轴向荷载作用下K型管节点应力集中系数研究

热点应力法是目前国内外学者研究焊接管节点疲劳寿命的主要方法,而应力集中系数是计算热点应力的主要参数。该文提出了焊缝模拟方法,建立了空心K节点及主管填充混凝土K节点的数值模型,研究了轴向平衡荷载作用下空心及填心K型管节点应力集中系数分布的规律,并与试验结果进行了比较,证明该文提出的数值模拟方法简单可靠,精确性好,可为管节点有限元分析提供借鉴;主管填充混凝土管节点的应力集中系数最大值减小为空心管节点的1/3,说明管内填充混凝土能有效提高管节点的疲劳寿命。     

轴向载荷下X型管节点应力集中系数研究

用有限元方法分析了X管节点承受轴向拉力作用时沿焊缝应力集中系数的分布,发现X型管节点最大应力集中系数出现在鞍点处。同时探讨了X节点几何参数对应力集中系数的影响,提出了用于计算X节点最大应力集中系数的参数公式,通过与国际疲劳指导委员会(Fatigue Guidance Review Panel)提出的评价标准比较,证明了所提公式的正确性。     

圆管-方管混凝土T型节点疲劳性能试验研究

基于热点应力法的管节点疲劳性能研究思路中,热点应力集中系数和疲劳寿命是描述焊接管结构节点疲劳性能的两个主要方面。对8个支管为圆管主管为内填充C50等级混凝土的方管节点试件（CT1～CT8）进行热点应力试验以确定其热点应力集中系数（SCF）,在此基础上进行疲劳试验以确定其疲劳寿命（N3）,并将试验结果与具有相同几何尺寸和受力条件的圆管-方管节点试验结果以及相关规范进行比较分析。研究表明,支管受轴向荷载下,圆管-方管混凝土节点较对应空钢管节点而言,节点区刚度分布有明显改善,前者SCF最大值均小于后者SCF最大值,具有更好的疲劳性能;已有规范中关于空钢管节点疲劳性能的S-N曲线不适用于进行圆管-方管混凝土T型节点的疲劳性能分析。     

基于拉弯比的焊缝名义应力的提取

为了获得任意拉弯组合载荷下焊缝的名义应力,利用纯弯和纯拉压载荷下的应力集中系数,引入拉弯组合名义应力换算系数,将基于纯拉压名义应力的焊缝疲劳性能数据,转换为疲劳损伤一致的、基于拉弯组合名义应力的焊缝疲劳性能数据．为了消除有限元建模导致的计算误差,引入单元尺寸影响因子,将拉弯组合的计算应力转换为拉弯组合的名义应力．通过上面2个转换,引入拉弯组合计算名义应力换算系数,将有限元中的拉弯组合计算应力转换为基于拉压载荷疲劳试验的名义应力,从而在具体的焊缝结构疲劳强度评估时可以直接使用拉压载荷下的疲劳试验数据．计算结果表明：拉弯组合计算名义应力换算系数与拉弯比、拉弯应力集中因子比和有限元模型中拉弯单元尺寸影响因子有关．通过选择合适的单元尺寸,使得拉压单元尺寸影响因子等于拉弯应力集中因子比,且弯曲单元尺寸影响因子等于1,可使得拉弯组合计算名义应力换算系数恒等于拉压单元尺寸影响因子,而与拉弯比无关．     

