Feasibility analysis for monitoring fatigue crack in hydraulic turbine blades using acoustic emission technique

In order to investigate the feasibility of monitoring the fatigue cracks in turbine blades using acoustic emission (AE) technique, the AE characteristics of fatigue crack growth were studied in the laboratory. And the characteristics were compared with those of background noise received from a real hydraulic turbine unit. It is found that the AE parameters such as the energy and duration can qualitatively describe the fatigue state of the blades. The correlations of crack propagation rates and acoustic emission count rates vs stress intensity factor (SIF) range are also obtained. At the same time, for the specimens of 20SiMn under the given testing conditions, it is noted that the rise time and duration of events emitted from the fatigue process are lower than those from the background noise; amplitude range is 49-74 dB, which is lower than that of the noise (90-99 dB); frequency range of main energy of crack signals is higher than 60 kHz while that in the noise is lower than 55 kHz. Thus, it is possible to extract the useful crack signals from the noise through appropriate signal processing methods and to represent the crack status of blade materials by AE parameters. As a result, it is feasible to monitor the safety of runners using AE technique.

Acoustic Emission Monitoring of Fatigue Crack Growth in a Slender Web of Bridge Girder

It is demonstrated that the initiation of fatigue cracks can be detected by sudden increases in cumulative acoustic emmission (AE) activities and in AE hit rate, and that the onset of catastrophic failure of structures can be detected by sudden decreases in average hit rate and average AE density. During the progression of fatigue cracks, a good linear relationship is obtained between the cumulative AE hits and the crack increment, and between the average AE hit rate and the average crack growth rate, indicating a potential for the quantitative assessment of fatigue crack growth. Location analysis can not only indicate the position of cracks, but also reflect the developing trends of crack ends.

Acoustic Emission Monitoring the Fatigue Crack Growth in SM50B-Zc Steel

The acoustic emission(AE)characteristics from the fatigue crack propagation in SM50B-Zc steel were strongly affected by the environment me- dium.The AE feature from the fatigue process both in air and in 3.5% NaCl aqueous solution mani- fested a periodicity which indicated the fatigue ex- tension was discontinuous.The fatigue striation and secondary cracking were the main AE sources during the fatigue crack growth in air.In contrast, AE during the fatigue crack propagation in 3.5% NaCl solution was released from the intergranular fracture and quasicleavage cracking.

Fatigue Crack Initiation Sizing Using Acoustic Emission

One of the major concerns in structures is early detection of a growing crack to prevent fracture, predict remaining useful life, schedule maintenance and reduce costly downtimes. AE （acoustic emission） is a non-destructive testing method with potential applications for locating and monitoring fatigue cracks. This paper focuses on in-situ monitoring of structural health, specifically detection of small crack growth and crack initiation in structures using AE technology. A probabilistic AE-based model for small fatigue cracks was developed and the uncertainties of the model were estimated. The paper discusses the methodology used, experimental approach, results obtained and predictive models developed.The developed model can be used to evaluate the integrity of structures and assess structural health by estimating the probability density function of the length of detected cracks. The outcome of this research has significant potential to be used for in-situ monitoring and evaluation of structural integrity.

Acoustic Emission Properties of Concrete Slab During Fatigue Testing with a Running-Wheel Load

AE （acoustic emission） signals from concrete slab during fatigue testing with a running-wheel load were evaluated. The signals were recorded by remote sensors connected to a computer network. The sensing equipment consisted of 60 kHz resonant-type AE sensors mounted on a reinforcing steel bar as a waveguide, together with a 16-channel sensor highway AE system. Because the detected AE signals included periodic mechanical noise from the motion of the wheel, these noises were eliminated by means of signal processing. The AE waveguide measurement over a length of 3 m detected fractures as vertical and horizontal cracks in the RC （reinforced concrete） slab. Those cracks were analyzed by correlating AE parameters with macroscopic distortions and the numbers of fatigue cycles. In the AE events and AE energy, two types of AE phenomena, active region and inactive region, were observed during fatigue testing. The vertical cracks were characterized by an AE amplitude of 58 dB, a peak frequency of 30 kHz, and a ratio of the rise time to the maximum amplitude value （RA） of 100. The horizontal cracks were characterized by an AE amplitude of 85 dB, a peak frequency of 60 kHz, and an RA value of 10.

