Showing papers on "Acoustic emission published in 1979"

Stress memory of crystalline rocks in acoustic emission

Abstract: In acoustic emission studies on granite under uniaxial compression, clear hysteresis of emission numbers related to the stress history (Kaiser effect) was observed below dilatancy onset stress. The maximum stress loaded in the past is memorized in rocks. Above dilatancy, we could not observe such a hysteresis. These features are explained by the nature of irreversible sudden advances of microcrack tips. Recovery process time of this hysteresis is over one month under room humidity and temperature conditions. The results in this paper cast foundation for the applicability of geostress measurements by the acoustic emission method.

Acoustic emission and transient waves in an elastic plate

Abstract: To study the mechanism of the source of acoustic emission, we analyze the transient waves which are generated by four kinds of point sources, a single force, double force, and center of dilatation, all inside an infinite plate, and a single force on the surface of the plate. Some of these forces or a combination of them could be used to model the dynamic process of material defects. The analysis is based on the generalized ray theory and Cagniard’s method. Transient solutions are obtained by evaluating the ray integrals with a complex algorithm. Numerical results are shown for receivers that are located at a distance up to six plate thickness, and for a duration less than ten transit time for a P‐wave to cross the thickness of a plate.

The crack-initiation threshold in ceramic materials subject to elastic/plastic indentation

Abstract: The threshold for indentation cracking is established for a range of ceramic materials, using the techniques of scanning electron microscopy and acoustic emission. It is found that by taking into account indentation plasticity, currant theories may be successfully combined to predict threshold indentation loads and crack sizes. Threshold cracking is seen to relate to radial rather than median cracking.

Energy absorbed by elastic waves during plastic impact

Abstract: During the normal impact of a hard particle on a metal surface, some of the kinetic energy of the particle is radiated into the target as elastic waves. The amount of energy involved is calculated for impacts where the metal is deformed plastically, and is found to be typically only a few per cent of the initial kinetic energy.

Stress effect on the magnitude of acoustic emission during magnetization of ferromagnetic materials

Abstract: The effect of stress on the magnitude of acoustic emission during magnetization of mild steel and nickel has been investigated. It has become clear that the magnitude of acoustic emission during magnetization shows a strong dependence on stress. This effect should have nondestructive testing applications suited for field inspection.

Acoustic Emission From a Brief Crack Propagation Event

Abstract: Acoustic emissions produced by elementary processes of deformation and fracture at a crack edge are investigated on the basis of elastodynamic ray theory. To obtain a two-dimensional canonical solution we analyze wavefront motions generated by an arbitrary distribution of climbing edge dislocations emanating from the tip of a semi-infinite crack in an unbounded linearly elastic solid. These wavefront results are expressed in terms of emission coefficients which govern the variation with angle, and phase functions which govern the intensity of the wavefront signals. Explicit expressions for the emission coefficients are presented. The coefficients have been plotted versus the angle of observation, for various values of the crack propagation speed. The phase functions are in the form of integrals over the emanating dislocation distributions. Specific dislocation distributions correspond to brittle fracture and plastic yielding at the crack tip, respectively. Acoustic emission is most intense for brittle fracture, when the particle velocities experience wavefront jumps which are proportional to the stress-intensity factors prior to fracture. An appropriate adjustment of the canonical solution accounts for curvature of a crack edge. Such effects as focussing, finite duration of the propagation event, and finite dimensions of the crack are briefly discussed. As a specific example, the first signals generated by brittle Mode I propagation of an elliptical crack are calculated.

