Fatigue crack growth model for assessing reliability of box-girders for cable-stayed bridge combining SHMS with strain data

Enhancement of the computer algorithm developed for the Runyang cable-stayed bridge (RYCB) in China is made by incorporating the fatigue crack growth analysis in contrast to the SN curve approach. Strain data obtained from the structural health monitoring system (SHMS) and finite element calculations are used. This provides the application of a deterministic method in addition to the probabilistic approach with the added feature of crack growth. The choice of selecting the two-parameter fatigue crack growth criterion was based on the definition of reliability index β such that the new results can be compared with those using the SN curves. A gradual drop of the reliability index β with time with an upper limit was obtained for the crack growth model in contrast to the linear time relation for the SN curve model that had no upper limit. This difference is significant and reveals the importance for selecting the fatigue failure criterion. Deterministic and probabilistic crack growth models are used to assess the differences. The results are based on the box-girder component strain history data of the Runyang Cable-stayed Bridge (RYCB) in China, stress history recorded by structural health monitoring system (SHMS) is analyzed using the monitored stress amplitude, mean stress and stress ratio. Finite element calculations are used to supplement data at locations not accessible for measurements. Additional improvement with reference to damage accumulation and the physical meaning of the reliability index will be studies in relation to the fatigue damage of box-girder of long cable-stayed bridges.     

基于实测荷载的铁路钢桁梁桥疲劳可靠性评估

为了对钢桁梁桥疲劳进行评估,引入可靠性理论,提出了基于实测荷载的桥梁疲劳可靠性评估方法。根据某铁路钢桁梁桥实测车辆荷载数据,建立了随机车辆荷载模型。并在考虑车辆荷载随机性的基础上,结合Monte-Carlo法与有限元,分析了钢桁梁桥构件疲劳应力谱,计算了构件疲劳可靠性随时间的变化,并探讨了车辆荷载及荷载效应变异性对构件疲劳可靠性的影响,最后采用β约界法与静力分析法对钢桁梁桥系统疲劳可靠性进行了研究。结果表明,基于随机车辆荷载的构件疲劳应力谱呈现单峰分布;构件疲劳可靠性随运营时间的增加而减小;车辆荷载的增长及荷载效应变异性的增加对构件疲劳可靠性影响较大,当车辆荷载增长率和等效应力变异系数分别增加到5%时,构件疲劳寿命大幅减小;β约界法结合静力分析法可快速确定钢桁梁桥失效模式,桥梁系统疲劳寿命小于构件疲劳寿命。总的来看,基于实测荷载的钢桁梁桥疲劳可靠性评估方法能有效地利用监测数据,对桥梁疲劳评估具有良好的适用性。     

基于荷载形函数的大跨桥梁结构移动荷载识别

大跨桥梁结构中的移动荷载识别是结构健康监测的重要组成部分之一,其作为动力学中的反问题,存在唯一性及稳定性等病态问题。本文首先推导出移动荷载作用下结构响应的卷积形式的离散公式,然后利用有限元理论中的形函数拟合移动荷载,推导出基于荷载形函数的移动荷载识别表达式。将移动荷载的识别转化为荷载形函数拟合系数的识别,降低了需要识别的未知量,减小了逆运算的计算量,消除或减弱逆问题病态性,并提高了计算效率。利用某大跨刚构-连续预应力混凝土桥梁修正后的有限元模型进行数值仿真,考虑路面粗糙度,由模态叠加法计算移动荷载作用下的响应;然后采用荷载形函数方法识别移动荷载,证明该方法在5%以下的噪声时能快速并精确识别复杂桥梁结构的移动荷载。     

基于LHS-Kriging法的正交异性钢桥面板疲劳可靠度分析

为解决随机车载下正交异性钢桥面板疲劳应力谱有限元求解耗时问题,采用拉丁超立方抽样（LHS）与Kriging方法,建立了快速获取随机车流作用下细节疲劳应力谱的LHS-Kriging有限元替代模型,并将此模型应用于南溪长江大桥正交异性钢桥面板疲劳可靠度计算。结果表明,基于LHS-Kriging方法的有限元替代模型, 不需要经过大量车辆荷载的有限元加载,可直接快速获取细节疲劳应力谱;与传统的响应面法（RSM）相比,Kriging法预测的细节等效疲劳应力更符合有限元计算结果;随着交通量增长率的增大,桥梁的疲劳可靠度显著减少;100年后,当交通量增长率为3%和5%时,正交异性桥面板与纵肋焊接处的细节疲劳可靠度小于2。     

