碳纤维增强复合材料加固钢板的边界元分析

采用边界元分析对碳纤维增强复合材料加固钢板的疲劳性能进行扩展研究。采用BEASY软件计算应力强度因子、裂缝扩展和疲劳周期。应用面单元模拟组合增强片和带裂缝的钢板,将粘结层作为连接增强片和钢板的界面单元。数值模拟结果与试验结果具有一致性,从而验证了本文方法的有效性。采用边界元方法,对影响应力强度因子和疲劳寿命的因素进行评估,这些因素包括粘结长度、粘结宽度、增强片配置、碳纤维增强复合材料的层数组合增强片模量、粘结组合模量。     

CFRP加固受拉钢构件的疲劳性能

对采用CFRP加固的钢结构的疲劳性能进行研究,完成了不同荷载条件下的初步试验以评估双侧加固试件的疲劳性能。由于粘合剂的连续剥离导致加固构件的刚度退化,因此,刚度退化被选择作为整体破坏的指标。观察发现,首先从板端部产生剥离,然后沿界面剥离直至破坏。对破坏后的粘合层检查发现,粘合剂-金属界面是加固系统的薄弱点。详细定义了S-N曲线,对试件的疲劳性能与欧洲规范3中详细分类的焊逢疲劳抗力进行比较,结果显示:钢板和碳纤维之间的粘合层疲劳抗力明显高于盖板焊缝的抗力(欧洲规范3:36*-56*条目)。     

Nonlinear 3D finite element simulation of reinforced concrete flat plates retrofitted with externally bonded CFRP strips

In this study, a comprehensive 3D non-linear finite element modelling procedure is presented for the simulation of reinforced concrete flat plates, retrofitted with externally bonded carbon fibre-reinforced polymer (CFRP) strips and subjected to punching shear loads. The presented modelling procedure may be used to conduct parametric numerical analyses on the response of reinforced concrete elements similar to considered in this study. The modelling procedure is applied on: (i) a solid, un-retrofitted flat plate and (ii) eight flat plates with openings and retrofitted with externally bonded CFRP strips. In the procedure, debonding of CFRP strips from concrete surfaces is accounted for by defining appropriate bilinear shear stress–slip and normal stress–gap relationships. In the model, the corner lift-offs observed at the corners of the flat plates are also considered by defining appropriate restrains and frictional support conditions. Results obtained from the proposed modelling procedure are verified using experimental data available in the literature. From the verification, accuracy of the proposed modelling procedure is confirmed in terms of structural responses such as load displacement relationships and crack propagations of the test specimens.     

Fatigue reliability assessment of retrofitted steel bridges integrating monitored data

This paper focuses on fatigue reliability assessment of retrofitting distortion-induced cracking in steel bridges integrating monitored data. The fatigue reliability assessment of the connection details is based on the approach used in the AASHTO standard design specifications with all necessary information from finite element modeling (FEM) and structural health monitoring (SHM). Both in-plane traffic loading and out-of-plane relative displacements are considered along with different connection boundary conditions. The primary cause of the observed fatigue cracks before retrofitting is identified as the out-of-plane relative displacements, while the potential fatigue cracking re-initiation after retrofitting depends on the boundary conditions and critical locations that can be identified from the validated FEM. When the identified critical locations are different from the SHM sensor locations, the original monitored data may be modified by using a spatial adjustment factor (SAF). The proposed approach is illustrated by using an actual bridge monitored by the Advanced Technology for Large Structural Systems (ATLSS) Center, a National Engineering Research Center at Lehigh University.     

