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     

LZ50车轴钢疲劳短裂纹萌生的数值模拟

针对LZ50车轴钢的疲劳短裂纹应用数值方法对疲劳短裂纹在LZ50车轴钢中的萌生进行了数值模拟.利用二维Voronoi图随机地生成了该材料的微观结构.根据对疲劳试样所施加的载荷,结合有限元法得到了该微观结构中应力和应变的分布规律.最后利用材料的疲劳S-N曲线和裂纹萌生的概率方法给出了在不同循环周次下LZ50钢中疲劳短裂纹的萌生过程.该数值模拟的结果可用于进一步分析LZ50车轴钢中疲劳短裂纹的扩展和群体演化行为.文中还指出在单向拉压的工况下,短裂纹的萌生方向主要受到与载荷方向相一致的应变影响,最大剪应变方向萌生方向的夹角为45°.     

金属材料疲劳损伤的宏细观理论

工程结构的疲劳损伤发展过程经历了由初始缺陷的形成、裂纹的稳态扩展直到最后失效的不同发展阶段,通常疲劳损伤的演化可以概括为以下几个阶段:(1) 亚结构和微观结构的变化引起永久损伤的形成,产生微观裂纹;(2) 微观缺陷的长大会合形成主裂纹;(3) 主裂纹稳态扩展;(4) 结构失稳或完全失效.首先论述了疲劳裂纹扩展的物理机制,并从细观和宏观两个方面总结了处理疲劳裂纹问题的最新研究成果,对位错力学在处理短裂纹扩展问题中的应用,以及无位错区(DFZ)在疲劳裂纹扩展中的作用进行了较详细讨论.      

疲劳短裂纹萌生及发展的细观过程和理论

本文综述了疲劳短裂纹研究的进展和现状,包括关于短裂纹的属性,短裂纹萌生和发展的细观过程以及描述短裂纹行为的若干模型。      

光滑试样表面疲劳裂纹演化的数值仿真

考虑裂纹形核，扩展，合并以及晶界的作用等因素，对光滑圆试样表面疲劳裂纹的演化进行了数值模拟。结果表明，疲劳裂纹系统的演化是由无序向有序转化的过程，演化诱致突变。这一过程与实验观察到的结果一致。疲劳裂纹系统的演化终态具有很大的分散性，主要表现为裂纹路径的随机性和突变寿命的分散性。样本的随机性是造成上述分散性的外因，系统对裂纹位形的敏感性是内因。     

缺口试样疲劳短裂纹行为研究

采用复型方法对中碳结构钢单边钝缺口试样的非穿透和穿透疲劳短裂纹的演化过程进行了研究。结果表明：试样的疲劳破坏主要是由形成于缺口根部的表面短裂纹扩展引起的；形成穿透裂纹以前的短裂纹形成和扩展过程占试样疲劳寿命的７０％以上；线弹性断裂力学不适合于描述表面裂纹和穿透裂纹早期阶段的扩展特性；某些条件下，裂纹初始几何形状的影响是导致穿透裂纹早期扩展出现奇异特性的主要原因。     

35CrMo合金钢疲劳短裂纹扩展规律的试验研究

本文通过试验研究了35CrMo 合金钢三点弯曲试样疲劳短裂纹的扩展特性;用电测法得到了裂尖处的载荷—应变标定关系。试验结果表明,短裂纹的扩展速率正比于局部应力幅的幂次和裂纹长度,即 da/dN=A(?),可以较好地描述疲劳短裂纹的扩展规律。     

45CrNiMoVA钢的低周疲劳特性和表面疲劳裂纹的在位观测

利用MTS810材料试验机对带中心圆孔的45CrNiMoVA钢低周疲劳特性进行了研究,并借助长焦显微镜和CCD摄像头对试样表面裂纹的演化规律进行了在位停机观测和在位不停机连续观测尝试,试验发现,裂纹均萌生于与拉伸应力垂直的圆孔边缘;疲劳裂纹的萌生与Lueders形变带密切相关,萌生期超过整个疲劳寿命的70%,主裂纹的发展既可以共面扩展的方式向前连续延伸,也可以裂纹连接的方式向前跳跃传播,应力控制的低周疲劳条件下,疲劳寿命与应力面形貌图像,频闪光源照明可以实现表面疲劳裂纹的在位不停机观测。     

单向加载,平均应力下的疲劳短裂纹生长

本文报道单向加载下平均应力和交变应力对疲劳短裂纹生长的影响。采用一种1.99%NiCrMo钢,在平均应力和交变应力不同组合的应力控制条件下进行室温疲劳试验,将疲劳过程划分为3个阶段:塑性局部化,微观组织短裂纹(MSC)生长和物理短裂纹(PSC)生长,研究了平均应力对上述3个阶段的影响并得到一组关系式,这些关系式可预测多种平均应力和交变应力组合条件下短裂纹的初始阶段和生长行为。获得了不同平均应力下疲劳承载曲线的满意的预测结果。本研究为仍依赖于传统的Goodman疲劳图的设计人员提供了一种以疲劳过程物理机制为基础的方法。     