基于非齐次复合Poisson过程的正交异性钢桥面板细节疲劳裂纹随机扩展研究

传统的正交异性钢桥面板疲劳损伤评估常采用确定性和可靠性分析方法,忽略了疲劳裂纹扩展的随机性影响,针对这一问题,提出钢桥面板细节疲劳随机扩展分析方法。本文以南溪长江大桥为工程背景,基于长期车辆荷载监测数据,建立了车辆荷载非齐次复合Poisson过程模型。建立钢桥面板有限元模型,采用瞬态分析方法将随机车辆荷载转化成细节疲劳应力,基于线弹性断裂力学理论推导U肋-顶板焊接细节疲劳裂纹扩展时变微分方程,实现宏观关系式疲劳应力幅次数-疲劳损伤至微观表达式应力时间序列-疲劳损伤转换,讨论了车载次序及超载对疲劳裂纹扩展的影响。研究结果表明,非齐次复合泊松过程模型能够较好描述随机车流运营状态,车辆荷载的次序对疲劳裂纹扩展速率的影响不可忽略,重车排序靠前时能够促使疲劳裂纹扩展增速,南溪长江大桥细节点的车辆超载迟滞效应修正系数取值0.804。     

Combined use of SHMS and finite element strain data for assessing the fatigue reliability index of girder components in long-span cable-stayed bridge

The design of long-span cable-stayed bridge involves a large number of loads, geometric and material parameters, all of which can interact in a random fashion. It is desirable to have a total measure of the operational reliability and safety of the structural components. Based on the box-girder component strain history data of the Runyang Cable-stayed Bridge (RYCB) in China, a computer algorithm is developed to evaluate the fatigue damage that is assumed to occur in increments, according to a lognormal distribution. The corresponding probability density function is then found to obtain a fatigue reliability index β for ranking the integrity of the girders. Emphases are placed on the overall scheme of structural reliability evaluation such that the different fatigue damage criteria, probability density functions, and strain measurement techniques can be made. Finite element calculations are also used to provide strain data at locations that are not conducive for installing strain gauges, while the compatibility of measured and calculated data is made empirically. Each of the subroutine in the fatigue reliability algorithm can be altered for improvement. The flexibility allows up-dating the prediction as the monitored strains are changed by the environmental conditions. Preliminary results are first obtained to test the selected damage increments in relation to the probability function and fatigue damage criterion. Particular attention has been given to test the sensitivity of the combined governing parameters. The highly non-linear behavior of numerical calculations related to fatigue failure necessitates an in-depth understanding of the physical model. The condition under which fatigue damage accumulation is needed in contrast to the linear sum of fatigue cycles will be left for the future. Justification should be given to include the more complex issues. The aim here is to seek a simple, and yet reliable index that can account for the fatigue damage of box-girder of long cable-stayed bridges.     

Features of long-term health monitored strains of a bridge with wavelet analysis

This paper analyses the five years' monitored strains collected from a long-term health monitoring system installed on a bridge with wavelet transform. In the analysis, the monitored strains are pre-processed, features of the monitored data are summarized briefly. The influences of the base functions on the results of wavelet analysis are studied simultaneously. The results show that the db wavelet is a good mother wavelet function in the analysis, and the order N should be larger than 20, but less than 46 in decomposing the monitored strains of the bridge. According to the strain variation features of concrete bridge, the proper decomposition level is 4 in the wavelet multi-resolution analysis. With the present method, the strains caused by random loads and daily sunlight can be accurately extracted from the monitored strains. The decomposed components of the monitored strains show that the amplitudes of the strains caused by random loads, daily sunlight, and annual temperature effect, are about 5 με, 25 με, and 50 με respectively. The structural response under random load is smaller than the other parts.     

