Fatigue behavior and life prediction of A7N01 aluminium alloy welded joint

Fatigue characteristics of A7N01 aluminium alloy welded joint were investigated and a fatigue crack initiation life-based model was proposed. The difference of fatigue crack initiation life among base metal, weld metal and heat affected zone (HAZ) is slight. Furthermore, the ratio of fatigue crack initiation life (Ni) to fatigue life to failure(Nf) is a material dependent parameter, 26.32%, 40.21% and 60.67% for base metal, HAZ and weld metal, respectively. Total fatigue life predicted using the presented model is in good agreement with the experimental data and that using Basquin’s model. The observation results of fatigue fracture surfaces, using scanning electron microscope (SEM), demonstrate that fatigue crack initiates from smooth surface due to welding process for weld metal, blowhole in HAZ causes fatigue crack initiation, and the crushed second phase particles play an important part in fatigue crack initiation in base metal.

Fatigue life prediction of gas metal arc welded crucifrom joints of AA7075 aluminium alloy failing from root region

Empirical relationship was developed to predict the fatigue life of gas metal arc welded （GMAW） cruciform joints failing from root region. High strength, age hardenable aluminium alloy of AA7075-T6 grade was used as the base material. The design of experiments concept was used to optimize the required number of fatigue testing experiments. Fatigue experiment was conducted in a servo hydraulic controlled fatigue testing machine under constant amplitude loading. The empirical relationship was developed. By using the developed empirical relationship, the fatigue life of GMAW cruciform joints failing from root region was predicted at 95% confidence level. The effect of cruciform joint dimensions on fatigue life was discussed in detail.

Improving fatigue performance of welded joints of X65 pipeline steel by UIT

Treating weld toes properly can improve the fatigue performance. Ultrasonic impact treatment （UIT） is a more effective and convenient method to enhance the fatigue strength of welded joints and suchlike structures. Fatigue tests were conducted on the specimens made of X65 pipeline steel. The test specimens were investigated on the fatigue strength and the fatigue life at the same stress range level by comparing the ones peened by UIT with the others without the treatment： the fatigue strength of the specimens as UIT, 90% of the fatigue strength of the base mental, is increased by 38% compared with that of as welded only; the fatigue life of the ones as UIT is prolonged by 11 multiples of the ones as welded only.

Review on fatigue life prediction models of welded joint

Fatigue assessment of welded joint is still far from being completely solved now,since many influencing factors coexist and some important ones should be considered in the developed life prediction models reasonably.Thus,such influencing factors of welded joint fatigue are firstly summarized in this work;and then,the existing life prediction models are reviewed from two aspects,i.e.,uniaxial and multiaxial ones;finally,significant conclusions of existing experimental and theoretical researches and some suggestions on improving the fatigue assessment of welded joints,especially for the low-cycle fatigue with the occurrence of ratchetting,are provided.

Fatigue life evaluation of girth butt weld within welded cast steel joints based on the extrapolation notch stress method

The fatigue life evaluation of the girth butt weld within the welded cast steel joint was studied based on the extrapolation notch stress method.Firstly,the mesh sensitivity of the finite element model of the welded cast steel joint was analyzed to determine the optimal mesh size.Based on the stress field analysis of the finite element model of the welded cast steel joint at the weld toe and weld root,the sharp model of the extrapolation notch stress method was applied to derive the effective notch stress of the rounded model belonging to the effective notch stress method,in which the key problem is to calculate the extrapolation point C,and the extrapolation point C has an exponential function relationship with the geometric parameters of the welded cast steel joint.By setting different values of geometric parameters,the corresponding value of parameter C is calculated,and then the functional relationship between the extrapolation point C and the geometric parameters can be obtained by the multiple linear regression analysis.Meanwhile,the fatigue life evaluation of the girth butt weld within welded cast steel joints based on the effective notch stress was performed according to the guideline recommended by the IIW(International Institute of Welding).The results indicate that the extrapolation notch stress method can effectively simplify the process of calculating the effective notch stress and accurately evaluate the fatigue life of the girth butt weld within welded cast steel joints.

FATIGUE DAMAGE AND LIFETIME PREDICTION OF AERONAUTIC WELDED STRUCTURES UNDER HIGH TEMPERATURE

The fatigue damage evolution equations and the relation of fatigue damage parameter with maximum cyclic stress of superalloy GH150 and its welded structures are established. The fatigue damage evolution equations in a multiaxial stress state are also given. By use of cyclic thermal elastoplastic damage constitutive relations, the fatigue damage and lifetime predictions are carried out for the welded combustion chamber of aeroengine.

