圆钢管混凝土T型焊接节点应力强度因子计算方法研究

主管内填充混凝土的圆管桁架是一种新型结构,鉴于断裂力学评估这种结构疲劳性能的需要,该文探讨了圆钢管混凝土T型焊接节点在支管轴拉力作用下的应力强度因子计算方法.首先,根据节点的热点应力分布和疲劳裂纹试验结果,提出了节点断裂力学分析的半椭圆表面裂纹模型.其次,推导了主管、支管及混凝土等部分的参数变换公式,并通过ANSYS软...     

K型钢管混凝土节点疲劳性能试验研究

以湘潭四大桥桁架拱为工程背景,进行了三种规格的K型钢管混凝土节点的疲劳试验,基于热点应力计算了K型钢管混凝土节点的疲劳荷载幅值,表明热点应力法能够较好地评估钢管混凝土节点的疲劳寿命;试验表明K型钢管混凝土节点的疲劳性能明显优于K型钢管节点。在此基础上初步提出了K型钢管混凝土节点的S—N曲线,对建立K型钢管混凝土节点疲劳寿命设计准则具有一定的参考意义。     

随机轮载下钢桥面顶板与纵肋焊缝疲劳裂纹扩展特性研究

车辆轮迹横向分布是钢桥面板焊缝处产生随机应力谱的一项关键因素,加之焊接缺陷形态的随机性,诱发钢桥面板疲劳裂纹随机扩展行为。为了研究钢桥面顶板焊缝处疲劳裂纹的随机扩展特性,基于断裂力学理论与扩展有限元方法分析了轮迹横向分布对钢桥面板顶板焊根和焊趾等效应力强度因子的影响规律,揭示了轮迹横向分布离散度、初始裂纹深度和初始裂纹形态比对焊缝处疲劳裂纹随机扩展路径分布的影响规律。结果表明：轮载中心处于U肋正上方中心和焊缝中心位置分别为顶板焊趾和焊根的最不利横向加载工况;车辆轮迹横向分布对顶板焊根和顶板焊趾疲劳裂纹前缘应力强度因子影响差异显著,顶板焊根最大等效应力强度因子为85.99 MPa·mm^1/2,比顶板焊趾增加了6.72%;轮迹横向分布离散度和初始裂纹深度与钢桥面板焊缝处疲劳裂纹随机扩展路径分布离散程度成正相关,初始裂纹形态比与其成负相关;焊缝细节初始裂纹深度越大,车辆荷载对其横向影响范围越大;焊缝细节初始裂纹形态比越大,车辆荷载对其横向影响范围变化不明显。     

某中承式钢管混凝土桁式拱肋节点疲劳开裂分析

为了分析某中承式钢管混凝土桁式拱肋节点疲劳开裂的原因,运用有限元软件MIDAS/CIVIL建立全桥模型进行有限元分析。分别研究了不同车辆荷载和不同拱肋节点几何参数情况下拱肋节点应力幅的变化,进而分析了车辆超载和车辆中、偏载布置对拱肋节点应力幅的影响,并讨论了拱肋节点几何参数变化对拱肋节点应力幅的影响。研究表明,当车辆荷载超载30%、50%、100%和150%时拱肋节点应力幅基本呈线性增大,应力幅最大值达155 MPa。BS5400疲劳车辆荷载作用下拱肋节点应力幅最大,中国550 k N车辆荷载偏载作用下拱肋节点应力幅为中载1.2倍。超载和偏载分别是造成该钢管混凝土桁式拱桥拱肋节点出现疲劳开裂和疲劳裂缝分布不对称的主要原因。节点应力幅随弦杆和腹杆间壁厚比的增大而减小,而随弦杆和腹杆间管径比、弦杆径厚比和腹杆径厚比的增大表现为先增大后减小的趋势。     

基于磁记忆方法的抽油杆裂纹扩展监测

基于拉-拉疲劳实验,利用磁记忆检测仪监测了抽油杆试件疲劳裂纹扩展的整个过程,提取了表征应力集中程度的磁信号合成梯度极限状态系数指标;并结合有限元方法计算了裂纹尖端的断裂力学参量,得到不同裂纹长度所对应的应力强度因子。结果表明:基于实验数据和仿真结果所建立的磁记忆信号特征指标与应力强度因子之间具有很好的一致性,可以有效反映疲劳裂纹的扩展过程以及累积损伤,为再制造前的毛坯筛选提供理论依据。     

