基于局域分析的疲劳短裂纹群体演化随机模型

采用局域裂纹数密度描述金属材料中不同局部区域的疲劳短裂纹群体损伤的发展情况通过考虑在不同局域存在的材料性质的随机涨落及局部损伤对损伤总量发展的影响，建立了局域裂纹数密度演化随机方程对方程数值求解从而模拟了材料的疲劳短裂纹损伤过程结果显示出主裂纹出现的随机性，并讨论了裂纹总数与最大裂纹尺度在统计意义上的演化特征     

混凝土疲劳分析方法综述

从探究物理机制的视角对混凝土疲劳研究进行了逻辑上的梳理,表明既有研究可分为3 类主要模型:基于疲劳试验分析的现象学模型、基于断裂力学的疲劳裂纹扩展模型和基于损伤力学的疲劳损伤演化模型. 通过解析具有典型性的研究,论述了不同类型研究的特点. 进而,从还原论的角度出发,阐明了速率过程理论可以作为解释混凝土材料疲劳损伤演化的物理基础. 注意到疲劳试验结果所表现出的显著随机性,阐述了应以物理随机系统思想研究混凝土疲劳问题的观点.     

疲劳裂纹形成,扩展统计特性的大子样试验研究

针对疲劳损伤过程的随机性，该文开展了等幅谱大子样疲劳裂纹形成，扩展至破坏全寿命过程的试验研究。在计算确立了裂纹形成，扩发界工程小裂纹长度的基础上，事理得到了裂纹形成，扩展寿命的统计分布数据，根据断口金相ＳＥＭ观察，进一步定性分析了疲劳条纹分布的相关性特征。     

随机疲劳累积损伤理论研究进展

对疲劳载荷、应力、寿命和强度的随机性分别进行了明确分类．然后,针对在疲劳理论发展史上有代表性的、目前在国内外仍被大量引用的（主要用于裂纹起始阶段的）疲劳累积损伤准则,从力学、概率统计等方面进行了较深入系统的分析．提出疲劳损伤累积具有两类不同的概率机制,即损伤在单一个体内具有的随机性以及损伤在同一母体中不同个体之间具有的分散性．最后,重点介绍了二维概率Miper准则以及有关随机疲劳寿命、随机疲劳强度等方面的研究进展．      

疲劳裂纹形成、扩展统计特性的大子样试验研究

针对疲劳损伤过程的随机性，该文开展了等幅谱大子样疲劳裂纹形成、扩展至破坏全寿命过程的试验研究．在计算确立了裂纹形成、扩展分界工程小裂纹长度的基础上，整理得到了裂纹形成，扩展寿命的统计分布数据．根据断口金相ＳＥＭ观察，进一步定性分析了疲劳条纹分布的相关性特征．     

考虑微动磨损的钢丝微动疲劳裂纹扩展寿命预测研究

微动疲劳是矿井提升钢丝绳主要失效形式之一,在钢丝微动疲劳过程中,微动磨损严重影响钢丝微动疲劳裂纹扩展特性,进而制约钢丝微动疲劳断裂机制,故开展考虑微动磨损的钢丝微动疲劳裂纹扩展寿命预测研究至关重要. 运用自制钢丝微动疲劳试验机开展钢丝微动疲劳试验和拉伸断裂试验,通过高速度数码显微系统揭示微动疲劳过程中钢丝微动磨损演化、裂纹萌生和扩展及断裂特性,基于摩擦学和断裂力学理论,运用有限元法、循环迭代法和虚拟裂纹闭合技术建立了考虑微动磨损的钢丝微动疲劳裂纹扩展寿命预测模型,并进行试验验证. 结果表明:采用微动疲劳过程稳定阶段磨损系数预测钢丝微动磨损演化可保证预测正确性,微动疲劳过程中钢丝主要为I型裂纹扩展模式,考虑微动磨损的钢丝微动疲劳裂纹扩展寿命预测值和试验值吻合较好,验证了预测模型正确性.      

