循环载荷下缺口板的弹塑性应变分析——塑性各向异性强化理论的应用

为了估算承受低周疲劳时工程构件的寿命,需要研究缺口件在低周交变负荷时的应变分布.由于循环载荷作用下材料的鲍兴效应不能忽略,需要应用塑性各向异性强化模型.从本文的计算结果与实验结果的比较可以看出即使是线性随动强化模型对解决循环加载下的应变计算还是很有效的.     

一种镍基单晶超合金高温低周疲劳的晶体取向相关性模型

在950℃t对[001]、[012]、[112]、[011]和[114]晶体取向的镍基单晶超合金DD3试样进行了对称循环低周疲劳(ICF)试验。应变率取1.0×10-2,1.33×10-3,0.33×10-3s-1.试验结果表明,LCF特性显着地取决于晶体取向和应变率。试样断口细观分析表明,除了[001]取向试样外,其余所有试样断口上均有明显的等间距疲劳纹。这些疲劳纹由微裂纹组成,其间距取决于试样的晶体取向和总应变范围。基于晶体滑移理论,建立了疲劳纹间距和总分切应变范围及取向和应变率函数的一个简单关系。对Lall-Chin-Pope(LCP)模型进行修正并推广应用于循环塑性和疲劳寿命研究,提出了一个晶体取向和应变率参数,该参数可以很好地描述镍基单晶超合金高温低周疲劳循环塑性和疲劳寿命的晶体取向和应变率相关性。     

泥质盐岩单轴蠕变寿命研究

岩石流变力学的研究中，蠕变寿命是一个重要问题.由于长期蠕变试验资料的缺乏，难以估计蠕变破坏时间.该文进行了泥质盐岩单轴全应力 应变压缩试验，并采用陈氏加载法进行了单轴蠕变试验.对蠕变曲线进行了处理，获得了不同应力水平下的蠕变曲线簇，进而得到了等时应力-应变曲线簇.通过拟合分析，建立了等时应力-应变曲线割线模量随时间变化关系模型和等时应力 应变曲线的数学模型.对等时应力 应变曲线与全应力-应变曲线之间的关系进行了分析，获得了蠕变破坏强度和破坏应变分别与蠕变寿命之间的数学表达式.该文研究成果可以估计泥质盐岩的蠕变寿命、长期强度、长期模量、蠕变破坏线和蠕变终止线，对相关岩石流变寿命的估计具有借鉴意义.     

一个用于低周疲劳寿命预测的损伤函数

选择循环塑性应变能作为损伤变量,建立了它的瞬态响应数字模型,采用损伤力学分析方法,导出了计及循环相关的非线性疲劳损伤函数,得到了用于低周疲劳寿命预测的数学公式。利用该损伤函数预测的低周疲劳寿命与试验结果符合较好。     

TFR、TRV和CE模型序加试验下WEIBULL分布产品的失效分布

本针对TFR模型，首次提出将步加试验推广至序加试验，就两参数Weibull分布给出了损伤因子函数，同时给出了产品寿命的残存函数，另外针对TRV模型，在序加试验下就两参数Weibull分布给出了损伤系数，同时给出了产品寿命的残存函数。     

典型循环本构模型对316L不锈钢单轴棘轮行为预测结果评估

分别采用Dafalias-Popov模型,AF模型,OW II模型,AF-OWΙ模型和AF-OW II五种典型循环本构模型对316L不锈钢单轴棘轮行为进行模拟和预测,并对预测结果进行评估。结果表明:Dafalias-Popov模型在循环前期对材料的棘轮应变演化描述得比较合理,但最终预测到安定的棘轮效应;AF模型对单轴棘轮的预测值偏大;AF-OWΙ模型不能较合理地描述棘轮应变率的演化;AF-OW II模型和OW II模型对单轴棘轮应变及其演化速率能够进行合理地预测,且OW II模型比AF-OW II模型预测的棘轮应变值更低。进一步在此基础上对AF-OWΙ模型进行了改进,提高了其对单轴棘轮行为的预测能力。     

短纤维复合材料的本征应变边界积分方程计算模型

提出了短纤维复合材料的本征应变边界积分方程计算模型,并采用新发展的边界点法进行了求解．模型依据Eshelby等效夹杂物的概念并借助Eshelby张量,采用迭代方法来计算基体中各种性能短纤维的本征应变,其中所需的Eshelby张量不难通过解析或数值方法获得．由于未知量只出现在边界上,与已有的有限元和边界元模型相比,提出的计算模型可极大地减小异质体问题的求解规模,提高计算效率．通过数值算例计算了代表性体积单元上各种短纤维复合材料的整体弹性性能,验证了计算模型和求解方法的正确性和有效性．     

