An acoustic emission energy analysis and its use to study damage in laminated composites

A new technique which uses the output of a true RMS voltmeter to measure the acoustic emission energy output of a transducer is presented. To demonstrate its use in a typical case, this procedure is used to measure acoustic emission energy during tensile tests on [0°/±30°/90°]_s glass-epoxy laminate uniaxial and 10° off-axis tensile coupons. The test results were compared with numerical predictions of laminate response and acoustic emission energy. The experiments indicate that acoustic emission energy can be used to indicate the onset of ply and interlaminar failure.     

Numerical calculations of acoustic emission

Two computer programs which solve the partial differential equations for sound propagation numerically are applied to the study of problems in acoustic emission. The programs use finite difference and finite element techniques to calculate sound fields due to distributions of sources in complex geometries in two dimensions. The potential to handle more complex geometries and to model more realistic sources is the main advantage of these numerical calculations over the analytic calculations. The main disadvantage of the numerical techniques is the cost of obtaining results since a large main frame computer or supercomputer is required. Both numerical methods are found to agree well with analytic calculations using Green's functions. The finite difference method agrees very well with the analytic calculations but, in its current implementation, is much slower and contains more numerical noise compared to the finite element method. The finite element method has the additional advantage of being capable of handling more complex geometries than the finite difference method.     

Fracture behavior of concrete-concrete interface using acoustic emission technique

The fracture behavior of concrete-concrete interface is characterized using acoustic emission (AE). Beams of different sizes having jointed interface between two different strengths of concrete are tested. The results of load, displacement, CMOD, AE-events and AE-energy are analyzed. The width of fracture process zone and damage zone are computed using AE-data and are found to be independent of size. It is observed that, as the difference in compressive strength of concrete on either side of interface increases, the load carrying capacity, number of AE-events, AE-energy, width of fracture process zone and damage zone decreases.     

Improved acoustic emission locations

Acoustic emission locations have been determined in a modest-size specimen of Westerly granite ( 50×50×200 mm) under uniaxial compressive loading to failure. Periodically during the experiment acoustic pulses were transmitted from a set of source transducers to an acoustic emission receiver array. Axial and transverse velocities were determined from these transmission data. The piezoelectric transducers used in this study were too large to be treated as points. A separate experiment was performed to determine the effect of finite transducer size and incident angle on observed arrival times. Velocity pulse arrival times were inverted for the known source transducer positions using (1) an isotropic, fixed velocity model, (2) the observed anisotropic, stress dependent velocity field, and (3) transducer size corrections. Incorporation of the observed velocity field and transducer size effects dramatically improved location accuracy. Acoustic emissions were located using this technique. Synthetic data, exact and with realistic errors, were inverted to determine location uncertainties at various locations throughout the sample. Suggestions are made for further improvements.     

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。     

Recovery of microcrack parameters in mortar using quantitative acoustic emission

A three-dimensional quantitative acoustic emission (AE) analysis of microcracking in unreinforced mortar beams was conducted. In order to facilitate the analysis of the large amounts of data generated by an AE test, a simplified method for the inversion of AE signals was developed. By applying the theoretical Green's function for an infinite space, the multichannel deconvolution normally required of AE data inversion reduces to a nonlinear curve-fitting problem. Using this procedure, microcracking in a mortar specimen was evaluated using a seismic moment tensor representation. Source-time functions for the microcracks were also recovered. The locations of the AE events were calculated, and damage localization was observed. The moment tensor analysis showed the dominant mode of microfracture to be mode II, with a limited number classified as mixed mode. A microstructural mechanism for this behavior is presented.     

Characterization of lead-based relaxor ferroelectric crystals by acoustic emission

The acoustic emission (AE) method is presented as a useful complementary tool for nondestructive characterization of selective relaxor ferroelectrics. Combined measurements of the dielectric permittivity and AE activity as a function of temperature and externally applied electric fields have been carried out using Pb(Mg_1/3Nb_2/3)O₃–33%PbTiO₃ (PMN-0.33PT) single crystals and Pb(Sc_0.5Ta_0.5)O₃ (PST) single crystals. Anomalous behavior in the electric field dependencies of the characteristic temperatures, T_m (diffuse permittivity maximum in PMN-PT) and T_n (formation of incommensurate antiferroelectric domains in PST), and the associated AE intensities has been observed. Both T_m and T_n exhibit minima coinciding with the AE activities maxima at critical values of the applied dc electric fields, 0.5 and 0.13 kV respectively. These phenomena are discussed mainly in terms of the interaction of the external field with the random electric fields originating from the polar nanoregions in relaxor ferroelectrics.     

