Showing papers on "Acoustic emission published in 1992"

Partial discharge. XIII. Acoustic partial discharge detection-fundamental considerations

Abstract: An overview of the basic acoustics theory required to understand the finer points of acoustic partial discharge (PD) detection systems is given. PD and acoustic test methods are discussed, and acoustic wave motion, impedance, and intensity are described. Wave propagation and signal absorption, and the velocity of sound are discussed. Acoustic characteristics of media gases, liquids, and solid materials are described. >

Chapter 1 Observations of Quasistatic Fault Growth from Acoustic Emissions

Abstract: The location of acoustic emission (AE) sources during deformation of rock has proven to be a useful non-destructive analytic technique. We present experimental results,based on AE observations, that show the nucleation and growth of macroscopic fault planes in granite and sandstone samples. By controlling axial stress to maintain constant AE rate rather than more conventional loading conditions such as constant strain rate or constant stress, we have been successful in retarding the failure process in triaxial experiments. As a result, the post-failure stress curve has been followed quasistatically, extending to minutes or hours the fault growth process that normally would occur violently in a fraction of a second. In addition to this novel control system, three-dimensional locations of AE events were determined by analyzing the relative arrival times of AE pulses recorded on a network of transducers attached to the sample. In this manner, as many as 40 000 AE events were located in the course of a single experiment. While the details of fault formation varied from experiment to experiment, a number of features were consistently observed. In all three granite experiments, the fault plane nucleated abruptly at a point on the sample surface soon after reaching peak stress. Prior to fault nucleation, microcrack growth was distributed evenly throughout the granite samples. From the nucleation site, the fault plane grew across the sample, accompanied by a gradual drop in axial stress. AE locations showed that the fault propagated as a fracture front (process zone) with in-plane dimension of 10-50 mm. As the fracture front passed, the AE from a given region would drop to a low level. If allowed to progress to completion, stress eventually dropped to the frictional sliding strength. Sandstone samples showed somewhat different response. In these experiments, a diffuse damage zone appeared prior to peak strength and gradually localized into an incipient fault plane. After passing through peak stress, this plane grew, as in the granite samples, to eventually bisect the sample.

A study of progressive failure of rock under cyclic loading by ultrasonic and AE monitoring techniques

Abstract: New experiments were conducted to expand the existing information on AE and pulse velocity changes In this study, a few Hyderabad granites were subjected to uniaxial compressive cyclic loading conditions at room temperature The laboratory tests were carried out by loading the rock to a constant stress maximum ranging between 40% and 80% of failure stress in the initial cycles, and then by progressively increasing the stress maximum, and also increasing the time gap between the cycles Compressional wave velocity and amplitude changes were monitored in directions perpendicular to the applied stress, and acoustic emission event count data were recorded continuously until the fatigue failure of the rock

Application of acoustic emission techniques in manufacturing

Abstract: This paper reviews applications of acoustic emission sensing techniques in manufacturing processes. Acoustic emission (AE) has been a tremendous growth in use in process monitoring and quality control in manufacturing processes since its discovery in the early 1950s in Germany. Originally applied to the destructive testing of structures (especially storage and pressure vessels) it has also become a monitoring technique in manufacturing due to its sensitivity to process parameters. This paper gives some background on the sensing needs in manufacturing for machine diagnostics as well as process monitoring, reviews the potential for AE application in manufacturing and illustrates some of the existing applications dating back to the first process monitoring in the 1970s. Special emphasis is on machining and tool condition monitoring.

Nondestructive evaluation of ferromagnetic cables and ropes using magnetostrictively induced acoustic/ultrasonic waves and magnetostrictively detected acoustic emissions

Abstract: A method and apparatus for the nondestructive evaluation of ferromagnetic and non-ferromagnetic materials, particularly wire ropes, cables, and strands, and pipes utilizing the magnetostrictive effect for measuring minute variations in magnetic fields and characterizing these minute variations as indicative of the acoustic/ultrasonic behavior of fractures, cracks, and other anomalies within a substance under evaluation. The apparatus and method contemplate both an active testing application, wherein a transmitting sensor generates an acoustic/ultrasonic pulse within a material through the magnetostrictive effect and a second receiving sensor detects reflected acoustic/ultrasonic waves within the material, again by the inverse magnetostrictive effect. The advantages of utilizing magnetostrictive sensors as opposed to well known piezoelectric sensors lies in the ability to generate and detect acoustic/ultrasonic waves without a direct physical or acoustical contact to the material. The apparatus and method of the present invention also anticipates the use of a passive monitoring system comprised only of a receiving magnetostrictive sensor that continuously monitors a ferromagnetic or non-ferromagnetic substance for acoustic emissions and either records this monitored information or alerts the appropriate personnel of the existence of an acoustic emission indicating deterioration within the structure.

