滑动轴承二维动态摩擦接触的分析研究

应用边界单元法研究了滑动轴承动态摩擦时的接触力学性质,揭示了接触压力分布的不对称性;讨论了载荷,摩擦系数等因素对不对称性的影响,认为摩擦系数可使峰值接触压力降低和接触面积增加,增大载荷会使接触压力分布的不对称性更为明显,接触压力分布的不对称意味着磨损分布的不对称。同时分析了实际工作中滑动轴承摩擦副间的当量摩擦系数,指出在滑动试验研究中应采了当量摩擦系数。     

PHASE FIELD SIMULATION OF DOMAIN SWITCHING IN FERROELECTRIC SINGLE CRYSTAL WITH ELECTRICALLY PERMEABLE AND IMPERMEABLE CRACKS

Domain switching around electrically permeable and impermeable cracks in ferroelectric single crystals subjected to a mechanical load is investigated by using a phase field model.It is found that the e...     

基于虚拟材料的T-B梁拓扑优化ESO方法

自从Zhou和Rozvany于2001年以T-B梁(Tie-Beam)问题质疑渐进结构优化方法ESO(Evolutionary Structural Optimization)的算法收敛性以来,结构最优化领域的研究者提出了多种算法以求解决这一问题。T-B梁问题仍是当前结构拓扑优化领域的研究热点,因为现有的各种T-B梁解法或是存有不足或是无法获得满意解答。本文在传统ESO算法中,将虚拟材料引入待删除单元,用以检测结构传力路径是否受到破坏,进而确定单元删除的合理性。算例表明,本文算法可有效防止ESO方法求解T-B梁问题时的失效并获得最优解,且本文算法只需在ESO迭代中附加一次检测,不改变ESO方法的迭代进程和寻优能力。     

高锁螺栓连接件动态拉伸响应与失效机理

飞机坠撞过程中结构的变形模式和吸能对乘员保护具有重要意义，而连接结构的载荷传递和失效形式是影响飞机结构变形的重要因素之一。为了获取航空高锁螺栓连接件在坠撞载荷下的动态响应和失效机理，基于抗剪型平头高锁螺栓设计了2种材料（2024-T3和7050-T7451）的单钉单搭接连接件，利用高速液压伺服材料试验机进行4种速度（0.01、0.10、1.00和3.00 m/s）下的拉伸测试，得到连接件的动态响应、极限载荷、能量吸收和失效模式随速度的变化规律，并分析了连接件的失效机理。结果表明，连接件的失效模式受母材和高锁螺栓/螺母材料强度影响较大，而受加载速度影响较小；当速度从0.01 m/s增加到3.00 m/s时，2024-T3连接件的极限载荷和能量吸收分别增加了2.17%和34.43%，7050-T7451连接件的极限载荷和能量吸收分别增加了5.53%和6.58%。     

On the crushing of aluminum open-cell foams: Part I. Experiments

This two-part study is concerned with the understanding and modeling of the compressive response of open-cell metallic foams. Part I presents experimental results from Al-6101-T6 foams of three different cell sizes with relative densities of about 8%. X-ray tomography is first used to characterize the geometry of the microstructure. The cells are irregular polyhedra of nearly uniform size that are somewhat elongated in one direction. The ligaments are nearly straight with convex, three-sided cross-sections and variable area distribution along their length. Foam specimens were compressed at slow displacement rates along the rise and transverse directions and the evolution of crushing in the specimens was monitored using X-ray tomography. In both directions, the response is initially nearly linear, terminating into a limit load that is followed by an extensive load plateau. At an average strain of about 55% the load increases monotonically again due to densification. The limit load is caused by plastification due to combined compression and bending of the ligaments. Beyond this point, cells start to buckle and collapse locally, forming bands that cover the full cross-section of the specimen. Contact of the collapsing cells arrests local deformation triggering collapse in neighboring cells. In this manner, crushing gradually spreads throughout the specimen and when this is achieved the load required for further deformation starts to rise. The initial elastic modulus, the stresses at the limit load and the plateau and the extent of the plateau have been measured as a function of relative density for both directions. The stress–displacement response in the transverse direction is generally somewhat lower than in the rise direction but the prevalent events were found to be similar in the two directions.     

