斜拉索风致动力疲劳损伤的研究

研究了斜拉索风致振动的疲劳损伤问题。首先,根据 Miner 线性累积损伤原理,给出了结构疲劳损伤的计算公式。其次,根据高周疲劳的特性,我们假设斜拉索的疲劳损伤符合 Basquin 疲劳估算模型。第三,在综合目前疲劳损伤既有研究的基础上,进一步提出评价不同风速和不同风攻角对斜拉索疲劳损伤影响的框架。最后,以松花江斜拉桥中的斜拉索为对象,基于动力时程法计算分析了在不同风速和不同风攻角情况下斜拉索上各节点的应力时程,再使用雨流计数法统计斜拉索的应力-应变循环历史,估算出在不同风速和不同风攻角下斜拉索的疲劳损伤。于是,定量地评价了在不同风速和不同风攻角对斜拉索疲劳损伤的影响。     

斜拉索面内参数振动的理论和试验研究

研究斜拉索在弦向位移激励下的面内非线性振动方程,该振动方程考虑拉索垂度、倾斜角、大位移、激励幅值、阻尼等影响因素,并应用龙格-库塔数值积分法求解该微分方程。在试验室建立了斜拉索模型,开展斜拉索受弦向位移激励的面内参数振动的试验研究,实现了斜拉索的参数振动。试验研究和数值计算表明斜拉索的参数振动与系统频率比、激励幅值、阻尼等因素有关,参数振动发生在一定的频率比范围内,斜拉索振幅与频率比关系曲线体现出非线性特性,系统阻尼使得斜拉索参数振动的激励幅值要超过一定的阀值。     

基于小波包分析的拉索损伤声发射信号特征提取

结合显式有限元和小波包分析技术开展了拉索损伤声发射信号特征提取的仿真分析。采用ANSYS/LS-DYNA模拟得到拉索损伤声发射信号的仿真信号,基于小波包能量谱对拉索声发射的有限元仿真信号进行了特征提取,从小波包分解层次、特征频带数量的选择及特征参数的噪声鲁棒性三个方面开展了讨论分析。结果表明:(1)通过选择适当的小波包分解层次,小波包能量谱可以精细地反映信号的特征;(2)选取少数特征频带就能使得小波包能量谱反映声发射信号的特征信息;(3)基于小波包能量谱的特征参数具有良好的损伤敏感性及噪声鲁棒性,能在强噪声影响下实现对拉索不同损伤类型的判别。     

基于最敏感设计参数的斜拉索有限元动力模型修正

基于较完善的斜拉索有限元模型,分析讨论了斜拉索张力、抗拉刚度、抗弯刚度以及线密度等不同设计参数对长索和短索计算频率的相对灵敏度,确定了最敏感设计参数.在此基础上,根据优化理论,对洞庭湖大桥拉索进行基于最敏感设计参数的动力模型修正,修正后的模型的动力特性更加趋近于实测值.修正后的斜拉索有限元模型可以作为拉索张力评估的基准.     

聚乙烯醇纤维混凝土损伤演化声发射监测及其评价

通过对掺入聚乙烯醇（PVA）纤维混凝土试件进行三点抗弯的声发射试验,来研究它的损伤失效机理。首先,通过三点弯曲试验,得到了PVA纤维腐蚀前后混凝土的抗弯强度。然后,基于加载全过程中声发射特征信号,根据声发射幅值、计数和能量相关图,并通过比较有无腐蚀PVA纤维混凝土声发射信号,揭示了PVA纤维腐蚀混凝土的抗弯强度下降原因及其损伤机理。 最后,借鉴NDIS-2421定量评定标准分析声发射信号,对混凝土试件的损伤情况做出定性评价,分析PVA纤维在腐蚀前后混凝土中的作用;此外,通过计算声发射特征信号裕度指标,发现PVA纤维混凝土弯曲损伤明显分为三个阶段,未腐蚀PVA纤维混凝土试件的裕度值较均匀。     

