Stress Wave Emission Research Articles

Detection of rational frequency of the sample incremental loading during its tests for endurance on the basis of a synergetically organized acoustic emission

The article considers results of the evaluation of rational frequency effecting the sample when implementing the method of determining the fatigue characteristics of materials based on synergistically organized emission of stress waves. The essence of this process lies in the fact that a flow of the emission signal is formed with a small-scale loading of the tested sample at each step of loading. At the same time, another series of dislocations is being generated, capable of reaching the crystal surface at the next moment of loading and emitting a stress wave. The magnitude of this signal characterizes the processes occurring in the material at a particular load, and allows the power parameters corresponding to such value as endurance limit to be recorded. The purpose of this work is to determine the frequency of small-scale loading, providing the maximum wave signal when implementing the method for determining the fatigue characteristics of materials based on synergistically organized emission of stress waves. The analysis of the movement of material elements was made. Based on previously published materials on the use of synergistically organized acoustic emission, the process of behavior of the metal structural components was analyzed; the process of the behavior of its grain under the influence of dislocation movements is identified and described. The strength of each such impact was represented by the delta function. The behavior of metal grains was described by the second order differential equation. The probability of a grain moving from impulse action from the side of lying crystals and from its own impulses is described by the density of movement probability of this grain. Considering jointly the dynamic and probabilistic description of the grain behavior, the Kolmogorov – Focker – Planck equation was obtained. Due to the fact that in the present work, the oscillatory nature of the metal grain movement was of interest, the above-mentioned equation was transformed into a wave-mechanical function of the process of grain behavior. The solution of wave-mechanical function is wave equation. As a result of consideration of the wave equation, natural frequency of material grain oscillations was revealed. This frequency falls in the range of frequencies that can be reproduced under stepwise loading of the test sample. This makes it possible to realize a resonance effect as applied to behavior of the metal crystal structure. Thus, frequency at which fluctuations in the material structure at the grain level will resonate with an external effect on the sample is determined. Resonant interaction of the material structure and external incremental loading of the sample will ensure a more powerful value of the emission signal at the same value of the steps under small-scale loading.

APPLICATION OF STRESS WAVES EMISSION FOR DETERMINATION OF FATIGUE CHARACTERISTICS OF MATERIAL

Results of experimental evaluation of the fatigue characteristics of tested samples material are considered based on emission of stress waves. Using previously published data on synergistically organized acoustic emission, an experiment was prepared and performed. In experiments on different materials, possibility of using acoustic emission signal for operative determination of mechanical characteristics and, above all, the limit of endurance were demonstrated. Samples for strength testing of materials were made of five steel grades and one grade of Br AZh9-4 bronze. Five experiments were conducted on each of the materials. The samples in the experiment underwent a fine-step loading, at each step of it radiation of signal occurred simultaneously, and another series of dislocations was prepared, that could reach surface of crystal and emit a stress wave at the next moment of loading. Thus, the joint radiation of energy dislocations prepared for movement was already formed. A comparison of experimental data, obtained on the basis of acoustic emission, with calculated values of endurance limit, obtained by empirical formulas through the ultimate strength of this material, performed by the Fisher criterion, has shown their adequacy at a significance level of 5 %. Evaluation of the experimental results of endurance limit determination on basis of acoustic emission by the Cochran test indicates that variances of measurement results in experiment are uniform for all types of used materials. The results have shown that such method on the basis of synergistically organized acoustic emission allows us to quickly obtain experimental values of endurance limit of material with sufficiently high degree of accuracy.

Shear-wave generation from cavitation in soft solids

The formation and dynamics of cavities in liquids lead to focusing of kinetic energy and emission of longitudinal stress waves during the cavity collapse. Here, we report that cavitation in elastic solids may additionally emit shear waves that could affect soft tissues in human bodies/brains. During the collapse of the cavity close to an air-solid boundary, the cavity moves away from the boundary and forms a directed jet flow, which confines shear stresses in a volume between the bubble and the free boundary. Elastographic imaging and high-speed imaging resolve this process and reveal the origin of a shear wave in this region. Additionally, the gelatin surface deforms and a conical crack evolves. We speculate that tissue fracture observed in medical therapy may be linked to the non-spherical cavitation bubble collapse.

