Showing papers on "Acoustic emission published in 2002"

A brief review: acoustic emission method for tool wear monitoring during turning

Abstract: Research during the past several years has established the effectiveness of acoustic emission (AE)-based sensing methodologies for machine condition analysis and process monitoring. AE has been proposed and evaluated for a variety of sensing tasks as well as for use as a technique for quantitative studies of manufacturing processes. This paper reviews briefly the research on AE sensing of tool wear condition in turning. The main contents included are: 1. The AE generation in metal cutting processes, AE signal classification, and AE signal correction. 2. AE signal processing with various methodologies, including time series analysis, FFT, wavelet transform, etc. 3. Estimation of tool wear condition, including pattern classification, GMDH methodology, fuzzy classifier, neural network, and sensor and data fusion. A review of AE-based tool wear monitoring in turning is an important step for improving and developing new tool wear monitoring methodology.

Elastic waves in bodies with initial (residual) stresses

Abstract: An analysis is made of the results obtained in the three-dimensional linearized theory of elastic waves propagating in initially stressed solids. Consideration is given to surface waves along planar and curvilinear boundaries and interfaces, waves in layers and cylinders, waves in composite materials, waves in hydroelastic systems, and dynamic problems for moving loads. The results were obtained in exact formulations.

Damage assessment of reinforced concrete beams qualified by acoustic emission

Abstract: Acoustic emission (AE) techniques have been widely studied in concrete engineering and are presently being applied to monitoring concrete structures in service. In order to assess the damage levels of the structures, a criterion based on the Kaiser effect of AE is proposed in the recommended practice recently published by the Japanese Society for Non-Destructive Inspection. New AE parameters of load and calm ratio are defined for qualification of the damages. Accordingly, the feasibility of the damage qualification is experimentally examined using reinforced concrete (RC) beams damaged under incremental cyclic loading. It was found that the damages qualified by the 2 ratios were in good agreement with actual damages of the beams. This suggests that the damages of such RC structures in service as bridges, docks, and buildings are quantitatively assessed by simply applying cyclic loading and monitoring AE activity.

Experiments to relate acoustic emission energy to fracture energy of concrete

Abstract: Acoustic emission (AE) was used to measure energy associated with fracture of standard concrete test specimens. The goal of the work was to identify ways in which AE could be used to quantify damage in generic laboratory structures for the purpose of tuning damage models. A series of mortar and concrete specimens of different compositions were tested for fracture energy G-sub-f, while simultaneously being monitored for acoustic emission energy release. Reasonable correlation between the 2 quantities was observed for fine-grained specimens; however, the relationship was not as good for coarse-grained specimens. Toughening mechanisms such as friction are suggested as being responsible for the poor relationship observed in the course-grained materials. It is further suggested that AE energy release can be related to actual crack formation energy but not to friction and other internal energy dissipation or toughening mechanisms.

Surface Acoustic Waves in Materials Science

Abstract: Laser-generated surface waves provide new tools for studying material properties, from linear elastic behavior to fracture.

Acoustic emission from paper fracture.

Abstract: We report tensile failure experiments on paper sheets The acoustic emission energy and the waiting times between acoustic events follow power-law distributions This remains true while the strain rate is varied by more than 2 orders of magnitude The energy statistics has the exponent beta approximately 125+/-010 and the waiting times the exponent tau approximately 10+/-01, in particular, for the energy roughly independent of the strain rate These results do not compare well with fracture models, for (brittle) disordered media, which as such exhibit criticality One reason may be residual stresses, neglected in most theories

Comparison of on-line partial discharge detection methods for HV cable joints

Abstract: The capacitive coupler, acoustic emission (AE) sensor and radio frequency current transducer (RFCT) have been used to detect partial discharge (PD) activity within a 132 kV prefabricated cable joint containing a known defect. Although each of the three methods has been applied individually under different situations, a comprehensive investigation and comparison of these three methods has not yet been made. Results obtained were compared to measurements made using the conventional electrical detection method. The quantification of the capacitive coupler measured signal in mV with a discharge apparent quantity in pC has been investigated and an on-line quantification method proposed. The capacitive coupler has good detection sensitivity and PD location can be realised by studying the time of Right between signals from two sensors. The difference in pulse shape, spectrum and time of flight between an internal discharge and external interference has been investigated. The AE approach has the advantage of being free from electrical interference. However, investigation indicated that AE signals were significantly attenuated within the cable joint. RFCTs were used to detect the discharge current flowing through the cable sheath. Where the detection sensitivity was low, a wavelet de-noising method was applied to process the RFCT signals and proved to tie effective in increasing the measurement signal to noise ratio.

Waveform based clustering and classification of AE transients in composite laminates using principal component analysis

Abstract: Acoustic emission (AE) transients in composite laminate tensile test specimens are analyzed by principal component analysis (PCA). Signals resulting from matrix cracking and local delaminations were acquired by broadband transducers. The aim was to study the clustering and classification ability of PCA based on the time history of the recorded AE events. An unsupervised clustering analysis showed that AE signatures from matrix cracking and local delaminations were separated in different clusters. Some differentiations within these two types of signals were also observed. From the classification based on a training set, a majority of the signals in the test set were given the correct class membership. The applied pattern recognition method is an objective method using the complete information in the AE transients and the investigation shows that it may be a useful complement in the field of non-destructive evaluation.

