The calculation method of forming limit of sheet metals based on M-K instability theory is proposed,and the method is applicable to different yield criterions and hardening models.The forming limit dia-grams of AA5754...The calculation method of forming limit of sheet metals based on M-K instability theory is proposed,and the method is applicable to different yield criterions and hardening models.The forming limit dia-grams of AA5754-O,AA6111-T4 aluminum alloy sheet and DP steel sheet under combined loading paths are obtained based on mixed hardening model with YLD2000-2D yield criterion proposed by Barlat in 2003 and L-C nonlinear kinematic hardening model proposed by Lemaitre and Chaboche.The results show that the forming limit diagram made up of limit strain(FLD-strain) is evidently influenced by the loading path.The forming limit diagram made up of limit stress(FLD-stress) is also influenced by loading path and it is not an only curve,which differs from the conventional view.The degree of the influence of loading path on FLD-stress is related with pre-strain.The larger the pre-strain is,the greater the influence of loading path on FLD-stress will be.The change of FLD-stress is small only when pre-strain is small.In addition,the hardening behavior of the material will influence the path-dependence of FLD-stress:The larger the proportion of kinematic hardening in the whole hard-ening is,namely the more obvious Bauschinger effect of the material,the greater the influence of loading path on FLD-stress will be.展开更多
The molecular dynamics(MD)model ofα-Al_(2)O_(3) nanowires in bending is established by using LAMMPS to calculate the atomic stress and strain at different loading rates in order to study the effect of loading rate on...The molecular dynamics(MD)model ofα-Al_(2)O_(3) nanowires in bending is established by using LAMMPS to calculate the atomic stress and strain at different loading rates in order to study the effect of loading rate on the bending mechanical behaviors of theα-Al_(2)O_(3) nanowires.Research results show that the maximum surface stress−rotation angle curves ofα-Al_(2)O_(3) nanowires at different loading rates are all divided into three stages of elastic deformation,plastic deformation and failure,where the elastic limit point can be determined by the curve symmetry during loading and unloading cycle.The loading rate has great influence on the plastic deformation but little on the elastic modulus ofα-Al_(2)O_(3) nanowires.When the loading rate is increased,the plastic deformation stage is shortened and the material is easier to fail in brittle fracture.Therefore,the elastic limit and the strength limit(determined by the direct and indirect MD simulation methods)are closer to each other.The MD simulation result ofα-Al_(2)O_(3) nanowires is verified to be valid by the good agreement with the improved loop test results.The direct MD method becomes an effective way to determine the elastic limit and the strength limit of nanoscale whiskers failed in brittle or ductile fracture at arbitrary loading rate.展开更多
The stability analysis of passive bolt-reinforced rock slopes under seismic loads is investigated within the framework of the kinematic approach of limit analysis theory.A pseudo-static method is adopted to account fo...The stability analysis of passive bolt-reinforced rock slopes under seismic loads is investigated within the framework of the kinematic approach of limit analysis theory.A pseudo-static method is adopted to account for the inertial forces induced in the rock mass by seismic events.The strength properties of the rock material are described by a modified Hoek-Brown strength criterion,whereas the passive bolts are modeled as bar-like inclusions that exhibit only resistance to tensile-compressive forces.Taking advantage of the ability to compute closed-form expressions for the support functions associated with the modified Hoek-Brown strength criterion,a rotational failure mechanism is implemented to derive rigorous lower bound estimates for the amount of reinforcement strength to prevent slope failure.The approach is then applied to investigating the effects of relevant geometry,strength and loading parameters in light of a preliminary parametric study.The accuracy of the approach is assessed by comparison of the lower bound estimates with finite element limit analysis solutions,thus emphasizing the ability of the approach to properly predict the stability conditions and to capture the essential features of deformation localization pattern.Finally,the extension of the approach to account for slipping at the interface between reinforcements and surrounding rock mass is outlined.展开更多
To improve the compensation for the inherent instability in a current mode converter, the adaptive slope compensation, giving attention to the problems of the traditional compensation on compensation accuracy, loading...To improve the compensation for the inherent instability in a current mode converter, the adaptive slope compensation, giving attention to the problems of the traditional compensation on compensation accuracy, loading capability and turning jitter, is presented. Based on the analysis of current loop, by detecting the input and output voltage, converting the adaptive slope compensation current, the compensation of the current loop is optimized successfully. It can not only improve the compensation accuracy but also eliminate the over compensation, the turning jitter and the poor loading capability in the reported slope compensation. A power supply chip with adaptive slope compensation has been fabricated in a 0.35 μm CMOS process. The measurement results show that the chip starts up and operates steadily with the constant current limit under conditions of 5 V input voltage, from 10% to 100% duty cycle.展开更多
