Showing papers on "Acoustic emission published in 1974"

Continuous monitoring of fatigue-crack growth by acoustic-emission techniques

Abstract: The application of acoustic emission to the detection of fatigue-crack propagation in 7075-T6 aluminum and 4140 steel is investigated. The relationship between crack-growth rate, cyclic stress-intensity factor, load-cycling rate and observed acoustic-emission behavior is presented. Crack-growth rates of less than 10−6 in./ cycle could be detected, and acoustic-emission counts per cycle were shown to be closely related to the energy released by crack extension per cycle. A quantitative relationship for the threshold conditions for detection of fatigue-crack growth is presented which agrees with experimental test results. The results also showed that fatigue-crack growth occurs in an accelerating and decelerating manner, even though the stress-intensity range remains uniform, and that the count rate posses through a peak that is believed to be associated with a plane strain-plane stress transition. The effects of instrumentation sensitivity and frequency bandpass are also investigated. The results obtained indicate that acoustic-emission techniques should be suitable for in-service monitoring of a variety of cyclically loaded structures, even in the presence of high background noises.

The role of inclusions in the fracture of ceramic materials

Abstract: The stress concentrations that occur at inclusions due to thermal expansion and elastic modulus mismatch are discussed and the stress intensity factors at interface cracks that result from these stresses are calculated. It is shown that conservative failure prediction based on an equivalence between inclusion size and crack size is usually acceptable if the shear modulusμ or thermal expansion coefficientα for the inclusion is larger than the matrix values. If, however,μ andα are smaller for the inclusion than the matrix, extensive cracking can develop at the inclusions which may lead to premature failure. For this case the only effective methods for failure prediction are techniques which give directly the maximum stress intensity factor, i.e., proof testing and/or acoustic emission.

Highlights of Progress in the Science of Fracture of Ceramics and Glass

Abstract: Rapid, catastrophic propagation of cracks is described in terms of stress concentration and energy balance conditions from a fracture-mechanics viewpoint. Slow crack propagation by stress corrosion or other mechanisms is also presented in fracture-mechanics terms. Data on slow crack propagation and accompanying acoustic emission in a variety of ceramics and glasses are reviewed. A survey is presented of the application of these results to the quantitative treatment of strength, dependence of strength on loading rate, time to failure, proof testing, acoustic emission monitoring, thermal shock, impact damage, and erosive machining. Some limitations in the application of fracture mechanics to ceramics are discussed.

Investigation of Acoustic Emission During Fatigue Loading of Composite Specimens

Abstract: Strain and load controlled fatigue tests were run on boron-aluminum and boron-epoxy angle-plied specimens. Acoustic emission data was recorded using a gating technique that eliminated most extraneous noise. Various material parameters were monitored during the test. A good corre lation between acoustic emission and damage extent and propagation was obtained. This was evidenced by an apparent first order linear relationship between dynamic compliance and totalized acoustic emission. A basis for an energy based failure model is formulated and discussed.

Acoustic emission —4: Spectral analysis of acoustic emission

Abstract: The author discusses the significance and measurement of the frequency and shape of acoustic emissions. He considers theoretical descriptions of acoustic emission and compares them with experimental evidence. He considers the implications of this understanding of the nature of acoustic emission and illustrates these implications by a description of a programme of investigation at AERE Harwell. This programme added to our knowledge of acoustic emission and the author considers the results in the light of other experiments. He concludes that spectral analysis could complement the use of amplitude distribution and ring-down counting but not enough is yet known of the general meaning of the spectrum of an acoustic emission.

Effect of loading variables on the acoustic emissions of fatigue-crack growth

Abstract: A series of experiments has been conducted to determine the effect of loading variables such as cyclic frequency, load ratio, and material on acoustic emission from fatigue-crack propagation. It is shown that the applied-stress intensity range (ΔK) is the controlling parameter for all materials studied while the other parameters have lesser effects. Two potential methods for engineering application of acoustic emission during fatigue loading are described.

