典型密封元件的失效分析与失效过程研究

飞机整体油箱结构的密封性对飞机安全性和续航能力有很大影响,所以密封胶失效分析和失效过程研究就显得相当重要．为了研究密封胶的失效,根据密封胶的受力特点,本文设计了典型元件的密封试验（平板型和T型）,通过试验得到了这两种结构的失效载荷和最大载荷．同时,利用有限元模拟软件ABAQUS对试验进行了模拟计算,使用cohesive模块实现对密封胶层的模拟．通过观察胶层失效过程,得到了胶层失效后的破坏路径,找到了容易发生泄漏的位置,并给出了失效载荷与最大载荷的关系．通过对比试验结果和模拟计算结果,验证了有限元模拟分析的可靠性,并对结果进行了讨论．     

杆系结构静强度和疲劳失效机理及可靠性分析

根据疲劳载荷造成的疲劳累积损伤对结构极限承载力的影响情况,讨论了结构系统承载能力的可靠度计算方法,分析了结构系统中每一个单元在静载和疲劳载荷作用下的两种失效模式,并考虑了二者之间的相关性对该单元可靠性的影响,具体分析了结构系统在这两种载荷作用下的失效机理,给出了在这两种载荷作用下结构系统的可靠性分析方法. 算例表明,在不同的使用年限内,静载和疲劳载荷对结构系统可靠性的影响是不同的;在结构系统主要失效路径中既有单元静强度失效又有单元疲劳失效,这是符合结构系统使用真实情况的.     

基于板理论的层级褶皱结构失效模式分析

本文基于弹性板理论和夹层板理论对二级层级褶皱结构失效模式进行了分析。通过对基本构件进行受力分析得到了载荷与结构变形之间的关系。根据6种失效模式的定义,从极限载荷或极限应力角度出发,分析了在压缩载荷和剪切载荷工况下的各种失效模式,给出了结构单胞对应的等效正应力和等效切应力表达式。由最小失效强度得到了各失效模式之间的占优关系,并构建了失效机理图来阐释这一机制。最后通过与有限元分析结果比较,分析了本文公式的精度,与数值解吻合较好。     

枚举模糊随机主要失效模式的准则法

基于模糊不确定性向随机不确定性的等价转换和有限元分析计算，建立了以失效模式相对重要度为依据的广义主要失效模式枚举的工程准则法．该方法可以综合考虑材料参数、结构几何以及外载荷等基本变量的随机和模糊两种不确定性因素对失效模式相对重要度的影响，通过合理选择门槛值，可以在保证计算精度的基础上尽可能的减小计算工作量．以平尾转轴的模糊随机可靠性分析为算例，给出了门槛值的经验值，并说明了所建工程准则法的合理性．     

爆炸载荷作用下加筋板的失效模式分析及结构优化设计

通过对爆炸载荷下具有1根加强筋的固支矩形加筋板的有限元模拟，探讨了抗爆加筋板结构优化设计方法，分析了加筋板的失效模式以及加强筋相对刚度和冲击载荷强度对加筋板失效模式的影响，指出了失效模式Ⅰ下的3种变形模态以及失效模式Ⅱ下的2种子失效模式，得到了失效模式Ⅰ下加强筋和加筋板最大挠度的近似计算公式，提出了单根加筋板的两种失效模式的判别条件，并对具有1根加强筋的固支矩形加筋板抗爆结构进行了优化设计。结果表明，通过数值模拟或模型实验可以求得任意加筋板结构由发生塑性大变形到发生破损的临界条件，从而确定抗爆性能最强时加筋板的质量与加强筋横截面尺寸及间距间的关系，实现对抗爆加筋板结构的优化设计。     

含缺口复合材料层合板的三维有限元失效分析

本文研究了复合材料层合板最终强度计算的有限元理论,讨论了修正的Newton-Raphson迭代方法在层合板失效过程中应力场计算的迭代过程.同时本文建立了带缺口的复合材料层合板三维有限元模型,充分考虑层合板的纤维断裂、基体开裂和分层三种失效模式,采用修正的三维Hashin准则作为失效判断的依据,计算了三种不同铺层的层合板最终失效载荷值,与试验值吻合得很好.鉴于层压板材料常数ν23的数值难于测定的特点,讨论其对最终失效载荷的影响.在三维有限元模型的基础上,实现了失效扩展仿真分析.     

