Acoustic Emission Transducers Research Articles

Ultrasonic characterization of ceramic fibres at ambient and elevated temperatures

A non-destructive laser-generated ultrasonic inspection system has been developed to evaluate the elastic properties of ceramic fibres. The approach uses a pulsed Nd:YAG laser to excite ultrasonic signals in fibres. The signal is detected by a piezoelectric acoustic emission transducer to obtain the appropriate frequency response suitable for an elastically one-dimensional sample. By using a differential time-of-flight system, a very accurate measure of the velocity can be obtained in the fibre, with a total scatter of less than 0.5%. This approach has been used to investigate the Young's modulus of polycrystalline carbon and boron fibres as a function of stress. Both types of fibres were found to have a Young's modulus increase as greater applied loads were imposed. The carbon and boron fibres, along with silicon carbide fibres, were evaluated at elevated temperatures up to 700 °C. The carbon fibres were found to have an immediate decrease in the Young's modulus as the temperature was increased, due to oxidation of the carbon. The Young's modulus of the boron fibres decreased only at temperatures higher than 200 °C, probably the result of a microstructural transformation or relaxation. The silicon carbide fibres were found to have no significant change in the elastic properties up to 700 °C. The ultrasonic technique was also applied to polycrystalline alumina fibres and fibre tows between ambient temperature and 1200 °C in a specially designed furnace. Using this technique, it was possible to distinguish the changes in the elasticity of the alumina fibres as they were processed into α-alumina. The change in the Young's modulus was readily apparent during phase transformations to α-alumina. In addition, the ultrasonic velocity can be used to infer information concerning any coatings that were applied to the alumina fibres. This can be used to aid in the quantification of the coating thickness and uniformity. The application of the ultrasonic inspection system has demonstrated the ability to determine rapidly and non-destructively the elastic properties in ceramic fibres. The information gained from the measurements can be used as a quality assurance technique, or can be modified to be a real-time process control/process monitoring system.

Removal of bone cement from the femoral canal using an acoustic emission-controlled milling device

The removal of bone cement from the femoral canal in the case of hip prosthesis loosening takes a considerable amount of time and carries the additional risk of complications, thus risking bone damage and perhaps even endangering the successful reimplantation of a new prosthesis. We have demonstrated experimentally that the milling of bone and bone cement produces acoustic emission waves of different intensity. These waves can be measured by a commercial acoustic emission transducer. This method enables us to control a milling device during the removal of bone cement from deep in the femoral canal, avoiding the serious damage to the bone and thus enabling a new prosthesis to be reimplanted.

Early Detection Of Room Fires Through Acoustic Emission

Acoustic emission (AE) has been shown to be a viable concept for the early indication of an open flame impinging on various structural materials. To assess its effectiveness in a more realistic environment, experiments have been performed in a 2.4 m cubical room constructed of gypsum board and wood. AE transducers were mounted on top of a ceiling joist and on a wall stud. The threats examined were a natural gas fire producing a thermal load up to 125 kW, and a charring condition achieved by attaching a 550 W electrical heater to a wall stud. A signal discernable above the background was recorded from at least one AE sensor in six of nine situations. In each case, measurable acoustic emission occurred before a noticeable increase in voltage from the thermocouple mounted adjacent to the AE sensor. The conclusion is that AE emission appears to be sufficiently sensitive to detect two particular threats, and that an overheated condition in a wall or ceiling can be detected if it is not more than 3 m from the transducer. Additional experiments are required to identify signals that are likely to complicate the differentiation between a false and a true fire event.

Absolute calibration of acoustic emission transducers by reciprocity method.

Absolute calibration of acoustic emission transducers by reciprocity method.