Fatigue cracks nucleation on steel, acoustic emission and fractal analysis

In this paper, a new acoustic emission (AE) diagnostic technique, for the study of fatigue cracks nucleation and propagation on steel, was investigated. Using the fractal analysis, and the box-counting method (BCM) in particular, it is possible to characterize the spatial distribution of the prime AE sources through the fractal dimension (D) that evolve with the number of fatigue cycles (N) of the specimen. D–N curves were found useful to identify the condition of incipient collapse due to the nucleation and propagation of fatigue cracks on steel. It is possible to use the fractal dimension as a damage parameter. In all tested specimens, the crisis occurs within the same range of values of fractal dimension. The results suggest that it is possible to anticipate the detection of crack beginning relating to the other theoretical or experimental techniques.     

A finite difference approach to acoustic tomography in anisotropic media

In this paper, a finite difference formulation for acoustic wave propagation is used as the basis for tomographic reconstruction. This approach offers some interesting advantages over traditional, ray based methods; particularly for anisotropic media. Since this approach provides information on the full acoustic field (not individual rays), it offers a conventional way to incorporate beam skew and ray bending phenomena directly into the problem formulation. Here, we present a tomographic reconstruction algorithm which is adapted from the algebraic reconstruction technique (ART) to take full advantage of the finite difference formulation of the problem. Results are presented to illustrate the utility of this approach.     

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.     

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。     

汽车起重机臂梁声发射检测技术初探

汽车起重机臂梁在长期的使用过程中，受交变负荷、压力等载荷作用，疲劳破坏导致各种缺陷，使其安全性下降。传统的无损检测方法费时、费力，而且在汽车起重机臂梁的检测中难于施行。声发射检测可以预测到由未知不连续活动缺陷引起系统的灾难性失效，通过加载、放置探头在起重机臂梁上，可对其整体进行快速的探伤检测。重点分析了汽车起重机臂梁现状，以及将声发射检技术应用于QY8C汽车起重机臂梁检测的方法及其试验结果，并指出在汽车起重机臂梁声发射检测存在的问题。     

Gabor小波时频分析在声发射信号处理中的应用

介绍Gabor小波时频分析的基本理论,用数值仿真举例验证Gabor小波时频分析可同时在时域和频域上来表征信号分布的特点。利用该方法可得到声发射源定位中传感器接收到的声发射信号在时域和频域上的分布,并用于区分声发射信号的传播模式和不同的信号组成,得到声发射波群的准确到达时间,从而提高时差定位的精度。     

Ferroic phase boundaries investigations by the acoustic emission method

 By the acoustic emission, dilatometric and optic methods the phase transitions have been investigated in PbTiO₃, PbZrO₃ and PbHfO₃ ferroelectric crystals. By the quantities of the acoustic emission activity the related coherence of the phase boundaries have been appraised. In general case the distribution on the transitions by coherence of the boundaries between the transforming phases in perovskite-type crystals are following: paraelectric-ferroelectric, paraelectric-antiferroelectric, ferroelectric-antiferroelectric, antiferroelectric-antiferroelectric. The received results have been discussed generalized in order of the similarity between the ferroelectric and martensitic phase transitions. It is shown, that acoustic emission method would be to apply for identification of the phase transition type in the multiaxial ferroic crystals fruitful. Received: 22 July 1998 / Reviewed and accepted: 25 September 1998     

Signal processing techniques for filtering acoustic emission data in prestressed concrete

The current state of infrastructure in the United States and worldwide has raised the need for reliable structural health monitoring techniques. Piezoelectric sensing, such as acoustic emission, has recently gained attention due to its high sensitivity and associated capability for early detection of damage. The high sensitivity of this method, however, results in the collection of data not directly related to damage growth. Current filtering procedures focus primarily on parametric analysis of the collected signals. This study focuses on developing more robust filtering techniques for acoustic emission data collected from a prestressed concrete specimen. Simulated data was generated to enable proper identification of the source of the collected signals. Filtering criteria were developed through characterization of the energy content using a wavelet transform. The developed filters were capable of separating the induced target signals from other signals with reasonable accuracy, and the results were verified through source location. The developed filters were validated using acoustic emission data collected during a load test.     

