Accurate prediction of stress-strain behavior of metals as a function of arbitrary temperature and strain rate paths has remained a challenge. The Mechanical Threshold Stress constitutive model is one formalism that h...Accurate prediction of stress-strain behavior of metals as a function of arbitrary temperature and strain rate paths has remained a challenge. The Mechanical Threshold Stress constitutive model is one formalism that has emerged following several decades of research. Vast experience has accumulated with the application of the Mechanical Threshold Stress model over a wide variety of pure metals and alloys. Out of this has arisen common trends across metal systems. The magnitude of activation energies presents one example of this, where these variables consistently increase in magnitude as the obstacle to dislocation motion transitions from short range to long range. Trends in strain hardening are also observed. In Face-Centered Cubic metals the magnitude of strain hardening scales with the stacking fault energy;trends in Body-Centered Cubic metals are less clear. Model parameters derived for over twenty metals and alloys are tabulated. Common trends should guide future application of the MTS model and further model development.展开更多
Combined bodies of rock-like material and rock are widely encountered in geotechnical engineering,such as tunnels and mines.The existing theoretical models describing the stress-strain relationship of a combined body ...Combined bodies of rock-like material and rock are widely encountered in geotechnical engineering,such as tunnels and mines.The existing theoretical models describing the stress-strain relationship of a combined body lack a binary feature.Based on effective medium theory,this paper presents the governing equation of the“elastic modulus”for combined and single bodies under triaxial compressive tests.A binary effective medium model is then established.Based on the compressive experiment of concretegranite combined bodies,the feasibility of determining the stress threshold based on crack axial strain is discussed,and the model is verified.The model is further extended to coal-rock combined bodies of more diverse types,and the variation laws of the compressive mechanical parameters are then discussed.The results show that the fitting accuracy of the model with the experimental curves of the concretegranite combined bodies and various types of coal-rock combined bodies are over 95%.The crack axial strain method can replace the crack volumetric strain method,which clarifies the physical meanings of the model parameters.The variation laws of matrix parameters and crack parameters are discussed in depth and are expected to be more widely used in geotechnical engineering.展开更多
By using GDS dynamic hollow cylinder torsional apparatus,a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initi...By using GDS dynamic hollow cylinder torsional apparatus,a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initial principal stress direction α0,the initial ratio of deviatoric stress η0,the initial average effective principal stress p0 and the initial intermediate principal stress parameter b0 on the threshold shear strain γt of Nanjing saturated fine sand are then systematically investigated. The results show that γt increases as η0,p0 and b0 increase respectively,while the other three parameters remain constant. α0 has a great influence on γt,which is reduced when α0 increases from 0° to 45° and increased when α0 increases from 45° to 90°. The effect of α0 on γt plays a leading role and the effect of η0 will weaken when α0 is approximately 45°.展开更多
Near-threshold fatigue crack propagation behaviour of a cold-worked copper tested with distinct fa- tigue testing systems under different stress ratios has been investigated.The emphasis was placed on the evaluation o...Near-threshold fatigue crack propagation behaviour of a cold-worked copper tested with distinct fa- tigue testing systems under different stress ratios has been investigated.The emphasis was placed on the evaluation of crack closure effect as well as the effective fatigue threshold,It was found that the fatigue threshold for R=0 is higher than those for R=-1 and R=0.4,for which almost the same threshold value was derived.Compared with the conventional closure evaluation method,the pro- posed new closure evaluation method can generally interpret the dependence of the fatigue thresh- old on stress ratio and loading condition,and leads to a higher and relatively constant effective fa- tigue threshold of about 3 MPa·m^(1/2) for the cold-worked copper at different stress ratios and loading conditions.展开更多
The threshold pressure gradient and formation stress-sensitive effect as the two prominent physical phenomena in the development of a low-permeable reservoir are both considered here for building a new coupled moving ...The threshold pressure gradient and formation stress-sensitive effect as the two prominent physical phenomena in the development of a low-permeable reservoir are both considered here for building a new coupled moving boundary model of radial flow in porous medium. Moreover, the wellbore storage and skin effect are both incorporated into the inner boundary conditions in the model. It is known that the new coupled moving boundary model has strong nonlinearity. A coordinate transformation based fully implicit finite difference method is adopted to obtain its numerical solutions. The involved coordinate transformation can equivalently transform the dynamic flow region for the moving boundary model into a fixed region as a unit circle, which is very convenient for the model computation by the finite difference method on fixed spatial grids. By comparing the numerical solution obtained from other different numerical method in the existing literature, its validity can be verified. Eventually, the effects of permeability modulus, threshold pressure gradient, wellbore storage coefficient, and skin factor on the transient wellbore pressure, the derivative, and the formation pressure distribution are analyzed respectively.展开更多
