Simulated Acoustic Emission Research Articles

Analysis of plate wave propagation in anisotropic laminates using a wavelet transform

A new approach is presented for the analysis of transient waves propagating in anisotropic composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time-frequency analysis of dispersive flexural waves in these plates. It can be shown that the peaks of the magnitude of WT in a time-frequency domain is related to the arrival times of the group velocity. Experiments were performed using a lead break as the simulated acoustic emission source on the surface of unidirectional and quasi-isotropic laminates. A method was developed to obtain the group velocity of the flexural mode as a function of frequency. Theoretical predictions were made using the Mindlin plate theory, which includes the effects of shear deformation and rotatory inertia. Our predictions on the dispersion of the flexural mode showed good agreement with the experimental results.

A Continuous Sensor to Measure Acoustic Waves in Plates

An active fiber composite tape with segmented electrodes was modeled for use as a continuous sensor for structural health monitoring of plate structures. The material has unidirectional piezoceramic fibers with interdigital electrodes on the top and bottom surfaces and is poled in the fiber direction. The elastic response of a plate with the modeled sensor was computed in closed form at small time steps, and the coupled piezoceramic constitutive equations were solved. Wave propagation responses and the corresponding sensor voltages were computed in the simulations. The simulations indicate that the continuous sensor can detect damage to the plate based upon high frequency wave propagation characteristics. A continuous sensor comprised of monolithic piezoceramic patches was built and tested to measure simulated acoustic emissions on panel structures. The monolithic patches were used to simplify fabrication of the sensor. The sensor detected simulated acoustic emissions in a fiberglass panel using only one channel of data acquisition. The continuous sensor can potentially reduce the cost, complexity, number of channels of data acquisition, and the weight of monitoring instrumentation to a level where it becomes practical to perform health monitoring on large structures.

Detection of simulated acoustic emission with Michelson interferometric fiber-optic sensors

Detection of simulated acoustic emission with Michelson interferometric fiber-optic sensors

Wavelet analysis of plate wave propagation in composite laminates

A new approach is presented for the analysis of transient waves propagating in composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time–frequency analysis of dispersive plate waves. It is shown that the peaks of the magnitude of WT in the time–frequency domain are related to the arrival times of group velocity. Experiments are performed using a lead break as the simulated acoustic emission source on the surface of quasi-isotropic and unidirectional graphite/epoxy laminates. For predictions of the dispersion of the flexural mode, Mindlin plate theory is shown to give good agreement with the experimental results. The planar source location based on the flexural wave is performed using a triangulation method. The use of frequency-dependent arrival time of output signal and angular dependence of group velocity provides accurate results of source location for anisotropic laminates.

Design of a self-calibrating simulated acoustic emission source

The use of conical piezoelectric transducers as point acoustic sources has been investigated. It has been shown that transducers based on a design originally developed at the National Institute for Standards and Technology in the USA can be used as point transmitters over the frequency range of interest in acoustic emission measurements (100 kHz to around 1 MHz). They should, therefore, be suitable for use in experiments to calibrate structures so that acoustic emission source strengths can be determined. It has also been shown that measurements of the response of the transmitting transducer backing can be used to assess the coupling efficiency, and hence to remove concerns about inconsistent coupling affecting the calibration measurements. The results indicate that the variation of the backing response with coupling is due to a shift in the resonance frequencies of the transducer with the mechanical load impedance. If other transducers can be shown to behave in a similar fashion this effect could be used to measure coupling in standard acoustic emission and ultrasonic transducers.

Non-contact measurement of acoustic emission in materials by laser interferometry

The non-contact measuring system of acoustic emission (AE) by laser interferometry was developed to detect AE signals and analyze microfracture quantitatively during materials testing. The capability of this system was estimated by comparison between simulated AE signals due to glass capillary breaking and calculations using the finite element method. The systemcould measure AE signals during practical tensile tests of carbon fiber reinforced plastics. This technique was also applied to the thermal cycle test of ceramic/metal coatings, and AE signals during cooling were successfully detected and analyzed by a deconvolution method to evaluate quantitatively the microfracture process.

Test Results of Resonant Integrated Microbeam Sensor (RIMS) for Acoustic Emission Monitoring

The feasibility of detecting stress-wave acoustic phenomena with Honeywell's resonant microbeam-based MEMS sensor technology is presented. The baseline technical approach described here incorporates an integrated silicon microstructure and a resonant microbeam with micron-level feature size, and features frequency sensitivity up to 500 kHz. The stress-wave acoustic MEMS concept has been demonstrated successfully in the laboratory test environment to sense simulated acoustic emission (AE) events for structural fatigue crack monitoring applications. The technical design approach and laboratory test results are presented.