It is confirmed that tandem-blade configurations have potential to enlarge the flow turning in two-dimension(2D) studies. However, the potential of tandem blades to enlarge the design space for highly loaded axial com...It is confirmed that tandem-blade configurations have potential to enlarge the flow turning in two-dimension(2D) studies. However, the potential of tandem blades to enlarge the design space for highly loaded axial compressors was rarely investigated in open literatures. The present work aims to show the capability of tandem blades to break the loading limit of conventional blades for highly loaded compressors. The 2D models of the maximum static pressure rise derived in previous work were validated by a large amount experimental data, which showed a good agreement. An E parameter was defined to evaluate the stall margin of compressor based on the theoretical models, which indicated that the tandem blade was able to increase the loading limit of axial compressors. A single-blade stage with a loading coefficient of 0.46(based on the blade tip rotating speed) was designed as the baseline case under the guidance of the E parameter. A tandem-blade stage was then designed by ensuring that the velocity triangles were similar to the single-blade stage. The performances of both stages were investigated experimentally. The results showed that the maximum efficiency of the tandem-blade stage was 92.8%, 1% higher than the single;the stall margin increased from 16.9% to 22.3%. Besides, the maximum pressure rise of tandem rotors was beyond the loading limit of 2D single-blade cascades, which confirmed the potential of tandem blades to break the loading limit of axial compressors.展开更多
Based on 3D, steady N-S equations and k-e turbulence model, Fluent was employed to do numerical simulation for lateral aerodynamic performance of 6-axis X2K double-deck container trains with two different loading form...Based on 3D, steady N-S equations and k-e turbulence model, Fluent was employed to do numerical simulation for lateral aerodynamic performance of 6-axis X2K double-deck container trains with two different loading forms, and speed limits of the freight trains were studied. The result indicates that under wind environment： 1） As for vehicles without and with cross-loaded structure, aero-pressure on the former is bigger, but air velocity around the latter is larger; 2） When sideslip angle θ=0°, the airflow is symmetry about train vertical axis; when θ〉0°, the airflow is detached at the top of vehicles, and the air velocity increases above the separated line but decreases below it; 3） With θ increasing, the lateral force on the mid vehicle firstly increases but decreases as θ=75°; 4） When the 6-axis X2K fiat car loads empty boxes of a 40 ft and a 48 ft at 120 km/h, the overturning wind speed is 25.19 m/s, and the train should be stopped under the 12th grade wind speed.展开更多
In analyzing seismic stability of a slope with upper bound limit analysis method, the slip surface is often assumed as a log-spiral or plane slip surface. However, due to the presence of a weak layer and unfavorable g...In analyzing seismic stability of a slope with upper bound limit analysis method, the slip surface is often assumed as a log-spiral or plane slip surface. However, due to the presence of a weak layer and unfavorable geological structural surface or a bedrock interface with overlying soft strata, the preexisting slip surface of the slope may be irregular and composed of a series of planes rather than strictly logspiral or plane shape. A computational model is developed for analyzing the seismic stability of slopes with pre-existing slip surfaces. This model is based on the upper bound limit analysis method and can consider the effect of anchor bolts. The soil or rock is deemed to follow the Mohr-Coulomb yield criterion. The slope is divided into multiple block elements along the slip surface. According to the displacement compatibility and the associated flow rule, a kinematic velocity field of the slope can be obtained computationally. The proposed model allows not only calculation of the rate of external work owing to the combined effect of self-weight and seismic loading, but also that of the energy dissipation rate caused by the slip surface, interfaces of block elements and anchorage effect of the anchors. Considering a direct relationship between the rate of external work and the energy dissipation rate, the expressions of yield acceleration and permanent displacement of anchored slopes can be derived. Finally, the validity of this proposed model is illustrated by analysis on three typical slopes. The results showed that the proposed model is more easily formulated and does not need to solve complex equations or time consuming iterations compared with previous methods based on the conditions of force equilibrium.展开更多
基金Supported by the National Natural Science Foundation of China (Grant No. 50475004)
文摘The calculation method of forming limit of sheet metals based on M-K instability theory is proposed,and the method is applicable to different yield criterions and hardening models.The forming limit dia-grams of AA5754-O,AA6111-T4 aluminum alloy sheet and DP steel sheet under combined loading paths are obtained based on mixed hardening model with YLD2000-2D yield criterion proposed by Barlat in 2003 and L-C nonlinear kinematic hardening model proposed by Lemaitre and Chaboche.The results show that the forming limit diagram made up of limit strain(FLD-strain) is evidently influenced by the loading path.The forming limit diagram made up of limit stress(FLD-stress) is also influenced by loading path and it is not an only curve,which differs from the conventional view.The degree of the influence of loading path on FLD-stress is related with pre-strain.The larger the pre-strain is,the greater the influence of loading path on FLD-stress will be.The change of FLD-stress is small only when pre-strain is small.In addition,the hardening behavior of the material will influence the path-dependence of FLD-stress:The larger the proportion of kinematic hardening in the whole hard-ening is,namely the more obvious Bauschinger effect of the material,the greater the influence of loading path on FLD-stress will be.