轮轨摩擦接触下钢轨多裂纹相互作用研究

利用热机耦合有限元法,建立了轮轨摩擦接触时钢轨表面多裂纹的热弹性平面应变有限元模型。数值模型中,考虑轮轨摩擦温升对轮轨材料参数的影响,通过移动载荷和热源来模拟运动车轮对钢轨的作用。分析了轮轨滑动接触时多裂纹相互作用和表面裂纹数量对钢轨疲劳裂纹扩展特性的影响。计算结果表明:与单个裂纹相比,多裂纹有降低钢轨疲劳裂纹扩展的作用;钢轨裂纹尖端应力强度因子K1和应力强度因子范围?K2均随裂纹数的增多而减小;钢轨表面裂纹数为5条时可以反映更多裂纹时的裂纹扩展特性。     

黏聚裂纹阻抗的弯曲梁承载力

在混凝土类软化材料断裂研究中,裂纹端部损伤区被简化为具有黏聚应力分布的非线性裂纹,该黏 聚力对裂纹扩展有阻抗作用。裂纹体的应力强度因子是断裂力学标志载荷作用与几何构型因素的量化表达指标; 黏聚力形成的阻抗强度因子数值,与黏聚裂纹长度和材料极值拉伸应力存在数量关系。通过双K断裂判据,以 带切口的三点弯曲梁为断裂力学模型,分析了裂纹黏聚阻力对断裂过程的影响规律,计算该弯曲梁结构断裂试 样的最大承担载荷;其结果比较符合实验数据。     

FRP布加固具有中心裂纹板条的断裂疲劳性能

研究了FRP布加固具有中心穿透裂纹板条在两端拉伸载荷作用下的断裂和疲劳,得到了FRP布加固板条的界面剪应力,利用叠加原理和断裂力学的基本结果,推导了FRP加固板条裂纹尖端的应力强度因子解析表达式。在此基础上,分别给出了FRP加固具有中心穿透裂纹板条Paris和Elber模型的疲劳寿命预测公式,通过实例计算发现,循环荷载作用下FRP加固具有中心穿透裂纹板条的裂纹闭合效应非常显著,应采用Elber模型预测其疲劳寿命,而对于未加固的裂纹板条,应采用Paris模型预测其疲劳寿命。同时,参数分析表明:FRP布加固长度存在最优值,且FRP刚度对应力强度因子幅值影响显著,应力强度因子幅值随着FRP刚度的增加而减小,因此,其疲劳寿命延长。     

基于断裂力学的圆钢管混凝土T型焊接节点疲劳寿命预测

该文应用线弹性断裂力学基本原理预测圆钢管混凝土桁架焊接T型节点的疲劳寿命。首先,进行了一些节点的疲劳试验,作为验证节点疲劳寿命预测是否可靠的参考数据;其次,建立了基于断裂力学的疲劳寿命数值模拟的模型和流程框图,采用ANSYS软件编制了APDL宏命令,实现了节点疲劳寿命的计算;最后,分析了节点疲劳裂纹的扩展特性。研究结果表明:断裂力学数值模型能较好地预测这种复杂的钢混组合节点的疲劳寿命;在正常的焊接质量条件下,不同的初始裂纹尺度对节点疲劳扩展寿命的影响不大;裂纹在长度方向的扩展速度大于在深度方向的扩展速度;裂纹深度在达到1/2主管壁厚之前,裂纹沿深度方向的扩展非常缓慢,大部分的疲劳寿命消耗在此阶段,之后裂纹沿深度方向的扩展较快。     

几何非线性扩展有限元法及其断裂力学应用

该文将扩展有限元方法应用到几何非线性及断裂力学问题中,并研制开发了扩展有限元Fortran程序。扩展有限元法其计算网格与不连续面相互独立,因此模拟移动的不连续面时无需对网格进行重新剖分。该文推导了几何非线性扩展有限元法的公式,在常规有限元位移模式中,基于单位分解的思想加进一个阶跃函数和二维渐近裂尖位移场,反映裂纹处位移的不连续性,并用2个水平集函数表示裂纹;采用拉格朗日描述方程建立了有限变形几何非线性扩展有限元方程;采用多点位移外推法计算裂纹应力强度因子并通过最小二乘法拟合得到更精确的结果。最后给出的大变形算例表明该文提出的几何非线性的断裂力学扩展有限元方法和相应的计算机程序是合理可行的,而且对于含裂纹及裂纹扩展的问题,扩展有限元法优于传统的有限元法。     

平面K型主圆支方钢管节点力学性能数值分析

以平面K型主圆支方钢管节点的试验数据为基础,建立有限元参数分析模型,进行了非线性有限元参数分析。研究揭示了节点受力全过程,破坏模式以及分布规律。重点考察了支管边长与主管直径比值β、主管的径厚比γ和支管与主管的壁厚比τ对节点极限承载力的影响。有限元参数分析结果表明：主管的径厚比γ和支管与主管的壁厚比τ对节点极限承载力影响...     

