Showing papers on "Acoustic emission published in 1985"

Subcritical crack growth, surface energy, fracture toughness, stick-slip and embrittlement

Abstract: It is proposed that the difficulties encountered with the meaning of subcritical crack growth arose from a misunderstanding of the Griffith equation. This equation is G=2γ for an equilibrium crack (stable or unstable) where γ is the intrinsic surface energy. When G>2γ the crack has a velocity v depending on the crack extension force G−2γ, even in a vacuum, and the following equation, well verified for adherence of elastomers, G−2γ=2γφT(v) where φT(v) is related to viscoelastic losses or internal friction at the crack tip, is generalized to other materials. At a critical speed vc, dφ/dv becomes negative; as a negative branch cannot be observed the velocity jumps to high values on a second positive branch, so that G=Gc is a criterion for crack speed discontinuity, not the Griffith criterion. The multiplicative factor 2γ on the right-hand side accounts for the shift of the v-K curves with environment. No stress corrosion is needed to explain subcritical crack growth. Subcritical crack growth in glasses and ceramics and velocity jump in brittle polymers are shown to agree with this proposal. This model can also explain stick-slip motion when a mean velocity is imposed in the negative branch. Occurrence of velocity jump or stick-slip depends on the geometry tested and the stiffness of the apparatus. A second kind of stick-slip associated with cavitation in liquid-filled cracks is discussed. When the surrounding medium can reach the crack tip and reduce the surface energy, even at the critical speed vc, the critical strain energy release rate Gc is reduced in the same proportion as γ, and a loading which would have given subcritical growth will give a catastrophic failure. Reduction of surface energy in the Rehbinder effect and in embrittlement by segregation is discussed. Finally, the evolution of ideas concerning the Irwin-Orowan formula and fracture toughness is examined.

Deformation and cracking behavior of protective oxide scales on heat-resistant steels under tensile strain

Abstract: The deformation and cracking behavior of oxide scales formed in air on four heat-resistant steels and on nickel 99.6 have been studied in constant-extension-rate tests at 800°C. The strain rates in the experiments ranged between 10−6 and 10−9 s−1. Acoustic emission (AE) was used as an instrument for detecting the beginning of scale cracking. Additionally, metallographic, SEM, and micro-probe investigations were performed which supported the results from the AE measurements. The strain-to-cracking of the scales did not exceed 0.5% except when lateral growth effects in the oxide scales occurred, leading to critical strains of up to nearly 2.5%. Also the crack distribution in the scales was measured. The deformation and cracking behavior of the scales investigated could be explained by model like considerations.

Localization of dilatancy in Ohshima granite under constant uniaxial stress

Abstract: The localization of dilatancy during creep of Ohshima granite under uniaxial compression was observed. Hypocenters of 3933 acoustic emission (AE) events were accurately located. It was found that the mechanical behavior of Ohshima granite was controlled by the localization of microcracks. During the stage of loading up to the creep stress, which is 83% of the average short-term fracture strength, the hypocenters of AE events were randomly distributed throughout the specimen. As soon as the primary creep began, abrupt migration and clustering of AE hypocenters into several near-surface zones were observed. AE events formed volumetric concentrations. This migration and clustering strongly suggested the rapid localized development of dilatancy at the very beginning of the primary creep stage. The distribution of AE hypocenters observed in this stage was unchanged until final faulting. By the end of the primary creep stage, AE events began to concentrate into one of these clusters, while the activities of other clusters gradually decayed. This change spread broadly and continuously in time during the creep. In the most active cluster, clustering of AE events by itself gave rise to more AE events. The shape of this cluster was spheroidal with the long axis parallel to the loading axis. No evidence directly related to planar focusing of dilatancy was found. Surface strains were mapped. The axial strain distributions in the loading interval showed that the state of stress within the sample was homogeneous. A large change in both axial and circumferential strain fields occurred during the early stage of the primary creep. After this drastic change, the pattern of strain distribution remained unchanged in the subsequent stage of the creep. The accelerated increase in one of the circumferential strain gauges during the tertiary creep stage showed strongly localized deformation preceding faulting. The development of localized dilatancy identified by hypocenter locations was confirmed by the surface strain mapping. The position where the anomalous acceleration in circumferential strain was observed was close to the active, remaining cluster. The migration and clustering of AE hypocenters to the circumferential surface at the beginning of primary creep in the uniaxial unjacketed specimen were explained in terms of stress corrosion. Since the surface is the most favorable for accessing atmospheric moisture, stress corrosion in the vicinity of the surface was facilitated.

