, Kuo Chu HwangAbstract:The indentation size effect in spherical indentation experiments is studied via the Conventional Theory of mechanism-based strain gradient plasticity (CMSG) established from the Taylor dislocation model. Two approaches are adopted in the present study. The first, an extension of Johnson’s [Johnson, K.L., 1970. The correlation of indentation experiments. Journal of the Mechanics and Physics of Solids 18, 115–126.] theoretical indentation model based on CMSG, fails to predict the experimental data for iridium. The finite element method for CMSG is used to characterize the indented material in the second approach. The predicted indentation hardness agrees well with the experimental data. A simple, analytic indentation model is established to give the indentation hardness H=H02+141α2μ2bR in terms of the radius R of the spherical indenter, where H0 is the indentation hardness without accounting for the tip radius effect (i.e., given by classical plasticity theories), μ is the shear modulus, b is the magnitude of the Burgers vector, and α is the empirical coefficient around 1/3 in the Taylor dislocation model.
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A study of particle size effect and interface fracture in aluminum alloy composite via an extended Conventional Theory of mechanism-based strain-gradient plasticity
Composites Science and Technology, 2005Co-Authors: Thomas Siegmund, Yonggang Huang, Fushi Zhang, Keh-chih HwangAbstract:Abstract Recent experiments have shown that the particle-reinforced composites display significant particle size effect. The classical plasticity theories have no intrinsic material lengths and cannot explain the observed size effects. The strain-gradient plasticity theories have been applied to study the particle size effects in composites, but they tend to predict the stress–strain curves in uniaxial tension that are lower than the experimental data at the small strain (
Huajian Gao - One of the best experts on this subject based on the ideXlab platform.
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A Conventional Theory of strain gradient crystal plasticity based on the Taylor dislocation model
International Journal of Plasticity, 2007Co-Authors: Huamiao Wang, Yonggang Huang, Chung-souk Han, Kuo Chu Hwang, B. Liu, G. Ravichandran, Huajian GaoAbstract:Single crystal metallic materials display strong size effects when the characteristic length of plastic deformation is on the order of microns. The classical crystal plasticity Theory cannot explain the size effects since its constitutive model possesses no intrinsic material length. The strain gradient crystal plasticity Theory [Han, C.S., Gao, H.J., Huang, Y., Nix, W.D., 2005a. Mechanism-based strain gradient crystal plasticity – I. Theory. Journal of the Mechanics and Physics of Solids 53, 1188–1203; Han, C.S., Gao, H.J., Huang, Y., Nix, W.D., 2005b. Mechanism-based strain gradient crystal plasticity – II. Analysis. Journal of the Mechanics and Physics of Solids 53, 1204–1222] has been modified to incorporate a new quasi rate-independent formulation for the slip rate. Its major advantage is that it is not necessary to distinguish plastic loading and unloading in a rate-independent formulation, and therefore avoids the complexity of determining the set of active slip systems in single crystals. The intrinsic material length is identified from the Taylor dislocation model as l=α2(μτ0)2b, where μ is the shear modulus, τ0 is the initial yield stress (critical resolved shear stress) in slip systems, b is the magnitude of Burgers vector, and α is an empirical coefficient between 0.3 and 0.5. For non-uniform plastic deformation with the characteristic length of deformation comparable to the intrinsic material length l, the present Theory gives higher plastic work hardening than the classical crystal plasticity Theory due to geometrically necessary dislocations.
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A Conventional Theory of mechanism-based strain gradient plasticity
International Journal of Plasticity, 2003Co-Authors: Yonggang Huang, Keh-chih Hwang, Huajian GaoAbstract:Abstract There exist two frameworks of strain gradient plasticity theories to model size effects observed at the micron and sub-micron scales in experiments. The first framework involves the higher-order stress and therefore requires extra boundary conditions, such as the Theory of mechanism-based strain gradient (MSG) plasticity [J Mech Phys Solids 47 (1999) 1239; J Mech Phys Solids 48 (2000) 99; J Mater Res 15 (2000) 1786] established from the Taylor dislocation model. The other framework does not involve the higher-order stress, and the strain gradient effect come into play via the incremental plastic moduli. A Conventional Theory of mechanism-based strain gradient plasticity is established in this paper. It is also based on the Taylor dislocation model, but it does not involve the higher-order stress and therefore falls into the second strain gradient plasticity framework that preserves the structure of Conventional plasticity theories. The plastic strain gradient appears only in the constitutive model, and the equilibrium equations and boundary conditions are the same as the Conventional continuum theories. It is shown that the difference between this Theory and the higher-order MSG plasticity Theory based on the same dislocation model is only significant within a thin boundary layer of the solid.
