基于细观损伤力学的梁柱焊接节点断裂破坏预测分析

应用细观损伤力学模型预测分析单调荷载作用下梁柱焊接节点的裂纹萌生与扩展。分别依据节点部位的母材、热影响区和焊缝金属缺口试样的单轴拉伸试验，标定材料细观损伤力学模型的参数；通过有限元分析模拟，预测焊接节点的开裂位置以及断裂破坏的过程，与已有试验结果以及基于微孔扩展模型和应力修正临界应变模型的有限元分析结果相比，建议的细观损伤力学模型的有限元分析结果具有更好的预测精度；针对不同焊接孔几何形状对节点断裂性能的影响进行有限元分析，结果表明，孔的几何形状对节点的断裂破坏模式的影响显著，长圆孔比圆孔具有更好的节点延性，开裂延缓，但是两种孔形的节点承载力差别很小。     

随机变幅荷载作用下焊接节点海水腐蚀疲劳裂纹扩展试验

本工作采用国产海上平台用钢的T型和十字形板状焊接节点试件,模拟导管架臂节点的弯曲受力状态,在随机变幅荷载下进行了海水腐蚀疲劳裂纹扩展试验,分别测得在海水自由腐蚀和阴极保护条件下的(~—da/dN)-AK~(NM)关系曲线。对于十字型焊接节点试件,根据试验测得的裂纹扩展速率曲线,用断裂力学方法对其海水腐蚀疲劳裂纹扩展寿命(有阴极保护)进行了估算,估算得到的裂纹扩展寿命曲线与试验数据,以及用线性累积损伤理论估算的寿命曲线,进行了分析比较和讨论。     

不同断裂模型在钢结构断裂破坏预测中的比较

为对比分析不同断裂模型在钢结构断裂破坏预测中的适用性,选取3类典型的断裂模型（微孔扩展模型、GTN模型、连续损伤模型）对钢结构的断裂破坏进行预测分析。首先基于试验数据标定了Q345钢材以及焊缝金属的断裂模型参数,然后将这3类断裂模型通过ABAQUS的用户子程序嵌入到有限元分析程序中,最后将这3类模型应用到开孔板以及梁柱焊接节点的断裂破坏分析中。结果表明:3类模型均能很好地预测开孔板和节点裂纹的起始和发展过程;3类模型对于开孔板断裂位移的预测精度相当,但对于相对复杂的梁柱焊接节点,GTN模型对裂纹起始位移的预测精度明显高于其他2类模型。     

Fracture prediction of welded steel connections using traditional fracture mechanics and calibrated micromechanics based models

Fracture resistance is of primary concern in the seismic design of beam-to-column connections in steel moment resisting frames (SMRFs). Micromechanics based fracture models such as the void growth model (VGM) and the stress modified critical strain (SMCS) model provide alternative approaches for ductile fracture prediction by relating micro-mechanisms of void nucleation, growth and coalescence to macroscopic stresses and strains. In this study, the VGM and SMCS models were calibrated for Q345 structural steel and the corresponding weld, through smooth notched tensile (SNT) tests and complementary continuum finite element models (FEMs). A series of seven local connections representing beam-to-column connections in SMRFs were tested under monotonic tensile loading and the specimen elongations at fracture critical point were obtained. The traditional J-integral based fracture mechanics and micromechanics based fracture models (VGM and SMCS) were applied to predict fracture in each tested local connection through refined three-dimensional FEM. Comparisons between these numerical approaches and experimental observations in prediction of fracture critical displacement, indicated that the VGM and SMCS models were able to predict fracture of welded connection with good accuracy, while the J-integral based approach resulted in quite conservative fracture prediction. This paper has bridged the gap between small-scale material tests and large-scale structural experiments in fracture evaluations.     

钢结构节点断裂的研究现状

对钢框架梁柱节点及钢管节点断裂的研究现状进行了综述,指出现有研究多是基于传统断裂力学方法采用应力强度因子、裂纹尖端张开位移（CTOD）或J积分等预测断裂,这种方法主要适用于研究脆性断裂或局部塑性损伤程度极其有限的伪脆性断裂问题,而对强震作用下构造无明显缺陷部位发生较大尺度屈服时的延性断裂问题并不适用;介绍了近年来发展的基于微观机制的断裂预测方法,可以考虑节点区显著的应力三轴度因素,且适用于无初始裂纹或处于明显塑性阶段的节点延性断裂预测问题,可用于预测强震作用下钢结构焊接节点的超低周疲劳断裂。     

旧水泥混凝土路面沥青加铺层疲劳断裂分析

在道路的使用过程中,沥青加铺层一直经历着交通荷载及温度变化的循环作用,沥青加铺层结构中的裂缝扩展呈现为疲劳断裂特征.沥青加铺层的使用寿命（从开始承载直至破坏所经历的车辆荷载及温度循环次数或作用时间）可分为疲劳裂缝形成寿命和疲劳裂缝扩展寿命两部分.疲劳裂缝形成寿命为由微观缺陷发展到宏观可检裂缝所对应的寿命,目前仍由传统疲劳理论的方法确定;而疲劳裂缝扩展寿命则为由宏观可检裂缝扩展到临界裂缝而发生破坏这段区间的寿命,用疲劳断裂力学方法确定.论文着重对沥青混合料与沥青路面的疲劳断裂进行分析.     

