Abstract:As a typical Al-Zn-Mg-Cu series aluminum alloys, 7A52 aluminum alloy is widely used in the struc- ture of the aircrafts, rockets, light armored vehicles and ships with its excellent mechanical properties and high specific strength, which is often present in the form of weldments during practical applications. However, due to the low melting point of aluminum alloy, large heat transfer coefficient and high thermal expansion rate, when welded by conventional fusion welding process, this alloy may have such defects as weld cracks, pores and soft- ening of welded joints. Moreover, the unevenness of the welded joint is the main cause of failure in service. Therefore, how to improve the strength of the welded joint and realize the uniformity of the microstructure of the welded joint is one of the key problems in the welding of the high-strength aluminum alloy. The surface nan- otechnology is expected to solve this problem, thereby improving the overall performance of the welded joint. This paper reviews the research results at home and abroad in recent years. Based on the overview of 7A52 alu- minum alloy, it focuses on the comparison of the alloy TANDEM MIG, electron beam welding, friction stir weld- ing, laser welding, plasma welding and composite welding. At the same time, it discusses the research status of surface nanocrystallization of metal and its welded joints in details, especially elaborating the use of high-energy shot peening and ultrasonic impact treatment to achieve surface nanocrystallization and improve the current situ- ation and development trend of welding fatigue strength. It points out that improving the strength coefficient of joint is the key research direction of aluminum alloy welding in the future, and ultrasonic impact treatment tech- nology is one of the important methods to realize the nanometerization of welded joint surface.

Abstract:Though the construction scale of tramcar transportation in China becomes larger and larger, the design of track and subgrade is at the exploratory stage. Tramways with 3-4 years’ operations from three different cities in China were explored in this study. Site investigation showed that at rail -highway crossing, cracking and breakage for bituminous concrete pavement around track were easily generated, and differential settlement for pavement and track was easy to be seen. Edge damage for the pavement at the sides of the rail was also includ- ed. Settlement of rail surface in some sections resulted in scratch between the axle box and objects under the axle box. Cracking and breakage for highway pavement were generated at the top of fastener cover. Unreasonable tramway foundation stiffness transition caused the fastener loose. All the track disease inducements were ana- lyzed and corresponding prevention and control suggestions were given, such as adding precast reinforced con- crete plate wrapped with profile steel on both sides of the rail, setting the rubber block between the rail and concrete plate, matching design between tramway and pavement, and strengthening the subgrade for both tramway and highway.

Abstract:In order to reduce such accidents as the rear-end collision and side scraping when vehicles enter the exit of interchange， a theoretical calculation model of the outflow angle of deceleration lane was established to analyze the influence of stopping sight distance on the outflow angle range of interchange deceleration lane at different design speed. Furthermore， the vehicle dynamics simulation model was set up by using CarSim to sim- ulate the process of vehicles entering the exit of interchange. Finally， from the perspective of anti-sliding and anti-overturning of the vehicle， the lateral stability of the vehicle during driving was analyzed so as to verify the rationality of the calculation model of the outflow angle.

Abstract:In the field of rail transit today，maglev trains are no longer limited by the traditional wheel-rail con- tact mode of travel，which uses electromagnetic force to overcome gravity and escapes from the ground. By way of maglev technology in railway transportation，the maglev train，as the most advanced mode of transportation，has undoubtedly become an important scientific and technological object on which all countries are competing for research. In order to improve driving safety and ride comfort，scholars at home and abroad have carried out a lot of researches on the dynamic problems related to maglev trains，and have obtained important results in theoreti- cal analysis，numerical simulation and so on. This paper reviews the existing achievements in dynamic modeling， dynamic characteristics and suspension control of maglev train，and then puts forward some suggestions and prospects for further research.

Abstract:Given the fact that urban rail train impacts the traction network voltage during the starting and brak- ing and brings about the negative impact of the grid voltage instability, this paper proposes a hybrid energy stor- age system composed of a super capacitor and a lithium titanate battery, which meets the demand of urban rail trains for high energy and high power. Taking the overall structure of the hybrid energy storage system as the re- search object, the design and simulation verification of the precharging circuit were introduced, which may sig- nificantly improve the performance of the hybrid energy storage device and prolong the service life. Based on the structure, an integrated control strategy was proposed, which realized the operation of the energy storage unit in the constant current charge and discharge state by controlling the working modes of the two bidirectional DC/DC converters in the main circuit. Finally, the simulation compared the proposed control strategy with the traditional voltage-current double closed-loop control strategy. The simulation waveform comparison analysis shows that the control strategy proposed in this paper can be more effective for the fluctuation of the network voltage caused by the sudden increase and decrease of large-capacity load such as traction locomotive. The rapid adjustment of the charging and discharging state of the hybrid energy storage system ensures that the fluctuation of the traction network pressure is small, and the operational reliability and stability of the urban rail train are improved.

