Showing papers on "Acoustic emission published in 1969"

Interaction between elastic surface waves in piezoelectric materials

Abstract: Nonlinear interaction between elastic surface waves and an almost uniform time‐dependent electric field is demonstrated in LiNbO3 at 17 MHz.

Acoustic emission prior to rockbursts and earthquakes

Abstract: On the basis of popular reports of animal agitation prior to earthquakes, a semiquantitative study has been carried out of the possibility of the emission of acoustic strain and fracture radiation prior to earthquakes. Such emission has been observed in the laboratory and in mines, and the bearing of these reports on the animal behavior reports is discussed. The comparative auditory/vibration response of humans and animals is discussed insofar as quantitative data are available. Theoretical estimates are derived of the frequency of the sound emitted by preliminary fracturing in a highly stressed region. The results for rockburst conditions in mines are consistent with the frequency spectra of the observed signals. Application to earthquake conditions indicates some emission should occur over high audible to low ultrasonic frequencies. Estimates of the intensity of such emission are also obtained to ascertain if it might be detectable on a practical basis. These results lead to transmission range estimates which, for rock Q-values at normal pressure, indicate a very short range with at best a marginal possibility of detection under certain very favorable circumstances. Since, however, rock Q-values are known to increase with pressure, it is pointed out that this may lead to a substantial increase in local transmission ranges within regions of high stress. This effect might permit the practical observation of preliminary emission for shallow earthquakes whose strain fields extend to the surface.

A Cyclic Cavitation Process

Abstract: A type of acoustic cavitation has been studied that occurs at the face of a vibrator with a small diameter. A gas bubble grows to near‐resonant size by accumulation of microbubbles. The process then reverses and the parent bubble rapidly throws off a cloud of microbubbles. This phase is accompanied by severe surface distortion of the main bubble, with emission of strong harmonics and a good deal of white noise. Under suitable conditions, a cyclic process is set up at a very regular repetition rate that depends on the acoustic pressure and the physical properties of the liquid. Acoustic streaming has an important role in maintaining the cycle. High‐speed movies have been taken, and these, along with measurements of the acoustic emission, show clearly the progress of the cycle. A technique is discussed for using this process to generate bubbles of defined size.

Thermally generated stress waves in a dispersive elastic rod

Abstract: The propagation of thermally generated stress waves in a dispersive elastic rod was investigated both experimentally and analytically.

Stress-wave emission in ndt.

Abstract: Acoustic emission (or ‘stress-wave emission’) is one of the most promising new techniques of non-destructive testing. It is currently in use in the aerospace and nuclear industries for monitoring the growth of cracks and flaws in pressure vessels. It is particularly useful for locating flaws which may be tens of feet from the detecting transducer. Failure strength prediction and metal condition diagnosis can be carried out by use of acoustic emission. It can also be used to predict the failure strength of adhesive bonds and to detect poor adhesion.

Detecting acoustic emission in the presence of hydraulic noise

Abstract: The acoustic emission produced as metals deform and fracture can be used to monitor the integrity of nuclear reactor pressure boundaries, as long as the emission data can be separated from background noise. Tests have shown that a sensing system using shear-wave sensors in the frequency range 1.5–2.5MHz can detect acoustic emission in the presence of turbulent flow and cavitation noise, and at distances of at least 3ft (1m) from the signal source. Further work is needed to improve the sensing technique, establish the maximum distance at which an emission signal can be reliably detected, and adapt the technique for practical use.

Fracture development in wood resulting from bending stresses and detected using the acoustic emission phenomenon

Abstract: A new approach to the problem of failure prediction of wood components and structures i s that of fracture mechanics. In order to implement this approach knowledge of flaw growth (or crack propagation) in wood subjected to various stress systems and environmental conditions i s required. The extension of flaws i s accompanied by the release of energy in the form of acoustic or stress wave emissions, which can be detected and have been found to be reliable indicators of crack growth. Using acoustic emissions as a measure, flaw growth was investigated in stressed bending specimens of three species. The test pieces contained a variety of knot configurations, seasoning checks and resin pockets. Three wood moisture contents were used; specimen size was 2-by-4-inch (nominal) cross section and 50-inch length. A detailed description of the system used for detecting, measuring and recording acoustic emissions i s given. Emission activity was measured using an electronic counter. Simultaneous count, load and deflection measurements were recorded and countdeflection and load-deflection curves plotted. The association between acoustic emissions and crack growth i s discussed. Four types of integrated count-

On the propagation of spherical waves due to large underground explosion

Abstract: When a high explosive pressure acts on the boundary of a spherical cavity in an elastic medium, the material in a region around the charge behaves plastically and thereafter an elastic wave is propagated. Not only that but the elastic plastic boundary moves also with a certain velocity which is less than the elastic wave velocitySabodash [3]2). In this paper, firstly the case when the elastic plastic boundary moves with a velocityV 0. (V 0