Q690D钢材十字形焊接接头高温后的断裂破坏试验研究

为研究Q690D高强度钢材及焊缝连接件在常温和高温后的断裂性能,选取代表实际梁柱节点局部焊接构造的十字形焊接节点试样,完成了常温和一系列高温后Q690D钢材和ER80-G焊缝金属的单轴拉伸试验,得到了钢材和焊缝金属在不同高温后的弹性模量、屈服强度、极限强度和延伸率。开展了常温和高温后十字形焊接接头的单调拉伸试验和超低周循环试验,研究了Q690D高强度焊接接头的断裂机理,探讨了过火温度、加载制度对焊接接头断裂性能的影响。结果表明:当钢材和焊接接头的过火温度高于600 ℃时,钢材和十字形焊接接头的强度降低,其变形能力开始增大; 800 ℃高温后Q690D钢材的强度降低,但ER80-G焊缝金属的力学性能无明显变化,导致焊接接头经受800 ℃高温后,在单调荷载作用下,其断裂破坏未发生在焊缝处,而发生在母材位置; 循环荷载作用下焊接接头的承载能力和变形能力都低于单调荷载的情况; 试验得到钢材、焊缝金属和焊接接头的力学性能指标,为发展考虑火灾后效应的断裂分析模型提供了基础试验数据。     

Fatigue Behavior Investigation and Stress Analysis for Rib-to-Deck Welded Joints in Orthotropic Steel Decks

The fatigue problems in orthotropic steel decks have raised widely concerns in recent years. This study focused on the root crack mechanism at rib-to-deck welded joints, based on the previous test results of sectional specimens and the matching FE analysis, the fatigue behaviors of structure detail were investigated by considering the effect of root gap shapes, weld penetrations, and plate thicknesses on crack initiation. Besides, various root crack depths were simulated in models to clarify the stress variations occurring during the propagation stage under cyclic loading. The results showed that the root gap shape and penetration rate have an impact on the root cracking direction and fatigue life at the initiation stage, but seem not directly related to the crack propagation mechanism; the higher penetration rate is advantageous for the prevention of root crack initiation. However, although the stiffness increased with the increase in plate thickness, the fatigue life of crack initiation might be reduced owing to the low fatigue strength of the thick deck plate, whereas the U-rib thickness has limited effect on the stress response of the root tip. Moreover, the significant difference between the 8 mm-crack model and other crack models is the high stress concentration around the crack tip. The stress conditions of root tip would be changed under loading cycles when a root crack propagated into half of deck plate thickness. Finally, the effect of structural dimensions on fatigue strength were also compared according to test results and FEA.     

Effects of weld profile and undercut on fatigue crack propagation life of thin-walled cruciform joint

Fatigue may occur in undercarriages and support systems of trailers, haymakers, graders and swing-ploughs made up of thin-walled tubular sections with wall thicknesses less than 4 mm. Little research has been done on the fatigue of thin-walled tubular sections below 4 mm thickness. The weld profile and weld undercut may affect the fatigue crack propagation life of welded joints especially for thin-walled sections. Numerical analysis of 2-dimensional non-load carrying thin-walled cruciform joints was performed to determine the effect of weld profile and weld undercut on fatigue crack propagation life under cyclic tensile loading. The cruciform joints analysed are made up of 3 mm thick plates, joined by fillet welds. The Boundary Element Analysis System Software (BEASY) is used, which uses fracture mechanics theory to carry out crack propagation analysis. The weld profiles and weld undercuts were measured from welded connections in thin-walled fillet welded sections. The measured weld profiles and undercut were used in the analysis. The results are compared with those of a similar study to determine the comparative reduction in fatigue crack propagation life between thin-walled cruciform joints (T=3 mm) and thicker walled cruciform joints (T=20 mm). This paper provides an understanding of the impact which weld toe undercut has on the fatigue crack propagation life of thin-walled cruciform joints. The presence of undercuts may reduce the benefits of the thickness effect in thin-walled joints of thicknesses less than 4 mm.     

