A PROCEDURE FOR FATIGUE LIFE PREDICTION OF SPOT WELDED JOINTS

Abstract— The problem of predicting the fatigue life of spot welded joints has not yet been satisfactorily resolved. Several approaches exist, but many of them are difficult to apply to general structures, which are widely different from simple laboratory specimens. A general criterion is proposed in this paper, based on a theoretical extrapolation of the radial stress. The criterion must be used together with a Finite Element model of the joint and so this paper presents the steps required to get the necessary FE data; steps that are also applicable to the case of complex joints. The advantage of the proposed method is its independence of notch effects near the spot-weld edge. The suggested procedure is applied to several specimens of different materials and the results obtained are successfully compared with experimental data.     

基于缺口应力法的焊接接头疲劳分析

等效缺口应力法作为焊接疲劳分析的一种局部方法,不仅克服了焊接结构名义应力难以确定和焊根结构应力无法定义的困难,而且能够反映焊接局部后处理对焊接接头疲劳强度的影响,因此近年来备受关注。该文建立了典型焊接接头的三维缺口应力模型,对焊趾根部的缺口应力集中系数进行了求解;通过对对接接头和纵向角接头在焊后未处理(AS-weld)和超声喷丸处理(UPT)两种状态下的疲劳试验数据进行分析处理,获得了两种焊接接头在缺口应力系统下统一的S-N曲线,并与目前国际焊接学会所推荐的具有相同存活率的疲劳寿命曲线(IIW:m=3,FAT=225)进行比较,结果表明,该曲线具有更高的疲劳等级和更低的斜度。     

基于断裂力学的圆钢管混凝土T型焊接节点疲劳寿命预测

该文应用线弹性断裂力学基本原理预测圆钢管混凝土桁架焊接T型节点的疲劳寿命。首先,进行了一些节点的疲劳试验,作为验证节点疲劳寿命预测是否可靠的参考数据;其次,建立了基于断裂力学的疲劳寿命数值模拟的模型和流程框图,采用ANSYS软件编制了APDL宏命令,实现了节点疲劳寿命的计算;最后,分析了节点疲劳裂纹的扩展特性。研究结果表明：断裂力学数值模型能较好地预测这种复杂的钢混组合节点的疲劳寿命;在正常的焊接质量条件下,不同的初始裂纹尺度对节点疲劳扩展寿命的影响不大;裂纹在长度方向的扩展速度大于在深度方向的扩展速度;裂纹深度在达到1/2主管壁厚之前,裂纹沿深度方向的扩展非常缓慢,大部分的疲劳寿命消耗在此阶段,之后裂纹沿深度方向的扩展较快。     

A REVIEW OF FRACTURE MECHANICS MODELS OF TUBULAR JOINTS

Fracture mechanics methods are used to assess the significance of cracking in any structure, but a tubular joint is very complex to analyse. However the improvements in numerical methods, such as stress analysis using finite elements, and extensive full-scale laboratory testing (UKOSRP, ECSC, etc.) have allowed various workers to propose fracture mechanics models of tubular joints. This paper presents a critical review and comparison of these models. It concludes that simple plate models can provide upper and lower bound stress intensity factor solutions for cracked tubular joints. Stress intensity factors from three dimensional finite element analysis of a tubular joint are in reasonable agreement with semi-empirical data. However, in the absence of a validated database the paper concludes that any analytical assessment of a crack in a joint should be viewed as tentative.     

A FRACTURE MECHANICS ANALYSIS OF LOW CYCLE FATIGUE IN A SUPERALLOY*

Abstract— This paper introduces an analytical procedure for predicting engineering crack formation life of high temperature turbine components. This procedure is based upon elastic-plastic fracture mechanics rather than linear elastic fracture mechanics. State of the art, some experimental results and future developments are discussed.     

船板钢焊接接头的断裂失效行为及GTN模型的数值分析

船舶钢结构皆采用焊接方法建造而成,在实际工况及环境载荷作用下,焊接接头的力学性能及其断裂强度,直接影响船舶整体结构的强度和寿命。该文针对常用的船板钢材料(Q345和Q690),首先对母材进行单向拉伸试验,获得其各自的应力-应变曲线,进而评估其断裂性能;基于Gurson-Tvergaard-Needleman (GTN)损伤模型,通过程序代码的调试和一系列的数值模拟分析,提出了母材本构关系的表达函数及最优GTN模型参数,且与实测的应力-应变曲线高度吻合。同时,针对满足焊接规范要求的船板钢对接焊接头,进行了单向拉伸试验获得其应力-应变曲线;考虑焊缝微观缺陷以及焊接残余应力的影响,提出修正GTN损伤模型中的初始空穴体积分数f0和材料的幂函数塑性强化参数,预测焊接接头的断裂强度,且与试验测量数据吻合一致。     

THE APPLICATION OF FRACTURE MECHANICS TO FRETTING FATIGUE

Abstract— A fracture mechanics model of fretting fatigue has been developed. This enables the calculation of the critical size of defect which can grow under fretting conditions. The model has been applied to the fretting of aluminium alloy 2014A fatigue specimens in contact with 3.5NiCrMoV steel fretting contact pads. The input parameters to the model include externally applied stresses, contact pressure between pad and specimen and frictional forces between the two contacting surfaces. The latter have been measured using a strain gauge technique. Small fretting defects in the fatigue specimens have been investigated metallographically and fractographically. There is good agreement between their size and critical defect sizes calculated using the fretting model.     

