Showing papers on "Acoustic emission published in 1981"

Quantitative Relationships for Acoustic Emission from Orthogonal Metal Cutting

Abstract: Theoretical relationships have been drawn between acoustic emission (AE) and the metal cutting process parameters by relating the energy content of the AE signal to the plastic work of deformation which generates the emission signals The RMS value of the emission signal is expressed in terms of the basic cutting parameters Results are presented for 6061-T6 aluminum and SAE 1018 steel over the range of speeds 252 to 372 sfm (0128 to 19 m/s) and rake angles 10 to 40 deg Good correlation has been found between predicted and experimental signal energy levels In addition, AE generation from chip contact along the tool face is studied and the AE energy level reflects the existence of chip sticking and sliding on the tool face, and indicates the feasibility of utilizing AE in tool wear sensing

The origin of acoustic emission during deformation of aluminium and an aluminium–magnesium alloy

Abstract: The acoustic emission power during deformation of pure aluminium single crystals and polycrystals, and specimens of A1 1·3wt% Mg, has been measured. In pure aluminium the acoustic emission activity increased with grain size to approach the levels of activity from single crystals, whereas the emission activity of the Al 1·3wt% Mg solid solution exhibited a peak at a grain size of ∼80 μm, and samples with large grain size generated almost no detectable emission. A model is presented for acoustic emission by a dislocation, and is used to explain the observed emission dependence upon grain size in both metals.

Magnetomechanical acoustic emission — a new method for non-destructive stress measurement

Abstract: This paper describes a newly found non-destructive testing technique for residual stress measurements, which is called magnetomechanical acoustic emission (MAE). It applies to ferromagnetic materials only, but has a greater depth of penetration than other common methods of residual stress analysis, such as X-ray diffraction and acoustic surface wave. Previous studies on MAE are reviewed, in which effects of metallurgical and magnetic parameters on MAE were evaluated. Our recent study has correlated magnetic properties of nickel and iron to MAE characteristics. Some of the observed features of MAe can be explained in terms of the domain theory, but many unresolved questions remain. Practical applications must await further developmental efforts, but some preliminary results of residual stress analysis for a welded steel plate and others are presented.

Acoustic Emission Behavior and Monitoring of Soils

Abstract: As far as nondestructive testing methods are concerned, acoustic emission techniques are relatively recent additions to the rock monitoring area (dating from the late 1930's) and to the metal testing area (dating from the 1950's). Its application to soils is an even more recent event with little activity prior to the 1970's. However, over the past five to ten years, interest has been generated in the soils area to the point where at least five equipment manufacturers are currently marketing acoustic emission systems specifically for geotechnical engineering applications. This activity is seemingly well founded, for acoustic emissions are indeed generated by deforming soil masses and technical feasibility is now firmly established. This state-of-the-art paper on acoustic emission activity in soils presents these findings on the basis of fundamentals, small-scale laboratory tests, and large-scale laboratory tests. Furthermore, the technique has been applied to field situations in a number of cases. These include slope stability monitoring of dams and embankments, soil movements arising from horizontal and vertical deformations, seepage monitoring, and grout/hydrofracture monitoring. Specific case histories in each group are presented. Collectively taken, the information available seems encouraging enough for many investigators to use the technique for a wide variety of applications. With a multifaceted attack, the current qualitative status of the technique (that is, no acoustic emission indicates stability; low acoustic emission indicates small movement; moderate acoustic emission indicates larger movement; high acoustic emission indicates instability) should move into a better defined quantitative status. In this latter case, acoustic emission signatures of different soils could lead to instant assessment of actual stress levels in any given situation. (Authors)

Identification of quench origins in a superconductor with acoustic emission and voltage measurements

Abstract: The acoustic emission and voltage measurements were used to identify quench sources in a current‐carrying superconductor. The sources identified were (i) conductor motion, (ii) epoxy cracking, and (iii) pure joule heating resulting from the conductor reaching critical current. The combination of these two techniques appears to be a promising diagnostic tool in probing premature quenches in superconductors and superconducting magnets.

