Research on stress concentration factor of tube joints under pure bending and torsion loading

For fatigue strength design and evaluation as well as for multiaxial fatigue tests analysis of welded structures, the stress concentration factor(SCF) at the weld must be known. In the present study, two common models of multiaxial fatigue tests, tube-plate(TP) model and tube-tube(TT) model are selected as the research objects. SCFs of the two models with various geometric parameters under pure bending and pure torsion loading have been investigated by conducting a series of three-dimensional(3 D) finite element models based on linear elasticity theory. The effect of weld width(w), plate thickness(t), weld toe notch radius(r), and reinforcement angle(θ) on SCF is assessed. The quadratic parametric equations based on t/r are proposed to predict SCFs of the two models, which reveal satisfactory accuracy.

Parametric equations for notch stress concentration factors of rib—deck welds under bending loading

The effective notch stress approach for evaluating the fatigue strength of rib-deck welds requires notch stress concentration factors obtained from complex finite element analysis.To improve the efficiency of the approach,the notch stress concentration factors for three typical fatigue-cracking modes(i.e.,root-toe,root-deck,and toe-deck cracking modes)were thoroughly investigated in this study.First,we developed a model for investigating the effective notch stress in rib-deck welds.Then,we performed a parametric analysis to investigate the effects of multiple geometric parameters of a rib-deck weld on the notch stress concentration factors.On this basis,the multiple linear stepwise regression analysis was performed to obtain the optimal regression functions for predicting the notch stress concentration factors.Finally,we employed the proposed formulas in a case study.The notch stress concentration factors estimated from the developed formulas show agree well with the finite element analysis results.The results of the case study demonstrate the feasibility and reliability of the proposed formulas.It also shows that the fatigue design curve of FAT225 seems to be conservative for evaluating the fatigue strength of rib-deck welds.

Analysis of the stress intensity factor of welded joints under prior corrosion and complex stress fields

Taking cruciform welded joints under different corrosion degrees, together with 45° inclined angle and full penetration load-carrying fillet, as the research object, the stress intensity factor of a quarter-circular comer crack of welded joints is calculated based on FRANC3D with ABAQUS. Effects of different corrosion pit sizes, crack aspect ratios, and crack depths on stress intensity factor are analyzed. The results show that pit depth plays a major role in stress intensity factor, while the effect of pit radius is relatively small. The cracking modes of the surface and the deepest point are mode I, and mixed modes I and II, respectively. Effects of pit depths, crack aspect ratios, and crack depths on the stress intensity factor at the surface point are greater than at the deepest point.

Stress concentration coefficients of optimized cope-hole welded detail

The fatigue performance of optimized welded detail has been investigated by fatigue experiments of three welded specimens under different loadings.In addition,local finite element models of this welded detail were established using finite element software ANSYS.The influences of different factors such as plate thickness,plate gap and initial geometric imperfections on the stress concentration coefficient(SCC) were discussed.The experimental results indicate that the fatigue life of three specimens for this welded detail is 736,000,1,044,200 and 1,920,300 times,respectively.The web thickness,the filler plate thickness and the initial geometric imperfection have relatively less effect on the SCCs of this welded detail.However,cope-hole radius is influential on the SCCs of the web and the weld.The SCC of weld is significantly affected by the weld size and plate gap,but the SCCs of other parts of the welded detail are hardly affected by the plate gap.

Use of Plastic Correction Formula to Improve Accuracy of Welding Residual Stress Test with Blind-Hole Method

The blind-hole method is the most widely used approach to experimentally determine the distribution of residual stress. This paper aims to improve test accuracy of welding residual stress and conducts an experimental study on the strain release factors involved when using the blind-hole method for Q235 and Q345, two steels commonly used in building structures. The ranges of strain release factors A and B in the elastic stage, the effects of strain release factors on residual stress calculated values, and the plastic corrected strain release factors are analyzed considering of the effect of plastic deformation around the blind hole on measurement accuracy. Finally, a simplified calculation formula to determine strain release factors is proposed for use with the blind-hole method. Results show that in the elastic stage, strain release factor A for Q235 and Q345 ranges from-0.399 to-0.525 and strain release factor B from-0.791 to-0.960. Changing the strain release factors A and B shows that calculated residual tensile stress varies in relation to a decrease in both factor values. However, there is a increase in calculated residual compressive stress with a decrease in the strain release factor A value, but there is an decrease with a decrease in strain release factor B value. Calculated residual stress applied to elastic strain release factors is compared with that applied to amended plastic strain release factors for Q235 steel. The maximum deviation between calculated residual stress and test stress is reduced from 21.1 to 1.0%,and for Q345 steel from 26.5 to 1.2%. It is thus evident that the plastic correction formula proposed in this paper can be used in calculations when conducting a residual stress test.

