Acoustic Emission Frequency Research Articles

Study on Spectrum Characteristics and Clustering of Acoustic Emission Signals from Rock Fracture

Acoustic emission signals are relevant to the process of rock failure. In this paper, acoustic emission waveform signals during rock loading are acquired through uniaxial compression test of granite in laboratory. Short-time Fourier transform is used to analyze the acoustic emission signals during rock fracture to obtain the peak frequency. Based on the peak frequency of acoustic emission, four types of acoustic emission signals are classified by using fuzzy C-means method. The parameters of different types of acoustic emission signals are analyzed, which include ring count, duration, amplitude and energy. Meanwhile, the progressive propagation of surface cracks in rock specimens is quantitatively analyzed by digital image correlation (DIC) technology. The result shows that different types of acoustic emission signals correspond to different strength of rock fracture. Before rock fracture, high-count, long-duration and high-energy precursory characteristic signals appear intensively. The event density $$ {\text{Den}}_{t} = 1 $$ is taken as the early warning threshold of rock fracture through the quantitative analysis of acoustic emission signal. The present results of the research show the usefulness of the DIC and acoustic emission techniques in experiment of that type.

Shedding frequency in cavitation erosion evolution tracking

Cavitation erosion is a concern for most hydraulic machinery. An especially damaging type of cavitation is cloud cavitation. This type of cavitation is characterized by a growth-collapse cycle in which a group of vapor bubbles first grows together in a low-pressure region and then collapses almost simultaneously when the pressure recovers. Measuring the frequency of these collapse events is possible by acoustic emission (AE), as demonstrated in this study, in which a cavitation tunnel is utilized to create cloud cavitation in the vicinity of a sample surface. These samples were equipped with AE sensors, and the initially high frequency AE signal was demodulated to detect the relatively low frequency cloud cavitation shedding. It was found that when the cavitation number is increased, AE successfully detects the changes in this frequency, confirmed by comparing the results to video analysis and to simulations from literature. Additionally, the frequency increases when cavitation erosion progresses, thus providing means to track the erosion stage. It is concluded that the presented method is suitable for both detecting the transition from cloud to sheet cavitation and the erosion evolution in the experimental cavitation tunnel. The method could probably be extended to non-intrusive hydraulic machine monitoring, as this type of cloud cavitation is common in hydrofoils.

Acoustic Emission Multi-Parameter Analysis of Dry and Saturated Sandstone with Cracks under Uniaxial Compression

In order to study the mechanics and acoustic emission (AE) characteristics of fractured rock under water-rock interaction, dried and saturated sandstone samples with prefabricated double parallel cracks were prepared. Then, uniaxial compression experiments were performed to obtain their AE signals and crack propagation images. The results show that water reduces the strength and fracture toughness of fractured sandstone and enhances plasticity. After saturation, the samples start to crack earlier; the cracks grow slowly; the failure mode is transformed from shear failure along the prefabricated cracks to combined shear and tensile failure; more secondary cracks are produced. The saturated samples release less elastic energy and weaker AE signals in the whole failure process. However, their AE precursor information is more obvious and advanced, and their AE sources are more widely distributed. Compared with dry specimens, the AE frequencies of saturated specimens in the early stage of loading are distributed in a lower frequency domain. Besides, the saturated samples release less complex AE signals which are dominated by small-scale signals with weaker multi-fractal characteristics. After discussion and analysis, it is pointed out that this may be because water makes rock prone to inter-granular fracture rather than trans-granular fracture. The water lubrication also may reduce the amplitude of middle-frequency band signals produced by the friction on the fracture surface. Multi-fractal parameters can provide more abundant precursory information for rock fracture. This is of great significance to the stability of water-bearing fractured rock mass and its monitoring, and is conducive to the safe exploitation of deep energy.