Characterisation of fatigue crack extension by quantitative acoustic emission

Abstract: A calibrated four-channel elastic wave recording system has been used to detect and characterise acoustic emission events associated with the growth of a fatigue crack in a compact tension specimen of 7010 aluminium alloy. The transducers sampled the elastic wave field in four independent directions in a plane perpendicular to the crack. The 2-D force dipole representation of each event, assumed to be a point source, was deduced by inversion of the Green's tensor. Each emission event was then characterised in terms of source type (e.g. microfracture, slip), orientation relative to fatigue crack and size (e.g. crack volume).

Generalized theory and source representations of acoustic emission.

Abstract: Source characteristics of acoustic emission (AE) are investigated on the basis of the generalized theory previously proposed The modeling of a crack in the generalized theory is correlated to the moment tensor representation, indicating that the principal vectors of a moment tensor represent the crack orientation and the principal values represent the principal axes of the radiation pattern emanating from the crack Since source representations in the generalized theory are directly based on the discontinuity of displacement components, an explicit displacement distribution due to a crack can be incorporated We studied penny-shaped cracks and rectangular moving cracks as AE sources AE waveforms due to tensile cracks and shear cracks are also synthesized using Fortran programs for Green's functions of the second kind in a half space Formulae to synthesize AE waveforms due to both the point and the moving dislocations and their results are presented Effects of inclined angles of the crack orientation on the calculated wavefonns and on the inverse problem are studied It is shown that crack orientation has considerable effects on the results of the deconvolution analysis The moving source is found to broaden peaks of AE waveforms These results confirm the utility and applicability of the generalized theory in elucidating fundamental mechanisms of AE

Diffuse elastic waves at a free surface

Abstract: For a diffusely vibrating elastic body, the participation of the surface in the general disturbance is evaluated. It is shown that in the vicinity of a free surface a diffuse acoustic field may legitimately be regarded as a sum of incoherent isotropic and homogeneous independent plane waves incident upon the surface together with their respective outgoing reflected consequences. The work contributes to a conceptual basis for the study of acoustic emission signals on time scales large compared to acoustic travel times across the structure.

Acoustical Technology Applications in Electrical Insulation and Dielectrics

Abstract: Acoustical waves travel by molecular interaction, and as a consequence, wave attenuation and transit time can be extremely sensitive to changes in the media in which they propagate. With careful acoustic measurements, a wide variety of media properties can be monitored. These range from variations in gas mixtures and pressure, to changes in material density, porosity, and crystal size and orientation. Acoustical technology can also be applied in the listening mode to sense the tiny acoustic emissions from partial discharges in dielectrics, and from stress, strain, and other events. Many of these acoustic techniques are ideal for applying to the science of electrical insulation and dielectrics, and in this paper these different methods are described and evaluated. The subject is introduced with an outline of ultrasonic partial discharge (PD) diagnostics and nondestructive evaluation, with emphasis on acoustic factors which affect accuracy. It is then shown how the use of acoustic waveguides can improve diagnostic PD measurements in hostile and inaccessible locations. In a completely different application, a technique is described in which sound velocity measurements in gas mixtures are used for predicting electrical strength. Other valuable acoustical applications which are outlined are the monitoring of sounds from bouncing particles on metal surfaces and from gas bubbles generated in liquid dielectrics. From this latter phenomenon the temperature of a hot metal surface in a liquid can be estimated. A related subject which is also discussed is the attenuation of sound waves by gas bubbles in a liquid dielectric.

Detection of breakaway oxidation and spalling in the oxide scales of 2 1/4Cr-1Mo steel using acoustic emission technique

Abstract: The onset of breakaway oxidation and in-situ cracking and spalling of the oxide layer formed on 2 1/4Cr-1Mo steel have been detected by the use of the acoustic emission (AE) technique. Various AE parameters, viz., acoustic emission event counts vs time, acoustic emission counts vs time, and voltage level vs time have been used to detect these phenomena. It has been found that variations in AE parameters are marginal during heating at 600, 700, and 800°C, and a sudden rise in these parameters occurs during cooling. Increase in AE activity during cooling has been related to spalling of the oxide layers. At 900 and 950°C, a considerable increase in AE parameters (except voltage level) has been detected after certain times at the respective temperatures. By comparing with corresponding thermogravimetric curves, this increase in AE parameters has been attributed to the onset of breakaway oxidation. A very large increase in AE parameters during cooling from these temperatures has been related to in-situ cracking.

