In-plane non-linear dynamics of the stay cables

Stay cables used in cable-stayed bridge and cable-stayed arch bridge are prone to vibration due to their inherent susceptibility to external deflection. The present work is devoted to the mitigation of a stay cable from the point of view of its nonlinear dynamics. The Galerkin integral, multiple scales perturbation method, and numerical techniques are applied to analyze the primary and subharmonic resonances of the stay cable. The nonlinear dynamic response of the stay cable subjected to parametrical and forced excitations is investigated numerically. The effects of some key parameters of the stay cable, such as initial tension force, damping and inclination angle, and the excitation frequency and amplitude are discussed. The carbon fiber reinforced polymers (CFRP) cable is also studied to understand the effect of the material properties of cable. The results show that these parameters have a considerable effect on the dynamic behavior of the cable. In particular, unreasonable tension force and inclination angle of stay cable may cause excessive vibration. It is suggested that CFRP cable replaces steel cable, which can mitigate the vibration of a stay cable.     

A3钢损伤累积过程的声发射监测

研究Ａ３结构钢，损伤累积过程中的声发射特性，对其在静、动载荷下声发射参数进行了分析。声发射振铃的上升、下降与材料内部的损伤程度有关。在一定的条件下，可以根据声发射信号的强弱、上升及下降的趋势，判别材料内部的损伤程度．     

Identification of Cracking Mechanisms in Scaled FRP Reinforced Concrete Beams using Acoustic Emission

Acoustic emission was used to monitor the cracking mechanisms leading to the failure of scaled concrete beams having Glass Fiber Reinforced Polymer (GFRP) longitudinal reinforcement and no shear reinforcement. Dimensional scaling included that of the effective depth of the cross section, which is a key parameter associated with the scaling of shear strength; and maximum aggregate size, which affects the shear-resisting mechanism of aggregate interlock along shear (inclined) cracks. Five GFRP reinforced concrete (RC) beams with effective depth up to 290 mm and constant shear span-to-effective depth ratio of 3.1 were load tested under four-point bending. Two types of failures were observed: flexural, due to rupture of the GFRP reinforcement in the constant moment region; and shear, due to inclined cracking in either constant shear region through the entire section depth. Acoustic emission (AE) analyses were performed to classify crack types occurring at different points in the load history. The results of this study indicate that appropriate AE parameters can be used to discriminate between developing flexural and shear cracks irrespective of scale, and provide warning of impending failure irrespective of the failure mode (flexural and shear). In addition, AE source location enabled to accurately map crack growth and identify areas of significant damage activity. These outcomes attest to the potential of AE as a viable technique for structural health monitoring and prognosis systems and techniques.     

Acoustic emission monitoring of crack propagation in additively manufactured and conventional titanium components

Additive manufacturing (AM) is a novel and innovative production technology that can produce complex and lightweight engineering products. In AM components, as in all engineering materials, fatigue is considered as one of the principle causes of unexpected failure. In order to detect, localise and characterise cracks in various material components and metals, acoustic emission (AE) is used as a non-destructive monitoring technique. One of the main advantages of AE is that it can be also used for dynamic damage characterisation and specifically for crack propagation monitoring. In this research, we use AE to monitor the fatigue crack growth behaviour of Ti6Al4V components under four-point bending. The samples were produced by means of AM as well as conventional material. Notched and unnotched specimens were investigated with respect to the crack severity and crack detection using AE. The main AE signal parameters –such as cumulative events, hits, duration, average frequency and rise time– were evaluated and indicate sensitivity to damage propagation in order to lead to a warning against the final fracture occurrence. This is the first time that AE is applied in AM components under fatigue.     