Research on AE and EMR response law of the driving face passing through the fault

In this paper, the response characteristics of acoustic emission (AE) and electromagnetic radiation (EMR) to the driving face crossing a fault are analysed and tested. The method of real-time monitoring of the fault in front of the driving face by AE and EMR form coal is put forward. The results are as follows: (1) When passing through the fault, the large increase of stress in mining and fault superimposed stress, which makes it more likely to accumulate and release energy in the coal seam, is the main reason for the high risk of coal and gas outburst. (2) During crossing the fault, AE and EMR signals vary in stages, both the intensity and amplitude of the main frequency points of AE and EMR increase first and then decrease. (3) When the driving face is close to the fault, the main frequency of AE and EMR signals decreases and the frequency band concentrates. The main frequency ranges of AE and EMR signals are 3.66–22.5 kHz and 0.3–17.3 kHz, respectively. (4) The amplitudes of the main frequency points of the AE and EMR signals to the fault response. The change of the main frequency amplitude of AE and EMR signals can be used as the response index of the fault, and the response range is about 10 m. The research provides an effective monitoring method for structures in coal seam, which is of great significance to the safety production and rapid excavation of coal mines.

An Experimental Investigation of Moisture-Induced Softening Mechanism of Marble Based on Quantitative Analysis of Acoustic Emission Waveforms

The decrease of strength after saturation of rocks is known as moisture-induced softening. To date, there are numerous studies on the mechanism of moisture-induced softening of different rocks. However, due to a lack of effective observational methods, the microcosmic mechanism of moisture-induced softening still needs to be understood. We collected and processed acoustic emission (AE) signals during the uniaxial compression test of marble specimens. The results of spectral and statistical analysis show that two dominant frequency bands of AE waveforms exist regardless of the specimen’s water content. Additionally, for the AE signals from the saturated specimens, the ranges of the low and high frequency bands are wider than dried rock samples. Besides, since the tensile and shear failures in the rock release low and high dominant frequency AE signals, respectively, the test results of this paper show that micro-shear and micro-tensile failures dominate the final failure of dried and saturated rocks, respectively.

Study on Multiparameter Precursory Information Identification of the Fracture of Yellow Sandstone

In order to explore the disaster caused by uncontrollable instability of coal and rock mass, a multiparameter fusion system is constructed to predict and predict disasters more accurately by identifying the mechanical and acoustic precursors of coal and rock fracture. In order to explore the precursor information of yellow sandstone rupture, the damage evolution process of yellow sandstone is analyzed from the four aspects of rock mechanics, acoustic emission time domain, frequency domain, and characteristic parameters, and the body strain, dissipated energy and acoustic emission counting, acoustic emission energy, average frequency, peak frequency, b value, and entropy value precursor information identification points are obtained, and 8 parameters are analyzed by time series fusion. The specific conclusions are as follows: body strain in the violent stage of damage evolution, the slope is zero, the zero end point is the precursor information identification point, the dissipative energy curve overall shows the “s” type, the early growth rate is faster—the medium‐term stability—the later period is slowed down, and the upper slope boundary point of the “s” type curve is used as the precursor information identification point. In the violent stage of damage evolution, the layered features of the acoustic emission count are obvious, the specific gravity shift is more obvious, and the high count appears as the precursor information identification point; the acoustic emission energy accumulates the high‐energy signal and is accompanied by the steady and rapid growth of energy as the precursor information identification point. The effects of shearing main cracks, shear microcracks, tensile cracks, and composite cracks on the acoustic emission count and energy in the damage evolution process are analyzed. The increase of medium‐ and high‐frequency signals and the reduction of high‐frequency signals predict the rupture. The average frequency signal change law is continuous high frequency‐blank‐continuous high frequency, with the blank period end point as the damage precursor identification point; the b value damage evolution stage shows a continuously steady increase to a rapid increase, with the continuous stable growth starting point as the crack identification point. In the process of damage evolution, the sample entropy presents an orderly, chaotic, disordered, and orderly process. The end of chaos and the beginning of disorder are used as the prejudging demarcation points. Based on the time sequence, an eight‐parameter comprehensive early warning system is constructed. The indicators are classified into five levels for early warning in the stage of severe damage evolution. The identification of multiparameter precursory information of yellow sandstone provides a new research idea and analysis angle and method for the failure of other coal and rock masses.