PREDICTION OF THE FATIGUE LIFE OF LASER WELDED STAINLESS STEEL JOINTS

Abstract— A fatigue scatter band has been computed for laser welded austenitic stainless steel joints. These laser welded steels have a very small heat affected zone. The unified scatter band provided by standards for welded structural steels does not adequately describe the trend of the experimental data of laser welded steels and this makes their design parameters scarcely realistic. The scatter band proposed in this paper has been computed by re-sorting experimental data relative to joints with high stress concentration factors and has subsequently been assessed with data relevant to butt welded joints.     

正交异性钢桥面板肋-面板焊缝疲劳验算的应力分析模型评估

以天津塘沽海河大桥的正交异性钢桥面板为例,建立钢箱梁节段的有限元分析(FEA)模型、简化FEA模型和基于钢箱梁节段的子模型进行肋-面板焊缝疲劳应力分析模型的评估。计算结果表明:对于远离纵腹板处的肋-面板焊缝,通过提高简化FEA模型中横隔板高度和约束横隔板底部翼缘的方式可减小与钢箱梁节段FEA模型结果的误差;对于靠近纵腹板处的肋-面板焊缝,所有简化FEA模型的应力计算结果均低于钢箱梁节段FEA模型的结果,且误差超过23%。基于钢箱梁节段的三跨子模型可以准确地给出钢桥面板任意位置的肋-面板焊缝的疲劳应力。它具有钢箱梁节段FEA模型的计算精度和简化FEA模型的计算效率,因此它可作为正交异性钢桥面板任意位置处肋-面板焊缝疲劳应力分析的准确和高效计算模型。     

EFFECT OF YIELD STRENGTH ON THE BASIC FATIGUE STRENGTH OF WELDED JOINTS

Abstract— The basic fatigue strength of welded joints in four steels having different yield strengths has been obtained by tests in which the maximum applied stress was held constant and equal to the yield strength, to simulate the tensile residual stress in real large-scale structures. In the long-life region superior properties occurred with a decrease in the yield strength. It is therefore suggested that both low yield strength steel, which can be produced by a thermo-mechanical control process without affecting the tensile strength, and steels or welding consumables which show a low transformation temperature, may have a high fatigue strength.     

阻力板断裂原因分析

应用扫描电镜、光学显微镜和光谱分析等手段．分析了RZG45熔模精密铸件产生脆性断裂的原因。结果表明，铸件的断裂系由呈链状沿晶分布的脆性相所致。提出了消除脆性相的措施。     

THE EFFECT OF MISALIGNMENT ON THE FATIGUE STRENGTH OF WELDED CRUCIFORM JOINTS

Abstract— The effect of axial misalignment on the fatigue strength of load-carrying transverse cruciform welded joints was investigated using experimental and fracture mechanics methods. Where failure occurred by cracking from the weld toe, misalignment significantly reduced the fatigue strength. The reduction could be predicted using a nominal stress concentration factor (SCF). Misalignment had less effect where failure was due to cracking through the weld metal; an expression was deduced for the SCF in this case. For fracture mechanics assessments, an expression for an effective stress intensity factor using the SCF and stress intensity factors for aligned welds was shown to agree with the finite element (FE) results. Predictions of the effect of misalignment using the FE results agreed with experimental data. Misaligned transverse load-carrying cruciform joints should be assessed for fatigue failure from the toe using the same SCF as for a butt weld with the same misalignment. For failure through the throat, an alternative expression for the SCF is recommended. Fracture mechanics assessments of misaligned joints should be carried out using an effective stress intensity factor derived from the SCF and stress intensity factors for aligned joints. These recommendations are now incorporated in British Standard PD 6493:1991.     

FATIGUE ANALYSIS AND PREDICTION IN FILLET WELDED JOINTS IN THE LOW THICKNESS RANGE

Abstract— The paper reports the results of a comprehensive research project concerning fatigue life prediction in fillet welded joints. Geometry variables such as main plate thickness, radius of curvature at the weld toe and leg to leg distance were analysed in detail. Fatigue life computations were carried out for semi-elliptical cracks using appropriate FE techniques. The range of results covered several types of welded joints loaded in tension and in bending. A comparison of results was made using two methods of stress intensity determination. Experimental data was also obtained and that included measurements of weld toe radius, monitoring of crack shape and S-N curves. Correlation of results with the theoretical predictions gave generally good agreement. A set of fatigue design curves for fillet welded joints is proposed and in these the designer can introduce the geometry of the weldment.     

