Determination of effective stress range and its application on fatigue stress assessment of existing bridges

This paper presents a unified approach on determination of the effective stress range based on equivalent law of strain energy and fatigue damage model, so as to provide an efficient approach for accurately assessing effective fatigue stress of existing bridge under traffic loading. A new theoretical framework to relate variable- and constant-amplitude fatigue is proposed in this paper. Different formulation for calculating effective stress range can be derived by the proposed theory, which include the effective stress range by the root mean square, by Miner's law and a new effective stress range based on the nonlinear fatigue damage model. Comparison of the theoretical results of fatigue damage under the effective stress range of the variable-amplitude stress spectrum and experimental data of fatigue damage under realistic traffic loading has confirmed the validity of the proposed theory. As a way to relate variable-amplitude fatigue data with constant-amplitude data, the effective stress range provides the most convenient way for evaluating fatigue damage under variable-amplitude loading. The proposed theory is then applied to provide an efficient approach for accurately assessing fatigue damage of existing bridges under traffic loading, in which online strain history data measured from bridge structural health monitoring system is available. The proposed approach is applied to evaluate the effective stress range for the purpose of the fatigue analysis of a deck section of a long-span steel bridge––the Tsing Ma Bridge in Hong Kong.     

Accumulative damage near crack tip for welded bridge members: fatigue life determination

The behavior of crack growth for the fatigue damage accumulation near tip where damage is most severe is analyzed. Fatigue life is assessed for the welded members of bridges under traffic loading. Two parts are considered. They consist of the development of a fatigue damage accumulation model for welded bridge members and a method for calculating the stress intensity factor that is needed for evaluating the fatigue life of welded bridge members with cracks. Based on the concept of continuum damage accumulation and fatigue and fatigue crack growth relations, results are obtained to describe the relationship between the cracking count rate and the effective stress intensity factor. Crack growth and fatigue life are found for two types of welded members assisted by using fatigue experimental results. The stress intensity factors are modified by correcting for the geometric shape of the welded members in order to reflect the influence of the weldment and geometry. This is accomplished via the stress intensity factor. The calculated and measured fatigue lives were generally in good agreement for the initial cracking conditions of two types of welded members widely used in steel bridges.     

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.     

正变异性钢桥面板疲劳验算时的结构分析

正交异性钢板面板直接承受车辆轮荷载的反复作用,容易疲劳开裂,因此疲劳验算是钢桥面板设计中的一项重要任务。疲劳验算首先要涉及应力分析。本文对钢桥面板在一辆重型货车作用下的应力状况进行了现场试验研究,根据试验结果提出了简化实用的结构分析模型。在该模型的基础上,结合应用有限元数值计算,能在理论上很好地掌握钢桥面板的应力状况。     

桥梁结构劣化与损伤过程的多尺度分析方法及其应用

研究桥梁结构多尺度模拟和分析方法对于发展重大桥梁结构损伤检测与状态评估方法以确保其安全运营是至关重要的.本文综述和总结了作者及其课题组多年来在桥梁结构劣化与损伤过程分析领域的研究工作.包括:系统地提出了针对大型桥梁结构损伤劣化过程分析需求的结构多尺度问题,研究证实了结构损伤与劣化过程分析可以通过结构一致多尺度模拟和时空多尺度分析来实现;提出了针对大型桥梁结构多尺度模型的多因素、多目标同步模型修正技术,指出多尺度模型修正必须对各分析目标敏感的多种因素同步优化,同时交叉验证修正后模型对该级修正目标和对其它模拟目标的复现与预测能力.提出了基于结构一致多尺度模拟和结构健康监测信息进行结构劣化分析的方法并应用于两个重大桥梁工程结构的疲劳损伤分析与评估.最后,指出在此领域中还要很多问题亟待解决,尤其是如何模拟损伤从材料与结构中的细观层次的缺陷演化发展到结构局部乃至整体失效的过程,建立结构损伤演化与失效过程的跨尺度分析理论与方法.     

