The current research of sandwich structures under dynamic loading mainly focus on the response characteristic of structure.The micro-topology of core layers would sufficiently influence the property of sandwich struct...The current research of sandwich structures under dynamic loading mainly focus on the response characteristic of structure.The micro-topology of core layers would sufficiently influence the property of sandwich structure.However,the micro deformation and topology mechanism of structural deformation and energy absorption are unclear.In this paper,based on the bi-directional evolutionary structural optimization method and periodic base cell(PBC)technology,a topology optimization frame work is proposed to optimize the core layer of sandwich beams.The objective of the present optimization problem is to maximize shear stiffness of PBC with a volume constraint.The effects of the volume fraction,filter radius,and initial PBC aspect ratio on the micro-topology of the core were discussed.The dynamic response process,core compression,and energy absorption capacity of the sandwich beams under blast impact loading were analyzed by the finite element method.The results demonstrated that the overpressure action stage was coupled with the core compression stage.Under the same loading and mass per unit area,the sandwich beam with a 20%volume fraction core layer had the best blast resistance.The filter radius has a slight effect on the shear stiffness and blast resistances of the sandwich beams.But increasing the filter radius could slightly improve the bending stiffness.Upon changing the initial PBC aspect ratio,there are three ways for PBC evolution:The first is to change the angle between the adjacent bars,the second is to further form holes in the bars,and the third is to combine the first two ways.However,not all three ways can improve the energy absorption capacity of the structure.Changing the aspect ratio of the PBC arbitrarily may lead to worse results.More studies are necessary for further detailed optimization.This research proposes a new topology sandwich beam structure by micro-topology optimization,which has sufficient shear stiffness.The micro mechanism of structural energy absorption is clarified,it is significant for structural energy absorption design.展开更多
The conventional methods of determining the dynamic resistance were mostly done by measuring the voltage and current at secondary side of transformer in resistance welding machines, in which the measuring set-up norma...The conventional methods of determining the dynamic resistance were mostly done by measuring the voltage and current at secondary side of transformer in resistance welding machines, in which the measuring set-up normally interferes with the movement of electrode, and the measuring precision is influenced by inductive noise caused by the high welding current. In this study, the dynamic resistance is determined by measuring the voltage at primary side and current at secondary side. This increases the accuracy of measurement because of higher signal-noise ratio, and allows to apply to in-process system without any wires connected to electrodes.展开更多
A self-developed welding dynamic arc wavelet analyzer was adopted to analyze and assess the welding process of two CO2 arc welding machines. The experimental results indicate that the instantaneous energy can reflect ...A self-developed welding dynamic arc wavelet analyzer was adopted to analyze and assess the welding process of two CO2 arc welding machines. The experimental results indicate that the instantaneous energy can reflect the influence of the welding current and voltage on dynamic arc characteristic synthetically. Through calculating and analyzing the instantaneous energy, the energy during arc ignition and short circuit in CO2 welding process can be confirmed rationally, thus the foundation for the accurate design and control of the welding current and voltage can be provided. By reducing the ripple disturbance of the dynamic resistance, avoiding peak current and voltage waveform , and enhancing the transition frequency of short circuit suitably, the stability of the welding arc and the weld appearance can be improved.展开更多
The DRM （dynamic contact resistance measurement） in high voltage circuit breakers is a manner of evaluating the internal ageing condition of the chamber. DRM is similar to static contact resistance measurement testi...The DRM （dynamic contact resistance measurement） in high voltage circuit breakers is a manner of evaluating the internal ageing condition of the chamber. DRM is similar to static contact resistance measurement testing, but instead of measuring a single value when the breaker contacts are closed （static value）, the ohmic resistance is measured at various contact positions, from the beginning of the contact opening until a complete separation of the contacts. The relationship between the contact resistances of the new circuit breaker and the ageing circuit breaker in operation provides subsidy for the evaluation of both the main and arcing contact conditions. This research aims to analyze the correlation between the various levels of degradation of the contacts and the configuration of the DRM curve. This work considers curve samples from new acceleration tests. breaker chamber contacts and different levels of degradation by展开更多
<span style="font-family:Verdana;">Several studies on PV solar cells are found in</span> <span style="font-family:Verdana;"><span style="font-family:Verdana;"><...<span style="font-family:Verdana;">Several studies on PV solar cells are found in</span> <span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">the </span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">literature which use static models. Those models are mainly one-diode, two-diode or three-diode models. In the dynamic modelling</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span><span><span style="font-family:;" "=""><span style="font-family:Verdana;"> a variable parallel capacitance is incorporated. Unlike the previous studies which do not clearly establish a relationship between the capacitance and the voltage, in the present paper, the link between the capacitance and the voltage is investigated and established. In dynamic modelling investigated in this paper, the dynamic resistance is introduced in the modelling of the solar cell. It is introduced in the current-voltage characteristic. The value of the dynamic resistance is evaluated at the maximum po</span><span style="font-family:Verdana;">wer point and its effect on the maximum power is investigated. The study</span> <span style="font-family:Verdana;">shows for the first time, that the dynamic resistance must be introduced in</span> <span style="font-family:Verdana;">the current-voltage characteristic, because it has an influence on the PV cell </span><span style="font-family:Verdana;">output</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">.</span></span></span>展开更多
