铝合金板式节点网壳稳定承载力试验研究

为研究铝合金板式节点刚度对单层球面网壳整体稳定性能的影响,制作一个跨度为8m、矢高为0.5m的K6型网壳模型,对网壳顶点施加4次单向循环荷载。试验结果表明,加载初期网壳表现出超刚性特征,即其刚度大于刚接节点网壳刚度;而后的节点螺栓滑移变形会降低网壳刚度,螺栓滑移引起的网壳变形很大且不可恢复。网壳的失稳属于整体跳跃失稳,但网壳屈曲后可继续承载,且荷载可进一步增大。最后一次单向循环加载时网壳因顶点杆件下翼缘的拉裂而破坏。另外,随着循环荷载逐级增大,网壳再加载初始刚度都有一定弱化。采用ANSYS有限元软件建立网壳分析模型,采用BEAM 188单元和COMBINE 39单元模拟考虑节点半刚性的铝合金板式节点网壳杆件,节点弯矩-转角关系曲线采用四折线模型,有限元分析结果与试验结果吻合较好。     

Stability analysis of an embedded single-walled carbon nanotube with small initial curvature based on nonlocal theory

Many experimental observations have shown that most nanostructures, such as carbon nanotubes, are often characterized by a certain degree of waviness along their axial direction. This geometrical imperfection has profound effects on the mechanical behavior of carbon nanotubes. In the present work, stability of a slightly curved carbon nanotube under lateral loading is investigated based on Eringen's nonlocal elasticity theory. Euler Bernoulli and Timoshenko beam theories are employed to obtain equilibrium equations. Winkler-Pasternak elastic foundation is used to approximate the effect of matrix. Effects of initial curvature, nonlocal parameter, beam length, and elastic foundation parameters on initiation of critical conditions are investigated.     

基于局部截面构型优化的双稳态结构跳跃阈值控制与设计*

双稳态结构正反行程跳跃阈值不相等引起的驱动困难是制约其工程应用的瓶颈问题。基于大变形有限元理论分析了影响双稳态结构跳跃过程的关键因素,引入跳跃阈值力、稳态保持力以及双稳态特征系数(二者的比值)作为描述双稳态特征的指标,提出了基于局部单元截面构型优化的双稳态跳跃阈值控制方法,建立了具有特定跳跃力学特征的双稳态结构优化设计模型,以局部单元的截面尺寸、个数及位置为设计变量,采用分步优化的方法,得到了几类具有不同双稳态特征系数的余弦梁新构型,分析了变截面单元厚度及布置方式对跳跃阈值及路径的影响关系。数值仿真结果表明,局部截面构型改变前后双稳态结构的跳跃时间及振动频率变化不大。考虑试验加载位置与夹持条件对双稳态特征系数的影响,仿真与试验结果基本一致,验证了所提出的双稳态结构跳跃力控制方法的有效性。     

自锁式平板折叠网架折展过程参数设计

针对自锁折叠结构的受力特点,分析了其自锁原理及其在折叠过程中的荷载位移路径.通过对自锁式四棱柱折叠单体采用ANSYS分析其折叠过程的受力状态,并考虑杆件稳定性及结构弹性设计的要求,明确了折叠结构几何设计的四个基本参数,给出了单体几何参数和杆件截面尺寸设计的推荐取值范围.     

Zero-Power Shape Retention in Soft Pneumatic Actuators with Extensional and Bending Multistability

Power-free shape retention enables soft pneumatic robots to reduce energy cost and avoid unexpected collapse due to burst or puncture. Existing strategies for pneumatic actuation cannot attain motion locking for trajectories combining extension and bending, one of the most common modes of operation. Here, a design paradigm is introduced for soft pneumatic actuators to enable zero-power locking for shape retention in both extension and bending. The underpinning mechanism is the integration of a pneumatic transmitter and a multistable guider, which are programmed to interact for balanced load transfer, flexural and extension steering, and progressive snapping leading to state locking. Through theory, simulations, and experiments on proof-of-concept actuators, the existence of four distinct regimes of deformation is unveiled, where the constituents first interact during inflation to attain locking in extension and bending, and then cooperate under vacuum to enable fully reversible functionality. Finally, the design paradigm is demonstrated to realize a soft robotic arm capable to lock at desired curvature states at zero-power, and a gripper that safely operates with puncture resistance to grasp and hold objects of various shapes and consistency. The study promises further development for zero-power soft robots endowed with multiple deformation modes, sequential deployment, and tunable multistability.     

