冲击荷载作用下单层网壳结构动力稳定性研究

针对冲击荷载不同于地震作用,而常规动力稳定性判定准则不适用问题,阐述求解冲击问题的基本理论及冲击荷载取值;据冲击荷载特性提出适合冲击碰撞问题的单层网壳结构动力稳定性判定准则;选K6型单层网壳结构模型,利用非线性有限元软件ANSYS/LS-DYNA进行结构冲击作用下动力稳定性分析,通过大量算例,分析其在不同冲击物质量比及速度作用下全过程动力响应,结合动力响应模式获得冲击荷载作用下单层网壳动力失稳的临界能量区域,并从矢跨比、跨度、杆件截面三方面对结构进行参数分析。结果表明,基于网壳动力响应模式与冲击能量相结合方法对单层网壳进行动力稳定性判定合理;结构刚度越小,冲击作用下动力稳定性越差;加大主肋利于提高结构动力稳定性。     

三边简支一边固支矩形薄板动力屈曲解析解

根据板的动力平衡方程和压缩波前附加约束方程,基于双特征参数法和应力波理论,求解了三边简支一边固支矩形薄板在面内轴向冲击载荷作用下动力屈曲位移的解析解。揭示了矩形薄板动力屈曲过程中板的厚宽比、屈曲模态、冲击载荷大小和临界屈曲长度之间的关系。计算结果表明,由于横向惯性效应的存在,动力屈曲的临界载荷要比静力屈曲的大得多。     

风荷载下单层柱面网壳的动力稳定

以一单层柱面网壳为例,利用Budiansky-Roth准则研究空间结构在风荷载下的动力稳定性。介绍了Budiansky-Roth准则,通过风洞试验获得单层柱面网壳上的风荷载并研究其动力稳定性,讨论了初始几何缺陷、风向角和风压系数的影响,并将动力失稳分析结果与我国规范和阵风响应因子法（GRF法）计算动力响应导致结构破坏的方法作了比较。研究结果表明,空间结构进行风荷载下的稳定性设计时有必要研究其在风荷载下的动力稳定性。     

大型地下厂房结构振动反应分析

建立锦屏一级水电站地下厂房结构三维有限元动力分析模型,计算结构自振特性,并复核结构共振。通过模型水力脉动试验,获得锦屏一级水电站流道脉动压力荷载特性,用动力法分析厂房结构在机组振动荷载与水力脉动荷载作用下的振动反应,并据国内外有关规程规范对振动反应进行评价。研究结果表明,地下厂房结构与各种振源频率相差较大,不存在共振的可能性。地下厂房结构整体刚度较大,在机组振动荷载及水力脉动荷载作用下,振动位移、应力、速度均较小,满足有关规程要求。     

积雪漂移对空间结构动力稳定性的影响

以一单层柱面网壳为例,以Budiansky-Roth准则为动力失稳判别准则,研究积雪漂移对风雪耦合作用下空间结构动力稳定性的影响。由风洞试验获得结构表面的风荷载时程,通过数值方法模拟风力下积雪在结构表面的漂移。结果表明,风力下网壳顶部的积雪发生侵蚀而迎风与背风面的积雪发生沉积,沉积区和侵蚀区之间形成与风向角一致的分界线。所有风向角下积雪漂移均使单层柱面网壳的动力稳定性降低。风吹雪时间越长,结构的动力稳定性越差,但影响程度随时间逐渐降低。随着初始积雪厚度的增加,结构的动力稳定性降低。     

基于强度折减法的边坡动力稳定性研究

基于强度折减法,利用有限元软件ANSYS建立二维边坡模型,采用Plane82单元进行网格划分,并对可能出现的滑坡体细化网格,研究爆破荷载作用下的边坡动力稳定性。以计算不收敛作为边坡失稳的判定准则,求得边坡静力安全系数为2.36;以边坡坡底节点位移突变作为边坡动力失稳的判定准则,求得爆破动荷载作用下的边坡动力安全系数为2.24。模拟结果表明:给定边坡在当前爆破动荷载作用下,其动力安全系数相对于静力安全系数降低了5.08%。通过对坡底节点位移时程曲线的变化规律进行研究发现,随着边坡抗剪强度参数的折减,边坡坡底节点的水平最大位移也在缓慢增加,主要是由于边坡强度折减造成的塑性位移增加。     

软土浅埋框架结构抗震计算方法评价

摘要：我国地下工程结构的抗震设计主要采用等效静力法,国外则提出了自由场变形法、反应位移法等方法。本文分析评价等效静力法、自由场变形法和反应位移法应用于软土地下结构抗震设计的差异。分析中设定表征软土地下结构的基准模型,并以动力时程分析给出地下结构的地震响应。假定地质条件和结构埋深不变,通过比较分析随基准模型的结构刚度变化时计算变形和内力的差异,评价三种拟静力地下结构抗震设计方法的适用性。结果表明,等效静力法对刚度较大的浅埋框架结构适用性较好,其它情况计算的结构变形和内力偏大;自由场变形法适用于浅埋框架结构刚度较小或与周围地层刚度接近的情况,且倒三角形分布力模式优于集中力模式;反应位移法可适用于不同刚度软土浅埋框架结构的抗震设计。     