Abstract:In order to establish the true shape mathematical model of the rail rolling contact fatigue (RCF) crack, the true shape point cloud data of the RCF crack at the rail gauge-rail shoulder was obtained by X-ray tomog- raphy. A method was proposed to model the shape of rail RCF crack by using the grid algorithm to determine the boundary point of crack tip and opening, fitting crack tip with cubic B-spline curve and using intersection of crack surface and rail surface to determine the shape of crack opening. It was found that the number of crack boundary feature points extracted by the grid algorithm was more than that extracted by the convex hull algo- rithm. Both the bumps and the concave points representing the crack boundary feature were identified by the grid algorithm and the crack boundary distortion was smaller. As the length of the grid was gradually reduced, the number of crack boundary points extracted by the grid algorithm was further increased and the missing point problem of the convex hull algorithm was solved. The extracted crack tip boundary points were fitted by the least square method, Bezier curve and cubic B-spline curve. It found out that the crack tip obtained by the cubic B- spline curve fitting was the closest to the real crack and the maximum deviation and standard deviation of data fitting with the cubic B-spline curve were both the smallest and the mathematical model of the true crack tip shape was obtained.

Abstract:Taking the fourth Yuanshui Bridge in Changde as the construction background， the space finite ele- ment models of semi-floating system， pylon and girder rigid fixity system and rigid frame system were estab- lished respectively to investigate the serviceability of structural systems for multi-span-short tower cable-stayed bridges. The action effects under static and dynamic conditions were analyzed and compared. The results indi- cated that the distributions of load effects were fairly even under the static force and seismic action due to the structural stiffness of rigid frame system. But the internal forces in piers and towers of rigid frame system were strong with global temperature rising. There were relatively large deformations and small temperature internal forces of semi-floating system and pylon and girder rigid fixity system. And the response of each pier under braking force or seismic action was obviously different from the semi-floating system and pylon and girder rigid fixity system. The structural system of the Fourth Yuanshui Bridge is a combination of rigid frame system and pylon and girder rigid fixity system， which solves the excessive stress of the abutment pier under thermal action. Moreover， the fixed piers are divided into two adjacent legs to reduce the thrust rigidity of pier， which do not increase the foundation quantity and also achieve fine aesthetic effect.

Abstract:Roundabout is mostly located in the downtown area, which can beautify the urban landscape and alle- viate traffic. However, there is a widespread traffic jam in the ring intersection, which brings heavy traffic capac- ity in the central area of the city, and a large number of traffic flows lead to serious human and car conflicts, re- sulting in great hidden danger to the traffic safety of the roundabout. By analyzing the traffic characteristics of pedestrian crossing in road crosswalk, this paper constructs a multi-objective optimization model of signal control and selects genetic algorithm. It shows that the model can effectively improve the traffic capacity of the round- about and alleviate traffic conflicts in China.

Abstract:In view of the fact that the mean method is not sensitive to road congestion changes for the existing traffic evaluation methods, this study proposes a congestion ratio evaluation index based on real-time traffic con- dition data. Firstly, the GIS analysis and processing software was used to classify the traffic information of the real-time road condition data, and the change state of each road in a certain period of time was evaluated ac- cording to the classification value of the road condition change. Based on the traffic data of different working pe- riods in five working days in Nanjing, the ratio of congestion to traffic congestion was calculated. According to the calculation results, the spatial and temporal distribution characteristics of congested roads in Nanjing working days were analyzed and evaluated, and some feasible suggestions for avoiding blockage and slow blocking were put forward. It maintains that the evaluation method can be applied to urban traffic operation analysis and road congestion change analysis.

Abstract:In order to study the influence of vehicle entering and leaving at the exit of the parking lot on the speed of vehicles on the connecting road, this paper analyzes the influencing factors such as the size of the park- ing lot, the exit rate and the length of the entrance road. It used the time obstacle rate to reflect the impact of the parking vehicle on road traffic. By taking the parking effect as the traffic impedance of the vehicles in the section, it established the main road vehicle speed model for different types of vehicles based on the model of BPR. Then, it simulated a parking lot and connecting road using the VISSIM simulation software. Comparing the result calculated by the speed model and the simulation result, it found that the speed model can accurately de- scribe the impact of parking lot on the vehicle speed of connecting road.

Abstract:In order to improve the overall network operation efficiency of urban rail transit and its service，an in- dex which measures the integrated importance of transfer nodes is put forward. Based on this，a coordinated opti- mization model and algorithm of network operation schedule are established with the consideration of passenger demand. This model aims at reducing the passenger transfer waiting time and facilitating the transfer of passen- gers in the transfer station. Finally，the proposed method and algorithm are applied in a real rail transit to vali- date the effectiveness. The results show that after the coordination step by step，the average waiting time of the whole network has been reduced by 12.42%，saving 0.39 minutes for passengers. It proves that the model and al- gorithm can effectively reduce passenger transfer waiting time，optimize the coordination of network operation schedule，and improve the transfer efficiency of the whole network.

