scispace - formally typeset
Search or ask a question

Showing papers on "Acoustic emission published in 1994"


Journal ArticleDOI
TL;DR: In this article, the damage mechanisms of four short-glass-fibre-reinforced polypropylenes are investigated in tensile, tensile fatigue and crack propagation tests.

353 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the scattering of plane sound waves by a vortex by solving the compressible Navier-Stokes equations numerically and analytically with asymptotic expansions.
Abstract: The scattering of plane sound waves by a vortex is investigated by solving the compressible Navier-Stokes equations numerically, and analytically with asymptotic expansions. Numerical errors associated with discretization and boundary conditions are made small by using high-order-accurate spatial differentiation and time marching schemes along with accurate non-reflecting boundary conditions. The accuracy of computations of flow fields with acoustic waves of amplitude five orders of magnitude smaller than the hydrodynamic fluctuations is directly verified. The properties of the scattered field are examined in detail. The results reveal inadequacies in previous vortex scattering theories when the circulation of the vortex is non-zero and refraction by the slowly decaying vortex flow field is important. Approximate analytical solutions that account for the refraction effect are developed and found to be in good agreement with the computations and experiments. The prediction of the sound produced by turbulent flow requires a detailed knowledge of acoustic source terms. Direct computation of both the acoustic sources and far-field sound using the unsteady Navier-Stokes equations allows direct validation of aeroacoustic theories. In a recent review by Crighton (1988), the difficulties involved in direct computations of aeroacoustic fields are discussed. These include: the large extent of the acoustic field compared with the flow field; the small energy of the acoustic field compared to the flow field; and the possibility that numerical discretization may introduce a significant sound source due to the acoustic inefficiency of low-Mach-number flows. In order to address these difficulties, Crighton proposed that direct computations be performed on elementary model aeroacoustic problems whose physics are well understood. For this reason, and to validate our numerical scheme for direct computation of aeroacoustic problems, we investigate the scattering of sound waves by a compressible viscous vortex. This problem has received significant attention, and thus provides a large database of theory, numerics and experiment with which detailed comparisons may be made. Yet there is significant disagreement amongst the various theories, which has not yet been fully rectified. Therefore, the purpose of the current work is twofold: to validate our numerical scheme for direct computation of aeroacoustic problems using the unsteady Navier-Stokes equations, and to investigate the scattering of sound waves by a compressible viscous vortex.

146 citations


Journal Article
TL;DR: In this article, the authors used lead breaks to generate simulated acoustic emission signals in an aluminum plate at angles of 0, 30, 60, and 90 degrees with respect to the plane of the plate.
Abstract: Lead breaks (Hsu-Neilsen source) were used to generate simulated acoustic emission signals in an aluminum plate at angles of 0, 30, 60, and 90 degrees with respect to the plane of the plate. This was accomplished by breaking the lead on slots cut into the plate at the respective angles. The out-of-plane and in-plane displacement components of the resulting signals were detected by broad band transducers and digitized. Analysis of the waveforms showed them to consist of the extensional and flexural plate modes. The amplitude of both components of the two modes was dependent on the source orientation angle. This suggests that plate wave analysis may be used to determine the source orientation of acoustic emission sources.

138 citations


Journal ArticleDOI
TL;DR: In this article, the authors use a modal-style approach for the analysis of the exterior radiation characteristics of structures and show that the accuracy of the acoustic modal representation depends on the number of degrees of freedom in the radiation operator.
Abstract: The use of a modal‐style approach for the analysis of the exterior radiation characteristics of structures has recently received increasing attention. This approach generally seeks to find a set of orthogonal functions, or acoustic modes, that diagonalizes a radiation operator. These acoustic modes are found through an eigenfunction or singular value decomposition of the radiation operator. The eigenvalue or singular value associated with a given acoustic mode is directly proportional to the radiation efficiency of that acoustic mode. The acoustic mode represents a particular velocity pattern on the surface of the radiator. As with the analogous problem of finding structural natural frequencies and mode shapes, the accuracy of the acoustic modal representation depends on the number of degrees of freedom in the radiation operator. The radiation efficiency of the most efficient acoustic mode has a finite upper bound, and converges fastest with increasing degrees of freedom. Each additional degree of freedom in a model introduces a new least efficient acoustic mode. In fact, the radiation efficiency of the least efficient acoustic mode may be forced arbitrarily close to zero by introducing sufficient degrees of freedom. The most efficient acoustic modes are least sensitive to perturbations in their velocity patterns, while the least efficient acoustic modes are most sensitive. These characteristics of the exterior modal acoustic representation have significant implications for the use of the modal representation in design, optimization, and active noise control. This paper explores these issues using the example of a finite baffled beam.

