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.     

CONSIDERATIONS OF THE EFFECT OF RESIDUAL STRESSES ON FATIGUE OF WELDED ALUMINIUM ALLOY STRUCTURES

A new class of large, high-speed seagoing ferry-boat is under development for service around the world. The ships, which are built entirely of aluminium-alloy plate and stiffeners, show a propensity for fatigue cracking of the welded structure. Cracks may occur in both the hulls and the superstructure early in their 20-year service life. Early appearance of fatigue cracks is shown to result from the combined stress and strain fields set up in weld zones by the static residual stresses and cyclic loads, beyond the effects of weld and detail geometry.  A numerical example demonstrates that conventional methods of fatigue analysis overestimate the lifetime of the welded aluminium structure, while damage tolerance analysis based on fracture mechanics leads to improved prediction.     

残余应力对金属疲劳强度的影响

残余应力对光滑试样高周疲劳极限的影响可以用Goodman关系来描述，但必须要得到残余应力作用系数m、合理地提取残余应力的表征值和区分开其它因素的影响。残余应力对缺口疲劳极限的作用大于对光滑试样的作用，是由于残余应力也存在应力集中现象，而且不易衰减。残余应力的应力集中系数不仅与缺口几何因素有关，还与材料特性有关。试验研究还表明，表层残余压应力对于承受轴向载荷且疲劳残纹萌生于表面的零件也十分有益。     

INFLUENCE OF STRESS RELIEVING BY VIBRATION ON THE FATIGUE BEHAVIOUR OF WELDED JOINTS IN COMPARISON TO POST-WELD HEAT TREATMENT

Abstract— Constant amplitude fatigue tests with welded specimens under fully reversed four-point bending as well as under axial loading have shown that vibration stress relieving does not lead to a fatigue life improvement of welded parts when compared to the as-welded state. Thus, a substitution of thermal stress relieving by a vibration treatment is not successful. This was also proved by residual stress measurements in the welded parts studied in this paper.     

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。     

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。     

FATIGUE STRENGTH OF NON-LOAD-CARRYING FILLET WELDED JOINTS: EFFECTS OF WELD RESIDUAL STRESSES AND STRESS CONCENTRATION

The fatigue strength of non-load-carrying fillet welded joints of KE36(TMCP) steel was studied. Both residual stress measurements and fatigue tests were carried out, with the plate thickness, the plate width and the heat input being varied. Specimens given a Post Weld Heat Treatment (PWHT) were also prepared. The plate width had no effect on the fatigue strength, because it hardly affected the transverse residual stresses at the weld toe. However, the heat input influenced the transverse residual stress distribution, and a significant difference in fatigue strength due to the heat input was observed, especially when N≥ 10⁶ cycles. It was also found that PWHT removed almost all the residual stresses at the weld toe, improving the fatigue strength drastically. In this study, the values of stress concentration factor K₂ were estimated by Machida's method and it was concluded that the thickness effect resulted from a combination of both stress concentration and residual stresses with the contribution of the latter being particularly significant for N≥ 10⁶ cycles.     

EFFECT OF RESIDUAL STRESSES OF SHOT PEENING ON THE FATIGUE BEHAVIOUR OF A HIGH STRENGTH STEEL*

Abstract— The effects of shot-peening residual stresses on bending fatigue behaviour of welded plate and surface-flawed plate were investigated. The results show that shot peening residual stresses may significantly increase fatigue strength as well as the threshold stress- intensity factor range of a surface-flawed plate. This paper presents a simple analytical method for determining the residual stresses. The calculated values are essentially in agreement with the experimental results.     

焊接接头应力强度因子计算公式的改进

针对焊接接头的半椭圆表面裂纹的应力强度因子SIF 的计算公式, 该文介绍了目前常用的Newman-Raju-Maddox 模型和Yamada-Hirt-Albert模型以及两种模型的异同性。经过分析, 发现Yamada-Hirt-Albert模型存在不足, 其SIF 公式难以合理地计算均匀受拉状态下的SIF 值。在对现有文献中研究数据的分析基础上, 改进了Yamada-Hirt-Albert 模型, 提出了新的SIF 计算公式。经与Benchmark 数据以及其他国外试验结果的比较, 验证了该文所提出的SIF 改进公式具有更好的可靠性。     

焊缝组织及应力对声速、幅值及谐波系数的影响

运用超声波(f=15MHz)水浸聚焦透射检测法,对消除应力退火前后的45钢摩擦焊接头进行了检测研究,发现了焊逢、热影响区组织及残余应力对超声声速、幅值及谐波系数的影响规律.     