基于能量法的多轴疲劳寿命预测方法

摘 要：有效的疲劳寿命预测方法是确保处于多轴循环载荷作用下的工程构件安全性的关键。结合临界平面思想,提出了一种基于能量法的多轴疲劳寿命预测模型;该模型针对不同的疲劳失效形式采用不同的临界面上能量参数作为损伤参量,可体现多轴加载条件下的平均应力效应以及临界面上各方向参量对材料疲劳损伤的影响。通过六种材料的多轴疲劳试验数据对所提出的模型及其它三种经典能量模型进行了评估与验证,结果显示所提出的模型相较于其他模型具有更好的寿命预测精度及工程适用性。     

STUDY ON THE SURFACE CRACK GROWTH BEHAVIOR IN 14MnNbq BRIDGE STEEL

Three-dimensional crack closure correction methods are investigated in this paper.The fatigue crack growth tests of surface cracks in 14MnNbq steel for bridge plate subjected to tensile and bending loadings are systematically conducted.The experimentally measured fatigue crack growth rates of surface cracks are compared with those of through-thickness cracks in detail.It is found that the crack growth rates of surface cracks are lower than those of through-thickness cracks.In order to correct their differences in fatigue crack growth rates, a dimensionless crack closure correction model is proposed.Although this correction model is determined only by the experimental data of surface cracks under tensile loading with a constant ratio R=0.05, it can correlate the surface crack growth rates with reasonable accuracy under tensile and bending loadings with various stress ratios ranging from 0 to 0.5.Furthermore, predictions of fatigue life and crack aspect ratio for surface cracks are discussed, and the predicted results are also compared with those obtained from other prediction approaches.Comparison results show that the proposed crack closure correction model gives better prediction of fatigue life than other models.     

Material plasticity dependence of mixed mode fatigue crack growth in commonly used engineering materials

In this paper, an investigation of fatigue crack propagation in rectangular plates containing an inclined surface crack is presented. A criterion for the three-dimensional stress state is proposed to predict fatigue crack initiation angles. It is assumed that the direction of crack initiation coincides with the direction of the minimum radius of the plastic zone defined by the von Mises yield criterion. The maximum energy release rate criterion, i.e., G_max criterion, is extended to study the fatigue crack growth characteristics of mixed mode cracks. A modification has been made to this criterion to implement the consideration of the plastic strain energy. Subsequently, this concept is applied to predict crack growth due to fatigue loads. Experiments for checking the theoretical predictions from the proposed criterion have been conducted. The results obtained are compared with those obtained using the commonly employed fracture criteria and the test data.     

A cyclic steady-state method for fatigue crack propagation: Evaluation of plasticity-induced crack closure in 3D

The numerical study of plasticity-induced crack closure using the node-release technique presents many difficulties widely studied in literature. For instance various rules, proposed for overcoming mesh sensitivity, are challenged by more recent studies. This paper intends to propose and evaluate a numerical method for the investigation of crack propagation under fatigue loading, and particularly for the assessment of plasticity-induced crack closure in three-dimension. The method is an extension of the “steady-state method” to cyclic loadings. The steady-state method allows a direct computation (on a fixed mesh, without releasing nodes) of stress and strain fields around the crack tip and in the wake for a steady crack growth. The method is extended to simulate crack propagation under fatigue loading. Therefore it constitutes a valuable numerical tool for gaining insight into the physics of crack propagation, as it provides accurate mechanical fields around the crack tip and their relation with crack growth rate, various loading modes and parameters. The proposed method is also compared with the classical node-release technique. A very good agreement between the two methods is found. However the steady-state method needs much less mesh refinement and computational time. Following an analysis of some features of the fatigue crack, a discussion on a crack closure criterion is opened, and a reliable criterion for the determination of local crack closure is proposed.     

Analysis for the optimal location of cable damping systems on stayed bridges

Computational models are increasingly being used for the dynamic analysis of structures with nonlinear or uncertain behavior, such as cables in stayed bridges, which nowadays are progressively more used as an alternative for long span and slim structures. In this work, a 3D nonlinear model is described to evaluate the wind dynamic effects on cables for this type of bridges under different scenarios, but also for health monitoring and structural simulation to guarantee performance, evaluate load capacity and estimate life prediction. Fatigue is one of the most relevant and complex failure causes in highway bridges, particularly on the anchorage elements of the cables in stayed bridges; where dampers may be used to minimize the dynamic behavior of the structure and reduce fatigue damage. With this nonlinear simulation model, different damper locations and configurations are evaluated to find the optimal position. A feasibility function is used as a weighting function to take into account the damper’s size and design. Analysis is particularly focused for a real cable stayed bridge in the state of Veracruz in México. Although the geometry, the forces and the stresses on cable structures are a challenge, even for structural specialists, the results from this work using the proposed 3D nonlinear model showed to be accurate for the simulation of many different wind scenarios, and damper’s location and orientations. Finally, the feasibility weighting function enabled the geometrical limitations to estimate the best location of a damper system to minimize the risk for fatigue failure.     