Determining the load-time history of fibre composite materials by acoustic emission

Abstract: Acoustic emission was monitored during the axial loading of unidirectional fibre composite tensile specimens. The material consisted of strong, brittle fibres (E glass) embedded in a viscoelastic matrix (epoxy). It was found that when the load was held constant the acoustic emission output continued, but at a decreasing rate with time at load. As the load level was increased, the acoustic emission output at load continued for a longer period. It is suggested that the acoustic emission under constant load is a result of fibre fracture which continues after loading ceases because of the viscoelastic nature of the matrix which allows stress redistribution with time. The experimental results from acoustic emission are compared with computer calculations for fibre fracture based on theoretical considerations. Good agreement is noted between the theoretical and experimental results.

The dependence of acoustic emission on strain rate in 7075-T6 aluminum: The aim of this experimental investigation was to determine the effect of strain rate on the acoustic emission from a polycrystalline metal

Abstract: The dependence of RMS acoustic-emission output on strain rate was determined during tensile tests on 7075-T6 aluminum over a range of strain rates from 0.015 min−1 to 0.230 min−1. A special normalization technique was developed to eliminate the inherent differences in the acoustic emission from supposedly identical specimens tested under identical conditions. It was found that the RMS acoustic-emission output increases approximately linearly with strain rate, and that the slope of the linear dependence decreased with increased plastic strain. The effect of pinned dislocation breakaway on the acoustic-emission output vs. plastic-strain relationship is the subject of a preliminary theory.

The measurement of energy in acoustic emission

Abstract: A circuit for measuring the energy in randomly occurring bursts of rf energy is described. The circuit is linear to within 10% (over a 15 dB dynamic range) for frequencies up to 1.5 MHz. Examples are given of its use with acoustic emission data from a martensitic phase transition in the indium thallium system and from a nondestructive test on a filament wound vessel.

Method and apparatus for detecting progressive cracks in materials

Abstract: An acoustic emission monitoring technique for detecting the growth or formation of cracks or other flaws in structures is disclosed. A growing crack emits acoustic signals which can be detected by an appropriate sensor on the structure surface. It has been found that the rate of amplitude buildup decreases as distance from a sensor to the site of the acoustic event increases. Thus it is possible by selecting only acoustic signals which have at least a selected build-up rate, to eliminate most background noise and other signals originating further than selected distance from the sensor. Thus, the formation and growth of cracks in the selected area can be accurately monitored in an acoustically noisy environment.

Acoustic emission in drilling wells

Abstract: Method of drilling a well in subterranean strata having an uneven fluid pressure distribution comprising periodically stopping drilling, observing the acoustic emission characteristics of the formation rock in the vicinity of the drill bit, and adjusting the weight of the drilling fluid to balance the formation pressure as expressed by the acoustic emission characteristics.

Origin and Use of Fracture Mechanics

Abstract: Numerous theoretical calculations have indicated that the tensile strength of materials should lie close to ∿ 1/10 of the material’s elastic modulus, E (see Kelley(l) for a compendium). On the other hand, due to the presence of cracks and flaws, measured strengths lie between E/103 and E/102. By eliminating pre-existing cracks, tensile strengths can approach theoretical estimates. This has been demonstrated, for example, by Ernsberger (2) for the case of glasses.

Failure Prediction in Brittle Materials Using Fracture Mechanics and Acoustic Emission

Abstract: Acoustic emission testing has found many uses in recent years and new applications are constantly being uncovered. The interesting areas for application lie in failure prediction and the characterization of the microscopic processes of yielding and fracture, and the macroscopic processes of slow crack growth and onset of fast crack propagation. This paper has described these applications and emphasized that the total counts and count rate depend on the energy released per event and the density of events per unit of deformation. Where models are presented, they should be regarded as first order approximations which await confirmation.

Acoustic emission response of dry soils

Abstract: An investigation into the acoustic emission response of a series of axially stressed dry soil samples is presented in this study. The correlation between acoustic emissions and strain is noted and strongly suggests that acoustic emission monitoring of soil masses may lead to a nondestructive field evaluation technique. Preliminary insight into the fundamental mechanism of acoustic emissions in dry soils is presented. It is seen that granular soils yield higher accumulated emission counts than fine-grained soils. The summary curves which are presented show this clearly defined trend and can be regarded as laboratory calibration curves for the soils which were tested.