Improving Fatigue Properties of 316L Stainless Steel Welded Joints by Surface Spinning Strengthening

The surface spinning strengthening(3S)mechanism and fatigue life extension mechanism of 316L stainless steel welded joint were systematically elucidated by microstructural analyses and mechanical tests.Results indicate that surface gradient hardening layer of approximately 1 mm is formed in the base material through grain fragmentation and deformation twin strengthening,as well as in the welding zone composed of deformedδ-phases and nanotwins.The fatigue strength of welded joint after 3S significantly rises by 32%(from 190 to 250 MPa),which is attributed to the effective elimination of surface geometric defects,discrete refinement ofδ-Fe phases and the appropriate improvement in the surface strength,collectively mitigating strain localization and surface fatigue damage within the gradient strengthening layer.The redistributed fineδ-Fe phases benefited by strong stress transfer of 3S reduce the risk of surface weak phase cracking,causing the fatigue fracture to transition from microstructure defects to crystal defects dominated by slip,further suppressing the initiation and early propagation of fatigue cracks.

Fatigue Behavior of a Dissimilar Aluminum Alloy Welding Joint With and Without Natural Defect

In order to investigate the influence of natural defect on the fatigue behavior of 5A06/7A05 dissimilar aluminum alloys welding joint,fatigue tests of two types of specimens with and without defects were carried out systematically under stress amplitude control conditions (stress ratio R=0.1) at normal temperature in laboratory air condition.Furthermore,a new parameter,i e,fatigue defect effect factor (FDEF) was introduced to assess the effect of defect on fatigue strength.The fatigue failure analysis was conducted as well to compare the fatigue and fracture behavior of the two types of specimens.The results show that:(1) natural defects have a strong effect on the fatigue lives of welding joint,and the differences between the specimens with and without defects can reach 80 times under a same theoretical net sectional stress;(2) the FDEF parameter introduced is effective to deal with the defect effect,and the FDEF decreases along with the increase of fatigue life.The mean relative error between the experimental data and predicted fatigue strength based on the FDEF is 10.2%;(3) the macro fracture of both types of specimens have three typical zones,i e,fatigue source zone,crack propagation zone and final fracture zone,while there are more than one fatigue sources for specimens with natural defects.The overall pattern of crack propagation zone and fracture zone are quite similar,but the morphologies are different in details.

Fatigue crack initiation for Al-Zn-Mg alloy welded joint

To investigate fatigue crack initiation characteristics of A1-Zn-Mg alloy welded joint, notched specimens were used in fatigue test for the base metal, welding bead and heat affected zone （HAZ）. The fatigue fracture surface near the fatigue crack initiation site was observed by scanning electron microscope （SEM）. The results show that the differences of fatigue crack initiation life among base metal, welding bead and HAZ are not obvious. Inhomogeneity in microstructure and mechanical performance of HAZ influences the fatigue crack initiation life. The ratio of fatigue crack initiation life （Ni） to fatigue failure life （Nf） for the base metal, welding bead and HAZ of A7N01 aluminium alloy welded joint are 26.32%, 40.21% and 60.67%, respectively. Fatigue crack initiation life can be predicted using a uniform model. Observation of fatigue fracture surfaces shows that for the welding bead a fatigue crack initiates from the smooth surface due to the welding process, the blowhole in HAZ causes fatigue crack and the crushed second phase particles play an important role in fatigue crack initiation for the base metal.

Fatigue Life Analysis of Longitudinal Welding Seam for Wind Turbine Tower

Tower is an important fundamental component of large-scale wind turbines.The fatigue performance of the tower welded part directly affects the running safety and reliability of wind turbines.A fatigue life predicting method for the longitudinal welding seam of the tower is proposed in this paper.Under the precondition of satisfying the limit strength,the time series stress of each working condition is obtained by DIN18800-4 section stress calculation.Combined with the rain-flow counting method and the Miner linear cumulative damage theory,the fatigue life of the tower longitudinal weld is predicted.The results show that the tower structure meets the design requirement,and the feasibility and effectiveness of the method are verified.

Microstructure characterization and HCF fracture mode transition for modified 9Cr-1Mo dissimilarly welded joint at different elevated temperatures

The high cycle fatigue（HCF） tests of modified 9 Cr-1 Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 108 cycles based on S-N curve can be estimated as a half of weld joint＇s yield strength for all conducted temperatures, which can be a reliable criterion in predicting the fatigue life. The results show that the inter-critical heat affected zones（IC-HAZs） of both sides are the weak zones due to their low hardness and inferior fatigue resistance property. HAZ of COST-FB2（BM2） is the weakest zone at room temperature due to the existence of numerously distributed defects and the initiation of cracks, either in the surface or interior zone, impacting a crucial effect on the fatigue life of the joint. While at elevated temperatures, fatigue life was controlled mostly by the intrusion-extrusion mechanism at the specimen surface under high stress level and subsurface non-defect fatigue crack origin（SNDFCO） from the interior material under low stress amplitude. With increasing temperature, more and more fatigue failures began to occur at the HAZ of COST-E（BM1） due to its higher susceptibility of temperature. Besides, it is found that the-ferrite in the BM1 has no harm to the HCF behavior of the joint at the conducted temperatures.

