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．     

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.     

含缺口试件疲劳全寿命预测一种建议的方法

传统的研究含缺口构件的疲劳的方法是将疲劳启裂和疲劳裂纹扩展两个过程完全独立起来,用不同的方法来模拟,相互间并没有定量的关系。本文是基于最新发展的多轴疲劳损伤理论,建立了一种适用于各种载荷条件下的疲劳启裂和裂纹扩展的普适方法。根据从弹塑性分析中得到的应力应变,确定疲劳损伤模型,建立能够预测疲劳启裂、裂纹扩展速率和扩展方向的新方法。整个模拟可以分为两步:弹-塑性应力分析得到材料的应力应变分布;再运用一个通用的疲劳准则预测疲劳裂纹启裂和裂纹扩展。通过对1070号钢含缺口试件的疲劳全寿命预测,得到了与实验非常吻合的模拟结果。     

Statistical analysis of high cycle fatigue crack growth: center crack in aluminum panel

High cycle fatigue crack growth results are obtained from a statistical analysis and compared with test data for an aluminum panel with a center crack under constant amplitude cyclic load. The analysis makes use of a generalized Fock-Planck equation that is satisfied by a union probabilistic density of stochastic variables of cycle number and crack length. Obtained are high degrees of modified expressions of the equation's coefficients and hence the analytical solution. A two-parameter fatigue crack growth rate relation is assumed together with a logarithmic average life distribution. Good agreement is obtained with test data for different initial crack lengths.     

沥青混合料疲劳过程的损伤力学分析

采用损伤力学方法研究沥青混合料的疲劳失效问题。针对悬壁梁弯曲疲劳试件,推导出疲劳过程中应力场、损伤场和疲劳裂纹形成寿命的工程封闭公式。根据沥青混合料特点,提出一种模拟疲劳裂纹扩展的特征单元失效模式,从而将疲劳裂纹形成与扩展两个阶段统一用损伤力学理论进行描述和分析。本文对沥青混合料试件的疲劳裂纹形成寿命与扩展寿命分段进行了预测,还对疲劳过程中刚度衰减及位移幅值的演化过程进行了数值模拟计算。理论预期与实验结果吻合良好。     

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．     

交变载荷下含两孔洞的铝合金长条的五种疲劳破坏型式

通过试验发现交变载荷下含两孔洞的铝合金长条,由于两孔洞的方位不同,至少有五种疲劳裂纹引起的破坏型式.在五种不同的破坏情况下,疲劳裂纹初始寿命和疲劳裂纹扩展寿命均不同,必须分阶段评估,这也直接影响到整体疲劳寿命的预计;此时常规的、经典的疲劳裂纹扩展理论难以直接应用.     

疲劳裂纹扩展分析的随机方法

本文介绍了近年发展起来的疲劳裂纹扩展分析的随机方法,即疲劳裂纹扩展随机模型建立方法和疲劳裂纹扩展统计分析方法。这一分析方法为金属构件疲劳裂纹扩展寿命及可靠性研究提供了有力的工具。      

焊接金属疲劳裂纹萌生寿命估算方法的研究

疲劳裂纹萌生阶段在整个疲劳破坏过程中占有极为重要的地位，而萌生阶段的裂纹体损伤规律用长裂纹断裂力学方法是无法确定的．以萌生阶段中塑性滞回能作为控制参量，根据实验数据拟合出疲劳损伤的连续曲线，得到裂纹萌生寿命估算公式．     

铝合金切口件在四组变幅载荷下3.5%NaCl溶液中腐蚀疲劳裂纹起始寿命试验研究与估算模型

试验研究了铝合金切口件在4组变幅块谱下、3．5％NaCl溶液中腐蚀疲劳裂纹起始寿命．结果表明，变幅载荷谱中的大超载会显著延长腐蚀疲劳裂纹起始寿命，并且加载顺序具有明显的影响．根据反映大小载荷交互作用的超载腐蚀疲劳裂纹起始寿命曲线和Miner累积损伤定则，建立了变幅载荷下切口件腐蚀疲劳裂纹起始寿命估算模型，应用该模型估算的结果与试验结果相吻合．     

