Showing papers on "Acoustic emission published in 1990"

Dislocation modeling and acoustic emission observation of alternating ductile/brittle events in Fe-3wt%Si crystals

Abstract: It has been established that hydrogen assisted subcritical crack growth in Fe-3wt%Si single crystals is discontinuous while accompanied by substantial plasticity. Prop osed micromechanisms of this process are addressed via observed fine-scale {100} cleavage features, acoustic emission tracking and computer simulation analysis. The near crack tip stress distribution in an elastic-plastic analysis enabled insight into how dislocation shielding led to mirocrack nucleation. Such discretized computational models include: the stress due to the dislocation self field, the stress from crack-dislocation interactions, the stress from the crack-external stress interaction and the external applied stress. Stress distributions in both macroscopic and microscopic scales of interest were also examined, appropriate to different slip systems. It was found that the stress at the crack tip became slightly compressive while the position of the maximum stress, approaching the theoretical strength shifts to about 2–30 nm from the crack tip. The high stress region may be correlated with the observed 1 μm discontinuous crack instabilities as detected by acoustic emission. The mutual feedback from experimental findings related to acoustic emission and crystallographical habits vis-a-vis theoretical aspects are analyzed. This approach results in a micromechanical model which has implications to both hydrogen embrittlement thresholds and the ductile-brittle transition.

Acoustic diagnosis of gas insulated substations: a theoretical and experimental basis

Abstract: Loose particles and discharges inside the ducts of a gas-insulated substation (GIS) are considered hazardous to the insulation system. These irregularities or flaws can be detected by using acoustic sensors placed on the enclosure. An elementary review of sound propagation in a GIS together with corresponding experimental results are presented. By using ultrasonic acoustic emission sensors, an excellent sensitivity to discharges and moving particles is obtained. The method offers possibilities for quantification of the flaws, and thereby for a risk analysis. However, the degree of certainty of such an analysis is still low, especially for particles. >

Fracture mechanisms of quasi-brittle materials based on acoustic emission

Abstract: Recently acoustic emission (AE) techniques have been used to study crack propagation in materials. The application of these techniques to heterogeneous, quasi-brittle materials such as concrete requires a better understanding of how the signal generated from a microfracture is transformed due to wave propagation and due to the transducer response. In this study, piezoelectric transducers were calibrated using displacement transducers. The validity of an elastodynamic Green’s function approach was examined for cement-based materials. The acoustic emission source was characterized using moment tensor analysis. Acoustic emission measurements were analyzed for center-cracked-plate specimens of mortar and concrete. It was observed that, as expected, the dominant mode of cracking was mode I (tensile). However, mode II (shear) and mixed mode cracks also occurred, perhaps due to grain boundary sliding and interface debonding. Microfractures appear to localize prior to critical crack propagation. Mode I cracks generally required more energy release than mode II and a smaller inclusion provided a stronger interface bond than the larger ones.

Experimental Investigation of Frictional Noise and Surface-roughness Characteristics

Abstract: An experimental investigation into the nature of the noise generated when a stylus travels over a frictional surface has been carried out using several engineering materials. The relation between the sound pressure level (SPL) and surface roughness under various contact loads was established. An acoustic device was designed and constructed to be used as a reliable tool for measuring roughness. For each tested material, it has been found that the filtered noise signal within a certain spectrum bandwidth contains a specified frequency at which the amplitude is maximum. This frequency was named the dominating frequency and was found to be a material constant independent of surface roughness and contact load. It was also found that the dominating frequency for a given material is proportional to the sonic speed in that material.