Theory of Acoustic Emission

Abstract: A theory of acoustic emission is presented based on a Green's function type of formalism, rather than on the conventional count rate concept. Sources are represented by stress drop rate tensors and conditions are derived from which the source can be considered small in terms of wavelength and distance to the transducer. These “pseudopoint” sources are examined over a restricted frequency bandwidth, called the “informative bandwidth.” Such a bandlimited system may be described by a tensor transfer function type of formalism, facilitating the analysis and reducing the inverse problem—where the source is not known a priori –to a deconvolution operation. Due to the tensor nature of the source, multiple transducer measurements are necessary to reconstruct the source stress drop. The difficulty of using spectral techniques for data analysis in the presence of multiple sources is discussed. The forward problem for acoustic emission to be expected from plasticity, phase transformations, and cracks is solved within the pseudopoint approximation and the concept of retarded dipole density is introduced to deal with the problem of time delays from propagating inhomogeneity sources. In addition, the strong directionality of the signal with respect to source type and orientation is illustrated by calculating the acoustic emission signals generated by loop expansion of slip and climb (prismatic collapse) type dislocations. This directionality is shown to be the same for isotropic materials as that associated with Mode II/III or Mode I cracks and a comparison is provided between emission from expanding penny-shaped cracks and that from dislocation loops. Finally, the concepts of stress-controlled and materials-controlled acoustic emission are introduced and their significance is discussed in terms of relating AE source descriptions and material mechanisms.

Acoustic emission and inclusion fracture in 7075 aluminum alloys

Abstract: For slow crack growth (da/dn ≈0.1 µm per load cycle) in 7075-T6 aluminum, quantitative agreement was found between the amplitude distribution of burst acoustic emissions and the area size distribution of intermetallic inclusions ≈10 (µm)2 in area as measured on thepolished fracture face. This observation permits the prediction of the amplitude distribution of acoustic emissions due to crack growth in a particular sample of 7075-T6 aluminum directly from a simple, standard metallographic observation performed on the material in question. It was also found that a reduction of the yield stress from that of 7075-T6 aluminum (503 MPa) to that of 7075-0 aluminum (103 MPa) completely eliminated burst acoustic emission activity due to crack advance in the amplitude range studied.

Stimulated and spontaneous emission of acoustic waves from shock fronts

Abstract: In certain materials, shock waves of sufficient intensity spontaneously emit transverse rarefaction waves and are therefore unstable. A criterion is derived by calculating the relative amplitudes of reflected acoustic waves incident on the shock from behind. This criterion is found to delimit the conditions under which both acoustic amplification and acoustic emission can occur; it agrees with earlier results by Kontorovich.

Dynamics approach for monitoring bridge deterioration

Abstract: In conjunction with a fatigue test of a full-scale in situ three-span highway bridge, an investigation was undertaken to evaluate the use of changes in dynamic properties of the bridge as a possible means of detecting structural deterioration due to fatigue cracks in the girders. Cyclic-loading tests (transient and steady-state) were conducted to determine the changes in dynamic properties. The loading was imposed by a moving-mass, closed-loop electro-hydraulic actuator system. Several different dynamic tests were employed in the investigation to determine the modal viscous damping ratios, stiffness, and mechanical impedance of the bridge at selected intervals during the fatigue loading. Acoustic emission sensors were also used to monitor the growth of fatigue cracks in the girders. The results show that changes in the bridge stiffness and vibration signatures in the form of mechanical-impedance plots are indicators of structural deterioration caused by fatigue. Stiffness coefficients were calculated from the experimental mode shapes on the basis of a multi-degree-of-freedom system that uses modified coupling. The average reduction in stiffness was approximately 20 percent. This reduction was attributed to the combined deterioration of the bridge deck and steel girders. Mechanical-impedance plots were made from frequency-sweep tests, which included five resonant modes. Early changes in the mechanical-impedance plots were related to the deterioration of the bridge deck. Subsequent changes in these plots correlated with the fatigue cracking in the steel girders. An evaluation of the acoustic emission data showed that the sensors were able to detect the rapid critical crack growth in one girder. (Authors)

Acoustic emission from dislocation motion in precipitation-strengthened alloys

Abstract: A model is proposed to explain the effect of precipitation on acoustic emission from dislocation motion in age-hardening systems. The model is based on the hypothesis that a substantial number of dislocations must move rapidly and nearly simultaneously within a small volume of material in order to create detectable acoustic emission. In alloy systems where cross slip is difficult, precipitates, if they are not too strong, can serve as breakable pins and lead to the formation of dislocation avalanches and thereby increase acoustic emission near the onset of plastic flow. In easy cross slip systems, dislocations tend to cross slip around precipitates rather than form pile-ups, so the precipitates do not lead to increased acoustic emission. These considerations do not apply if the pins are Cottrell atmospheres, and high levels of acoustic emission may be observed in either easy or difficult cross slip systems when solute atmospheres provide the pinning. Acoustic emission measurements have been made ...