Numerical stress analysis of crack at welded beam-to-column connection

In the past, brittle fracture of steel structure was reported rarely under earthquake. However, recent earthquakes, especially Northridge Earthquake (USA) and Hyogoken Nanbu earthquake (Japan), astonished engineers in the field of construction. The experience from recent earthquakes of USA and Japan shows that brittle fracture of welded steel structure always starts from high stress zone with welded crack [1～5] . As backing bar for grooved weld on beam flange exists, artificial crack is formed because of lack of fusion at the root of flange weld. In this paper stress distribution of connection is computed with FEM, and stress concentration at the root of flange weld is also analyzed. Stress intensity factors (SIFs), K I, at the root of flange weld are computed in the method of fracture mechanics. The computation shows that stress intensity factor on bottom flange weld is obviously higher than that on top flange weld. It is proved by the fact that brittle fracture is liable to start at the root of bottom flange weld on actual earthquake [1,4] . Finally measures are brought forward to avoid fracture of weld structure against earthquake.

Evaluation of stress concentration for planar tubular joints

In offshore tubular structures, a typical tubular joint may be subjected to three different types of basic loadings: axial, in-plane bending and out-of-plane bending, through its brace members. Each type will cause a different stress distribution at the joint intersection of structures. Moreover, the actual load condition of a tubular joint can be any combination of the above three basic load cases, for this reason, a combined loading was investigated in addition to these three basic loadings. This load is composed of an axial loading combined with a continuation of rotational bending loading obtained while rotate center of the brace around a circle. Different types of planar joints such as T, Y, X, K, DT, DY, DX, TY, TK, DTX, DTDY, and DTDK, with braces subjected to combined loading, were numerically analyzed to study the effect of those different cases of loading and different types of joints, on the stress concentration zone and values.

焊接残余应力对高强钢点焊接头裂纹扩展及疲劳性能的影响

点焊接头因高度应力集中,极易于熔核边缘萌生疲劳裂纹,点焊产生的残余应力会影响其扩展。为探究点焊残余应力对疲劳裂纹扩展的影响规律,建立DP600高强钢点焊接头三维裂纹有限元仿真模型。基于热-弹塑性理论求解点焊残余应力场,在此基础上计算有无残余应力时折线裂纹的局部应力强度因子(Stress Intensity Factor,SIF)解,通过分析两种条件下裂纹最深点局部SIF随裂纹深度比变化的曲线,研究点焊残余应力对折线裂纹扩展的影响规律;基于断裂力学理论,采用改进的双参数模型预测有无残余应力时的疲劳寿命。结果表明,点焊残余应力使局部SIF显著提高,且随着裂纹深度比的增加,残余应力对其的影响程度也增加;考虑残余应力后裂纹整体扩展趋势不变但扩展速率显著增加,疲劳寿命由35616次下降至12564次。

钢桥面板对接焊缝表面多缺陷疲劳效应研究

为探明钢桥面板对接焊缝焊趾区域共面及异面表面多缺陷在裂纹扩展过程中的形态变化及相互作用机制,以钢桥面板U肋下翼缘对接焊缝简化后的基本焊接构造为研究对象,探明对接焊缝焊接区域的应力强度因子KI分布,并在验证ABAQUS与FRANC3D数值模拟方法可靠性的基础上,对焊趾区域植入的共面或异面多裂纹进行多裂纹扩展分析。研究结果表明:含余高对接焊缝在轴拉荷载作用下,焊趾线附近存在应力强度因子KI的峰值点;相邻共面裂纹在扩展至临界深度的过程中存在裂纹融合前、裂纹融合时及裂纹融合后3个典型的扩展阶段,不同阶段裂纹前沿形态及扩展速率da/dN变化可通过等效应力强度因子幅值ΔK_(eff)的分布来体现;共面裂纹融合前,单裂纹靠近另一裂纹长轴端点的裂纹相互作用比例因子随净间距s1与裂纹深度a比值呈幂次负相关性;对接焊缝异面裂纹中焊趾处裂纹对热影响区裂纹存在抑制作用,并随着裂纹尺寸差异的累计增加,焊趾处裂纹对热影响区的抑制作用呈线性扩大,最终使得热影响区裂纹ΔK_(eff)低于门槛值ΔKth而失去活性停止扩展,上述抑制作用随异面净间距s2的增加而减弱。

