组合薄壁直梁的耐撞性研究

针对汽车前部纵梁结构的耐撞性要求,设计了在矩形空心管内部填充成组圆管的组合梁结构,研究了组合直梁在轴向冲击载荷下的能量吸收与变形特性,并进一步研究了填充圆管直径和长度等参数变化对组合梁的性能影响。结果发现,矩形薄壁梁内部填充圆管以后,结构的碰撞吸能特性得到较大程度的提高;通过合理的改变填充圆管的数量和长度,可以较好地调整结构压溃过程中的碰撞力峰值载荷和均值载荷。     

A novel 5-DOFs dynamic model of aerostatic spindle considering the effect of process damping in ultra-precision machining

The existing aerostatic spindle dynamic model only analyses the effects of mass imbalance and external load, ignoring the influence of cutting system on the spindle dynamic characteristics under cutting conditions. In this paper, a 5-DOFs aerostatic spindle dynamic model is established considering the influence of the micro-scale non-linear dynamic performance of the aerostatic spindle and cutting process damping. First, an analytical identification model of process damping with blunt circular cutter is established. Then, the micro-scale dynamic characteristics of the aerostatic spindle are analyzed and a 5-DOFs aerostatic spindle dynamic model is established considering the influence of process damping. Finally, the model is simulated and the influence of process damping on the dynamic characteristics of aerostatic spindle is analyzed. The simulation results show that the process damping of the cutting system has a significant influence on the dynamic characteristics of the aerostatic spindle. This study can provide theoretical guidance for coupling research of cutting system and spindle system.

     

Stability of tapered bars of circular cross-sections under semi- and quasi-tangential torques

A torque may be classified as semi- or quasi-tangential, depending on whether it is generated by two couples or one couple of direct forces. Both types of torque may cause a torsionally loaded bar to buckle at certain critical values. This paper presents an analytical approach for investigating the instability of tapered bars of circular cross-sections subjected to torques of the semi- or quasi-tangential type. The analysis results indicate that a distinction must be made in practice between the two types of torque, as their critical loads have a difference of a factor of two. For the special case of a bar with constant cross-sections, the present results reduce to those given by Ziegler.     

Modeling of interfacial friction damping of carbon nanotube-based nanocomposites

Carbon nanotube-based composite is becoming increasingly popular and offers great potential for highly demanding practical high strength and high damping applications. The excellent damping capacity of CNTs is primarily due to the interfacial friction between carbon nanotubes and polymer resins and the extremely large interfacial surface area over a given specific mass (specific area). In this paper, damping characteristics of carbon nanotube-based composites have been investigated, with an objective of developing an effective and accurate analytical model, which can be used as a design tool for the damping design of such materials. Based on the interfacial slips between the resin and nanotubes and between the nanotubes themselves, a micro stick-slip damping model has been developed. Such a physically derived model is believed to be appropriate and representative of the actual complex damping mechanism of the material system. The model, developed for the first time, is analytical and relates explicitly the material properties of the resin and nanotubes and the processing parameters to the overall material damping loss factor and hence it offers the possibility for material engineers to possibly optimize the damping for required applications. Due to the nonlinear force–displacement relationship derived under the micro stick-slip, a harmonic linearization method, the Describing Function method, has been employed to analyse its vibration characteristics and to derive the required damping loss factors. From the analytical formula, it can be seen that the damping loss factor of the material system depends on the individual material properties of the resin and the nanotubes, structural deformation, nanotube volume fraction and the critical shear stresses at which interfacial slips take place. By taking careful considerations of these design parameters, optimized carbon nanotube-based composites for advanced damping applications can be developed.Extensive numerical simulations have been carried out to establish the practical applicability of the proposed analytical model. Based on realistic material properties of carbon nanotubes and polymer resins, damping characteristics have been predicted which compare well with existing results from open literature. The results have shown that for a volume fraction as small as 1%, a damping loss factor as high as 20% can be achieved which is adequate for most practical applications. The model has been further developed to deal with bending vibrations where different parts of the material are subject to different vibration strain levels. A practical case of cantilevered beam vibration has been employed to demonstrate the practical application of the proposed model.     

环形阀片局部加载变形研究

提出了减振装置环形节流阀片受局部均载作用时挠曲变形量的一种研究方法,根据节流阀片物理模型以及薄板力学的相关理论,推导了阀片受局部均载作用时挠曲变形量的解析式,并整理得出了便于理论分析和工程应用的表达形式.与有限元计算结果对比验证了公式以及推导过程的正确性,研究了不同载荷、载荷作用半径以及节流阀片相关结构参数和材料特性对其任意半径处变形量的影响规律,得出的解析式和结论为减振装置阻尼阀的精确设计提供了依据.     