侵彻过程中弹引螺纹连接结构振动特性研究

针对侵彻过程中的弹引系统,对弹引螺纹连接结构振动特性进行了研究,建立了弹引螺纹连接结构弹性模型。在模型中,充分考虑了螺纹载荷分布不均匀的特性,不但给出了螺纹载荷分布规律,还给出了螺纹连接结构的等效刚度和振动频率;同时,为了验证模型的正确性,对弹引螺纹连接结构的拉伸和冲击过程进行了有限元仿真和试验,分别通过对弹引系统各结构振动特性的计算和对实测过载信号进行时频分析得到了系统的频率特性;将弹引系统的振动频率与实测过载信号的时频分析结果进行了对比。分析计算和试验结果发现:与静载荷时相比,冲击载荷作用下第一扣螺纹承受的载荷更大;螺纹连接结构的刚度明显小于固连结构;增加螺纹材料刚度、增加螺纹旋合长度、减小螺距能够有效增加螺纹连接结构固有频率;在侵彻过载测试信号的时频分析结果中明显存在与螺纹连接结构的振动频率一致的振动信号,并且该频率成分的信号幅值很高,对过载信号影响很大。     

圆筒装置内爆炸压力载荷特性实验研究

为了研究密闭环境压力载荷波形特征和分布规律，开展了长径比2∶1圆筒装置的TNT装药内爆炸实验，获取了圆筒壁面、端盖板表面的压力载荷数据，结合仿真计算结果，研究了圆筒壁面、端盖板上压力载荷波形特征和形成机理，建立了压力载荷峰值的表征模型，并开展了模型验证。结果表明，圆筒壁面与盖板压力载荷波形不完全相同，壁面压力载荷按照爆心距由近到远，从显著的单波峰向多峰值变化，在近场区域压力载荷衰减特性与一定炸高自由地面冲击波类似；盖板上压力载荷呈多波峰特点，中心区域压力载荷最大峰值大于首峰值的3倍，角隅区域大于首峰值6倍；圆筒壁面上的压力载荷最大峰呈现凹型分布规律。该研究结果可为密闭环境内爆载荷的特性分析和结构毁伤评估提供参考。     

An investigation of the heat transfer and static pressure on the over-tip casing wall of an axial turbine operating at engine representative flow conditions. (II). Time-resolved results

This article reports the measurements of time-resolved heat transfer rate and time-resolved static pressure that have been made on the over-tip casing of a transonic axial-flow turbine operating at flow conditions that are representative of those found in modern gas turbine engines. This data is discussed and analysed in the context of explaining the physical mechanisms that influence the casing heat flux. The physical size of the measurement domain was one nozzle guide vane-pitch and from −20% to +80% rotor axial chord. Additionally, measurements of the time-resolved adiabatic wall temperature are presented. The time-mean data from the same set of experiments is presented and discussed in Part I of this article. The nozzle guide vane exit flow conditions in these experiments were a Mach number of 0.93 and a Reynolds number of 2.7 × 10⁶ based on nozzle guide vane mid-height axial chord. The data reveal large temporal variations in heat transfer characteristics to the casing wall that are associated with blade-tip passing events and in particular the blade over-tip leakage flow. The highest instantaneous heat flux to the casing wall occurs within the blade-tip gap, and this has been found to be caused by a combination of increasing flow temperature and heat transfer coefficient. The time-resolved static pressure measurements have enabled a detailed understanding of the tip-leakage aerodynamics to be established, and the physical mechanisms influencing the casing heat load have been determined. In particular, this has focused on the role of the unsteady blade lift distribution that is produced by upstream vane effects. This has been seen to modulate the tip-leakage flow and cause subsequent variations in casing heat flux. The novel experimental techniques employed in these experiments have allowed the measurement of the time-resolved adiabatic wall temperature on the casing wall. These data clearly show the falling flow temperatures as work is extracted from the gas by the turbine. Additionally, these temperature measurements have revealed that the absolute stagnation temperature within the tip-gap flow can be above the turbine inlet total temperature, and indicates the presence of a work process that leads to high adiabatic wall temperatures as a blade-tip passes a point on the casing wall. It is shown that this phenomena can be explained by consideration of the flow vectors within the tip-gap, and that these in turn are related to the local blade loading distribution. The paper also assesses the relative importance of different time-varying phenomena to the casing heat load distribution. This analysis has indicated that up to half of the casing heat load is associated with the over-tip leakage flow. Finally, the implications of the experimental findings are discussed in relation to future shroudless turbine design, and in particular the importance of accounting for the high heat fluxes found within the tip-gap.     

An experimental validation of load distribution in screw threads

The thread load distribution has been examined, as is known, in literature both theoretically and experimentally. in the present paper the load distribution is validated by strain-gage measurements. Starting from the theoretical load distribution the stresses on the outer surface of the female member of a threaded connection are calculated. The theoretical and experimental stress values obtained are reasonably close.     