考虑焊接残余应力的波形钢腹板PC梁关键细节疲劳可靠度研究

焊接残余应力对波形钢腹板细节疲劳寿命的影响不可忽略,本文以头道河大桥为工程背景,基于ABAQUS有限元软件,建立波形钢腹板焊接构件数值模型,提出不同波折角度的波形钢腹板残余应力模型。采用瞬态分析方法,分析6种典型车辆作用下波折角区域焊缝细节的应力时程。在此基础上,构建考虑焊接残余应力和车辆荷载共同作用下的波形钢腹板细节疲劳极限状态方程,讨论不同波折角度和交通增长率对疲劳可靠度指标的影响规律。研究表明,翼缘板-波形钢腹板焊接构件斜边两侧的纵向残余应力呈对称分布,焊接细节转角区域圆弧外侧的纵向残余应力整体高于圆弧内侧,随着波折角度的增加,两侧焊缝附近的纵向残余应力变大;在桥梁设计基准期取100年时,30°波折角度焊缝细节疲劳可靠度是60°波折角度的1.05倍;交通量的线性增长对头道河大桥焊缝细节的疲劳可靠度影响较大,不考虑交通增长率的焊缝细节疲劳可靠度是α=5%的1.84倍。     

静风荷载作用下大跨度钢拱桥施工稳定性的参数研究

随着钢拱桥跨径的不断增大,钢拱桥在静风荷载作用下的施工稳定性问题已引起了人们的广泛重视。本文以上海在建的主跨550m的中承式钢拱桥为例,采用几何和材科非线性分析法详细分析了大跨度钢拱桥在两个不同施工阶段下的静风稳定性。结果表明,拱肋最初合拢状态是拱桥施工过程中最不利的状态。最后,详细探讨影响大跨度钢拱桥施工过程中静风稳定性的主要参数。     

吊车荷载作用下钢结构吊车梁的疲劳可靠寿命评估

从Miner累积损伤的定义出发,视累积损伤为随机过程,临界损伤为随机变量,基于疲劳动态可靠性理论,提出了在役钢结构吊车梁疲劳剩余寿命的可靠寿命评估法。之后与现行《钢结构检测评定与加固技术规程》(YB9257-96)中安全期限寿命评估法进行了分析比较,说明了安全寿命评估方法的概率意义。算例分析结果表明,本文提出的可靠寿命评估方法更加合理。     

A quasi-secant continuous model for the analysis of long-span cable-stayed bridges

In this paper a non-linear continuous model for the statical analysis of long-span cable-stayed bridges with fan scheme and H-shaped towers is proposed. This model is based on a quasi-secant large-displacement formulation of the stays-deck interaction and involves an Euler-Bernoulli/De Saint Venant model for the girder. An approximate closed-form solution of the statical problem, in which flexural and torsional terms are coupled, is obtained by means of a perturbative technique and according to the prevailing “truss-like” structural behavior. Results for some study-cases are compared with those relevant to both classical models and numerical solutions, proving effectiveness and applicability of the proposed model.     

Experimental investigation of impact of environmental temperature and optimal baseline for thermal attenuation in structural health monitoring based on ultrasonic guided waves

Structural health monitoring (SHM) of any mechanical component is compulsory for its efficient and long-term performance. One of the major challenges to apply SHM technique in real-time inspections is variation in environmental and operating conditions (EOCs). Sometimes the effect of this variation in EOCs is so severe that it influences the SHM system’s response and reduces the accuracy of the inspection process. The goal of current research is to investigate experimentally the impact of environmental temperature on the ultrasonic guided wave signal during damage detection. According to the characteristic of breathing phenomenon of fatigue crack caused by the applied temperature (30 °C–180 °C) under operation condition, behavior of reflection and transmission signal is analyzed in terms of amplitude and group velocity. Based on experiment findings, a wave velocity function has been generated in the Matlab® environment to compute the velocity of acquired signal considering the effect of both temperature and excitation frequency. A corresponding sequence curve is drawn which illustrates that the proposed function is valid when the operating temperature is less than 130 °C because sensor bonding’s characteristics are affected by the further increment in temperature and consequently it would become difficult to illuminate the sole impact of temperature on damage detection results. Impact of temperature on examined material properties and sensor’s bonding strength is also observed in the current study. Analysis of dispersion curves is performed to examine the individual behavior of S0 and A0 wave modes with temperature and to determine the temperature invariant points to reduce the influence of environmental temperature in SHM. Hence current study not only evaluates the impact of temperature on damage detection but also provides an optimal baseline for thermal attenuation in real-time ultrasonic guided wave inspections.     