In flux pumps,motors and superconducting magnets,the high temperature superconductor(HTS)coated conductor frequently carries a DC transport current when an oscillating magnetic field is present in the background.Under...In flux pumps,motors and superconducting magnets,the high temperature superconductor(HTS)coated conductor frequently carries a DC transport current when an oscillating magnetic field is present in the background.Under this circumstance,the interesting effect of dynamic resistance takes place,which can affect the operating performance of superconducting devices:heat accumulation can contribute to the rising temperature of the HTS tape and the dynamic resistance voltage can change accordingly.This article explores the time‐dependent development of the dynamic resistance voltage using a numerical modeling considering the thermal effects.After a validation against experimental results,this work investigates the effects of several factors on the structure of the HTS tape on the time‐dependent development of the dynamic resistance,thus providing insights toward a better understanding of the time‐dependent behavior of HTS tapes under external magnetic fields.展开更多
In order to explore the influence of welding parameters and to investigate the Al alloy (AA) nugget formation process, a comprehensive model involving electrical-thermal-mechanical and metallurgical analysis was estab...In order to explore the influence of welding parameters and to investigate the Al alloy (AA) nugget formation process, a comprehensive model involving electrical-thermal-mechanical and metallurgical analysis was established to numerically display the resistance spot welding (RSW) process within multiple fields and understand the AA-RSW physics. A multi-disciplinary finite element method (FEM) framework and a empirical sub-model were built to analyze the affecting factors on weld nugget and the underlying nature of welding physics with dynamic simulation procedure. Specifically, a counter-intuitive phenomenon of the resistance time-variation caused by the transient inverse virtual variation (TIVV) effect was highlighted and analyzed on the basis of welding current and temperature distribution simulation. The empirical model describing the TIVV phenomenon was used for modifying the dynamic resistance simulation during the AA spot welding process. The numerical and experimental results show that the proposed multi-field FEM model agrees with the measured AA welding feature, and the modified dynamic resistance model captures the physics of nugget growth and the electrical-thermal behavior under varying welding current and fluctuating heat input.展开更多
Based on previous achievements,a dynamic pressure-sinkage equation for saturated clay is established.First,aquasi-static penetration rate is selected,and the ratio of the dynamic penetration rate to the quasi-static r...Based on previous achievements,a dynamic pressure-sinkage equation for saturated clay is established.First,aquasi-static penetration rate is selected,and the ratio of the dynamic penetration rate to the quasi-static rate is used to characterize the degree of dynamic effect,then theβth power of the ratio is used to quantify the dynamic effect of sinkage.The dynamic effect exponentβis obtained using penetration tests with different penetration rates.Then,a dynamic motion resistance equation for a tracked vehicle is established based on the dynamic pressure-sinkage equation.The equation incorporates both penetration and bulldozing resistance.Finally,a series of simulation experiments with varying travel speeds and slip rates is carried out.The results show that an increase in the speed leads to stronger terrain stiffness,resulting in a decrease in sinkage and motion resistance.However,the enhancement effect becomes weaker with an increase in the travel speed.展开更多
The end value of the dynamic resistance curve of stainless steel was proved to have strong correlation with nugget size by experiments, so it was an important factor for estimation of weld quality. BP neural network w...The end value of the dynamic resistance curve of stainless steel was proved to have strong correlation with nugget size by experiments, so it was an important factor for estimation of weld quality. BP neural network was employed to estimate the weld quality, The end value of the dynamic resistance curve, welding current and welding time were selected as the input variables while the nugget diameter, which is closely related to weld quality, was selected as the output variable. Testing results shows that such network has fine fault tolerance and real-time quality estimation is possible.展开更多
Dent resistance of automobile body panels is an important property for automobile design and manufacture, but the study on this aspect is not still profound. This study is to summarize the testing methods and physical...Dent resistance of automobile body panels is an important property for automobile design and manufacture, but the study on this aspect is not still profound. This study is to summarize the testing methods and physical significations of static and dynamic dent resistance of automobile body panels combined with the author＇s study, and to analyze the dent behaviors in the round. Several influence factors on dent resistance are expatiated including the mechanical properties of materials, stress states after forming, bake hardening ability, modulus, methods of testing, and structure of specimens and so on. The automotive lightweight and application of high strength steel sheets and aluminum alloys sheets are analyzed, and the significance of testing of dent resistance, especially for dynamic dent resistance of auto-panels, and the finite element simulation analysis are emphasized. To explain the physical phenomenon of dent behaviors, the latest and concerned study results are also discussed. According to this study, a dent resistance test and evaluation standard of Society of Automotive Engineers of China for automotive body panel is presented and is being carried out, and an industry conference is hold to discuss the working-out of the standard, a primary schedule of this standard is confirmed now. The study can guide the further testing and study of dent resistant of auto-panels.展开更多
