不同程度损伤的CFRP加固钢板的疲劳性能研究

为进一步探讨碳纤维加固复合材料(CFRP)板对钢结构疲劳寿命的延长效果,进行了试验研究和理论分析。利用不同长度的人工裂缝代表不同程度的疲劳损伤。试验结果表明:采用CFRP修复能够有效减缓裂纹扩展并延长疲劳寿命。提出用于预测试件疲劳寿命的理论模型。随后,对其进行了参数分析,研究不同损伤程度下钢板的疲劳性能。深化了对在裂纹扩展不同阶段采用CFRP进行修复的理解,并针对这种加固方法提出了一些有益的建议。     

钢筋蚀坑对其疲劳断裂特性的影响

基于降载勾线法,对人工钻孔模拟蚀坑的3根HPB300钢筋试件开展了疲劳裂纹扩展试验,并结合FRANC3D软件对钢筋疲劳裂纹扩展规律进行了数值分析.在此基础上,以不同应力幅值对12种尺寸的半椭圆形蚀坑钢筋模型的裂纹扩展过程进行了数值分析.结果表明:钢筋裂纹扩展速率前期缓慢增长,后期迅速增大;初始裂纹深度较大的钢筋疲劳寿命明显缩短;初始裂纹深宽比越大的钢筋疲劳寿命越短;应力幅值增大会使缺陷钢筋疲劳寿命缩短,并且初始裂纹深宽比越大的钢筋,其后期裂纹扩展速率增长越快,疲劳寿命缩短越明显.     

频率效应对Q345钢疲劳行为的影响

本文对结构用钢Q345的低周、超低周以及高频试验疲劳行为进行了研究。实验在常温下岛津电液伺服疲劳试验机和QBG-100高频疲劳试验机上进行。低周疲劳试验采用轴向应变控制方法,恒定应变速率为0.005 s~(-1),应变比为-1。超低周疲劳试验采用加载频率控制。高频试验通过增减砝码质量(激励质量)的数目来改变系统的谐振频率,应用共振原理测试高频下试件的载荷-周期(S-N)试验数据。实验结果表明,在同一应力水平下,加载频率更高的高频试验条件下的试件寿命总体上比低频率的低周疲劳试验试件的寿命长。超低周疲劳试验中,随着加载应力的提高,试件寿命曲线逐渐下降,通过频率-寿命(f-N)图可以看出,随着加载频率的增加,试件寿命有所提高。通过SEM扫描电镜对三种试件断口进行观察,可以发现低周及超低周疲劳试验试件的裂纹多从表面起源,高频试验既有表面起源的裂纹也有从内部萌生的裂纹,破坏形式主要取决于加载应力和材料性质,与加载频率无太大关系。     

海洋石油钢结构的疲劳寿命问题

本文归结了几种疲劳寿命的估算方法与模式,论述了估算疲劳寿命过程中不确定性的解决方法,和运用概率断裂力学方法解决的几个问题(如裂纹扩展试验、疲劳寿命的分布规律及其失效概率)以及考虑模糊性在内的模糊疲劳可靠度等高难课题。     

预切口沥青混合料小梁疲劳试验

沥青路面在初始裂纹形成后的裂纹扩展规律对于沥青路面的设计和养护具有重要的理论与实践意义。本试验采用疲劳荷载加载模式,进行预切口小梁疲劳断裂试验,其结果显示为,对于I-II复合型裂纹的扩展初期表现出复合裂纹扩展特性,随着裂纹的继续扩展,表现出I型裂纹特征,随着预切口到加载中心距离的增大,沥青混合料小梁的疲劳寿命增大;对于连续级配沥青混合料,最大粒径越大,小梁的疲劳寿命越长。     