Determining the Fatigue Characteristics of Material on the Basis of Stress-Wave Emission

Assessment of the fatigue of test samples on the basis of stress-wave emission is considered. Experiments are conducted on the basis of previously published material regarding the use of synergetically organized acoustic emission. Experimental data for various materials demonstrate that the acoustic-emission signal permits ongoing determination of mechanical characteristics and, in particular, the fatigue. The samples for fatigue tests are prepared from five grades of steel and also from BrAZh9-34 bronze. Five experiments are conducted for each metal. The experimental samples are subjected to loads in small increments. In each step, the emission signal is recorded, and the next series of dislocations is prepared for subsequent loading of the crystal surface and the emission of a stress wave. Thus, energetically prepared dislocations ensuring sufficient power for stable signal recorded are emitted. The experimental data obtained on the basis of the synergetically organized acoustic emission are compared with the fatigue limit of the material determined from empirical formulas in terms of the strength. According to the Fisher test, the experimental results are adequate at the 5% significance level. Assessment of the experimental data for the fatigue limit obtained from the acoustic-emission signal by means of the Cochran test indicates that the dispersion of the experimental measurements is uniform for all the materials considered. The conclusion is that determination of the fatigue limit of a material on the basis of synergetically organized acoustic emission yields real-time experimental data on the fatigue limit, with sufficient precision.

Transient thermal stress wave and vibrational analyses of a thin diamond crystal for X-ray free-electron lasers under high-repetition-rate operation.

High-brightness X-ray free-electron lasers (FELs) are perceived as fourth-generation light sources providing unprecedented capabilities for frontier scientific researches in many fields. Thin crystals are important to generate coherent seeds in the self-seeding configuration, provide precise spectral measurements, and split X-ray FEL pulses, etc. In all of these applications a high-intensity X-ray FEL pulse impinges on the thin crystal and deposits a certain amount of heat load, potentially impairing the performance. In the present paper, transient thermal stress wave and vibrational analyses as well as transient thermal analysis are carried out to address the thermomechanical issues for thin diamond crystals, especially under high-repetition-rate operation of an X-ray FEL. The material properties at elevated temperatures are considered. It is shown that, for a typical FEL pulse depositing tens of microjoules energy over a spot of tens of micrometers in radius, the stress wave emission is completed on the tens of nanoseconds scale. The amount of kinetic energy converted from a FEL pulse can reach up to ∼10 nJ depending on the layer thickness. Natural frequencies of a diamond plate are also computed. The potential vibrational amplitude is estimated as a function of frequency. Due to the decreasing heat conductivity with increasing temperature, a runaway temperature rise is predicted for high repetition rates where the temperature rises abruptly after ratcheting up to a point of trivial heat damping rate relative to heat deposition rate.

Stress wave emission from plasmonic nanobubbles

Stress wave emission from the collapse of cavitation nanobubbles, generated after irradiation of single-spherical gold nanoparticles with laser pulses, was investigated numerically. The significant parameters of this study are the nanoparticle radius, laser pulse duration, and laser fluence. For conditions comparable to those existing during plasmonic photothermal therapy, a purely compressive pressure wave is emitted during nanobubble collapse, not a shock. In the initial stage of its propagation, the stress wave amplitude is proportional to the inverse of the stress wave radius. The maximum amplitude and the duration of the stress wave decreases with the laser fluence, laser pulse duration, and gold nanoparticle radius. The full width at half maximum duration of the stress wave is almost constant up to a distance of 50 µm from the emission center. The stress wave amplitude is smaller than 5 MPa, while the stress wave duration is smaller than 35 ns. The stress wave propagation results in minor mechanical effects on biological tissue that are restricted to very small dimensions on a cellular or sub-cellular level. The stress wave is, however, able to produce breaching of the human cell membrane and bacterial wall even at distances as large as 50 µm from the emission centre. The experimentally observed melting of gold nanoparticles comes from the large temperature reached inside the nanoparticles during laser irradiation and not from the propagation of the stress wave into the surrounding liquid during nanobubble rebound.