The dynamic effective shear strength of saturated sand under cyclic loading is discussed in this paper.The discussion includes the transient time depen- dency behaviors based on the analysis of the results obtained in...The dynamic effective shear strength of saturated sand under cyclic loading is discussed in this paper.The discussion includes the transient time depen- dency behaviors based on the analysis of the results obtained in conventional cyclic triaxial tests and cyclic torsional shear triaxial tests.It has been found that the dy- namic effective shear strength is composed of effective frictional resistance and viscous resistance,which are characterized by the strain rate dependent feature of strength magnitude,the coupling of consolidation stress with cyclic stress and the dependency of time needed to make the soil strength sufficiently mobilized,and can also be ex- pressed by the extended Mohr-Coulomb's law.The two strength parameters of the dynamic effective internal frictional angle φd and the dynamic viscosity coefficient η are determined.The former is unvaried for different number of cyclic loading,dy- namic stress form and consolidation stress ratio.And the later is unvaried for the different dynamic shear strain rate γt developed during the sand liquefaction,but increases with the increase of initial density of sand.The generalization of dynamic effective stress strength criterion in the 3-dimensional effective stress space is studied in detail for the purpose of its practical use.展开更多
REBCO tapes carry DC current under AC magnetic fields in proposed HTS fusion applications. AC loss will be generated in the process and it is important to understand the AC loss behaviour for safe operation of the fus...REBCO tapes carry DC current under AC magnetic fields in proposed HTS fusion applications. AC loss will be generated in the process and it is important to understand the AC loss behaviour for safe operation of the fusion magnets. In this work, magnetisation loss (Qm), dynamic resistance (Rdyn), and total loss (Qtotal) in four different REBCO tapes are numerically studied, using the measured and , for the magnetic field amplitude applied perpendicularly up to 8 T at 20 K and 50 K, where represents the magnetic field and field angle () dependent critical current density. The peak of Theva data is different from that of other tapes. We artificially shifted the ab-plane peak of Theva to the left by 25° to match the peak value. The newly shifted data is named as Theva-shift, which was also investigated to study the influence of the Theva peak shift on AC loss. The normalised DC transport current level (i = It/Ic0) ranges from 0.05 to 0.9, where the DC current amplitude and the self-critical current of the tape are represented by It and Ic0, respectively. The simulation results show that the AC losses deviate significantly from the Brandt-Indenbom (BI) equation at high magnetic fields. Jc and instantaneous loss curves for different tapes show correlation at high magnetic fields. The simulation results also show how different characteristics for different tapes influence AC losses. When AC loss values are scaled by the self-field critical current, Qm without current and Qtotal with current in the different tapes show a good agreement. It implies that the temperature dependence of the two types of loss can be calculated from a known loss at one temperature and the self-field critical current.展开更多
The objective of this paper was to find new modifier to improve the aging resistance and low temperature cracking resistance of asphalt. To investigate the aging resistance of modified asphalt binders, mesoporous nano...The objective of this paper was to find new modifier to improve the aging resistance and low temperature cracking resistance of asphalt. To investigate the aging resistance of modified asphalt binders, mesoporous nano-silica（doping Ti^（4＋）） was used as a asphalt modifier. Some physical properties including penetration, ductility, and softening point of asphalt were analyzed with RTFO（Rotating thin film oven） aging and ultraviolet aging. Moreover, the performances of high and low temperature of modified asphalt binders with pressure aging were tested by dynamic shear rheometer（DSR） test and bending beam rheometer（BBR） test. These results showed that the penetration decreased, low temperature ductility, and softening point increased when adding mesoporous nano-silica to base asphalt. After ultraviolet radiation aging, the penetration loss and ductility loss of modified asphalt decreased than that of original asphalt, the increase of softening point was also significantly reduced than that of base asphalt. Furthermore, The test results of DSR and BBR showed that the G＊sinδ and creep modulus‘s＇ of pressure aged asphalt decreased, but the creep rate ‘m＇ increased. It can be concluded that the aging resistance and cracking resistance of modified asphalt are improved by adding mesoporous nano-silica, especially the doping of Ti^（4＋） could improve the aging resistance obviously.展开更多
With the continuous increase of train speed,undulations of catenary and vibrations of the pantograph head result in generating pantograph- catenary arc frequently,intensifying the abrasion between pantograph strip and...With the continuous increase of train speed,undulations of catenary and vibrations of the pantograph head result in generating pantograph- catenary arc frequently,intensifying the abrasion between pantograph strip and catenary wire,which has seriously influenced the current collection and safety of electric multi units(EMU). It is necessary to study the pantographcatenary arc in immediately. Some researchers develop a few pantograph- catenary arc testing equipment,which couldn’t really reflect the operating condition of pantograph-catenary system. In this paper,the pantograph-catenary arc test apparatus was developed,which simulated the flexible and straight contact of pantograph strip and catenary wire,based on the coupling relationship between pantograph and catenary. The equipment was used to research the electrical parameters of the pantograph-catenary arc and the dynamic contact resistance.展开更多