Частотный спектр акустической эмиссии в ходе накопления деформации в алюминиево-магниевом сплаве в условиях термомеханического нагружения

  In this paper, results of a study of acoustic emission frequency spectra obtained during accumulation of deformation in an aluminum-magnesium alloy under thermo-mechanical loading from room temperature up to 5500С are presented. The spectra of acoustic emission signals are analyzed for the conditions of active deformation accumulation: the monotonic accumulation - in the region of low temperatures (up to a temperature of deformation and structural transition), and the quasi step-like accumulation - in the high-temperature region. Analysis of spectral density of acoustic emission signals indicates the redistribution of the vibrational energy of the primary acoustic signal by the resonant vibrations of the standing shear and longitudinal waves of the resonator, the geometry of which corresponds to the region of deformation localization (sample flow). It is shown that no resonances are formed in the selected resonant frequencies in the spectrum of signals at monotonic accumulation of deformation before the structural transition (in the region of low temperatures). In the high-temperature region of the quasi step-like accumulation of deformation, noticeable energy of oscillations of standing waves (both for shear and longitudinal waves) is accumulated in the resonator. In the crystal medium, the oscillations of the standing wave activate elementary deformation shifts in a certain volume, which is associated with the length of the standing wave, which determines the macroscopic scale of the correlation.

Friction, acoustic emission, and wear mechanisms of a PEKK polymer

Abstract The tribological behavior of a steel/PEKK ball-on-flat contact was studied regarding the load, the velocity and the sliding distance in reciprocating or unidirectional motion. The friction measurements were synchronized with an acoustic emission (AE) device. The results show a behavior change associated with an adiabatic effect on the polymer when the mechanical energy input increases. Two interfacial mechanisms are also characterized: Schallamarch ridges and longitudinal ploughings. These mechanisms are related to two AE frequencies of 0.2 MHz and 1 MHz. The discussion on these two AE populations brings a stronger analysis and completes the friction and the wear mechanisms results.

Energy distribution and fractal characterization of acoustic emission (AE) during coal deformation and fracturing

In this paper, the acoustic emission (AE) signals of coal ruptures were evaluated under the condition of uniaxial compression, and then the spectral, energy and fractal features of the AE waveform under different stress levels were analyzed. Results show that coal damage is progressive under loading and that the corresponding AE is paroxysmal. The AE signals are closely related to the load and deformation of coal rock. AE counts reaches the maximum at the peak stress and it reduces once the coal ruptures. During the compaction load stage, the coal structure is stable, it produces only bits of AE events with low frequency and spectrum. In addition, the proportion of low-frequency band energy is greater, and the correlation dimension is the maximum. However, as the loading progresses, the coal becomes unstable, the frequency and high-frequency band energy increase. Whereas, the correlation dimension decreases. Near the dilatation of coal volume, the AE frequency, amplitude and high-frequency band energy reach the peak values, and the correlation dimension decreases to a minimum value. At the stress peak, coal ruptures, the AE frequency reduces, low-frequency band energy increases, and the correlation dimension reaches to a low value. During the initial load time, AE is primarily influenced by small energy events. Near the dilatation of coal volume, the proportion of large-energy incidents primarily affects the AE characteristics. At the stress peak, the crack size primarily influences these characteristics. Before the peak load, the main deformation of coal is fracturing, and frictional force dominates. The AE waveform characteristics can be used to evaluate the process of coal rupture and the form of ruptured coal.