93 citations


Journal ArticleDOI
TL;DR: In this article, a correlation between acoustic emission and the stress intensity factor was derived which allows prediction of the ΔK from acoustic emission measurements, at very low values of ΔK, corresponding to the onset of fatigue crack growth, was expected that acoustic emission would detect this event.

89 citations


Journal ArticleDOI
TL;DR: A microfracture mechanism of the dental resin composites containing spherically-shaped filler particles is proposed based on the results obtained from the AE-releasing pattern, two-dimensional AE location, and fracture surface findings.
Abstract: The effects of spherically-shaped filler particles on bending strength, bending elastic modulus, and fracture toughness of resin composites were studied. The filler content was changed by 0, 20, 40, 60, and 70 wt%. Bending properties and fracture toughnesses were determined on three-point bending specimens. Acoustic Emission (AE), i.e., the elastic wave released from a localized source in the material, was detected by sensors of a high-sensitivity and low-noise resonance type during the fracture toughness test. Detected acoustic emission signals were analyzed for parameters such as amplitude, events, and locations. The fractured surf ace was examined by a scanning electron microscope.The bending strength and the fracture toughness showed almost the same trend in their increasing rates by filler content, but the elastic modulus showed a much higher increasing rate. A microfracture mechanism of the dental resin composites containing spherically-shaped filler particles is proposed based on the results obtain...

88 citations


Journal ArticleDOI
TL;DR: In this paper, the authors suggest that acoustic emission sensing may be a useful tool for controlling the grinding process, and the following observations can be made: fluid coupling to the grinding zone is easy to implement and is effective; AE is superior to force for detecting wheel/work contact at start of cycle.

85 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the crack propagation in electrostrictor, piezoelectric and phase-change antiferroelectric ceramic multilayer actuators with an interdigital electrode configuration.
Abstract: Destruction mechanisms were investigated in electrostrictor, piezoelectric and phase-change antiferroelectric ceramic multilayer actuators with an interdigital electrode configuration. Simultaneous observations were done by three different methods; visual observation with a charge coupled device (CCD) microscope, field-induced strain and acoustic emission (AE) measurements. During a cyclic electric field application, the crack was initiated from the edge of an internal electrode and propagated obliquely to another electrode in the piezoelectric sample, while in the antiferroelectric, the crack was initiated between a pair of electrodes and propagated parallel to the electrodes. The field-induced strain curve exhibited asymmetric and enhanced strains in all the samples during the crack propagation, probably due to the internal delaminations. The AE count also increased drastically before the destruction.

73 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used acoustic emission doublets (AE doublets) in the triaxial hodogram method to evaluate the direction and distance of subsurface extension cracks.
Abstract: We have developed a precise relative source location technique using acoustic emission doublets (AE doublets) in the triaxial hodogram method to evaluate the direction and distance of subsurface extension cracks. An AE doublet is a pair of acoustic emissions with similar waveforms and adjacent locations on the same crack but which occur at different times. The relative source location is estimated by an analysis in the frequency domain. The relative distance between two AE sources is determined from the difference of P-S arrival time delays by cross-spectrum analysis. The relative direction is derived using a spectral matrix from the difference in P-wave polarization directions. We also propose a method to optimize the estimated relative location by using a group of AE doublets. The accuracy of the estimated source location was confirmed by performing field experiments. The relative locations of artificial wave sources about 150 m from a triaxial detector can be estimated with distance errors of less than 1 m, and direction errors of less than 3.8 degrees in both azimuth and inclination. Results of the application of this analysis on AE doublets in a geothermal field demonstrate its ability to evaluate deeper subsurface fractures.

67 citations


Patent
Walter Zörner1
17 Feb 1994
TL;DR: In this article, a method for the early detection and location of a change in a component of a turbine, in particular in a turbine blade, in the event of a deviation from a standard value in at least one measured value determined during operation of the turbine, an acoustic spectrum generated by the component is measured in the interior of turbine and compared with a reference spectrum.
Abstract: In a method for the early detection and location of a change in a component of a turbine, in particular in a turbine blade, in the event of a deviation from a standard value in at least one measured value determined during operation of the turbine, an acoustic spectrum generated by the component is measured in the interior of the turbine and compared with a reference spectrum. In order to increase the intensity of signals in the acoustic spectrum, the components of the turbine, preferably runner blades, are excited from outside to acoustic emission (resonance test). An apparatus has a number of sensors for recording operating parameters of the turbine. The sensors are connected to a device for measured value conditioning. In order to detect and locate changes, an acoustic measurement probe which can be inserted from the outside into the interior of the turbine, is connected to the device.