钢框架梁柱节点焊缝损伤性能研究Ⅱ：理论分析和有限元模拟

采用断裂力学理论,依据9个钢框架全焊接梁柱节点局部试件的单向拉伸试验结果,对节点试件的焊缝损伤进行了理论分析和有限元模拟。理论计算表明,简化计算式能够简洁地反映节点开裂情况,但对约束条件、裂纹尺寸的简化会影响计算精度。有限元分析结果表明,焊接孔附近的应力集中和热影响区材料的退化导致节点焊缝断裂易发生在梁腹板焊缝位置和梁翼缘焊缝热影响区,两处开裂概率接近,与试验结果吻合;试件开裂位置由初始缺陷的尺寸、位置和材料断裂韧性共同决定;弹塑性有限元分析能够较准确分析试件开裂位置和启裂荷载,精度较高,为研究梁柱节点焊缝损伤性能提供了一种损伤判断方法和准则。     

Prediction of brittle fracture initiating at ends of CJP groove welded joints with defects: study into applicability of failure assessment diagram approach

A series of tests, which reproduced brittle fracture starting from the region around the terminations of complete joint penetration groove welded joints, was conducted. The joint models under study represented a portion of welded beam flange-to-through diaphragm joints in beam-to-column connections. Various artificial weld defects were installed in the welded joints prior to testing. Test results were evaluated based on the failure assessment diagram approach specified in BS 7910. The conventional failure assessment diagrams predicted occurrences of brittle fracture erring on the very conservative side as expected, especially when applied to surface defects. The failure assessment diagram approach was modified by considering the effect of enhanced apparent toughness of material due to the loss of crack tip constraint. The modified failure assessment approach was found to predict the test results well.     

Mode III fracture of a large-scale welded extruded aluminum panel

As part of the research on deformation and fracture of the large-scale welded thin-walled AA 6061 panels, this paper presents the experimental and numerical investigation on its fracture under Mode III loading. The proposed local fracture approach is applied by incorporating the calibrated damage input data into the finite element model. The investigations demonstrate that the present finite element model can successfully predict the global force–deflection response, the deformation modes, as well as the crack formation and propagation, in this quite sophisticated problem involving welded joints.     

Failure criteria and cumulative damage models for steel components under cyclic loading

In this paper the behaviour of welded beam-to-column connections under low-cycle fatigue is investigated in terms of failure criteria. A damage index and a failure criterion for steel components under cyclic loading, based on the capacity of the structural details to dissipate energy, are presented and discussed.After a review of some cyclic test results obtained for steel beam-to-column connections, a comparison is presented between the cumulative damage model based on the damage index and failure criterion previously introduced, and other models and failure criteria available in the literature.     

基于OpenSEES数值模拟的工字形截面支撑低周疲劳性能研究

为研究工字形截面支撑低周疲劳性能,进行了18根Q235和21根ST12材质焊接工字形钢支撑在等幅轴向循环位移作用下的OpenSEES有限元分析,在验证模拟结果和试验结果吻合较好的同时,提出利用支撑几何参数(长细比、宽厚比、高厚比、屈服点)修正的表征应变作为预测支撑低周疲劳寿命的控制参量,给出了对应修正公式和寿命预测公式。表征应变与支撑低周疲劳寿命满足对数线性关系,据此预测的支撑低周疲劳寿命离散性较小且基本位于试验值1.5倍分散带内。对于变幅加载条件下的支撑寿命预测,利用Miner线性损伤理论对39组支撑试验数据进行以裂纹萌生寿命为终止点的损伤值统计,给出了容许损伤建议值作为预测寿命对应损伤值。而后,引入5组单斜工字钢支撑框架低周疲劳试验结果进行验证,其预测寿命均接近试验裂纹萌生寿命,可以较为保守地评估该类支撑的低周疲劳寿命,且支撑截面损伤分布与试验破坏模式保持一致。以上均证明该数值模拟方法和低周疲劳寿命预测方法的准确性和高效性,可为框架支撑结构低周疲劳性能的研究提供借鉴。     

Q460D高强钢及其焊缝连接疲劳性能试验研究

在疲劳荷载作用下钢结构焊缝区易发生疲劳断裂,通过Q460D高强钢及其焊缝连接的疲劳性能试验研究,结合试验数据,拟合了Smax-N曲线预测其疲劳寿命,根据疲劳损伤理论分析了疲劳破坏程度,并通过断口形貌揭示了疲劳裂纹扩展规律。研究结果表明：Q460D母材具有较高的疲劳抗力;对接焊缝接头Smax-N的95%保证率曲线与ANSI/AISC 360-10的疲劳设计曲线吻合较好;GB 50017—2003的设计曲线能较好预估循环次数大于40万次以上十字接头的疲劳寿命。损伤指标能够较好地表征疲劳破坏过程中构件内部状态的变化,缺口系数越大,损伤发展越快。瞬断前裂纹扩展规律与损伤发展一致,随着损伤发展疲劳条带间距逐渐变大。     