Kuo Chu Hwang - One of the best experts on this subject based on the ideXlab platform.
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A Conventional Theory of strain gradient crystal plasticity based on the Taylor dislocation model
International Journal of Plasticity, 2007Co-Authors: Huamiao Wang, Yonggang Huang, Chung-souk Han, Kuo Chu Hwang, B. Liu, G. Ravichandran, Huajian GaoAbstract:Single crystal metallic materials display strong size effects when the characteristic length of plastic deformation is on the order of microns. The classical crystal plasticity Theory cannot explain the size effects since its constitutive model possesses no intrinsic material length. The strain gradient crystal plasticity Theory [Han, C.S., Gao, H.J., Huang, Y., Nix, W.D., 2005a. Mechanism-based strain gradient crystal plasticity – I. Theory. Journal of the Mechanics and Physics of Solids 53, 1188–1203; Han, C.S., Gao, H.J., Huang, Y., Nix, W.D., 2005b. Mechanism-based strain gradient crystal plasticity – II. Analysis. Journal of the Mechanics and Physics of Solids 53, 1204–1222] has been modified to incorporate a new quasi rate-independent formulation for the slip rate. Its major advantage is that it is not necessary to distinguish plastic loading and unloading in a rate-independent formulation, and therefore avoids the complexity of determining the set of active slip systems in single crystals. The intrinsic material length is identified from the Taylor dislocation model as l=α2(μτ0)2b, where μ is the shear modulus, τ0 is the initial yield stress (critical resolved shear stress) in slip systems, b is the magnitude of Burgers vector, and α is an empirical coefficient between 0.3 and 0.5. For non-uniform plastic deformation with the characteristic length of deformation comparable to the intrinsic material length l, the present Theory gives higher plastic work hardening than the classical crystal plasticity Theory due to geometrically necessary dislocations.
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The indentation size effect in the spherical indentation of iridium: A study via the Conventional Theory of mechanism-based strain gradient plasticity
International Journal of Plasticity, 2006Co-Authors: Yonggang Huang, George M. Pharr, Kuo Chu HwangAbstract:The indentation size effect in spherical indentation experiments is studied via the Conventional Theory of mechanism-based strain gradient plasticity (CMSG) established from the Taylor dislocation model. Two approaches are adopted in the present study. The first, an extension of Johnson’s [Johnson, K.L., 1970. The correlation of indentation experiments. Journal of the Mechanics and Physics of Solids 18, 115–126.] theoretical indentation model based on CMSG, fails to predict the experimental data for iridium. The finite element method for CMSG is used to characterize the indented material in the second approach. The predicted indentation hardness agrees well with the experimental data. A simple, analytic indentation model is established to give the indentation hardness H=H02+141α2μ2bR in terms of the radius R of the spherical indenter, where H0 is the indentation hardness without accounting for the tip radius effect (i.e., given by classical plasticity theories), μ is the shear modulus, b is the magnitude of the Burgers vector, and α is the empirical coefficient around 1/3 in the Taylor dislocation model.
Keh-chih Hwang - One of the best experts on this subject based on the ideXlab platform.
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A study on the Conventional Theory of mechanism-based strain gradient plasticity for mixed hardening by the method of characteristics
Engineering mechanics, 2009Co-Authors: Keh-chih Hwang, Yonggang HuangAbstract:The Conventional Theory of Mechanism-based Strain Gradient Plasticity (CMSG) is of lower-order strain gradient that retains the essential structure of classical plasticity Theory. It does not require additional non-classical boundary conditions, thus it can be easily applied in numerical analysis. The constitutive relations of CMSG Theory for mixed hardening are established, and its well-posedness is studied by the method of characteristics. For an infinite layer in shear, the "domain of determinacy" for CMSG Theory at different mixed hardening states is determined. Within the "domain of determinacy", the presented results agree well with the numerical solution obtained by the finite difference method. Outside the "domain of determinacy", the solution may not be unique, in that case, the additional, non-classical boundary conditions are needed for the well-posedness of CMSG Theory. As the applied shear stress increases, the "domain of determinacy" shrinks and eventually vanishes.