Fracture strength for a high strength steel bridge cable wire with a surface crack

In this paper, the fracture strength of a cracked suspension bridge wire is determined based on linear elastic fracture mechanics (LEFM). The wire is 5 mm in diameter, with an original ultimate strength of 1725 MPa and ultimate elongation that ranges between 5.5% and 6%. The average value of for the wire fracture toughness, K_C, was recently evaluated by the author. The state of practice is to use the ultimate strength of the cracked wire as obtained from tensile tests. This approach may overestimate the strength of the wire due to possible delamination and crack tip plasticity. A case study for a group of in situ wire breaks retrieved from a suspension bridge cable is presented. The failure analysis is performed based on both the fracture toughness criterion and the net section theory. The fracture toughness criterion produced more realistic results for the fracture strength of the wire. The decline in the fracture toughness and the corresponding reduction in the fracture strength of cracked degraded wire are predicted making use of the strain energy density criterion.     

正交异性钢桥面系的多目标离散优化设计

将大跨径钢桥的正交异性钢桥面板和其上的铺装层作为钢桥面铺装体系整体进行多目标优化设计.取钢桥面铺装体系结构总重量最轻及铺装层使用年限最长两类指标构造目标函数,在此基础上采用功效函数法确定了目标函数的权重系数.应用正交异性钢桥面系力学性能控制指标和疲劳寿命预测公式,建立多目标优化设计的数学模型.引入蚂蚁算法,开发了钢桥面铺装体系结构多目标离散优化设计程序.以国内某大跨径钢桥为对象, 采用多目标优化设计方法, 给出钢桥面铺装体系中各参数的优化值.结果表明, 应用多目标优化设计方法对大跨径钢桥面铺装体系设计是可行的.研究成果可为大跨径钢桥面铺装体系结构的设计提供理论依据.     

Fatigue crack growth of cable steel wires in a suspension bridge: Multiscaling and mesoscopic fracture mechanics

Intrinsically, fatigue failure problem is a typical multiscale problem because a fatigue failure process deals with the fatigue crack growth from microscale to macroscale that passes two different scales. Both the microscopic and macroscopic effects in geometry and material property would affect the fatigue behaviors of structural components. Classical continuum mechanics has inability to treat such a multiscale problem since it excludes the scale effect from the beginning by introducing the continuity and homogeneity assumptions which blot out the discontinuity and inhomogeneity of materials at the microscopic scale. The main obstacle here is the link between the microscopic and macroscopic scale. It has to divide a continuous fatigue process into two parts which are analyzed respectively by different approaches. The first is so called as the fatigue crack initiation period and the second as the fatigue crack propagation period. Now the problem can be solved by application of the mesoscopic fracture mechanics theories developed in the recent years which focus on the link between different scales such as nano-, micro- and macro-scale.On the physical background of the problem, a restraining stress zone that can describe the material damaging process from micro to macro is then introduced and a macro/micro dual scale edge crack model is thus established. The expression of the macro/micro dual scale strain energy density factor is obtained which serves as a governing quantity for the fatigue crack growth. A multiscaling formulation for the fatigue crack growth is systematically developed. This is a main contribution to the fundamental theories for fatigue problem in this work. There prevail three basic parameters μ^∗, σ^∗ and d^∗ in the proposed approach. They can take both the microscopic and macroscopic factors in geometry and material property into account. Note that μ^∗, σ^∗ and d^∗ stand respectively for the ratio of microscopic to macroscopic shear modulus, the ratio of restraining stress to applied stress and the ratio of microvoid size ahead of crack tip to the characteristic length of material microstructure.To illustrate the proposed multiscale approach, Hangzhou Jiangdong Bridge is selected to perform the numerical computations. The bridge locates at Hangzhou, the capital of Zhejiang Province of China. It is a self-anchored suspension bridge on the Qiantang River. The cables are made of 109 parallel steel wires in the diameter of 7 mm. Cable forces are calculated by finite element method in the service period with and without traffic load. Two parameters α and β are introduced to account for the additional tightening and loosening effects of cables in two different ways. The fatigue crack growth rate coefficient C₀ is determined from the fatigue experimental result. It can be concluded from numerical results that the size of initial microscopic defects is a dominant factor for the fatigue life of steel wires. In general, the tightening effect of cables would decrease the fatigue life while the loosening effect would impede the fatigue crack growth. However, the result can be reversed in some particular conditions. Moreover, the different evolution modes of three basic parameters μ^∗, σ^∗ and d^∗ actually have the different influences on the fatigue crack growth behavior of steel wires. Finally the methodology developed in this work can apply to all cracking-induced failure problems of polycrystal materials, not only fatigue, but also creep rupture and cracking under both static and dynamic load and so on.     

