LZ50车轴钢疲劳长裂纹扩展的概率模型

试验研究了随机疲劳长裂纹扩展率概率模型．首先发展了考虑平均应力影响和低应力强度因子范围趋于门槛值现象的扩展率方程．进而提出了同时考虑试验数据分散性规律与样本数量对概率评价影响的概率模型,模型参量由线性回归法结合极大似然统计分析原理测定．LZ50钢的试验数据分析,验证了方法的有效性与实用性．     

金属材料疲劳裂纹扩展速率估计的分层模型

裂纹扩展速率与应力强度因子幅值的关系曲线,是金属构件损伤容限设计及寿命预测的重要疲劳性能数据．为了充分、合理地运用在不同测试条件下获得的试验数据,分层随机样本模型,将总体样本分为若干层,每一层样本都是在相同的测试条件下获得的试验数据,结合压力容器的实际工作状况,对每一层样本赋予适当的权重,从而对疲劳裂纹扩展速率方程做出更符合实际要求的估计．实际计算表明,对不同炉号试样提供的Q235A级钢材的裂纹扩展速率数据,运用分层随机样本模型得到的结果,明显地优于运用简单随机样本模型分别处理每个炉号试样提供的数据所得的结果．     

铝合金高温断裂韧性的研究

通过6063铝合金的标准试验,确定了常温和高温下材料的弹性模量、屈服强度、破坏强度和应力强度因子。分别采用双线性和理想弹塑性模型,在ABAQUS平台上对标准CT试样的断裂韧性测试过程进行了数值模拟,计算了J积分,得到的应力强度因子和试验值误差4%,模拟的失稳扩展现象与实验所观察的现象一致。     

寿命服从对数正态分布、有置信度时概率疲劳S-N曲线的合理重构

当概率疲劳S-N曲线以特定存活概率(P)和置信度(C)的方式给出并无法重做试验时,除特定P-C外无法做其它概率水平的可靠性分析.因此,需要广泛适用的曲线模型.建立了疲劳寿命服从对数正态分布时疲劳试验S-N数据及广义曲线的Monte Carlo模拟重构方法.为了避免现有人为放大样本到数千给出偏危险评价,从实际试验情况出发,采用了材料小试样每组样本7至20、结构试样每组样本至多10个、还原统计参量误差小于5%的模拟策略.然后,依据模拟数据利用回归法重建了可实现任意P-C水平可靠性分析的P-C-S-N曲线.铁路60Si2Mn高强度弹簧钢概率曲线的重构实践说明了方法的有效性与适用性.     

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．     

结构疲劳长裂纹扩展速率新模型研究

比较现有疲劳长裂纹扩展速率模型的特点和不足,基于裂纹扩展钝化复锐理论推导,提出了广义钝化复锐疲劳长裂纹扩展速率模型(GPLFCPRM),并推导获得了疲劳长裂纹扩展速率模型通项.该模型克服了现有疲劳长裂纹扩展速率模型的缺点,能有效地描述自门槛值到断裂点全过程的裂纹扩展规律,具有明确的物理含义,且可反映材料的强度特性、断裂特性和热处理状态等因素对疲劳裂纹扩展速率的影响.通过LZ 50钢、A533-B、AlZnMgCu 0.5和0.5Cr 0.5Mo 0.25V钢疲劳裂纹扩展速率试验,结果反映该模型与试验结果十分吻合,且具有更广的通用性和应用推广价值.     

带裂纹三点弯曲试样的动态应力强度因子分析

提出了计算带单边裂纹三点弯曲试样动态应力强度因子的新方法．首先由权函数的普遍形式和两种参考载荷下的应力强度因子,得到了带单边裂纹三点弯曲试样的权函数,然后考虑试样的转动惯性和剪切变形,根据振动理论推导出无裂纹梁内的动应力响应和分布,最后由权函数的思想推导出了带裂纹三点弯曲试样动态应力强度因子公式．通过有限元数值计算,验证了该方法的正确性,结果比较表明公式具有较高的精度．另外,还研究了冲击载荷下三点弯曲试样的动态应力强度因子随裂纹长度和加载速率的变化规律．     