Electrically assisted friction stir welding is a promising way to improve liquidity of viscoplastic metal. However, this local heat source only can take into effect in a certain contact interface status. Semi-analytic...Electrically assisted friction stir welding is a promising way to improve liquidity of viscoplastic metal. However, this local heat source only can take into effect in a certain contact interface status. Semi-analytical model and FEM are adopted to analyze materials flow in electrically assisted FSW. Semi-analysis results show that Joule heating is validity with rotation speed 1 400 r/min and travel speed 400 mm/min. The maximum flow velocity is 0.033 m/s when travel speed is 400 mm/min, and it increases to 0.043 m/s with current assistant. Visco-plasticizing efficiency is a key factor to decide whether Joule heat is significant, and viscoplastic flow can periodically make circuit short. Periodic torque, axial force and onion rings surface show dynamic flow process, which can be used to illustrate the dynamic contact resistance feature and correct the calculation equation of Joule heat. This paper is aim to point out the potential relation between viscoplastic flow, periodic torque and Joule heating validity.展开更多
The sound absorbing performance of the sintered fibrous metallic materials is investigated by employing a dynamic flow resistivity based model,in which the porous material is modeled as randomly distributed parallel f...The sound absorbing performance of the sintered fibrous metallic materials is investigated by employing a dynamic flow resistivity based model,in which the porous material is modeled as randomly distributed parallel fibers specified by two basic physical parameters:fiber diameter and porosity.A self-consistent Brinkman approach is applied to the calculation of the dynamic resistivity of flow perpendicular to the cylindrical fibers.Based on the solved flow resistivity,the sound absorption of single layer fibrous material can be obtained by adopting the available empirical equations.Moreover,the recursion formulas of surface impedance are applied to the calculation of the sound absorption coefficient of multi-layer fibrous materials.Experimental measurements are conducted to validate the proposed model,with good agreement achieved between model predictions and tested data.Numerical calculations with the proposed model are subsequently performed to quantify the influences of fiber diameter,porosity and backed air gap on sound absorption of uniform(single-layer)fibrous materials.Results show that the sound absorption increases with porosity at higher frequencies but decreases with porosity at lower frequencies.The sound absorption also decreases with fiber diameter at higher frequencies but increases at lower frequencies.The sound absorption resonance is shifted to lower frequencies with air gap.For multi-layer fibrous materials,gradient distributions of both fiber diameter and porosity are introduced and their effects on sound absorption are assessed.It is found that increasing the porosity and fiber diameter variation improves sound absorption in the low frequency range.The model provides the possibility to tailor the sound absorption capability of the sintered fibrous materials by optimizing the gradient distributions of key physical parameters.展开更多
The application of advanced high strength steels in automotive industry has highlighted the need for research into spot weldability of these steels.Using weld lobe diagrams,the spot weldability of DP600 steel was foun...The application of advanced high strength steels in automotive industry has highlighted the need for research into spot weldability of these steels.Using weld lobe diagrams,the spot weldability of DP600 steel was found to be poor with conventional weld schedules.An enhanced weld schedule consisting of two pulses with reduced current on the second pulse gave a substantial increase in the lobe width;the first pulse removed the zinc coating and the second pulse controlled the nugget growth.A data acquisition system was designed to monitor weld expulsion during the weld operation.Of the three possible control strategies proposed,especially with AC welding equipment,the dynamic resistance signal is easily obtained and the least expensive.Expulsion phenomena,microstructural characterization and mechanical properties of spot-welded hot dipped galvanized DP600 steel and interstitial free steel were investigated.Further work on dissimilar welds in DP 600 and HSLA 350 was also conducted with emphasis on tensile and fatigue properties and fracture characteristics.The performance of dissimilar spot welds was different from that of the similar spot welds in each of the HSLA350 and DP600 steels.The DP600 weld properties played a dominating role in the hardness and tensile properties of the dissimilar spot welds.However,the fatigue performance of the dissimilar welds was similar to that of the HSLA welds.Details will be presented at the conference.展开更多
Gallium nitride(GaN)field-effect transistors have low ON resistance and switching losses in high-frequency(>MHz)resonant wireless power transfer systems.Nevertheless,their performance in the system is determined by...Gallium nitride(GaN)field-effect transistors have low ON resistance and switching losses in high-frequency(>MHz)resonant wireless power transfer systems.Nevertheless,their performance in the system is determined by their characteristics and operation mode.A particular operating mode in a 6.78-MHz magnetic resonant wireless transfer system that employs class-D GaN power amplifiers in the zero-voltage switching mode is studied.Two operation modes,the forward mode and the reverse mode,are investigated.The nonideal effect under the device-level dynamic resistance and thermal effect are also analyzed.The dynamic resistance under different operation modes is demonstrated to have different generation mechanisms.Finally,the device characteristics with system operating conditions are combined,and the effects of temperature and dynamic resistance under different operating conditions are evaluated.展开更多