Mechanical anisotropy associated with beddings in shale under Brazilian test conditions:Insights from acoustic emission statistics

A comprehensive understanding of shale’s bedding anisotropy is crucial for shale-related engineering activities,such as hydraulic fracturing,drilling and underground excavation.In this study,seven Brazilian tests were conducted on shale samples at different bedding orientations with respect to the loading direction(0°,45°and 90°)and the disc end face(0°,45°and 90°).An acoustic emission(AE)system was employed to capture the evolution of damage and the temporal-spatial distribution of microcracks under splitting-tensile stress.The results show that the Brazilian tensile strength decreases with increasing bedding inclination with respect to the disc end face,while it increases with the angle between bedding and loading directions.Increasing the bedding inclination with respect to the end face facilitates the reduction in b value and enhances the shale’s resistance to microcrack growth during the loading process.Misalignment between the bedding orientation and the end face suppresses the growth of mixed tensile-shear microcracks,while reducing the bedding angle relative to the loading direction is beneficial for creating mixed tensile-shear and tensile cracks.The observed microscopic failure characteristics are attributed to the competing effects of bedding activation and breakage of shale matrix at different bedding inclinations.The temporal-spatial distribution of microcracks,characterized by AE statistics including the correlation dimension and spatial correlation length,illustrates that the fractal evolution of microcracks is independent of bedding anisotropy,whereas the spatial distribution shows a stronger correlation.The evolution features of correlation dimension and spatial correlation length could be potentially used as precursors for shale splitting failure.These findings may be useful for predicting rock mass instability and analyzing the causes of catastrophic rupture.

Investigating fatigue behavior of gear components with the acoustic emission technique

A novel method is presented to evaluate the complicated fatigue behavior of gears made of20Cr2Ni4 A.Fatigue tests are conducted in a high-frequency push-pull fatigue tester,and acoustic emission（AE）technique is used to acquire metal fatigue signals.After analyzing large number of AE frequency spectrum,we find that：the crack extension can be expressed as the energy of specific frequency band,which is abbreviated as F-energy.To further validate the fatigue behavior,some correlation analysis is applied between F-energy and some AE parameters.Experimental results show that there is significant correlation among the Fenergy,root mean square（RMS）,relative energy,and hits.The findings can be used to validate the effectiveness of the F-energy in predicting fatigue crack propagation and remaining life for parts in-service.F-energy,as a new AE parameter,is first put forward in the area of fatigue crack growth.

On the Bump Tests of Cracked Shafts Using Acoustic Emission Techniques

An investigation about the application of Acoustic Emission (AE) techniques to analyze the dynamic response of different cracked shafts rendered in bump tests is presented in this work. The experimental apparatus devised for this work complies of six shafts with different transverse crack sizes and a high-frequency data acquisition system. The AE signals generated in the bump tests performed on the different cracked shafts are captured by a wideband AE transducer. Those signals are treated by using statistical moments, wavelet transforms, and frequency- and time-domain procedures. A transverse crack of predetermined depth is etched into each shaft. The experimental results show that the values of kurtosis and skewness estimated for the AE signals can be used to identify the crack size.

Acoustic Emission Characterization of Matrix Damage Initiation in Woven CFRP Composites

In this paper, an experimental investigation is performed using acoustic emission (AE) technique to study the behaviour of crack initiation and propagation of woven carbon fibre reinforced plastic (CFRP) composites under static tensile loading. Three laminates with three plies each were manufactured and tested under tensile static loading. Damage was being monitored using AE technique with instantaneous AE hits energies being used as the damage parameter. Results showed that angle ply laminates experienced crack initiations at about 50% of ultimate tensile strength whereas cross ply laminates and non-conventional angle ply laminates having one cross ply laminate experienced initiation of cracks at about a third of ultimate tensile strength. This early cracks initiation is believed to be caused by the presence of 90° plies that cannot take much load transversely.