Transfer functions for acoustic emission transducers using laser interferometry

A high-fidelity, broadband laser interferometer is used to develop a transfer function for a piezoelectric, acoustic emission transducer. The transfer function, which converts the relative transducer output to an absolute measurement of surface velocity, uses a methodology that accounts for the transducer’s frequency bias. This technique treats the piezoelectric transducer as a corruptive channel and develops a filter that models this bias on a measured acoustic emission signal. This filter, which is developed with a signal processing software package, is based on comparisons between signals measured with a laser interferometer and those made with a piezoelectric transducer. The accuracy of this filter is verified and it is used to remove the transducer-related features from simulated acoustic emission signals.

Development of Interferometer for Acoustic Emission Testing

The characterization of the exact response of an acoustic emission sensor is of fundamental importance for the quantitative nondestructive evaluation of engineering components. To effectively determine an acoustic emission source, the influence of the sensor on the measured waveform must be determined. A heterodyne interferometer is developed and tested for the detection of acoustic emission signals. This optical device makes localized absolute measurements of out‐of‐plane surface velocities with very high fidelity. An important feature of this system is that a linear frequency modulation (FM) discriminator is used to demodulate the carrier signal; as a result, the interferometer behaves as a Doppler‐shift detector. A series of experiments was performed to test the response of commercially available, piezoelectric acoustic emission transducers in order to understand their potential influence on a measured acoustic emission waveform.

New test method for obtaining softening response of unnotched concrete specimen under uniaxial tension

A new method was developed to obtain the complete stress-deformation response of an unnotched concrete specimen. This method employs a digitally controlled closed-loop testing machine and five control channels: stroke LVDT and four LVDTs mounted on the specimen. The test portion of the specimen was fully spanned by these four LVDTs. The outputs of these control channels and the load cell were monitored during the test by a computer equipped with LabTech's Notebook software. A ‘C’ language program was written to enable quick switching of the mode of control from one LVDT to another. The problem of uncertainty in the location of the major crack was tackled by the said LVDT arrangement and the computer program. It was demonstrated that it is always possible to obtain stable post-peak response provided one ensures that at any time during the test, the feedback used is the LVDT that exhibits, at that time, the largest slope of the response-time curve. An acoustic emission (AE) measurement system of six channels was also used in the experiments. Monitoring of signals from the AE transducers provided valuable information which helped in making the decision to switch control.

Dynamic Crack Microscope and Its Application

A new generation acoustic emission (AE) source characterization system combined with detected wave analyzing system was developed for materials evaluation. The system consists of six AE transducers. parallel processing units. a host CPU and a desk-top computer subsystem. AE signals from each transducer are digitized by a high-speed ND converter in each processing unit. From digitized waveform data, AE parameters such as arrival time, peak amplitude, rise time and phase of the first motion of longitudinal wave are extracted by a Digitial Signal Processor (DSP) installed in each processing unit. The data of arrival time in each channel is fed into the host CPU to determine AE source location. The location result together with both AE parameters and waveform data are transferred to the desk-top computer, on which AE source functions are calculated by single deconvolution method using both the dynamic Green's function of a medium and the transfer function of the measuring system. Then the size and mean nuclea...

Deconvolution of Acoustic Emission and Other Causal Time Series.

A new technique, root projection (RP), is given for quantitative deconvolution of causal time series in the presence of moderate amounts of noise. Deconvolution is treated as a well-conditioned but underdetermined problem and a priori information is employed to obtain comparable noise reduction to that achieved by singular value decomposition (SVD) techniques while providing more accurate frequency information about the inverse. Two detailed examples arc given. The first gives noise analysis for alternate methods for deconvolution with a Gaussian kernel. The second example presents a model acoustic emission transducer calibration problem with typical noisy and incomplete output data. This example is treated by the use of a robust cross-cutting algorithm combining both the RP and SVD methods.