用概率断裂力学方法预测管状接头的疲劳寿命

本文研究了Ｔ型管状接头在面内和面外弯曲循环载荷作用下的疲劳强度问题，通过将材料常数和初始裂纹尺寸作为随机变量，采用ＭｏｎＣａｒｌｏ模拟方法对疲劳寿命进行了统计分析．同时，对计算结果进行了线性回归，并与实验结果作了比较。     

有关《三角形的全等判定》教学的一个想法

在现行课本中 ,三个三角形全等判定的公理顺序为 :边角边 ,角边角 ,边边边 .在此我有一个教学的想法 ,将前两个公理的教学顺序交换一下 .这个想法来源于我对角边角公理的一次教学过程的设计 .1　角边角公理教学过程设计的中心内容对于角边角公理的教学过程我分了三个部分 :公理的引入 ,公理的明确 ,公理的巩固 .与教材不同的是 ,我用一个生活中的实例设计问题情景引入公理 .这就是问题一 :有一块三角形玻璃碎成如图所示的两块 ,如果要将其复原 ,是不是两块都要带去 ?面对这样的问题学生有了兴趣而且议论纷纷 ,答案不一 .在此时教师应提出问…     

描述低周疲劳裂纹扩展速率的循环J积分新参量

探讨了低周疲劳加载条件下的应力增量．应变增量关系，提出了模拟裂纹疲劳扩展的二维模型以建立新的循环．积分参量，详细阐述了该积分参量的定义、主要特点、物理意义以及数值计算方法，并通过紧凑拉伸试样的疲劳试验检验该积分参量的有效性．结果表明：该积分参量能够较好描述恒幅低周疲劳裂纹的扩展速率．此外，基于积分参量体系，从能量的角度解释了疲劳迟滞现象．     

Mechanics of Pb40/Sn60 near-eutectic solder alloys subjected to vibrations

Pb40/Sn60 is the most commonly used solder alloy for microelectronics packaging. It is well understood that, heat generated by the circuits when a semiconductor device is on and the coefficient of thermal expansion mismatch between the soldered layers lead to the thermal fatigue of the solder joints. On the other hand, there is very little research done to understand failure mechanics of solder joints when microelectronics devices are subjected to vibrations. In this study, it is shown that dynamic stresses contribute to the failure mechanism and in certain circumstances they can become the dominant failure cause when semiconductor devices are used in a vibrating environment. The purpose of this study is to understand the mechanics of the vibration induced damage in solder interconnects. A material nonlinear time domain dynamic analysis of a solder joint was performed using a damage mechanics based constitutive model. The analysis was done for isothermal condition and no thermal loads were considered. The study included a wide range of frequency and acceleration combinations.     

The effective permeability of the underfill flow domain in flip-chip packaging

In order to reach the goals of high electrical performance and dense packaging within the limited space, the flip-chip technology becomes popular in electronics packaging. In the flip-chip assembly, difference between thermal expansion coefficients of the chip and substrate may cause thermal fatigue at solder joints. To avoid this thermal fatigue, epoxy encapsulant is filled into the gap between the substrate and chip by the capillary force. Because of the small space in the flow domain, the underfilling flow can be assumed as a flow in porous medium. Permeability is used to characterize the flow field of the space among the substrate, chip, and solder bumps. In this study, a numerical method is used to determine the effective permeability for the underfilling flow domain. Analysis of the three dimensional flow in a unit cell of the underfill flow domain is performed. The resulting average velocity and pressure gradient are used to calculate the apparent permeability. Comparison with the analytical approximation for the permeability in literature is also performed. The effective permeability calculated using the proposed numerical method gives reasonable prediction of the underfill flow as compared to the experimental result.     