Fatigue behaviors of square-to-square hollow section T-joint with corner crack. I: Experimental studies

Four full-size square-to-square hollow section (SSHS) T-joints were subjected to static and fatigue tests under combined loads. Static tests were first carried out to investigate the hot spot stress (HSS) distribution along the brace-chord intersections. Fatigue tests were then performed until extensive cracks appeared and the cracks development were monitored by the alternating current potential drop (ACPD) technique. The cracked joints were then opened up and the shapes of the crack surfaces were measured. Fatigue test results and crack geometry measurements showed that under cyclic loading, cracks will be initiated at one of the four corners of the brace-chord intersection and propagate mainly in the direction parallel to the chord wall. Furthermore, branches and secondary cracks, which seldom appear in circular hollow section (CHS) joints, were formed near the end of the test. The test results also indicated that a more stringent criterion should be used to define the fatigue life the joint. Finally, the shapes of the surface cracks of the tested joints were found to be more complicated than that for CHS joints and any of the existing geometrical model used in numerical modeling of CHS joint is deemed to be inadequate.     

Stress concentration factor prediction by the multi-dimensional Lagrangian interpolation method

A new approach based on the multi-dimensional Lagrangian interpolation method, which is widely used in the finite element method, is proposed for the prediction of stress concentration factor for tubular joints. When comparing with the parametric regression method, this new method uses the same set of numerical parametric study results as its inputs. The accuracy of the proposed method is validated by applying it to predict the stress concentration factor and hot spot stress of partially overlapped circular hollow section K-joints. Numerical results indicated that the new method is more reliable and accurate than the parametric regression method.     

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.     

Size effects in the fatigue behavior of welded steel tubular bridge joints

Tubular space trusses for bridge applications use thick‐walled tubes. The reduction in fatigue resistance due to geometrical size effects is thus an important issue. In order to carry out a thorough study, both fatigue tests on large‐scale specimens and advanced 3D crack propagation modelling were carried out at ICOM/EPFL. The study is limited to circular hollow sections (CHS) K‐joints. An alternate current potential drop (ACPD) system is used to measure crack depth on nodes of the tested truss specimens. The results obtained from the tests are given in the paper in terms of S‐N data, crack depth versus number of cycles and deduced crack propagation rates. The numerical model was developed using the dual boundary elements method (DBEM), software BEASY?, and was validated with fatigue tests data. The stress intensity factors (SIF) along the doubly curved crack front at different crack depths were obtained. With this model, a parametric study investigates the influence of geometry, size and load case on fatigue life. The results of both proportional and non‐proportional sizing effects on fatigue strength are presented. The paper shows that size effects (proportional and non‐proportional) can be expressed as a function of the non‐dimensional parameters and chord thickness.     

Numerical models verification of cracked tubular T, Y and K-joints under combined loads

This paper summarizes the key steps involved in the construction of an accurate and consistent finite element model for general cracked tubular T, Y and K-joints. The joint under consideration contains a surface crack which can be of any length and located at any position along the brace-chord intersection. Welding details along the brace-chord intersection, compatible with the American Welding Society (AWS) specifications [American Welding Society. Structural welding code-steel, ANSI/AWS, 15th ed., Miami, 2000], are included in the geometrical model. In order to develop a systematic and consistent modelling procedure, the whole process is divided into four key steps. They are, namely, (1) construction of a consistent geometrical model of the joint with welding details, (2) determination of cracked surface to define the semi-elliptical surface crack profile, (3) generation of well-graded finite element meshes, and (4) stress intensity factor studies around the crack front. To produce a well-graded finite element mesh, a sub-zone technique is used in the mesh generation whereby the entire structure is divided into several sub-zones with each zone consisting of different types of elements and mesh densities. The stress intensity factors (SIFs) are evaluated using the standard J-integral method. Two full-scale T and K-joint specimens were tested to failure under axial load (AX), in-plane bending (IPB), and out-of-plane bending (OPB). In the tests, the rate of crack propagation was monitored carefully using the alternating current potential drop (ACPD) technique. Using the known material parameters C and m, the experimental SIFs were obtained, and they are found to be in complete agreement with the computed SIFs obtained from the generated models. Hence, the proposed finite element models are both efficient and reliable.     

Stress intensity factors for fatigue cracking of round bars

The stress intensity factor for a single edge crack of either straight or circular front in a round bar has been determined using both the degenerated quarter-point isoparametric finite element and experimental fatigue crack growth data, and compared with values found by earlier investigators.The results of this study confirm that the stress intensity factors for straight edged surface cracks are lower in round bars than in square bars and a comparison of finite element and experimental results indicates that the effective stress intensity factor at the centre of the fatigue crack front in a round bar is 17% greater than its theoretical value.A correction function is proposed to account for the effect on the stress intensity factor of the circular boundary of a round bar.     

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。