Acoustic emission transducer and mounting adapted for monitoring metalcutting tools

An acoustic emission transducer has an improved design and mounting arrangements for using it to monitor machine tool operating condition. The transducer has a hollow housing with external threads capable of making a threaded mounting connection through which acoustic emission energy generated by operation of a machine tool propagates and is then transmitted by the housing. Also, a wear plate is attached to one end of the externally-threaded housing and supports a piezoelectric detecting element in the housing. The plate is capable of transmitting acoustic emission energy and the detecting element is capable of detecting it and generating an electrical signal in response thereto. In a surface mounting arrangement of the transducer, an adapter is used in mounting the transducer to a surface of the machine tool support structure. In an integral mounting arrangement of the transducer, the housing of the transducer is threaded directly into an internally threaded cavity in the machine tool support structure. In both mounting arrangements, a contact ball is disposed in a recess formed in the machine tool support structure below the transducer plate for providing acoustic emission energy transmitting contact between the transducer plate and the machine tool support structure.

Characterization of particle impact by quantitative acoustic emission

Acoustic emission (AE) is proposed as a method of monitoring hard particle impact on surfaces. In a preliminary study, small spherical particles of nominal diameter 53–63 μm and 75–90 μm of bronze and glass respectively were dropped in vacuum onto steel or aluminium target plates at selected velocities between 2.5 and 7.1 m s −1. At these low velocities the collisions are entirely elastic and no erosion occurs. However, a quantitative AE signal analysis method was developed which enabled various parameters from the particle impacts to be determined, with the potential for measuring the degree of plasticity of impact in later experiments. AE signals were detected with a calibrated broad bandwidth piezo-electric transducer positioned on the opposite face of the target plate at the impact epicentre. The Green's function for the plate, together with the AE transducer and system response obtained by calibration using a capacitance transducer, were deconvolved from AE signals to yield the impact force function of the particle impact. The velocity dependence of the impact time and peak impact force thus determined agreed with that predicted from semi-empirical models of elastic particle impact. The absolute magnitude of these two parameters also compared very well with theoretical values. The quantitative acoustic technique developed was capable of sizing the particles, and the results compared favourably with the particle size distribution measured optically, giving the correct mean particle diameter to within 10% and the distribution spread to within 26%. This fundamental approach provides a framework for later extension to plastic and partially plastic particle impact Thus the technique could be developed for on-line erosion monitoring.

Role of disk surface roughness on slider flying height and air-bearing frequency

The flying heights and air-bearing frequencies during the transition from flying to sliding were measured as a function of disk speed for thin-film disks with various roughness magnitudes using a multichannel laser interferometer and a laser-Doppler vibrometer. An acoustic emission transducer was also employed to detect head-disk contact. No significant differences in the flying heights or air-bearing frequencies were observed for different disk roughnesses at the tested speeds, but the landing speeds for the smoother disks were lower. It was found that the landing speed of a slider does not correlate with RMS surface roughness, but is more a function of asperity peak heights. There was no observed dependence of the air-bearing frequency on surface roughness at any speed. It is concluded that the effect of the surface roughness on the flying height and air-bearing frequencies is minimal, and the presence of isolated asperities affects the slider landing speed. >

Natural frequencies of sliders and transducers used to detect slider-disk contacts

The natural frequencies of the various transducers and sliders were studied by finite element modeling and experimentation. Finite element analysis shows that the lowest ringing frequency of a 3370 slider is around 310 kHz. This value has been verified by the laser Doppler vibrometer and AE (acoustic emission) measurements. It is determined that the lowest natural frequency of a mini-Winchester slider is around 130 kHz. The 140-kHz frequencies reported in the literature using the two-rail sliders with piezoelectric transducers attached to the slider and the 160-kHz frequencies reported using AE transducers attached to the suspension mount are associated with the natural frequencies of these transducers and not of the sliders.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monitoring of crack growth and crack closure behavior by a simulated acoustic emission technique.

A new nondestructive method using a simulated acoustic emission (AE) technique is proposed for the monitoring of crack growth and crack closure behavior. Simulated AE signals, which propagated through a crack, were detected and the waveform changes with the crack growth were examined. It became clear that the amplitudes of the first peak of the waveforms have a strong correlation with the crack length and the state of crack closure. The numerical simulation by a finite differential method showed that about ±10mm of crack length can be measured with a pair of AE transducers, and the correlation curve between crack length and amplitude of the simulated AE shows little change with the materials. These results suggest that the simulated AE technique is applicable to the evaluation of both crack propagation and crack closure behavior.