Method for predicting destruction of a bearing utilizing a rolling-fatigue-related frequency range of AE signals

Abstract: A method for predicting destruction of a bearing preliminarily finds frequency range of acoustic emission signals indicating fatigue of bearings. In order to find the fatigue-related frequency range, frequency components of electrical noise, mechanical noise, acoustic emissions (AE) due to slip of a bearing, and AE due to plastic-deformation of the bearing are respectively obtained under corresponding conditions, using a test equipment. Then, under a certain condition, combined AE signals of electrical noise, mechanical noise, slip-related AE, plastic-deformation-related AE and rolling-fatigue-related AE are generated. By subtracting the frequency components of electrical noise, mechanical noise, slip-related AE, plastic-deformation-related AE from frequency components of the combined AE signals, the fatigue-related frequency range is obtained. This frequency range is used to predict destruction of a bearing. AE signals in the preliminarily found frequency range are extracted from all acoustic emission signals detected. The extracted AE signals are converted into digital signals and compared with a predetermined reference value. When the digital signals exceed the predetermined reference value, a preliminary indication of inner destruction of the bearing is discriminated.

Acoustic emission: A probe into dislocation dynamics in plasticity

Abstract: Acoustic emission (AE) from deforming copper and its dilute alloys was recorded at varied frequencies at room temperature and up to 500°C. All results proved to be compatible with the spurt-like dislocation movement between two bounds being the AE generating event. AE presents a maximum in the beginning of stage II deformation when event lifetimes in the range of microseconds correspond to the recording frequency. Furthermore, AE decreases rapidly with decreasing event size (by work-hardening) resp. decreasing dislocation mobility (by elevated temperature or solute content). In copper viscous dislocation movement achieves some m/s. As event bounds dislocation or cell walls should act. Solution hardening increases the stress exponent in dislocation mobility, but it is demonstrated, that doubt is advisable in some of such results. So AE proves to serve as probe into dislocation dynamics inside the bulk of a work-hardening metal.

Bubble oscillations in the vicinity of a nearly plane free surface

Abstract: The linear oscillation frequency of a bubble in the vicinity of a distorted plane free surface is calculated by a perturbation method. The approximate expression found is compared with numerical results valid for surface deformations of arbitrary magnitude. It is found that the approximate analytical result is quite good, provided that the deformation is small compared with the depth of immersion of the bubble. It is also shown that, unless the deformation of the free surface extends to distances at least of the order of an acoustic wavelength, the ‘‘image’’ bubble has the same source strength of the real bubble so that a dipolar acoustic emission can be expected in spite of the deformation of the surface.

Laboratory study of fluid pressure diffusion in rock using acoustic emissions

Abstract: In many geological situations, there is a need to monitor fluid migration and associated phenomena. Acoustic emissions (AE) that result from fluid pressure diffusion from a borehole can potentially provide useful information about migration paths, local stress states and local strength distributions. However, AE may also result from regional differential stress as well as mechanical stress concentrations associated with the injection borehole. To attempt to differentiate between these possible AE generating situations, we performed several experiments in unjointed granite rock samples containing a borehole. Fluid diffusion into and out of the borehole was achieved, under hydrostatic to high deviatoric far-field stress states, while concurrently recording digitized waveforms of AE at 20 transducer locations and measuring acoustic velocity changes. Over 2000 AE events were located with a precision better than 2 mm, and focal mechanisms were obtained for the majority of them. Most acoustic emission events could be modeled by a shear displacement source mechanism giving rise to a quadrature polarity pattern usually associated with a double couple. A significant fraction of the events, however, were not so simply modeled and may indicate a combination of extension and shear displacement at the source. Very few pure dilatational or compressional events occurred. Results indicate that diffusing fluids, at pressures less than the local tensile strength, produce useful accompanying AE only in the presence of a large differential stress state surrounding the borehole. It is difficult, however, to distinguish events produced solely as a result of fluid pressure diffusion from those resulting from regionally applied shear stresses. A high-pressure fluid leak at a metal-rock interface was easily detected using AE, but mapping of isobaric pore pressure fronts in the presence of background seismicity was less successful. A model is developed to explain the difficulty. The spatial patterns of the acoustic emission locations appeared to be fractal and might be used to infer information about the magnitude and spatial distributions of local strength in the rock.