基金Supported by National Natural Science Foundation of China (Grant Nos.12072219,12202303,12272254)Shanxi Provincial Excellent Talents Science and Technology Innovation Project of China (Grant No.201805D211033)。
文摘The current research of sandwich structures under dynamic loading mainly focus on the response characteristic of structure.The micro-topology of core layers would sufficiently influence the property of sandwich structure.However,the micro deformation and topology mechanism of structural deformation and energy absorption are unclear.In this paper,based on the bi-directional evolutionary structural optimization method and periodic base cell(PBC)technology,a topology optimization frame work is proposed to optimize the core layer of sandwich beams.The objective of the present optimization problem is to maximize shear stiffness of PBC with a volume constraint.The effects of the volume fraction,filter radius,and initial PBC aspect ratio on the micro-topology of the core were discussed.The dynamic response process,core compression,and energy absorption capacity of the sandwich beams under blast impact loading were analyzed by the finite element method.The results demonstrated that the overpressure action stage was coupled with the core compression stage.Under the same loading and mass per unit area,the sandwich beam with a 20%volume fraction core layer had the best blast resistance.The filter radius has a slight effect on the shear stiffness and blast resistances of the sandwich beams.But increasing the filter radius could slightly improve the bending stiffness.Upon changing the initial PBC aspect ratio,there are three ways for PBC evolution:The first is to change the angle between the adjacent bars,the second is to further form holes in the bars,and the third is to combine the first two ways.However,not all three ways can improve the energy absorption capacity of the structure.Changing the aspect ratio of the PBC arbitrarily may lead to worse results.More studies are necessary for further detailed optimization.This research proposes a new topology sandwich beam structure by micro-topology optimization,which has sufficient shear stiffness.The micro mechanism of structural energy absorption is clarified,it is significant for structural energy absorption design.
文摘The conventional methods of determining the dynamic resistance were mostly done by measuring the voltage and current at secondary side of transformer in resistance welding machines, in which the measuring set-up normally interferes with the movement of electrode, and the measuring precision is influenced by inductive noise caused by the high welding current. In this study, the dynamic resistance is determined by measuring the voltage at primary side and current at secondary side. This increases the accuracy of measurement because of higher signal-noise ratio, and allows to apply to in-process system without any wires connected to electrodes.
文摘A self-developed welding dynamic arc wavelet analyzer was adopted to analyze and assess the welding process of two CO2 arc welding machines. The experimental results indicate that the instantaneous energy can reflect the influence of the welding current and voltage on dynamic arc characteristic synthetically. Through calculating and analyzing the instantaneous energy, the energy during arc ignition and short circuit in CO2 welding process can be confirmed rationally, thus the foundation for the accurate design and control of the welding current and voltage can be provided. By reducing the ripple disturbance of the dynamic resistance, avoiding peak current and voltage waveform , and enhancing the transition frequency of short circuit suitably, the stability of the welding arc and the weld appearance can be improved.
文摘The DRM （dynamic contact resistance measurement） in high voltage circuit breakers is a manner of evaluating the internal ageing condition of the chamber. DRM is similar to static contact resistance measurement testing, but instead of measuring a single value when the breaker contacts are closed （static value）, the ohmic resistance is measured at various contact positions, from the beginning of the contact opening until a complete separation of the contacts. The relationship between the contact resistances of the new circuit breaker and the ageing circuit breaker in operation provides subsidy for the evaluation of both the main and arcing contact conditions. This research aims to analyze the correlation between the various levels of degradation of the contacts and the configuration of the DRM curve. This work considers curve samples from new acceleration tests. breaker chamber contacts and different levels of degradation by
文摘<span style="font-family:Verdana;">Several studies on PV solar cells are found in</span> <span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">the </span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">literature which use static models. Those models are mainly one-diode, two-diode or three-diode models. In the dynamic modelling</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">,</span></span></span><span><span><span style="font-family:;" "=""><span style="font-family:Verdana;"> a variable parallel capacitance is incorporated. Unlike the previous studies which do not clearly establish a relationship between the capacitance and the voltage, in the present paper, the link between the capacitance and the voltage is investigated and established. In dynamic modelling investigated in this paper, the dynamic resistance is introduced in the modelling of the solar cell. It is introduced in the current-voltage characteristic. The value of the dynamic resistance is evaluated at the maximum po</span><span style="font-family:Verdana;">wer point and its effect on the maximum power is investigated. The study</span> <span style="font-family:Verdana;">shows for the first time, that the dynamic resistance must be introduced in</span> <span style="font-family:Verdana;">the current-voltage characteristic, because it has an influence on the PV cell </span><span style="font-family:Verdana;">output</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">.</span></span></span>
基金the National Natural Science Foundation of China(Grant No.52107022).
