FATIGUE LIFE PREDICTION OF RC BEAMS STRENGTHENED WITH PRESTRESSED CFRP UNDER CYCLIC BENDING LOADS

The application of prestressed carbon reinforced polymer(prestressed CFRP)in reinforced concrete(RC)members can improve the mechanical properties of strengthened structures and strengthening efficiency.This paper proposed a semi-empirical prediction formula of fatigue lives of the RC beams strengthened with prestressed CFRP under bending loads.The formula is established based on the fatigue life prediction method of RC beams and fatigue experimental data of non-prestressed CFRP reinforced beams done before.Fatigue effect coefficient of the formula was confirmed by the fatigue experiments of the RC beams strengthened with prestressed carbon fiber laminate(prestressed CFL)under cyclic bending loads.Fatigue lives of the strengthened beams predicted using the formula agreed well with the experimental data.     

FATIGUE LIFE PREDICTION OF SUS 630 （H900） STEEL UNDER HIGH CYCLE LOADING

The fatigue life prediction of high strength steel SUS 630 （H900） under high cycle loading is conducted with consideration of a characteristic fatigue length of material. Based on the WShler curve of smooth materials, a modified method for fatigue life prediction is approached. The characteristic fatigue length of material under cyclic loading is associated with the polycrystalline material. Rather than the stress at a point, the average stress within the characteristic fatigue length is implemented for the fatigue life prediction. The method can be applied to both the smooth and the defected material. The fatigue life prediction is also verified experimentally by specimens with various small circular holes. Through the comparison, it is found that the method can be adopted to predict the fatigue lives with different size effects.     

A criterion for high-cycle fatigue life and fatigue limit prediction in biaxial loading conditions

This paper presents a criterion for high-cycle fatigue life and fatigue strength estimation under periodic proportional and non-proportional cyclic loading. The criterion is based on the mean and maximum values of the second invariant of the stress deviator. Important elements of the criterion are: function of the non-proportionality of fatigue loading and the materials parameter that expresses the materials sensitivity to non-proportional loading. The methods for the materials parameters determination uses three S–N curves: tension–compression, torsion, and any non-proportional loading proposed. The criterion has been verified using experimental data, and the results are included in the paper. These results should be considered as promising. The paper also includes a proposal for multiaxial fatigue models classification due to the approach for the nonproportionality of loading.     

EFFECT OF LOADING TYPE ON FATIGUE CRACK GROWTH BEHAVIOR OF SOLDER JOINT IN ELECTRONIC PACKAGING

Fatigue cracking tests of a solder joint were carried out using in-situ scanning electron microscopy （SEM） technology under tensile and bending cyclic loadings. The method for predicting the fatigue life is provided based on the fatigue crack growth rate of the solder joint. The results show that the effect of the loading type on the fatigue crack growth behavior of a solder joint cannot be ignored. In addition, the finite element analysis results help quantitatively estimate the response relationship between solder joint structures. The fatigue crack initiation life of a solder joint is in good agreement with the fatigue life （N50%） of a totally electronic board with 36 solder joints.     

Fatigue reliability based on residual strength model with hybrid uncertain parameters

The aim of this paper is to evaluate the fatigue reliability with hybrid uncertain parameters based on a residual strength model. By solving the non-probabilistic setbased reliability problem and analyzing the reliability with randomness, the fatigue reliability with hybrid parameters can be obtained. The presented hybrid model can adequately consider all uncertainties affecting the fatigue reliability with hybrid uncertain parameters. A comparison among the presented hybrid model, non-probabilistic set-theoretic model and the conventional random model is made through two typical numerical examples. The results show that the presented hybrid model, which can ensure structural security, is effective and practical.     

ADVANCES IN TRANSFORMATION RATCHETING AND RATCHETING-FATIGUE INTERACTION OF NITI SHAPE MEMORY ALLOY

The accumulation of inelastic deformation occurring in NiTi shape memory alloy under the stress-controlled cyclic loading condition is named transformation ratcheting, since it is mainly caused by the solid-solid transformation from austenite to martensite phase and vice versa. The transformation ratcheting and its effect on the fatigue life (i.e., transformation-fatigue interaction) are key issues that should be addressed in order to assess the fatigue of NiTi shape memory alloy more accurately. In this paper, the advances in the studies on the transformation ratcheting and ratcheting-fatigue interaction of super-elastic NiTi shape memory alloy in recent years are reviewed: First, experimental observation of the uniaxial transformation ratcheting and ratcheting-fatigue interaction of super-elastic NiTi alloy under the stress-controlled cyclic loading conditions is treated, and the detrimental effect of transformation ratcheting on the fatigue life is addressed; Secondly, two types of cyclic constitutive models (i.e., a macroscopic phenomenological model and a micromechanical one based on crystal plasticity) constructed to describe the transformation ratcheting of super-elastic NiTi alloy are discussed; Furthermore, an energy-based failure model is provided and dealt with by comparing its predicted fatigue lives with experimental ones; Finally, some suggestions about future work are made.     