周期荷载作用下煤岩声发射特征的颗粒流模拟

为研究周期荷载应力水平对煤样声发射特征的影响,采用PFC数值软件开展了 3种不同应力水平的等幅周期荷载数值模拟试验,分析了周期荷载应力水平对煤样破坏循环次数、声发射计数及损伤演化特征的影响.研究结果表明:周期荷载作用下,煤样破坏过程中的声发射活动呈现初始、相对平静和活跃三阶段演化规律,且在煤样破坏前的周期荷载卸载阶段及...     

Acoustic emission for fatigue damage characterization in metal plates

Acoustic emission (AE) supplies information on the fracturing behavior of different materials. In this study, AE activity was recorded during fatigue experiments in metal coupons. The plates were characterized by a symmetric V-shape notch and were loaded in tension-tension fatigue until final failure with concurrent AE activity monitoring. The relatively broad bandwidth of the sensors enabled the recording of a wide range of frequencies up to 1 MHz. AE parameters like energy and duration exhibited a certain increase with the accumulation of damage although the hit rate was not significantly influenced. Furthermore the behavior of RA value (ratio of rise time to amplitude of the waveforms) which quantifies the shape of the first part of the AE signals and has been used for characterization of the cracking mode, showed a certain shift indicating the transition from tensile mode to shear which can be confirmed by the visual observation of the crack geometry after the experiment. The time history of RA is similar to the crack propagation rate (da/dN) curve but exhibits the rapid hyperbolic growth consistently about 1000 cycles earlier than final failure. Therefore, the use of acoustic emission parameters is discussed both in terms of characterization of the damage mechanisms, as well as a tool for the prediction of ultimate life of the material under fatigue.     

高径比对砂岩单轴压缩声发射特性的影响研究

岩石尺寸效应对声发射特性的影响对于利用声发射技术监测岩石失稳破坏具有重要的实践意义.本文选择不同高径比的长石细砂岩试样开展单轴压缩声发射试验,探讨尺寸效应对岩石声发射事件数、振铃计数、峰值频率等声发射特性参数的影响.研究结果表明:根据岩石试件撞击数的增长速率,将试件破坏阶段分为突变期、平静期、爆发期三个阶段,随着岩石试...     

Experimental study on AE behavior of hydraulic concrete under compression

The research work of this paper is to quantitatively evaluate the ongoing damage degree of existing concrete structures being loaded by applying acoustic emission (AE) monitoring. On the basis of some approximations and simplifications, the relation between the AE parameter (AE event) and the scalar damage parameter for concrete structures under uniaxial compression was derived, and was modeled as a simply linear correlation by combining the rate process theory and the traditional parameter-based technique of AE, Continuous Damage Mechanics (CDM) and nonlinear regression analysis. Meanwhile, the AE event-based stress-stain relation was also modeled too. The measured data of AE monitoring system and the strain dynamical strain system used in this work confirms significantly the effectiveness of the AE event-based damage evolution assessment for existing hydraulic concrete structures. The results of the experiments show that the AE event-based method permits a fast and effective in situ assessment of the ongoing damage phenomena in hydraulic concrete structures.     

Damage process detection of a ceramic tile system by acoustic emission

The debonding and falloff of ceramic tile systems are common failure modes for ceramic finishes of buildings and infrastructures due to the damage accumulation caused by application of mechanical load or changes in environmental conditions. To better understand the mechanism of the damage, a newly developed push-off test was conducted and a quantitative acoustic emission (AE) technique was developed to detect the damage processes during the loading history. The source locations of internal damage were determined by a new three-dimensional algorithm. Test results show that the growth of damage and the onset of failure can be monitored by AE technique. Also, the experimental results indicate that a probability density function of AE activity can be used to quantitatively describe the degree of damage of a material. It was found that the ceramic tile system with a lower strength adhesive displayed a higher AE rate than that with a higher strength adhesive at the same loading level prior to failure.     