Analytical and experimental investigations into the controlled energy absorption characteristics of thick-walled tubes with circumferential grooves

In this paper, the energy absorption characteristics of grooved circular tubes are investigated under quasi-static loading condition. For experiments, thick-walled tubes with circumferential grooves are prepared. The grooves divide the thick-walled tube into several shorter thin-walled portions. Specimens are subjected to axial crushing load to observe the effect of distribution of circular grooves on the deformation mechanism and energy absorption capacity. Geometrical parameters of the specimens are designed utilizing the Taguchi method to cover a reasonably wide range of groove length-to-wall thickness ratios. An analytical approach based on the concept of energy dissipation through the plastic hinges is applied. Taking the effect of strain hardening into account, the obtained analytical results are in good agreement with the experimental ones. The agreement between analytical and experimental results may indicate the validity of the proposed analytical approach. Desirable mechanism of deformation observed justifies the pre-forming method for obtaining favorable energy absorption characteristics.     

Study of fluid damping effects on resonant frequency of an electromagnetically actuated valveless micropump

As fluid flow effects on the actuation and dynamic response of a vibrating membrane are crucial to micropump design in drug delivery, this paper presents both a mathematical and finite-element analysis (FEA) validation of a solution to fluid damping of a valveless micropump model. To further understand the behavior of the micropump, effects of geometrical dimensions and properties of fluid on the resonant frequency are analyzed to optimize the design of the proposed micropump. The analytical and numerical solutions show that the resonant frequency decreases with the slenderness ratio of the diffuser and increases with the opening angle, high aspect ratio, and thickness ratio between the membrane and the fluid chamber depth. A specific valveless micropump model with a 6-mm diameter and 65-μm thickness polydimethylsiloxane (PDMS) composite elastic membrane was studied and analyzed when subjected to different fluids conditions. The resonant frequency of a clamped circular membrane is found to be 138.11 Hz, neglecting the fluid. For a gas fluid load, the frequency is attenuated by slightly shifting to 104.76 Hz and it is significantly reduced to 5.53 Hz when the liquid fluid is loaded. Resonant frequency remarkably shifts the flow rate of the pump; hence, frequency-dependent characteristics of both single-chamber and dual-chamber configuration micropumps were investigated. It was observed that, although the fluid capacity is doubled for the latter, the maximum flow rate was found to be around 27.73 μl/min under 0.4-A input current with an excitation frequency of 3 Hz. This is less than twice the flow rate of a single chamber of 19.61 μl/min tested under the same current but with an excitation frequency of 4.36 Hz. The proposed double-chamber model analytical solution combined with the optimization of the nozzle/diffuser design and assuming the effects of damping proved to be an effective tool in predicting micropump performance and flow rate delivery.     

Nonlinear in-plane elastic buckling of shallow circular arches under uniform radial and thermal loading

This paper presents a nonlinear in-plane elastic buckling analysis of circular shallow arches that are subjected both to a uniform temperature field and to a uniform radial load field. A virtual work method is used to establish nonlinear equilibrium equations and buckling equilibrium equations, and analytical solutions for the limit instability and bifurcation buckling loads are obtained. It is found that the temperature influences the limit instability, bifurcation buckling and postbuckling behaviour of shallow arches significantly. The limit instability and bifurcation buckling loads increase with an increase of the temperature. A maximum temperature is shown to exist for the occurrence of bifurcation buckling of shallow arches, and when the temperature is higher than this value, bifurcation buckling of an arch is not possible.An arch geometric parameter is introduced to define switches between the limit instability and bifurcation buckling modes, and between buckling and no buckling. Formulae and methods for the calculation of the limiting values of the arch geometric parameter are developed. It is also found that the limiting values of the arch geometric parameter decrease with an increase of the temperature.     

Closed form solutions of axisymmetric bending of circular plates having non-linear variable thickness

A new analytical method for evaluation of elastic stresses and deformations in axisymmetric plates having variable thickness according to a power of a linear function, either solid or annular, subjected to symmetrical bending due to lateral loads either distributed on upper surface or distributed along the inner or the outer edges. The proposed procedure is based on two independent integrals of the hypergeometric differential equation describing the rotation field and constitutes the generalization of the one found in the literature. This method allows to study a wide range of plates, be they solid or annular, converging or diverging with linear or non-linear thickness function, convex, concave or linear tapered, without the restrictions of the known procedures. Analytical results obtained by using this method utterly match both theoretical results which may be obtained in the specific case known (constant-thickness circular plate, linear variable thickness annular circular plate) and numerical results obtained by using FEA.     