Fractal Structure of the Hypocenter Distributions and Focal Mechanism Solutions of Acoustic Emission in Two Granites of Different Grain Sizes

Abstract: Coarse-grained Inada granite (grain size 5 mm in average) and fine-grained Oshima granite (grain size < 2 mm) samples were deformed in triaxial compression experiments. Acoustic emission (AE) was monitored using 20 transducers in real time. In each experiment, hypocenters of some thousands of events were determined using anisotropic velocity model based on measured data. The spatial distributions of AE hypocenters in both rocks had fractal structure. Their fractal dimensions were 2.3 and 2.7 in average for Inada granite and Oshima granite, respectively. Focal mechanisms of AE showed an important difference between two granites. In Inada granite, type-S (emitted by shear fracturing) was dominant throughout the fracturing process. However, in Oshima granite, fracture types were dependent on stress levels. At lower stress stage, type-C (assigned to implosive fracturing) was dominant (although only few in number). At the stress level below 80% of the fracture strength, type-T (emitted by tensile fracturing) was dominant, whereas above this stress level, type-S gradually became dominant.

The propagation characteristics of the plate modes of acoustic emission waves in thin aluminum plates and thin graphite/epoxy composite plates and tubes

Abstract: Acoustic emission was interpreted as modes of vibration in finite aluminum and graphite/epoxy plates. The `thin plate'' case of classical plate theory was used to predict dispersion curves for the two fundamental modes described by the theory and to calculate the shapes of flexural waveforms produced by a vertical step function loading. There was good agreement between the theoretical and experimental results for the aluminum. Composite materials required the use of a higher order plate theory (Reissner-Mindlin) combined with lamination theory in order to get good agreement with the measured velocities. Plate modes were shown to be useful for determining the direction of motion of a source. Thus, with a knowledge of the material, it may be possible to ascertain the type of the source. For example, particle impact on a plate could be distinguished from a crack growing in the plate. A high fidelity transducer was needed to distinguish the plate modes. After evaluating several types of transducers, a broadband ultrasonic transducer was found which satisfied the fidelity requirement and had adequate sensitivity over the 0.1 to 1 MHz range. The waveforms were digitized with a 5 MHz transient recorder. The dispersion curves were determined from the phase spectra of the time dependent waveforms. The aluminum plates were loaded by breaking a 0.5 mm. pencil lead against the surface of the plate. By machining slots at various angles to the plane of a plate, the direction in which the force acted was varied. Changing the direction of the source motion produced regular variations in the measured waveforms. Four composite plates with different laminate stacking sequences were studied. To demonstrate applicability beyond simple plates, waveforms produced by lead breaks on a thin-walled composite tube were also shown to be interpretable as plate modes. The tube design was based on the type of struts proposed for Space Station Freedom''s trussed structures.

Damage mechanics with long-range interactions: correlation between the seismic b-value and the fractal two-point correlation dimension