Analysis of aluminum oxynitride AlON (Abral®) abrasive grains during the brittle fracture process using stress-wave emission techniques

This article presents the properties of a new generation of abrasive grains made from aluminum oxynitride AlON (Abral®), as well as the methodology and application of acoustic emissions as a measurement analysis method for those stress waves generated during the brittle fracture process. The methodology of evaluation of grain properties presented in the article mostly consists of examining the resistance to fracture as a result of the force applied and analyzing the registered acoustic emission signals. The applied solution involves using a tension machine and conducting compression tests upon AlON grains and, as a point of comparison, white fused alumina 99A grains, microcrystalline sintered corundum SG™, and green silicon carbide 99C. What was analyzed were the registered compression force values and acoustic emission signals within the time and frequency domains. The characteristics within the time function involve determination of the event and ring-down parameters for single acoustic emission impulses. In the case of the frequency analysis, the signal amplitude and phase characteristics were determined. The research results indicate that stress fractures appear during grain compression tests, which generate elastic waves of various characteristics. The recording and analysis of these waves, in the form of an acoustic emission signal, turned out to be an efficient tool for analyzing the process of abrasive grain cracking and made it possible to differentiate their structure. The research results obtained point to the necessity for further analyses into stress-wave emission, especially with reference to the selection of the most effective methods for analyzing the signal frequency spectrum.

S1107-1-2 摩擦・摩耗現象下におけるアコースティックエミッションと接触電圧の計測([S1107-1]トライボロジーにおける実験・計測・解析手法の新展開)

Acoustic Emission (AE) is the emission of elastic stress waves resulting from deformation and fracture of materials. The AE signals depend on the intensity of deformation and fracture of materials. A tribological phenomenon is a kind of deformation and fracture occurred at material surfaces. Therefore, it could be possible to observe the tribological phenomena by measuring AE signals. In this paper, we measured AE signals and voltage of contact in dry rubbing. A state of the contact influences tribological phenomena. Accordingly measuring contact voltage makes evaluation of the contact state effective. The AE signals were analyzed by short time Fourier transform. We examined frequency spectrum of the AE signals accompanied by the different shifts of the contact state. In consideration of that, we discussed the relationship between AE signals and tribological phenomena.

Stress wave emission and cavitation bubble dynamics by nanosecond optical breakdown in a tissue phantom – CORRIGENDUM

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The relationship between acoustic emissions and wear particles for repeated dry rubbing

Acoustic emission (AE) is the emission of elastic stress waves resulting from the deformation and fracture of materials. The application of AE measurement techniques permits in-process monitoring of tribological characteristics. For practical applications, it is necessary to determine the relationship between AE signals and tribological phenomena, so we measured AE signals generated by tribological actions for repeated dry rubbings. The experimental results indicate that the features of AE parameters differ depending on the type of wear particles that are produced. The amplitude variation in the AE count rate is proportional to the surface damage at a friction interface. There is a good correlation between the AE cumulative count and the number of wear particles. In addition, there may be a negative correlation between the AE mean value and the elastic energy stored in wear particles between sliding surfaces. The generation and actions of wear particles between sliding surfaces has a marked effect on AE signals detected under repeated dry rubbing.