62 citations


Journal ArticleDOI
TL;DR: In this paper, the application of the acoustic emission (AE) technique to locate and monitor damage growth in titanium matrix composites (TMC) was investigated using several optical techniques including a long focal length, high magnification microscope system with image acquisition capabilities.
Abstract: The application of the acoustic emission (AE) technique to locate and monitor damage growth in titanium matrix composites (TMC) was investigated. Damage growth was studied using several optical techniques including a long focal length, high magnification microscope system with image acquisition capabilities. Fracture surface examinations were conducted using a scanning electron microscope (SEM). The AE technique was used to locate damage based on the arrival times of AE events between two sensors. Using model specimens exhibiting a dominant failure mechanism, correlations were established between the observed damage growth mechanisms and the AE results in terms of the events amplitude. These correlations were used to monitor the damage growth process in laminates exhibiting multiple modes of damage. Results revealed that the AE technique is a viable and effective tool to monitor damage growth in TMC.

Journal ArticleDOI
TL;DR: In this article, an approach is proposed for measuring the grinding wheel geometric characteristics by using the dressing tool as a touch probe and the acoustic emission level generated in the contact as a trigger for the dimensional measurement.

Journal Article
TL;DR: In this paper, a computer model for elastic Lamb waves in cylindrical symmetry was used to study the influence of the characteristics of the source stress on the resulting Lamb waves, and the dependence of the results on the cell size used in the computer model was studied.
Abstract: The overall objective of this work is the study of the structure of Lamb waves produced in thin plates and the techniques used to measure such waves. Laboratory experiments were conducted using a lead break source on an aluminum plate. Tgese experiments were used to vslidate a computer model for elastic waves in cylindrical symmetry. The computer model was used to study the influence of the characteristics of the source stress on the resulting waves. The rise time, source width, and the distribution of the stress over the some were all considered. The dependence of the results on the cell size used in the computer model was studied, and the computer requirements for an exten-sion of this work to three dimensions were considered. The effect of aperture size on the signal fidelity from potential scoustic sensors was examined. Validation of the computer model allows the model to be used for the study of certain acoustic emission phenomena which are difficult or impossible to measure in the laboratory

Journal ArticleDOI
TL;DR: In this paper, the fast Fourier transform spectrum of acoustic emission during CO2 laser welding of Al 1100 shows frequency components in the 3-9 kHz range that can be identified with the presence of a keyhole and correlate with penetration.
Abstract: The fast Fourier transform spectrum of acoustic emission during CO2 laser welding of Al 1100 shows frequency components in the 3-9 kHz range that can be identified with the presence of a keyhole and correlate with penetration. In addition, a study of the effect of anodization and surface pretreatment of Al 1100 with excimer laser radiation has shown that acoustic emission at 9-10 kHz arises from burning off of surface oxide. A comparison of these results with those predicted from simple thermal and fluid dynamical models yields good agreement with theory. These results indicates that acoustic emission over specific frequency ranges may be highly diagnostic of laser processing conditions.

Journal ArticleDOI
TL;DR: In this article, an on-line monitoring tool, Acoustic Emitting (AE), is used to measure and measure the fatigue damage progression in composite materials, and an attempt is made to estimate the cumulative damage to predict residual life.
Abstract: Fatigue damage progression in composite materials is governed by different failure mechanisms, each of which contributes to the damage to a different extent. To assess the cumulative damage undergone by the material and to estimate the residual life, it is necessary to discriminate and characterize the failure mechanisms. This necessitates an on-line technique which can be used to monitor and measure the damage progression as it occurs. Acoustic emission (AE), an on-line monitoring tool, is ideally suited for this purpose. To understand fatigue damage in terms of different failure mechanisms using this technique, it becomes necessary to identify and establish their AE characteristics. This paper discusses the experimental investigations carried out on unidirectional glass fibre reinforced plastic (GFRP) composite specimens in which the acquired AE data was analysed utilizing pattern recognition (PR) techniques. The results obtained from the experiments show that three different failure mechanisms which primarily contribute to the damage at any given stage can be discriminated and characterized. Further, an attempt is made to estimate the cumulative damage to predict residual life.