真实随机环境中大型结构的强度可靠性分析方法

将标准有限元和已有的迭代响应面法相结合,并将标准有限元与已有的改进的均值法相结合,给出了这两种适用于多随机性和多失效模式的真实环境中大型复杂结构的强度可靠性分析的方法。文中以算例分析了这两种方法的有效性和适用范围,指出解决高度非线性隐式极限状态方程问题可能的较为精确的方法。最后以真实的复杂飞机结构为例,分别用两种方法进行了考虑载荷和材料性能参数随机性的强度可靠性分析,并得到具有工程意义的结论。     

约束修正失效评定曲线及内压管道失效分析

用A2参数描述裂纹尖端约束效应,考虑裂纹尖端约束水平对断裂阻力的影响,对R6失效评定曲线进行修正。以X80钢为例,根据不同约束水平试件测试得到的JR–△a曲线,给出了与裂纹尖端约束相关的失效评定曲线。在使用有限元对带裂纹内压管道进行弹塑性分析的基础上,利用失效评定曲线对失效载荷进行预测;与试验结果比较表明:采用约束修正失效评定曲线,在对存在缺陷的构件进行安全性评定、寿命评估、失效概率计算时,考虑约束效应对断裂韧度的影响可以更充分地发挥构件的强度潜力。     

关于结构塑性动力失效若干问题的研究进展

重点分析常见工程结构元件（如梁、板、圆柱壳、圆环和夹层及复合材料结构）在强动短时载荷下塑性动力失效的研究方法、进展及尚待解决的问题，这些结构元件的失效模式及分析方法具有普遍意义，建议借用无量纲数──损伤数（ｄａｍａｇｅｎｕｍｂｅｒ）──来统一表征结构塑性动力失效的判据，最后以海洋平台和桁架为例简单介绍工程中复杂结构动力失效分析的情况。     

端头带有质量块的悬臂梁在冲击载荷下的两种剪切失效模型

剪切失效是强动载荷作用下结构失效的重要模式。本文给出了计及梁的转动惯量时端头带有质量块的悬臂梁结构受到冲击载荷作用后发生剪切失效的无量纲判据。分析表明，在初始速度间断面上是否发生剪切失效取决于质量块初始动能和质量块尺寸与梁厚之比，而与梁的长度无关。梁的转动惯量对于剪切失效具有不可忽略的影响。     

非完全相关荷载下钢筋混凝土框架结构体系可靠度分析

目前框架结构体系可靠度分析大多是在荷载完全相关的假定下完成的,对于非完全相关荷载作用下的体系可靠度研究则鲜有涉及,对于非理想弹塑性或弹脆性的结构尤其如此.本文在随机系统分析的概率密度演化理论的基础上,结合结构非线性全过程分析的位移控制算法,推导了基于归一化位移参数的结构非线性发展概率密度演化方程和承载力裕度的概率密度演化方程.采用纤维梁柱单元进行非线性分析,研究了钢筋混凝土框架结构在非完全相关荷载下的体系可靠度,并与Monte Carlo法进行了对比分析,证明了文中建议方法的可行性.基于文中方法,分析了荷载相关性对可靠度的影响,计算结果表明,荷载相关性对体系可靠度有比较明显的影响.     

A general failure criterion for spot welds under combined loading conditions

The circumferential failure mode of spot welds is investigated under combined loading conditions. Failure mechanisms of spot welds under different loading conditions are first examined by the experimental observations and a plane stress finite element analysis. An approximate limit load analysis for spot welds is then conducted to understand the failure loads of spot welds under combinations of resultant forces and resultant moments with consideration of the global equilibrium conditions only. The approximate limit load solution for circumferential failure is expressed in terms of sheet thickness, nugget diameter and combinations of loads. Failure contours are generated for spot welds under opening and shear loading conditions. The results indicate that failure contours become smaller when the ratio of the sheet thickness to the nugget diameter increases. Based on the approximate limit load solution, a general quadratic failure criterion for spot welds under combined three resultant forces and three resultant moments is proposed with correction factors determined by fitting to the experimental results of spot welds under combined loading conditions. The failure criterion can be used to characterize the failure loads of spot welds with consideration of the effects of sheet thickness, nugget diameter and combinations of loads. Experimental spot weld failure loads under combined opening and shear loading conditions and those under combined shear and twisting loading conditions are shown to be characterized well by the proposed failure criterion. Finally, a simplified general failure criterion for spot welds under three resultant forces and three resultant moments is proposed by neglecting the coupling terms of the resultant forces and moments for convenient use of the failure criterion for engineering applications.     