The stress-wave radiation from growing cracks

Abstract: Explicit formulae are given for the stress discontinuities radiated by a suddenly starting two-dimensional crack under tension, in an ideal elastic body. These formulae also give, with a change in sign, the stress discontinuities radiated by a suddenly stopping crack, and, with a geometrical deformation, those radiated by a three-dimensional crack. The stress in the primary radiation due to short crack-jumps is thus shown to vary in essentially the same manner as does the crack speed during the jump. The diffraction of the primary radiation from one tip of a centre-crack by the other tip produces a secondary radiation, whose properties depend mainly on the nature of the surface wave associated with the primary radiation. Since this first diffraction can lead to crack extension, further diffractions would be difficult to study analytically. If it is possible to isolate the primary radiation experimentally, however, it should give directly the essential characteristics of the source. This could be used to separate source-characteristics from specimen and transducer effects in acoustic emission studies of fatigue cracking, or stress-corrosion cracking.

Correlations between light emission, acoustic emission and ion density in premixed turbulent flames

Abstract: An experimental study was performed to investigate the correspondence between fluctuating ion density, light emission and acoustic emission in open premixed, turbulent propane/air flames. To date the kinds of measurements employed have been a) pointwise, cross section average and volume average of the ion, density fluctuation, b) cross section average and volume average of the C 2 radical light emission fluctuations and c) the far field acoustic pressure fluctuations. It is shown that the measurements may be interchanged so that they are all indicators of the same quantity-heat release rate fluctuations.

Emission of electrons and positive ions upon fracture of oxide films

Abstract: During tensile deformation of oxide‐coated aluminum, small cracks a few hundredths of a mm in length occur in the oxide film. During and following the appearance or elongation of these cracks, electron emission (EE) and positive ion emission (PIE) are detected in the surrounding vacuum. Crack propagation in the oxide coating can be detected with an acoustic emission (AE) transducer. Correlations between charged particle emission and film fracture can then be determined. A comparison of rates of EE, PIE, and AE, the distribution of the number of electrons or ions per burst, and the time distributions relative to crack propagation of both EE and PIE are presented. The time distributions indicate distinct differences in the rate limiting steps governing EE and PIE.

Relationship between microstructure and acoustic emission in Mn-Mo-Ni A533B steel

Abstract: The sources of acoustic emission in a steel of the A533B composition with low sulphur content have been investigated using two different recording techniques. First, systematic narrow band (0.1–1.0 MHz) measurements were made of acoustic emission activity during uniaxial tensile tests of specimens with a range of controlled microstructures. Mechanical properties and fracto graphic data were analysed, and the most probable deformation and fracture mechanisms deduced for these microstructures. It appears that only a few processes generate detectable emission in this material. For instance, high-velocity dislocation motion can be detected during the yield of slow-cooled or long-tempered specimens, and cleavage fracture in rapidly quenched specimens. A model is presented to relate the magnitude of the emission to the dynamic operation of dislocation and fracture sources. Second, very wide band (25 MHz) calibrated measurements were made of the most energetic emission sources in rapidly quenched materia...

Acoustic Emission Signatures of Various Rock Types in Unconfined Compression

Abstract: An investigation of ten rock types in unconfined compression has led to four distinct types of stress versus acoustic emission response curves, that is, signatures. These are the originally proposed Mogi type, and three variations of this generalized behavior, that is, unstable, dense, and dense unstable types. They have been correlated to conceptual ideas on brittle rock behavior. Frequency analysis on all rock types at each stage of the various stress ranges has also been performed. Trends in peak frequency shifting are presented with implications as to the use of this information. (Authors)

Acoustic Emission in the Drying of Hardened Cement Paste and Mortar

Abstract: Acoustic emission (AE) during the drying of cement and mortar was studied extensively; AE in direct fracture of notched beams was also measured. A new method for observing the hydration of cement by measuring acoustic conductivity was developed and used to aid in the interpretation of the drying experiments. Acoustic emission due to drying increased rapidly with the time of hydration. It reached a maximum value at ∼60 days for hardened cement paste and 240 days for a standard mortar. It is shown to be caused by fractures in structures that develop, at least in part, during the drying process.