Progress in Ultrasonic Measurements Research in 1990: Transient Sources for Acoustic Emission Work

Abstract: : Many techniques have been used by acoustic emission workers to produce rapid transient disturbances in elastic media. Techniques employing transient sources are necessary for characterizing many acoustic emission system components such as receiving transducers, transfer media, and other sources as well. Wave forms from seven types of sources were studied by performing experiments in which the source and a high fidelity receiver were mounted on opposite sides of an aluminum plate. Soundspeeds in the plate had been previously measured to provide information necessary for the calculation of the plate Green's function. In each experiment, a source was triggered and the ensuing output from the receiver was captured by a transient recorder. The force waveform of the source was calculated by convolving the inverse of the plate Green's function with the captured waveform.

Acoustic emission in martensitic transformations

Abstract: This work deals with the study of the acoustic emission generated during martensitic transformations. We propose a source model containing both a shear and a volume change mechanism. The dynamic Green's function formalism for a continuous elastic media enables, in the far field approximation, one to obtain the radiation (pattern and kinematics) characteristics of the source. The experiments corroborate that the acoustic radiation pattern in such transformations corresponds to a predominant shear mechanism in the (110) 〈1 1 0〉 system and a volume change. The kinematics of growth for martensite plates has been experimentally obtained by making use of the Doppler effect for acoustic emission waves allowing us to obtain the velocity, depth and growth length of transformation steps.

Acoustic emission in single filament carbon/polycarbonate and Kevlar®/olycarbonate composites under tensile deformation

Abstract: An analysis of envelope signals of acoustic emission (AE) produced from carbon/polycarbonate and Kevlar®/polycarbonate composites undergoing tensile deformation has been carried out to identify the sources of emission. The Kaiser effect was reproduced to validate the AE technique. Two different fiber failure mechanisms, i.e., fiber fracture and fiber degradation in Kevlar®/polycarbonate composite have been identified. A one-to-one correspondence between acoustic emission signals and fiber fracture and degradation has been established. It is shown that the critical length of the fiber, evaluated by using acoustic emission signals, is helpful in understanding the fracture behavior of the composites, as affected by surface treatments of the fibers.

Acoustic emission during crystallization of polymers

Abstract: Ultrasonic emission was found to accompany spherulitic crystallization of polymers from the melt. The source of acoustic waves was identified as an abrupt negative pressure release in regions of melt occluded by spherulites (weak spots) in the course of crystallization. The buildup of a negative pressure in occluded areas is a result of a density change during crystallization. When the level of stress approaches the limit related to melt cohesion, cavitation occurs, the stress is released, and acoustic wave is emitted. On the basis of a statistical approach to the description of spherulitic structure formation, it was predicted that most acoustic events should occur at a high degree of conversion—close to the end of crystallization. The ultrasonic emission was found during isothermal crystallization of isotactic polypropylene and poly(methylene oxide). In the case of Nylon 6, acoustic emission was attributed to environmental stress cracking and degradation. No acoustic events were recorded during crystallization of high-density polyethylene and during similar thermal treatment of a series of noncrystallizable polymers. Addition of a nucleating agent to crystallizable polymers drastically reduces the acoustic emission during crystallization, which indicates that the spherulite size is one of the most important parameters. Depending on the size and geometry of weak spots, melt properties, and mechanical properties of spherulites, the stress in weak spots is either released by cavitation or remains frozen in the material. It was found that the time distribution of acoustic emission is located between the time distributions of the formation of weak spots and the formation of quadruple boundary points between spherulites.

