Nonlinear Dynamics of an Electrically Driven Impact Microactuator

We study the dynamics of an electrostatically driven impact microactuator. Impact between moving elements of the microactuator is modeled using the coefficient of restitution. Friction between the microactuator and its supporting substrate is modeled using the Amonton–Coulomb law. We consider the bifurcations under changes in the driving voltage and frequency. Grazing bifurcations introduce discontinuous transitions between different motions. It is also found that impacts dramatically change the characteristics of the frequency-response curve. Finally, we discuss the evolution of incomplete chatter to complete chatter, that is, sticking.Contributed by Prof. G. Rega.     

The Dynamics of Thin-Walled Structures Under Nonstationary Loads

Studies are reviewed in which nonstationary loads determining the dynamics of thin-walled structures are found and the initial concepts are formulated for the theory and methods of stress–strain analysis of shells with holes and different discrete reinforcements, as well as for coupled problems of hydro- and aeroelasticity. Special attention is drawn to the design and strength and dynamic analyses of thin-walled blasting chambers. The basic investigations conducted at the S. P. Timoshenko Institute of Mechanics of the National Academy of Sciences of Ukraine are outlined     

斜冲击界面动力学研究

讨论了界面受到斜冲击时的动力学问题。有效冲击摩擦系数被处理成冲击过程及初始冲击角的函数 ;界面的法向响应描述计及弹性接触、弹性变形极限以及依赖于应变、应变率及温度的完全塑性接触 ;界面在冲击过程中的构形变化也予考虑 ,并采用平均应变、应变率及最大温升的概念与估算。这种新的斜冲击界面动力学模型用于数值模拟刚性球对于延性靶的斜冲击实验 ,计算与实验比较 ,结果是令人满意的。     

拓扑互锁结构冲击防护研究及优化设计

拓扑互锁结构是一种通过具有特殊形状的元素（块）彼此啮合拼接组成的整体结构,具有很好的增韧和抗冲击特性．目前对拓扑互锁结构的静荷载分析研究逐渐成熟,而对冲击条件下结构的强度、稳定性等性能分析以及对结构吸能性能的定量分析尚有发展空间;另外,对影响拓扑互锁结构力学性能的因素、结构设计优化等研究也有待完善．本文选择四面体元素拓扑互锁结构,通过ABAQUS有限元仿真软件对其进行冲击试验仿真,得出拓扑互锁结构在冲击作用下的变形机制,探索了元素互锁角度、元素间摩擦对冲击性能的影响;指出结构被冲穿的决定因素为冲击能量而非冲击速度;最后研究了局部榫卯对结构力学性能的影响．结果验证了拓扑互锁结构较好的缓冲吸能特性,同时表明拓扑互锁结构的力学性能是各个因素综合作用的结果,实际应用中可以根据使用场景和功能需求等对拓扑互锁元素进行调整优化．     

转子系统瞬时撞击刚度的定量计算方法

通过对转子与定子碰撞过程的线性化处理，得到了转子系统瞬时碰撞刚度的定量计算公式。由该公式确定的碰撞刚度不但与转子和定子的杨氏模量有关，而么与转子的径向速度有关。因此该公式较准确地确定了转子系统的碰撞过程。本文通过实例对如何运用此公式确定转子系统的碰撞刚度进行了详细的论述。     

The Stress—Strain State of Thin-Walled Structures Exposed to Radiation

Mathematical models of radiation-induced physical processes in materials are analyzed. An approximate method is proposed to determine thermal and radiation strains depending on the energy spectrum and components of radiation. The stress–strain state of thin plates is studied with allowance for radiation effects     

浮筏系统中的碰撞动力学离散模型(2):随机模型

本文建立了浮筏系统在部分冲击受损后形成的碰撞振动系统的离散动力学随机模型.在此随机模型中,浮筏系统的运动满足高斯分布,零部件的运动方程中的非线性是由于空隙单元产生,可以采用非高斯截断技术导出随机系统的平均碰撞庞加莱映射.实例分析指出,随机模型中由于增加了噪声的摄动,表现出的动力学特性比确定性模型更丰富,噪声的影响有时会增加零部件与设备之间的碰撞次数,也会抑制零部件与筏体之间的碰撞次数,噪声将增加系统动力学特性的复杂性.     