考虑二阶效应的拱结构面内弹性屈曲

对拱结构的极值点屈曲及分支点屈曲进行了详细阐述。利用有限元分析程序ANSYS对拱进行非线性分析,考虑结构屈曲前变形对临界力的影响。选取线性屈曲前几阶模态作为扰动位移,对拱的整个受载过程进行跟踪分析,得到拱的屈曲临界力。将矢跨比、跨径及矢高与截面回转半径的比值作为影响参数,分析竖向均布力及径向均布力作用下,无铰拱和两铰拱的屈曲性能。分析表明,屈曲前变形降低了坦拱的屈曲临界力,考虑二阶效应后拱的屈曲临界力与线性屈曲临界力的比值,随矢跨比的减小而减小,随跨径的增大而增大;对于矢跨比较小的坦拱,矢高与截面回转半径的比值将决定拱的屈曲形式。     

Nonlinear deterministic and random response of shallow shells

Deterministic and random vibrations are considered for the case of shear deformable shallow shells composed of multiple perfectly bonded layers. The nontrivial generalization of the flat plate vibrations is expressed by the fact of “small amplitude” vibrations existing about the curved equilibrium position together with the snap-through and snap-buckling type large amplitude vibrations about the flat position. The geometrically nonlinear vibrations are treated by applying Berger’s approximation to the generalized von Karman-type plate equations considering hard hinged supports of the straight boundary segments of skew or even more generally shaped polygonal shells. Shear deformation is considered by means of Mindlin’s kinematic hypothesis and a distributed lateral force loading is applied. Application of a multi-mode expansion in the Galerkin procedure to the governing differential equation, where the eigenfunctions of the corresponding linear plate problem are used as space variables, renders a coupled set of ordinary time differential equations for the generalized coordinates with cubic and quadratic non-linearities. For reasons of convergence, a light viscous modal damping is added. The nonlinear steady-state response of shallow shells subjected to a time-harmonic lateral excitation is investigated and the phenomenon of primary resonance is studied by means of the “perturbation method of multiple scales”. The use of a nondimensional formulation and introduction of the eigen-time of the basic mode of the associated linearized problem provides a unifying result with respect to the planform of the shell. Within the scope of random vibrations, it is assumed that the effective forces can be modelled by uncorrelated, zero-mean wide-band noise processes. Considering the set of modal equations to be finite, the Fokker-Planck-Kolmogorov (FPK) equation for the transition probability density of the generalized coordinates and velocities is derived. Its stationary solution gives the probability of eventual snapping after a long time has elapsed. However, the probability of first occurrence follows from the (approximate) integration of the nonstationary FPK equation. The probability of first dynamic snap-through is derived for a single mode approximation with the influence of higher modes taken into account. Using the two-mode expansion, the probability distribution of the asymmetric snap-buckling is also evaluated.     

双稳定矩形非对称复合材料层板的跳变研究

对双稳定矩形非对称复合材料层合板两种稳定构型的跳变行为进行了实验和数值研究。通过实验测得中心加载下层合板两种稳定构型跳变的临界载荷, 建立了两种稳定构型的非线性有限元分析模型, 成功预报了层合板固化后的两种稳定构型, 并进一步预报了两种稳定构型的跳变临界载荷, 预报结果与实验结果吻合良好。实验和有限元模拟结果表明, 中心加载下矩形非对称层合板两种构型的临界载荷相差较大, 并且跳变的过程有所差异。