机车车体枕梁关键含缺陷结构疲劳寿命评估

为研究枕梁含缺陷结构服役安全问题,首先,基于有限元仿真对比不同尺寸下的底架枕梁结构刚度、挠度,根据最佳结构试验尺寸开展等效疲劳台架试验,获取焊缝关注点的应力分布状态及潜在裂纹位置,并验证枕梁结构与仿真模型的一致性;然后,基于BS 7910:2019标准中结构完整性评估方法,通过引入当量化裂纹缺陷获得结构临界失效状态,实现关键焊缝区域的寿命预测,同时结合断裂力学数值仿真方法,从仿真角度实现局部模型结构的裂纹扩展模拟,对含裂纹缺陷结构的损伤容限进行分析;最后,对比分析基于标准评估的理论方法与数值仿真方法结果的可靠性和差异性。研究结果表明:枕梁子模型结构台架试验与仿真模型的应力分布具有较好的一致性,枕梁结构薄弱区域集中在垂向减振器角焊缝位置,基于BS 7910:2019标准和数值仿真计算得到的减振器座角焊缝表面缺陷临界深度分别为6.50 mm和7.85 mm,对应疲劳载荷循环数分别为1.029×105和1.121×105次,两者评估结果具有较好的一致性。

锆合金管塞结构压力电阻焊接头的应力集中系数及影响因素

采用Abaqus有限元分析软件建立了锆合金管塞结构压力电阻焊接模型与应力集中系数计算模型,通过与热模拟试验得到的挤出金属高度进行对比,对焊接模型进行验证,模拟分析了内部挤出金属过渡角、端塞行进量和包壳管内壁倒角对接头应力集中系数的影响。结果表明:内部挤出金属高度模拟结果和试验结果的相对误差为9.58%,外部挤出金属高度的相对误差为6.65%,说明建立的焊接模型准确;随着内部挤出金属过渡角增加,接头应力集中系数先降低,当过渡角增大至60°后增加,增大至120°时又下降;随着端塞行进量的增加,内部挤出金属过渡角降低,应力集中系数增加,接头熔合线长度增大;与未倒角接头相比,包壳管内壁倒角深度不大于0.50 mm时应力集中系数降低,最大降低了16.00%,倒角深度大于0.50 mm时应力集中系数增大,最大提高了91.49%。

基于XFEM的X80管线钢焊接接头三维裂纹萌生扩展仿真分析

利用有限元软件ABAQUS,以X80管线钢不同坡口形式的焊接接头为研究对象,基于扩展有限元方法(XFEM),对X形和V形两种不同形式坡口的焊接接头在拉伸载荷下的裂纹扩展形态进行数值模拟分析。结果表明,裂纹尖端首先出现了应力集中,应力分布趋势与理论计算的塑性区形状大致相同。利用XFEM方法模拟裂纹尖端自由扩展,X形坡口和V形坡口的焊接接头裂纹首先出现在焊缝并靠近熔合线处,且二者的扩展路径一致,在相同条件下,X形坡口的焊接接头焊缝处最先出现裂纹,V形坡口的焊接接头焊缝处裂纹出现的时间较X形坡口出现裂纹的时间晚。

基于等效结构应力法的高强钢焊接结构低温主S-N曲线

现有的疲劳S-N曲线已不再适用于北极的低温环境,为了评估北极海洋工程设备材料的低温疲劳寿命,有必要建立金属结构尤其是焊接结构的低温疲劳S-N曲线。本文基于等效结构应力法,计算出Q690高强钢管环焊缝的主S-N曲线,并通过共振疲劳试验对该方法进行了验证。在此基础上,结合大量的高强钢焊接结构试验结果,将温度敏感因子c引入到低温疲劳主S-N曲线的推导中,首次建立了基于低温金属焊接结构的主S-N曲线,并对文献中的疲劳S-N试验数据和ASME中的修正方法进行了对比和验证。结果表明,等效结构应力法能够较准确地计算出焊缝疲劳S-N曲线,推导出的金属焊接结构低温主S-N曲线与试验曲线吻合较好,能够满足低温地区工程要求,可为高强钢焊接结构在北极地区低温环境中的广泛应用提供理论指导。该方法可以为金属焊接结构的低温疲劳研究节省大量的成本,减少非标准试验操作造成的不必要的误差。这对北极海洋工程设备的设计、安全运行和疲劳评估具有重要意义。