Vector calibration of ultrasonic and acoustic emission transducers

The independent transient responses of a transducer to displacements polarized in each of the three orthogonal directions have been determined using a new method that assumes only linearity and no angular impingement dependence of the transducer response. The modeling of the transducer response is thus converted from a vector problem to three much more tractable scalar problems, thereby forming the basis for vector calibration of acoustic emission and ultrasonic transducers as receivers.

Wide plate crack arrest testing

Five wide-plate crack arrest tests have been performed between September 1984 and December 1985 on an ASTM A533B quenched and tempered steel. These tests were done in the 26 MN Universal Testing Machine located at the National Bureau of Standards. The specimens were fractured in a thermal gradient to make the crack initiate at a preexisting notch in a cold, brittle region, and arrest in a hot, tough region. To obtain a great deal of information from these tests, each specimen was thoroughly instrumented with thermocouples, strain gages, crack-mouth-opening displacement gages, timing wires, and/or acoustic emission transducers. Fast data response was obtained from these sensors during the run-arrest events, which generally lasted for less than 10 ms. Estimates of two types of important data were obtained: (1) crack velocity as a function of time, and (2) initiation and arrest toughness. Thus, a successful data collecting system was developed for acquiring the basic data required for the prediction of the behavior of nuclear pressure vessels subjected to pressurized thermal shock.

Analysis of Acoustic Emissions Generated During Stress Corrosion Cracking

Abstract Stress corrosion cracking of 300-M steel (Unified Numbering System [UNS] K4420) was monitored using an acoustic emission transducer. Stress corrosion cracking properties were altered through heat treatments and the relationships between the resulting microstructure and acoustic emission responses were studied. Prior austenite grain size exerted the maximum influence on the acoustic emission characteristics. Models were proposed to explain the grain size dependency of emission event peak amplitude and the relationships between crack velocity and the number of counts per event. The number of counts per event was also governed by material constants, applied stress intensity, and threshold level of detection of the acoustic emission system.

Determination of the duration of transient phenomena by frequency-domain analysis of acoustic emission

Abstract A method of estimating the duration of transient phenomena is proposed using frequency analysis of the acoustic emission (AE). The generation and propagation characteristics of the AE through an infinite elastic medium are examined. The time dependent distortion of the AE source is assumed to be a Gaussian error function. The relation between the rise time of the source (δτ) and the log-log gradient, m, of the observed AE spectrum is represented as m = ω2δτ2/8 for an infinite medium using a flat frequency response AE transducer, where ω is the angular frequency. This expression suggests that for a particular frequency, it is possible to obtain a value for the rise time, δτ, which corresponds to a higher frequency. This parameter has been examined experimentally in a quasi-infinite plate, and is compared with theoretical predictions.

A comparison of high-performance acoustic emission transducers

The constructions of a capacitive (C) transducer, an improved wide-band PZT transducer and two PVDF transducers are presented. For calibration purposes, the breaking of a brittle capillary tube on a steel plate is applied. The responses of the transducers to displacement in the epicenter of the source on a steel plate show that the signals from the capacitive (C) and PZT transducers represent displacement, while the signal from the PVDF transducer represents velocity. The sensitivities of the transducers are characterized by transient response functions calculated by deconvolution of the measured signals and a theoretically determined plate response function. C, PZT and larger PVDF transducers are of comparable performance, while the miniature PVDF transducer is inferior due to its much smaller sensitivity.

Acoustic Emission Transducers for the Vibration Monitoring of Bearings at Low Speeds

The use of acoustic emission transducers for the vibration monitoring of rolling element bearings at low speeds is explored. The frequency response and the base strain and bending sensitivities of the transducers are shown to be important.