STOCHASTIC PROPAGATION OF SEMI-ELLIPTICAL SURFACE CRACK

The semi-elliptical surface crack growth of structural components withuncertain material resistance under random loading is studied by using the stochasticaveraging principle.The FPK equation governing the transition probability densityfunction of crack lengths is derived.The analytical solution of the FPK equation forthe case of that the equations for the crack growth in the surface and depth directionsare uncoupled is obtained.The effects of the parameters of the stress process and of thematerial property on the behavior of semi-elliptical fatigue crack growth of thecomponents with deterministic resistance to crack growth in the stationary Gaussianstress process are examined.The comparison of the analytical result with digitalsimulation shows the effectiveness of the present method.     

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.     

INVESTIGATION OF FRACTURE DESIGN FOR MEDIUM CARBON STEEL UNDER EXTRA-LOW CYCLIC FATIGUE IN AXIAL LOADING

The extra-low cyclic fracture problem of medium carbon steel under axial fatigue loading was investigated. Several problems, such as the relations of the cycle times to the depth and tip radius of the notch, loading frequency, loading range and the parameters of fracture design for medium carbon steel on condition of extra-low axial fatigue loading were discussed based on the experiments. Experimental results indicated that the tension-pressure fatigue loading mode was suitable for extra-low cyclic fatigue fracture design of medium carbon steel and it resulted in low energy consumption, fracture surface with high quality, low cycle times, and high efficiency. The appropriate parameters were as follows： loading frequency 3-5 Hz, notch tip radius r = （0.2-0.3） mm, opening angle α = 60°, and notch depth t = （0.14-0.17）D.     

Bi-variable damage model for fatigue life prediction of metal components

Based on the theory of continuum damage mechanics,a bi-variable damage mechanics model is developed,which,according to thermodynamics,is accessible to derivation of damage driving force,damage evolution equation and damage evolution criteria. Furthermore,damage evolution equations of time rate are established by the generalized Drucker’s postulate. The damage evolution equation of cycle rate is obtained by integrating the time damage evolution equations,and the fatigue life prediction method for smooth specimens under repeated loading with constant strain amplitude is constructed. Likewise,for notched specimens under the repeated loading with constant strain amplitude,the fatigue life prediction method is obtained on the ground of the theory of conservative integral in damage mechanics. Thus,the material parameters in the damage evolution equation can be obtained by reference to the fatigue test results of standard specimens with stress concentration factor equal to 1,2 and 3.     

循环载荷下碳纤维薄板增强RC梁的疲劳性能试验研究

以碳纤维薄板(CFL)增强RC梁为研究对象,通过对循环载荷作用下增强梁的三点弯曲疲劳试验研究,得到了增强梁的线性对数疲劳寿命曲线和跨中挠度的演化规律,外推得到了极限疲劳强度和抗弯刚度的演化规律,揭示了增强梁的疲劳破坏机理。循环载荷下CFL增强RC梁的破坏模式包括混凝土开裂、碳纤维薄板与混凝土界面剥离、主筋屈服等模式,疲劳破坏过程具有明显的损伤成核、稳定扩展、失稳扩展三阶段发展规律。与普通RC梁相比较,CFL增强RC梁的裂缝分布较均匀、密集,粘贴CFL对增强梁的初始抗弯刚度提高幅度较小,对疲劳损伤阶段的抗弯刚度则提高了约一倍。根据CFL增强RC梁的疲劳寿命曲线得到其极限疲劳强度为25.42 kN,为三点弯曲静载下极限承载力的58.5%。     

随机载荷下碳纤维薄板增强RC梁试验研究

桥梁等钢筋混凝土结构在运营期所受活载为随机载荷。研究随机载荷作用下碳纤维薄板(Carbon Fiber Laminate简称CFL)片材增强钢筋混凝土构件的疲劳性能,对于采用碳纤维薄板技术加固桥梁等混凝土结构有重要的指导意义。本文通过随机载荷作用下碳纤维薄板增强RC梁的三点弯曲疲劳试验,得到了增强梁的S-N曲线和跨中挠度的演化规律,揭示了随机载荷下增强梁的疲劳破坏机理。随机载荷下碳纤维薄板增强RC梁的破坏模式包括钢筋断裂、碳纤维薄板剥离、混凝土压坏等破坏形态,疲劳破坏过程具有明显的损伤成核、稳定扩展、失稳扩展的三阶段发展规律。     