爆炸与冲击载荷下结构和材料动态响应研究的新进展

对激波与高应变率现象的基础问题及应用国际学术会议 (EXPLOMET 2 0 0 0 )和第 2 0届国际理论与应用力学大会 (ICTAM 2 0 0 0 )上有关爆炸冲击载荷下结构和材料动态响应的研究论文进行了回顾和综合分析。分别从应力波、结构的高应变率响应和材料的高应变率响应三个方面就其新进展和新动向作了述评。     

Mo／2Si混合粉末的摩擦化学效应

利用 ZJM10 T型搅拌机和 X射线衍射仪研究了 Mo/ 2 Si混合粉末在机械球磨过程中的摩擦化学效应 .结果表明 :在球磨初期 ,晶粒尺寸明显减小 ,显微应变和有效温度系数明显增加 ;在球磨 3h后 ,Mo和 Si粉末的晶粒尺寸和显微应变变化幅度减小 ;球磨 10 h后 ,有效温度系数趋于极限值 ,晶粒尺寸和显微应变几乎保持不变 .塑性 Mo颗粒的机械活化以变形或晶格畸变加剧为主 ,而脆性 Si颗粒的机械活化以细化或比表面积增加为主 .球磨过程中未检测到摩擦化学变化     

浮筏系统中的碰撞动力学离散模型(1):确定性模型

本文研究浮筏中刚性连结的零部件冲击受损后在有限空间的碰撞振动,应用碰撞动力学理论建立其确定性离散动力学模型,导出确定性系统的碰撞庞加莱映射.实例分析指出,零部件的质量和冲击后外激励的频率不影响零部件与设备之间的碰撞,碰撞主要发生在零部件与筏体之间,而且随着零部件质量的增加碰撞次数发生有限次的增加.冲击后外激励的频率对零部件与筏体之间的碰撞次数有一定的影响.从碰撞庞加莱图可以看出碰撞系统的振动表现为亚谐振动.     

Some Issues of Continuum Mechanics and Mathematical Problems in the Theory of Thin-Walled Structures

Consideration is given to some issues of continuum mechanics and mechanical problems arising in the theory of thin plates and shells. The main research areas are analyzed. The results obtained in the linear and nonlinear theories of plates and shells are reviewed and some open issues and unsolved problems of those theories are formulated.     

高速干铣削钛合金时涂层硬质合金刀具磨损机理研究

采用CVD涂层硬质合金可转位立铣刀对钛合金(Ti-6Al-4V)进行了高速干铣削试验,采用扫描电子显微镜(SEM)观察刀具的磨损形貌,通过能谱分析(EDS)分析失效刀具表面的元素分布,并对刀具的主要磨损机理进行了分析.研究结果表明:使用涂层硬质合金刀具高速干铣削Ti-6Al-4V时,刀具的失效机理主要为磨粒磨损、粘结磨损、氧化磨损、扩散磨损和热-机械疲劳磨损的综合作用.刀具刚参与切削时,刀具后刀面会产生粘结和由于摩擦引起的擦伤,粘结层在断续冲击作用下的脱落过程还会造成后刀面涂层的剥落;随着刀具进一步的磨损,涂层剥落、粘结磨损及磨粒磨损伴随整个刀具失效过程,且还会出现氧化磨损、扩散磨损和疲劳裂纹等.切削速度越高,新产生的钛合金切屑就越容易燃烧,使刀具粘结、氧化和扩散以及热-机械疲劳等磨损加剧.     

一维应力预加载下红砂岩冲击力学特性试验研究

为研究高地应力下再受动力扰动的岩石力学特性,利用改进的岩石动静组合加载SHPB试验装置,对红砂岩进行一维预应力状态下岩石的冲击力学特性试验。选取4个轴压水平,进行不同应变率下的动加载试验。研究结果表明:不同应变率下,岩样在破坏阶段出现"峰后塑性"、"应力跌落"、"应变回弹"三种类型,岩样的割线模量随轴压的增大先增大后减小;相同轴压下,岩样的动态抗压强度和单位体积吸收能随着应变率的增大而增大,分别呈现出指数函数关系和线性关系;相同应变率下,岩样的动态抗压强度和单位体积吸收能随着轴压的增大呈先增大后减小的趋势;无轴压条件下,岩样为拉伸破坏模式,有轴压条件下,岩样为剪切破坏模式。     