Modeling Stress Intensity Factor of Rail Steel under Situation of Growing Fatigue Crack—A Novel Technique

Simulation of stress intensity factor as function of rolling contact fatigue cracks of railway tracks and the vehicle load is made with the help of COMSOL Multiphysics software. It is found that the critical stress intensity factor i.e. 41.6 MPa. m1/2 is reached at a stress level of 32 MPa and at the crack size 11.5 × 10-2 m.Noting the power law variation of acoustic emission count with increase in crack size (analogous to Paris Law), the simulation was further carried out to model the dependence of measured AE count with the stress intensity factor ahead of a growing RCF crack tip. It is demonstrated that AE measurement can be effective to trigger a control loop for avoidance of fatigue failure of railway track. In view of potential difference in the intensity of back scattered light from surface irregularities, a model is developed to find out the threshold intensity of scattered light that insures safety in the railway system against fatigue failure.

Relationship between the Initial Fracture Stress and Fatigue Limit—Simple Prediction Method of Tensile Fatigue Limit of Composite

This article presents an experimental study that clarifies the relationship between the initial fracture stress and fatigue limit of glass fiber reinforced unsaturated polyester resin specimens with a laminated structure taken from a pultruded square pipe. Quasi-static bending and tension tests are performed with acoustic emission (AE) measurements to identifying the occurrence of initial fracture during testing. AE and observation results have clarified the occurrence of initial fracture was detected by maximum acoustic energy values and corresponding fiber breakage in the unidirectional (UD) bundles. Moreover, the ratio of initial fracture stress to ultimate strength is 32% in bending and 26% in tension, when comparing stress and strains on the tension side of the UD layer. These values are in good agreement with each other and with the measured tensile fatigue limit when the cyclic stress is at 25% of the tensile strength. Initial fracture stress obtained by static tests is close values to the fatigue limit which will greatly contribute to the prediction of the fatigue limit.

声发射技术在复合材料壁板压-剪复合载荷疲劳试验损伤扩展监测中的应用

利用声发射技术对某飞机复合材料机身壁板压-剪复合载荷疲劳试验中位于壁板长桁上的两处初始损伤开展了连续跟踪监测,试验过程中对各监测通道声发射信号的幅值、能量、计数等特征参数进行实时统计分析,在试验中成功监测到其中一处损伤的扩展,为试验结论分析提供了重要数据支撑。

疲劳裂纹摩擦声发射信号特性研究

由于声发射波的产生、传播以及与结构相互作用的复杂性,使得声发射波形分析变得困难。为了降低裂纹扩展监测过程中疲劳裂纹摩擦信号的干扰,文章对金属构件裂纹摩擦引起的声发射(AE)信号进行了剖析。通过给予含预制裂纹试件30 Hz与180 Hz的振动激励,对裂纹结合面运动进行诱导,从而产生声发射信号。用多个距裂纹不同距离的R15a换能器记录不同共振频率下的声发射信号,剖析所记录波形。在不同共振条件下,变化的裂纹结合面运动会引发具备不同特征的声发射信号。通过多传感器实验,对离裂纹不同距离处裂纹摩擦声发射信号差异及其几何传播规律进行了研究与总结。

基于声发射b值演化的脆性岩石裂纹起裂应力评价方法

岩石裂纹起裂应力的识别和研究对认识脆性岩石受载变形及破坏过程具有重要意义。系统总结了现有的裂纹起裂应力确定方法(裂纹体积应变法、侧向应变法、移动点法、声发射振铃计数法)及存在的问题,并以单轴压缩条件下岩石破裂过程中声发射幅值及b值分析为基础,提出了一种基于声发射b值演化的脆性岩石裂纹起裂应力评价方法。采用GAW2000单轴刚性试验机和声发射测试仪开展了一系列变辉长岩和花岗岩的单轴加载试验,根据力学及声发射测试结果,对所提方法及现有方法进行了评价。研究结果表明:声发射b值演化法具有明确的物理机制及意义,可以方便准确地确定起裂应力大小,测定应力结果与现有方法所得结果具有一致性。通过方差分析可知,所提方法与现有方法不存在统计差异,验证了该方法测定裂纹起裂应力的可靠性。2类岩样的起裂应力均与单轴抗压强度呈正线性相关,使用所提方法确定的变辉长岩起裂应力约为单轴抗压强度的0.51倍,花岗岩起裂应力约为单轴抗压强度的0.48倍。所提方法及研究成果对于丰富岩石裂纹起裂应力测试手段和探究岩石脆性破坏机制具有一定的价值。