Acoustic emission signal detection by ceramic/polymer composite piezoelectrets embedded in glass‐epoxy laminates

AbstractPiezoelectric composites, consisting of a ferroelectric ceramic powder of calcium‐modified lead titanate dispersed in two different polymer matrices, viz. from vinylidene fluoride trifluoroethylene and an epoxy, have been fabricated. The composites show a mixed connectivity structure, and the results of electromechanical characterization are given. Thin films of these materials have been incorporated into surface mounted acoustic emission sensors and their frequency response and ability to detect plate waves, generated by a simulated acoustic emission source, has been examined. In situ transducers embedded into glass‐reinforced laminate plates have also been examined and have been shown suitable for acoustic emission sensors.

Stabilization of an embedded fiber optic Fabry-Perot sensor for ultrasound detection

A method is proposed to stabilize an intrinsic fiber optic Fabry-Perot interferometric sensor embedded in a solid material for the purpose of detecting ultrasound. Without stabilization the interferometer drifts out of quadrature due to the presence of low-frequency dynamic strains. Stabilization is achieved by using active homodyne stabilization which tunes the laser frequency to maintain quadrature. A control loop shifts the laser frequency by 10 GHz at rates less than 25 Hz in order to compensate for induced drifts. The stabilization procedure was tested for a sensor embedded in an epoxy plate. Ultrasonic pulses, generated by a 5 MHz piezoelectric transducer, were detected with the embedded fiber sensor stabilized in the presence of applied low frequency strains. Improvements in sensitivity which result from stabilization are demonstrated. Additionally, a simulated acoustic emission signal, generated by a lead pencil break (Hsu-Neilson source), was detected with the sensor stabilized in the presence of dynamic strains.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Application of Green’s function modeling to ultrasonic probing of a planar interface

The use of plane‐wave ultrasound to probe interface properties based on angular spectrum analyses is common in the nondestructive testing of materials. Space‐time domain Green’s function modeling is an alternative approach to frequency domain modeling. it is better suited for the implementation of short duration pulses, small aperture transducers, and time‐resolved pulse‐echo methods. However, a computation based on the Green’s function, though conceptually simple, is more involved than a computation based on the model of a plane wave of fixed frequency. Recently, an explicit Green’s function for layered media has been derived. Efficient computer programs have also been developed for cases which can be easily tested experimentally and have immediate applications. As examples, comparison results between theory and experiment are reported here for three test configurations: (1) a solid plate on a half‐space with different interface conditions, with a point step‐function source and a point detector located on top of the plate; (2) a liquid/solid interface, with both the point source and point detector in the liquid; and (3) a line focus transducer probing a liquid/solid interface. Practical applications to laser ultrasound, simulated acoustic emission, and the acoustic microscope are illustrated.

Plate mode velocities in graphite/epoxy plates

Measurements of the velocities of the extensional and flexural plate modes were made along three directions of propagation in four graphite/epoxy composite plates. The acoustic signals were generated by simulated acoustic emission events (pencil lead breaks or Hsu–Neilsen sources) and detected by broadband ultrasonic transducers. The first arrival of the extensional plate mode, which is nondispersive at low frequencies, was measured at a number of different distances from the source along the propagation direction of interest. The velocity was determined by plotting the distance versus arrival time and computing its slope. Because of the large dispersion of the flexural mode, a Fourier phase velocity technique was used to characterize this mode. The velocity was measured up to a frequency of 160 kHz. Theoretical predictions of the velocities of these modes were also made and compared with experimental observations. Classical plate theory yielded good agreement with the measured extensional velocities. For predictions of the dispersion of the flexural mode, Mindlin plate theory, which includes the effects of shear deformation and rotatory inertia was shown to give better agreement with the experimental measurements.

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.

Extensional and Flexural Waves in a Thin-Walled Graphite/Epoxy Tube*

Simulated acoustic emission signals were induced in a thin-walled graphite/epoxy tube by means of lead breaks (Hsu-Neilsen source). The tube is of similar material and layup to be used by NASA in fabricating the struts of Space Station Freedom. The resulting waveforms were detected by broad band ultrasonic transducers and digitized. Measurements of the velocities of the extensional and flexural modes were made for prop agation directions along the tube axis (0 degrees), around the tube circumference (90 degrees) and at an angle of 45 degrees. These velocities were found to be in agreement with classical plate theory.

Study of simulated acoustic emission in unidirectional carbon fibre reinforced plastic

A lead break source and an impactor source have been used to simulate two different types of acoustic emission events in unidirectional carbon fibre reinforced plastic. The observed transient signals were detected after travelling different distances in the material using conventional wide-band transducers. The signals were analysed in both the time and frequency domains, and it was found that the distribution of amplitude peaks and the frequency spectra provided the most useful characteristics for discriminating between the two types of source. The attenuation of the signals was found to be high, especially in directions perpendicular to the fibres. The transients were also analysed using a LOCAN system, and it was concluded that the most useful parameters for distinguishing between the two types of source were the event duration, the rise-time and the event count.