覆冰斜拉索驰振稳定性的理论研究

覆冰斜拉索可能会发生驰振振动并影响到斜拉桥的安全。在进行风洞试验对6种覆冰拉索模型测力的基础上,推导了覆冰拉索1阶模态驰振的运动微分方程,并采用龙格-库塔法进行求解,得到了拉索的驰振响应规律。以海南洋浦大桥为工程背景,研究了中跨最长拉索M18的驰振临界风速,研究了覆冰长度、拉索阻尼比等对驰振临界风速的影响。结果表明,在新月形、扇形和D形覆冰条件下,中跨最长拉索M18的驰振临界风速在10-15m/s范围内,远小于基本风速,存在发生驰振可能性;随着阻尼比的增加、覆冰长度的变短以及拉索长度的变短,驰振临界风速增加。     

斜拉索杠杆质量减振器理论及模型试验研究

为解决斜拉桥跨度增大带来的拉索振动问题,采用杠杆质量减振器(LMD)对斜拉索进行减振。介绍了LMD的构造特性及作用机理,建立了斜拉索-LMD系统振动方程,推导了LMD等效阻尼力模型及LMD与斜拉索各阶模态质量比,并对斜拉索前15阶模态对数衰减率进行了理论计算及模型试验研究。研究结果表明：LMD由于惯性项、刚度项及阻尼项的三重反馈作用,同时由于杠杆的调节放大作用,其减振效果优于相同安装高度的粘性剪切型阻尼器(VSD)。LMD对桥梁景观影响较小、安装维护方便,可为千米级的斜拉桥拉索减振提供更可靠的解决方案。     

O型套环对斜拉索涡激振动影响的试验研究EI北大核心CSCD

斜拉索的涡激振动起振风速低,发生频繁,可能造成斜拉索的疲劳破坏,因此,斜拉索涡激振动的控制措施研究和设计十分重要。根据斜拉索涡激振动机理,提出O型套环这种气动措施来控制涡激振动,通过风洞试验,比较了标准斜拉索和套环斜拉索的涡激振动响应,研究了套环几何参数对斜拉索涡激振动的控制效果和影响规律,并初步分析了抑制振动的机理。结果表明:套环可以有效抑制斜拉索涡激振动,不同试验参数套环可将原振幅减小13%~98%;从总体趋势来说,套环宽度和厚度越大、间距越小,套环对斜拉索涡激振动的控制效果越好;安装O型套环之后,斜拉索外形表现出三维几何特征,流场的三维性被加剧,进而卡门涡的规则脱落和涡激振动被减弱。     

带有纵向肋条斜拉索的风雨激振减振机理研究

基于滑移理论,推导耦合的水膜运动方程和斜拉索运动方程,得到了可用于分析任意横断面外形斜拉索的风雨激振理论模型;并建立带有两个纵向肋条的斜拉索外形,通过对方程组进行求解分析水膜形态、斜拉索升力及斜拉索振动响应之间的相互联系,探究其减振机理。结果表明:带有两个肋条的斜拉索虽不能抑制上水线的形成,但与圆形横断面斜拉索的相比,使上水线厚度变薄,并且阻碍水线沿斜拉索表面的周期性滑落,扰乱其与斜拉索振动的频率相关性,使得斜拉索升力周期性特性减弱,从而抑制斜拉索在横风向的大幅度振动,阻碍风雨激振现象的产生。     

表面损伤的斜拉索平均气动阻力特性的试验研究

斜拉索作为斜拉桥的重要受力构件,其风荷载设计在桥梁抗风设计中具有重要意义。斜拉索在生产、运输和安装过程中有可能受到损伤,针对不同程度表面损伤的斜拉索,利用风洞测力试验,研究了亚临界、临界和超临界雷诺数区域损伤程度对斜拉索气动阻力的影响,并得到了表面损伤斜拉索气动阻力的计算方法。结果表明:在一定的风向范围内,斜拉索表面损伤对其气动阻力影响显著,在亚临界区,表面损伤模型阻力系数大于光滑模型;进入临界区后,表面损伤模型的阻力系数则小于光滑模型;在超临界区,二者又比较接近。随着划痕的加深,临界区起始雷诺数提前,且在临界雷诺数区,随着雷诺数的增大,阻力系数减小。对不同损伤程度斜拉索的阻力系数进行拟合,得出相应的计算公式,以方便斜拉索气动阻力的估算。     