Acoustic emissions in stressed soil samples

Abstract: Although acoustic emission techniques have been used in a wide variety of materials, scarcely any attention has been given to particulate systems such as soils. Yet, it is in such particulate materials where the concept of strain is quite fictitious and an alternate parameter is greatly needed. The results of this study suggest that acoustic emission might be such a parameter. Included herein is a laboratory study of the acoustic‐emission response of a series of soil samples at different water contents and under different stress states. The stress states are hydrostatic compression and triaxial shear compression tests. In each type of test the soil samples are laterally confined by pressure which simulates the soil in its natural state. The results show that high confinement pressure and high water content both produce high levels of acoustic emission. The emissions begin instantaneously upon the application of the stress. In all cases, the emissions cease when the soil samples reach an equilibrium position under the applied stress state. Some insight as to the wide range of problems which might be solved using acoustic‐emission techniques is also noted.

Acoustic emission and the Portevin-Le Chatelier effect

Abstract: Many metals and alloys which exhibit repeated discontinuous yielding (Portevin-Le Chatelier effect) also emit rather interesting acoustic energy during work hardening. Both phenomena are dramatic in dead-weight extensions of annealed specimens of brass or aluminum. The acoustic emission from such specimens was monitored and correlated with the features of the Portevin-Le Chatelier effect. It is shown that, when the discontinuous yielding subsides in aluminum, so does the acoustic emission; in fact, smooth continuous flow can occur in these materials with no detectable acoustic emission. Data are presented which are consistent with the hypothesis that, at room temperature, elastic energy released during a yield increment is proportional to the elastic energy stored since the last yield increment. This is not observed at elevated temperatures. It is concluded that additional studies of the acoustic-emission phenomena associated with plastic deformation can aid in achieving a better knowledge of the strain-hardening process for crystalline solids.

A multichannel acoustic emission monitoring system with simultaneous multiple event data analyses

Abstract: A multichannel acoustic emission monitoring system has been developed recently. The system possesses high sensitivity for the sound pressure level of the emission and is concerned with the frequency selection to overcome various background noise disturbances. The signal and data processing unit can process multiple acoustic emission events simultaneously for analyses. The cathode ray tube screen of a computer is used for a real time graphic display of the number of events at individual locations of the emission sources.

Thermo-acoustical waves in thermo-plastic materials

Abstract: The propagation velocities and the variation of the amplitudes of thermo-acoustical waves in thermo-plastic materials are theoretically investigated. The constitutive equations of anisotropic thermo-plastic materials are derived from the concept of imaginary decomposition of the deformation rate tensor into the elastic and plastic contributions and from that of the plastic potential. From generalized Vernotte's heat conduction law the propagation condition of the jumps of the velocity gradients and of the temperature rate is obtained. In isotropic materials and in the case of a normal stress vector on the wave front we have two purely mechanical transverse waves and two thermo-longitudinal coupled waves. Formulae for the velocities and amplitudes are quite similar with those for thermo-elastic materials. The variation of the amplitude is discussed. There are, in general, three effects on the variation, that is, the non-planar, heat conduction and plastic flow effects. The transverse waves are subjected only to the non-planar effect, while the thermo-longitudinal waves may grow or decay according to the above three effects.

Correlation of Barkhausen effect type measurements with acoustic emission in fatigue crack growth studies

Abstract: Discontinuous changes in the magnetization of a ferromagnetic sample similar to the rapid changes in magnetization which take place in the Barkhausen Effect are caused by the propagation of a fatigue crack. These changes were detected through induced voltages on a pick-up coil surrounding the specimen, which can be quantitatively related to the crack growth. Simultaneous acoustic emission measurements confirmed that the magnetization changes were due to the fatigue crack. In some cases the resolution of the Barkhausen technique is superior to the resolution of acoustic emission measurements.

Acoustic emission from Al2O3-Mo fibre composites

Abstract: Acoustic emission has been used to investigate the fracture processs in Al2O3 and an Al2O3-Mo fibre composite. The acoustic emission data indicate extensive micro-fracturing at loads considerably less than that required to cause fracture in Al2O3. The acoustic emission emanating from the composite in the early stages of fracture is very similar to that observed in Al2O3. However, as final fracture is approached the acoustic emission profiles differ considerably. The acoustic emission profiles are discussed in terms of the fracture mechanisms.