查看更多>>摘要：For three kinds of Aluminum-Silicon (Al-Si) alloy piston materials with different alloying element contents, the parameters such as thermal diffusion coefficient, specific heat capacity and thermal conductivity were measured, and the effects of Cu and Ni on the thermal properties of the ma-terials were studied. Under the same processing conditions, three kinds of engine aluminum alloy pistons of different materials were manufactured. The piston surface temperature under the rated working condition of the diesel engine was measured by the hardness plug temperature cali-bration method. The distribution of piston surface temperature field was simulated by ANSYS finite element analysis, and the surface temperature analysis results of key parts of the piston were obtained. The results show that the finite element simulation results of piston surface temperature field are consistent with the temperature measurement results of hardness plug test, and the temperature change of key specific locations of piston possesses the similar trend. The thermal conductivity of aluminum alloy piston material increases with the increase of tempera -ture. There is an obvious temperature gradient from the piston head to the piston skirt. High thermal conductivity can reduce the surface temperature of the piston head. The addition of alloying elements such as Cu and Ni will reduce the thermal conductivity of aluminum alloy materials and increase the surface temperature of piston. Therefore, proper controlling of the content of alloying elements can control the heat resistance and high temperature performance of piston materials.
查看更多>>摘要：The clamping force of the bolted joint is a key factor in maintaining its service performance. However, when the torque wrench is removed after tightening, there will always be a certain degree of loss in the pre-tightening force. Herein, the stress state of the bolt and the nut during tightening and service was analyzed. The repeatability experiment of the threaded fasteners during repeated tightening was carried out to investigate the pre-tightening force loss after the removal of torque wrench (initial loss) and the pre-tightening force loss in a period of service (service loss). Different forms of lubrication was considered, namely without lubrication, thread lubrication and nut surface lubrication. The influence of the pre-tightening force, repeated tightening and lubrication on the initial loss and the service loss was analyzed according to the experimental results. It showed that the pre-tightening force and its loss have a large dispersion. At the first tightening, the pre-tightening force may increase after the initial loss stage. When the lubricant is used, the service loss has a significant positive correlation with its pre-tightening force. The use of lubricant at the threads is easier to lead the service loss of pre-tightening force.
查看更多>>摘要：The gear system is an essential component of railway vehicles. However, wheel flat often affects the gear system's fault diagnosis and fatigue life and further jeopardizes vehicle operation safety. This paper develops a railway vehicle model equipped with a helical gear system. The model validation uses the experimental data under various operating conditions. The discussion of the simulation results involves the time-domain and time-frequency domain responses of the gear system of a vehicle with the wheel flat under variable speed conditions. Simultaneously, the analysis of its regularity uses the peak-to-peak value as characteristic vibration indicators. The results show that the wheel-rail contact status has two regions: the continuous contact region and the intermittent contact region. The vibration of the gear system is relatively stable in the continuous contact region. However, it fluctuates sharply in the intermittent contact region. In the time-frequency domain results, the fundamental frequency of the wheel flat and its multiplier gradually replaces the healthy gear meshing frequency and its multiplier as the primary contributor to energy. Furthermore, the peak-to-peak value shows that the gear system vibrates significantly under critical conditions. The above results can help improve the gear system's stability and thus the running stability and safety of railway vehicles.
查看更多>>摘要：In this investigation, it is found that the generated elastic stress during the unconstraint vibration treatment process can act as the dislocation driving stress on the dislocation in the micro-scale. When the sum of the dislocation driving stress, the image stress and the back stress exceeds the motion resistance of the dislocations in a pile-up, the dislocations begin to reverse back, and the pile-up disappear. Based on this principle, a fatigue damage-healing model under unconstraint vibration condition was proposed. The verified results showed that the damaged copper film can be effectively healed by the unconstraint vibration guided by the model calculation.
查看更多>>摘要：The tempered laminated windshield used in high speed train is different with that used in auto. This work aims to study the crack propagation and dynamic impact responses of tempered laminated windshield. To achieve this point, a cohesive element based finite element model that take residual stress into consideration was built and validated by safety hammer impact experiment. Afterwards, the crack features and propagation process were characterized. The simulation results show that the fracture fronts in the interior tensile zone is ahead of that in the near surface compression zone and the fragment is much smaller than that of float glass, which demonstrate residual stress has significant influence on fracture mechanism and crack modes. And then, a parametric discussion about the influence of impact velocity and thickness of tempered laminated glass's components on the impact performances were carried out and correlated. The results show that the impact responses are velocity sensitivity. The front glass layer (G1) has more significant effects on the energy absorption and peak acceleration comparing with back glass layer (G2). In practical engineering application, increasing the thickness of PVB film appropriately could not only reduce peak acceleration but also consume more impact kinetic energy.
查看更多>>摘要：The valve serves as a pipeline attachment to relieve pressure, adjust and control the conveying medium. It is essential component in the fluid delivery system. As the fluid passes through the valve, the change of flow area results in dramatic changes in the fluid pressure, which leads to the occurance of cavitation and inevitably causes the damages to the valve body and the pipeline. Therefore, suppressing cavitation and minimizing its adverse impact are still of an important subject. In this study, the different throttling structures in an perforated cage are used to control flow. The effect on cavitation and the uniformity of flow are numerically investigated. The results show that the types of throttling stage clearly influence the intensity of cavitation. The different arranged types with the same number of stage and orifice diameter have the different location of cavitation. The forward two-stage throttling has the most severe cavitation in the downstream of the cage. In addition, the flow characteristics and the pressure uniformity are quite different with various throttling structures. Finally, some suggestions are given in the choice of a valve in the practical applications.