Fatigue Failure Process Characterization for Carbon Fiber Sheet Reinforced Steel Rods Using the Acoustic Emission Technique

Acoustic emission (AE) technique is a widely used CM technique. In this paper, AE technique was used to characterize the fatigue failure process for carbon fiber sheet (CFS) reinforced steel rod. The AE signals at the frequency band of 50–400 kHz are detected by using AE sensors mounted on the steel rod and analyzed by both parameter analysis and spectrum analysis in order to investigate the feasibility of using the AE technique to identify various failure modes during fatigue failure process for CFS reinforced steel rods. Tension-tension fatigue experiments were carried out on both CFS reinforced and unreinforced steel rods. Based on AE energy parameter analysis using wavelet decomposition method, failure process was initially divided into three stages for unreinforced specimen and seven stages for CFS reinforced specimen. The frequency contents within the frequency band of 50–400 kHz for various failure modes in each stage including crack initiation, developing, final rupture in steel rod and matrix crack, debonding in CFS were revealed by fast Fourier transform (FFT) method. Further wavelet transform (WT) analysis was performed to illustrate the sequences of the failure modes and main failure mode in each stage by the occurrence time and longest duration time, respectively. This work indicates that the proposed method is promising for distinguishing failure stages qualitatively and identifying failure modes quantitatively in CFS reinforced steel rods.     

刀具磨损声发射信号小波分析中小波基的选取

针对在用小波理论分析刀具磨损声发射（AE）信号时选取不同的小波基对分析结果有重要影响的问题,通过对小波基性质和刀具磨损AE信号特点的研究,从理论上分析了小波分析中刀具磨损AE信号处理中小波基选取的方法。在试验验证过程中,根据信号在小波包分解前后遵循能量守恒的原理,用四种小波基对刀具磨损AE信号进行三层小波包分解。以经小波包分解后AE信号各频带上的频带能量为特征参数,比较四种情况下特征参数的变化,验证了理论分析的正确性。     

Damage Modes Recognition and Hilbert-Huang Transform Analyses of CFRP Laminates Utilizing Acoustic Emission Technique

Discrimination of acoustic emission (AE) signals related to different damage modes is of great importance in carbon fiber-reinforced plastic (CFRP) composite materials. To gain a deeper understanding of the initiation, growth and evolution of the different types of damage, four types of specimens for different lay-ups and orientations and three types of specimens for interlaminar toughness tests are subjected to tensile test along with acoustic emission monitoring. AE signals have been collected and post-processed, the statistical results show that the peak frequency of AE signal can distinguish various damage modes effectively. After a AE signal were decomposed by Empirical Mode Decomposition (EMD) method, it may separate and extract all damage modes included in this AE signal apart from damage mode corresponding to the peak frequency. Hilbert-Huang Transform (HHT) of AE signals can clearly illustrate the frequency distribution of Intrinsic Mode Functions (IMF) components in time-scale in different damage stages, and can calculate accurate instantaneous frequency for damage modes recognition to help understanding the damage process.     

Tension–tension fatigue behavior of layer-to-layer 3-D angle-interlock woven composites

The tension–tension fatigue behavior of (the layer-to-layer) three-dimensional angle-interlock woven composite (3DAWC) was investigated using the acoustic emission (AE) technique, examination of damaged regions via light microscopy and micro-CT, and finite element analysis (FEA). AE events occurred during the entire fatigue process, and damage modes were determined based on cumulative fatigue damage of the 3DAWC undergoing tension-tension cyclic loading. FEA was employed to determine stress distribution and specific regions of stress concentrations within the composite structure. Debonding occurred at the warp tow-matrix-weft tow interface where the warp tows exhibited a maximum in amplitude of undulation. In addition, interrupted tests were performed to investigate fatigue damage modes of the 3DAWC at specific points in the fatigue life. Fatigue damage occurred in three distinct stages with characteristic damage modes.     