钢箱梁正交异性桥面板疲劳评估全空间S-N曲线研究

全空间S-N曲线是利用时序准则进行钢箱梁正交异性桥面板疲劳评估的基础。首先基于钢桥设计规范给出的有限疲劳寿命区S-N曲线,从理论上推导了覆盖低周疲劳寿命区、有限疲劳寿命区和高周疲劳寿命区的一般全空间S-N曲线模型。进一步针对小型汽车和日常风等荷载引起的低水平应力幅以及地震和强风等荷载引起的高水平应力幅,引入高周疲劳损伤累积因子和低周疲劳强度调整因子分别对低水平应力幅和高水平应力幅造成的疲劳损伤进行计算,提出一种适用于钢箱梁正交异性桥面板疲劳评估的全空间S-N曲线模型。在此基础上对模型参数的几何意义进行了解释。最后采用正交异性桥面板顶板-U肋疲劳试验数据对模型参数进行了估计,得到了具有最佳拟合度的全空间S-N曲线和具有99%保证率的疲劳评估全空间S-N曲线。研究结果表明,该文提出的全空间S-N曲线模型能准确描述钢箱梁正交异性桥面板在各种应力幅下的疲劳损伤累积特性。研究结果可为钢箱梁正交异性桥面板的疲劳评估奠定基础。     

悬臂弯曲加载10CrNiMo钢的S-N曲线试验研究

采用有限元方法,计算了在悬臂弯曲加载方式下,不同载荷作用时板状光滑试样施力点处的挠度;采用成组试验法,通过载荷控制模式,进行了应力比为-1时10CrNiMo钢的S-N曲线试验,并对试样断口进行了观察和分析。结果表明:在试验载荷范围内,所设计试样施力点处的挠度变化,在试验机作动器的有效行程内,可满足S-N曲线试验,通过试验获得了两种常用置信度下10CrNiMo钢的S-N曲线;在大载荷往复作用时,表面裂纹从试样上、下两个表面萌生并扩展所形成的面积基本相当,但在小载荷循环作用下,表面裂纹通常先从试样的一个表面萌生并扩展,当裂纹扩展至一定程度时,试样另一个表面才开始萌生疲劳裂纹,并协同前一表面的裂纹共同扩展至试样断裂。产生这种现象的原因,和不同大小的载荷开动试样上下两个表面材料内部滑移系的数量、材料内部组织的不均匀性,以及裂纹在扩展过程中前缘应力状态的变化有关。     

基于三参数威布尔分布的自动调整臂疲劳寿命的P-S-N曲线研究

通过疲劳试验研究了自动调整臂矩形压缩弹簧的疲劳寿命、疲劳变形以及破坏特征等。试验结果表明:疲劳应力载荷作用下,矩形压缩弹簧中间应力集中部位会产生裂纹,随着应力水平提高,裂纹变大,使得其传递转矩能力减弱;利用最小二乘法及最大相关系数法拟合自动调整臂疲劳寿命,可知其疲劳寿命服从三参数威布尔分布;根据三参数幂函数形式的S-N曲线方程,建立自动调整臂在不同应力下的P-S-N曲线,通过自动调整臂试验与理论疲劳极限寿命的比较表明,两者相对误差较小。     

深水港码头轻型导管架结构在船舶撞击下的疲劳分析

结合大连新港新建30万吨级(兼靠45万吨)进口原油码头工程,研究离岸深水港码头轻型导管架结构在系泊船舶撞击下的疲劳损伤。采用通用有限元程序ANSYS计算船舶撞击作用下导管架平台的动力反应,得到结构构件的名义应力,乘以相应的应力集中系数,得到管节点热点应力的时程曲线。分别应用美国石油学会(API)规范、挪威船级社(DNV)规范和中国船级社(ZC)规范中相应的S-N曲线对船舶撞击作用下的疲劳损伤进行了计算。计算结果表明,对应用于离岸深水港码头的导管架结构,由船舶撞击平台所引起的疲劳损伤较大,不能忽略,若只考虑波浪荷载引起的疲劳损伤会导致过高地估计结构的疲劳寿命。结果还表明,按API规范与ZC规范计算得到的疲劳损伤基本一致,按DNV规范计算得到的疲劳损伤较其他两种规范偏大。     