3508 アコースティックエミッション法による旋削加工状態の認識に関する研究 : 切りくず生成過程と工具摩耗の評価(S65 加工計測・評価システム,S65 加工計測・評価システム)

Acoustic Emission (AE) is the emission of elastic stress waves resulting from the deformation and fracture of materials. From viewpoints such as the maintenance of machining accuracy and the fail-safe, monitoring of cutting states is very important. By measuring AE signals generated during turning process, the state estimation of machining becomes possible. This report discusses about correlation between the AE signals and the phenomena on turning process. The results indicate that there is a good negative correlation between the shear angle and the AE signal level. The measurement of the AE total energy will permit the estimation of tool wear. When TiAIN coated tool or the workpiece of different hardness was used, the similar results were obtained.

Stress wave emission and cavitation bubble dynamics by nanosecond optical breakdown in a tissue phantom

Stress wave emission and cavitation bubble dynamics after optical breakdown in water and a tissue phantom with Nd: YAG laser pulses of 6 ns duration were investigated both experimentally and numerically to obtain a better understanding of the physical mechanisms involved in plasma-mediated laser surgery. Experimental tools were high-speed photography with 50000 frames s, and acoustic measurements. The tissue phantom consisted of a transparent polyacrylamide (PAA) gel, the elastic properties of which can be controlled by modifying the water content. Breakdown in water produced a purely compressive stress wave. By contrast, in stiff PAA samples and for sufficiently large pulse energies, the compression wave was followed by an intense tensile wave, similar to the behaviour previously observed in cornea. The elastic/plastic response of the medium led to a significant decrease of the maximum size of the cavitation bubble and to a shortening of its oscillation period which was found to be related to the generation of the tensile stress wave upon breakdown. For increasing elastic modulus of the PAA, both the amplitudes of the bubble oscillation and of the stress wave emitted during bubble collapse decreased until the bubble oscillation was so strongly damped that no collapse stress wave was emitted. Numerical simulations were performed using a spherical model of bubble dynamics which includes the compressibility and elastic/plastic behaviour of the medium, viscosity, density and surface tension. The calculations revealed that consideration of the elastic/plastic behaviour of the medium surrounding the bubble is essential to describe the experimentally observed bipolar shape of the stress wave emitted upon optical breakdown. Water is a poor tissue model because the shape of the emitted stress waves and the bubble dynamics differ strongly for both materials. The mechanical properties of PAA were also found to be quite different from those of tissues. Experimental and numerical results provided evidence that the dynamic mechanical properties relevant for optical breakdown in PAA and tissue differ by as much as two orders of magnitude from the static values. The discovery of a tensile stress wave after optical breakdown in tissue-like media is of great importance for the assessment of collateral damage in laser surgery because biological tissues are much more susceptible to tensile stress than to compressive stress.

An extended Rayleigh model of bubble evolution

An extended Rayleigh model for laser generated bubbles in water and soft tissue is presented. This model includes surface tension, viscosity, a realistic equation of state, material strength and failure, stress wave emission, and linear growth of interface instabilities. The model is validated by comparison to detailed compressible hydrodynamic simulations using the LATIS computer program. The purpose of this study is to investigate the use of the extended Rayleigh model as a much faster and simpler substitute for the detailed hydrodynamic simulations when only limited information is needed. It is also meant to benchmark the hydrosimulations and highlight the relevant physics. The extended Rayleigh model and the hydrosimulations are compared using both a 1D spherical geometry with a bubble in the center and a 2D cylindrical geometry of a laser fiber immersed in water with a bubble formed at the end of the fiber. Studies are done to test the validity of the material strength and failure, stress wave emission, and the interface instability terms in the extended Rayleigh model. The resulting bubble radii, material damage radii, the emitted stress wave energies, and the size of the interface distortions are compared. Many of the trends found in the hydrosimulations are illuminated by the extended Rayleigh model owing to its relative simplicity. The extended Rayleigh model is very useful since it is accurate over a large range of parameters and it is computationally much faster than the hydrosimulations.