A Study of Fracture Scenarios on the Basis of Acoustic Emission Data

A system of equations for studying the fracture of solids has been formulated using a physico-mathematical model that describes the heat and mass transfer in heterogeneous media. The derived relations are used to analyze the energy balance equation and to study the fracture of a concrete sample under uniaxial compression. The analysis is performed by a quantitative interpretation of experimental data on two acoustic emission amplitude and frequency spectra recorded during sample loading. The spectra provided data on the time evolution of the structural characteristics of the material and were used to evaluate the relationship between the rates of change of surface energy and acoustic emission intensity per unit volume of the compressed concrete sample at the time of recording the spectra. Conditions for the occurrence of possible fracture scenarios by the time of recording the second spectrum were investigated. With experimental data on acoustic emission amplitude-frequency spectra for more than two time points, it is possible to study in more detail the changes in the structural characteristics, surface energy, and acoustic emission intensity per unit volume of a solid. The beginning of particular fracture stages can be predicted with higher accuracy.

Superelasticity and acoustic emission of Ni46Mn28Ga20Co3Cu3 microwire

Noise is a common occurrence in many structural and functional materials during plastic deformation processes. The acoustic signal is hard to obtain for traditional materials due to the one-off nature of the deformation. But for shape memory alloys, this phenomenon can be analyzed systematically during cyclic tensile experiments, owing to the shape memory effect. Acoustic emission (AE) is studied in Ni46Mn28Ga20Co3Cu3 shape memory microwires with various diameters during stress-induced martensitic transformation. The microwires exhibit fully reversible strain larger than 10% and obvious serration behavior at room temperature. The AE waves obtained during tensile processing reflect the phonon softening and lattice vibration during stress-induced phase transformation. Analysis of the spectra shows that AE frequency concentrates in certain selected frequency ranges, and this result reveals the characteristic nature of collective atom movement. The correlations between the amplitude, cut-off frequency and size effect of the microwires were also analyzed, respectively. Focusing on the relationship between the acoustic properties and deformation behavior of shape memory alloys may provide a new perspective for materials science.

Damage monitoring of three-point bending concrete specimens by acoustic emission and resonant frequency analysis

The crack propagation process in pre-notched concrete beam specimens, tested in the lab under three-point bending, was monitored by the Acoustic Emission (AE) technique and the extraction of resonant frequencies. The loading process was carried out in displacement-controlled conditions up to the final failure of the specimen. Average frequency vs. RA value analysis was used to characterize the crack propagation mode, whereas the cumulative AE energy and the variation in the resonant bending frequencies were selected as the main parameters to monitor the damage progress due to crack advancement. Moreover, finite element models were implemented to reproduce the load–deflection curve and to evaluate the resonant frequencies for different notch depths. An inverse procedure was therefore applied to estimate the crack advancement based on the measured and calculated frequencies.

Effects of the welding inclusion and notch on the fracture behaviors of low-alloy steel

Effects of the welding inclusions and notches on the deformation and fracture behaviors of low-alloy steels were investigated by combining the tensile performance testing and acoustic emission (AE). The micro-structures and fracture surfaces of the hot-rolled and welded samples were observed by OM and SEM. It was found out that the welding inclusions and notches would result in negative impacts on the mechanical properties of low-alloy steel. During the fracture process of the welds, there were four groups of the AE frequencies, representing three kinds of deformation and fracture behaviors: plastic deformation of the welding matrix, fracture of the welding matrix, and fracture or interface detachment of the welding inclusions. Plastic deformation of the welding matrix mainly took place in the plastic stage and the late hardening stage. Fracture of the welding matrix and inclusions were concentrated in the first half of the hardening stage.

Dynamic in-process characterization method based on acoustic emission for topographic assessment of conventional grinding wheels

The main goal of the present research is to evaluate an acoustic emission (AE)-based quick test-method for in-process determination of the topographic characteristics of a fused aluminum oxide grinding wheel. The in-process evaluation is carried out by scanning the effective width of the grinding wheel with an instrumented diamond tip under interferences in the realm of the elastic deformation range. These contact interferences generate broad frequency AE signals, which are detected by an AE transducer and converted into a proportional electric tension as a time-dependent raw signal, AERAW. The AERAW signals are simultaneously transferred to the employed hardware for online conditioning and signal sampling. A software has been developed to extract in-process quantified information from the sampled AERAW signals. The obtained quantitative information is based on both time and frequency domain analyses, resulting in fast extraction of in-process information related to the grinding wheel topography without stopping the grinding process for post-analyses. The presented method permits obtaining the information of the grinding wheel topography at usual cutting speeds. The validation of the proposed AE-based quick-test method is accomplished through a post-process evaluation of both the grinding cutting force components and the effective roughness of the grinding wheel.