CFRP索斜拉桥的索-浅拱模型及面内自由振动分析

本文针对斜拉桥的受力特点,基于索和浅拱的经典动力学运动方程,结合拉索与浅拱之间的耦合边界条件,并且考虑两者的几何非线性,建立了斜拉桥的多索-浅拱面内自由振动模型。将浅拱分段处理,结合索、浅拱连接处的动态平衡条件,应用分离变量法,建立多索-浅拱模型的面内自由振动理论。以双索浅拱模型为例,求解其特征值问题。同时,建立了相应的有限元模型,有限元计算结果与本文理论分析吻合良好。最后针对CFRP索斜拉桥的关键参数,基于本文的索-浅拱理论,对面内自由振动进一步研究。研究表明：浅拱的矢高在一定范围内变化,仅对某一阶频率产生影响,而其他各阶频率几乎没影响;CFRP拉索能显著改善索-浅拱组合结构的基本动力学特性。     

An Acoustic Emission Study for Monitoring Anterior Cruciate Ligament Failure Under Tension

The analysis of the acoustic signals produced during anterior cruciate ligament (ACL) failure could be useful in understanding its behavior. The purpose of the present study was to evaluate the role of coupling conditions of the sensors and to determine the value of newly introduced acoustic emission (AE) parameters. Seven femur-ACL-tibia complex (FATC) specimens were fixed in a universal tensile testing machine and load was applied. Different coupling conditions were applied in two groups of specimens. The load-time curve was monitored, with the simultaneous recording of the acoustic signals and the failure mode. During ACL tear, detectable changes in the load-time curve occurred linked to changes in the macroscopic sequence of events and the measured AE parameters irrespective the coupling conditions. AE provides information on the determination of the moment (or load) when crucial irreversible damage occurs. Furthermore, specific AE indices exhibit changes throughout the testing, and imply shift of the failure mechanisms.     

Classification of cracking mode in concrete by acoustic emission parameters

The study occupies with acoustic emission monitoring of several types of concrete during bending. The signals emitted at the different fracturing stages exhibit distinct signatures. Specifically, frequency and shape parameters of the acquired waveforms shift during the experiment, closely following the sequence of fracture mechanisms from tensile micro-cracking to brittle macro-cracking and fiber pull out. A number of AE indices are proposed, the use of which will enable classification of the cracks according to their mode. The study sheds light to the fracture process of cementitious materials, and enables a warning against the final failure. The simplicity of the scheme renders it applicable in situ.     

损伤因素对单层平面索网静力性能影响研究：有限元分析与试验验证

运用通用有限元软件ANSYS建立三维有限元模型,对横索与竖索连接节点失效、索预应力损失和锚固端失效这三种损伤因素的不同损伤工况下,单层平面索网结构的受力性能进行了非线性有限元分析,并与相应的试验结果进行了全面对比分析。结果表明：本文的有限元模型能够准确地分析计算上述三种损伤因素对单层平面索网结构受力性能的影响,包括结构...     

Q345R钢蠕变过程声发射特性实验研究

声发射测试技术由于实时、连续、在线监测的特点,被越来越多地应用于材料性能的研究,但由于蠕变实验温度过高,超过传感器使用温度限制,因此在金属材料蠕变损伤领域还尚属空白。本文以Q345R钢为例,设计蠕变声发射监测专用的夹具导波机构,进行Q345R钢蠕变声发射监测实验。监测结果表明,蠕变损伤过程的声发射活动表现出与蠕变曲线相类似的阶段性特征。蠕变初期撞击数较多,声发射较为活跃;随着损伤的演进,试件进入稳态蠕变阶段,声发射活动渐趋平稳,日平均撞击数趋于稳定值;蠕变后期,能量快速释放,声发射活动加剧,试件发生蠕变断裂。     

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.     