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。     

储气库注采管柱疲劳可靠性多级加速载荷试验研究

针对储气库注采管柱在高速气流作用下易于发生疲劳失效的问题,开展注采管柱N80材料疲劳可靠性试验研究。在MTS810试验机上完成针对N80材料储气库注采管柱的拉-拉疲劳实验。通过多级加载疲劳试验法,获得N80储气库注采管柱疲劳分组加速试验数据,采用概率分析模型得到N80材料的S-N曲线。提出基于古德曼曲线预估材料疲劳寿命极限的方法,采用该方法可以比较可靠地预估材料不同加载条件下对应的最大应力水平、应力幅值及应力比等参数,为加速疲劳试验、提高试验效率提供一种参考方法。给出基于概率统计方法进行多级加载疲劳可靠性分析模型,通过多级加载疲劳试验获得储气库注采管柱N80油管试件的疲劳寿命样本数据,并采用可靠性模型给出试验材料的概率分布规律以及相应的P-S-N曲线。     

Stress concentration factor analysis for welded,notched tubular T-joints under combined axial,bending and dynamic loading

The finite element analysis will be used in this study to predict the location of hot-spot stresses in a welded tubular T-joint. The fillet weld has been modeled all around the joint. Using symmetry, the tubular T-joint is submitted to axial, in-plane bending (IPB) and out-of-plane bending (OPB) loadings. The finite element method analysis shows that stresses are very high on the brace member in the vicinity of the fillet weld and gradually decrease, with a quasi-stable difference, in the direction of the brace extremity. Both on the brace member and along the fillet weld (from crown to saddle), stresses are high at the crown toe, decrease in the middle and increase once again at the saddle point. From a general perspective, this stress distribution analysis reveals that hot-spot stresses (HSS) are located at the crown and saddle points. Dynamic loading greatly increases the stress concentration factor at the hot-spot stress (HSS) located on the brace member where fatigue damage is capable of appearing quickly. In the U-notch, this stress concentration factor (SCF) increases as notch width decreases. In a general way therefore, stress concentration factors decrease on the brace and chord members (in the vicinity of the weld) and increase considerably in the notch, which underscores the deleterious nature of such a defect. Consequently, these zones (HSS) require reinforcement solutions in order to ensure a sufficiently long fatigue life for offshore structures.     

Numerical modelling and experimental investigation on welding residual stresses in large‐scale tubular K‐joints

This paper is devoted to the experimental and numerical assessment of residual stresses created by welding in the region surrounding the weld toe of tubular K‐shaped joints (i.e. region most sensitive to fatigue cracking). Neutron‐diffraction measurements were carried out on K‐joints cut from large‐scale truss beams previously subjected to high cycle fatigue. Tri‐axial residual stresses in the transverse, longitudinal and radial direction were obtained from the weld toe as a function of the depth in the thickness of the tube wall. In addition, thermomechanical analyses were performed in three‐dimensional using ABAQUS and MORFEO finite element codes. Experimental and numerical results show that, at and near the weld‐toe surface, the highest residual stresses are critically oriented perpendicularly to the weld direction, and combined with the highest externally applied stresses. Based on a systematic study on geometric parameters, analytical residual stress distribution equations with depth are proposed.     

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.     

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 strength assessment of tubular welded joints by an alternative structural stress approach

The tubular joints, frequently employed in the offshore industry, are submitted to stresses resulting from elementary loadings: tension/compression, in-plane bending and out-of-plane bending. This work concerns the analysis of the recommendations commonly used for the fatigue design of welded joints submitted to combined loadings. Particularly, it deals with the fatigue behaviour of T-joints submitted to deviated-bending: first, a finite element analysis was developed and a post-processing based on the structural stress approach, as proposed by the International Institute of Welding (IIW). Then, fatigue tests were conducted on T-joints submitted to deviated-bending. Comparisons between experimental and numerical results showed that this kind of recommendations is not systematically conservative. Thus, an alternative approach based on structural stresses and taking into account the multi-axial stress state in the weld toe was developed in order to complete the recommendations for the fatigue design of tubular welded joints.     