高弹模CFRP粘贴法修复钢桥板疲劳裂纹

针对钢桥中钢板疲劳裂纹进行了一系列疲劳裂纹修复试验研究,以确定不同修复方法下结构的疲劳性能。高弹模CFRP（碳纤维增强复合材料）修复法是通过粘贴高弹模CFRP布来增大结构刚度,以延长结构疲劳寿命,且碳纤维材料基本不会产生疲劳裂纹,具有较好的抗疲劳效果。采用数值模拟和试验研究相结合的方法,首先对修复钢板的应力分布和特点进行了详细的分析,然后通过3组对比疲劳试验,确定了不同修复方法对损伤钢板疲劳寿命的增强效果。最后利用修复试验值与各国规范规定的S-N曲线进行比较,分析已修复钢板的疲劳性能。研究结果表明：在止裂孔上覆盖粘贴高弹模CFRP布可有效减小孔边应力集中,具有延长结果疲劳寿命的效果,修复后的裂纹钢板的疲劳寿命是仅用止裂孔修复的裂纹钢板疲劳寿命的8倍以上。仅用止裂孔法修复损伤钢板不足以满足各国规范规定的疲劳强度要求,而在止裂孔法上进一步采用高弹模CFRP布修复后的损伤钢板的抗疲劳性能更优,其疲劳强度等级高于美国规范AASHTO中的A类细节等级。     

高速铁路隧道基底软岩动力累积损伤特性

利用相似材料模拟高速铁路隧道基底软岩,采用荷载控制和非对称正弦波循环加载方式对软岩试件进行动三轴疲劳与损伤检测综合试验。通过试验结果分析,建立了软岩3参数多项式疲劳寿命计算模型和4参数多项式累积损伤参量计算模型,获得了软岩疲劳损伤特性,即:软岩疲劳破坏表现为端部拉-剪复合破坏和中部压-剪复合破坏两种模式;疲劳破坏全过程表现为初始微孔隙压密、裂纹发生与稳定扩展以及损伤裂纹加速发展3个发展阶段;软岩疲劳寿命主要取决于本身强度和动应力水平,强度愈高、动力应力水平愈低,其疲劳寿命就越长;当动应力水平相同时,软岩疲劳寿命与其弹性模量呈线性增长关系。     

采用声发射技术预测钢桥中的疲劳裂纹发展

声发射信号与裂纹发展间的关系对正常工作状态下桥梁的健康检测非常重要。以正常工作状态下的工况为例,对钢桥进行试验研究,并建立声发射绝对能量率和裂纹发展率间的关系。由于在实际桥梁中不好确定应力集中区域,而本方法又与应力集中区域无关,故能简化寿命预测过程。采用ASTM A572G50碳钢制作紧凑拉伸试件,进行疲劳试验,并从疲劳裂纹中侦测声发射信号。通过联合Swansong II过滤器和波形研究,过滤由噪声引起的声发射信号。波形研究适用于过滤数据和解释现场试验。基于试验数据和本模型,预测裂纹发展和疲劳寿命,与实际裂纹吻合,这也验证了本模型的正确性。研究表明:在疲劳裂纹预测中,采用声发射完全能量率比采用计数率更经济。     

CFRP加固含疲劳裂纹钢板的有限元参数分析

CFRP加固含疲劳裂纹钢板可显著提高其疲劳寿命。采用有限元分析方法,详细研究了CFRP长度、宽度、厚度、弹性模量,胶层的剪切模量和厚度等各种参数对加固效果的影响。结果表明:提高CFRP的弹性模量、厚度,采用适当的胶层剪切模量、厚度,对提高加固效果显著。根据有限元计算结果,归纳得出应力强度因子幅的计算经验公式,该经验公式与有限元计算结果和试验结果符合较好,可用于计算CFRP加固含疲劳裂纹钢板疲劳寿命的计算。     