气门疲劳断头失效分析

采用宏、微观检验、化学成分分析和硬度测试等方法对断头的排气门进行了分析.结果表明,气门断裂方式属于疲劳失效.结合气门服役受力情况,指出气门失效主要由于气门间隙调整过大使气门盘部与座圈的落座力过大,气门与座圈经长时间撞击,在气门根部产生金属疲劳.气门杆端面在运行中不断与摇臂撞击磨损使该现象进一步加剧,导致气门颈部撞击疲劳断裂失效.     

基于断裂力学的钢框架梁柱节点抗震性能分析

介绍了8 个足尺节点试验的结果与连接焊缝断裂破坏的现象。采用二维有限元断裂模型,研究了抗弯钢框架梁柱节点的断裂行为。以I 型裂纹尖端应力强度因子K_I和J积分为定量评价指标,分析了焊接孔形式、初始缺陷尺寸及位置、焊接垫板、角焊缝补强等设计细节对节点材料断裂韧性需求的影响。弹性分析表明,切除垫板并以角焊缝补强的措施,能最有效地降低对材料韧性的要求。弹塑性分析表明,焊接高匹配时热影响区裂纹比界面裂纹对材料韧性的要求更高,且局部塑性应变累积使材料的断裂韧性降低;解释了节点试验中断裂易发生在梁下翼缘焊缝热影响区的现象。基于断裂分析得到的节点抗弯承载力与试验中节点的极限弯矩吻合较好,该文的研究为钢框架梁柱节点的防断设计提供了参考。     

管节点表面裂纹疲劳扩展的数值模拟

本文采用线弹簧单元与ＡＤＩＮＡ程序结合计算了表面裂纹在交变载荷作用下的应力强度因子，首次提出了预埋线弹簧单元法模拟表面裂纹疲劳扩展，并发展了相应的技术和软件。     

CF-62钢焊接接头断裂韧性分布的试验研究

对ＣＦ－６２钢焊接接头Ｊ－Ｒ阻力曲线和断裂韧性Ｊ１Ｃ进行了８测试，并用参数假设检验及概率级等方法，对试验结果进行了分析和检验，得到了Ｊ１Ｃ的分特征值，为高强钢焊接接头断裂韧性的测试方法提供了依据。     

CYCLIC CRACK RESISTANCE OF WELDED JOINTS WITH NON-UNIFORM FIELDS OF RESIDUAL WELDING STRESSES

Abstract— The use of linear elastic fracture mechanics to describe the kinetics of fatigue fracture of welded joints with high welding residual stresses (WRS) is experimentally evaluated in this paper. A correction analysis is used to show that the crack propagation rate of cracks in joints, as a function of the applied stress intensity factor, is linear on a log-log scale in the Paris regime when non-uniform fields of WRS are superimposed on the applied cyclic loading. It is shown that crack growth rates in joints with high WRS do not depend on the characteristics of the loading cycle. The parameters of the Paris exponential equation are determined by the initial WRS distribution, by the range of cyclic stresses and by the load ratio. A method for calculating the cyclic crack resistance of joints is proposed which explicitly allows for a non-uniform field of WRS that influences the fatigue crack growth rate.     

钢箱梁桥焊接细节的疲劳断裂可靠性分析

联合线弹性断裂力学和长期监测数据提出了大跨桥梁焊接细节疲劳可靠度评估方法, 并以润扬大桥钢箱梁的焊接细节为对象开展了应用研究。首先, 采用长期监测数据建立了疲劳荷载效应的概率模型;其次, 推导了焊接细节疲劳断裂抗力的表达式, 分析了初始裂纹深度和极限裂纹深度对断裂抗力的影响规律, 采用随机数值模拟的方法得到了断裂抗力的概率模型;最后研究了交通荷载增长条件下焊接细节疲劳可靠度的时变规律。研究表明:断裂抗力服从对数正态分布, 且对初始裂纹深度极为敏感;当考虑交通荷载增长, 桥梁的服役期还未满设计基准期时, 焊接细节的可靠度就将低于目标可靠度水平。     

FATIGUE ANALYSIS OF SURFACE CRACKS AT FILLET WELDED TOES

This paper reports a study of fatigue crack growth and coalescing behaviour at semi-elliptical cracks in the stress concentration region of steel plates with fillet shoulders or fillet welds. Fatigue tests were carried out on machined plate specimens with a fillet geometry similar to a fillet welded joint. These specimens were notched and precracked to provide single and multiple coplanar semi-elliptical surface cracks at the fillet toe region. Finite element stress analysis results were used to obtain approximate Mk factors (i.e.: stress concentration magnification factors) for the fillet toe geometry with a semi-elliptical surface crack. An analytical model was developed to simulate crack shape development and growth to failure in the case of multiple coplanar semi-elliptical cracks. In this model, a simple crack coalescing procedure is applied to merge coplanar cracks when they meet by recharacterising the coplanar cracks into a single semi-elliptical crack. Alternative crack growth laws were investigated and comparisons made between actual and predicted shape developments and lives.