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FRICTION EFFECT ON INDENTATION
Engineering mechanics, 2006Co-Authors: Fan Zhang, Keh-chih Hwang, Yonggang Huang, Jiang QinAbstract:Microindentation is a classical experiment which shows a size-dependent material behavior under nonuniform deformation at micron and submicron scales. Many investigations on microindentation have been carried out under frictionless assumption while less discussion has been undertaken on the friction effect. Based on a Conventional Theory of mechanism based strain gradient plasticity (CMSG), a FEM analysis was developed to investigate the friction effect on micro-indentation. For the widely used Berkovich indenter, which can be simplified to a conical indenter with 140.6 cone angle, the result shows that the friction effect was negligible and the problem could be simplified to be frictionless.
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A study of particle size effect and interface fracture in aluminum alloy composite via an extended Conventional Theory of mechanism-based strain-gradient plasticity
Composites Science and Technology, 2005Co-Authors: Thomas Siegmund, Yonggang Huang, Fushi Zhang, Keh-chih HwangAbstract:Abstract Recent experiments have shown that the particle-reinforced composites display significant particle size effect. The classical plasticity theories have no intrinsic material lengths and cannot explain the observed size effects. The strain-gradient plasticity theories have been applied to study the particle size effects in composites, but they tend to predict the stress–strain curves in uniaxial tension that are lower than the experimental data at the small strain (
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Fracture analysis in the Conventional Theory of mechanism-based strain gradient (CMSG) plasticity
International Journal of Fracture, 2004Co-Authors: Shaoxing Qu, Peidong Wu, Hanqing Jiang, Yonggang Huang, Keh-chih HwangAbstract:In a remarkable series of experiments, Elssner et al. (1994) and Korn et al. (2002) observed cleavage cracking along a bimaterial interface between Nb and sapphire. The stress required for cleavage cracking is around the theoretical strength of the material. Classical plasticity models fall short to reach such a high stress level. We use the Conventional Theory of mechanism-based strain gradient plasticity (Huang et al., 2004) to investigate the stress field around the tip of an interface crack between Nb and sapphire. The tensile stress at a distance of 0.1 µm to the interface crack tip reaches 13.3σY ,w hereσY is the yield stress of Nb. This stress is nearly 4 times of that predicted by classical plasticity Theory (3.6σY ) at the same distance to the crack tip, and is high enough to trigger cleavage cracking in materials and interfaces. This is consistent with Elssner et al.'s (1994) and Korn et al.'s (2002) experimental observations.
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A Conventional Theory of mechanism-based strain gradient plasticity
International Journal of Plasticity, 2003Co-Authors: Yonggang Huang, Keh-chih Hwang, Huajian GaoAbstract:Abstract There exist two frameworks of strain gradient plasticity theories to model size effects observed at the micron and sub-micron scales in experiments. The first framework involves the higher-order stress and therefore requires extra boundary conditions, such as the Theory of mechanism-based strain gradient (MSG) plasticity [J Mech Phys Solids 47 (1999) 1239; J Mech Phys Solids 48 (2000) 99; J Mater Res 15 (2000) 1786] established from the Taylor dislocation model. The other framework does not involve the higher-order stress, and the strain gradient effect come into play via the incremental plastic moduli. A Conventional Theory of mechanism-based strain gradient plasticity is established in this paper. It is also based on the Taylor dislocation model, but it does not involve the higher-order stress and therefore falls into the second strain gradient plasticity framework that preserves the structure of Conventional plasticity theories. The plastic strain gradient appears only in the constitutive model, and the equilibrium equations and boundary conditions are the same as the Conventional continuum theories. It is shown that the difference between this Theory and the higher-order MSG plasticity Theory based on the same dislocation model is only significant within a thin boundary layer of the solid.