Numerical and experimental investigations of phase segragation effects in binary brazing solders

Solder materials occupy many of fields for technical application (e.g. solder joints in automotive control units or in microelectronic packages). They are required to provide electrical and mechanical connections between different components. Due to their wide range of applications solder alloys are subject to a great variety of microstructural changes such as phase separation and coarsening processes. The micromorphological variations influence strength and life expectation of solder materials, in particular, in very small components such as solder joints in microelectronic packages. In order to analyze the microstructural evolution with a diffusion theory of heterogeneous solid mixtures we employ an extended Cahn-Hilliard phase field model. The diffusion equation under consideration constitutes a partial differential equation involving spatial derivatives of fourth order. Thus, the variational formulation of the problem requires approximation functions which are piecewise smooth and globally C1-continuous. In our contribution we fulfil the continuity requirement by means of rational B-spline finite element basis functions. To illustrate the versatility of this approach numerical simulations of phase decomposition and coarsening controlled by diffusion and by mechanical loading are discussed and compared with experimental results. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

表面组装焊点形态模型的建立及求解

基于SnAgCuRE钎料焊点成形时受力平衡方程,根据表面组装RC1206元件焊点结构,采用数学分析方法建立了焊点形态的数学模型,预测了片式元件的SnAgCuRE钎料焊点形态.     

Void dynamics in lead-free Sn-Ag-Cu solder joints

As a result of thermal and mechanical loading of Sn-Ag-Cu solder joints pores, flaws and voids may nucleate and grow. Such void nucleation is studied here by means of a phase-field approach which accounts for the decomposition of the solder into phases described by its concentration c. In this investigation, the void growth results from boundary inward flux only, effects due to grain size and interstitials are not involved, cf. [1]. The free energy functional is approximated by a second order Taylor expansion and thus composed of a bulk free energy density and a Ginzburg-type gradient term. The bulk free energy density ϕ follows a classical Ginzburg-Landau double-well potential. The gradient-energy coefficient κ depends on ϕ and is calculated similar to [2]. Experimental data have been adapted for the modeling of the temporal evolution of the concentration of voids located on a square domain in 2D. The simulation is based on a B-spline finite element analysis. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stochastic modeling of fatigue crack propagation

This paper presents a stochastic model of fatigue-induced crack propagation in metallic materials. The crack growth rate predicted by the model is guaranteed to be non-negative. The model structure is built upon the underlying principle of Karhunen–Loève expansion and does not require solutions of stochastic differential equations in either Wiener integral or Itô integral setting. As such this crack propagation model can be readily adapted to damage monitoring and remaining life prediction of stressed structures. The model results have been verified by comparison with experimental data of time-dependent fatigue crack statistics for 2024-T3 and 7075-T6 aluminum alloys.     

基于LiToSim平台的疲劳寿命评估LtsFatigue软件开发及应用

针对结构的疲劳问题,考虑随机载荷作用,在自主开发的LiToSim平台基础上,嵌入结构疲劳分析数值计算程序。基于LiToSim平台开发LtsFatigue疲劳软件,运用时域疲劳算法,通过雨流计数法对应力时程曲线处理并计算结构疲劳寿命;引入频域疲劳算法,基于应力响应功率谱根据应力循环分布估算疲劳寿命;通过齿轮算例进行疲劳分析,与商业软件对比,验证了LtsFatigue定制化疲劳软件时域法、频域法的计算精度,同时,频域算法有效提高了计算效率,凸显了LtsFatigue软件的优势。基于LiToSim平台的LtsFatigue定制化疲劳软件开发,对大型复杂结构的疲劳仿真具有重要的应用价值。     

Improved difference schemes for the Dirichlet problem of Poisson's equation in the neighbourhood of corners

Summary The standard 5-point difference scheme for the model problem u=f on a special polygonal domain with given boundary values is modified at a few points in the neighbourhood of the corners in such a way that the order of convergence at interior points is the same as in the case of a smooth boundary. As a side result improved error bounds for the usual method in the neighbourhood of corners are given.     

Statistical analysis of progressive stress accelerated life test for the product of two-parameter Laplace BS fatigue life distribution under inverse power law model

Based on the product of two-parameter Laplace Birnbaum-Saunders fatigue life distribution, its failure distribution mode is theoretically derived under the progressive stress accelerated life test with inverse power law model, and then three-parameter generalized Laplace Birnbaum-Saunders fatigue life distribution is introduced. The basic properties of three-parameter generalized Laplace Birnbaum-Saunders fatigue life distribution are analyzed, and the image characteristics of its density function, failure rate function and average failure rate function are investigated. Meanwhile, the point estimate method is given for three parameters, and then the point estimates of parameters are obtained for the product of two-parameter Laplace Birnbaum-Saunders fatigue life distribution under the progressive stress accelerated life test with inverse power law model. In addition, the practical example and simulation examples are illustrated to show the feasibility of the proposed method.