General analytical shakedown solution for structures with kinematic hardening materials

The effect of kinematic hardening behavior on the shakedown behaviors of structure has been investigated by performing shakedown analysis for some specific problems. The results obtained only show that the shakedown limit loads of structures with kinematic hardening model are larger than or equal to those with perfectly plastic model of the same initial yield stress. To further investigate the rules governing the different shakedown behaviors of kinematic hardening structures, the extended shakedown theorem for limited kinematic hardening is applied, the shakedown condition is then proposed, and a general analytical solution for the structural shakedown limit load is thus derived. The analytical shakedown limit loads for fully reversed cyclic loading and non-fully reversed cyclic loading are then given based on the general solution. The resulting analytical solution is applied to some specific problems: a hollow specimen subjected to tension and torsion, a flanged pipe subjected to pressure and axial force and a square plate with small central hole subjected to biaxial tension. The results obtained are compared with those in literatures, they are consistent with each other. Based on the resulting general analytical solution, rules governing the general effects of kinematic hardening behavior on the shakedown behavior of structure are clearly.     

高速动车组减振器载荷特征研究

减振器的主要功能是提供阻尼力以衰减和抑制车辆系统振动,对高速动车组动力性能有十分重要的影响。既有研究主要将减振器处理为阻尼力以研究车辆系统动力性能,极少从动力学和结构可靠性角度关注减振器自身承受的载荷。制作某型高速动车组转向架抗蛇行减振器、轴箱减振器、二系横向和垂向减振器测力元件,在大同-西安高速线路上测试并获得该型动车组运行过程中四种减振器载荷引起的应变信号。对测试数据进行处理和分析,获得高速动车组运行工况下四种减振器载荷的时间历程,分析减振器载荷的时域和频域特征。采用雨流计数法统计减振器载荷峰谷值和频次,获得不同速度等级下载荷分布。结果表明,高速动车组抗蛇行减振器载荷最大、二系横向减振器载荷最小。轴箱减振器相对速度最大、二系横向减振器相对速度最小。减振器载荷总体上呈正态分布,而且一般有列车运行速度越高减振器载荷越大。列车正线行驶时曲线半径对轴箱减振器、二系垂向减振器以及二系横向减振器载荷影响不明显,列车速度和线路小半径曲线对抗蛇行减振器载荷影响明显。     

Dynamic response analysis of a cantilevered beam due to an elastic impact

The beam structure models with impact or contact parts under impact forces have been applied to the design of mechanical and electronic accessories. Switches, pick-ups and sensors are the typical examples of the structure to be designed to colliding with other parts. The damping characteristics and stiffness of an impacter have a great effect on dynamic responses of structures subjected to impact forces. Since the response characteristics of structures have an effect on the generation of impact forces, both theoretical and experimental investigations for the dynamic relationships between the impacter and impact mechanism are required for the structural design to reduce impact forces or eliminate unwanted vibration modes. In this paper, in order to examine the relationship between the changes of the stiffness and damping of an impacter and the variations of the dynamic characteristics of the impact model of a cantilevered beam with an impacter, the impact force of the impacter and response characteristics of the cantilevered beam were analyzed by both numerical simulation and experiments. Since the stiffness and damping of the impacter are highly nonlinear, the contact model using revised Hertz-model was established by experiments. Also, the results of numerical analyses for the dynamic response and impact force of a cantilevered beam with an impacter have a good agreement with experimental results. It is believed that the mathematical model and the analytical method presented here can be used for the design analysis and the improvement of performances of the beam-shape switches and pickup devices.     

Mode of collapse and energy absorption characteristics of constrained frusta under axial impact loading

The energy absorption performance of right circular frusta subjected to dynamic axial load is studied and compared with the results of quasi-static tests. Frusta of different geometric ratios and end constraints were axially crushed using a drop hammer at initial velocities in the range of 2–5 m/s. The effect of heat treatment on the collapse behaviour and energy absorption is also investigated. The experimental observations indicate that the effects of the end constraints and heat treatment on the energy absorption were qualitatively similar to those observed under quasi-static testing. Due to inertia effects, the absolute values of the energy absorbed by similar frusta were higher under dynamic loads than under quasi-static loads. It has been established that constraining the frusta enhances their energy absorption capacity under static and dynamic loading particularly at the top (smaller diameter). The optimum geometric parameters for maximum energy absorption performance are identified when residual stresses and strain hardening characteristics, arising from spinning the frusta, were removed.     

A revised design of circular hydrostatic bearings for optimal pumping power

A modification of the circular flat externally pressurized thrust bearing is presented in this paper. This involves the addition of a central step in the bottom of the rotating pad for the purpose of restricting the lubricant flow and minimizing the pumping power required for continuous operation. The performance of such a bearing configuration under static loads is analysed and the key design parameters are discussed, grouped and presented in a design chart. In addition to the minimization of pumping power, the proposed configuration provides higher thrust loads and enables the bearing to withstand radial loads, an advantage which could not be achieved in other hydrostatic thrust bearing configurations. The bearing is also able to restore concentricity due to the lateral damping provided by the squeezed lubricant film in the clearance between the step and the recess.     