基于OpenSEES数值模拟的工字形截面支撑低周疲劳性能研究

为研究工字形截面支撑低周疲劳性能,进行了18根Q235和21根ST12材质焊接工字形钢支撑在等幅轴向循环位移作用下的OpenSEES有限元分析,在验证模拟结果和试验结果吻合较好的同时,提出利用支撑几何参数(长细比、宽厚比、高厚比、屈服点)修正的表征应变作为预测支撑低周疲劳寿命的控制参量,给出了对应修正公式和寿命预测公式。表征应变与支撑低周疲劳寿命满足对数线性关系,据此预测的支撑低周疲劳寿命离散性较小且基本位于试验值1.5倍分散带内。对于变幅加载条件下的支撑寿命预测,利用Miner线性损伤理论对39组支撑试验数据进行以裂纹萌生寿命为终止点的损伤值统计,给出了容许损伤建议值作为预测寿命对应损伤值。而后,引入5组单斜工字钢支撑框架低周疲劳试验结果进行验证,其预测寿命均接近试验裂纹萌生寿命,可以较为保守地评估该类支撑的低周疲劳寿命,且支撑截面损伤分布与试验破坏模式保持一致。以上均证明该数值模拟方法和低周疲劳寿命预测方法的准确性和高效性,可为框架支撑结构低周疲劳性能的研究提供借鉴。     

Fatigue and fracture simulation of welded bridge details through a bi-linear crack growth law

This paper deals with the application of a probabilistic fracture mechanics approach to predict the fatigue life of welded steel details in the presence of cracks under bridge spectrum loading. It is based on a recently proposed bi-linear relationship to model fatigue crack growth and incorporates a failure criterion to describe the interaction between fracture and plastic collapse. The formulation leading to the expected number of cycles to failure is first outlined, followed by a simple example on a butt-welded detail. Uncertainty modelling, especially on fatigue crack growth parameters, is undertaken with the aid of recently published data in support of the bi-linear crack growth relationship. Results pertaining to fatigue reliability and fatigue crack size evolution are presented using simulation with Latin Hypercube Sampling, and emphasis is placed on a comparison between linear and bi-linear crack growth models. The latter is found to lead to higher fatigue life estimates and significantly different crack size distributions, both of which have implications on inspection schemes for steel bridge components.     

Prediction of fatigue crack growth in steel bridge components using acoustic emission

The correlation of acoustic emission (AE) signal characteristics with crack growth behavior is of paramount importance to structural health monitoring and prognosis for in-service steel bridges. Relationships between AE absolute energy rate and crack growth rate are developed and presented. The relationships are based on experimental investigations intended to represent conditions found for in-service steel bridges. The approach presented is independent of the stress intensity range, which may simplify the life prediction procedure because stress intensity range is not always well defined in actual bridge components. Fatigue tests were performed to detect AE signals from fatigue cracks using compact tension (CT) specimens made of ASTM A572G50. Noise induced AE signals were filtered through a combined approach involving Swansong II Filters and investigation of waveforms, which are appropriate for data filtering and interpretation of field tests. Based on the experimental data and presented model, procedures for predicting crack extension and remaining fatigue life were carried out. Agreement between the predicted cracks and actual cracks verified the presented model and procedure. The study indicates that AE absolute energy rate may be more suitable than count rate in fatigue life prediction for the material of interest.     

Stochastic fatigue crack growth in steel structures subject to random loading

Uncertainty in fatigue crack growth under service load conditions arises from the statistical characteristics of crack growth under constant amplitude loading and from random variable amplitude loading. This study generalizes previous stochastic fatigue crack growth models by incorporating a time-dependent noise term described by arbitrary marginal distributions and autocorrelations to model the uncertainty in the crack growth under constant amplitude loading. A computationally efficient approach for handling wide-band random loadings based on the rainflow method of stress cycle identification also is developed. The method is illustrated with a fatigue reliability analysis of a steel miter gate at a lock and dam facility operated by the US Army Corps of Engineers.     

Fatigue analysis of a pre-fabricated orthotropic steel deck for light-weight vehicles

An orthotropic steel deck designed for carrying light-weight vehicles was investigated to assess its fatigue life. Fracture mechanics analysis was performed with consideration of pre-existing flaws at the fatigue-sensitive region. Three-level models were used to accurately evaluate the stress-intensity factors at the rib-to-diaphragm connection. The crack growth was simulated by numerical integration of the Paris formula. In order to assess the severity of pre-existing flaws at the welded joint of the rib-to-diaphragm connection, the fatigue life of the deck was investigated with different sizes of initial crack. This approach provides a rational quality assessment method for the orthotropic steel deck.     