Large-strain deformations introduce several confounding factors that affect the application of the Mechanical Threshold Stress model. These include the decrease with the increasing stress of the normalized activation ...Large-strain deformations introduce several confounding factors that affect the application of the Mechanical Threshold Stress model. These include the decrease with the increasing stress of the normalized activation energy characterizing deformation kinetics, the tendency toward Stage IV hardening at high strains, and the influence of crystallographic texture. Minor additions to the Mechanical Threshold Stress model are introduced to account for variations of the activation energy and the addition of Stage IV hardening. Crystallographic texture cannot be modeled using an isotropic formulation, but some common trends when analyzing predominantly shear deformation followed by uniaxial deformation are described. Comparisons of model predictions with measurements in copper processed using Equal Channel Angular Pressing are described.展开更多
The Wenchuan Ms8.0 earthquake and the Lushan M7.0 earthquake occurred in the north and south segments of the Longmenshan nappe tectonic belt, respectively. Based on the focal mechanism and finite fault model of the We...The Wenchuan Ms8.0 earthquake and the Lushan M7.0 earthquake occurred in the north and south segments of the Longmenshan nappe tectonic belt, respectively. Based on the focal mechanism and finite fault model of the Wenchuan Ms8.0 earthquake, we calculated the coulomb failure stress change. The inverted coulomb stress changes based on the Nishimura and Chenji models both show that the Lushan MT. 0 earth- quake occurred in the increased area of coulomb failure stress induced by the Wenchuan Ms8. 0 earthquake. The coulomb failure stress increased by approximately 0. 135 - 0. 152 bar in the source of the Lushan M7.0 earthquake, which is far more than the stress triggering threshold. Therefore, the Lushan M7.0 earthquake was most likely triggered by the coulomb failure stress change.展开更多
In this paper, we demonstrate that thermal stress is the main mechanism in the process of paint removal by Q-switched Nd：YAG laser （λ = 1064 nm, τ = 10 ns）. A theoretical model ofpaint removal by short-pulse lase...In this paper, we demonstrate that thermal stress is the main mechanism in the process of paint removal by Q-switched Nd：YAG laser （λ = 1064 nm, τ = 10 ns）. A theoretical model ofpaint removal by short-pulse laser is established from the perspective of thermal stress. Thermal stress is generated by thermal expansion, and the temperatures of different samples are calculated according to the one-dimensional （1D） heat conduction equation. The theoretical cleaning threshold can be obtained by comparing thermal stress with the adhesion of paint, and the theoretical damage threshold is obtained by calculating the temperature. Moreover, the theoretical calculations are verified by experimental results. It is shown that the thermal stress model of the laser cleaning is very useful to choose the appropriate laser fluence in the practical applications of paint removal by Q-switched Nd： YAG laser because our model can validly balance the efficiency of laser cleaning and the safety of the substrate.展开更多
This paper studies the degradation of device parameters and that of stress induced leakage current （SILC） of thin tunnel gate oxide under channel hot electron （CHE） stress at high temperature by using n-channel me...This paper studies the degradation of device parameters and that of stress induced leakage current （SILC） of thin tunnel gate oxide under channel hot electron （CHE） stress at high temperature by using n-channel metal oxide semiconductor field effect transistors （NMOSFETs） with 1.4-nm gate oxides. The degradation of device parameters under CHE stress exhibits saturating time dependence at high temperature. The emphasis of this paper is on SILC of an ultra-thin-gate-oxide under CHE stress at high temperature. Based on the experimental results, it is found that there is a linear correlation between SILC degradation and Vh degradation in NMOSFETs during CHE stress. A model of the combined effect of oxide trapped negative charges and interface traps is developed to explain the origin of SILC during CHE stress.展开更多
Post-processing can effectively improve the resistance to laser damage in multilayer films used in a high power laser system. In this work, HfO_2/SiO_2 multilayer films are prepared by e-beam evaporation and then β-r...Post-processing can effectively improve the resistance to laser damage in multilayer films used in a high power laser system. In this work, HfO_2/SiO_2 multilayer films are prepared by e-beam evaporation and then β-ray irradiation is employed as the post-processing method. The particle irradiation affects the laser induced damage threshold(LIDT),which includes defects, surface roughness, packing density and residual stress. The residual stress that is relaxed during irradiation changes from compressive stress into tensile stress. Our results indicate that appropriate tensile stress can improve LIDT remarkably. In view of the fact that LIDT rises from 8 J/cm^2 to 12 J/cm^2, i.e., 50% increase, after the film has been irradiated by 2.2×10^(13)/cm^2 β-ray, the particle irradiation can be used as a controllable and desirable postprocessing method to improve the resistance to laser induced damage.展开更多
文摘Accurate prediction of stress-strain behavior of metals as a function of arbitrary temperature and strain rate paths has remained a challenge. The Mechanical Threshold Stress constitutive model is one formalism that has emerged following several decades of research. Vast experience has accumulated with the application of the Mechanical Threshold Stress model over a wide variety of pure metals and alloys. Out of this has arisen common trends across metal systems. The magnitude of activation energies presents one example of this, where these variables consistently increase in magnitude as the obstacle to dislocation motion transitions from short range to long range. Trends in strain hardening are also observed. In Face-Centered Cubic metals the magnitude of strain hardening scales with the stacking fault energy;trends in Body-Centered Cubic metals are less clear. Model parameters derived for over twenty metals and alloys are tabulated. Common trends should guide future application of the MTS model and further model development.