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.     

疲劳可靠性二维概率MINER准则

给出在裂纹起始阶段疲劳损伤演化规律的四个基本假设，即单调增、无耦合、可分高、不相交，由损伤力学原理得到等幅加载下个体Ｄ－Ｓ＿ａ－Ｓ＿ｍ－Ｎ等损伤曲面，推导出二维个体Ｍｉｎｅｒ准则，进而建立了母体在变幅以及随机时间历程加载下的二维概率Ｍｉｎｅｒ准则（Ｔｗｏ－ｄｉｍｅｎｓｉｏｎａｌＰｒｏｂａｂｉｌｉｓｔｉｃＭｉｎｅｒ’ｓＲｕｌｅ，即ＴＰＭＩＮＥＲ），并用测度论给予了证明．试验数据验证结果显示ＴＰＭＩＮＥＲ的预测值与试验值吻合良好，对工程结构疲劳可靠性分析有较好的应用价值．     

随机车辆冲击作用下简支梁桥疲劳可靠度评估

为了研究考虑随机车辆冲击效应的简支梁桥疲劳安全水平,提出了基于车-桥耦合振动与随机车流的桥梁疲劳应力谱模拟方法,并应用于疲劳可靠度评估。基于某高速公路桥梁动态称重数据建立随机车流模型,采用小样本车辆数据拟合桥梁等效疲劳应力范围的插值响应函数,最后由高斯混合模型拟合大样本随机车流作用于桥梁构件的疲劳应力谱。分析了25 m标准跨径简支T梁桥底部普通钢筋的疲劳应力谱,评估了考虑路面劣化与交通量增长的桥梁疲劳可靠度。数值分析结果表明,基于随机车流模拟的疲劳应力谱具有典型的多峰分布特征,包含了超载车辆产生的疲劳应力;车辆对桥梁的冲击效应致使等效疲劳应力放大系数略大于冲击系数,当路面等级为一般时,采用规范冲击系数将低估车辆冲击效应的疲劳损伤;路面劣化与交通量增长均会导致桥梁运营期内的疲劳可靠指标显著降低,由路面劣化导致车辆对简支梁桥的冲击效应不可忽略。     

基于顺序雨流法和临界平面法的多轴变幅低周疲劳寿命评价

 在多轴变幅疲劳寿命预测过程中,合适的雨流计数法对复杂加载历程分析非常重要,但是大多数雨流计数过程往往无法保持原始的加载顺序特性,进而会导致非保守的疲劳损伤和寿命预测．本文提出一种考虑加载顺序效应并基于临界面概念的多轴实时顺序雨流法,该方法既具有实时顺序计数特点,同时与Bannantine-Socie多轴雨流法结合,可以实现对主要通道内的载荷历程实时的雨流计数．基于Morrow 模型,提出一种新的考虑加载顺序的线性损伤累积方法．相对于传统雨流计数法需要得到完整的载荷数据后才能进行分析的特点,新方法计算效率更高,实用性更强．通过对316L 不锈钢的多轴疲劳试验数据的分析,验证了该方法在多轴疲劳寿命预测过程中的有效性．     

Scaling of size/time/temperature — Part 2: Progressive damage in uniaxial compressive specimen

This work is concerned with predicting the fatigue failure initiation of a wing/fuselage bolt assembly. Accounted for in the analysis are both the influence of energy dissipation and damage accumulation as the structure is subjected to repeated cyclic loading. Results involving the location and number of cycles to initiate a fatigue crack 10⁻² in. are obtained. They agreed both qualitatively and quantitatively with the experimental findings. Also discussed is the influence of pre-torque in the bolt which tends to decrease the number of cycles to fatique crack initiation. Fatigue life may be extended by altering the load path so as to decrease the accumulation of energy near the site of failure initiation. This can be accomplished without major modification of the design. The methodology that makes use of the strain energy density criterion can be used to optimize the fatigue strength of other structural sub-assemblies by appropriate combination of material and geometry for specified load conditions.     

