Acoustic Emission Frequency Research Articles

Experimental study on acoustic emission frequency characteristics of granite under different cyclic loading and unloading

In Granite acoustic emission tests were performed during three different cyclic loading and unloading processes. The relationship between the acoustic emission frequency distribution pattern and the loading and unloading mode of granite during the cyclic loading and unloading process was discussed. The distribution pattern of the peak frequency with the stress evolution is analyzed qualitatively and quantitatively. Studies have shown that: in the first cycle of loading and unloading at each stage of the three cyclic loading and unloading processes, the acoustic emission ringing count and peak frequency are significantly increased, which has a significant Kaiser effect. It proves that the Kaiser effect essentially remembers the extent of the damage inside the rock previously. In the process of loading and unloading in the fourth stage cycle, the AE ring count showed a “U”-shaped change law, showing a significant Felicity effect. Comparing the three different cyclic loading and unloading modes, with the increase of the lower limit stress, the stress amplitude decreases, and the AE ringing count corresponding to the lower limit stress value of the unloading gradually decreases, and the peak frequency distribution shows more dense or concentrated characteristics. At the beginning of the third stage cycle, the proportion of high frequency band (140∼162kHz) gradually decreased, and the proportion of lower frequency band (11-18kHz) gradually increased. With the increase of the lower limit of cyclic loading and unloading, the lower frequency band (11-18kHz) in the fourth-stage cyclic loading and unloading process takes up a higher proportion, even as high as 95%. It indicates that the rock samples at this stage mainly undergo large-scale failure, and the rock samples are in a critical failure state. These provide the basis for exploring the evolution mechanism of rock mechanical behavior and the prediction method of instability and damage.

Evaluation of the acoustic emission caused by the chip segmentation frequency during machining of titanium alloy

Abstract This work investigates the relationship between acoustic emission and chip segmentation frequency of Ti-6Al-4V at the external longitudinal turning process. Therefore, several sensors like structure borne sensors, microphones and a force dynamometer have been installed in a vertical turning machine. To induce a change of the segmentation frequency, several experiments with different feed rates have been carried out. From each experiment the acoustic emissions have been recorded and the generated chips have been analyzed. Since the chips get stretched or compressed during the chip formation the change in the length is calculated to get an estimation of the segmentation frequency. The comparison of the spectral analysis of the acoustic emission signals and the chip analysis has shown that both methods show the same tendency. The segmentation frequency decreases with increasing feed.

Analysis of the AE signal parameters during granite compression process based on the K-means clustering method

The average frequency (AF), rise-time divided by amplitude (RA), and peak frequency of acoustic emission (AE) generated during the granite uniaxial and triaxial compression failure process are taken as the main factors of the type of sample crack. Further, AE signals can be characterized based on the K-means clustering method. Results show that there is a large difference between the AE signals generated by uniaxial and triaxial compression. The former has more diversity in the category, e.g., better clustering effect and uniaxial or triaxial compression. AE signal parameters can be divided into four categories based on the K-means clustering method. Acoustic signals A, B, C, D in the two compression modes are substantially similar and uniaxial compression mainly generates tensile crack before the peak stress, and shear crack appears after the peak stress. During the triaxial compression process, tensile and shear cracks are both generated before the peak stress, but shear crack is no longer present after the peak stress.

Understanding the damage evolution of sapphire under scratching from AE signals

In this paper, the study on damage evolution of sapphire induced in scratching process using acoustic emission (AE) technology was carried out. Single-grit scratches with ramp scratch depth were conducted on C-plane of sapphire with AE signal monitoring. Groove microstructure after scratching shows that the damage evolution process is mainly determined by the interactions among cracks, specimen surface and groove boundary. To evaluate the damage evolution, the AE waveform indexes, i.e. fractal and frequency characteristics, were fully analyzed and correlated with groove microstructure of specimen after tests. It is found that the fractal dimension (FD) can identify the three basic damage modes as well as various damage behaviors during scratching process. In addition, the model of damage evolution is further proposed. The frequency characteristics of AE signals are distinctive to various damage behaviors during their evolution, especially to the occurrences of the critical damages and removal modes. Furthermore, the characteristic frequency band (CFB) of AE signals was extracted. It is found that FD of the reconstructed AE signals in CFB (86.8–161.4 KHz herein) can highlight damage details and features during initial damage stage of sapphire in scratch test effectively.