基金the Major Program of National Natural Science Foundation of China(No.41941019)Shaanxi Province Innovative Talent Promotion Plan-Science and Technology Innovation Team(No.2021TD-55)Central University Natural Science Innovation Team(No.300102262402)。
文摘Combined bodies of rock-like material and rock are widely encountered in geotechnical engineering,such as tunnels and mines.The existing theoretical models describing the stress-strain relationship of a combined body lack a binary feature.Based on effective medium theory,this paper presents the governing equation of the“elastic modulus”for combined and single bodies under triaxial compressive tests.A binary effective medium model is then established.Based on the compressive experiment of concretegranite combined bodies,the feasibility of determining the stress threshold based on crack axial strain is discussed,and the model is verified.The model is further extended to coal-rock combined bodies of more diverse types,and the variation laws of the compressive mechanical parameters are then discussed.The results show that the fitting accuracy of the model with the experimental curves of the concretegranite combined bodies and various types of coal-rock combined bodies are over 95%.The crack axial strain method can replace the crack volumetric strain method,which clarifies the physical meanings of the model parameters.The variation laws of matrix parameters and crack parameters are discussed in depth and are expected to be more widely used in geotechnical engineering.
基金supported by the Key Research Project of National Natural Science Foundation of China under grant No. 90715018the Special Fund for the Commonweal Industry of China under grant No. 200808022the Key Basic Research Program of Natural Science of University in Jiangsu Province under grant No. 08KJA560001
文摘By using GDS dynamic hollow cylinder torsional apparatus,a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initial principal stress direction α0,the initial ratio of deviatoric stress η0,the initial average effective principal stress p0 and the initial intermediate principal stress parameter b0 on the threshold shear strain γt of Nanjing saturated fine sand are then systematically investigated. The results show that γt increases as η0,p0 and b0 increase respectively,while the other three parameters remain constant. α0 has a great influence on γt,which is reduced when α0 increases from 0° to 45° and increased when α0 increases from 45° to 90°. The effect of α0 on γt plays a leading role and the effect of η0 will weaken when α0 is approximately 45°.
文摘Near-threshold fatigue crack propagation behaviour of a cold-worked copper tested with distinct fa- tigue testing systems under different stress ratios has been investigated.The emphasis was placed on the evaluation of crack closure effect as well as the effective fatigue threshold,It was found that the fatigue threshold for R=0 is higher than those for R=-1 and R=0.4,for which almost the same threshold value was derived.Compared with the conventional closure evaluation method,the pro- posed new closure evaluation method can generally interpret the dependence of the fatigue thresh- old on stress ratio and loading condition,and leads to a higher and relatively constant effective fa- tigue threshold of about 3 MPa·m^(1/2) for the cold-worked copper at different stress ratios and loading conditions.
基金Project supported by the National Natural Science Foundation of China(Grant No.51404232)the China Postdoctoral Science Foundation(Grant No.2014M561074)the National Science and Technology Major Project,China(Grant No.2011ZX05038003)
文摘The threshold pressure gradient and formation stress-sensitive effect as the two prominent physical phenomena in the development of a low-permeable reservoir are both considered here for building a new coupled moving boundary model of radial flow in porous medium. Moreover, the wellbore storage and skin effect are both incorporated into the inner boundary conditions in the model. It is known that the new coupled moving boundary model has strong nonlinearity. A coordinate transformation based fully implicit finite difference method is adopted to obtain its numerical solutions. The involved coordinate transformation can equivalently transform the dynamic flow region for the moving boundary model into a fixed region as a unit circle, which is very convenient for the model computation by the finite difference method on fixed spatial grids. By comparing the numerical solution obtained from other different numerical method in the existing literature, its validity can be verified. Eventually, the effects of permeability modulus, threshold pressure gradient, wellbore storage coefficient, and skin factor on the transient wellbore pressure, the derivative, and the formation pressure distribution are analyzed respectively.