Development, characterization and test of an ultrasonic vibration-assisted ball burnishing tool

This paper presents the design and characterization of an ultrasonic vibration-assisted ball burnishing (VABB) tool on Ti-6Al-4V. This process is based on the modification of conventional ball burnishing, by means of the addition of a 40-kHz vibratory force to the burnishing preload exerted by the spring inside the tool. For the purposes of successfully executing the process, a new tool is designed through the assembly of different modules responsible for the various aspects involved in it. That design is hereby presented. Then, a methodology comprising acoustic emission and high frequency sampling is proposed to characterize the functioning of a prototype manufactured according to the previously presented design. The set of techniques deployed to measure the performance of the VABB prototype is presented as a feasible means of characterizing this sort of advanced manufacturing tools, especially like this one which is governed by ultrasonic frequencies. Last of all, the prototype is tested on a Ti-6Al-4V surface to validate it. The superior results of VABB, compared to the non-assisted version of the process, is shown in terms of average and total surface roughness, as well as surface hardness. The highest improvement is achieved by applying the VABB process with 300 N and 5 passes. The effectiveness of the designed prototype is confirmed, and places this kind of VABB tools as inexpensive systems to successfully execute an innovative finishing process for industrial components.

Investigation of active vibration drilling using acoustic emission and cutting size analysis

This paper describes an investigation of active bit vibration on the penetration mechanisms and bit-rock interaction for drilling with a diamond impregnated coring bit. A series of drill-off tests (DOTs) were conducted where the drilling rate-of-penetration (ROP) was measured at a series of step-wise increasing static bit thrusts or weight-on-bits (WOBs). Two active DOTs were conducted by applying 60 Hz axial vibration at the bit-rock interface using an electromagnetic vibrating table mounted underneath the drilling samples, and a passive DOT was conducted where the bit was allowed to vibrate naturally with lower amplitude due to the compliance of the drilling sample mountings. During drilling, an acoustic emission (AE) system was used to record the AE signals generated by the diamond cutter penetration and the cuttings were collected for grain size analysis. The instrumented drilling system recorded the dynamic motions of the bit-rock interface using a laser displacement sensor, a load cell, and an LVDT (linear variable differential transformer) recorded the dynamic WOB and the ROP, respectively. Calibration with the drilling system showed that rotary speed was approximately the same at any given WOB, facilitating comparison of the results at the same WOB. Analysis of the experimental results shows that the ROP of the bit at any given WOB increased with higher amplitude of axial bit-rock vibration, and the drill cuttings increased in size with a higher ROP. Spectral analysis of the AEs indicated that the higher ROP and larger cutting size were correlated with a higher AE energy and a lower AE frequency. This indicated that larger fractures were being created to generate larger cutting size. Overall, these results indicate that a greater magnitude of axial bit-rock vibration produces larger fractures and generates larger cuttings which, at the same rotary speed, results in a higher ROP.

An Experimental Investigation on the Relationship between MS Frequency Response and Coal and Gas Outburst

