疲劳缺口系数Kf与理论应力集中系数Kt之间的关系

疲劳缺口系数Kf是表征缺口部位疲劳强度降低程度的参量,这一系数在结构抗疲劳设计中经常被使用.一般认为Kf与理论应力集中系数Kt呈线性关系,线性的斜率与不同材料类别(铝合金、钛合金和钢)有关.通过对大量不同材料、不同应力比和不同缺口试样形式的高周疲劳极限的系统分析,发现大多数情况下K f与Kt的呈线性关系,但有些情况下二者不服从线性关系.Kf与Kt的关系除与材料类别有关外还与实验的应力比有关.     

Prediction of Fatigue Characteristics of Metals at Different Loading Frequences

The procedure for predicting fatigue characteristics by high-frequency test results over lifetime ranges up to 10¹⁰ cycles is proposed. The procedure is based on the fatigue fracture model accounting for the loading frequency and stress ratio. The potentials of the method are exemplified by the tests of smooth specimens and specimens with a stress concentrator from different materials (nickel-, aluminum-, and titanium-base alloys). The prediction results for different loading frequencies (35–10,000 Hz) and stress ratios (from -1 to 0.5) are shown to vary by about 10% from experimentally obtained data.     

错位板节点疲劳强度的估算方法

本文考虑了材料、受载形式、尺寸、错位量、焊接残余应力以及焊趾根部短裂纹对错位板节点疲劳强度的影响，在Ｐｅｔｅｒｓｏｎ公式和Ｔｏｐｐｅｒ公式的基础上，分别建立了两个估算这类节点疲劳强度的有效应力集中系数的公式。并与实验结果进行了比较，估算值与实验值符合较好。本文所建立的方法对工程应用有参考价值。     

Improving fatigue life for aluminium cruciform joints by weld toe grinding

The increase of fatigue life in aluminium cruciform joints by weld toe grinding was the focus of the current study. The test data are presented by both a nominal stress range approach and by the more refined structural and notch stress range approaches. The influence of the weld toe angle, weld leg length and weld toe radius on the structural and notch stress concentration factor (SCF) was systematically studied by means of finite element analysis. Experimental data based on 18 pieces of as-welded and 13 pieces of weld toe-ground specimens made of 12 mm thick plates showed a significant improvement in fatigue life in aluminium by grinding the weld toe and confirmed the permitted improvement in fatigue life by design codes.     

铸造钛合金投梭棒断裂分析

铸造钛合金投梭棒在５Ｈｚ的交变载荷下长期工作。在推广使用中,其中一件仅运转２００多小时就发生断裂。介绍了对该断裂件的损伤分析方法及结果。分析结论认为该件断裂的主要原因是铸件补焊时存在有焊接缺陷,引起应力集中而导致疲劳断裂。     

A discussion on various estimations of elastic stress distributions and stress concentration factors for sharp edge notches

The elastic stress distributions in some edge-notched geometries have been estimated from Creager and Paris's and Neuber's expressions. Close to the notch root, these approximate solutions agree well with the finite element results. Further away from the notch, the approximate methods give overestimations. A simple formula derived from Creager and Paris's expression provided accurate stress concentration factor solutions for some edge-notched geometries.     

论疲劳缺口减缩系数Kf

本文简要回顾了疲劳减缩系数Kf的定义和主要表达式,在此基础上本文从疲劳损伤的基本机理出发推导了定量描述Kf的基本公式,讨论了影响Kf值的主要因素,并用此理论分析了疲劳实验数值,获得了较为满意的结果。     

Threshold Fatigue and Fretting Fatigue of Metals

General regularities in the variation of threshold stress intensity factors of metals are considered taking into account the influence of various factors. Justification is offered of the possibility of predicting fatigue limits for smooth specimens and specimens under conditions of fretting from the known characteristics of threshold stress intensity factors.     

Fatigue of cantilevered pipe fittings subjected to vibration

Vibration fatigue failures in piping systems often occur at cantilevered small‐bore fittings (SBF) such as pressure tappings and drain valves. Piping vibration has often been assessed by measuring vibration displacement or velocity. However, in the case of straight cantilevered fittings with a concentrated mass, a better method exists. This paper presents a simple robust method of calculating vibration induced stress for cantilevered fittings at both the fitting branch neck and the main pipe. The method is based on measurement of acceleration at the concentrated mass (e.g., valve) and simple mechanics calculations. Extensive laboratory and field verification of the technique is presented. The choices of a fatigue strength criterion, such as ASME BPVC VIII Div. 2, BS7608, or a fracture mechanics approach such as BSI PD6493, are discussed. Several case studies are presented as well as simple ‘good engineering practice’ rules‐of‐thumb for SBF design. These ‘rules‐of‐thumb’ are now incorporated in the piping specifications of the author's company.     