Surface-roughness measurement using dry friction noise

Abstract: A new technique for determining surface roughness is introduced. It is based upon measuring the generated noise due to dry friction of a metallic blade which travels over the surface under consideration. If the frictional force is made small enough to excite the blade, and not the entire system, then the noise will be proportional to surface roughness, and independent of the measured specimen size and material. A prototype acoustic device was designed and constructed and then used to substantiate the technique. Several specimens were machined by different machining processes to obtain a wide range of roughnesses. The specimen surface roughness was measured by a widely used commercial instrument (Talysurf 10), and the prototype transducer. A straight line correlation (on a logarithmic graph) between sound pressure levels (SPL), obtained from the transducer, and roughnesses, obtained from the Talysurf instrument, was found over the range from 0.025 to 100 μm. The coefficient of correlation between the measured values of SPL and roughness was as high/as 98 percent. Therefore, the technique could be adopted for measuring roughness and the prototype may thus be considered as a new roughness measuring device. The prototype device has the advantages of being sensitive, accurate and compact. It also has the capability of measuring the roughness of almost any type of surface regardless of its shape, material and complexity. Hence it is considered adequate for most engineering applications.

Crack Growth and Acoustic Emission in Ceramics During Thermal Shock

Abstract: It is recognized that microcracks may contribute to failure of ceramics undergoing thermal shock, although there is little direct experimental evidence. The combination of acoustic emission (AE) and SEM observations provides such evidence. In this work AE data were analyzed after rapid quenching of samples in silicone oil. A thermal shock resistant material, alumina, and a material resistant to thermal damage, advanced zirconia refractory, have been examined. Cracks were detected by analysis of AE amplitudes and durations and their growth was monitored by systematic SEM observations as thermal shocks of increasing severity were applied. Three-point bending strengths were determined in air after quenching. For the first time SEM images are presented showing early stages of crack initiation for temperature differences less than ΔTcrit, i.e., where the fracture was not believed to occur. Further development of the cracks leads to abrupt strength reduction in alumina and controlled strength loss in zirconia, although AE data did not indicate any particular pattern of catastrophic crack propagation when substantial loss of strength occurred.

What are the observed high-frequency solar acoustic modes?

Abstract: Jefferies et al. (1988) observe discrete peaks up to ≈7mHz in the power spectra of their intermediate degree solar intensity oscillation data obtained at South Pole. This is perhaps surprising since waves with frequency greater than the acoustic cut-off frequency at the temperature minimum (≈ 5.5mHz), unlike their lower frequency counterparts, are not trapped in the solar interior. We propose that the observed peaks are associated with what are principally progressive waves emanating from a broad-band acoustic source. The geometrical effect of projecting observations of these progressive waves onto spherical harmonics then gives rise to peaks in the power spectra. The frequencies and amplitudes of the peaks will depend on the spatial characteristics of the source. Partial reflections in the solar atmosphere modify the power spectra, but in this picture they are not the primary reason for the appearance of the peaks. We estimate the frequency and power which would be expected from this model and compare it with the observations. We argue that these high frequency “mock-modes” are not overstable, and that they are excited by acoustic emission from turbulent convection.

A self‐exciting process of acoustic emission occurrence in steady creep of granite under uniaxial stress

Abstract: Clustering of acoustic emissions (AE) in space and time was found in a laboratory experiment. To investigate the stochastic process of AE occurrence, event times of AE were measured during steady creep phases under different stress levels. Oshima granite was continuously subjected to constant uniaxial stress of different intensity. The occurrence time of AE event was analyzed as a combination of a Poisson process and a self-exciting part; in the latter, events are excited by preceding events. The self-exciting character was more enhanced under higher crack density, in accordance with a stronger concentration in hypocenter distribution at higher stress.

Acoustic emission simulator

Abstract: The invention simulates acoustic emission signals produced by crack growth within a structure so that listening equipment may properly be calibrated to listen for acoustic emission signals from actual structural crack growth. The invention has several embodiments which generate acoustic emission signals from a crack growing in a specimen such that the acoustic emission signals have a broad bandwidth as the crack grows. The acoustic emission signals produced by the growing crack can be introduced into a structure being monitored through a cone-shaped portion which is bonded to the structure being monitored.