Acoustic Emission From Corroding Electrodes

Abstract: Acoustic emission measurements have been performed on unstressed Al alloys which were freely corroding or coupled to Cu, 4130 steel, Cd or Zn in 3,5% NaCI. The rate of acoustic emission is in qualitative agreement with the observed corrosion rates. The acoustic emission is probably a result of hydrogen bubble evolution in the acidified pits. The rate of acoustic emission of 4130 steel in HCI also follows the corrosion behavior, decreasing significantly with addition of organic corrosion inhibitor. Stainless steel Type 304 was studied freely corroding or at applied constant anodic or cathodic currents. The highest rates of acoustic emission were determined at cathodic currents; when an anodic current was applied, the acoustic emission rate was higher than for a freely corroding sample and showed wide fluctuations which continued for some time when anodic polarization was discontinued. For localized corrosion of Al alloys and stainless steel, the acoustic emission process seems to reflect the stati...

The resistance to crack growth of asbestos cement

Abstract: The crack resistance of sheet asbestos cement has been characterized in terms of anR-curve which can accomodate effects which often influence the measurement of the critical stress intensity factorKc. The detection and location of the acoustic emission (AE) obtained from the asbestos cement has shown that it originates from microcracks in a zone just in front of the crack. The size of this zone increases to a maximum during slow propagation of the major crack and afterwards remains of constant size during the final crack growth. The form of theR-curve has been explained in terms of the mechanisms of fracture with the aid of AE and fractography studies. An analytical study has related the experimentalR-curve to a theoreticalR-curve and, hence, to the volume fraction, fibre aspect ratio and the strength of the fibre—matrix interface. It has been shown that the microcracking zone can be considered as a theoretical extension, of about one third of the zone length, to the real crack length.

Acoustic emissions from stress-induced dauphiné twinning in quartz

Abstract: Dauphine (electrical) twins have been introduced into quartz crystals by means of suitably oriented mechanical stresses. Stress-induced twinning was observed optically by means of the photoelastic effect. Accompanying acoustic emission, first detected by ear, was quantitatively measured with a piezoelectric transducer and a suitable filtering and amplification system. Synchronized sound cinematography has been used to correlate the acoustic emission with visual observation of twinning. The resultant motion picture has been used to determine the stress levels required for twinning in Brazilian quartz and to set limits on the twin wall velocity. The possible relevance to rock noise and earthquake precursory phenomena is discussed.

The temperature dependence of the fracture toughness and acoustic emission of polycrystalline alumina

Abstract: The technique of acoustic emission has been shown to be suitable for the monitoring of fracture-toughness tests over a range of temperatures Commercial polycrystalline alumina has been tested at temperatures up to 1000° C to determine the effect of microstructure and impurity content on fracture toughness and acoustic emission For a given alumina there was no significant variation in acoustic response or fracture toughness up to 650° C The emissions observed prior to fracture in this temperature range were attributed to subcritical crack growth The number of emissions depended on the amount of subcritical crack growth, the grain size, and the presence and amount of porosity Above 650° C the fracture behaviour changed due to the flow of a grain-boundary glassy phase This was associated with a peak in the temperature dependence of the apparentKIC and was accompanied by a large number of acoustic events of low amplitude and low pulse width At these elevated temperatures the extent of grain-boundary glassy flow, and hence the acoustic response, increased with decreasing grain size and increasing impurity content

Magnetomechanical Acoustic Emission of Iron and Steels.

Abstract: : The motion of magnetic domain walls in ferromagnetic materials produces acoustic emission (AE). This type of AE was detected during magnetization of a nickel and during elastic loading of iron. Recently, this was also found to depend on applied stress. Varying AE outputs were observed on several ferromagnetic materials under alternating magnetic field. This magnetomechanical AE phenomenon has a potential of performing nondestructive measurements of residual stresses in structures, components and weldments, and we examined systematically effects of applied stress and magnetic field strength in ferritic steels. This measurements of residual stresses in structures, components and weldments, and we examined systematically effects of applied stress and magnetic field strength in ferritic steels. Several carbon steels, A 533 B steel and commercially pure iron were tested in annealed or normalized condition. By employing two AE transducers of different resonant frequencies, rms voltages were measured at two frequency ranges. Maximum stress level was 350 MPa. It was found that 1020 steel shows the highest AE response among the materials tested. Residual stress levels can be determined by monitoring the ratio of the outputs of two AE transducers for a given material condition. The amount of prior cold work and the difference in heat treatment can also be determined by monitoring this AE phenomenon. Several exploratory experiments for the developing a new method of nondestructive evaluation of residual stress and material conditions have also been conducted. Results of these experiments are described and discussed in detail. (Author)