考虑破坏时复合材料结构的分析

本文考虑复合材料结构发生局部破坏时的分析问题,这是一个非线性问题。本文引入带步长限制的变步长增量载荷方案作为解决这一问题的工程途径。用有限元法作为结构分析的基本手段。经试验结果对比,初步表明该方案是可行的。     

Crystallographic failure analysis of film near cooling hole under temperature gradient of nickel-based single crystal superalloys

In this paper, a unit cell model with a film cooling hole has been set up to analyze the crystallographic stress characterization and failure behavior under temperature gradient of nickel-base single crystallographic superalloys (SC). The aim of this work is to study the failure behavior of SC blades with film cooling. The distribution of cooling air pressure on the hole side surface and the distribution of the temperature around the hole are obtained from the fluid analysis. The result of the temperature distribution is then transferred to the finite element model (cell model) by the interpolation method. The cell model is analyzed by the crystallographic rate dependent finite element method (FEM). Special attention is put on the influence of temperature gradient. The influence of the loading boundaries, i.e. displacement loading and stress loading, on the stress characterization around hole is also taken into consideration. The results show that temperature gradient hole has much influence on the stress characterization. Different types of loading boundaries result in different types of stress and strain distributions. There is clear stress concentration near the hole under displacement loading, while there is clear strain concentration under stress loading. The failure characterization has been studied by the strain energy density criterion. It is shown that the temperature gradient has influence on the failure behavior.     

Numerical simulation of the failure process of unreinforced masonry walls due to concentrated static and dynamic loading

Finite element analyses of brick masonry subjected to in-plane concentrated static and dynamic loads are carried out to study crack initiation and propagation during the failure process of unreinforced masonry walls. The numerical model is firstly validated by the experimental tests by using the same materials parameters and loading conditions. Then, the static and dynamic concentrated loads are applied to the mortar joints and brick, respectively, and numerical simulations are used to compare the fracture characteristics for these loads. In addition, a comparison of fracture mechanisms for the concentrated loads on the mortar joint and brick is also given. Finally, the effect of dynamic pressure (P_max) on the failure mechanism of brick masonry is considered.     

Limit analysis of space grids

The paper presents generalized, unique solutions for the limit design of pin-connected, double-layer space grids of regular construction, simply-supported along the sides of a rectangle and carrying a uniform concentration of normal nodal loading applied to the upper layer of the structure. The principal assumption upon which the foregoing results are obtained is that member forces increase monotonically with the applied loads, therefore insuring against the inducement of higher than ultimate loads in the compressive elements at anytime prior to structure failure. The virtual work method of the plastic analysis has been employed in conjunction with the techniques of the finite difference calculus to study the various collapse modes of the system. The work is concluded with an illustrative example to demonstrate the simplicity of the proposed solutions.     

基于夏比冲击试验的材料失效模型参数

结合夏比冲击试验和ABAQUS显式动力数值模拟,对Q370d钢进行了Johnson-Cook失效模型参数研究。首先,在不考虑材料失效的情况下,通过3种不同厚度的无缺口试件冲击实验对有限元模型参数设置和材料本构模型的准确性进行了验证,同时还讨论了试件断裂区网格的合适尺寸;在此基础上,基于正交设计,通过大量的有限元数值模拟得到失效模型参数样本,利用回归分析求得冲击功与失效模型参数的回归方程组;最后结合夏比V型缺口冲击试验,求解Q370d钢的失效模型参数,并对断裂截面的力学特性进行了分析,可为工程应用提供参考。     

复合材料失效判据有效性验证方法研究

为系统地验证复合材料失效判据计算精度和有效性范围,给出了4种材料体系、6种铺层形式的层合板在单轴、双轴载荷下的失效试验数据,用以评估复合材料失效判据在单向层合板失效包线、多向层合板初始失效包线、多向层合板最终失效包线、层合板变形及层合板的破坏特性等五个方面的预测能力。并根据验证方法和有效性评估策略对失效判据计算精度进行量化考核,给出了失效判据在五个层面上的计算精度。     

Damage analysis by boundary element method

The response of cracked bodies subjected to loading was investigated by the boundary element method in this paper. The two-law elastic-cohesive-softening model was used for crack propagation analysis. The interface conditions for uncracked, craze, open crack, adhesive crack and slid crack parts were discussed and the corresponding incremental iteration algorithm was given. A simplified damage propagation model was presented. The technique has been applied to some specific examples which give the evidence that the method is satisfactory and efficient.     

网架结构拟夹层板法的有限元验证

用拟夹层板法和有限元法对网架结构进行分析,对三类屋面网架（正放四角锥网架、两向正交正放网架和正放抽空四角锥网架）进行了均布荷载、局部荷载（半跨均布荷载）作用下的静力分析以及固有振动分析,对三类竖向承重网架墙体进行了稳定性分析。通过与有限元法分析结果的对比,表明了拟夹层板法作为一种简化的计算方法,其精度是比较高的,绝大多数的误差都小于5％,是可以直接用于工程结构设计的一种有效方法。此外,拟夹层板法还可作为一种在宏观上检验有限元建模正确与否的实用方法。