Journal ArticleDOI
TL;DR: In this paper, a European project for the measurement of AE signals during the grinding process was presented and analyzed in a frequency range up to 3.5 MHz, where different wear phenomena can be readily distinguishable by their individual AE signature at these frequencies.

Journal ArticleDOI
TL;DR: In this article, the effect of tool wear and chip-form change in face milling was investigated and the results indicated that the ringdown counts, rise time increase with progressive tool wear; however number of events and event duration are sensitive to chip form.

Journal ArticleDOI
TL;DR: In this paper, the fracture initiation at hydrides in smooth tensile specimens of Zr-2.5Nb pressure tube material was investigated at elevated temperatures up to 300 °C using an acoustic emission (AE) technique.
Abstract: Crack initiation at hydrides in smooth tensile specimens of Zr-2.5Nb pressure tube material was investigated at elevated temperatures up to 300 °C using an acoustic emission (AE) technique. The test specimens contained long, radial hydride platelets. These hydrides have their plate normals oriented in the applied stress direction. Below~100 °C, widespread hydride cracking was initiated at stresses close to the yield stress. An estimate of the hydride’s fracture strength from this data yielded a value of ~520 MPa at 100 °C. Metallography showed that up to this temperature, cracking occurred along the length of the hydrides. However, at higher temperatures, there was no clear evidence of lengthwise cracking of hydrides, and fewer of the total hydride population fractured during deformation, as indicated by the AE record and the metallography. Moreover, the hydrides showed significant plasticity by being able to flow along with the matrix material and align themselves parallel to the applied stress direction without fracturing. Near the fracture surface of the specimen, transverse cracking of the flow-reoriented hydrides had occurred at various points along the lengths of the hydrides. These fractures appear to be the result of stresses produced by large plastic strains imposed by the surrounding matrix on the less ductile hydrides.



Journal ArticleDOI
TL;DR: In this paper, the authors reported that there was a certain relation between the indentation hardness of rock and the AE parameters, such as accumulated number of events, peak RMS and integrated RMS.

Journal ArticleDOI
TL;DR: In this paper, the failure behavior and the damage zone growth in a glass fiber mat-reinforced thermoplastic polypropylene (GMT-PP) were studied on static tensile loaded single-edge notched (SEN-T) and compact-te...
Abstract: The failure behavior and the damage zone growth in a glass fiber mat-reinforced thermoplastic polypropylene (GMT-PP) were studied on static tensile loaded single-edge notched (SEN-T) and compact-te...

Journal ArticleDOI
TL;DR: In this article, a tensile fracture model for unidirectional composites has been proposed, where the interface contribution of SiC filaments embedded in an aluminium (1050 and 5083 alloys) or a titanium (Ti-6Al-4V) matrix has been analyzed in an effort to obtain the interfacial contribution in terms that could be incorporated into a Tensile Fracture Model.
Abstract: Fragmentation tests of single SiC filaments embedded in an aluminium (1050 and 5083 alloys) or a titanium (Ti-6Al-4V) matrix have been analysed in an effort to obtain the interface contribution in terms that could be incorporated into a tensile fracture model for unidirectional composites. Depending on the matrix, two regimes of interfacial stress transfer can be distinguished within the whole range of tested temperatures. For the SCS2/5083 system, plastic deformation of the alloy limits the stress transfer, and the interface contribution thus finds its expression in the shear stress of the matrix. for the SCS6/Ti-6Al-4V system, friction is the leading process and the interface contribution strongly depends on the stress state around the fibre. Assuming a temperature dependent compressive radial stress up to 925‡C, an effective transfer shear stress may be easily calculated for unidirectional SCS6/Ti-6Al-4V composites.

