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。     

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。     

物理短裂纹疲劳扩展的Markov链模拟

使用Ｍａｒｋｏｖ链理论，基于１６Ｍｎ钢小试样疲劳裂纹扩展试验，构造了一个物理短裂纹扩展的概率演化模型。该模型对裂纹扩展的循环数分布以及分布的演化过程的模拟，表明了与实验结果良好的吻合程度，从而为物理短裂纹扩展的概率分析及可靠性评价提供了手段。     

超声疲劳载荷下球墨铸铁裂纹扩展速率研究

论文采用超声疲劳试验技术测试了发动机曲轴用球墨铸铁的超高周疲劳强度及超声疲劳载荷下的裂纹扩展速率.考虑高频载荷下疲劳裂纹扩展过程中的温升效应,测试了球墨铸铁超声疲劳裂纹扩展过程中的温度变化,研究了超声微幅循环载荷下球墨铸铁的疲劳裂纹萌生与扩展机制.根据超声疲劳试验控制位移的加载特点及超声疲劳试样几何形状特征,数值计算了超声疲劳裂纹扩展应力强度因子,给出了该球墨铸铁超声疲劳裂纹扩展速率曲线.     

高温疲劳表面短裂纹的分形研究

对2.25Cr-1Mo合金钢进行了高温疲劳实验，采用中断试验与复膜技术相结合的方法观测了试样表面疲劳短裂纹萌生，扩展及合体的演化过程，并用数据格子方法研究了含裂纹表面的分形特征。结果表明：疲劳短裂纹的萌生扩展行为具有分形特征，可以用分形理论加以描述；分形维数随着疲劳过程的继续而稳定地成比例增加；分形维数可以作为把握材料总体损伤状态的参数，可作为寿命评价的依据。     

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。     

谱载荷下疲劳裂纹扩展随机规律的实验研究

简要介绍了谱载荷下疲劳裂纹扩展试验的理论基础、方法和过程。讨论了各种数据处理方法对参数估计的影响,研究了疲劳裂纹扩展的随机规律。由试验得到的疲劳裂纹扩展试验数据估计了裂纹扩展方程的参数,计算出结构可靠度曲线,通过对试验结果的分析难证了以下结论：以时间为参量的裂纹扩展随机过程模型和以裂纹长度为参量的模型在一定条件下是统一的;数据处理方法的选择与可靠性分析的结果有密切的联系;裂纹扩展寿命受裂纹扩展队机     

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.      

复合材料疲劳Ⅱ型层间裂纹扩展门槛值试验方法研究

疲劳裂纹扩展门槛值在结构损伤安全设计中具有重要作用，该文提出了复合材料Ⅱ型疲劳裂纹扩展门槛值的试验方法，并实测了两种复合材料的疲劳裂纹扩展门槛值。     

Ⅱ型裂纹疲劳扩展中超载行为的研究

利用铝合金ＬＹ１２ＣＺ制十字型试样进行Ⅱ型裂纹的疲劳扩展试验，施加若干次短瞬的Ⅱ型超载。发现Ⅱ型裂纹的超载行为与Ⅰ型裂纹的不同，裂纹的扩展速率可几倍于未超载时相同应力强度因子幅度下的扩展速率；而且超载后的短瞬期间，裂纹在塑性区内仍保持较快的速率扩展。试验研究表明：Ⅱ型超载和Ⅰ型超载也许分别受不同的机制控制；它更类似于Ⅲ型超载。     

一种经验型疲劳小裂纹扩展模型

金属材料疲劳寿命由裂纹萌生和裂纹扩展寿命两部分组成，其中对于萌生寿命中的小裂纹分析是精确描述裂纹萌生寿命的关键．而小裂纹在扩展过程中由于尺寸相对较小，导致传统线弹性断裂力学预测方法失效，需要对其进行改进，考虑裂纹尖端塑性区引起的残余压应力对小裂纹扩展速度的影响．本文针对此问题进行了初步分析，通过对塑性区引起的残余应力的量化，结合小裂纹门槛值特性，提出了一种经验型修正的小裂纹扩展模型，用于定量预测裂纹的萌生寿命．使用铝合金6082-T6缺口试样进行了疲劳实验，并与理论结果进行了对比，验证了所提模型的有效性．     

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 statistical investigation of the behavior of short fatigue crack in medium carbon steels

Rotary bending fatigue tests were carried out by using smooth specimens of medium carbon steel with two ferrite grain sizes. In order to clarity the physical background of the scatter in fatigue behaviour, the statistical characteristics of the initiation and growth of short fatigue crack were investigated. Results show that the short crack initiation and growth life distribution can be express by two-or three-parameter Weibull distribution. The Weibull distribution of the mixed type is well fitted to the crack length distribution for both steels. The knee values of Weibull plots are related to the grain size. The crack length distribution before the knee point can be expressed by a two-parameter Weibull distribution, and that after the knee point can be represented by two-or three-parameter Weibull distribution. It was found that the initiation Life, growth life and crack length distribution were affected by the grain size.     

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.     

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.     

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     

基于随机点过程理论的疲劳裂纹随机扩展模型

本文根据疲劳裂纹扩展特性，运用随机点过程理论导出了求取裂纹扩展随机过程低阶矩的公式。同时根据二阶矩近似法用Ｗｅｉｂｕｌｌ分布近似给定寿命下的疲劳纹长度分布。给出了求取给定寿命下裂纹超值概率分布和给定裂纹长度下疲劳寿命分布公式。最后，用小样本孔边短裂纹扩展数据对模型适用性进行了初步验证。     

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。     

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.     

Investigation on 3D fatigue crack propagation in surface-cracked specimens

In the present work the fatigue crack growth in AISI304 specimens is investigated experimentally. In 3D finite element analysis the virtual crack closure technique is applied to calculate distributions and variations of the stress intensity factor along the surface crack front. It is confirmed that the stress intensity factor along the surface crack front varies non-uniformly with crack growth. Crack growth rate is proportional to the stress intensity factor distribution in the 3D cracked specimen. The fatigue crack growth in surface cracked specimens can be described by the Forman model identified in conventional compact tension specimens. For crack growth in the free specimen surface the arc length seems more suitable to quantify crack progress. Geometry and loading configuration of the surface cracked specimen seem to not affect the fatigue crack growth substantially.