基金the National Natural Science Foundation of China(No.12162010)the Science Technology Base and Talent Special Project of Guangxi,China(No.AD19245143)Natural Science Foundation of Guangxi,China(No.2021GXNSFAA220087).
文摘The molecular dynamics(MD)model ofα-Al_(2)O_(3) nanowires in bending is established by using LAMMPS to calculate the atomic stress and strain at different loading rates in order to study the effect of loading rate on the bending mechanical behaviors of theα-Al_(2)O_(3) nanowires.Research results show that the maximum surface stress−rotation angle curves ofα-Al_(2)O_(3) nanowires at different loading rates are all divided into three stages of elastic deformation,plastic deformation and failure,where the elastic limit point can be determined by the curve symmetry during loading and unloading cycle.The loading rate has great influence on the plastic deformation but little on the elastic modulus ofα-Al_(2)O_(3) nanowires.When the loading rate is increased,the plastic deformation stage is shortened and the material is easier to fail in brittle fracture.Therefore,the elastic limit and the strength limit(determined by the direct and indirect MD simulation methods)are closer to each other.The MD simulation result ofα-Al_(2)O_(3) nanowires is verified to be valid by the good agreement with the improved loop test results.The direct MD method becomes an effective way to determine the elastic limit and the strength limit of nanoscale whiskers failed in brittle or ductile fracture at arbitrary loading rate.
基金financial support from Ecole des Ponts et Chaussées-ParisTech(France)the French Institute of Tunisia (French Embassy-Tunisia)Laboratoire de Génie Civil (ENIT) through project SSHN2015-ENPC/ENIT
文摘The stability analysis of passive bolt-reinforced rock slopes under seismic loads is investigated within the framework of the kinematic approach of limit analysis theory.A pseudo-static method is adopted to account for the inertial forces induced in the rock mass by seismic events.The strength properties of the rock material are described by a modified Hoek-Brown strength criterion,whereas the passive bolts are modeled as bar-like inclusions that exhibit only resistance to tensile-compressive forces.Taking advantage of the ability to compute closed-form expressions for the support functions associated with the modified Hoek-Brown strength criterion,a rotational failure mechanism is implemented to derive rigorous lower bound estimates for the amount of reinforcement strength to prevent slope failure.The approach is then applied to investigating the effects of relevant geometry,strength and loading parameters in light of a preliminary parametric study.The accuracy of the approach is assessed by comparison of the lower bound estimates with finite element limit analysis solutions,thus emphasizing the ability of the approach to properly predict the stability conditions and to capture the essential features of deformation localization pattern.Finally,the extension of the approach to account for slipping at the interface between reinforcements and surrounding rock mass is outlined.
基金Project supported by the National Defense Pre-Research Project of China(No.51308010610)
文摘To improve the compensation for the inherent instability in a current mode converter, the adaptive slope compensation, giving attention to the problems of the traditional compensation on compensation accuracy, loading capability and turning jitter, is presented. Based on the analysis of current loop, by detecting the input and output voltage, converting the adaptive slope compensation current, the compensation of the current loop is optimized successfully. It can not only improve the compensation accuracy but also eliminate the over compensation, the turning jitter and the poor loading capability in the reported slope compensation. A power supply chip with adaptive slope compensation has been fabricated in a 0.35 μm CMOS process. The measurement results show that the chip starts up and operates steadily with the constant current limit under conditions of 5 V input voltage, from 10% to 100% duty cycle.