On-line analysis of hydro-abrasive erosion of pre-cracked materials by acoustic emission

Acoustic Emission (AE) sensing technique is used as a tool for on-line monitoring of hydro-abrasive erosion (HAE) of pre-cracked multiphase materials. As reference materials, five types of concrete materials were used for the experimental study. Compression tests were performed to determine the mechanical properties and the failure behavior of these materials. Erosion parameters, such as abrasive particle velocity, local exposure time, and abrasive mass flow rate were varied during the experiments and AE-signals were acquired. The trends exhibited by the time domain and frequency domain AE-signals with change in process parameters and material properties were analyzed. The results indicate that acoustic emission signal is capable of revealing the different material removal mechanisms occurring in pre-cracked multiphase materials when subjected to hydro-abrasive erosion. Visualization studies performed on the erosion site provide more insight into the physics of the process and verify the observations made from the AE-signals. Finally, it is concluded that due to its capability to quantify the amount of material removed, AE_RMS could be considered as a parameter for monitoring the material removal process.     

Acoustic emission assessment of interface cracking in thermal barrier coatings

In this paper, acoustic emission (AE) and digital image correlation methods were applied to monitor interface cracking in thermal barrier coatings under compression. The interface failure process can be identified via its AE features, including buckling, delamination incubation and spallation. According to the Fourier transformation of AE signals, there are four different failure modes: surface vertical cracks, opening and sliding interface cracks, and substrate deformation. The characteristic frequency of AE signals from surface vertical cracks is 0.21 MHz, whilst that of the two types of interface cracks are 0.43 and 0.29 MHz, respectively. The energy released of the two types of interface cracks are 0.43 and 0.29 MHz, respectively. Based on the energy released from cracking and the AE signals, a relationship is established between the interface crack length and AE parameters, which is in good agreement with experimental results.     

Monitoring damage in holed CFRP laminates using embedded chirped FBG sensors

This study proposes a new approach to monitoring the damage process in holed CFRP laminates using an embedded chirped fiber Bragg grating (FBG) sensor. To this end, we experimentally and numerically investigated the damage process and the damage-induced changes in the spectrum shape. It was experimentally confirmed that multiple types of damage (e.g., splits, transverse cracks and delamination) appeared near a hole, and that the spectrum shape of the embedded chirped FBG sensor changed as the damage extended. Our proposed simulation for the reflection spectrum considering the damage agreed with the experiments. Furthermore, this study investigated the effect of each damage pattern on the changes in the spectrum shape. Finally, based on these discussions, we present simple damage identifications with the embedded chirped FBG for the holed CFRP laminates under completely unloaded conditions.     

碳纤维复合材料拉索的非线性力学性能

采用解析法分析碳纤维复合材料(CFRP)拉索的非线性力学性能,通过与钢拉索对比分析,得出了CFRP拉索的受力特点.由于密度低,CFRP拉索自重应力、垂度约为钢拉索的1/5,承载效率也比钢拉索高许多,其承载极限长度为钢拉索的7倍,且随跨径增大,钢拉索的等效弹性模量下降非常快,而CFRP索仍保持较高值.由于CFRP线胀系数比钢材的线胀系数要小得多,约为其1/14,在温差与约束相同时,无垂度CFRP索温度应力仅为无垂度钢索的1/23.当有垂度时,钢索的温度应力降低,CFRP索的温度应力变化很小.     

斜拉桥主梁振动对拉索阻尼器减振效果的影响

应用拉索索端阻尼器是大跨度斜拉桥拉索减振的主要措施之一. 将主梁、索与阻尼器组合起来作为一个振动体系,通过理论分析与试验研究相结合的方法初步研究了主梁振动对拉索附加阻尼器减振效果的影响. 建立了由索、梁和Kelvin阻尼器组成的简化理论模型; 设计了索、梁和阻尼器组合系统的简化力学试验模型; 详细研究了主梁振动对拉索附加阻尼器减振效果的影响. 理论与试验分析结果表明: 对于容易发生索、梁耦合振动的拉索,主梁振动明显降低拉索附加阻尼器的减振效果;在大跨度斜拉桥拉索的减振设计中,需考虑主梁参与振动的影响.