Dynamic crack initiation and growth in thick unidirectional graphite/epoxy plates

In this work we present some preliminary results of optical experiments performed on dynamically deforming thick polymeric-composite laminate plates. The composites used for this study are unidirectional graphite/epoxy fiber-reinforced composite plates, consisting of 48 plies (6 mm thickness). Edge-notched plates are impact loaded in a one-point bend configuration using a drop-weight tower. The lateral shearing interferometer of coherent gradient sensing (CGS), in conjunction with high-speed photography, is used to obtain real-time interferograms of the near-tip deformation during dynamic crack initiation and growth. Initiation fracture toughness data are obtained and reported. Crack growth speeds of 900 m s −1 are recorded and significant dynamic effects are observed through emission of stress waves from the propagating crack tip.

セラミック基複合材料の応力波放射型式による微視破壊様式の分類

セラミック基複合材料の応力波放射型式による微視破壊様式の分類

ＡＥによる摩擦・摩耗の研究 銅の摩耗形態とＡＥ周波数

It is well known that acoustic emission (AE) is the emission of elastic stress waves resulting from the deformation and fracture of materials. This paper describes the characteristics of AE signals which generated by friction and wear phenomena. The experiments carried out changed the finishing method of plate specimens for produce a typical wear mode. In addition, tensile tests carried out for study of the AE signal. The main results of experiments are summarized as follows : (1) In the friction and wear experiments, AE frequency range is very wide (0.02 to 1.5 MHz). (2) In the friction without wear, AE waves are only low frequency spectrum of below 500 kHz. (3) AE waves resulting from wear are characterized by the high frequency spectrum range (0.5 to 1.5MHz).

ＡＥによる摩擦，摩耗の観測 危険予知について

It is well known that acoustic emission (AE) is the emission of elastic stress waves resulting from the deformation and fracture of materials. This paper deals with a monitoring technique of the friction and wear behavior applied with the AE. Moreover, investigation was carried out about the prediction of unusual phenomenon of the friction and wear. The results are summarized as follows: (1) A little discrepancy of the friction and wear behavior can be detected sensitively by AE signals. (2) AE signals are used effectively in monitoring the run out state of lubricating oil film into frictional surfaces. (3) AE signals can be used for prediction of unusual phenomenon such as seizure.

Emission of stress waves during fracture

Emission of both longitudinal and surface (Rayleigh) waves during fracture of plates under conditions of plane stress and plane strain were studied experimentally. The non-equilibrated tensile stress in the fractured section of the plate creates an elastic wave, which travels radially along the plate at the sound speed. Moreover, the high surface deformation around the crack tip, due to the high stress concentration there, propagates as a surface wave following fracture of this zone, at the respective Rayleigh wave speed with a circular wavefront. The influence of the thickness of the plate and the type of fracture (brittle or ductile) was examined and interesting results were derived, by utilizing a high speed photography technique.

Energy processing techniques for stress wave emission signals

This paper describes the construction of two circuits capable of measuring the energy of transient signals, in particular, stress wave emission (SWE) signals. Energy of SWE signals cannot be measured by conventional energy meters since these meters require a repetitive signal, and by nature SWE signals are nonrepetitive. Compared to the more popular threshold-crossing technique, the energy processor gives a more accurate indication of the energy released from a system under stress. The constructions are based on the theory that there is a definite linear relationship between the true signal energy and the area under the envelope of the squared signal. A theoretical development of this relationship and experimental results showing the linearity for one of the circuits is also included.

Method and apparatus for distinguishing stress wave emission from mechanical noise

Signals emanating from a weld are detected during the post weld period and excursions of the detected signal above a predetermined threshold are counted. Simultaneously, an envelope of the signal is developed and excursions thereof above the threshold are also counted. A ratio of the signal excursion count to the envelope excursion count is formed. The ratio so formed is compared to known ranges of ratio values to determine whether the detected signal is mechanical noise or a stress wave emission signal.