Monitoring of thermal shock induced crack growth in a feed water nozzle corner of a reactor pressure vessel by acoustic emission

In the framework of a reactor safety research programme, thermal shocking of a clad feed water nozzle has been carried out during simulated operating conditions (temperature = 300°C, pressure = 11 MPa). The objective of these trials is to contribute to the understanding of crack initation and propagation under realistic thermal shock conditions, and to monitor crack initiation and growth by NDT on-line as well as off-line. Up to now the nozzle corner has been shocked 5200 times. The shocked area was monitored by acoustic emission (AE). During the last 800 thermal shocks new AE equipment with optimized capabilities for detection, location of AE sources and interpretation of AE signals has been used. This AE system and the results obtained during the last 800 shocks will be described. One important result is that it is possible to separate crack-growth signals from friction noise of the cracks by signal analysis when the AE signals are received with broad-band transducers.     

基于近场波束形成法的声发射源定位研究

引入近场波束形成法处理声发射信号,研究在小区域布置少量传感器来实现声发射源定位的新方法.首先,利用板波理论进行声发射信号传播特性研究,对比了板中声发射信号与空气中声波传播的差异性,分析了波束形成法应用于声发射源定位的问题.并在板上进行断铅实验,验证了近场波束形成法进行声发射源定位的有效性.最后,在旋转机械碰摩模拟实验台上进行了碰摩实验,利用波束形成法进行碰摩声发射源定位,所提出的方法能有效地定位旋转机械初始碰摩位置.所得结果可为声发射源定位提供一种新途径.     

Photoacoustic tomography using a Mach-Zehnder interferometer as an acoustic line detector

A three-dimensional photoacoustic imaging method is presented that uses a Mach-Zehnder interferometer for measurement of acoustic waves generated in an object by irradiation with short laser pulses. The signals acquired with the interferometer correspond to line integrals over the acoustic wave field. An algorithm for reconstruction of a three-dimensional image from such signals measured at multiple positions around the object is shown that is a combination of a frequency-domain technique and the inverse Radon transform. From images of a small source scanning across the interferometer beam it is estimated that the spatial resolution of the imaging system is in the range of 100 to about 300 mum, depending on the interferometer beam width and the size of the aperture formed by the scan length divided by the source-detector distance. By taking an image of a phantom it could be shown that the imaging system in its present configuration is capable of producing three-dimensional images of objects with an overall size in the range of several millimeters to centimeters. Strategies are proposed how the technique can be scaled for imaging of smaller objects with higher resolution.     

Experimental and analytical characterization of acoustic emission signals

The identification of crack growth signals is extremely important in order to effectively use acoustic emission techniques to detect, locate, and determine the significance of an internal flaw. The results of an analytical methodology, incorporating a source model that is an actual crack propagation and arrest event, are presented in this paper to predict a time dependent acoustic emission signal. The integral equation method is used to calculate the dynamic Mode I stress caused by a crack propagating with a prescribed velocity, after which the displacements at any point are calculated. These time dependent displacements are the analytical form of the acoustic emission waveforms. An experimental procedure, which uses a laser interferometer to measure velocities normal to the specimen's surface, is used to investigate acoustic emission waveforms in a compact tension specimen. The experimental results are used to verify the analytical model.     

声发射检测中利用能量进行定位的新方法

声发射检测过程中,对缺陷的定位是重要的一个步骤,传统的声发射检测利用时差定位方法,而时差定位里重要的参数是声速。通过推导,得到一种利用声衰减特性和能量参数对声发射源进行定位的新方法,并用声发射仪对普通钢件以铅笔芯折断作为模拟源进行测试,证明这种无需声速测量的新方法可行,且准确性可以得到保证。该方法的提出为声发射定位技术拓展了新方向,对提高声发射检测质量有一定的帮助。     

基于AR模型的声发射信号到达时间自动识别

声发射信号到达时间的信息,对于声发射事件的定位、识别以及声发射源机理分析都是非常重要的。实际应用中,常用人工读取或通过设定幅值阈值来获取信号的到达时间。针对以上常用方法的缺点,本文结合噪声信号的AR模型和声发射信号的AR模型,应用Akaike信息准则,实现了对声发射信号到达时间的自动识别。对实验数据的识别结果显示,该方法对信号的幅频特性变化比较敏感。在相同信噪比的情况下,该方法识别的偏差要小于阈值法。当信噪比较低时,阈值法可能会给出错误的结果,而该方法仍然能够给出较准确的结果。     