粒子群遗传混合算法求解考虑传输时间的FJSP

在某些生产制造场景中,工件在不同机器间的传输时间对车间调度的总拖期具有重要影响,本文基于此扩展了总拖期最小的柔性作业车间调度模型。针对问题模型的复杂性,采用粒子群优化算法和遗传算法的混合算法进行求解。在初始化过程以一定概率优选加工时间和传输时间短的机器并排除调度频繁的机器,使种群在保持多样性的前提下尽量选择优化结果好的个体;采用线性调整的方式动态改变交叉概率和变异概率的值,使种群在遗传算法的不同阶段具有不同的搜索强度;采用粒子群优化算法进行局部搜索,弥补了遗传算法局部搜索能力的不足。最后采用本文方法和其他方法求解柔性作业车间调度问题实例,并对比不同水平层次传输时间下的总拖期,验证了本文方法的有效性。     

修正阻尼最小二乘法在确定脑血流量系数中的应用

本文运用修正阻尼最小二乘法,通过以测定局部脑血流量的试验数据进行数据拟合建立双指数型数学模型 不仅确定了脑血流量系数α的估计值,而且确定了所有模型参数的满意估计。     

左截断右删失下Weibull分布参数估计及其应用

中国出口贸易关系持续性问题一直备受各界学者关注.基于生存分析方法研究左截断右删失数据下Weibull分布的参数估计,并对2003年至2015年中国大陆出口澳门农产品的贸易持续时间进行实证分析,模型拟合效果良好.结果表明中国大陆与澳门贸易失败的概率是逐年递减的,具有负时间依存性.未来应考虑如何跨越门槛值,以延长中国大陆出口贸易关系的持续时间.     

Discontinuous cellular automaton method for crack growth analysis without remeshing

A numerical technical of discontinuous cellular automaton method for crack growth analysis without remeshing is developed. In this method, the level set method is employed to track the crack location and its growth path, where the level set functions and calculation grids are independent, so no explicit meshing for crack surface and no remeshing for crack growth are needed. Then, the discontinuous enrichment shape functions which are enriched by the Heaviside function and the exact near-tip asymptotic field functions are constructed to model the discontinuity of cracks. Finally, a discontinuous cellular automaton theory is proposed, which are composed of cell, neighborhood and updating rules for discontinuous case. There is an advantage that the calculation is only applied on local cell, so no assembled stiffness matrix but only cell stiffness is needed, which can overcome the stiffness matrix assembling difficulty caused by unequal degrees of nodal freedom for different cells, and much easier to consider the local properties of cells. Besides, the present method requires much less computer memory than that of XFEM because of it local property.     

Hexahedral finite element with an open crack

A method of creating the stiffness matrix of a hexahedral eight-node finite element with a single, nonpropagating, transverse, one-edge crack at half of its length is presented in this paper. The crack was modelled by adding an additional flexibility matrix to that of the noncracked element. The terms of the additional matrix have been calculated by use of the laws of fracture mechanics. Employing the elaborated element a numerical test has been worked out, the results of which are compared with the data of analytical solutions accessible in the literature, and a high conformity with them has been obtained. The element presented in the paper may be applied to the static and dynamic analysis of different types of structural elements with material defects in the form of cracks. The described method of creating the stiffness matrix of the element allows to create different kinds of finite elements with cracks provided that the stress intensity factors for a given type of crack are known.     

Identification of the material symmetries with anisotropic damage evolution due to a growing mixed-mode crack

A numerical study of a growing mixed-mode internal crack in a unit cell was undertaken with the help of a finite element simulation. The model enables us to measure the components of the elastic compliance tensor modified by damage as the crack grows, showing the evolution of the anisotropic damage and the evolution of the type of material symmetries. The evolution of the elasticity tensor shows that the damage associated with a growing elliptical crack changes the virgin isotropic properties into orthotropic ones and by crack growth the axes of orthotropic symmetry, initially aligned with the local coordinates of the crack, rotate towards the principle loading axes. Crack propagation is simulated using the stepwise method, which consists of the succession of straight segments and crack growth is governed by the principle of maximum driving force which is a direct consequence of the variational principle of a cracked body in equilibrium and considers the effect of all three stress intensity factors. Without any ad hoc assumption, the crack growth rate is calculated using its thermodynamic duality with the local maximum driving force. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simulation of short crack propagation under hydrogen influence using a hybrid boundary element technique for anisotropic elastic solids

A two-dimensional model for stage I short crack propagation on multiple slip planes under the influence of hydrogen is presented. It considers elastic-plastic material behaviour by allowing sliding on the active slip planes in the corresponding slip directions. A crack propagation law based on the crack tip sliding displacement is used to simulate crack growth. The activation of slip bands and the sliding on these active slip bands will be influenced by the local hydrogen concentration. The model is solved numerically using the boundary element method. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