乐音准则法及其在结构损伤诊断中的应用研究

为了寻求一种简便、高效的结构损伤识别方法,组织实施了瓷盘、钢壳及5层钢框架的模型试验,根据试验结果,提出了一种基于欧姆听觉定律的结构损伤识别方法,即“乐音准则法”。该方法对于诊断均匀、规整的一维（杆、索）或二维（板、壳）结构的损伤十分有效。试验结果表明,均匀、规则结构的固有振动频谱具有与乐音相类似的特征,在其发生损伤后,固有振动频谱发生明显变化,据此定义了“乐音准则法”。试验证明,该方法对识别瓷盘损伤敏感有效;对钢壳径向损伤有效,对环向损伤不敏感,主要原因是环向损伤对壳体结构刚度影响较小。在5层钢框架的损伤识别试验中,根据乐音准则法,首次提出运用频率比作为结构损伤诊断指标,成功地识别出了结构损伤。     

单层穹顶网壳在地震作用下的动力稳定分析

根据网壳结构的力学稳定判别准则，分析了单层穹顶网壳在ｐａｓａｄｅｎａ波（取不同时间压缩化）作用下的稳定性能，并将其动力稳定临界荷载与静力临界荷载相对比，得出一些重要结论。     

湿度对手机用纸浆模塑缓冲包装能量吸收特性的影响

基于动态冲击试验,分析了纸浆模塑制品在冲击高度为30 cm、50 cm、80 cm及相对湿度为50%、65%、80%、90%条件下的荷载-位移数据,建立了名义应力-应变曲线和能量吸收曲线。结果表明:冲击高度一定时,随着相对湿度的增大,纸浆模塑制品的名义应力-应变曲线下移,弹性模量、屈曲临界应力明显下降,最佳能量吸收肩点向左下方移动,以RH=50%为基准,RH=65%、80%、90%时的平均第二屈曲临界应力分别下降14%、24%和42%;相对湿度一定时,随着冲击高度的增加,纸浆模塑制品的名义应力-应变曲线上移,弹性模量、屈曲临界应力明显提高,最佳能量吸收肩点向右上方移动,并且冲击高度的影响随相对湿度的增加显著增强;制品在承受冲击荷载时发生两次明显屈曲。     

OPTIMIZATION OF GEOMETRICALLY NONLINEAR STUCTURES WITH RESPECT TO BOTH BUCKLING AND POSTBUCKLING CONSTRAINTS

In this paper the optimization of structures against instability is carried out, and nonlinear behaviour of designed elements is taken into account. The form of the deformation path for an optimal element is analysed to check whether that behaviour is stable or not. In the latter case the formulation of the design problem is modified with a view to include constraints that take care of a stable form of the postbuckling path of the structure. A suitable form of the postbuckling constraints is proposed depending on the type of instability. If bifurcation occurs the constraints will force the structure to behave in a stable way after buckling. When a limit point is reached without bifurcation of the equilibrium the formulation is modified in such a way that the resulting structure does not iose its stability at all. For structures for which design can modify postbuckling behaviour without affecting buckling load, the postbuckling constraints are chosen so as to ensure stable behaviour of the structures after buckling. When the critical state does not exist and instability occurs at finite displacements the minimum load value of the deformation path is maximized.     

Dynamic buckling of limit-point systems under step loading

The nonlinear dynamic buckling response of discrete systems under step loading of infinite duration is thoroughly discussed by using one-degree-of-freedom models. The analysis is based on the exact nonlinear differential equations of motions and refers to those systems which when subjected to the same loading applied statically exhibit a limit-point instability. It is found that an unbounded motion may start for the smallest step load which forces the system to pass through an unstable equilibrium state of the postbuckling path with zero total potential energy. This leads to dynamic buckling criteria which allow the determination of exact dynamic buckling loads without solving the corresponding nonlinear differential equations of motion. A comparison of dynamic buckling estimates of previous works with those obtained herein shows that the latter are exact regardless of the magnitude of the initial imperfection. Moreover, some additional results provide a better insight into the actual mechanism of nonlinear dynamic buckling associated with the foregoing type of loading.     

Dynamic buckling of limit-point systems under step loading

The nonlinear dynamic buckling response of discrete systems under step loading of infinite duration is thoroughly discussed by using one-degree-of-freedom models. The analysis is based on the exact nonlinear differential equations of motions and refers to those systems which when subjected to the same loading applied statically exhibit a limit-point instability. It is found that an unbounded motion may start for the smallest step load which forces the system to pass through an unstable equilibrium state of the postbuckling path with zero total potential energy. This leads to dynamic buckling criteria which allow the determination of exact dynamic buckling loads without solving the corresponding nonlinear differential equations of motion. A comparison of dynamic buckling estimates of previous works with those obtained herein shows that the latter are exact regardless of the magnitude of the initial imperfection. Moreover, some additional results provide a better insight into the actual mechanism of nonlinear dynamic buckling associated with the foregoing type of loading.     