基于声子晶体的声发射波源检测概率研究

声发射(acoustic emission,AE)检测就是利用采集和分析声发射现象所产生的信号从而实现对材料进行无损评价的技术,该技术具有实时监测并定位破坏源的能力但是由于检测过程中声发射源信号未知,因此评价检测可靠性的检测概率(probability of detection,POD)研究仍然是空白。进一步来说,在利用声发射监测损伤过程时,声发射波源直接决定其检测结果的可靠性,因此波源的检测概率可以成为量化声发射检测有效性的一个重要指标。为了确定声发射波源的检测概率,利用声子晶体(phononic crystals,PCs)的特性,通过部署声子晶体形成物理带隙,滤除噪声信号,从而获得铝合金试件在紧凑拉伸试验下由于疲劳裂缝活动而引起的声发射信号,最终确定声发射源的检测概率及其影响因素。结果表明,检测概率会随着疲劳裂缝扩展而提高,但裂缝长度达到一定值后开始下降,最终趋于一个平稳值,另外通过设置阈值和选择适当的滤波器,能有效提高检测概率,保证声发射评价结构损伤的有效性。该研究对声发射源识别和检测可靠性评价具有重要意义。

高抗折矿渣硫铝酸盐水泥混凝土抗疲劳性能研究

传统水泥混凝土公路路面面临的主要问题是路面易开裂劣化和抗疲劳性能较差,这些问题大大缩短了路面的使用寿命,严重制约了水泥混凝土路面的发展和推广应用。矿渣硫铝酸盐水泥(Slag-Calcium Sulfoaluminate Cement,S-CSA)是一种具有高抗折强度的新型胶凝材料,利用该水泥制备的混凝土路面具有解决现有路面抗疲劳性能差的潜力。通过三点弯曲疲劳试验和声发射试验,对比分析了高抗折S-CSA混凝土和普通硅酸盐水泥(Ordinary Portland Cement,OPC)混凝土的抗疲劳性能。研究结果表明:相同荷载作用下,高抗折S-CSA混凝土粗骨料断裂数量更多;不同强度等级下,高抗折S-CSA混凝土的疲劳寿命均高于OPC混凝土,且提升幅度随着强度等级的提高而显著增加,最高可达到约7.2倍;钙矾石的生成有效改善了界面过渡区,从而提升了高抗折S-CSA混凝土的抗疲劳性能。

金属疲劳损伤在线监测方法综述

疲劳在线监测的主要目标是监测疲劳裂纹萌生及获得疲劳裂纹扩展速率,从而提供预警。声发射在线监测技术因其能实现对构件的疲劳失效历程开展实时、在线监测,而在疲劳失效在线监测方面得到了普遍的使用。本文简要介绍了应变片监测和光纤Bragg光栅监测技术,重点介绍了声发射技术在发电设备方面的疲劳失效过程的在线监测研究进展。

7050铝合金腐蚀疲劳裂纹扩展的声发射监测

为定量评估材料在服役过程中的损伤演化过程,识别材料损伤状态,确保材料的安全服役,开展了铝合金腐蚀疲劳裂纹扩展(CFCG)的声发射(AE)监测试验。利用层次聚类提取特征参数后进行K-means聚类,结合聚类结果和扫描电镜(SEM)图识别CFCG过程中的不同损伤类型,通过对各类损伤的累计计数和累计能量的分析,评估铝合金CFCG的损伤演化过程。试验结果表明,结合使用层次聚类选取的AE参数的无监督聚类和SEM图,识别出了铝合金CFCG过程中的析氢、第二相和夹杂物断裂、塑性变形微裂纹、裂纹扩展和点蚀等损伤模式。

管线钢焊接接头疲劳裂纹扩展的声发射信号特征分析

为明确X52管线钢焊接接头疲劳损伤特性与规律,对接头的母材、焊缝和热影响区进行了疲劳裂纹扩展的声发射(AE)实时监测研究。结果表明:母材的疲劳裂纹扩展速率高于焊缝和热影响区,而焊缝高于热影响区;AE测试结果显示,焊缝裂纹扩展的AE振铃计数率高于热影响区和母材,且信号强度也高于母材和热影响区,这是焊缝晶粒粗大使材料脆化倾向增加导致。同时,接头不同组织裂纹扩展的AE信号波形也不同,焊缝波形主要为300KHz左右的高频成分,而母材和热影响区为100KHz左右的低频成分。声发射信号特征参数可以用来定性区分接头不同位置的损伤。