基金the support of National Natural Science Foundation of China(Nos.51806004 and 51790511)National Science and Technology Major Project,China(No.2017-Ⅱ-0001-0013)。
文摘It is confirmed that tandem-blade configurations have potential to enlarge the flow turning in two-dimension(2D) studies. However, the potential of tandem blades to enlarge the design space for highly loaded axial compressors was rarely investigated in open literatures. The present work aims to show the capability of tandem blades to break the loading limit of conventional blades for highly loaded compressors. The 2D models of the maximum static pressure rise derived in previous work were validated by a large amount experimental data, which showed a good agreement. An E parameter was defined to evaluate the stall margin of compressor based on the theoretical models, which indicated that the tandem blade was able to increase the loading limit of axial compressors. A single-blade stage with a loading coefficient of 0.46(based on the blade tip rotating speed) was designed as the baseline case under the guidance of the E parameter. A tandem-blade stage was then designed by ensuring that the velocity triangles were similar to the single-blade stage. The performances of both stages were investigated experimentally. The results showed that the maximum efficiency of the tandem-blade stage was 92.8%, 1% higher than the single;the stall margin increased from 16.9% to 22.3%. Besides, the maximum pressure rise of tandem rotors was beyond the loading limit of 2D single-blade cascades, which confirmed the potential of tandem blades to break the loading limit of axial compressors.
基金Project supported by Scholarship Award for Excellent Doctoral Student granted by Ministry of Education,ChinaProject(2012QNZT029) supported by the Fundamental Research Funds for the Central Universities of China+1 种基金Project(CX2010B122) supported by Hunan Provincial Innovation Foundation for Postgraduate,ChinaProject(2010ybfz088) supported by the Foundation of Excellent Doctoral Dissertation of Central South University,China
文摘Based on 3D, steady N-S equations and k-e turbulence model, Fluent was employed to do numerical simulation for lateral aerodynamic performance of 6-axis X2K double-deck container trains with two different loading forms, and speed limits of the freight trains were studied. The result indicates that under wind environment： 1） As for vehicles without and with cross-loaded structure, aero-pressure on the former is bigger, but air velocity around the latter is larger; 2） When sideslip angle θ=0°, the airflow is symmetry about train vertical axis; when θ〉0°, the airflow is detached at the top of vehicles, and the air velocity increases above the separated line but decreases below it; 3） With θ increasing, the lateral force on the mid vehicle firstly increases but decreases as θ=75°; 4） When the 6-axis X2K fiat car loads empty boxes of a 40 ft and a 48 ft at 120 km/h, the overturning wind speed is 25.19 m/s, and the train should be stopped under the 12th grade wind speed.
基金financially supported by the NSFC-ICIMOD joint project(41761144077)the Light of West“Belt&Road”international cooperation team project of Chinese Academy of Sciences(Su Lijun)+1 种基金the Hundred Talents Program of Chinese Academy of Sciences(Su Lijun)the NSFC(National Natural Science Foundation of China)project(51278397)
文摘In analyzing seismic stability of a slope with upper bound limit analysis method, the slip surface is often assumed as a log-spiral or plane slip surface. However, due to the presence of a weak layer and unfavorable geological structural surface or a bedrock interface with overlying soft strata, the preexisting slip surface of the slope may be irregular and composed of a series of planes rather than strictly logspiral or plane shape. A computational model is developed for analyzing the seismic stability of slopes with pre-existing slip surfaces. This model is based on the upper bound limit analysis method and can consider the effect of anchor bolts. The soil or rock is deemed to follow the Mohr-Coulomb yield criterion. The slope is divided into multiple block elements along the slip surface. According to the displacement compatibility and the associated flow rule, a kinematic velocity field of the slope can be obtained computationally. The proposed model allows not only calculation of the rate of external work owing to the combined effect of self-weight and seismic loading, but also that of the energy dissipation rate caused by the slip surface, interfaces of block elements and anchorage effect of the anchors. Considering a direct relationship between the rate of external work and the energy dissipation rate, the expressions of yield acceleration and permanent displacement of anchored slopes can be derived. Finally, the validity of this proposed model is illustrated by analysis on three typical slopes. The results showed that the proposed model is more easily formulated and does not need to solve complex equations or time consuming iterations compared with previous methods based on the conditions of force equilibrium.