Journal ArticleDOI
TL;DR: In this article, the formation and evolution of faulting in three initially intact, oil-saturated specimens of Clashach sandstone is examined under conditions of constant strain rate loading at three different confining pressures, simulating the effect of tectonic loading at different depths in the Earth's upper crust.
Abstract: SUMMARY The formation and evolution of faulting in three initially intact, oil-saturated specimens of Clashach sandstone is examined under conditions of constant strain rate loading at three different confining pressures, simulating the effect of tectonic loading at different depths in the Earth’s upper crust. After a fault is formed the specimens are slid for a time, and then the initial confining pressure is increased to simulate the long-term recovery of strength expected in the Earth. The differential stress u and natural acoustic emissions (AE) are measured during the three separate phases of fault nucleation, sliding and strengthening. At the end of each individual phase the fluid permeability is measured by a pulse-decay technique at constant stress. The AE are interpreted using a mean field theory for damage evolution which calculates a mean crack length (c) from the seismic event rate N and the b-value, and a mean energy release rate (G) from u and (c). (c) and (G) both increase non-linearly in the nucleation stage, and are associated with large fluctuations in the scaling exponent b. In contrast, the b value remains almost constant at a value near unity in the sliding phase, even when the fault is mechanically strengthened. Small fluctuations in ( G ) observed during sliding may be associated with the breaking of individual asperities. The relative stability of b, combined with a relatively constant sliding stress, implies that frictional sliding has all the hallmarks of a self-organized critical (SOC) phenomenon. In contrast, the fresh fracture of intact rock shows a continuum of states with power-law scaling at a variety of b values, where b is negatively correlated to (G). During the fault nucleation stage the implied subcritical crack growth index n’ obtained from the AE data shows a distinct break of slope during the failure of the intact specimens, from n’ 2, consistent with a theoretical transition from stable damage to dynamic instability. A possible mechanism is the transition from local mechanisms dominated by dilatant hardening (negative feedback) to microcrack coalescence (positive feedback). Interpreted in this way, the results imply that the organized coalescence of microcracks to form a fault occurs before the ultimate strength (peak stress) is reached. This emphasizes the fact that local mechanisms of softening or hardening do not always scale simply to the macroscopic rheology in a heterogeneous medium. The fluid permeability reduces steadily throughout all phases of deformation, testament to the self-sealing nature of deformation in porous sedimentary rocks.

Journal ArticleDOI
TL;DR: In this article, a small four-point-bend jig was used in a scanning electron microscope (SEM) to monitor the tensile fracture processes in iron and nickel oxide scales in situ.
Abstract: A small four-point-bend jig has been used in a scanning electron microscope (SEM) to monitor the tensile fracture processes in iron and nickel oxide scales in situ. The scales were 3–40 μm thick, the strain rate was 4×10−5 sec−1 and acoustic emission (AE) was used to correlate signals with specific cracking events. The technique provided detailed information of the failure processes, and several micrographs were taken as the crack pattern developed during testing. Failure started with short random through-scale cracks. These cracks soon formed a regular pattern. Spallation only occurred at much higher strains and resulted in very energetic AE signals. These signals were used together with the SEM observations to determine the strain to cause spallation. Measurements of the crack spacing as a function of strain showed that plastic stress relaxation by interfacial slip and/or substrate yielding processes affected cracking. Thus, the oxide remained attached to the substrate longer than would be expected from purely elastic behavior. An analysis based on the observed crack spacing at the onset of spallation gave ratios of 0.8–1.9 and ∼0.9 between tensile and interfacial strength for iron and nickel oxides scales, respectively.

Journal ArticleDOI
TL;DR: In this article, the application of a Gaussian neural network for the monitoring of manufacturing processes is described, where the network memory is formed by a self-organized learning process which is stimulated by a multi-component input comprised of acoustic emission (AE) signals and process parameters.


Proceedings ArticleDOI
01 May 1994
TL;DR: In this paper, the authors investigated two acoustic emission (AE) sensing techniques for monitoring, detecting and locating flaw growth in structural components, and defined the goals that need to be achieved in AE technology before it can successfully be implemented into a smart structural health monitoring system (SHMS).
Abstract: A smart structural health monitoring system (SHMS) requires various sensing technologies to detect and locate flaws, and assess their criticality to the structural integrity of the aircraft. To realize its full potential, a SHMS must be capable of remotely sensing flaw growth and location. Acoustic emission (AE) is one of the few sensing technologies that is capable of direct and remote sensing of flaw growth. Currently, there are two AE sensing techniques used for monitoring, detecting and locating flaw growth in structural components. In one technique, specific AE event parameters are captured by narrowband transducers and are studied to identify their source and location. The other technique studies the whole AE waves captured by wideband transducers and then detects and locates flaw growth based on waveform analysis and the wave propagation characteristics of the structure being monitored. This paper investigates both AE techniques, establishes their limitations, and defines the goals that need to be achieved in AE technology before it can successfully be implemented into a SHMS.

Journal ArticleDOI
TL;DR: In this paper, an analysis of acoustic sensing for laser grooving and cutting processes is presented, where the interaction between the coaxial gas jet and the erosion front were developed using the wave equation to relate acoustic emission to the geometry of the groove or kerf.