Implosion of an underwater spark-generated bubble and acoustic energy evaluation using the Rayleigh model.

Abstract: The growth, collapse, and rebound of a vapor bubble generated by an underwater spark is studied by means of high-speed cinematography, simultaneously acquiring the emitted acoustic signature. Video recordings show that the growth and collapse phases are nearly symmetrical during the first two or three cycles, the bubble shape being approximately spherical. After 2–3 cycles the bubble behavior changes from a collapsing/rebounding regime with sound-emitting implosions to a pulsating regime with no implosions. The motion of the bubble wall during the first collapses was found to be consistent with the Rayleigh model of a cavity in an incompressible liquid, with the inclusion of a vapor pressure term at constant temperature within each bubble cycle. An estimate of the pressure inside the bubble is obtained measuring the collapse time and maximum radius, and the amount of energy converted into acoustical energy upon each implosion is deduced. The resulting value of acoustic efficiency was found to be in agreement with measurements based on the emitted acoustic pulse.

A wavelet transform applied to acoustic emission signals: part 1: source identification #

Abstract: A database of wideband acoustic emission (AE) modeled signals was used in Part 1 to examine the application of a wavelet transform (WT) to identify AE sources. The AE signals in the database were created by use of a validated three-dimensional finite element code. These signals represented the out-of-plane displacements from buried dipole sources in aluminum plates 4.7 mm thick and of small and large lateral dimensions. The surface displacement signals at three far-field distances were filtered with a 40 kHz high-pass filter prior to applying the WT. The WTs were calculated with AGU-Vallen Wavelet, a freeware software program. The effects of propagation distance, AE source type, and the depth of the AE source below the plate surface were examined. Specifically, a ratio of the WT magnitude (WT coefficient) from the fundamental anti-symmetric mode to that from the fundamental symmetric mode was studied for correlation with the AE source type. The WT magnitudes were those corresponding to a particular group velocity and signal frequency for each mode. For sources in the large plate located at the same depth, the ratios were able to distinguish different source types and exhibited only small changes with increasing propagation distance. But, when the variable of depth of the source was introduced, the ratios did not uniquely classify the AE source type. In the case of the small coupon plate specimen, reflections from the specimen edges distorted and complicated the WTs. Since the current coupon database excludes (except for one case) the parameter of changes in the distance of the source from the coupon sides, a full examination of these complications was not possible.

Fatigue crack monitoring of riveted aluminium strap joints by Lamb wave analysis and acoustic emission measurement techniques

Abstract: Statistics show that fatigue crack development comes first and foremost as a damage source in aerospace metallic structures. Currently, widespread methods are available to inspect these structures, but they are quite time-consuming, costly and require the structural system to be idle. Next, attempts to develop damage detection integrated systems are paramount for the safety and cost of such structures. This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. In this experimental analysis, Lamb waves are excited and received outside the joint area using piezoelectric transducers coupled onto the plates. The detected damage is cracks in joint resulting from fatigue loading. The collected signals on the piezoelectric transducers are analysed using Hilbert transform and time–frequency analysis. It is shown that the final interpretation of Lamb wave analysis may provide a means of sizing the defects and following the crack development. In addition to that, an acoustic emission system is used jointly with the Lamb wave analysis in order to discuss results and damage development. Finally, it is demonstrated that both methods can work together and the results obtained are in good agreement.

Acoustic Emission Monitoring of Small Wind Turbine Blades

Abstract: Wind turbine blade certification tests, comprising a static test, a fatigue test, and finally a residual strength test, often involve sudden audible cracking sounds from somewhere within the blade, without the operators being able to locate the noise source, or to determine whether damage (minor or major) has occurred. A current EC-funded research project is looking at the possibility of using acoustic emission (AE) monitoring during testing of fibre composite blades to detect such events and assess the blade condition. AE can both locate and characterise damage processes in blades, starting with non-audible signals occurring due to damage propagation at relatively low loads. The test methodology is discussed in the context of the blade certification procedure and results are presented from a series of static and fatigue blade tests to failure in the laboratory. Inferences are drawn about small differences in the manufacture of the nominally identical blades and conclusions are presented for the application of the methodology.Copyright © 2001 by The American Institute of Aeronautics and Astronautics, Inc. and ASME

Fundamentals for remote structural health monitoring of wind turbine blades - a preproject