Characteristics of Acoustic Emission Wave Attenuation in Short Fiber Reinforced Plastics

The attenuation coefficient (α) of acoustic emission (AE) waves was evalu ated for short glass fiber reinforced poly(ethylene terephthalate) (SGFR-PET) as a func tion of fiber volume fraction ( V f) using simulated AE waves. The coefficient α decreased with increasing Vfmore rapidly than expected from a simple linear relation between α and Vf. The effect of wave attenuation was taken into account in a quantitative analysis of the peak amplitude distribution of AE waves which were detected from each zone partitioned in a specimen gage portion. The amplitude distribution which was obtained by the zoning method was compensated for the attenuation loss stated above. The compensated ampli tude distribution was shown to be almost similar in every zone of the specimen. The com pensated amplitudes of AE waves became much larger as the specimen deformation pro ceeded. Consequently, the AE peak amplitude obtained from short fiber reinforced plastics was shown to be considerably influenced by the wave attenuation.

Acoustic emission testing of fiber composite materials

Acoustic emissions (AE) can be used to nonintrusively monitor and, in some instances, characterize the progression of damage in composite materials during deformations. In the present investigation, AE from graphite epoxy tape laminates and acrylonitrile butadiene styrene (ABS) specimens with and without short glass fibers were monitored and analyzed during cyclic loadings and monotonic proof tests. Usual AE parameters such as hits, counts, and amplitude were correlated with spectral analyses of acoustic emissions recorded during various portions of the loading waveform. Due to bandpass limitations of the acoustic emissions device, few correlations of the spectral content of the acoustic emissions could be made with the loadings or mechanisms of damage. However, some useful information for users of the AE technique was obtained from a wideband spectral analysis measurement made with the acousto-ultrasonic (AU) technique. With the simulated acoustic emission events generated with the AU technique, it was shown that physical parameters such as transducer resonance, source-to-receiver distance, specimen geometry, and material properties such as fiber orientation and composition have distinct and reproducible effects on the measured spectral response.

Transient response of a fluid‐filled borehole due to the passage of transient elastic waves

The interaction of transient elastic waves with a fluid‐filled borehole is investigated both experimentally and theoretically. The elastic waves incident from various angles are partially scattered by the borehole, while an acoustic wave field is introduced in the borehole fluid. Experiments were conducted in a solid block with a circular borehole inside. The transient elastic waves were generated by using either a simulated acoustic emission source or an ultrasonic transducer. The acoustic pressure in the borehole was measured by a commercial PZT hydrophone and by a home‐made PVDF hydrophone. The effect of the borehole on the elastic waves is identified by comparing the measured acoustic responses with the undistorted incident pulse, calculated by the generalized ray theory without the borehole. The theory of plane wave scattering by a borehole is used to compare with the experimental data. The transient results from both the experiments and the borehole theory agree with each other very well, and they both exhibit resonances due to the multiple reflections inside the borehole fluid.

Application of an intelligent signal processing system to acoustic emission analysis

This article describes a novel approach for analyzing elastic wave signals to obtain a solution to simple inverse source problems. By using a simulated intelligent system that resembles the structure of a neural network and an acoustic source with its associated field of elastic waves in a plate, a set of pattern vectors is generated. The memory of the system is formed through a learning process in which a systematic series of experiments is presented to the system. In the experiments described here, the system was trained using simulated acoustic emission signals generated by a normal force acting on a thick plate to which information about the source related to its location and characteristics was appended. It is demonstrated, using such a trained system, that one can recover the characteristics of an unknown source from the acoustic signals emitted by it. It is also shown that one can synthesize from the memory the acoustic signals corresponding to an arbitrary source without using any elastodynamic theory.

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.

アコースティック・エミッションの波形解析理論とモーメントテンサー解に関する考察

The development of mathematical theory of acoustic emission (AE) is summarized, and a waveform analysis of the moment tensor solution is proposed. On the basis of the theory, vertical surface motions (AE) in the disbonding process of stainless overlay were simulated, determining source characteristics from the observation of a micrograph. It is confirmed that a simulated displacement agrees quite well with a detected AE waveform up to the arrival of reflection waves, including the scale of amplitude.To check the effect of the inclined angle on results of the deconvolution analysis, simulated waveforms were deconvoluted with error on the inclination angle. According to the results, unless the inclination angle is correctly known, the source time functions obtained may include considerable error except for a rise time. It implies that the determination of crack orientation is of significant importance, before attempting the deconvolution analysis.The applicability of the moment tensor solution was investigated by numerical experiments for inclined tensile and shear cracks. Simulated AE waveforms in a half space were employed, and then components were determined within 8% discrepancy. The results of the eigenvalue analysis show a great promise of the moment tensor solution for determining the crack type and the crack orientation.