Rapid evaluation of fatigue limit in AZ31B magnesium alloy using acoustic emission

Acoustic emission (AE) was used in a fatigue experiment to characterise AE signals and to rapidly determine the fatigue limit of AZ31B magnesium (Mg) alloy. The AE signals during fatigue were characterised according to waveforms and frequency. Meanwhile, the energy dissipation in the process of fatigue, which was represented by the accumulative AE energy, can be used to determine the fatigue limit. Based on the AE parameters, the fatigue limit was 97?MPa, with an 8% error value when compared with the results obtained by the conventional S–N curve method. This model only requires the accumulated energy of the signals during strain hardening. Therefore, the fatigue limit can be determined rapidly.     

Damage evolution and real-time non-destructive evaluation of 2D carbon-fiber/SiC-matrix composites under fatigue loading

Electrical resistance acquisition, acoustic emission (AE) monitoring and infrared thermography were employed to evaluate damage evolution of 2D carbon-fiber/SiC-matrix composite under fatigue loading. Damage evolution was discussed on the basis of the calculation results of the modulus and mechanical hysteresis variation. At lower stress levels, the majority of damage was produced in the first few cycles and then the rate of damage accumulation gradually approached a steady value as the cycles proceeded. When the applied stress exceeded the endurance fatigue limit, extensive damage took place and led to failure of the composite. Changes of composite electrical resistance, AE activity and surface temperature had fairly well agreement with the modulus and hysteresis responses. It can be concluded that it is possible to employ these real-time non-destructive evaluation methods as in-situ damage evolution indicators for this kind of composites under fatigue loading.     

EVALUATION OF FATIGUE DAMAGE ACCUMULATION BY ACOUSTIC EMISSION

Improvements in acoustic emission (AE) test techniques have lead to developments whereby reliable acoustic emission measurements as a fatigue damage indicator were made at room temperature on Incoloy 901 smooth specimens, over a stress-ratio range of ?1.0≤R≤0.2. Fatigue damage accumulation was evaluated in terms of an AE damage definition and a damage function was developed. Thus, various damage phases in the cumulative damage process were identified by their AE signature, and then formulated and analyzed in terms of cumulative AE ringdown counts.     

Monitoring of impact damages in composite laminates using wavelet transform

Low-velocity impact damage is a major concern in the design of structures made of advanced laminated composites, because such damage is mostly hidden inside and cannot be detected by visual inspection. It is found that the acoustic emission (AE) waves generated by impact loads are undistinguishable from each mode and amount of damage by the conventional analysis methods in time or frequency domain. The wavelet transform (WT) can decompose the AE signals in time and wavelet scale domains, and catch the differences in these waves. It enables to distinguish the damage modes and size. This paper presents the application of the WT to detect the impact damage. As a fundamental approach, the characteristics of the AE signals due to matrix cracks and the evolution of free-edge delamination in [±45₂/0₂/90₂]_S Gr/Ep laminates under tensile load were analyzed by the WT. Then the characteristics of impact damages of quasi-isotropic laminates were studied using the WT.     

碳纤维增强环氧树脂复合材料螺栓连接结构在拉伸载荷下损伤过程的声发射分析

采用声发射技术对不同几何尺寸的碳纤维增强环氧树脂复合材料（CFRP）螺栓连接结构在静力载荷下破坏行为进行了试验研究,比较了不同几何构型下的连接结构的破坏行为与声发射信号之间的映射关系。采用声发射技术对结构损伤过程中的声发射信号进行全程采集与转换,结合CFRP螺栓结构的载荷-位移曲线和宏/细观破坏形貌,分析了幅值、熵曲线和Andrews曲线与破坏行为之间的关系。结果表明:挤压与剪切破坏试件的载荷-位移曲线均呈现出较明显的塑性特征。结构发生挤压和剪切破坏时,声发射信号以中幅值信号为主,并伴随少量高幅值信号;结构发生拉伸破坏时对应的幅值为中幅值信号。根据熵曲线特征将CFRP连接结构破坏过程分为四个阶段,在损伤演化阶段发生纤维断裂、分层等失效模式,在结构失效阶段以分层失效为主。基于Andrews曲线分析得到挤压和拉伸失效模式在损伤演化阶段会出现多种损伤类型,剪切失效模式在结构失效阶段会出现多种损伤类型。