Evaluation of in situ fracture toughness of ceramic coatings at elevated temperature by AE inverse analysis

This paper describes acoustic emission (AE) measurement at elevated temperature by laser techniques. AE signals originated from microfracture of Al₂O₃/SUS304 plasma sprayed coatings were detected using a laser interferometer during thermal cycling. The radii of microcracks were evaluated by the inverse analysis of AE signals. Microcrack radii were also studied by the numerical analysis based on the delamination model for coating materials with various fracture toughness and an initial crack size at elevated temperature. In situ fracture toughness for microcracking was evaluated by combining the results of AE inverse analysis with those of numerical analysis. This approach revealed that the in situ toughness of the coatings is 40–50 J/m², consistent with the results of a double cantilever beam test.     

Fracture source location in thin plates using the wavelet transform of dispersive waves

A new signal processing approach was presented for acoustic emission source location using the dispersive waves in a thin plate. For wave propagation in dispersive media, the accuracy of source location can be improved by using the arrival times of a single frequency component in the output signals at an array of sensors. The wavelet transform (WT) was used to resolve this problem. By utilizing the time-frequency data of the WT, the frequency-dependent arrival time traveling with the group velocity was shown to be easily determined. Experiments were performed using a lead break as the simulated fracture source on the surface of an aluminum plate. Two plate modes corresponding to the S(0) and A(0) Lamb waves were identified, and their group velocities were accurately measured. The source location results based on the WT method agreed well with the true locations. The WT method was also compared with the cross correlation technique, and both methods provide similar results.     

High-spatial-resolution sub-surface imaging using a laser-based acoustic microscopy technique

Scanning acoustic microscopy techniques operating at frequencies in the gigahertz range are suitable for the elastic characterization and interior imaging of solid media with micrometer-scale spatial resolution. Acoustic wave propagation at these frequencies is strongly limited by energy losses, particularly from attenuation in the coupling media used to transmit ultrasound to a specimen, leading to a decrease in the depth in a specimen that can be interrogated. In this work, a laser-based acoustic microscopy technique is presented that uses a pulsed laser source for the generation of broadband acoustic waves and an optical interferometer for detection. The use of a 900-ps microchip pulsed laser facilitates the generation of acoustic waves with frequencies extending up to 1 GHz which allows for the resolution of micrometer-scale features in a specimen. Furthermore, the combination of optical generation and detection approaches eliminates the use of an ultrasonic coupling medium, and allows for elastic characterization and interior imaging at penetration depths on the order of several hundred micrometers. Experimental results illustrating the use of the laser-based acoustic microscopy technique for imaging micrometer-scale subsurface geometrical features in a 70-μm-thick single-crystal silicon wafer with a (100) orientation are presented.     

Pattern Recognition Method for the Source Location of Acoustic Emission Generated During the Damage of Hydraulic Concrete

Abstract: To overcome the disadvantages of current acoustic emission (AE) source location methods, such as classical approaches based on times of arrival and artificial neural networks based on AE signal features, support vector machines (SVM)‐based models have been employed to recognise AE source regions in structures. However, in some circumstances, it seems that a more accurate positioning of AE sources is needed. This study concerns the spatial three‐dimensional (3D) positioning (i.e. coordinates) for damages in hydraulic concrete structures using the least squares SVM (LS‐SVM) regression with AE signal features. The data of artificial discrete AE sources were acquired from simulated AE events on a hydraulic concrete specimen. Various combinations of signal features were chosen to adequately excavate effective information and to obtain the multi‐output LS‐SVM regression model of the best performance. The training and testing results show that the proposed model can realise the accurate spatial 3D positioning of damages in hydraulic concrete structures in laboratory situations and reduce human factors (e.g. judgment of AE propagation velocity, etc.) in the AE source location process. Meanwhile, the work remaining in taking this idea to a practical implementation was discussed.     