Multiparametric sufficient criterion of quasi-brittle fracture for complicated stress state

Behaviour of an atomic structure in the vicinity of the crack tip is simulated. Buckling and postbuckling deformation of three- and four-atomic cells at generalized tension is studied. A discrete-integral fracture criterion is proposed for normal rupture cracks when stress fields have a singular component, vectors of fields of stresses and deformations being collinear. When formulating the proposed criterion, we used the new rank of solutions, which differ from solutions applied in formulating the classical sufficient fracture criterion, in conformity with Novozhilov’s hybrid model. The proposed criterion permits limiting passage to necessary criterion when energetic characteristics of postbuckling deformation of cells can be neglected within the limit. Values of critical loads obtained in conformity with the sufficient criterion essentially differ from those obtained in conformity with the necessary criterion.     

A stiffness parameter and truncation error criterion for adaptive path following in structural mechanics

We explore a truncation error criterion to steer adaptive step length refinement and coarsening in incremental-iterative path following procedures, applied to problems in large-deformation structural mechanics. Elaborating on ideas proposed by Bergan and collaborators in the 1970s, we first describe an easily computable scalar stiffness parameter whose sign and rate of change provide reliable information on the local behavior and complexity of the equilibrium path. We then derive a simple scaling law that adaptively adjusts the length of the next step based on the rate of change of the stiffness parameter at previous points on the path. We show that this scaling is equivalent to keeping a local truncation error constant in each step. We demonstrate with numerical examples that our adaptive method follows a path with a significantly reduced number of points compared to an analysis with uniform step length of the same fidelity level. A comparison with Abaqus illustrates that the truncation error criterion effectively concentrates points around the smallest-scale features of the path, which is generally not possible with automatic incrementation solely based on local convergence properties.     

扁拱的面内非线性稳定与突变分析

通过位移、应变能突变分析扁拱结构受简谐荷载作用特性。采用谐波平衡法由扁拱非线性振动方程获得位移、频率关系尖点突变模型,据位移突变判别式分析扁拱非线性响应;用系统能量原理及有限元软件,借助突变理论导出结构失稳的应变能突变准则,并比较位移与应变能突变判别方式差异。结果表明,扁拱受简谐荷载作用的位移或应变能会发生突变,跨度、矢高及荷载对突变均有影响;用位移、应变能突变判别式计算结果基本一致,且各有优势及不足。       

Capturing mode-switching in postbuckling composite panels using a modified explicit procedure

A postbuckling blade-stiffened composite panel was loaded in uniaxial compression, until failure. During loading beyond initial buckling, this panel was observed to undergo a secondary instability characterised by a dynamic mode shape change. These abrupt changes cause considerable numerical difficulties using standard path-following quasi-static solution procedures in finite element analysis. Improved methods such as the arc-length-related procedures do better at traversing certain critical points along an equilibrium path but these procedures may also encounter difficulties in highly non-linear problems. This paper presents a robust, modified explicit dynamic analysis for the modelling of postbuckling structures. This method was shown to predict the mode-switch with good accuracy and is more efficient than standard explicit dynamic analysis.     

Experimental and finite element analysis of fracture criterion in general yielding fracture mechanics

Efforts made over the last three decades to understand the fracture behaviour of structural materials in elastic and elasto-plastic fracture mechanics are numerous, whereas investigations related to fracture behaviour of materials in thin sheets or general yielding fracture regimes are limited in number. Engineering simulative tests are being used to characterize formability and drawability of sheet metals. However, these tests do not assure consistency in quality of sheet metal products. The prevention of failure in stressed structural components currently requires fracture mechanics based design parameters like critical load, critical crack-tip opening displacement or fracture toughness. The present attempt would aim to fulfill this gap and generate more information thereby increased understanding on fracture behaviour of sheet metals. In the present investigation, using a recently developed technique for determining fracture criteria in sheet metals, results are generated on critical CTOD and fracture toughness. Finite element analysis was performed to support the results on various fracture parameters. The differences are within 1 to 4%. At the end it is concluded that magnitude of critical CTOD and/or critical load can be used as a fracture criterion for thin sheets.     

构件跌落碰撞的数值模拟研究

针对构件跌落碰撞问题,将Hertz-damp模型与有限元方法结合,给出了结构碰撞位移突加约束条件,建立了碰撞结构体系的动力方程。该方法能考虑碰撞体质量分布对碰撞过程的影响,同时可描述结构碰撞过程中的刚度非线性和能量损失。建立了构件跌落碰撞模型,初步分析了跌落构件质量、质量分布以及跌落高度等参数对碰撞过程的影响。研究表明:结构碰撞模拟中碰撞体质量沿接触面法线轴向不对称时应考虑其质量分布,否则会高估碰撞反应;碰撞荷载峰值和单次碰撞持续时间与跌落构件质量和跌落高度均成指数关系,碰撞荷载峰值随跌落构件质量和跌落高度增大而增大,而单次碰撞持续时间则随跌落构件质量增大而增大,随跌落高度增大而减小。这些规律能为减轻结构碰撞破坏研究提供参考。