对接接头缺口应力集中系数不确定性分析

[目的]以典型对接接头为研究对象进行参数化建模,建立焊缝几何形状存在不确定性的有限元模型,来研究焊缝几何存在不确定性的对接接头焊趾处缺口应力集中系数K_(t),并统计分析其分布规律。[方法]首先,对以往采用的网格尺寸和单元类型进行改进,通过有限元网格敏感性分析,研究不同类型的单元以及网格尺寸对K_(t)的影响;其次,沿对接接头焊缝法向提取节点应力,通过绘制应力分布图,确定焊趾处最大应力集中所在部位;然后,通过引入控制点描述对接焊缝几何形状的变化来真实反映焊缝几何形状的不确定性,结合实际测量的焊缝形状数据的分布规律,随机抽样得到大量的焊缝几何参数数据;最后,基于缺口应力法计算K_(t),并对其进行统计分析,探究对接接头在焊缝几何参数存在不确定性时K_(t)的分布规律。[结果]结果表明,K_(t)的分布满足正态分布。[结论]通过探究焊趾处的应力分布,对于分析疲劳强度和提高疲劳寿命预测精度具有重要意义。

考虑残余应力的钢桥面板-肋双面焊裂纹应力强度因子计算方法

建立焊接分析有限元模型,对顶板-纵肋双面焊构造的焊接过程进行数值模拟,拟合得到顶板焊趾细节沿板厚方向分布的横向残余应力分布经验公式;建立钢桥面板断裂力学数值模型,结合统一的权函数表达式,推导适用于顶板焊趾处裂纹最深点和表面点应力强度因子的新权函数,并将权函数计算的应力强度因子与有限元计算的应力强度因子进行对比。研究结果表明:顶板-纵肋双面焊顶板焊趾处残余应力沿板厚方向处于拉—压—拉状态,呈正弦函数分布;在二次应力分布下,权函数法与有限元法计算所得顶板焊趾处裂纹最深点应力强度因子最大相对误差为7.4%,表面点应力强度因子最大相对误差为4.1%;在焊接残余应力场下,权函数法与有限元法计算所得顶板焊趾处裂纹最深点应力强度因子最大相对误差为7.6%,表面点应力强度因子最大相对误差为8.6%;权函数法能有效计算钢桥面板-肋双面焊顶板焊趾处疲劳裂纹应力强度因子。

Q355B直缝焊管开裂原因

某公司使用Q355B热轧带钢制作直缝焊管,焊接后发现部分焊管的焊缝区域发生开裂现象。采用宏观观察、金相检验、扫描电镜和能谱分析等方法研究焊管的开裂原因。结果表明:熔炼时带钢的硫化物含量较高,且连铸时带钢内弧1/4厚度处发生夹杂物偏聚现象,硫化物破坏了基体的连续性,在微裂纹处出现应力集中,最终造成焊管开裂。

风力发电机塔筒筒节焊接应力集中因数分析

采用热点应力法对风力发电机塔筒筒节的焊接应力集中因数进行分析,得到焊接应力集中因数随板厚、板厚差、外径的变化规律。通过分析确认,将板厚差从14 mm增大到20 mm,可以减小风力发电机塔筒的质量,并且不会降低风力发电机塔筒的疲劳强度设计等级。分析结果可以为减小风力发电机塔筒质量提供参考。

某挖掘机动臂焊缝开裂原因

某大型液压挖掘机的动臂在使用过程中发生开裂现象,采用宏观观察、化学成分分析、扫描电镜分析、力学性能测试、金相检验、有限元分析等方法对动臂开裂原因进行分析。结果表明:开裂的主要原因是内部焊缝的底部与母材存在未熔合缺陷,使焊接区域尖角应力集中,同时非标大负载铲斗使焊缝表面的应力明显加大,最终在循环大载荷的作用下,焊缝发生疲劳开裂。

桥梁钢对接焊缝接头残余应力分布影响因素研究

为了研究不同因素对桥梁钢对接焊缝接头残余应力分布的影响,建立了Q345qDNH桥梁钢对接焊接有限元模型,分析了在材料属性和焊缝余高2个因素影响下的残余应力分布规律。结果表明:未考虑应变随温度变化的钢板试件,其焊接残余应力最大值分布区域比考虑应变随温度变化情况下的残余应力分布区域大,焊接热影响范围较广,而考虑应变变化的试件,其残余应力数值相对较大;焊缝余高的存在会使焊接接头焊趾处出现明显的应力集中现象.

金属结构件焊接失效的原因及预防措施

焊接是金属结构件使用时常见的连接技术,受各方面因素影响,结构件容易出现焊接失效问题,降低其使用寿命。首先对金属结构件焊接失效的条件进行分析,然后从塑性断裂、脆性断裂、疲劳断裂三个层面详细介绍了焊接失效的原因,最后针对三种常见的失效形式提出了针对性的预防措施。对于提升金属结构件的焊接质量,降低焊接失效概率具有一定的意义。