天然关节软骨与医用不锈钢摩擦磨损行为研究

采用天然关节软骨与不锈钢摩擦副在往复运动试验机上进行关节软骨的摩擦学试验研究,探讨载荷、速度、润滑和作用时间对摩擦磨损行为的影响并分析其作用机理,并对摩擦磨损前后的软骨表面进行分析.结果表明:随着载荷从10 N增至22 N,软骨与不锈钢间的摩擦系数从0.147降至0.117;在同样的润滑和压力下,速度越大软骨和不锈钢的摩擦系数越小;透明质酸溶液可以有效降低软骨与不锈钢之间的摩擦,长时间试验后摩擦系数基本保持在0.23左右.试验后软骨表面出现磨损并伴有大量磨损颗粒,表面有明显的划痕出现,磨粒的粒径大小分布范围较窄,小尺寸的磨粒数目较多.     

Two-dimensional fractal-like finite element method for thermoelastic crack analysis

The fractal-like finite element method has been proved to be very efficient and accurate in two-dimensional static and dynamic crack problems. In this paper, we extend our previous study to include the thermal effect for two-dimensional isotropic thermal crack problems. Both the temperature intensity factor and thermal stress intensity factor can be calculated directly. The temperature distribution is first found, which is imposed thereafter as a thermal load in the elastic problem. The transformation function used in the study has been found analytically. The effects of different thermal loading on the thermal stress intensity factor are presented. The numerical examples are compared with the results from other methods and find to be in good agreement.     

Nondestructive shell-stability estimation by a combined-loading technique

Accurate nondestructive procedures for cylindrical-shell-stability investigations have long been a goal of experimentalists and practicing engineers. Stability criteria based on stress and deflection have been investigated intensively by researchers, but lateral-stiffness variation has been largely ignored. It is this aspect which is the foundation of the present work. The variation of specimen lateral stiffness with compressive axial loading is studied experimentally and it is demonstrated that buckling loads can be predicted for monocoque shells from such data. A study of the initial specimen geometry is presented and an evaluation is made of its effect on the distribution of stiffness at zero axial load. When the lateral test force used to termine the stiffness is considered as a destabilizing load in combination with the axial compression, it is shown that critical values of the force can be estimated by the “Southwell plat” procedure. These critical forces can be correlated with axial load and extrapolated to yield an accurate estimate of the buckling load. The approach greatly reduces the compressive load necessary for stability predictions over that required for other techniques.     

A method of torque control for independent wheel drive vehicles on rough terrain

Our previous research has revealed that, for vehicles with independently driven wheels, a torque distribution based on the ratio of the vertical load of each wheel to the total vehicle load is efficient for driving on flat ground. In this research, this method of torque distribution was extended to electric off-road vehicles driving on rough ground. In order to examine the driving efficiency of these vehicles, a numerical vehicle model was constructed in the pitch plane. Simulations using the numerical vehicle model on rough ground were conducted with a proposed torque distribution and control method. The numerical results from these simulations were compared with those of a conventional vehicle to evaluate the driving efficiency and trafficability on ground with various profiles. A comparison between the simulations demonstrated that the proposed method of torque distribution to the front and rear wheels based on the ratio of the vertical load is efficient for driving on rough ground.     

Stress concentrations in bolt-thread roots

The apparent geometric origin of fatigue fractures in oversize shank bolts is considered in this report. Bolt-nut interaction between the bolt and both a standard nut and a special nut are investigated. The “frozen stress” method of three-dimensional stress analysis was used to determine the magnitudes and locations of points of stress concentration in plastic models of both the oversize shank bolt and a standard bolt in combinations with standard and special nuts. The results indicate that, for installations in which the special nut is used, bolt geometry may be the controlling factor in determining the fatigue life of the boltnut combination, because the stress-concentration factors due to the nut loading are of the same order of magnitude as the stress-concentration factors due solely to bolt geometry. On the other hand, for installations in which standard nuts are used, the nut-induced stress-concentration factors are much larger than those due to bolt geometry; so, bolt geometry will have less effect on the fatigue performance of the combination. If the nut is threaded onto the bolt so far that the first loaded thread of the nut is in the thread-runout area of the bolt, fatigue performance of the installation will be greatly reduced due to the combination of stress-concentration factors from thread loading and section change and, again, bolt geometry may be a prominent causative factor.     