文摘In flux pumps,motors and superconducting magnets,the high temperature superconductor(HTS)coated conductor frequently carries a DC transport current when an oscillating magnetic field is present in the background.Under this circumstance,the interesting effect of dynamic resistance takes place,which can affect the operating performance of superconducting devices:heat accumulation can contribute to the rising temperature of the HTS tape and the dynamic resistance voltage can change accordingly.This article explores the time‐dependent development of the dynamic resistance voltage using a numerical modeling considering the thermal effects.After a validation against experimental results,this work investigates the effects of several factors on the structure of the HTS tape on the time‐dependent development of the dynamic resistance,thus providing insights toward a better understanding of the time‐dependent behavior of HTS tapes under external magnetic fields.
基金Projects (11202125, 61175038) supported by the National Natural Science Foundation of China
文摘In order to explore the influence of welding parameters and to investigate the Al alloy (AA) nugget formation process, a comprehensive model involving electrical-thermal-mechanical and metallurgical analysis was established to numerically display the resistance spot welding (RSW) process within multiple fields and understand the AA-RSW physics. A multi-disciplinary finite element method (FEM) framework and a empirical sub-model were built to analyze the affecting factors on weld nugget and the underlying nature of welding physics with dynamic simulation procedure. Specifically, a counter-intuitive phenomenon of the resistance time-variation caused by the transient inverse virtual variation (TIVV) effect was highlighted and analyzed on the basis of welding current and temperature distribution simulation. The empirical model describing the TIVV phenomenon was used for modifying the dynamic resistance simulation during the AA spot welding process. The numerical and experimental results show that the proposed multi-field FEM model agrees with the measured AA welding feature, and the modified dynamic resistance model captures the physics of nugget growth and the electrical-thermal behavior under varying welding current and fluctuating heat input.
基金Supported by the National Natural Science Foundation of China(51005018)
文摘Based on previous achievements,a dynamic pressure-sinkage equation for saturated clay is established.First,aquasi-static penetration rate is selected,and the ratio of the dynamic penetration rate to the quasi-static rate is used to characterize the degree of dynamic effect,then theβth power of the ratio is used to quantify the dynamic effect of sinkage.The dynamic effect exponentβis obtained using penetration tests with different penetration rates.Then,a dynamic motion resistance equation for a tracked vehicle is established based on the dynamic pressure-sinkage equation.The equation incorporates both penetration and bulldozing resistance.Finally,a series of simulation experiments with varying travel speeds and slip rates is carried out.The results show that an increase in the speed leads to stronger terrain stiffness,resulting in a decrease in sinkage and motion resistance.However,the enhancement effect becomes weaker with an increase in the travel speed.
文摘The end value of the dynamic resistance curve of stainless steel was proved to have strong correlation with nugget size by experiments, so it was an important factor for estimation of weld quality. BP neural network was employed to estimate the weld quality, The end value of the dynamic resistance curve, welding current and welding time were selected as the input variables while the nugget diameter, which is closely related to weld quality, was selected as the output variable. Testing results shows that such network has fine fault tolerance and real-time quality estimation is possible.
基金supported by National Hi-tech Research and Development Program of China(863 Program, Grant No. 2007AA03z551)Chongqing Municipal Technology Project of China (Grant No. 2007AA4008-4-4)
文摘Dent resistance of automobile body panels is an important property for automobile design and manufacture, but the study on this aspect is not still profound. This study is to summarize the testing methods and physical significations of static and dynamic dent resistance of automobile body panels combined with the author＇s study, and to analyze the dent behaviors in the round. Several influence factors on dent resistance are expatiated including the mechanical properties of materials, stress states after forming, bake hardening ability, modulus, methods of testing, and structure of specimens and so on. The automotive lightweight and application of high strength steel sheets and aluminum alloys sheets are analyzed, and the significance of testing of dent resistance, especially for dynamic dent resistance of auto-panels, and the finite element simulation analysis are emphasized. To explain the physical phenomenon of dent behaviors, the latest and concerned study results are also discussed. According to this study, a dent resistance test and evaluation standard of Society of Automotive Engineers of China for automotive body panel is presented and is being carried out, and an industry conference is hold to discuss the working-out of the standard, a primary schedule of this standard is confirmed now. The study can guide the further testing and study of dent resistant of auto-panels.