Abstract:In view of the limitation of the traffic environment influence on the traveling route choices, the corre- sponding traffic assignment model was built and the related augmented Lagrange multiplier method was designed. Firstly, based on the features of traffic environment influence, the constraints of limiting environment influence were divided into independent link constraint, independent node constraint and block limitation constraint. Sec- ondly, by adding the environment influence limitation into the classical user equilibrium model, the traffic as- signment model with environment influence was obtained. Through defining the generalized travel time and using the KKT conditions, the principle of selecting route corresponding to the new model was analyzed. Finally, an augmented Lagrange multiplier method embedding the Frank -Wolfe algorithm was designed to solve the new model. The numerical example verified the effectiveness of the new model and method. This research not only extends the research viewpoint of the traffic assignment theory, but also provides the theoretical support for traf- fic administrators when environment influence is considered.

Abstract:In order to study the deformation and bearing characteristics of first-order variable diameter pile under vertical load，a set of first-order variable diameter piles and equal diameter piles were selected for comparative static load test. This test made an analysis by comparing the axial force distribution curve of the pile body，the stress at the pile end and the stress variation curve at the variable section，the pile side frictional resistance a- long the pile body distribution curve and the pile top P-S curve. The results showed that the first-order vari- able-section piles had higher efficiency per unit volume of material，8% higher than that of equal diameter piles. The load transfer of the variable-section piles was still dominated by the lateral friction. When the soil layer was weak at the variable section，it softened at the variable diameter as the load increased，and the resistance at the variable diameter tended to be stable，with limited function.

Abstract:Based on the method of the design for six sigma (DFSS)， the brake pedal force and brake pedal travel was optimized in the condition of the brake deceleration speed below 0.3 g， to solve the softness of the brake pedal feel in the vehicle development. Firstly， the survey data of users’ new cars performance and the voice of customer (VOC) were used to obtain the customer concerning performance index. Secondly， the relationship be- tween the above index and the brake deceleration was acquired by means of brake performance tests. The exper- imental result was analyzed to ensure target values. Thirdly， the optimal plan for brake system was determined by development experience as well as the Pugh concept selection. Finally， the control factors and noise factors were analyzed and identified. The optimal values of the brake pedal force and brake pedal travel were obtained by orthogonal experiment. The result showed that the optimal values were well consistent with the target values， which may provide an economical and effective way to optimize brake pedal feel further.

Abstract:The graphitic carbon nitride has been widely used in energy and environmental protection owing to its excellent properties. This paper discussed the mechanism and the preparation and modification methods of graphitic carbon nitride. In addition, its applications in both pollutant removal and detection were reviewed, such as the degradation of organic pollutants, reduction of heavy metal ions and the inactivation of microorganisms. Then, it introduced the development of some photocatalytic coupling technologies of graphitic carbon nitride in water treatment. It maintains the graphitic carbon nitride application in the future is prospective, especially in the direction of preparation methods optimization, recovery efficiency improvement and combination with other technologies.

Abstract:With the increasing application of PPP financing mode in high-speed rail construction， the risk prob- lem is becoming more and more prominent. In order to evaluate the risk of high-speed railway PPP project rea- sonably in China， a comprehensive risk evaluation system of high-speed railway PPP project was established in this study. The weight of each evaluation index was calculated with AHP， and the risk evaluation model of high- speed railway PPP project was set up based on grey correlational analysis. The key risk factors were determined by the grey correlation degree ranking. Finally， the evaluation model was applied in Jinan-Qinghai high-speed railway project. The application results show that the model is reasonable and effective.

Abstract:Taking catenary as the research object，a comprehensive evaluation model of catenary health status based on variable weight matter-element extension was established. Combined with the matter-element extension theory and the relevant index standard of catenary，the classical domain of the state information of catenary was set up，and the state parameter matter-element matrix was formed. Meanwhile，using the linear combination of entropy weight method and analytic hierarchy process，the initial weight with both subjectivity and objectivity was obtained. Re-correction using the variable weight theory made the weight reveal the active participation of the evaluation object in the general evaluation process. Finally，the closeness degree function was introduced to evaluate the health level of catenary，and was checked by tests. The example proves the correctness and practi- cability of this model in comprehensive evaluation of catenary health status.

Abstract:The integrated operation of wind/thermal/pumped-storage can realize energy complementation and im- prove wind power utilization. However，the uncertainty of wind power would have a negative impact on the econ- omy and reliability of the optimal dispatch for the integration system. In order to solve this problem and minimize the overall operating cost of the system，based on the probability distribution function of wind speed，the uncer- tainty distribution probability of wind power output was obtained. The confidence interval method was used to generate wind power of the extreme scenario，and an economic dispatch model of the wind/thermal/pumped-stor- age integration system based on the extreme scenario was established. In addition，this study analyzed the relia- bility constraints of the extreme scenario，the effects of confidence interval and the pumped-storage capacity on the combined operating cost and reliability of the multi-energy units with an example application. The results showed that the proposed method could effectively solve the problem of low permeability，and it worked better in economy and reliability.