In situ monitoring of microfracture during plasma spray coating by laser AE technique

Atmosphere plasma spray coating materials include many pores and lamellar boundaries formed by flattened particles during spraying process although high reliability are required in ceramic coatings for turbines. These boundaries become an origin of the microcracks and following crack growth. As it is known that spraying parameters strongly affect the microstructure and strength of coating, it is expected to establish in situ monitoring technique for coating process. However, there is a limit to apply the existing non-destructive evaluation techniques to real-time monitoring at elevated temperature. We have investigated a non-contact measuring system to detect acoustic emission (AE) signals due to microfractures using a laser interferometer, and applied this technique for understanding microfracture process of ceramic coating at elevated temperature. In this paper, we evaluated the effect of several spraying parameters on the initiation and growth process of microcrack by detecting AE signals during coating process using a non-contact laser AE technique.     

In situ monitoring of microfracture during plasma spray coating by laser AE technique

Atmosphere plasma spray coating materials include many pores and lamellar boundaries formed by flattened particles during spraying process although high reliability are required in ceramic coatings for turbines. These boundaries become an origin of the microcracks and following crack growth. As it is known that spraying parameters strongly affect the microstructure and strength of coating, it is expected to establish in situ monitoring technique for coating process. However, there is a limit to apply the existing non-destructive evaluation techniques to real-time monitoring at elevated temperature. We have investigated a non-contact measuring system to detect acoustic emission (AE) signals due to microfractures using a laser interferometer, and applied this technique for understanding microfracture process of ceramic coating at elevated temperature. In this paper, we evaluated the effect of several spraying parameters on the initiation and growth process of microcrack by detecting AE signals during coating process using a non-contact laser AE technique.     

一种声发射源的新型平面定位方法研究

声发射现象中产生的弹性波在传播时声压遵从指数衰减规律,文中提出一种利用声衰减特性对声发射源进行平面定位的能量定位新方法。从理论上导出了该方法的定位计算公式,并用AE21C型声发射仪在高分子合成纤维平板上以铅笔芯折断产生信号为模拟源进行了实验测量,证明这种新的源定位方法可行,并具有无须测量传声媒质的声衰减和声速等优点。     

Simulation of Acoustic Emission in Planar Carbon Fiber Reinforced Plastic Specimens

The simulation of acoustic emission waveforms resulting from failure during mechanical loading of carbon fiber reinforced plastic structures is investigated using a finite element simulation approach. For this investigation we focus on the dominant failure mechanisms in fiber reinforced structures consisting of matrix cracking, fiber breakage and fiber-matrix interface failure. To simulate the failure process accurately, we present a new acoustic emission source model that is based on the microscopic source geometry and micromechanical properties of fiber and resin. We demonstrate that based on this microscopic source model these failure mechanisms result in excitation of macroscopic plate waves. The propagation of these plate waves is described using a macroscopic three-dimensional model geometry which includes contributions of reflections from the specimen boundaries. We further present a model of the acoustic emission sensors used in experiments to simulate the influence of aperture effects. To enhance the understanding of correlation between macroscopically detectable acoustic emission signals and microscopic failure mechanisms we simulate the response to different source excitation times, crack surface displacements and displacement directions. The results obtained show good agreement with fundamental assumptions about the crack process reported by various other authors. The simulated acoustic emission signals obtained are compared to experimentally measured waveforms during four-point bending experiments of carbon fiber reinforced plastic structures. The simulated signals of fiber-breakage, matrix-cracking and fiber-matrix interface failure show systematic agreement with the respective experimental signals.     

Acoustic emission monitoring of a fatigue crack in 50D steel in a sea-water environment

Four specimens of 4360-50D steel plates were fatigued under a range of simulated marine environments and loads. AE events were detected using four broadband point-contact transducers and located in 3D (< 1 mm accuracy) relative to the crack front. This information was used together with the load at which each emission occurred to distinguish between ‘primary’ and ‘secondary’ source events.Little or no emission was observed from a dry fatigue crack, indicating that ductile crack growth by mode I loading in 50D steel plate is relatively quiet. In the presence of sea water, however, secondary emissions were observed at discrete points behind the crack tip where subsequent chemical analysis indicated a high concentration of calcium carbonate deposit. A third, cathodically protected, specimen gave much more AE, consistent with an increase in calcium carbonate between the crack faces. The emissions occurred at loads between zero and maximum tension. The characteristics of the AE signals did not change on reducing the maximum load, confirming that crack advance was not responsible. Crack face debonding, arising from fracture of calcareous deposits present, is believed to be the source of AE. A fourth specimen, also cathodically protected, was fatigued entirely in tension and gave less AE, consistent with much weaker crack face bonding.Measurements of the compressional wave arrival strengths at each probe were compared with calculated radiation patterns for different types of AE source. The experimental radiation patterns were always consistent with a double monopole combined with a microcrack source in varying proportions.Under these testing conditions, secondary events associated with corrosion product fracture dominated the emission. Whilst little or no primary emission was detected during fatigue crack growth in parent plate, this may not be the case in welds, regions of low toughness or where the stress fields are more complex.     