活塞式压缩机联轴器连接螺栓断裂原因分析

某型号活塞式压缩机联轴器的连接螺栓在停车维修拆卸时发现断裂。通过化学成分分析、硬度测试、金相检验、断口分析及能谱分析对断裂原因进行了分析。结果表明：螺栓根部存在结构不连续的退刀槽,导致螺栓应力集中产生初始裂纹,在长期服役后发生疲劳断裂。     

变电站阻波器吊环断裂原因分析

变电站阻波器吊环在受力较小的情况下异常断裂。为查清断裂原因,运用宏观检验、扫描电镜及能谱分析、金相检验等方法进行了深入探讨。结果表明：吊环为疲劳断裂,断裂源位于吊环外表面,该部位同时存在机械损伤及应力集中;材料中较多的夹杂物及晶间析出物进一步降低了材料的疲劳强度,以上诸因素共同作用,最终导致了吊环的断裂失效。     

Ultra high molecular weight polyethylene deformation and fracture behaviour as a function of high strain rate and triaxial state of stress

The dynamic response and fracture characteristics of Ultra High Molecular Weight Polyethylene (UHMWPE) were investigated both experimentally and numerically. The strain rate sensitivity of the material was studied by carrying out tensile tests on smooth cylindrical specimens over a range of high strain rate conditions using the purpose built `flying wedge' testing machine at separation velocities up to 9 m/s. The effect of the initial stress triaxiality conditions on the material's ductility at different strain rates was studied using pre-notched cylindrical specimens with different notch radii. The true stress-strain results indicated that the tested material is highly sensitive to strain rate changes. Post-fracture geometric measurements of the fractured specimens indicated that the ductility of UHMWPE is strongly dependent on both the initial stress triaxiality conditions and the strain rate. Numerical simulations of the quasi-static and high strain rate tests were used to predict, for different notch radii, variation of the centre-most element radial strain and stress triaxiality factor with the average radial strain. Based on the combined numerical and experimental results, a simple relation for the ductile fracture of UHMWPE was derived as a function of stress triaxiality and strain rate.     

Analysis of ductile tearing using a local approach to fracture

In this paper, the Rice and Tracey (RT) model based on growth of cavities is verified in order to describe the ductile tearing in stainless steel 12Ni6Cr and aluminium alloy. Experimentally, central crack panel and compact tension specimens were tested. The comparison of the experimental results with the classical RT model shows that this model cannot make a good prediction using J −Δ a curve. On the basis of fitting procedure and numerical simulations, we have proposed a modification of this model by introducing corrective coefficients to improve the classical RT model. The proposed model, although very simple, is able to approximate with good accuracy the experimental results and that the approximation is, as expected, better than using the RT approach directly.     

On the Effect of Stress Triaxiality on Void Coalescence

Failure of ductile materials is often related to coalescence of microscopic voids. The stress triaxiality is one of the primary factors that influence the coalescence. In the present work, a 3D unit cell model is employed to investigate this effect. The cell model contains two aligned voids. A coalescence criterion is proposed in which the critical void volume fraction is expressed in terms of stress triaxiality.     

汽车定子盘开裂原因分析

某汽车制动系统定子盘使用不到六个月即在焊接热影响区部位发生开裂,采用金相检验、化学成分分析、硬度测试及断口分析等方法对该定子盘开裂的原因进行了分析。结果表明:引起开裂的主要原因是焊接区域及母材应力集中严重,加上焊缝与母材熔合不良,定子盘在使用过程中承受扭力,最终造成了疲劳开裂。     

电厂外供泵轴断裂原因分析

某外供泵在运行期间其泵轴发生断裂。通过宏观和微观检验、化学成分分析以及硬度测试等方法对泵轴断裂的原因进行了分析。结果表明:该轴的热处理没有达到要求,使各项强度指标显著降低,加上在应力集中部位键槽根部产生了疲劳裂纹,并进一步扩展,最终导致泵轴断裂。     

多孔Ti-6Al-4V合金在模拟体液中的力-电化学有限元分析

多孔Ti-6Al-4V合金植入人体内将长期处于体液腐蚀环境中,且不可避免地承受人体自重和运动产生的载荷,易引发应力腐蚀,增加无菌失败和严重并发症的风险。为了明确载荷水平及孔隙特性对多孔Ti-6Al-4V合金应力腐蚀性能的影响,本文通过数值仿真方法研究其力-电化学响应规律。基于应力腐蚀理论,将由孔隙特性引发的结构应力耦合到电极反应的平衡电位和交换电流密度表达式,建立了力-电化学耦合有限元模型,通过数值仿真方法分析了植入物试样承受弯曲载荷时,应力、电流密度以及电极腐蚀厚度等的力-电化学响应规律。结果表明：随着承载水平增加,应力集中明显的孔隙边缘区域腐蚀更为剧烈,最大腐蚀厚度均出现在孔梁边缘处。与无应力试样相比,超过屈服应力试样的5年腐蚀厚度增加了22%。孔隙尺寸显著影响应力分布、电化学腐蚀速率及最终的电极腐蚀厚度,随着孔隙的增大,电极5年腐蚀厚度逐渐减小,且大孔应力腐蚀速率低于小孔。     

叉车门架断裂原因分析

采用化学成分分析、力学性能测试、金相检验以及断口分析等方法对某叉车门架断裂的原因进行了分析。结果表明:由于在门架焊缝收弧部位存在凹坑缺陷和裂纹,形成缺口效应,产生缺口应力集中,加之门架的不对称工作应力的作用,最终造成门架发生了疲劳断裂失效。