Fatigue life prediction of heavy mining equipment. Part 1: Fatigue load assessment and crack growth rate tests

Frequent fatigue failure of heavy mining equipment has created the need for fatigue life prediction of its cracked components. As the first step, field monitoring of a cable shovel boom that had a history of chronic fatigue cracking was conducted. A set of load spectra for the boom was constructed from the collected field data. A finite element model of the boom was prepared and calibrated against the test data to predict the stress and strain histories. Eight crack growth rate coupon tests were carried out to obtain the fatigue properties of the material used. Constant amplitude equivalent stress ranges were calculated based on Palmgren-Miner’s cumulative damage rule and crack growth rate tests results.     

A comparative study of fatigue inspection methods

Bridges are inspected, regularly or otherwise, for fatigue cracks using a variety of different methods. However, for all of these methods, inspection does not necessarily imply detection due to a number of factors including the inspector’s experience and the physically inherent limitations of the method. Consequently, traditional inspection methods do not have a limitless capacity for crack detection. As fatigue is a phenomenon involving crack growth over time, application of a particular method will have a time-dependent probability of detecting a crack. In this paper, crack growth, as this may be observed in a typical bridge fatigue detail, is quantified using fracture mechanics and the performance of four different inspection methods over time is compared in terms of their probability of detection. Although the results presented here are pertinent to the particular type of bridge detail and loading conditions, fracture mechanics may also be applied to a wide variety of different details in order to compare detection capabilities at different time instances.     

Prediction of the fatigue life of slender web plates using fracture mechanics concepts

The results of a series of fatigue tests, on slender plate girders subjected to repeated shear loading, are summarised and used to establish a lower bound fatigue strength curve for the welded web boundary, based on geometric or principal surface stress ranges. The propagation of a semi-elliptical surface crack, through the thickness of a plate, is studied using fracture mechanics concepts. An extensive parametric study indicates that the initial crack size and aspect ratio, the geometric stress concentration at the weld toe, and the plate thickness, all have a significant influence on fatigue strength. It is concluded that the fracture mechanics approach requires specification of a number of parameters which are difficult to determine in practice. However, having specified a realistic crack size and aspect ratio and an approximate stress concentration at the weld toe, the solution can be calibrated against available experimental fatigue strength curves.     

T形方管节点的疲劳裂纹扩展特性分析

在矩形焊接节点的支撑和弦的结合处通常都是应力集中区域,所以在这些地方疲劳裂纹也更容易产生和扩展。采用边界元素方法模拟T形焊接方管节点的三维裂纹。根据三维分析,基于Paris法则和应力强度等因素,可以构建模型来预测疲劳裂纹的发展。试验结果与预测的裂纹产生以及形状发展都是一致的。基于裂纹扩展的分析,可以预测构件的疲劳程度,通过对比标准的S-N曲线,发现标准S-N曲线是安全的,但略带保守。     

Stable and unstable fatigue prediction for A572 structural steel using acoustic emission

The association of acoustic emission (AE) signals with crack growth behavior in the material of interest is the basis for monitoring the fatigue damage of in-service steel structures with the AE method. A model including the absolute energy rate of AE, stress intensity, fracture toughness and load ratio is presented to predict crack extension and remaining fatigue life for stable and unstable crack stages. The model is based on the Forman equation, and the balance between AE signal energy and the energy released due to crack growth. Results from AE-monitored fatigue tests with load ratios of 0.02, 0.1 and 0.7 are utilized to validate the presented model. To separate AE signals associated with crack growth from noise, a combined approach involving pattern recognition and analysis of waveform features was employed. Prediction procedures are demonstrated based on the presented model and experimental data. Reasonable agreement exists between the observed and predicted test results. The presented model conservatively estimates fatigue damage and remaining fatigue life.