Accumulation of damage in GRP laminates

Abstract: A study has been made of the accumulation of damage in GRP laminates during tensile loading. Changes in dynamic elastic properties resulting from microstructural damage events are related to the acoustic emission patterns recorded during deformation. The effects of laminate construction, loading geometry and exposure to water have also been examined. It is found that the shapes of AE/stress curves show well-defined characteristics that can be related to laminate structure. There is a good correlation for one particular laminate between changes in resonant frequency resulting from tensile loading and the integrated acoustic emission count. On the other hand, as far as ring-down counting is concerned, there is clearly no unique relationship between the integrated count at fracture and such mechanical properties as modulus or failure stress. Neither has it been possible to use damping (peak-width measurements) as a reasonable indicator of accumulated damage.

Method and apparatus for analyzing selected material properties with magnetomechanical acoustic emissions

Abstract: A method and apparatus for analyzing selected properties of a ferromagnetic material by magnetically inducing acoustic emissions in the material. The acoustic emission response is measured at a plurality of values of a selected response parameter, the response parameter being characterized by an acoustic emission response which, for each of its selected values, varies in mutual nonlinearity with the acoustic emission responses for the other selected values. The measured acoustic emission responses are compared to each other and to standard responses, thereby providing a more reliable measurement of the selected property than is attainable by analyzing at only one value of the response parameter. Comparing the ratio between acoustic emission responses at different values of the selected response parameter, with standard response ratios for the same response parameter values, further enhances reliability of the measurement. In preferred embodiments the selected response parameter is the frequency of the acoustic emission response, the signal threshold level for obtaining an acoustic emission count rate, or the magnetic field strength. Properties selected for analysis in the preferred embodiments are stress, plastic deformation, microstructure and chemical composition. The invention further includes structure in which transducers for measuring acoustic emission responses at different response parameter values are coupled to a specimen by a unitary coupling mechanism, thereby reducing spurious differences between the measurements at the various response parameter values.

Optical Probing of Acoustic Emission Waves

Abstract: Piezoelectric transducers are commonly used to monitor acoustic emission events. However, the information they yield is modified by the response characteristics of the transducers themselves, as well as by those of the medium which acoustically couples the transducer to the workpiece. Optical probing methods offer many advantages over piezoelectric transducers for monitoring acoustic emission events. Among these advantages are: (1) optical probes require no contact with the specimen and, obviously, no couplant; (2) they do not disturb the acoustic waves being detected; (3) they have broad frequency response limited only by the electronic amplifier and they are free from mechanical resonances; (4) because they can be absolutely calibrated, they measure actual acoustic displacements; and (5) they can measure the instantaneous local displacements within a few hundredths of a millimeter of a crack or other emission source. It is the purpose of the present chapter to present a comparative evaluation of optical methods for probing acoustic emission signals and to describe an acoustic emission interferometer which is superior in many respects to the other methods.

An acoustic emission study of plastic deformation in polycrystalline aluminium

Abstract: Acoustic emission experiments were performed on polycrystalline and single crystal 99.99% aluminum while undergoing tensile deformation. It was found that acoustic emission counts as a function of grain size showed a maximum value at a particular grain size. Furthermore, the slip area associated with this particular grain size corresponded to the threshold level of detectability of single dislocation slip events. The rate of decline in acoustic emission activity as grain size is increased beyond the peak value suggests that grain boundary associated dislocation sources are giving rise to the bulk of the detected acoustic emissions.