Dynamics of the flow around colliding spheres

An investigation of the flow resulting from the collision of two spheres at low Reynolds numbers is presented. Each sphere starts from rest and traverses a distance of 5 sphere diameters to the point of contact. Experimental and numerical results are compared for a symmetric collision; that is, a collision between two spheres of the same diameter and travelling with the same velocity. The flow consists of two axisymmetric recirculation zones which become a pair of colliding vortex rings, expanding radially from the collision point. Several examples of unbalanced collisions are also presented numerically, with one or both of the velocity and diameter of the spheres altered. These collisions break the symmetry, altering the post-collision expansion of the vortex rings.     

纳米TiAIN涂层硬质合金刀具高速铣削AerMet100钢的磨损机理

采用纳米TiAIN结构涂层硬质合金刀具对新型难加工材料AerMet100钢进行高速铣削试验,并对实验获得的数据从刀具磨破损形态及其磨损机理2方面进行系统地分析和研究。研究表明纳米TiAIN结构涂层硬质合金刀具在高速面铣削AerMet100钢时磨损破损形式主要为前刀面磨损、后刀面磨损、涂层材料的破损、微崩刃、边界沟槽磨损,贝壳状崩落;磨损机理主要是磨粒磨损、粘结磨损、氧化磨损和扩散磨损。此外,研究发现,高速铣削AerMet100钢时,由于工件材料中的Co含量较高,刀具中的Co元素不但没有扩散流失,反而增加。     

复合材料冲击损伤及冲击后压缩强度的等效实验方法

简要论述了复合材料低速冲击及冲击后压缩的研究情况,比较了几种不同的冲击后的压缩实验方法,选取一种小尺寸试件方法,对复合材料低速冲击后的压缩行为进行研究,并将结果与有关文献进行了比较,实验结果与文献结果具有一致的变化规律。研究结果显示,对于材料研究,该方法可用来表征复合材料冲击后的压缩特性。使用该方法可以大大节省实验费用及周期。     

A Mechanics Based Model for Study of Dynamics of Milling Operations

A unified mechanics based model with multiple degrees of freedom is developed and numerically simulated to study workpiece-tool interactions during milling of ductile workpieces with helical tools. A refined orthogonal cutting model is used at each section of the tool, and the milling forces are determined by using a spatial integration scheme along the axis of the tool. Both regenerative and loss of contact effects are considered in determining the cutting forces, which makes the model well suited for a wide range of milling operations. The model also allows for partial engagement of a tool with a workpiece, which is an important feature needed for milling operations with helical tools. Time domain simulations are carried out by using the developed model to predict the stability boundaries in the space of the tool spindle speed and the axial depth of cut. Poincaré sections are used to determine loss of stability from period-one motions to other motions such as two-period quasiperiodic motions, as a control parameter is varied.     

弹性杆与结构接触冲击的冲击力计算研究

本文提出了一种将杆与结构的接触冲击问题简化为集中力与集中阻尼的常规结构动力学问题的计算模型。对于大型结构，可采用一种特殊的人工边界，只需对人工边界域内的结构用有限元法进行计算。这种人工边界是齐次的，可根据冲击影响区的大小人为地确定     

On the Dynamics of a Harmonic Oscillator Undergoing Impacts with a Vibrating Platform

This paper deals with a model for repeated impacts of a massattached to a spring with a massive, sinusoidally vibrating table. Thismodel has been studied in an attempt to understand the cantilever-sampledynamics in atomic force microscopy. In this work, we have shown thatfor some values of the frequency of the vibrating table, there arecountably many orbits of arbitrarily long periods and the system issensitive to the initial conditions with which the experiments areconducted. We have also shown existence of complex dynamics in the caseswhen the natural frequency of the spring-mass system is very low; andwhen it is the same as the oscillating frequency of the table.     

Formation of Symmetric Structures in the Dynamics of Repelling Particles

We consider Hamiltonian systems corresponding to the motions of a system of N repelling particles evolving in space under the action deriving from a very long range potential energy; the asymptotic behavior of the system is analysed for the cases U=− ln r and . Only special “asymptotic?shapes” are reached, which may present quite interesting symmetries and correspond to the critical points of a gradient system. The relationships between the original Hamiltonian and the asymptotic gradient system are discussed. Accepted: May 25, 1999