Elastic wave emission from damage processes

The theory of elastic wave emission (i.e., acoustic emission; AE) from damage processes such as slip and microcracking is discussed. Analogous developments in the literature on earth-quake seismology and dynamic dislocation theory are noted and utilized. A general representation of the displacement field of an AE event is given in terms of the double-couple response to a distribution of moment density tensor in the source region. Results are specialized to a point source model and to a general far-field analysis of outgoing elastic waves, and conditions for validity of such representations and their low-frequency specializations are noted. Emitted wave fields are compared for tensile opening and slip events, and procedures which might enable the approximate determination of the size or area increase of tensile microcracks are discussed.     

Wavelet transform of acoustic emission signals in failure of model composites

The fundamental characteristics of acoustic emission (AE) signals, such as the attenuation, and frequency dependency of AE signals, were investigated and the fracture process of the single fiber composite (s.f.c.) was examined. As a result, the frequencies of AE signals were almost unchanged, while the amplitudes attenuated greatly with the increment of the propagation length. This proved that the frequency analysis is an effective way in processing AE signals of composite materials. In the fracture process of the s.f.c., the number of AE events was in a good agreement with the number of fiber breakages, and the sources of AE signals were the failure modes at fiber breakages. Using the proposed time-frequency method of wavelet transform (WT) to process AE signals, the microfailure modes at a fiber breakage and the microfracture mechanism, such as the sequence of each failure mode and their interaction, were made clearer. These indicated that both processing methods of AE signals, fast-Fourier transform and WT, were powerful for identifying the microfailure modes and for elucidating the microfracture mechanisms in composite materials.     

Damage classification in carbon fibre composites using acoustic emission: A comparison of three techniques

Classifying the type of damage occurring within a structure using a structural health monitoring system can allow the end user to assess what kind of repairs, if any, that a component requires. This paper investigates the use of acoustic emission (AE) to locate and classify the type of damage occurring in a composite, carbon fibre panel during buckling. The damage was first located using a bespoke location algorithm developed at Cardiff University, called delta-T mapping. Signals identified as coming from the regions of damage were then analysed using three AE classification techniques; Artificial Neural Network (ANN) analysis, Unsupervised Waveform Clustering (UWC) and corrected Measured Amplitude Ratio (MAR). A comparison of results yielded by these techniques shows a strong agreement regarding the nature of the damage present in the panel, with the signals assigned to two different damage mechanisms, believed to be delamination and matrix cracking. Ultrasonic C-scan images and a digital image correlation (DIC) analysis of the buckled panel were used as validation. MAR’s ability to reveal the orientation of recorded signals greatly assisted the identification of the delamination region, however, ANN and UWC have the ability to group signals into several different classes, which would prove useful in instances where several damage mechanisms were generated. Combining each technique’s individual merits in a multi-technique analysis dramatically improved the reliability of the AE investigation and it is thought that this cross-correlation between techniques will also be the key to developing a reliable SHM system.     

Evaluation of the repair on multiple leaf stone masonry by acoustic emission

Aim of this research was to evaluate the possible application of the AE technique to study the response of multiple leaf masonry repaired with different techniques. The results of shear tests carried out on three specimens repaired after failure are presented on an explorative basis; on two of them the acoustic emission (AE) technique was also used. Through a number of sensors and by triangulation, this technique makes it possible to identify the location of the damage, unknown initially, and subsequently to assess the stability of its evolution. Using the AE technique the area of the material where the cracks propagated during shear tests was identified. These analyses made it possible to evaluate the effectiveness of repair interventions through parameters other than stress and strain, and contributed to the identification of the most suitable methodology for their optimisation.     

Real-time damage mechanisms assessment in CFRP samples via acoustic emission Lamb wave modal analysis

This paper proposes a very promising acquisition-analysis procedure to evaluate real-time damage in carbon fiber-reinforced polymer (CFRP) composite plates by means of the acoustic emission (AE) method. It shows how, by using appropriate acquisition frequency filters and very narrow time windows, it is possible to avoid reflection at boundaries and successfully split the A0 and S0 Lamb modes of the AE signals. After that, an appropriate algorithm —based on the comparison of strength of both modes in time and frequency domains— allows one to associate each AE event to a particular damage mechanism (delamination, fiber breaking and matrix micro-cracking). Experimental results from three point bending tests carried out on 22-layer CFRP samples, with delamination artificially induced by a Teflon film, clearly demonstrate the real-time evaluation of the induced delamination and the beginning and growth of new ones.