A micromechanical explanation of the mean stress effect in high cycle fatigue

A micro–macro approach of multiaxial fatigue in unlimited endurance is presented in this study, as an extension of a previous model recently proposed by the authors [Monchiet, V., Charkaluk, E., Kondo, D., 2006. A plasticity–damage based micromechanical modelling in high cycle fatigue. C.R. Mécanique 334 (2), 129–136]. It allows to take into account coupling between polycrystalline plasticity and damage mechanisms which occur at the scale of persistent slip bands (PSB) during cyclic deformation. The plasticity–damage coupled model is obtained by adapting the Gurson [Gurson, A.L., 1977. Continuum theory of ductile rupture by void nucleation and growth: part I – yield criteria and flow rules for porous ductile media. J. Eng. Mater. Technol. 99, 2–15] limit analysis to polycrystalline materials to take into account microvoids growth along PSBs. The macroscopic fatigue criterion corresponds to microcracks nucleation at the PSB–matrix interface. It is shown that this criterion accounts for the effect of the mean stress and of the hydrostatic pressure in high cycle fatigue. Such features of HCF are related to the damage micro-mechanisms. Finally, some illustrations concerning the particular case of cyclic affine loadings are presented and comparisons of the predictions of the fatigue criterion with experimental data show the relevance of this new approach.     

A continuum damage model for the high-cycle fatigue life prediction of styrene-butadiene rubber under multiaxial loading

In the present contribution, the relationship between the fatigue life of styrene-butadiene rubber (SBR) and the stretch amplitude was established. Focusing on the multiaxial loading effect on the life duration of SBR, experimental tests were conducted using cylindrical specimens subjected to tension and torsion loadings under constant and variable amplitudes. Based upon the continuum damage mechanics approach, a three-dimensional model was derived and coupled with the cracking energy density criterion to predict the fatigue life of SBR. The capabilities of the model, which requires only three damage parameters to be identified, were analysed and a good agreement between predicted values and experimental data were clearly highlighted for tension and torsion loadings both in constant and variable amplitudes.     

多轴随机载荷下的疲劳寿命估算方法

现代工业的发展使得更多的构件承受着复杂的载荷形式, 将单轴疲劳模型应用到多轴载荷情况已不能满足现代工业的设计要求, 多轴随机载荷下的疲劳寿命计算日益引起人们的重视. 多轴随机载荷的寿命预测中, 如何计算载荷循环次数是其基础,目前广泛使用的是雨流计数方法, 现在已能成功的应用于多轴载荷的情况. 累积的疲劳损伤分析在各种构件和结构的载荷历史中都起着重要的作用. 自从线性损伤律提出以来已发展了数十种损伤律, 变幅载荷引起的疲劳损伤可以由许多不同的累积损伤律来计算, 虽然发展了许多损伤模型, 由于问题的复杂性, 每个模型的应用范围也是随具体情况而定. 线性损伤律方法尽管有很多不足之处, 但在设计使用中仍占有重要的位置. 两载荷水平及模式下的损伤累积以及损伤与物理机制的关系在本文中也做了介绍. 针对近年来提出的描述多轴随机载荷下疲劳寿命估算方法进行了详细的评述, 对各模型的应用范围和预测能力进行了讨论, 并对今后的工作提出了建议.      

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。     

A new distortion energy-based equivalent stress for multiaxial fatigue life prediction

A new equivalent stress amplitude expression has been developed for the assessment of fatigue life in components under multiaxial loading. The expression was generated by incorporating non-linear/plastic stress–strain relation into a mechanical energy calculation, and then applying the calculation to the distortion energy theory for a cyclic loading case. Therefore, the new uniaxial equivalent stress expression determines an appropriate stress amplitude value for multiaxial cyclic loading. The purpose of the equivalent stress value is to determine multiaxial fatigue failure using an energy-based fatigue life prediction criterion. The governing understanding behind the criterion states that the physical damage quantity for failure is equal to the accumulated strain energy in a monotonic fracture, which is also equal to the accumulated strain energy during fatigue failure. Using the new equivalent stress amplitude expression and the energy-based life prediction method, a comparison is made between prediction results and multiaxial empirical data. The multiaxial data was acquired by a vibration-based biaxial bending fatigue test and a torsion fatigue test with an assumed axial misalignment. The results of the comparison provide encouragement regarding the capability of the newly developed equivalent stress amplitude expression for fatigue life prediction.