Acoustic emission source location in composite materials using Delta T Mapping

The location capability of the acoustic emission (AE) technique is often considered its most powerful attribute. However, assumptions made in the calculation of location by current algorithms can be limiting in complex geometries and materials. This work forms a detailed study into the use of a novel mapping technique for AE source location in fibre reinforced composite materials. Both the performance and the robustness of the approach are assessed using artificial and real AE sources. Furthermore a large fatigue specimen was used to demonstrate detection and location of damage onset and development, where findings were validated using a thermo-elastic stress analysis (TSA) system. Substantial improvements in location accuracy were observed and early detection of damage onset was seen to outperform TSA.     

Intralaminar fracture mechanism in unidirectional CFRP composites: Part I: Intralaminar toughness and AE characteristics

Three types of representative carbon fiber reinforced unidirectional composite materials were used and their intralaminar fracture behavior was investigated using the double-cantilever beam specimen with a simultaneous acoustic emission measurement. The intralaminar fracture toughness was evaluated by both the compliance method and energy area method. As a result, it was found that the intralaminar fracture toughness without bridging fibers had a constant value during crack propagation but it increased greatly when bridging fibers were present. The effect of bridging fibers on the intralaminar fracture toughness was estimated quantitatively by cutting the bridging fibers. Distinct differences in load–displacement curves, compliance, crack propagating behavior and acoustic emission signal characteristics between these three types of unidirectional composite materials were observed. It was also found that bridging fiber failure generated relatively large power spectra and contributed to the peak frequencies of 600–700 kHz in the spectrum analysis of acoustic emission (AE) signals. This suggested that the bridging fibers were also an important source of AE signals. Furthermore, a linear relationship between crack length and normalized cumulative AE event count rate was obtained.     

The characteristics of acoustic emission from a hole in a composite laminate

Holes in composite laminates are sources for the onset of delamination and matrix cracking. Early discovery of delamination growth makes it possible to repair the damage. Monitoring of acoustic emission and analyzing the signals may enable the recognition of the onset of delamination and its location by using a few sensors and calculation of arrival times. It is shown that acoustic emission signals originating from holes can be characterized by using a ‘pattern recognition’ scheme. Thus the acoustic emission which results from the presence of a hole can be differentiated from that generated by fiber breakage.     

Measurement of material anisotropy by dual-probe laser interferometer

A novel dual-probe laser interferometer, which has the advantage that it measures the surface displacements of the same surface wave at two locations, has been used to measure the speeds of surface waves and pseudo-surface waves on a silicon single crystal. Two configurations have been investigated. When the distance between the two points of detection is large relative to the length of the acoustic wave, the two signals detected by the probes are separated in the time domain. Hence the signal travel time can be found by the autocorrelation method. When the distance between the two points of detection is short relative to the acoustic wave, a composite signal is obtained consisting of two overlapping signals. Then the power cepstrum method is employed to extract the signal travel time between the probe points. The speeds for both types of surface waves are shown as functions of direction relative to the [001] crystal axis. The results have been compared with theoretical calculations and line-focus acoustic microscopy measurements.     

互易法校准压电型声发射传感器的研究与实现

声发射传感器的校准是实现声发射定量技术的前提,依据电声换能器互易原理,在计算互易常数的基础 上,建立了适用于压电型声发射传感器表面波和纵波互易的校准系统。通过设置特定的激励信号波形,依据接收 电压信号与激励电流信号之间的时间延迟,准确获取电流信号与电压信号对应的特征值,实现了声发射传感器的表面波和纵波互易校准。由于传感器的尺寸效应,传感器在高频时的表面波速度灵敏度低于纵波灵敏度,不确定度评定结果表明,声发射传感器速度灵敏度的互易法校准不确定度为1. 2 dB。