基金The project supported by the National Natural Science Foundation of China (10172070)
文摘The dynamic effective shear strength of saturated sand under cyclic loading is discussed in this paper.The discussion includes the transient time depen- dency behaviors based on the analysis of the results obtained in conventional cyclic triaxial tests and cyclic torsional shear triaxial tests.It has been found that the dy- namic effective shear strength is composed of effective frictional resistance and viscous resistance,which are characterized by the strain rate dependent feature of strength magnitude,the coupling of consolidation stress with cyclic stress and the dependency of time needed to make the soil strength sufficiently mobilized,and can also be ex- pressed by the extended Mohr-Coulomb's law.The two strength parameters of the dynamic effective internal frictional angle φd and the dynamic viscosity coefficient η are determined.The former is unvaried for different number of cyclic loading,dy- namic stress form and consolidation stress ratio.And the later is unvaried for the different dynamic shear strain rate γt developed during the sand liquefaction,but increases with the increase of initial density of sand.The generalization of dynamic effective stress strength criterion in the 3-dimensional effective stress space is studied in detail for the purpose of its practical use.
基金NZ Royal Society Marsden under Grant MFP-VUW2205New Zealand Ministry of Business,Innovation and Employment under the Advanced Energy Technology Platform program“High power electric motors for large scale transport”under Grant RTVU2004.
文摘REBCO tapes carry DC current under AC magnetic fields in proposed HTS fusion applications. AC loss will be generated in the process and it is important to understand the AC loss behaviour for safe operation of the fusion magnets. In this work, magnetisation loss (Qm), dynamic resistance (Rdyn), and total loss (Qtotal) in four different REBCO tapes are numerically studied, using the measured and , for the magnetic field amplitude applied perpendicularly up to 8 T at 20 K and 50 K, where represents the magnetic field and field angle () dependent critical current density. The peak of Theva data is different from that of other tapes. We artificially shifted the ab-plane peak of Theva to the left by 25° to match the peak value. The newly shifted data is named as Theva-shift, which was also investigated to study the influence of the Theva peak shift on AC loss. The normalised DC transport current level (i = It/Ic0) ranges from 0.05 to 0.9, where the DC current amplitude and the self-critical current of the tape are represented by It and Ic0, respectively. The simulation results show that the AC losses deviate significantly from the Brandt-Indenbom (BI) equation at high magnetic fields. Jc and instantaneous loss curves for different tapes show correlation at high magnetic fields. The simulation results also show how different characteristics for different tapes influence AC losses. When AC loss values are scaled by the self-field critical current, Qm without current and Qtotal with current in the different tapes show a good agreement. It implies that the temperature dependence of the two types of loss can be calculated from a known loss at one temperature and the self-field critical current.
基金Funded by the China Scholarship Council(201506375019)
文摘The objective of this paper was to find new modifier to improve the aging resistance and low temperature cracking resistance of asphalt. To investigate the aging resistance of modified asphalt binders, mesoporous nano-silica（doping Ti^（4＋）） was used as a asphalt modifier. Some physical properties including penetration, ductility, and softening point of asphalt were analyzed with RTFO（Rotating thin film oven） aging and ultraviolet aging. Moreover, the performances of high and low temperature of modified asphalt binders with pressure aging were tested by dynamic shear rheometer（DSR） test and bending beam rheometer（BBR） test. These results showed that the penetration decreased, low temperature ductility, and softening point increased when adding mesoporous nano-silica to base asphalt. After ultraviolet radiation aging, the penetration loss and ductility loss of modified asphalt decreased than that of original asphalt, the increase of softening point was also significantly reduced than that of base asphalt. Furthermore, The test results of DSR and BBR showed that the G＊sinδ and creep modulus‘s＇ of pressure aged asphalt decreased, but the creep rate ‘m＇ increased. It can be concluded that the aging resistance and cracking resistance of modified asphalt are improved by adding mesoporous nano-silica, especially the doping of Ti^（4＋） could improve the aging resistance obviously.
基金supporting program of the National Science Foundation for Distinguished Young Scholars of China(Project No.51325704)the State Key Program of National Natural Science of China(Project No.U1234202)。
文摘With the continuous increase of train speed,undulations of catenary and vibrations of the pantograph head result in generating pantograph- catenary arc frequently,intensifying the abrasion between pantograph strip and catenary wire,which has seriously influenced the current collection and safety of electric multi units(EMU). It is necessary to study the pantographcatenary arc in immediately. Some researchers develop a few pantograph- catenary arc testing equipment,which couldn’t really reflect the operating condition of pantograph-catenary system. In this paper,the pantograph-catenary arc test apparatus was developed,which simulated the flexible and straight contact of pantograph strip and catenary wire,based on the coupling relationship between pantograph and catenary. The equipment was used to research the electrical parameters of the pantograph-catenary arc and the dynamic contact resistance.