校直对厢形梁焊接残余应力的影响

介绍不同焊接工艺对厢桥梁焊接余应力的影响及校直前后的其残余应力的变化。     

钢桥面板纵肋与横隔板焊接细节表面缺陷及疲劳效应研究

初始焊接缺陷是影响结构件疲劳性能的关键因素之一。在断裂力学评估框架下引入特征化初始焊接缺陷,结合相互作用积分法与复合断裂准则解决由表面缺陷所导致的复合型疲劳裂纹扩展问题,在此基础上编写裂纹扩展模拟程序,建立表面焊接缺陷效应评价方法,通过分析揭示了不同形态和尺度的初始焊接缺陷对于钢桥面板纵肋与横隔板构造细节裂纹扩展关键性度量指标和疲劳寿命预测的影响。结果表明：所建立的方法可有效用于评估焊趾部位表面焊接缺陷对于疲劳性能的效应;面状缺陷对于裂纹扩展度量指标和疲劳寿命预测结果的影响更为显著,其初始缺陷深度和形态均是影响疲劳性能的关键因素,体积型缺陷对于疲劳寿命的影响主要由深度方向的缺陷尺寸决定;焊接缺陷的形态和几何参数取值应根据工程实际和规范建议值共同确定,直接简化为面状缺陷会低估结构件的疲劳寿命;考虑焊接缺陷不确定性的可靠度评估方法尚需进一步研究。     

纵肋与横隔板交叉构造细节穿透型疲劳裂纹扩展特性及其加固方法研究

纵肋与横隔板交叉构造细节是正交异性钢桥面板最易发生疲劳开裂的构造细节,通过建立有限元数值模型,采用断裂力学方法,研究栓接角钢加固方式对该处疲劳易损细节穿透型裂纹的加固效果。基于疲劳试验足尺节段模型相对应有限元模型,建立了纵肋与横隔板焊接处穿透型疲劳裂纹模型,针对栓接角钢和纵肋外侧栓接钢板两种加固技术的加固效果进行评估。研究结果表明：钢桥面板纵肋与横隔板交叉构造细节的疲劳裂纹扩展至一定长度后将发展成穿透型裂纹,裂纹面受力复杂,纵肋腹板内外侧疲劳裂纹扩展特性表现的不一样,但是随着裂纹扩展的逐步进行,裂纹尖端的开裂模式均以复合型开裂为主;栓接角钢加固方式主要抑制纵肋与横隔板交叉构造细节易损部位疲劳裂纹的I型开裂,因此能很好地抑制短裂纹的扩展,但对于该细节处以复合形式扩展的穿透型疲劳裂纹的加固效果并不显著;在纵肋外侧栓接半U形钢板的加固方法能有效改善穿透型疲劳裂纹的等效应力强度因子,并且加固之后均保持在裂纹扩展阈值以下,表明该加固方式对穿透型疲劳裂纹有良好加固效果。     

焊接球节点刚度对网壳结构有限元分析精度的影响

采用与焊接空心球半径等长的刚臂来模拟节点的体积,在刚臂与杆件之间设置弹簧单元来模拟节点的刚度,推导了考虑刚臂和节点刚度的梁的单元刚度矩阵;根据焊接球节点刚度的回归公式,计算出符合施工构造要求的焊接球节点对杆件轴向刚度和弯曲刚度的影响范围,发现当焊接球的直径小于杆件几何中心长度的12%时,焊接球对轴力刚度的影响较小,而对弯曲刚度的影响很大;采用能量范数分析了弯曲刚度和轴力刚度误差应力解和位移解的影响,给出了焊接球对网壳位移解的误差估计公式。采用多尺度有限元法验证了该结论。     

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS-STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.     

热残余应变和微裂纹对晶须增韧陶瓷力学性能的影响

热残余应变和微裂纹是影响晶须增韧陶瓷基复合材料力学性能的两个重要因素,本文将综合考虑这两种因素的影响,定量研究晶须和微裂纹在基体内随机分布时多种因素对复合材料模量和强度的影响,包括晶须和微裂纹含量、晶须长径比和微裂纹尺寸以及组分性能的影响.理论结果表明,上述各种因素对材料力学性能均有不同程度的影响,所得结论将为材料的优化设计提供一定的指导作用.     

DESIGN OF EXPERIMENTS TO INVESTIGATE RESIDUAL STRESSES AND FATIGUE LIFE IMPROVEMENT BY A SURFACE TREATMENT

Abstract— The careful design of experiment (DOE) technique has been utilized to analyze the residual stress state and to investigate the fatigue life improvement of a material (nitriding steel) subjected to thermal and mechanical treatment.
Nitriding treatments have been performed on several specimens which have been subsequently shot- peened, varying the main parameters controlling the process. The design of experiment method has been accomplished in order to evaluate the influence of the main shot-peening parameters on the distribution and values of the residual stresses close to the surface, and also in order to estimate the influence of these parameters on fatigue resistance.     

X射线衍射法测残余应力的“突变”现象与定峰技巧探讨

X射线衍射法（XRD）是一种常用的测试金属表面残余应力的无损检测法。依据X射线应力测试的原理,对试验中遇到的衍射强度“突变”现象进行了初步探讨,并对其产生的原因和定峰中的处理方法进行了具体分析,为进一步发展和完善铝合金焊接残余应力的X射线分析技术提供了参考。