Microseismic (MS) frequency response is an important part of high‐efficiency data mining to achieve the aim of coal and gas outburst (CGOB) early warning. Based on the variation pattern of acoustic emission (AE) signal in the coal failure process, the experimental characteristics of MS activity and typical signals CGOB were obtained in this study. First, the AE behavior of coal failure experiment was studied, and an explanation of laws was provided as follows: the fracture behavior of coal sample exhibits certain characteristics of AE response in terms of AE event count, signal amplitude, and frequency; each stage has its own physical meaning during the process of loading test. Based on these laws, CGOB experiments were carried out using a large CGOB physical simulation system with a MS monitoring system. Notching filter and wavelet packet transform technique were used in the denoising and feature extraction of six typical MS events (signals). The features of each stage, including the time‐frequency domain, were extracted and quantitatively expressed. We finally arrive at the following conclusions: (1) CGOB exhibits significantly periodic characteristics, and each CGOB stage corresponds to the significant response characteristics of MS. CGOB presents varying characteristics, such as “valley‐peaks‐valley”. (2) From the incubation stage to happen stage of outburst, the spectrum significantly moved from extremely low frequency (100‐200 Hz) to high‐frequency band (approach to 1600 Hz). During the residual stage, MS frequency manifested the concentration distribution (50 Hz) and offered the advantage of energy concentration. (3) The phenomenon of signal energy also shows the trend of energy transform low to high and to low modes along with the process. Signals total energy distribution (42.81%, 1,437.5‐1,812.5 Hz) in the happen stage are markedly larger than those of events in incubation stage (7.01%) and residual stage (1.44%). The methodology presented in this paper for CGOB signal analysis provides a new method to obtain MS response precursor and predict CGOB disaster. This approach can be useful for rockburst anticipation and control during mining in gas and highly stressed coal mines.

Do high frequency acoustic emission events always represent fibre failure in CFRP laminates?

When damage in carbon fibre reinforced composites (CFRP) is monitored by acoustic emission (AE), it is a common belief that high frequency AE events originate from fibre failure. This shows that this statement may not correspond to the reality, and matrix cracks can emit high frequency AE signals. Quasi-static tension of [−452/02/+452/902]s laminates was monitored by AE, Digital Image Correlation (DIC) on the surface of the sample and in-situ optical microscopy on the sample’s polished edge. Unsupervised k-means clustering algorithm was applied to the AE results. By comparison with the direct DIC and microscopic observations, the AE cluster with high frequency and low amplitude was found to correspond to directly observed matrix cracks.

Acoustic Emission and Modal Frequency Variation in Concrete Specimens under Four-Point Bending

The Acoustic Emission (AE) and Dynamic Identification (DI) techniques were applied simultaneously, in an original way, to examine the stress dependent damage progress in pre-notched concrete beams tested in four-point bending. The damage mechanisms were characterized by analyzing the AE signals registered during the tests, conducted by increasing the specimen’s vertical deflection. In particular, the dominant fracture mode was identified, and correlations between dissipated and emitted energies were investigated. Moreover, variations in the natural bending frequencies, produced by the crack advancement under loading, were detected and put in relation with the cumulated AE energy. Two different types of piezoelectric (PZT) sensors, operating in well distinct frequency ranges, were used to measure AE and modal signals. This study may be of interest with an outlook on possible correlations between a multi-parameter structural monitoring and the solution of inverse problems by numerical models.

Deformation and Fracture of Copper and Silicon During Indentation Acoustic Emission Measurements

The acoustic emission (AE) signals during different phases in indentation (loading, dwell, and unloading times) of ductile Cu and brittle Si were studied. The effect of load and time of loading on the AE parameters were studied. The waveforms and dominant frequency ranges were analyzed. In order to clarify the effect of friction during indentation on the AE signals, the indentation was performed through lubricant film. It was shown that the main AE signals were observed just in the loading phase in indentation of Cu. The low-amplitude wide band noise-like high-frequency-range AE signals, observed during indentation of Cu, can be associated with breakaway and jump in the motion of dislocations. Much larger duration of the AE signals in indentation of Cu indicates a substantially larger dislocation glide in annealed Cu in comparison with limited deformation in Si. The waveform during the indentation of brittle Si presents strong burst-type AE signals. Indentation of Si is accompanied by high-energy and low frequency AE signals characterizing development of the dominant damage and limiting level of plastic deformation. Indentation through the lubricant film leads to the formation of flaky cleavage and swelling around the impression.