Capacitance transducer with a point-like probe for receiving acoustic emissions

Boler, Frances M.; Spetzler, H.; Getting, I. C.

doi:10.1063/1.1137918

Cited by 15 publications

(7 citation statements)

References 12 publications

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“…In 1971, Elber reported the discovery of a crack closure phenomenon that occurs with fatigue. He noted that closure of the crack planes near the crack tip can occur while the applied stress is still tensile (1). The existence of a closure stress opens the way of defining an effective stress intensity factor, Keff, given by…”

Section: Resultsmentioning

confidence: 99%

“…The resulting change in capacitance constitutes the signal. A detailed description of the instrument is given in [1].…”

Section: Capacitive Transducermentioning

confidence: 99%

“…The resulting change in capacitance constitutes the signal. A detailed description of the instrument is given in [1].There are several advantages of the capacitive transducer compared with the registration of ultrasonic waves via piezoelectric transducers:1.) The transducer is non contacting and does not interfere with the displacements of the free surface of the sample.…”

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confidence: 99%

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Characterization of Cracks in Thick Plates by Lamb Wave Diffraction Studies

Paffenholz

Getting

Spetzler

1988

Review of Progress in Quantitative Nondestructive Evaluation

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INTRODCCTIONThe object of the work is a combined theroretical and experimental study of diffraction of Lamb waves by two-and three-dimensional cracks in a plate. An important motivation for this project is the desire to develop a technique capable of detecting and characterizing cracks in weldments in plates. We want to analyze the dependence of the scattered signals on the dimension, angle of inclination and depth of burial of the crack. The basic measurement techniques requires the accurate determination of minute normal surface displacements at ultrasonic frequencies. Such measurements have recently been made possible by the development in our laboratory of a new surface displacement capacitive tranducer Ilj.The measured displacements will be compared with the theoretical predictions. EXPERIMENT ConfigurationThe first generation of measurements is being made on a 2 dimensional model. The specimen is a glass piece of 6.35 mm thickness, 25.4 mm height, and 2286 mm length. The signal in the specimen is generated by a steeel ball bearing with a diameter of 1.59 mm diameter being dropped from a height of 165 mm onto the smallest dimension edge of the specimen. Normal displacements are measured along the impacted surface of the specimen at various locations. For these idealized tests the specimen is mechanically decoupled from its supports and from the receiver transducer housing. This provides isolation from floor vibrations and prevents loss of the signal into the support structure. 1521Aluminium-coated coverslides of 0.18 mm thickness are glued on the plate to provide electrically conductive and smooth surfaces required by our capacitive transducer. The receiver output is recorded on a 20 MHz digital oscilloscope with pretriggering capability and can be transferred to a computer for analysis. Capacitive TransducerThe principle of our capacitance transducer is simple. The grounded conducting sample surface constitutes one electrode of a capacitor and the capacitance probe the other. A surface displacement changes the gap between the sample and the capacitance probe. The resulting change in capacitance constitutes the signal. A detailed description of the instrument is given in [1].There are several advantages of the capacitive transducer compared with the registration of ultrasonic waves via piezoelectric transducers:1.) The transducer is non contacting and does not interfere with the displacements of the free surface of the sample.2.) This eliminates problems with the reproducibility of the transducer-sample contact as found with piezoelectric transducers.3.) Because the transducer has no mechanical resonances and the detection circuits are suitably broad-band the response function is flat over a wide frequency range.4.) Due to the small probe diameter of 0.3 mm the transducer acts as a true point receiver over the entire frequency spectrum of interest. Thus problems with incoherent arrival of energy, which occur with larger sensing areas are avoided. In it's present configuration, it measures only that componen...

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Section: Resultsmentioning

confidence: 99%

“…The resulting change in capacitance constitutes the signal. A detailed description of the instrument is given in [1].…”

Section: Capacitive Transducermentioning

confidence: 99%

mentioning

confidence: 99%

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Characterization of Cracks in Thick Plates by Lamb Wave Diffraction Studies

Paffenholz

Getting

Spetzler

1988

Review of Progress in Quantitative Nondestructive Evaluation

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“…With this simple change in capacitor shape, measurement uncertainty arising from the electrode tilting can be minimized. Hemispherical [6] and spherical [7] electrodes have been used in capacitive sensors for measuring acoustic emission, where the hemispherical (or spherical) probe electrode was positioned near the conducting surface of a sample under test. Once the sample surface was set in motion by acoustic emission, the resulting vertical displacement was measured by the probe electrode as a change in capacitance.…”

Section: Introductionmentioning

confidence: 99%

SAW-based capacitive sensor with hemispherical electrode for nano-precision gap measurement

Kim¹,

Lee²,

Park³

et al. 2010

Sensors and Actuators A: Physical

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“…This assumption, however, was not confirmed experimentally, and therefore it is still necessary to prove whether the AE is radiated from tensile cracks or shear cracks. Maeda (1981) and Boler et al (1984), on the other hand, tried to develop new sensors. Their sensors have a flat frequency response in the high frequency range, but are too big for ordinary high pressure vessels.…”

Section: Ae Source Parametersmentioning

confidence: 99%

Fracture Mechanics of Rocks.

Kusunose

1995

J,Phys,Earth

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This paper reviews recent studies on (1) AE (acoustic emission) and cracking models, (2) failure processes and (3) frictional sliding processes, mainly based on work carried out in Japan. Techniques for AE data acquisition and hypocenter location have been greatly improved; one system can record twenty-one channels of waveforms and can locate the AE hypocenter automatically. Another system can also record the occurrence time and the maximum amplitude of the AE event without dead time. On the basis of these data, we are able to discuss the relation between the distribution of the hypocenters, the occurrence intervals, and the experimentally controlled physical parameters. For this purpose, many studies have tried to develop quantitative expression for the statistical characters of these distributions. Techniques for evaluating AE source parameters are still being developed; and there has been a great deal of improvement in our knowledge about cracking mode of AE. The focal mechanisms have been systematically studied based on the space distributions of the initial motion directions. The studies showed that shear type cracking becomes dominant with increasing axial stress. These mechanism solutions agree well with the local stress field suggested by the fracture plane. Increasing of the failure strength of rocks with increasing stress and strain rates under relatively low confining pressure has been studied experimentally. The failure process and the rate dependency of the fracture strength in the low pressure regime are discussed on the basis of a stress corrosion cracking model. The failure mechanism under higher confining pressures of up to 3GPa is also examined. Some behavior including the variation of AE activity with axial stress differs between low and high confining pressures although the stress-strain relations clearly show brittle deformation in both regimes. On the basis of these differences, the researchers proposed that 'high-pressure' brittle deformation was different from ordinarily observed brittle behavior at low pressure, and examined the failure micromechanisms through an optical and an electron microscopes. Frictional sliding has also been intensively examined in the past decade. Experiments using large samples have demonstrated that the slip propagation process is well described by a slip-weakening model. The relations between the dynamic parameters of slip propagation process and the physical parameters of slip surfaces is becoming clearer.

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Capacitance transducer with a point-like probe for receiving acoustic emissions

Cited by 15 publications

References 12 publications

Characterization of Cracks in Thick Plates by Lamb Wave Diffraction Studies

Characterization of Cracks in Thick Plates by Lamb Wave Diffraction Studies

SAW-based capacitive sensor with hemispherical electrode for nano-precision gap measurement

Fracture Mechanics of Rocks.

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