查看更多>>摘要：Coiled tubing life is predominantly limited by fatigue as it undergoes cyclic bending strains well into the plastic regime leading to cyclic ratchetting due to plastic strain accumulation. Furthermore, small defects occurring in the oil field environment are capable of reducing coiled tubing fatigue life significantly. Small defects are macroscopic flaws that have small dimensions relative to the tubing wall thickness. Very shallow flaws at depths of only a few percent of the wall thickness can have a substantial, detrimental influence on fatigue life. Accordingly, the influence of surface ball defects on the magnitude of the fluctuating strains at the root of these defects was the subject of this investigation. Accurate cyclic strain magnitude predictions are essential to ultimately quantify the fatigue life of coiled tubing. The effects of defect radius and depth were both studied, and the strains at the notch root were evaluated for coiled tubing having a diameter of 2.375 in. and a wall thickness of 0.156 in.. A matrix of ball defects with various depths and radii were examined. The defects were introduced at the surface of a model having the same thickness as the actual coiled tubing. The FEA results provided detailed information about the magnitude of cyclic strains at the root of a defect in coiled tubing that were previously unavailable. The notch root strain results were used to bring about a modified Neuber's rule since conventional notch strain analysis approaches did not lead to accurate notch root strain range predictions. A new modified rule was therefore proposed and validated based on a detailed statistical analysis. This modified rule led to a substantial reduction in the percentage difference between the numerical notch root strain range and the estimated analytical one to within 8%.
查看更多>>摘要：Potential-induced degradation (PID) causes a significant drop in the efficiency of a photovoltaic (PV) module, which results in module failure before the expected lifetime. Electroluminescence (EL) imaging has been a commonly used technique in laboratory and field studies for the early diagnosis of PID. This article presents the analysis and insight of EL imaging in qualitative and quantitative assessment of PID. Circuit analysis is presented to discuss the method of visually inspecting defects and the role of EL current in diagnosing PID. The performance of the conventional pattern-based PID detection method suggested in IEC standards is experimentally evaluated. Results show that low current EL imaging acquires characteristic PID pattern early hence useful in the simple monitoring of PID progression. Pattern-based detection becomes unreliable when PID progresses uniformly, or a module contains other shunting defects, which shows a similar appearance to PID in the EL images. Also, visually inspecting PID from the EL images is the sluggish approach when the aim is to detect PID early before it results in significant power loss. The potential of EL imaging in quantifying the impact of PID on module performance is also discussed. It reveals that the use of a high EL current can give maximum PID power loss estimation in the range of 25-75%. This work gives new insights that would help to develop EL methods for effectively diagnosing PID in early stages and preventing module failure.
查看更多>>摘要：The ballasless track has been massively applied nowadays, while the interface damage evolution of ballastless tracks is still rarely investigated. This work aims to investigate the damage evolution property of ballastless track under low-cycle fatigue load condition. The application of cyclic cohesive model as a key approach to simulated the damage evolution of ballastless track interface. Therefore, a low-cycle fatigue cohesive model (LCFC) that follows a bilinear law is established by combining and modifying existed cohesive models. The LCFC model is then implemented by a secondary development in Abaqus through implicit user-defined material subroutine (UMAT), and the validity of the model is verified though a double cantilever beam (DCB) model. To obtain the key parameters of the LCFC model for analyzing the fatigue damage of CRTS III (Chinese Railway Track System type III) ballastless track, the split tension test of specimen that is composed of the self-compacting concrete and the slab concrete is conducted, and the damage evolution process is captured through digital image correlation (DIC) technique. Further, a simulation model that involves the LCFC model is created for simulating and analyzing the interface fatigue damage of CRTS III- ballastless track under cyclic temperature gradient for the first time. The results show that the established LCFC model is suitable for studying the interface fatigue damage of CRTS III ballastless track under extreme temperature gradient cycles. The interface near the slab edge will be completely damaged within only several cycles when the cyclic temperature gradient is +/- 61 degrees C/m. The failure mode of the interface element is a kind of mixed mode failure which gradually changes from normal-dominated failure mode to shear-dominated failure mode with the increase of temperature gradient loading cycles.
查看更多>>摘要：Under temperature loading, the contact interface of cement asphalt mortar (CAM) layer and Tshaped joint are two vulnerable components of China Railway Track System type II (CRTS II) slab ballastless track. The purpose of this study is to develop six types of three-dimensional (3D) nonlinear finite element (FE) models for investigating the thermal behaviors of CAM and Tshaped joint under various temperature conditions. This was achieved by modelling CAM and Tshaped joint using cohesive zone model (CZM) and concrete damaged plasticity model (CDPM), respectively. Using five CRTS II slab tracks on a simple-supported box girder as a case study, the displacement and stress responses of the track structure were investigated and compared under six cases of overall temperatures and temperature gradients, respectively. Results show that CZM and CDPM have the capabilities of simulating the interlaminar debonding and damage behavior development in CRTS II track under various temperatures conditions. The combination of CZM and CDPM could provide the worst damage scenarios, and the CDPM of the joint could significantly reduce the structure responses. In addition, the increase of overall temperature together with temperature gradient could significantly increase the deformation and stress in CAM layer, and the influence of the magnitude of overall temperature is more obvious than that of temperature gradient.
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Elsevier