外贴CFRP板加固锈蚀钢板疲劳性能试验研究

为研究外贴CFRP板对锈蚀钢板疲劳性能的影响,开展16个CFRP板加固锈蚀钢板试件的疲劳拉伸试验,研究锈蚀程度、CFRP刚度、CFRP预应力度、端部锚固及疲劳应力幅对CFRP板加固锈蚀钢板疲劳破坏模式、疲劳寿命及裂纹扩展规律的影响。研究结果表明,外贴CFRP板可以显著提升锈蚀钢板疲劳寿命,对质量损失率为9.38%、13.39%、16.61%及21.24%的锈蚀钢板,端部锚固条件下双面粘贴CFRP板加固试件疲劳寿命相对于未加固试件分别提高了18.4倍、9.3倍、8.9倍及11.4倍;加固方式对疲劳加固效果影响显著,端部锚固可以有效延缓黏结界面失效,增大CFRP板加固刚度或采用预应力加固均可显著降低裂纹扩展速率。     

Fatigue life prediction of heavy mining equipment. Part 2: Behaviour of corner crack in steel welded box section and remaining fatigue life determination

Frequent fatigue failure of mining equipment has created a need for fatigue life prediction of several components that suffer from frequent recurrence of fatigue cracking. The focus of this paper is the frequent occurrence of fatigue cracks in the boom of cable shovels, which consists of a twin box girder joining together near both extremities. After the determination of load spectrum and the material fatigue properties by field monitoring and material testing, the fatigue behaviour of a cracked shovel boom was investigated. Two-tip corner cracks were of particular interest since they are commonly observed in the field. These cracks typically initiate in the flange at the toe of web to flange or flange to diaphragm welds and propagate into the web. Detailed finite element models with corner cracks were developed for the boom and stress intensity factors for various crack configurations were evaluated from the finite element analysis results. The growth behaviour of corner cracks was studied and a simplified method is presented for the fatigue life prediction. Fracture toughness tests were conducted on single edge notch bend specimens at room temperature and low temperature (−50 ^°C). The remaining life of a cracked boom is then determined based on brittle fracture or unstable crack growth.     

FRP单面加固带裂纹钢板的应力强度因子分析

对单面粘贴FRP的带裂纹钢板进行了线弹性断裂力学分析,研究了裂纹尖端的应力强度因子变化,分析结果表明,经过FRP加固后,钢板的应力强度因子显著降低;随着FRP刚度的增加,应力强度因子逐渐减小。     

Crack analysis in orthotropic media using the extended finite element method

An extended finite element method has been proposed for modeling crack in orthotropic media. To achieve this aim a discontinuous function and two-dimensional asymptotic crack-tip displacement fields are used in a classical finite element approximation enriched with the framework of partition of unity. It allows modeling crack by standard finite element method without explicitly defining and re-meshing of surfaces of the crack. In this study, fracture properties of the models are defined by the mixed-mode stress intensity factors (SIFs), which are obtained by means of the domain form of the interaction integral (M-integral). Numerical simulations are performed to verify the approach, and the accuracy of the results is discussed by comparison with other numerical or (semi-) analytical methods.     

Behavior of bridge girders with corrugated webs under monotonic and cyclic loading

The focus of this study is to investigate the behavior of plate girders with trapezoidally corrugated webs under fatigue loading. Six girders were tested under both monotonic and cyclic four-point loading arrangements. A relationship between the stress range, F_r, and the number of cycles to failure, N, was developed for the girders. The results from the fatigue tests were related to the current AASHTO specifications and compared with the test results of different types of girders carried out by others. Nonlinear finite element analyses were conducted to study the effects of the different geometrical parameters of this type of girders on the stress concentrations at different locations and to explain the findings of the experimental study. Two sources of stress concentration leading to fatigue cracking were isolated and evaluated; one is dependent on the radius of curvature between the web folds and the other depends on the inclination angle of the inclined fold. An analytical technique based on fracture mechanics is also proposed to enable designers to determine the fatigue life of corrugated web steel girders.     

Establishing stress state of cylindrical metal silos using finite element method: Comparison with ENV 1993

There is at present a great deal of interest in developing new methods and calculation tools for use in the study and measurement of stress states in shell metal structures. The finite element method (FEM) is a numerical method which permits this kind of study, and which is put forward in the European experimental standard, ENV 1993, as an accurate and reliable calculation tool. This study presents different three-dimensional models whose distinguishing feature is the simulation of both stored granular material and silo walls, without resorting to simplifications. The models developed predict the stress state of cylindrical metal silos flat bottomed, subjected to the action of stored granular material in their interior. The behaviour assigned to the stored material is elastic, and that assigned to the structure is the classical bilinear elastic-perfectly plastic, typical of metallic materials such as steel. Two geometric parameters are analysed: height and thickness of the wall. The results obtained from numerical methods (hoop, meridional or vertical, normal and shear stresses) are compared with those obtained via ENV 1993-1-6.     