Creep and creep-fatigue behavior of high chromium steel weldment

Strength of welded joints of high chromium steels is one of the important concerns for fabricators and operators of ultra supercritical thermal power plants. A number of creep as well as creep-fatigue tests with tensile hold have been carried out on the welded joints of two types of high chromium steels widely used in Japan, i.e. Grade 91 and 122 steels. It was found that failure occurred in fine grain heat-affected zone in all the creep-fatigue tests, even at a relatively low temperature and fairly short time where failure occurred in plain base metal region in simple creep testing. Four procedures were used to predict failure lives and their results were compared with the test results. A newly proposed energy-based approach gave the best estimation of failure life, without respect of the material and temperature.

基于等效裂纹法的铝合金T形焊接接头疲劳寿命评估

针对焊接缺陷的多样性、复杂性及随机分散性等特征,提出一种以一组高应力疲劳试验结果作为参考点的等效裂纹法开展焊接结构的疲劳寿命研究。以疲劳试验结果作为参考,假设一个完整板中的初始裂纹等效焊接结构中的缺陷及应力集中效应,基于线弹性断裂力学和Paris公式计算等效裂纹扩展寿命作为焊接结构的疲劳失效寿命。提出基于等效裂纹法的结构疲劳寿命计算流程;开展铝合金T形焊接接头的疲劳试验以及拟合P-S-N曲线;基于T形焊接接头的焊根失效模式采用等效裂纹方法预测T形接头的计算P-S-N曲线,并与试验拟合曲线进行对比,相对误差为10%,验证了该方法的可行性与有效性;最后,通过一组变幅载荷谱疲劳试验与预测P-S-N曲线对比,相对误差均小于15%,同时也验证了该方法对于变幅载荷的适用性,实现了基于一组疲劳试验结果预测全范围T形焊接接头疲劳寿命的评估。以较少的测试和成本实现确定的疲劳寿命,简化了概率疲劳寿命的评估过程,该方法可以推广于其他焊接接头寿命评估中。

实测数据驱动的挖掘机工作装置疲劳寿命预测

以某型挖掘机为研究对象,建立其工作装置刚柔耦合动力学模型,基于实测油缸位移数据驱动该模型,得到其主要性能参数和典型工况危险部位应力.根据强度理论、动力学仿真结果和工程经验,分析挖掘机动臂和斗杆的易开裂部位,得到典型焊缝高危点,并通过实测应力应变数据进行验证.以刚柔耦合动力学仿真所得的铰点载荷作为输入,利用nCode疲劳分析软件仿真预测挖掘机动臂和斗杆的疲劳寿命.结果表明,实测数据驱动的刚柔耦合动力学仿真可以准确获取挖掘机实际挖掘过程的动力学特性,基于该仿真模型提取铰点载荷并用于预测疲劳寿命的方法切实可行.

喷丸强度对船舶典型Q345钢十字型焊接接头残余应力分布的影响

对船舶典型Q345钢十字型焊接接头进行了不同喷丸强度(0.167,0.183,0.198 mm)下的喷丸处理,研究了喷丸后接头残余应力表面和沿深度方向的分布以及疲劳寿命,确定了最优喷丸强度。结果表明:喷丸处理消除了焊接接头表面残余拉应力,引入了较高水平残余压应力,改善了表面残余应力分布均匀性,增加了残余应力影响层深度,延长了疲劳寿命;随着喷丸强度增加,引入的残余压应力增大,分布均匀性先提升后减小,残余应力影响层深度增加,疲劳寿命先延长再缩短;最优喷丸强度为0.183 mm,此时焊接接头表面残余压应力较大,分布均匀性最好,残余应力影响层深度较大,疲劳寿命最长。

基于超声扩散场的焊接节点疲劳损伤早期检测及剩余寿命评估

钢结构焊接节点在疲劳载荷作用下易发生损伤、破坏,目前基于超声技术检测疲劳损伤的方法已得到初步验证,但早期裂纹萌生预警和剩余寿命评估仍存在困难和挑战。该研究基于超声扩散场能量密度提出一种检测焊接节点疲劳损伤的方法。该方法通过计算有损和无损状态下超声能量密度的差异,定义焊接节点疲劳损伤指数;采用对接焊缝和十字形传力角焊缝试件,验证损伤指数对焊接节点疲劳损伤的敏感性和普适性;而后,对早期裂纹萌生预警和剩余寿命评估与棘轮应变检测结果进行对比。试验结果表明:所提出的损伤指数变化趋势与棘轮应变检测结果吻合度高;焊接节点疲劳裂纹扩展时,损伤指数增长尤为快速;当裂纹萌生时,损伤指数表现出突然增大的趋势,据此定义早期裂纹萌生预警判据,实现对焊接节点疲劳全寿命60%~80%的早期剩余寿命评估;同时,根据损伤指数曲线明显的谷值点,实现焊接节点寿命30%~50%的早期剩余寿命评估。