基于失效评定图(FAD)研究含疲劳裂纹T型圆钢管节点的安全性

失效评定曲线(FAD)常用来评价焊接结构在出现裂纹后的安全性,为了验证这种曲线在评价焊接管结构在节点部位出现疲劳裂纹后安全性的适用性,采用实验测试和有限元分析的方法研究了3个含疲劳裂纹的T型管节点试件在静力作用下的极限承载能力及破坏过程。3个T型管节点试件首先进行疲劳实验在焊趾处产生表面裂纹,然后通过在支管端部施加轴向拉力作用检测节点的破坏过程。基于自行开发的含表面裂纹T型管节点的有限元网格自动产生程序以及ABAQUS分析软件,研究了在管节点破坏过程中表面裂纹最深点的应力强度因子大小,并通过实验的荷载-位移曲线确定了T节点试件的塑性极限承载力。在这些结果的基础上,验证了FAD在评价含疲劳裂纹的焊接管节点安全性方面的适用性。研究结果标明:FAD在评价含疲劳裂纹管节点的安全性方面是安全可靠的,但偏于保守。     

Fatigue crack growth analyses of offshore structural tubular joints

This study investigated various aspects of a fatigue crack growth analysis, ranging from the stress intensity factor solutions to the simulation of a fatigue crack coalescence process of a tubular joint weld toe surface flaw. Fracture mechanics fatigue crack growth analyses for offshore structural tubular joints are not simple, because of the difficulty to calculate the stress intensity factors due to their geometric complexity. The fully mixed-mode stress intensity factors of nine weld toe surface cracks of an X-shaped tubular joint under tension loading were calculated by detailed three-dimensional finite element analyses. Using these stress intensity factor solutions, a fatigue crack growth study was performed for the X-joint until (the crack surface length grew to two times the tube thickness. Through this study, the crack shape change during the fatigue crack propagation was investigated in detail. Fatigue life calculations were also performed for a range of crack geometries using the stress intensity factor solutions of the nine flaws. These calculations indicate that the natural fatigue crack growing path for a crack is its quickest growing path. The study demonstrated that detailed fracture mechanics fatigue analyses of tubular joints can be practical using the finite element method.     

承受平衡轴力的空间KK管节点应力集中系数研究

采用有限元法分析了空间KK型管节点在承受平衡轴力作用下的应力集中系数。在有限元分析中,首先采用分区网格法产生KK节点的有限元网格,这种方法可以针对不同应力梯度的区域形成不同质量和精度的网格。然后在分区网格法的基础上,采用ABAQUS(2000)通用软件分析计算了KK型管节点在平衡轴力作用下的应力集中系数的大小和分布。最后,通过对30个KK节点模型的有限元分析,研究了KK节点几何参数对其应力集中系数大小以及分布情况的影响。     

Rib loading effects on weld root fatigue failure modes at rib‐to‐deck welded joint

The service life of orthotropic steel decks is dependent on the fatigue resistance of rib‐to‐deck welded joints, which is often tested using two kinds of experimental models in terms of the rib loading condition. Different weld root fatigue failure modes have been observed in the different models, but the role of rib loading remains unclear. This paper aims to clarify the effect of rib loadings on the weld root fatigue failure modes at rib‐to‐deck welded joints. The loadings are decomposed into the deck loadings and rib loadings according to the principle of superposition. Formulae of the weld root notch stress intensity factors and T‐stress under rib loadings are developed by multiparameter regression analysis and subsequently used for the local stress analysis. The fatigue failure modes determined from the local stress field agree well with the experimental results. The results reveal that the weld root failure modes depend on the rib loadings but are independent of the weld geometries. The averaged strain energy density (SED) that can capture both weld geometry and loading condition effects is used to correlate the fatigue test data of different weld root failure modes. The SED is capable of evaluating the fatigue strength of the rib‐to‐deck welded joint failed by different weld root failure modes with a narrow scatter band.