Abstract: SUMMARY A modified Griffith criterion for a two-dimensional array of aligned elliptical cracks with a long-range interaction potential is presented. In accordance with observation, the pattern of cracks is assumed to be fractal, with a two-point correlation dimension Dc indicating a power-law distribution of crack spacings r, and a power-law exponent D of the crack length distribution. From a simple dislocation theory of the seismic source D is proportional to the seismic 6-value if an individual earthquake or acoustic emission is produced by displacement on a specific fault or crack in the population. As a result, the theory is applicable to incremental damage rather than the long-term evolution of crack systems with large displacements. The long-range interaction between cracks is taken to be elastic, implying a positive interaction potential proportional to r-'. Two models are presented for the spatio-temporal evolution of the resulting seismicity due to: (A) progressive alignment of epicentres along an incipient fault plane; and (B) clustering of epicentres around potential nucleation points on an existing fault trace. The modified Griffith criterion predicts either an increase or a decrease in the potential energy release rate G', depending on the sign of aDc/aD and the nature of the concentration of deformation. For model (A), if aDc/aD > 0 (corresponding to an implied positive correlation between the b-value and Dc), then G' increases in the presence of an interaction potential. In contrast G' increases if aD,/aD O. The mechanical hardening (lowering G ') is associated with geometrically distributed damage in either case. Equivalently this can be seen as a shielding effect, with the zone of damage reducing the local stresses on a particular crack. If there is no correlation the interaction potential has a slight mechanical hardening effect with no strong geometric effect. These predictions are also consistent with the usual tenets of damage mechanics, in which early crack growth is stable, distributed and is associated with mechanical hardening, and material failure occurs later in the cycle due to localized, unstable crack coalescence, associated with mechanical weakening. The main difference between the theory presented here and standard damage mechanics is that crack coalescence is organized, and hence instability can develop at lower crack densities.

Ultrasonic tomography and acoustic emission in hydraulically fractured Lac du Bonnet Grey granite

Abstract: Ultrasonic tomography and acoustic emission (AE) data were obtained during laboratory hydraulic fracturing tests on two large, unconfined cylinders of Lac du Bonnet grey granite. The cylinders were internally pressurized over four to five cycles prior to final failure. Compressional velocities were measured before and after each pressurization cycle with an array of 16 evenly spaced transducers around the central, cross-sectional plane of each sample. Sixteen channels of whole waveform AE data were recorded during most pressurization cycles and for a period of about 1 hour after final failure in one sample. Compressional velocities were found to be strongly anisotropic, with the in situ vertical direction being the most rapid direction in both samples. The velocity anisotropy is related to the rock's preexisting microcrack fabric. Owing to radial penetration of fluid into the rock, compressional velocities rose over the course of the experiment. A regression analysis showed that the velocity changes can be explained by variations in crack density, inferred from initial velocities, and radial distance from the borehole. Saturation levels consistent with the observed velocity changes were calculated on the basis of the O'Connell and Budiansky theory. Acoustic emissions reoccurred in a few distinct zones over several pressurization cycles. The AE locations allowed two distinct fracture planes to be sharply delineated in one sample. The fracture plane orientations were controlled by the preexisting microcrack fabric in both samples. AE occurred too rapidly during peak pressure failure to permit us to isolate distinct events. Source mechanism analysis of the AE which occurred prior to peak pressure failure in both samples, and during postfailure monitoring in one sample, showed a predominance of double-couple sources. Compressive sources, thought to be related to crushing of asperities during crack closure, and tensile sources, related to mode I crack growth, were also recorded, as well as more complex sources that could not be modeled by the simple source types listed above.

Acoustic emission monitoring of steel railroad bridges

Abstract: Results from acoustic emission (AE) monitoring of 36 steel railroad bridges are presented. The demonstrated successes of using AE to find new cracks, to identify active cracks, to validate the effectiveness of repairs, and to provide damage assessments to assist with repair prioritization are discussed. The challenges presented in using AE for fatigue-life estimation; for the assessment of pins, hangers, and complex weldments; and for crack assessment of severely corroded members are outlined. Finally, new ideas for refined monitoring and enhanced data analysis are presented.

Acoustic Emission During Pitting and Transgranular Crack Initiation in Type 304 Stainless Steel

Abstract: The acoustic emission (AE) response of a low-carbon type 304 stainless steel (SS) (UNS S30400) during pitting and transgranular stress corrosion cracking (TGSCC) has been measured. Tests w...

Tool monitoring of small drills with acoustic emission

Abstract: The risk of fracture is higher in the case of small-diameter drills than with any other cutting tool. Analysis of cutting forces or motor current is fundamentally unsuitable for practical applications, owing to the low forces or low sensitivity involved. Suitably-located AE sensors can, however, be used to obtain signals which allow tool wear and fracture to be monitored. The monitoring system realized in the study allows the tool-life dispersions of the drills, which are generally in a ratio higher than 1:3, to be exploited. Tool fractures due to high wear can also be avoided.

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

Abstract: 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 propagation 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.