Acoustic emission for detection and monitoring of crack initiation and propagation in materials

Abstract: Initiation and propagation of cracks in materials is detected and monitored by (a) adhering an acoustical emitter to the area of the material to be monitored, and (b) monitoring for acoustical signals emitted by the adhered emitter, said signals produced by cracks in the material initiating in or propagating into the acoustical emitter. An acoustical emitter such as brittle plastic, glass rods or strips and metal such as tin or cadmium can be used.

Temperature‐stimulated acoustic emission in the niobium‐hydrogen system

Abstract: The hydride precipitation in the Nb‐H system has been studied by the acoustic‐emission technique varying the temperature between 330 and 225 K at hydrogen concentrations ranging from 3.8 to 5.8 at.%. Concomitantly, elastic‐modulus and internal‐friction measurements have been conducted in order to reveal the hydride‐precipitation temperatures and the plastic‐deformation phenomena occurring during the formation of the hydride misfitting precipitates. In the virgin material, acoustic emission is absent during evolution of the precipitation although the dislocation‐multiplication processes are operative. When, due to a severe precipitation, a certain degree of deterioration of the material has been reached, acoustic activity characterized by discrete bursts is then stimulated by the hydride formation and definitively persists also after the high‐temperature annealing. The temperatures of the onset of the acoustic emission are lower than those corresponding to the hydride precipitation and may fit a line paral...

Acoustic Emission of Fiber Reinforced Plastics

Abstract: Because most nondestructive test methods are not suitable for use with plastics, and in light of the poor performance record of FRP structures, a practical nondestructive test could be of great value. The paper reports results of a series of tests designed to evaluate the suitability of acoustic emission for testing of FRP structures. In order to characterize FRP, tensile and flexural tests were carried out using various FRP constructions and a range of load levels. In addition, a test of a full-size FRP reactor was undertaken. The tests demonstrate that level of emission, first damage, Kaiser effect, emission during a load hold, amplitude spectrum, and the effect of different frequencies on wave attenuation provide useful information on the structural integrity of FRP. Recommendations for future test work are presented with an assessment of the applicability of acoustic emission for nondestructive testing of FRP structures.

Fracture Modes and Acoustic Emission of Composite Materials

Abstract: Unidirectional fibrous composite material laminae are the construction material of a laminate. Four failure modes can occur in this material, and the identification of these modes is as important as the identification of onset of fracture. Acoustic emission (AE) pulses, generated by the change in the stress field resulting from cracking in a material, have a unique pattern of amplitude distribution that depends only on the mode of fracture and the type of material. By using a tapered double-cantilever beam as grips for the unidirectional lamina, a crack with a constant stress intensity factor was induced, thus eliminating the factor of the stress level. Placing the lamina in different directions relative to the grips imposed a different failure mode. By plotting the log of the relative number of AE pulses above some level versus the log of the relative level, a single constant coefficient can be measured for each material and mode of fracture. The AE energy is proportional to the energy released by the cracking; this proportionality depends on the medium where the AE waves have to travel, and not on the cracking mode.

Acoustic emission interpretation of ductile fracture processes

Abstract: Detailed analyses of acoustic emission from several material classes have established that the predominant source in tensile testing is particle decohesion and fracture. Acoustic emission is a smoothly varying parameter with plastic strain in 4340 steel where small carbides predominate; however, in an IN 718 superalloy, it is both bimodal and exhibits a burst phenomena where very large carbides and nitrides as well as a medium size laves (δ) phase contribute. From microscopic and acoustic emission observations, it is found that the fracture process is sulfide decohesion followed by void sheet instability associated with carbides in 4340 steel. In IN 718, large carbide or nitride fractures are followed by void sheets associated with laves phase. Identification of the major particle nucleation sites have allowed an initial interpretation of ductile hole growth models. Application of McClintock hole growth and a Hahn and Rosenfield void sheet instability criterion to the ductile fracture process has provided a good correlation to tensile ductilities, plane-strain crack tip ductilities, and plane-strain fracture toughness, K IC.