钢框架梁柱节点焊缝损伤性能研究Ⅰ：试验研究

选取20个钢框架全焊接梁柱节点局部试件进行单向拉伸和拉压循环试验,记录了试件的破坏模态和应力 应变曲线,分析了材料强度、加载方式、板件厚度、宽度等参数对节点局部焊缝性能的影响,研究了焊缝开裂对节点损伤的影响。试验表明,循环加载会显著削弱焊接试件承载能力和变形能力,导致焊缝的开裂和破坏;焊缝在循环荷载作用下,受压的塑性发展会削弱焊缝延性;焊缝开裂和板件屈曲是节点损伤的两大主要因素,焊缝中裂纹发展导致节点承载力下降和刚度损伤;板件屈服后的局部屈曲对节点的损伤有待进一步研究。     

Fatigue life evaluation of welded joints considering the local multiaxial stress/strain states and numerically determined residual stresses/strains

This paper puts forward a kind of calculation method of estimating fatigue crack initiation life by considering welding residual stresses as initial stresses. The numerical method could be used to quantitatively analyze the influence of residual stresses on the cumulative fatigue damage. In order to gain the distribution of weld-induced residual stresses, the FE analyses as well as measurements were carried out. Based on critical plane approach, analysis of damage parameters was performed considering both welding residual stress and biaxial loads. Subsequently, validity of the proposed numerical method is verified by analyzing fatigue test results for several typical welded joints. It is concluded that the result of the present method is closer to the experimental value than that obtained with empirical formula method in the estimation of fatigue crack initiation life of welded structures.     

焊接工字形截面钢支撑低周疲劳性能试验研究

通过21根铰支焊接工字形截面支撑在6倍试件的受拉屈服位移幅值下的对称循环轴向加载试验,研究了具有不同几何特性的该类支撑构件的低周疲劳和耗能性能。研究发现,支撑翼缘直到断裂的疲劳损伤发展过程可分为可见裂纹萌生,穿透裂纹形成和穿透裂纹扩展3个阶段,并根据试验数据提出了估算支撑构件疲劳寿命和循环耗能能力的经验公式。统计分析表明,裂纹萌生疲劳寿命与弹塑性局部屈曲密切相关,当支撑翼缘的宽厚比越小,支撑的长细比越大时,其裂纹萌生疲劳寿命就越长。试验还表明,低周疲劳性能较优的支撑构件亦具有较高的耗能能力。为了防止框架-中心支撑结构中的支撑构件在强震下过早发生断裂破坏,在实际设计中除要限制板件的宽厚比之外,还应适当放宽现行规范对支撑构件长细比的限制。     

使用基于微观力学的断裂模型预测钢梁-柱节点的超低周疲劳

抗弯钢框架的梁-柱节点在地震中可能会遭受超低周疲劳(ELCF)。使用循环孔洞增长模型(CVGM)——1个基于微观力学的断裂模型——预测梁-柱连接的ELCF断裂。9个足尺连接试件的试验结果验证了该模型的有效性。此外,采用精细有限元模型模拟连接件的循环性能,基于应力和应变的时间历程计算CVGM裂缝指数。CVGM预测的出现ELCF断裂前的循环次数和累积变形与试验结果相符。该方法能够准确预测非弹性循环荷载作用下梁-柱节点的ELCF断裂。     

Development of fatigue test method and improvement of fatigue life by new functional steel plates for welding of trough rib and deck plate of orthotropic decks

Under heavy traffic conditions, some fatigue cracks have been found at welding joints between U-shaped ribs “trough ribs” and deck plates. Fatigue cracks along thickness direction of deck plate from root tip are important because visual crack detection from outside is limited. In this paper, new functional steel plates with initiation resistance of fatigue crack at weldment and propagation resistance at base material are applied to deck plate to evaluate improvement of fatigue life. We conducted fixed-point fatigue tests with real scale partial models consisted with one trough rib and a transverse girder, and the elastic FEM (Finite Element Method) analysis corresponding to the fatigue test specimen. Based on these results, we studied the fatigue properties which focused on the difference between conventional steel plates and newly developed steel plates, FCA (Fatigue Crack Arrester). Fatigue cracks were initiated from root tip of welded joints between a trough rib and a deck plate. It is clarified that FCA steel plates instead of conventional steel plates can improve the fatigue life under the same welding conditions. This improvement effect becomes much clearer in the high cycle region, which is noticeable at the operation and management stage of bridges.