Acoustic emission during fatigue of Ti-6Al-4V: Incipient fatigue crack detection limits and generalized data analysis methodology

Abstract: The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6Al-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 10 μm. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.

An Experimental Study of the Parameters That Determine Slider-Disk Contacts During Dynamic Load-Unload

Abstract: The parameters that appear to determine if slider-disk contact occurs during dynamic loading are the relative loading velocity at the instant of load and the initial pitch and roll of the slider at its unloaded state. A dual-beam LDV system is employed in this study to measure the displacement, pitch, and roll during dynamic loading for five different standard 3380-type sliders in order to investigate the effects of the initial pitch and roll. The effects of the initial height and control loading speed are also examined by using the dual-beam LDV and acoustic emission (AE)

Acoustic emission characterization of a glass-matrix composite

Abstract: Acoustic emission (AE) analyses have been performed on a high compliance, high failure strain randomly oriented carbon reinforced glass matrix composite. The composite exhibits a unique nonlinear stress-strain behavior with high failure strain during both uniaxial tensile and flexural testing. The extensive matrix cracking of this material during loading provides an excellent opportunity to study the fracture mechanism. In this study, two types of composites using similar constituents but different processing procedures resulting in different mechanical behavior, were studied. One composite showed a very pronounced non-linear stress-strain behavior during tensile testing. The curve consists of an initial linear portion, followed by a deviation from linearity and a large amount of longitudinal strain is produced with very little increase in the stress. The second composite did not show similar behavior with the slope of the stress-strain decreases continuously. The apparent different mechanical be...

Acoustic emission as an indicator of material-removal regime in glass micro-machining

Abstract: Acoustic emission (AE) spectra were recorded during microgrinding of brittle materials. It was found that the specific AE energy (i.e., the measured AE energy divided by the material removal rate) was lower for fracture-dominated grinding than for plastic flow-dominated grinding. Two subsequent experiments were performed to measure AE energy while holding the material-removal rate constant. By controlling either the critical depth of cut (for ductile-brittle transition) of the workpiece material, or the actual depth of cut of the grinding machine, the sensitivity of AE energy to grinding regime was investigated for grinding with a constant material-removal rate. Contrary to conventional thinking about the relative contributions of plastic flow and fracture in generation of AE activity, it was found that the AE energy was larger in ductile-regime grinding than in brittle-regime grinding, for identical material removal rates. As a result of the experiments described in this paper, it can be concluded that AE energy measured during microgrinding is sensitive to changes in the mechanism of material removal. For a given volume of material removed, there is more AE energy in a plastic flow-dominated process than in a fracture-dominated process. The relationship found between AE energy and material removal regime could lead to an in-process sensing strategy for controlling grinding ductility.

Mechanical interrogation of interfaces in monofilament model composites of continuous SiC fiber-aluminum matrix

Abstract: The interfacial region between continuous SiC fiber and aluminum alloy matrix, in monofilament metal matrix composites (MMC), has been characterized The study utilized two SiC fibers, produced by Textron (SCS-2) and Sigma (Σ) and two aluminum alloys: A11100 and A16061 Characterization methods employed included: optical and electron microscopy (ie SEM and EPMA), mechanical testing of as-received and heat treated single fiber samples and monitoring of acoustic emission (AE) during tensile tests In addition, interfacial shear strength (ISS) was experimentally determined by t wo different techniques, indentation and fragmentation Indentation tests were carried out in the temperature range of 0–320°C Results indicate that load is transferred from the matrix to the fiber, primarily by frictional stresses The friction between fiber and matrix during loading of composite stems residual compressive thermal stresses, which result from the temperature differential between consolidation and testing temperatures, and the difference between the thermal expansion coefficients of the fiber and matrix Consolidation had no effect on fiber strength and no reaction zones formed Exposure for 7000 h at 6000δC had no significant effect on ISS Fiber fractures were accompanied by intense AE The breaks produced a small piece between adjacent larger fragments, and gave rise to characteristic AE with distinct waveforms for the two different fibers (Σ and SCS-2)

Study of pseudoelastic behaviour of polycristallin shape memory alloys by resistivity measurements and acoustic emission

Abstract: The determination of the hyperelastic behavior of polycristalline Cu Zn Al shape memory alloys is performed using loading-unloading tension and tension-compression tests with resistivity and acoustic emission measurements. In the case of uniaxial mechanical loading, these measurements show, on the one hand, that the hyperplastic strain is proportional to the volume fraction x of martensite, and on the other hand, that the obtained acoustic emission data allows the evolution of x during the mechanical test, to be followed.