分块盖板屈曲约束钢板剪力墙受力性能与设计方法研究

为考察分块盖板屈曲约束钢板剪力墙的抗震性能,采用非线性有限元分析软件ABAQUS对其受力机理进行深入研究。将板墙的高宽比、盖板厚度比等参数作为变量,对屈曲约束钢板墙在水平往复荷载作用下的抗震性能进行分析,考察内嵌钢板面外变形与应力、盖板变形与钢筋应力。分析结果表明:采用非线性有限元数值模拟得到屈曲约束钢板剪力墙的滞回性能、内嵌钢板变形特性均与试验结果较为接近。分块盖板屈曲约束钢板剪力墙在层间位移角接近1/300时进入屈服;加载至层间位移角1/50时,滞回曲线接近于理想弹塑性。当盖板厚度比t_c/t_w不小于7时,可以形成对内嵌钢板面外变形的有效约束,钢板屈曲半波长度与钢板厚度之比λ/t_w基本保持不变。在此基础上,建立内嵌钢板与分块盖板简化计算模型,根据内嵌钢板主压应力推导出挤压应力计算公式,从而可以方便地确定对拉螺栓的内力与分块盖板的弯矩。简化计算方法与有限元数值模拟得到混凝土盖板弯矩的分布规律非常接近,简化计算方法偏于安全。     

梁转动对屈曲约束开缝钢板剪力墙受力性能影响有限元分析

利用有限元软件ABAQUS,研究梁转动对屈曲约束开缝钢板剪力墙受力性能的影响,并以钢板与两侧约束部件的间隙Δ_c为参数,针对开缝墙面外约束条件进行分析。结果表明：在其它条件不变的情况下,开缝墙初始刚度以及耗能能力均与梁转动刚度成正比,与梁转动影响系数λ（λ为开缝墙屈服时刻梁转角与屈服层间位移角的比值）成反比,当λ<0.18时,初始刚度可达到理想条件下的85%,而当λ>2.00时,能量耗散系数至少下降25%;开缝墙滞回曲线饱满程度以及屈服荷载均与面外约束缺陷因子η成反比,η为Δ_c与两侧约束部件（钢筋混凝土板）所需最小厚度t_c,min的比值,当η<0.22时,开缝墙能够先于屈曲达到屈服,滞回曲线饱满,屈服荷载没有降低,而当η≥1.30时,开缝墙则较早发生面外屈曲,导致滞回曲线捏拢严重,屈服荷载下降25%。根据上述分析结果,由试验结果算得λ及η（分别取λ=0.40,η=0.30和λ=1.00,η=0.23）,得到的有限元曲线与试验曲线吻合较好。     

Fatigue life estimation of steel girder of Yangpu cable-stayed Bridge due to buffeting

As the main span of modern cable-stayed bridges becomes longer and longer, the buffeting-induced fatigue damage problem of steel girders located in strong wind regions may have to be taken into consideration in the design of the bridge. This paper presents a method in the mixed frequency–time domain for estimating the fatigue life of steel girders of the Yangpu cable-stayed Bridge due to buffeting. In the suggested method, the joint probability density function of wind speed and wind direction at the deck level of the bridge is first established. The power spectra of the critical stress of the girder are then derived from the power spectra of the generalized coordinates of the bridge for different wind speeds and wind directions. The derived stress spectra are no longer a narrow spectrum when the background component of stress response is included. Thus, the time histories of the critical stress are simulated from their power spectra and the stress cycle distributions are estimated in terms of rainflow count method. The formulae derived based on the modified Miner law and the random vibration theory are finally used for estimating the fatigue life of the bridge girder. The results show that the effects of wind direction on the fatigue life of the Yangpu Bridge are significant. The predicted fatigue life due to buffeting is much longer than the design life of the bridge.