螺栓连接件微动疲劳特性分析

工程中钢结构构件经常采用螺栓连接,被连接部位多处于复杂受力状态,单轴疲劳理论已无法满足该 形式下构件的寿命评估需求．针对现有理论不足,本文建立了螺栓连接件有限元模型,研究了不同工况下被连 接件微动疲劳裂纹萌生位置,并基于临界平面的多轴疲劳理论,比较了四种常用模型的适用性以及预测了连接 件的疲劳寿命．结果表明：(1) 裂纹萌生位置位于受拉端螺栓孔附近的滑移粘着区,在相同螺栓预紧力下,该 位置与施加的疲劳载荷大小无关;(2) 基于临界平面方法的四种模型均可以较好判断裂纹萌生位置,其中SWT (Smith-Watson-Topper)模型对不同载荷水平下的螺栓连接件微动疲劳寿命预测效果较好,大部分预测结果位于± 2倍分散带之内,预测结果优于其他三种模型;(3) 在规范规定的螺栓预紧力范围内,被连接件裂纹萌生区域距 孔边的距离与预紧力大小无关,可能是由于预紧力变化范围内的粘着滑移区未发生明显变化所致,并且随着预 紧力减小,被连接件的寿命预测值反而增大     

海洋平台K型管节点的疲劳裂纹扩展分析Ⅰ:试验测试

对承受疲劳载荷的海洋平台K型管节点首先进行了静力测试,确定了沿着焊缝的热应力区的应力分布及热应力区最大应力点的位置,从而判断裂纹产生的位置;然后通过专用测试设备提供循环疲劳载荷,用ACPD(Alternating Current Potential Drop,即交流电流势能落差法)技术检测裂纹的产生和增长过程,得到裂纹最深点,用S-N曲线估算其疲劳寿命。对已有裂纹的K型管节点,用应力强度因子估计其剩余寿命。同时用测试的结果验证了S-N的准确性和可靠性。     

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．     

NEW METHOD FOR MEASURING RANDOM THRESHOLDS F LONG FATIGUE CRACK PROPAGATION

A so-called "local probabilistic Paris relation method" was presented for measuring the random thresholds of long fatigue crack propagation. A check was made to the conventional method, in which the thresholds were measured statistically and directly by the test data. It was revealed that this method was not reasonable because the test data have seldom a unified level of crack growth rates. Differently,in the presented method the Paris-Erdogan equation was applied to model the local test data around the thresholds. Local probabilistic relations with both the survival probability and the confidence were established on a lognormal distribution of the stress density factors.And then, the probabilistic thresholds were derived from the probabilistic factors with a given critical level of growth rate. An analysis on the test data of LZ50 axle steel for the Chinese railway vehicles verifies that the present method is feasible and available.     

New method for measuring random thresholds of long fatigue crack propagation

A so-called "local probabilistic Paris relation method" was presented for measuring the random thresholds of long fatigue crack propagation. A check was made to the conventional method, in which the thresholds were measured statistically and directly by the test data. It was revealed that this method was not reasonable because the test data have seldom a unified level of crack growth rates. Differently,in the presented method the Paris-Erdogan equation was applied to model the local test data around the thresholds. Local probabilistic relations with both the survival probability and the confidence were established on a lognormal distribution of the stress density factors. And then, the probabilistic thresholds were derived from the probabilistic factors with a given critical level of growth rate. An analysis on the test data of LZ50 axle steel for the Chinese railway vehicles verifies that the present method is feasible and available.     

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.     

焊趾表面裂纹的形态发展曲线与疲劳寿命预测

以作者建立的焊地椭圆表面裂纹应力强度因子数据库以及复杂应力场中焊践半随圆表面裂纹前缘应力强度因子分布计算的基本模式法为基础上,给出了复杂应力场中焊践表面表纹在疲劳扩展过程中形态变化规律及寿命的工程分析方法。     

停歇对疲劳裂纹扩展超载迟滞效应影响的研究

本文用 16 Mn和 A 537 两种普通低合金钢试件,较系统地研究了停歇对疲劳超载迟滞效应的影响。研究表明,疲劳载荷中的停歇对超载迟滞效应有明显影响,其影响的程度大小与停歇时的载荷 P_H,停歇时间 t_H 等因素密切相关;不但可能使试件的疲劳寿命减少,也可能使试件的疲劳寿命增加。在工程结构疲劳设计、分析及应用超载迟滞效应时应予以注意。     

Constant amplitude and variable load history fatigue test results and predictions for cruciform and lap welds

Cruciform and lap welds were fatigue tested under constant amplitude axial load and SAE Bracket spectrum load conditions. For the cruciform joints, fatigue cracks generally initiate at the root but may initiate at the toe if higher bending stresses are induced by joint distortion. For lap welds, the stress ratio ($\text{R}$) and weld shape are the major factors influencing the fatigue crack initiation site.The fatigue test results were compared with predictions made using an analytical model developed by the authors, and good agreement between experiment and theory was observed. The model for the predictions assumes that the fatigue crack initiation period, which is the number of cycles for the initiation of a fatigue crack and its early growth and coalescence into a dominant fatigue crack, is the main portion of the total fatigue life at long lives.     

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