基金supported by State Key Lab of Advanced Welding and JoiningHarbin Institute of Technology(AWJ-20-Z02)+1 种基金Chongqing Research Program of Basic Research and Frontier Technology (CSTC 2016jcyjA0692)Fuzhou University Testing Fund of precious apparatus (No. 2019T010 and 2019T012)。
文摘Electrically assisted friction stir welding is a promising way to improve liquidity of viscoplastic metal. However, this local heat source only can take into effect in a certain contact interface status. Semi-analytical model and FEM are adopted to analyze materials flow in electrically assisted FSW. Semi-analysis results show that Joule heating is validity with rotation speed 1 400 r/min and travel speed 400 mm/min. The maximum flow velocity is 0.033 m/s when travel speed is 400 mm/min, and it increases to 0.043 m/s with current assistant. Visco-plasticizing efficiency is a key factor to decide whether Joule heat is significant, and viscoplastic flow can periodically make circuit short. Periodic torque, axial force and onion rings surface show dynamic flow process, which can be used to illustrate the dynamic contact resistance feature and correct the calculation equation of Joule heat. This paper is aim to point out the potential relation between viscoplastic flow, periodic torque and Joule heating validity.
基金supported by the National Basic Research Program of China("973"Project)(Grant No.2011CB610300)the National Natural Science Foundation of China(Grant Nos.11102148,11321062 and 51134003)the Fundamental Research Funds for Central Universities of China(Grant No.xjj2011005)
文摘The sound absorbing performance of the sintered fibrous metallic materials is investigated by employing a dynamic flow resistivity based model,in which the porous material is modeled as randomly distributed parallel fibers specified by two basic physical parameters:fiber diameter and porosity.A self-consistent Brinkman approach is applied to the calculation of the dynamic resistivity of flow perpendicular to the cylindrical fibers.Based on the solved flow resistivity,the sound absorption of single layer fibrous material can be obtained by adopting the available empirical equations.Moreover,the recursion formulas of surface impedance are applied to the calculation of the sound absorption coefficient of multi-layer fibrous materials.Experimental measurements are conducted to validate the proposed model,with good agreement achieved between model predictions and tested data.Numerical calculations with the proposed model are subsequently performed to quantify the influences of fiber diameter,porosity and backed air gap on sound absorption of uniform(single-layer)fibrous materials.Results show that the sound absorption increases with porosity at higher frequencies but decreases with porosity at lower frequencies.The sound absorption also decreases with fiber diameter at higher frequencies but increases at lower frequencies.The sound absorption resonance is shifted to lower frequencies with air gap.For multi-layer fibrous materials,gradient distributions of both fiber diameter and porosity are introduced and their effects on sound absorption are assessed.It is found that increasing the porosity and fiber diameter variation improves sound absorption in the low frequency range.The model provides the possibility to tailor the sound absorption capability of the sintered fibrous materials by optimizing the gradient distributions of key physical parameters.
基金the Natural Sciences and Engineering Research Council(NSERC)of Canada for providing financial support
文摘The application of advanced high strength steels in automotive industry has highlighted the need for research into spot weldability of these steels.Using weld lobe diagrams,the spot weldability of DP600 steel was found to be poor with conventional weld schedules.An enhanced weld schedule consisting of two pulses with reduced current on the second pulse gave a substantial increase in the lobe width;the first pulse removed the zinc coating and the second pulse controlled the nugget growth.A data acquisition system was designed to monitor weld expulsion during the weld operation.Of the three possible control strategies proposed,especially with AC welding equipment,the dynamic resistance signal is easily obtained and the least expensive.Expulsion phenomena,microstructural characterization and mechanical properties of spot-welded hot dipped galvanized DP600 steel and interstitial free steel were investigated.Further work on dissimilar welds in DP 600 and HSLA 350 was also conducted with emphasis on tensile and fatigue properties and fracture characteristics.The performance of dissimilar spot welds was different from that of the similar spot welds in each of the HSLA350 and DP600 steels.The DP600 weld properties played a dominating role in the hardness and tensile properties of the dissimilar spot welds.However,the fatigue performance of the dissimilar welds was similar to that of the HSLA welds.Details will be presented at the conference.
基金Supported by the TSV 3D Integrate Micro/Nanosystem Lab(ZDSYS201802061805105)the Natural Science Foundation of Shenzhen(JCYJ20190808155007550)Shenzhen Science Plan(JSGG20180504170016884).
文摘Gallium nitride(GaN)field-effect transistors have low ON resistance and switching losses in high-frequency(>MHz)resonant wireless power transfer systems.Nevertheless,their performance in the system is determined by their characteristics and operation mode.A particular operating mode in a 6.78-MHz magnetic resonant wireless transfer system that employs class-D GaN power amplifiers in the zero-voltage switching mode is studied.Two operation modes,the forward mode and the reverse mode,are investigated.The nonideal effect under the device-level dynamic resistance and thermal effect are also analyzed.The dynamic resistance under different operation modes is demonstrated to have different generation mechanisms.Finally,the device characteristics with system operating conditions are combined,and the effects of temperature and dynamic resistance under different operating conditions are evaluated.