Dynamic Characteristics and Stability Analysis of a Wavy Thrust Bearing

A numerical procedure to analyze wavy thrust bearings is described. The numerical model is developed by assuming that two circular plates rotate relative to each other. The upper plate is assumed to be flat and rotating, whereas the lower plate is assumed to be stationary and wavy in surface geometry. A Reynolds-equation-based procedure is used to simulate the dynamics engendered by various wavy geometries and loading conditions. The equilibrium position of the journal results from the equilibrium between the forces generated by the fluid-film pressures and the externally applied loads. A numerical small perturbation technique is applied to calculate the linear stiffness and damping characteristics of the bearing at the equilibrium position. Using a three-degrees-of-freedom system with one axial and two rotational displacements, nine linear stiffness coefficients (three principal and six cross-coupled coefficients) and nine linear damping coefficients are calculated. These linear coefficients are then used to calculate the eigenvalues of the system by solving the homogeneous equations of motion. The stability of the bearing system is then expressed using the lowest logarithmic decrement obtained from these eigenvalues. Using this procedure, a parametric study is carried out to examine the effects of external load, location of the applied load, bearing number, and bearing wave amplitude on journal equilibrium position, bearing linear stiffness, damping characteristics, and bearing stability.     

NON-LINEAR DYNAMIC BEHAVIOR OF THERMOELASTIC CIRCULAR PLATE WITH VARYING THICKNESS SUBJECTED TO NON- CONSERVATIVE LOADING

The non-linear dynamic behaviors of thermoelastic circular plate with varying thickness subjected to radially uniformly distributed follower forces are considered. Two coupled non-linear differential equations of motion for this problem are derived in terms of the transverse deflection and radial displacement component of the mid-plane of the plate. Using the Kantorovich averaging method, the differential equation of mode shape of the plate is derived, and the eigenvalue problem is solved by using shooting method. The eigencurves for frequencies and critical loads of the circular plate with unmovable simply supported edge and clamped edge are obtained. The effects of the variation of thickness and temperature on the frequencies and critical loads of the thermoelastic circular plate subjected to radially uniformly distributed follower forces are then discussed.     

On the lateral instability of a thin beam under periodic bending loads

The lateral instability of a thin beam under periodic bending loads was investigated. Physical evidences of the instability were observed previously by experiments. But an analytical study has not been reported. The object of this study is to demonstrate the nature and existence of dynamic lateral instability. The harmonic balance method is applied to bifurcation modes which result from the stability change of torsional mode of a beam and then compared with numerical simulations. It is found, in a certain frequency range, that a small bending load results in the lateral instability when damping is small. Inha University     

Parametric combination resonance instability characteristics of laminated composite curved panels with circular cutout subjected to non-uniform loading with damping

The dynamic instability of laminated composite doubly curved panels with centrally located circular cutout, subjected to non-uniform compressive in-plane harmonic edge loading is investigated. The present work deals with the problem of the occurrence of combination resonances in contrast to simple resonances in parametrically excited antisymmetric angle-ply and symmetric cross-ply laminated composite doubly curved panels with central circular cutout. The method of multiple scales is used to obtain analytical expressions for the simple and combination resonance instability regions. It is shown that other cases of the combination resonance can be of major importance and yield a significantly enlarged instability region in comparison to the principal instability region. The effects of non-uniform edge loading, centrally located circular cutout, damping, number of layers, orthotropy, the static load factor and the width-to-thickness ratio on dynamic instability behavior of simply supported laminated composite doubly curved panels are studied. The results show that under localized edge loading, combination resonance instability zones are as important as simple resonance instability zones. The effects of damping show that there is a finite critical value of the dynamic load factor for each instability region below which doubly curved panels cannot become dynamically unstable. A central circular cutout has the destabilizing effect on the dynamic stability behavior of laminated composite doubly curved panels subjected to non-uniform edge loading. This example of simultaneous excitation of two modes, each oscillating steadily as its own natural frequency, may be of considerable interest in vibration testing of actual structures.     

Evaluation of shakedown loads for plates

A kinematic method is developed to evaluate the shakedown limits for plates subjected to variable (quasistatic as well as dynamic) loads. The shakedown kinematic theorem is transformed into a reduced form, to which the upper-bound method involving construction of kinematically admissible mechanisms with yield curves used in plastic limit analysis can be applied. As an example, inadaptation of a circular plate clamped at the edge under variable uniform loading is considered.     

厚壁筒受正弦分布压力之解及圆筒无限长时的极限

对厚壁圆筒在柱面上受正弦分布压力的情况,给出了解析解。应用一个新的应力函数,解决轴对称问题。同时,给出了压力沿轴向不变时的极限,导出了厚壁圆筒著名的Lame解答。