表面裂纹疲劳扩展的数值模拟(Ⅱ)

根据Paris疲劳裂纹扩展规律,对拉伸和纯弯曲疲劳载荷下表面裂纹扩展进行了数值模拟。数值模型中,用三次样条函数曲线拟合裂纹尖端,在裂纹扩展增量计算中考虑了裂纹闭合影响。裂纹形状演化的模拟结果与Newman和Raju经验公式预测结果进行了比较,表明了所采用的数值模拟方法的实用性。研究发现,裂纹闭合对疲劳裂纹扩展过程中的裂纹形状演化以及裂纹尖端的应力强度因子(SIF)分布都有明显影响。同裂纹形状演化一样,疲劳裂纹扩展过程中裂纹尖端的SIF分布表现出明显的特征。最后,建议了一个简单函数来统一描述表面裂纹尖端的SIF分布。     

疲劳裂纹随机扩展模型进展

本文分析了疲劳裂纹扩展具有分散性的主要来源，对疲劳裂纹随机扩展模型从唯象学的角度作了较为详细的分析和回顾．尽管疲劳裂纹随机扩展问题已有了很多的研究结果，但还有许多问题有待进一步的深入研究．     

表面裂纹疲劳扩展形状演化预测

应用新的应力强度因子经验公式预测了拉伸和纯弯曲载荷下疲劳扩展过程中表面裂纹形状的演化,并与Newman和Raju的经验公式以及实验结果进行了比较。结果表明:虽然对应力强度因子估算有一定差别,但两者对裂纹形状演化的预测有很好的一致性,预测结果与实验结果也比较吻合,从而表明所提出的经验公式能够用于预测疲劳扩展过程中表面裂纹形状演化。     

增材制造微结构演化及疲劳分散性计算

为了预测增材制造中工艺参数?微结构?力学性能之间的关联规律, 提出了集成离散元、相场模拟、晶体塑性有限元和极值概率理论的计算方法, 揭示了激光扫描速度对微结构演化、屈服应力和疲劳分散性的影响. 首先, 采用离散元实现了重力作用下粉床在已凝固层表面上的逐层铺设; 其次, 通过热?熔体?微结构耦合的非等温相场模拟, 获得了熔体、气孔、晶界、晶粒取向等的时空演化以及最终形成的多晶微结构; 然后, 应用晶体塑性有限元计算了增材制造多晶微结构的宏观力学响应, 并得到表征疲劳裂纹萌生驱动力的疲劳指示参数(FIP); 最后, 采用极值概率理论分析了增材制造多晶微结构的FIP极值分布规律及疲劳分散性. 以316L不锈钢选区激光熔化增材制造为例的计算结果表明: 增材制造微结构的宏观屈服强度随激光扫描速度的增加而降低, 且呈各向异性; FIP极值符合Gumbel极值分布规律, 激光扫描速度增加可降低增材制造微结构疲劳分散性, 但会导致FIP极值升高, 使得疲劳裂纹萌生驱动力增加, 疲劳寿命降低.      

Grain–boundary interactions and orientation effects on crack behavior in polycrystalline aggregates

A dislocation-density grain–boundary interaction scheme has been developed to account for the interrelated dislocation-density interactions of emission, absorption and transmission in GB regions. The GB scheme is based on slip-system compatibility, local resolved shear stresses, and immobile and mobile dislocation-density accumulation at critical GB locations. To accurately represent dislocation-density evolution, a conservation law for dislocation-densities is used to balance dislocation-density absorption, transmission and emission from the GB. The behavior of f.c.c. polycrystalline copper, with different random low and high angle GBs, are investigated for different crack lengths. For aggregates with random low angle GBs, dislocation-density transmission dominates at the GBs, which can indicate that the low angle GB will not significantly change crack growth directions. For aggregates with random high angle GBs, extensive dislocation-density absorption and pile-ups occur. The high stresses associated with this behavior, along the GBs, can result in intergranular crack growth due to potential crack nucleation sites in the GB.     

STOCHASTIC PROPAGATION OF SEMI-ELLIPTICAL SURFACE CRACK

The semi-elliptical surface crack growth of structural components withuncertain material resistance under random loading is studied by using the stochasticaveraging principle.The FPK equation governing the transition probability densityfunction of crack lengths is derived.The analytical solution of the FPK equation forthe case of that the equations for the crack growth in the surface and depth directionsare uncoupled is obtained.The effects of the parameters of the stress process and of thematerial property on the behavior of semi-elliptical fatigue crack growth of thecomponents with deterministic resistance to crack growth in the stationary Gaussianstress process are examined.The comparison of the analytical result with digitalsimulation shows the effectiveness of the present method.     