大跨度钢桁桥模型的精细化损伤定位模拟和试验研究

在役大跨钢桁桥数日众多,其杆件往往较长,所以识别出损伤杆并找出局部损伤所处位置对安全评定和加固尤为重要。针对这一问题,参考我国在役钢桁梁桥,基于健康监测的试验目的和相似理论设计制作了贝雷梁式筒支钢桁桥Benchmark模型。根据其结构形式、具体尺寸,并经试验结果修正,建立了基准Matlab有限元模型。环境激励下,首先隔...     

正交异性钢桥面板轮载横向效应的解析分析方法

在分析正交异性钢桥面板构造特点的基础上,将轮载影响范围内的桥面板简化为弹性支撑的平面框架,建立了正交异性钢桥面轮载横向效应的解析分析模型,推导了纵肋弹性支撑刚度和车轮荷载集度等效计算方法,提出了桥面板与U肋交接位置处横向弯曲应力的解析公式,讨论并明确了影响桥面板横向弯曲应力峰值的关键敏感影响因素,并以某钢箱梁为例证明了本文算法的合理性。研究发现,本文方法计算得到桥面板与U肋相交位置的横向应力值与有限元结果相差不超过10%,证实了本文算法的正确性,也为正交异性钢桥面的初步设计提供了极大的方便;正交异性钢桥面板的横向轮载应力随U肋厚度和高度增加而增大,但随顶板厚度和横隔板间距增大而减小;相对而言,顶板内横向拉应力受顶板厚度的影响最为显著,对腹板倾角和U肋腹板厚度的变化并不敏感。     

Optimal placement of sensors for sub-surface fatigue crack monitoring

This paper discusses the scattering of stress waves by defects in representative aircraft structures with multi-layered construction and geometry variation. The approaches for determining and enhancing the probability of detection of non-surface-penetrating defects in such structures as well as minimising the contributions of multi-layered construction and geometry variation to false indications are presented. The results demonstrate the importance of selecting the appropriate frequency and location of the sensor in monitoring sub-surface defects on these structures. The findings suggested that a computer solution of the problem may be required to determine the optimal combination of frequency and sensor location. This study suggests the possibility of incorporating structural health monitoring into the design of future structures which will constitute a significant leap in the current knowledge base of structural health monitoring.     

网壳结构健康监测中的传感器优化布置

针对网壳结构健康监测提出了一种以损伤可识别性与模态可观测性相协调为目标的传感器优化布置的方法.由于模态数目的选取对基于损伤灵敏度分析的传感器优化布置有很大的影响,因此本文建立了一种同时包含模态独立性信息和损伤灵敏度信息的Fisher信息矩阵,并选取合适的模态数目,然后发展了一种以信息矩阵最大和条件数最小为准则的多目标优化算法.空间网壳数值算例表明,本文提出的传感器优化方法能简单、有效地为空间结构传感器优化布置提供可行方案.     

Structural health monitoring and damage detection using an intelligent parameter varying (IPV) technique

Most structural health monitoring and damage detection strategies utilize dynamic response information to identify the existence, location, and magnitude of damage. Traditional model-based techniques seek to identify parametric changes in a linear dynamic model, while non-model-based techniques focus on changes in the temporal and frequency characteristics of the system response. Because restoring forces in base-excited structures can exhibit highly non-linear characteristics, non-linear model-based approaches may be better suited for reliable health monitoring and damage detection. This paper presents the application of a novel intelligent parameter varying (IPV) modeling and system identification technique, developed by the authors, to detect damage in base-excited structures. This IPV technique overcomes specific limitations of traditional model-based and non-model-based approaches, as demonstrated through comparative simulations with wavelet analysis methods. These simulations confirm the effectiveness of the IPV technique, and show that performance is not compromised by the introduction of realistic structural non-linearities and ground excitation characteristics.