随机寿命间的TTT序关系的一个非参数检验

本文先建立了一个具有TTT序关系的两个非负随机变量之间距离的概率度量，然后构造了一个非参数检验方法以判断严格的TTT序关系，利用L-统计理论获得了检验统计量的渐近分布，数值模拟的结果也表明该方法的性能是令人满意的．     

The Ex-Post Evaluation of the Minimum Local Reliability Level: An Enhanced Probabilistic Location Set Covering Model

In order to evaluate the behavior of three probabilistic location set covering models we propose a new method that allows the ex-post measurement of the so called minimum local reliability level, both under the independence and under the dependence assumption. We show experimentally, by means of a set of test problems, that the proposed loss-system version of Ball and Lin's model (1993) does almost always achieve the required reliability level. Moreover, if we compare this new version with the other two already known probabilistic models and consider as a second additional criteria the least number of required vehicles, we show that the new version has a better behavior both under the independence and the dependence assumption. In this paper we further propose a new model with the aim of reducing the number of required vehicles while satisfying the fixed reliability level. Our new model is formulated like Ball and Lin's model incorporating the servers workload estimate of the Binomial PLSCP of ReVelle and Hogan (1988). Finally, we check the precision of our ex-post evaluation method over the four considered models through a simulation study.     

考虑流固耦合效应的重力坝水力劈裂模拟

裂缝的高压水力劈裂是混凝土高坝安全评估的重要部分,研究其过程中的流固耦合作用是准确预测在各种情况下裂纹扩展路径和危险程度的关键.该文利用扩展有限元法在模拟裂纹扩展方面的优势,对大坝的裂纹进行水力劈裂模拟研究.裂纹中的水压分布模型采用Brühwiler和Saouma水力劈裂试验的成果,体现了水压和裂纹宽度的耦合关系,给出了扩展有限元在裂纹面上施加水压力荷载的实施方法,对一典型重力坝裂纹的水力劈裂进行了数值模拟分析.研究结果表明:采用扩展有限元法模拟水力劈裂,克服了常规有限元法存在的缺点,裂纹扩展时不用重新划分网格,裂纹的实时宽度可以由加强节点的附加自由度得到,裂纹面上水压的施加也变得简单易行.当考虑裂纹内的流固耦合效应时,裂纹的扩展路径相比不考虑耦合效应时的扩展路径(均布全水头水压),扩展角变大,扩展距离变短.     

平面弹性裂纹分析的一种有效边界元方法

提出了一种简单而有效的平面弹性裂纹应力强度因子的边界元计算方法．该方法由Crouch与Starfield建立的常位移不连续单元和闫相桥最近提出的裂尖位移不连续单元构成A·D2在该边界元方法的实施过程中,左、右裂尖位移不连续单元分别置于裂纹的左、右裂尖处,而常位移不连续单元则分布于除了裂尖位移不连续单元占据的位置之外的整个裂纹面及其它边界．算例(如单向拉伸无限大板中心裂纹、单向拉伸无限大板中圆孔与裂纹的作用)说明平面弹性裂纹应力强度因子的边界元计算方法是非常有效的．此外,还对双轴载荷作用下有限大板中方孔分支裂纹进行了分析．这一数值结果说明平面弹性裂纹应力强度因子的边界元计算方法对有限体中复杂裂纹的有效性,可以揭示双轴载荷及裂纹体几何对应力强度因子的影响．     

Growth of arterial cracks in polymers subject to static and fatigue loading

The growth kinetics of artificial and natural cracks (the former arising from notches) are studied in viscose, di- and triacetyl cellulose, nitrocellulose, polymethylmethacrylate, polystyrene, polypropylene, and carpon films by a micro-motion-picture method over a wide temperature range under both static and cyclic (fatigue) loading. In all the cellulose materials studied, the time required to form the nuclei of visible cracks is much shorter than the total life of the sample. The initial rate of crack growth depends exponentially on the applied stress and test temperature, so that the equation for the growth rate of an arterial crack is analogous to the general life equation. The different ways in which stress and test temperature affect the parameters of the equation describing the kinetics of crack growth enable us to distinguish the effect of local heating and that of the relaxation processes in the fatigue problem.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 253–265, March–April, 1970.