弯曲疲劳短裂纹扩展规律研究

本文对３５ＣｒＭｏ钢和４２ＣｒＭｏ钢带穿透短裂纹（初始裂纹长度ａ＝０．１～０．５ｍｍ）的试件进行了弯曲疲劳试验研究，得到了在弯曲疲劳加载下短裂纹扩展速率的计算表达式以及应力比，试件厚度对短裂纹扩展的影响。     

Experimental and theoretical study on numerical density evolution of short fatigue cracks

Fatigue testing was performed using a kind of triangular shaped specimen to obtain the characteristics of numerical density evolution for short cracks at the primary stage of fatigue damage. The material concerned is a structural alloy steel. The experimental results show that the numerical density of short cracks reaches the maximum value when crack length is slightly less than the average grain diameter, indicating grain boundary is the main barrier for short crack extension. Based on the experimental observations and related theory, the expressions for growth velocity and nucleation rate of short cracks have been proposed. With the solution to phase space conservation equation, the theoretical results of numerical density evolution for short cracks were obtained, which were in agreement with our experimental measurements. The project supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.     

狭长缺口根部萌生的小裂纹扩展分析

根据伴随着小裂纹延展过程中不同力学参数相互作用的分析研究基础之上,我们就疲劳小裂纹的扩展特性进行了分析,根据不扩展裂纹性质对小裂纹的长度范围进行了讨论,探讨了狭长缺口根部萌生的小裂纹扩展速率, 并研究了不扩展裂纹形成机理,推演出不扩展裂纹长度计算方法,发展出一种预测小裂纹扩展行为的方法,借助这种方法可以实现裂纹萌生寿命的预测,与疲劳设计相结合可以进一步提高构件安全性与可靠性．不扩展小裂纹的长度可以通过计算定量得到,它与材料的疲劳极限、裂纹门槛值及缺口尺寸等参数紧密相关．以理论数据为依据,可以设计出针对一定材料及尺寸的试样,使其在一定荷载条件下不扩展裂纹长度达到宏观可观测的范围,进而降低小裂纹测量方面的技术难度,进一步验证不扩张裂纹形成机理．     

A theoretical analysis on stochastic behaviour of short cracks and fatigue damage of metals

In this paper, the closed form of solution to the stochastic differential equation for a fatigue crack evolution system is derived, and the relationship between metal fatigue damage and crack stochastic behaviour is investigated. It is found that the damage extent of metals is independent of crack stochastic behaviour if the stochastic deviation of the crack growth rate is directly proportional to its mean value. The evolution of stochastic deviation of metal fatigue damage in the stage close to the transition point between short and long crack regimes is also discussed.     

穿透短裂纹扩展趋偏现象的模型分析和实验研究

本文揭示了穿透短裂纹疲劳扩展中的一个特性－趋偏扩展特性。这一特性是穿透短裂纹预测和测试中产生偏问题的根本原因。通过对长、短裂纹扩展特性及主要影响因素的分析，提出了进行穿透短裂纹试验时必须注意的若干事项。     

Extension of Mixed‐Mode Fatigue Cracks in Steel Samples

Experimental studies of the extension of mixedmode fatigue cracks were performed on rail steel samples. The direction of growth of fatigue cracks was studied for three types of loading: alternate transverse shear and transverse and longitudinal shears under compressing stresses. For all tested samples, the following common feature in the extension of fatigue cracks was established: the direction of crack growth coincides with the propagation direction of the principal stresses having minimum absolute values.     

STUDY ON THE SURFACE CRACK GROWTH BEHAVIOR IN 14MnNbq BRIDGE STEEL

Three-dimensional crack closure correction methods are investigated in this paper.The fatigue crack growth tests of surface cracks in 14MnNbq steel for bridge plate subjected to tensile and bending loadings are systematically conducted.The experimentally measured fatigue crack growth rates of surface cracks are compared with those of through-thickness cracks in detail.It is found that the crack growth rates of surface cracks are lower than those of through-thickness cracks.In order to correct their differences in fatigue crack growth rates, a dimensionless crack closure correction model is proposed.Although this correction model is determined only by the experimental data of surface cracks under tensile loading with a constant ratio R=0.05, it can correlate the surface crack growth rates with reasonable accuracy under tensile and bending loadings with various stress ratios ranging from 0 to 0.5.Furthermore, predictions of fatigue life and crack aspect ratio for surface cracks are discussed, and the predicted results are also compared with those obtained from other prediction approaches.Comparison results show that the proposed crack closure correction model gives better prediction of fatigue life than other models.