Acoustic emission and microstructure in aluminium alloys 7075 and 7050

Abstract: The relationship between microstructure and acoustic emission (AE) detected during tensile deformation has been investigated for the commercial aluminium alloys 7075–T7351 and 7050–T7351. Also, AE measurements made during tensile deformation of 7075–T7351 and high-purity aluminium have been compared with those made during compressive deformation. It is concluded that the fracture of brittle inclusions is the source of AE detected during tensile deformation of the alloys. An observed variation in AE activity from specimen to specimen (of both alloys) has been attributed to the inhomogeneous distribution of these inclusions in the plates used for specimen manufacture.

Fundamental developments for quantitative acoustic emission measurements

Abstract: This report describes a research program supported jointly by the Electric Power Research Institute and the National Bureau of Standards. The intent of this report is to present an in depth description of research and results of this program for the specialist; additional details are contained in the referenced papers resulting from this program. The work under Phase 1 of the EPRI/NBS AE program has focused on: improved test standardization through the development of a calibration capability for AE sensors; improved sensor concepts and techniques for field and laboratory calibration; an improved basis for understanding and predicting AE behavior through the development of a mathematical framework for AE (transfer function formalism) through specific theoretical solutions to AE generation, transmission and inversion problems and the successful application of these theories to actual events in glass; an improved basis for assessing defect significance through the development of improved signal processing and inversion methods and through experimental results from AE in pressure vessel steels; the implementation of experiments to establish the feasibility of using causal methods, based on theoretical mechanics, to obtain source information in structural steels.

Origins of acoustic emission in superconducting wires

Abstract: Origins of acoustic emission (AE) in superconducting wires have been investigated. Our experimental results indicate that wire motion is a dominant source of AE in current carrying wires, both superconducting and nonsuperconducting. Furthermore, the results show that flux motion, except during flux jumping, produces no discernable acoustic signals. Both of these conclusions differ from the conclusions of earlier AE results, which attributed the major source of AE signals in superconductors and superconducting magnets to flux motion.

Acoustic-emission crack monitoring in fracture-toughness tests for AISI 4340 and SA533B steels: Purpose of this investigation was to examine the features of acoustic emission associated with the onset of subcritical crack extension and crack propagation

Abstract: The feasibility of using acoustic-emission techniques for the characterization of fracture resistance in AISI 4340 and SA533B steels is examined. The critical value of J-integral (JIC) is measured with a single small specimen (compact tension) loaded in the elastic-plastic range at room temperature. Initiation is detected during loading by acoustic emission. A new AE procedure for crack-growth monitoring in fracture-toughness specimens has been proposed. The possibility of discriminating AE signals from noncritical sources, such as void nucleation during crack-tip plastic deformation and signals from a growing crack, is discussed.

Influence of microstructure on acoustic emission during deformation of aluminium alloys

Abstract: Acoustic emission (AE) has been measured during the deformation of pure aluminium and aluminium alloys with a range of microstructures. In polycrystalline aluminium ‘continuous’ emission activity was observed which increased with grain size to approach that of single crystal aluminium. The data were consistent with dislocation glide as the source of emission. In the binary solid solution, Al–1·3Mg, ‘burst’ emission activity was observed which increased rapidly with grain size to a peak value at 80 μm before decreasing at larger grain size. The cooperative escape of dislocations from solute atoms is proposed as the source of emission. In the binary age-harden able alloy, Al–4Cu, there was a peak in burst emission activity in underaged samples (2 h at 170°C). Here the proposed source of emission was the formation of coarse slip bands induced by the shear of weak Guinier–Preston zones. Studies of Al–5·5Zn–2·5Mg and Al–5·5Zn–2·5Mg–1·6Cu alloys indicated a similar source. Large burst emissions were det...