温度-随机载荷下CFL加固RC梁疲劳性能的实验研究

考虑亚热带地区公路桥梁的服役环境温度与车辆载荷的作用效应,本文以碳纤维薄板(CFL)加固钢筋混凝土(RC)桥梁结构为研究对象,利用本课题组构建的实验平台,提出了温度与车辆随机载荷耦合作用下CFL加固RC梁的疲劳实验方法。在3个温度和3个载荷水平下实施了温度-随机载荷耦合作用下的三点弯曲疲劳实验,初步探讨了温度-随机载荷作用下加固梁的疲劳破坏机理,并提出了温度-随机载荷耦合下加固梁疲劳寿命的半经验公式。     

FRP片材在土建修复加固工程中应用的力学问题

纤维增强复合材料(FRP)片材由于其优异的力学性能和施工的便捷性而备受国内外土木建筑领域的高度关注,并得到了迅速的推广应用.本文结合国内外的研究现状及本课题组的研究工作,对FRP片材(纤维布、纤维板和纤维薄板)在土木建筑修复加固工程中应用的主要力学问题-FRP片材加固钢筋混凝土(RC)构件的静力学性能、疲劳性能、界面的力学性能、以及构件的耐久性等进行综述和讨论,并试图指出今后有关FRP片材修复加固RC构件力学性能的研究方向.     

先张预应力CFL加固RC梁预应力损失的实验研究

由于预应力碳纤维增强复合材料(CFRP)加固钢筋混凝土(RC)结构技术能够充分发挥其优异的力学性能而备受关注。本文以预应力碳纤维薄板(CFL)加固RC桥梁结构为研究对象,研制了采用先张法对CFL施加预应力的实验装置,提出了先张预应力CFL加固RC梁的实验方法及其预应力损失的监测方法,并在3个预应力水平下对RC梁实施了预应力CFL加固,测试和分析了CFL的预应力损失演化规律。研究结果表明,本文提出的先张预应力CFL加固RC梁以及预应力损失测试的方法是可行和有效的。     

一种FRP累积损伤模型及其在结构疲劳寿命估算中的应用

推荐了一种应变损伤累积模型，能够考虑单向板面内多轴应力和平均应力的影响。只需要单向板在确定应力比下的若干典型疲劳试验结果，就可以预测相同材料体系多向层压结构在不同应力比的循环载荷下的疲劳寿命，有助于降低试验成本和工作量。研究了适用于多向层压结构剩余强度估算和疲劳寿命预测的步骤和程序。针对碳纤维／树脂基T300／QY8911复合材料，试验测定了三组典型单轴循环应力（[0]16拉－拉、[90]16拉－拉和[0/90]4S剪－剪）下的S－N曲线。以此为输入，预测四种多向铺层板在各种拉－拉循环应力下的疲劳寿命，寿命预测结果和相应的试验结果吻合良好。采用了保持计算和试验的载荷／强度比相对等值的方法来近似抵消层合效应对疲劳寿命的影响。强调了进一步发展能够定量估计层间应力影响与分层扩展过程的疲劳损伤模型的重要性。     

随机载荷下疲劳裂纹的闭合和扩展估算模型

本文分析了裂尖前,后方塑性区对闭合性能的影响,提出从裂尖塑性钝化量和尾迹区残余塑性变形两个方面来确定裂纹面的残余变形,并讨论了压缩载荷对闭合应力的影响,由此建立了一个疲劳裂纹闭合模型,然后通过模型“有限记忆”性质的假定将它应用到随机加载情况。用此模型对铝合金2219——T851受飞机谱载荷的CCT平面应力试件进行了疲劳寿命估算,估算值与实验结果接近。     

基于损伤分布模型的震损RC框架力学性能分析与实证研究

震损结构的剩余荷载位移响应分析十分重要，是准确评估震后钢筋混凝土(RC)框架结构剩余性能的基础，然而震损结构损伤的不均匀分布为其荷载位移响应的定量分析带来了困难。本文基于纤维梁柱模型提出了一种能够考虑地震损伤不均匀分布的震损RC框架数值模型。基于震损RC框架的损伤现象对其损伤分布进行了量化，考虑了材料损伤沿柱高度与截面深度的分布；基于量化结果分别确定了RC框架中各纤维梁柱单元中不同位置处纤维的损伤本构关系，并对其剩余荷载位移响应进行了定量分析；以一个一榀三层四跨的RC框架试验为算例，对模型进行了验证。结果表明，本文模型能够较为准确地模拟震损RC框架的剩余荷载位移响应，其中最大剩余承载力、刚度和屈服强度等各项指标计算误差均值均在10%以下，因此本文模型能够为震后RC结构的评估工作提供一种新的思路。     

随机载荷作用下结构疲劳损伤的累积

本文提出了一种计算在随机载荷作用下具有随机疲劳强度的结构的累积疲劳损伤与疲劳寿命的概率密度函数及可靠性函数的方法.对在平稳窄带高斯随机应力作用下的金属与钢筋混凝土构件,给出了上述函数的解析表达式.数例计算表明,理论结果与数字模拟结果颇为吻合.