文摘Large-strain deformations introduce several confounding factors that affect the application of the Mechanical Threshold Stress model. These include the decrease with the increasing stress of the normalized activation energy characterizing deformation kinetics, the tendency toward Stage IV hardening at high strains, and the influence of crystallographic texture. Minor additions to the Mechanical Threshold Stress model are introduced to account for variations of the activation energy and the addition of Stage IV hardening. Crystallographic texture cannot be modeled using an isotropic formulation, but some common trends when analyzing predominantly shear deformation followed by uniaxial deformation are described. Comparisons of model predictions with measurements in copper processed using Equal Channel Angular Pressing are described.
基金the financial supports from the National Key Laboratory of Science and Technology for National Defence on High-Strength Lightweight Structural Materials of China(No.20190104)State Key Laboratory of High-Performance Complex Manufacturing of Central South University,China(No.ZZYJKT2020-03)the National Key Research and Development Program of China(No.2016YFB0300801)。
基金supported by the Director Foundation of Institute of SeismologyChina Earthquake Administration(201056088)
文摘The Wenchuan Ms8.0 earthquake and the Lushan M7.0 earthquake occurred in the north and south segments of the Longmenshan nappe tectonic belt, respectively. Based on the focal mechanism and finite fault model of the Wenchuan Ms8.0 earthquake, we calculated the coulomb failure stress change. The inverted coulomb stress changes based on the Nishimura and Chenji models both show that the Lushan MT. 0 earth- quake occurred in the increased area of coulomb failure stress induced by the Wenchuan Ms8. 0 earthquake. The coulomb failure stress increased by approximately 0. 135 - 0. 152 bar in the source of the Lushan M7.0 earthquake, which is far more than the stress triggering threshold. Therefore, the Lushan M7.0 earthquake was most likely triggered by the coulomb failure stress change.
基金supported by the National Natural Science Foundation of China(Grant No.61067002)the Foundation for Department of Education of Jiangxi Province,China(Grant No.GJJ12581)+1 种基金the Natural Science Foundation of Jiangxi Province,China(Grant No.20132BAB212008)the Foundation for GannanNormal University,China(Grant No.11kyz12)
文摘In this paper, we demonstrate that thermal stress is the main mechanism in the process of paint removal by Q-switched Nd：YAG laser （λ = 1064 nm, τ = 10 ns）. A theoretical model ofpaint removal by short-pulse laser is established from the perspective of thermal stress. Thermal stress is generated by thermal expansion, and the temperatures of different samples are calculated according to the one-dimensional （1D） heat conduction equation. The theoretical cleaning threshold can be obtained by comparing thermal stress with the adhesion of paint, and the theoretical damage threshold is obtained by calculating the temperature. Moreover, the theoretical calculations are verified by experimental results. It is shown that the thermal stress model of the laser cleaning is very useful to choose the appropriate laser fluence in the practical applications of paint removal by Q-switched Nd： YAG laser because our model can validly balance the efficiency of laser cleaning and the safety of the substrate.
基金Project supported by the National Natural Science Foundation of China (Grant Nos 60736033 and 60506020)
文摘This paper studies the degradation of device parameters and that of stress induced leakage current （SILC） of thin tunnel gate oxide under channel hot electron （CHE） stress at high temperature by using n-channel metal oxide semiconductor field effect transistors （NMOSFETs） with 1.4-nm gate oxides. The degradation of device parameters under CHE stress exhibits saturating time dependence at high temperature. The emphasis of this paper is on SILC of an ultra-thin-gate-oxide under CHE stress at high temperature. Based on the experimental results, it is found that there is a linear correlation between SILC degradation and Vh degradation in NMOSFETs during CHE stress. A model of the combined effect of oxide trapped negative charges and interface traps is developed to explain the origin of SILC during CHE stress.
基金Project supported by the National Natural Science Foundation of China(Grant No.11405085)the Jiangsu Provincial Natural Science Fund,China(Grant No.BK20130789)
文摘Post-processing can effectively improve the resistance to laser damage in multilayer films used in a high power laser system. In this work, HfO_2/SiO_2 multilayer films are prepared by e-beam evaporation and then β-ray irradiation is employed as the post-processing method. The particle irradiation affects the laser induced damage threshold(LIDT),which includes defects, surface roughness, packing density and residual stress. The residual stress that is relaxed during irradiation changes from compressive stress into tensile stress. Our results indicate that appropriate tensile stress can improve LIDT remarkably. In view of the fact that LIDT rises from 8 J/cm^2 to 12 J/cm^2, i.e., 50% increase, after the film has been irradiated by 2.2×10^(13)/cm^2 β-ray, the particle irradiation can be used as a controllable and desirable postprocessing method to improve the resistance to laser induced damage.