面内双轴载荷下T形焊接接头的疲劳失效行为

船舶中的“十字”和“T形”焊接接头在服役中会受到复杂的多轴疲劳载荷作用,易在焊缝等薄弱区域失效。通过设计的T形焊接接头和试验装置开展试验,以得到T形接头在不同面内双轴载荷下的疲劳寿命、起裂位置及断口特征;利用有限元方法分析了不同面内双轴载荷条件下T形接头焊趾处的应力分布状态,研究了影响焊趾应力状态的因素;结合修正的W9hler曲线法(modified W9hler curve method,MWCM)预测T形接头的疲劳寿命。结果表明:横向载荷一定时,纵向载荷的增加导致T形接头的疲劳寿命呈指数降低;鉴于T形接头的几何特性,裂纹总是萌生于下焊趾,焊趾过渡圆弧半径对焊趾的应力状态有显著影响,增大焊趾过渡圆弧半径可提高疲劳寿命;T形接头焊趾处的应力较高时,MWCM具有较高的预测精度。

高强钢焊接接头疲劳寿命的提高方法进展

随着人们对装备性能和环保的追求高强钢被越来越多地应用,对于高强钢而言焊接作为一种重要的材料连接方法应用非常广泛,然而接头疲劳性能并不随着钢材强度升高而提高,因此提高焊接接头的疲劳寿命对于高强钢的应用具有重要意义.基于近年来国内外相关研究,总结了影响高强钢焊接接头疲劳性能的因素,包括应力集中、焊接残余应力、焊接方法和焊材选择等因素.综述了多种提高高强钢焊接接头疲劳寿命的方法,根据其对接头疲劳性能改善机理进行分类,第1类方法主要基于对焊缝几何形貌的改进,通过降低应力集中系数减缓疲劳裂纹的萌生提高寿命.如TIG熔修、激光熔修、喷涂沉积和仿形加工和打磨等方法.其中一些方法在改进焊缝几何形貌的同时还引入对疲劳寿命有益的残余压应力.第2类寿命提高方法主要基于对焊缝残余应力进行调整,通过减小残余拉应力或引入残余压应力减缓裂纹扩展速度.如焊后热处理(postweld heat treatment,PWHT)、高频机械冲击(high frequency mechanical impact,HFMI)、激光冲击强化(laser shock processing,LSP)、滚压和搅拌摩擦等方法,其中一些方法同时还会改善接头表面硬度和晶粒尺寸从而影响疲劳裂纹萌生.并对这些方法的使用提出建议和展望.

超声冲击对SMA490BW耐候钢焊接接头腐蚀疲劳的影响

采用USP-300超声表面加工装置对SMA490BW耐候钢表面进行超声冲击处理。研究超声冲击前后试件的显微组织、显微硬度和残余应力,比较超声冲击前后SMA490BW耐候钢焊接接头的腐蚀速率和腐蚀疲劳寿命。通过光学显微镜观察超声冲击后SMA490BW耐候钢的显微组织,采用测试仪器测试焊接接头的表面硬度和残余应力,分析超声冲击提高SMA490BW耐候钢抗腐蚀性及其疲劳寿命的机理。结果表明,超声冲击处理能使材料表层晶粒细化形成致密的塑性变形层,提高显微硬度,并在材料表面引入有效的残余压应力,降低腐蚀速率,减小失重量。同时,超声冲击处理后裂纹扩展路径变得更加复杂,裂纹扩展速率降低,焊接接头腐蚀疲劳寿命得到提高。

TC4焊接接头多元竞争失效分析及寿命预测

针对TC4焊接接头多裂纹源竞争失效机制,提出一种超高周疲劳寿命预测方法。在应力比为-1以及应力幅值300 MPa下对接头开展超高周疲劳试验,以探究其超高周疲劳性能。通过扫描电镜对TC4焊接接头的疲劳断口观察可知,断口内部总计有三处裂纹起裂点,并在载荷持续作用下均得到了不同程度的扩展。进一步结合应力强度因子计算以及断口形貌,绘制断口内部的多元竞争机理图。分析表明,三处缺陷诱发的疲劳裂纹均在不同程度上降低了焊接接头的疲劳寿命,其中1号气孔诱发的裂纹在TC4焊接接头的失效过程中占据主导位置。基于竞争失效分析,构建了一种适用于多裂纹源疲劳失效的寿命预测方法,模型预测结果较好。