Abstract: This summary-report describes the results of a pre-project that has the aim of establishing the basic technical knowledge to evaluate whether remote surveillance of the rotor blades of large off-shore wind turbines has technical and economical potential. A cost-benefit analysis was developed, showing that it is economically attractive to use sensors embedded in the blade. Specific technical requirements were defined for the sensors capability to detect the most important damage types in wind turbine blades. Three different sensor types were selected for use in laboratory experiments and full-scale tests of a wind turbine blade developing damage: 1) detection of stress wave emission by acoustic emission, 2) measurement of modal shape changes by accelerometers and 3) measurement of crack opening of adhesive joint by a fibre optics micro-bend displacement transducer that was developed in the project. All types of sensor approaches were found to work satisfactory. The techniques were found to complement each other: Acoustic emission has the capability of detecting very small damages and can be used for locating the spatial position and size of evolving damages. The fibre optics displacement transducer was found to work well for detecting adhesive failure. Modelling work shows that damage in a wind turbine blade causes a significant change in the modal shape when the damage is in the order of 0.5-1 m. Rough estimates of the prices of complete sensor systems were made. The system based on acoustic emission was the most expensive and the one based on accelerometers was the cheapest. NDT methods (ultrasound scanning and X-ray inspection) were found to be useful for verification of hidden damage. Details of the work are described in annexes. (au)

Investigating the Portevin–Le Châtelier effect by the acoustic emission and laser extensometry techniques

Abstract: The Portevin–Le Châtelier (PLC) effect is a spectacular effect of dynamic strain aging in many alloys deformed in certain intervals of strain rates and temperatures. The main feature of the PLC effect is a negative strain rate sensitivity of stress, which is linked with stress fluctuations, a macroscopic spatio-temporal localization of plastic deformation (nucleation and propagation of deformation bands) and an intense acoustic emission. Recent theoretical studies have pointed out that cooperative dislocation motion is a necessary condition for plastic instabilities to occur under conditions of negative strain rate sensitivity. In this work the potential of acoustic emission and laser extensometry to monitor in situ cooperative dislocation motion due to PLC effect is reviewed and examined experimentally in an Al–1.5 wt.% Mg alloy. At the conditions of testing, the alloy exhibits a Luders phenomenon followed by the C- and/or B-type of the PLC effect. The results indicate that different dislocation processes are responsible for the Luders phenomenon, the nucleation of PLC bands and the propagation of PLC bands.

Monitoring of impact damages in composite laminates using wavelet transform

Abstract: 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.

Gear fault diagnosis using energy-based features of acoustic emission signals

Abstract: In this work, energy-based features are introduced for monitoring and diagnosis of machine conditions in spite of speed and load variations. The basic feature, termed here the energy index (EI), is a statistical measure of relative energy levels of segments of a time domain signal over a cycle. The properties of the EI are discussed and its different forms are derived. A procedure is presented for fault diagnosis of gears using the proposed features. As an illustration, time domain acoustic emission (AE) signals of a test gearbox have been processed to extract these features and to test their relative significance in the diagnostic process. The proposed technique is compared with some of the existing methods using the same AE data for early fault detection. The applicability of the proposed technique is also studied using a set of vibration data of a helicopter drivetrain system gearbox. The results show the effectiveness of the proposed features in monitoring and diagnosis of machine conditions, ...

Optical fibre acoustic emission sensor for damage detection in carbon fibre composite structures

Abstract: In this note we describe a proof-of-principle acoustic emission based damage detection experiment. A large carbon fibre composite structure was made to resemble an aerospace component. This structure was dynamically loaded after damage. Optical fibre sensors detected acoustic emission events during the load cycles. These trials demonstrate that acoustic emission is suitable for damage detection in carbon fibre composite structures and that optical fibre sensors are sensitive enough for the task.

Acoustic emission analysis for tool wear monitoring in face milling

Abstract: Monitoring the condition of the cutting tool in any machining operation is very important since it will affect the workpiece quality and an unexpected tool failure may damage the tool, workpiece and sometimes the machine tool itself. Advanced manufacturing demands an optimal machining process. Many problems that affect optimization are related to the diminished machine performance caused by worn out tools. One of the most promising tool monitoring techniques is based on the analysis of Acoustic Emission (AE) signals. The generation of the AE signals directly in the cutting zone makes them very sensitive to changes in the cutting process. Various approaches have been taken to monitor progressive tool wear, tool breakage, failure and chip segmentation while supervising these AE signals. In this paper, AE analysis is applied for tool wear monitoring in face milling operations. Experiments have been conducted on En-8 steel using uncoated carbide inserts in the cutter. The studies have been carried out with on...

Monitoring extremely slow rolling element bearings: part II

Abstract: The most established technique for monitoring the integrity of rolling element bearings is vibration analysis. However, at extremely slow rotational speeds monitoring the health of rolling element bearings is fraught with difficulty. Part I of this work details difficulties with monitoring rolling element bearings rotating at speeds of less than 2 rpm. Furthermore, it presents a study of the high frequency acoustic emission (AE) technique as a means of monitoring low-speed rolling element bearings. The application of AEs to monitor operational slow-speed bearings with bore diameters of 120 mm is presented. It is concluded that the mechanism of AE generation was the relative movement between bearing elements experiencing loss of mechanical integrity, for example, the rolling/sliding action of a roller on a defective outer race.