随机应力过程下半椭圆型表面裂纹的扩展

本文应用随机平均原理研究了在随机载荷作用下具有随机抗裂特性的构件所含半椭圆型表面裂纹的疲劳扩展。导出了支配半椭圆型表面裂纹尺寸的转移概率密度的ＦＰＫ方程，给出了裂纹扩展方程在表面和深度两个方向互不耦合情形下的解析解。通过数例详细考察了具有确定性抗裂特性的构件所含半椭圆型表面疲劳裂纹在平稳窄带高斯应力作用下的扩展行为，并有和数字模拟验证方法的有效性。     

随机疲劳裂纹扩展新模型

疲劳裂纹扩展的随机特性是工程结构可靠性分析和制定检查、维修计划野性 须考虑的因素,提出疲劳裂纹随机扩展模型,不仅考虑了时间参数影响,而且还考虑了初始裂纹尺寸分布影响。模型适应范围广,由于采用了一次二阶矩近似方法简化随机过程计算,便于工程应用。     

Fatigue damage in polycrystals – Part 2: Intrinsic scatter of fatigue life

Part 2 deals with the evolution of plastic flow resistance with crack growth from its minimum value (fatigue limit) towards its saturated bulk value (cyclic yield stress). The far-field stress level, the geometry of the crack and the grain size distribution of the material are those parameters that control the area of crack tip plasticity and hence the rate towards saturation. The implication of the far-field stress is held responsible for the violation of the similitude concept and the failure of the stress intensity factor to describe conditions of short cracking. However, an engineering tool based on the stress intensity factor and being able to predict the fatigue life of short cracks can be constructed, considering that the distribution of crack growth rates is intrinsically defined by the material itself. The above allows the development of a set of equations able to construct the fatigue life scatter of the material.     

Quasi-static tensile deformation and fracture behavior of a highly particle-filled composite using digital image correlation method

Polymer bonded explosives (PBXs) are highly particle-filled composite materials. This paper experimentally studies the tensile deformation and fracture behavior of a PBX simulation by using the semi-circular bending (SCB) test. The deformation and fracture process of a pre-notched SCB sample with a random speckle pattern is recorded by a charge coupled device camera. The displacement and strain fields on the observed surface during the loading process are obtained by using the digital image correlation method. The crack opening displacement is calculated from the displacement fields, the initiation and propagation of the crack are analyzed. In addition, the damage evolution and fracture mechanisms of the SCB sample are analyzed according to the strain fields and the correlation coefficient fields at different loading steps.     

Statistical evolution of small fatigue crack in 1Cr18Ni9Ti weld metal

Statistical evolution of small fatigue crack behavior is analyzed. Three considerations are given. They consist of the total fit, the consistency of statistical parameters with test data, and the practice of commonly used distributions, namely Weibull (two- and three-parameters), normal, lognormal, extreme minimum, extreme maximum, and exponential. The crack growth rate data of 1Cr18Ni9Ti stainless steel-weld metal reveal that fatigue damage is a gradual process involving an initial non-ordered (chaotic) or load history-independent random state to an ordered load history-dependent random state. The seven distributions are examined. Different from existent conclusion, appropriate assumed distribution for characterizing the evolution data is the normal or extreme minimum value model rather than the Weibull two- or three-parameter. The other distributions do not fit the data as they violated two or all three of the mentioned considerations.     

考虑T-stress的非正态随机参数疲劳裂纹扩展寿命的可靠性分析

基于断裂力学的疲劳裂纹扩展寿命问题的研究常常将裂纹尖端应力展开项的高次项忽略,引起了裂纹扩展模拟的误差,本文考虑高次项T-stress对裂纹扩展角的影响,对裂纹扩展过程做了数值模拟,结果显示相同裂纹扩展长度下,考虑T-stress会延长裂纹扩展寿命。文章首先采用修正的Paris-Erdogan 公式计算了两端承受均布拉伸载荷的边缘斜裂纹板的疲劳裂纹扩展寿命,裂纹扩展方向采用两参数修正的最大拉应力准则。由于结构尺寸,材料特性和载荷等因素具有不确定性,导致疲劳裂纹扩展过程带有一定的随机性,本文以材料属性和载荷为随机变量,在随机有限元法的基础上,结合计算可靠度的四阶矩法,Edgeworth级数展开方法,提出随机参数服从任意分布时的结构疲劳裂纹扩展寿命可靠度的计算方法。分析了参数为非正态分布时的平板裂纹扩展寿命可靠度随裂纹扩展的变化过程。本文方法可预测工程中板裂纹的扩展行为,以及预测裂纹板的可靠度。     

On the role of the process zone in dynamic fracture

The evolution of the fracture process zone and its influence on dynamic crack propagation are investigated using a simple constitutive assumption to model the material in the fracture process zone. The mode III problem of anti-plane shear is examined using a finite difference scheme to obtain the full field solution. Unlike the classical approach of determining the steady state solutions for imposed constant speed of crack propagation, here the fracture criterion is imposed along with the formulation of the boundary-initial value problem and the coupled equations are solved numerically to determine the generation and growth of the fracture process zone as well as the crack tip. The results of the simulation indicate that the constitutive behavior of the process zone material (or equivalently the evolution of the process zone itself) plays a key role in determining the dynamics of fast fracture.