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摘 要：利用大型通用有限元软件ANSYS10.0，首先对钢筋混凝土试件梁采用了三维的建模方式，然后对其抗弯性能进行了非线性数值模拟，所模拟的量包括各临界荷载、挠度、应力分布、中和轴的移动等，模拟结果与实测值吻合得很好。该方法可以用来分析钢筋混凝土梁板构件的承载能力，也可以应用在某些院校的教学中。     

An instrumented fastener for shear force measurements in joints

A preliminary investigation has been conducted on instrumented fasteners for use as sensors to measure the shear loads transmitted by individual fasteners installed in double-splice joints. Calibration and load verification tests were conducted for instrumented fasteners installed at three fastener torque levels. Results from calibration tests show that the shear strains obtained from the instrumented fasteners vary linearly with the applied load and that the instrumented fasteners can be effectively used to measure shear loads transmitted by individual fasteners installed in double-splice joints. Tests were also conducted with three instrumented fasteners installed in a typical double-splice joint. The test results showed that the load distribution between individual fasteners is dependent on the location of the fastener in the joint and the fastener torque level. The fastener located near the end of the joint with the single plate carried more load than the fasteners located near the end of the joint with the two plates. Installing the fasteners with a torque greater than finger tight results in a significant amount of the load being carried by friction between the faying surfaces of the plates even if the faying surfaces are polished and lubricated. Increasing the fastener torque increases the load being carried by friction between the faying surfaces of the joint. Increasing friction between the faying surfaces of the joint. Increasing the fastener torque also results in a more uniform distribution of the loads between the individual fasteners for joints in aluminum plates with two fasteners, but does not have a significant effect for joints in steel plates with three fasteners. Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11.     

开槽对大跨悬挑平屋盖结构风荷载的影响

本文结合一大跨悬挑平屋盖模型的风洞试验研究，分析了此类结构屋盖的平均局部体型系数和极值局部体型系数的分布特性；并根据其风压的分布特性，提出在悬挑屋盖承受较大负压的屋檐和角点附近这些遇强风易发生破坏的部位采取开槽气动抗风措施，进一步研究了该气动抗风措施对大跨悬挑平屋盖结构风荷载的影响，试验结果显示，它们可以有效地削减屋盖风敏感处的风荷载值，使平均局部体型系数大约降低50％，极值局部体型系数大约降低25％，所得的这些结果对于结构的抗风设计有一定的指导意义。     

Non-Newtonian effects of second-order fluids on double-layered porous Rayleigh-step bearings

Theoretical study of double-layered porous Rayleigh-step bearings with second-order fluid as lubricant is presented. An approximate method for the solutions of the governing fluid film equations for a porous region is proposed. The expressions for the pressure distribution, load capacity and frictional coefficient are obtained in compact form. Calculations of the dimensionless load capacity, frictional force and frictional coefficients are presented for specific values of the material parameters. It is found that the double porous layer yields an increase in the load capacity and ensures decrease in frictional force at the porous lining as compared with the conventional porous Rayleigh-step bearings. The maximum dimensionless load-carrying capacity is found to occur at a slightly larger step ratio as compared with the conventional porous Rayleigh-step bearings.     

On effects of thickness on ductile crack growth in mild steel

Critical crack opening displacement (COD) values have been examined for a range of specimen thicknesses. The COD at the initiation of fracture δ₁ is found to be constant, given a plane-strain crack-tip stress-state, whereas the COD at maximum load δ_max decreases with increasing thickness. The loads required to produce instability are found to vary with thickness, in a way analogous to behaviour observed under linear elastic conditions. Crack growth under constant load for a range of specimen thicknesses has been examined, and failure has been found to occur at loads below that associated with Δ_max,; the minimum load per unit thickness required to cause failure decreasing with specimen thickness.     

THE ANALYSES OF THE THREE-DIMENSIONAL STRESS STRUCTURE NEAR THE CRACK TIP OF MODE Ⅰ CT SPECIMENS IN ELASTICPLASTIC STATE(Ⅱ)—THE ANALYSES OF THE STRESS STRUCTURE

Based on[1],the stress structures of the smooth region and shear lip of thespecimens have been investigated in the paper.The characteristics of the stress structure inthe smooth region have been found that the variable z can separated out;the stressesin the midsection can be obtained by the plane strain FEM results or HRR structuremodified by the stress triaxiality.The effects of load level and thickness on the stressstructure can be reflected by the distribution of CTOD along the thickness direction.The obtained expressions of the stresses are very simple and visualized.The analysesof the stress structure in the shear lip show that the stresses can be obtained bydifferent methods of interpolation to a certain precise degree.A new degree parameter of the plane strain state has been put forward andstudied.The parameter can reflect relatively well the variation of the kind andthickness of the specimen as well as the load level.The fracture parameter has alsobeen investigated to be sure that it can be obta