Micromechanics and acoustic emission analysis of the failure process of thermoplastic composites

Abstract: The finite element method (FEM) and acoustic emission technique (AE) were applied to the micromechanics analysis of the failure process of composites with thermoplastic matrix materials. FEM calculations to local stress-strain distribution and the influence of very different intermediate layer properties are interpreted with regard to microscopic failure mechanisms in composite materials. The strongly differing AE behaviour of both chalk-filled Polyvinylchloride and high density polyethylene and short-glass-fibre reinforced polypropylene, polyamide, PBTP, SAN and ABS in tensile test experiments is demonstrated. Representative loading limits are derived from the nature and extent of the dominating failure mechanisms by comparison of theoretical and experimental results. The influence of critical strain, shear strength and fracture toughness properties of the modified matrix as well as the composite morphology and phase adhesion on significant deformation and failure stages is discussed. Finally some conclusions are drawn about a possible critical long-term strain of composites.

High speed thin plate fatigue crack monitor

Abstract: A device and method are provided which non-destructively detect crack length and crack geometry in thin metallic plates. A non-contacting vibration apparatus produces resonant vibrations without introducing extraneous noise. Resulting resonant vibration shifts in cracked plates are correlated to known crack length in plates with similar resonant vibration shifts. In addition, acoustic emissions of cracks at resonance frequencies are correlated to acoustic emissions from known crack geometries.

Acoustic emission from plastic deformation of a pure single crystal

Abstract: Acoustic emission (AE) during plastic deformation is analyzed for a pure single crystal neglecting the effects of grain boundaries, impurities, and second‐phase particles. Acceleration of a moving dislocation is considered to be the principal AE source. There are two major mechanisms of dislocation motion related to acceleration, initial, and continuous oscillatory motion. Initial motion induced by the creation of mobile dislocations is modeled as a step function of velocity. Continuous oscillatory motion produced by interactions with neighboring dislocations is modeled as a harmonic function. These mechanisms vary with strain and strain rate due to dislocation multiplication. AE can thus be described in terms of strain and strain rate. Annihilation at a free surface is also regarded as an AE source in addition to the initial and oscillatory motions. The kinetic and strain energies stored around a moving dislocation are dissipated during annihilation, and can be related to AE. The frequency spectrum of AE...

Rate process analysis of acoustic emission activity in core test of concrete

Abstract: AE activity under uniaxial compression is closely related with the existence of microcracks in concrete. When concrete contains a number of critical microcracks, AE events occur actively from low stress level. In contrast, AE activity in sound concrete is observed just prior to final failure. The discrepancy of AE generating behavior is quantitatively evaluated, introducing the rate process theory. To gauge an applicability of AE rate process analysis to the uniaxial compression tests of core samples, experiments on newly cast concrete and core samples are carried out. The core samples of controlled damage were taken from reinforced concrete slabs which were tested by cyclic fatigue loading. Thus, the correlation with the strength, the age, and the damage level of concrete is clarified.

The Effect of Strain Rate on Damage Mechanisms in a Glass/Polypropylene Composite:

Abstract: Changes in damage accumulation and energy absorption were studied as a function of strain rate in a glass reinforced polypropylene composite during uniaxial ten sile testing. The polypropylene matrix was reinforced with long discontinuous glass fibers, and was manufactured using a du Pont proprietary wet-forming process. Acoustic emission monitoring was combined with mechanical testing and microscopic observation to study details of the damage accumulation process. Maximum stress, strain at maximum stress, and energy absorption capability all increased with increasing strain rate. Mechanical re sponse, acoustic emission signatures and micrographs of the fracture surfaces all showed a strain rate dependent transition in failure mechanism. Results show that the "apparent" fiber-matrix interface properties changed as a function of strain rate, and were closely coupled to the balance between the time scale of fiber pull-out and the characteristic time scale of resin deformation. Also, a systematic change in t...