An evaluation of acoustic emission from fibre-reinforced composites

Abstract: An analysis of acoustic emission(AE) from epoxy matrices of different amounts of hardener and model composites containing a glass bead, carbon and glass fibres has been carried out to identify the sources of emission A few AE events generated by microcracking were observed for epoxy matrix near the final fracture strain From microscopic and emission observations it was found that the emission was generated by interfacial debonding at the pole for the model composite containing a single particle of the glass bead, and that the source of AE bursts for a continuous single carbon fibre/epoxy composite was succeeding fibre fractures along fibre length The high AE activity due to fibre fracture was observed for a composite consisting of a bundle of glass fibres The total of AE events was in agreement with the number of fibre fracture counted with the aid of a microscope in a carbon/epoxy composite The shear strength at the carbon/epoxy interface was evaluated by a critical length of the fractured fibres using the AE results

Electron emission and acoustic emission from the fracture of graphite/epoxy composites

Abstract: In past studies it has been shown that the fracture of materials leads to the emission of a variety of species, including electrons, ions, neutral molecules, and photons, all encompassed by the term 'fractoemission' (FE). In this paper, electron emission (EE) from the fracture of single graphite fibers and neat epoxy resin is examined. Measurements of EE are also combined with the detection of acoustic emission (AE) during the testing of graphite-epoxy composite specimens with various fiber orientation. The characteristics of these signals are related to known failure mechanisms in fiber-reinforced plastics. This study suggests that by comparing data from AE and FE measurements, one can detect and distinguish the onset of internal and external failure in composites. EE measurements are also shown to be sensitive to the locus of fracture in a composite material.

Determining peak stress history using acoustic emissions

Abstract: As part of the test program at the Nevada Test Site, there is a need for determining the peak stress induced by explosions in tuff Standard techniques make use of various gages grouted into the tuff prior to the test These are difficulties in interpreting the output of these gages and there is always the chance that the gage will not survive long enough to allow a stress determination to be made As an alternative, we have been testing a passive technique for determining peak stress as a function of distance from a test Using core samples retrieved from the vicinity of an explosion, we have tested for the existence of a threshold stress for the onset of acoustic emissions, the Kaiser effect (Kaiser 1950) From laboratory results it is known that for many rock types, the previously applied peak stress can be detected by restressing a sample while monitoring acoustic emissions An abrupt onset for acoustic emission activity typically occurs at a stress state close to the previous peak The point of this work was to determine if the Kaiser effect occurred in tuff and, if so, whether it could be used to determine the peak stresses induced bymore » an explosion On the basis of four tests it is only possible to draw tentative conclusions There does seem to be enough evidence of the existence and stress dependence of the Kaiser effect in the tuff to allow hope for the use of this technique in studying past stress states In future tests the saturation state of the samples should be carefully controlled Tests should be done on variously oriented subcores from a main core to determine the other components of the in situ stress tensor If the components are indeed decoupled, as these few tests indicate, then a complete stress determination can be made this way It would be interesting to use the same technique to try and determine in situ stresses in tuff 7 refs, 6 figs« less

Acoustic wave generation by thermal excitation of small regions

Abstract: Reciprocity theory is used to derive a general formalism for generation of acoustic waves by thermal excitation. It is shown that the most important parameters contributing to the generation include the thermal expansion of the material and its interaction with the volume dilation associated with the acoustic field of the receiving transducer. The excitation of a longitudinal acoustic wave, as a function of angle to the surface normal, is derived directly.

Acoustic emission spectra due to leaks from circular holes and rectangular slits

Abstract: Acoustic waves emitted during a leak are studied both theoretically and experimentally. Two geometries of the leak are considered: a circular hole and a rectangular slit. An expression for the dependence of the frequency of the emitted signal on the pressure difference of the fluid across the leak and the dimensions of the hole or slit is derived. The expression is found to agree with the experimental results obtained for the case of a circular hole over a wide range of pressure differences and hole diameters. It is concluded that a leak may be treated as a resonator that emits acoustic waves at a sharp resonant frequency, besides its well‐known characteristics of producing low‐frequency noise due to the increase of turbulence in the fluid around the area of the leak.

Acoustic Emission Monitoring of Damage Progression in Graphite/Epoxy Laminates

Abstract: The use of an acoustic emission system to conduct a test program involving five dif ferent graphite/epoxy laminates is presented. The system includes count-rate and amplitude distribution capabilit...

A study of continuous/discontinuous chip formation using acoustic emission

Abstract: Chip formation control is an important problem in unmanned machining operations. Short, discontinuous chips are often most desirable to avoid entanglement with tooling and to aid with mechanized removal systems. Chip formation conditions can change during machining, especially with single-point turning. Experiments were conducted on a machining center. Conditions of feedrate-induced segmented chips correlate well with the count rate of acoustic emission (AE). The sensitivity of AE signals to chip congestion or entangling due to continuous chip formation is illustrated.