Modelling Acoustic Emissions induced by dynamic fluid-asperity shearing in hydrodynamic lubrication regime

This paper investigates the mechanism and characteristics of Acoustic Emission (AE) generated from the dynamic Fluid-Asperities Shearing (FAS) in the hydrodynamic lubrication (HL) regime. Firstly, a FAS model is derived to take into account the dynamic effect of surface asperities. Then, the influence of surface profiles, lubricants and operating conditions are illustrated on AE characteristics, i.e. magnitudes and frequency bandwidths. It has been found that the correlation length of surface roughness parameters and shear rate are two main factors affecting FAS behaviours and consequently AE characteristics. Finally, the corresponding experiments are carried out based on a rheometer rig, which validate the predictability of the model and new findings, paving the foundation for developing AE based monitoring techniques.

Evaluation of crack growth stage of sapphire under scratching based on AE signals

For hard-brittle materials, crack initiation and growth is an important factor to determine the surface quality of components during machining process. In this paper, the crack growth stage of sapphire was investigated based on acoustic emission (AE) signals through single-grit scratch experiments. The AE waveform indexes, i.e. fractal and frequency characteristics, were fully analyzed and correlated with groove microstructure of specimen after tests. The research shows that the specimen undergo four stages of crack growth successively as scratch depth increasing. It is found that the fractal dimension (FD) and frequency characteristics of AE signals are distinctive to various damage behaviors and be used to evaluate crack growth stage induced in machining process effectively. Especially, the FD distribution is sensitive to the occurrences of the critical damages and removal modes resulting from crack growth in scratch procedure. This paper can shed light on the control of surface integrity and subsurface damage of hard-brittle materials in ultra-precision manufacturing.

Influence of supercritical CO2 saturation on the failure process of hot dry rock with acoustic emission monitoring

To investigate the effect of supercritical CO2 saturation on acoustic emission (AE) signal properties in hot dry rock failure process, AE tests were conducted on granite under uniaxial compression. The results revealed that the strength of granite was obviously decreased after supercritical CO2 saturation, and new frequency bands appeared in the high-frequency region of AE and dominant frequency dispersion was enhanced. Dominant frequency mode was evolved from single mode dominated by high frequency to complex mode distributed in multiple frequencies indicating the failure modes diversified. Furthermore, supercritical CO2 saturation resulted in more uniform distribution of energy events and wider distribution of magnitude during granite failure. The decrease of b value as the prediction basis for failure might not be applicable. In addition, the degradation of the cementation degree of granite mineral particles and particle size were considered to be the fundamental reasons for the variation of AE properties.

Simultaneous audio-visual-thermal characterization of transition boiling regime

Safety concerns due to the highly unstable nature of boiling during the transition from the nucleate to the film boiling regime has limited investigations on this topic. Here we perform simultaneous characterization of the thermal, acoustic, and the bubble visualization data during the transition boiling regime. We show that the key features missed due to the high latency in the thermal data are captured in the audio (0-2kHz). We correlate the acoustic emissions with the high-speed visualization to elucidate the presence of an explosive boiling sub-regime, also referred to as the microbubble emission boiling (MEB). This regime is typically observed after the critical heat flux (CHF) and is characterized by numerous small sized bubbles (≈300±50μm) emanating from an unstable vapor film on the heater surface. We correlate this change in bubble behavior with the corresponding increase in the intensity and peak frequency of acoustic emissions up until the onset of a completely silent and stable film boiling regime. Conversely, MEB features are also observed during the transition from film boiling to the nucleate boiling regime in quenching experiments. We modify the heater surface and the boiling fluid, and vary the test conditions, to confirm the existence of MEB in all experiments over a wide range of subcoolings ranging from 2°C to 25°C. Our experiments suggest that the extent of the MEB increases with, (i) the increase in subcooling, (ii) the decrease in the surface tension of the boiling fluid, and (iii) the increase in surface roughness/wettability of the heater surface. We believe these unique acoustic features of the transition boiling regime can be used as a feedback to design measures to avoid catastrophic failures in boiling based energy and thermal management systems.

Acoustic emission propagation characteristics and damage source localization of asphalt mixtures

Fracture of asphalt materials has been a critical issue that affects the fatigue performance and service life of asphalt pavements. Acoustic emission (AE) technique, as a kind of non-destructive testing (NDT) method, can effectively detect minor damage in various materials. The objective of this paper was to propose a damage source localization method suitable for understanding the fracture behavior of asphalt mixtures by AE detection. Firstly, the pencil-lead-break (PLB) tests were conducted on asphalt mixture beams to explore the propagation characteristics of AE signals, and a reasonable layout scheme of AE sensors was determined for monitoring the AE process of asphalt mixtures. Secondly, the wavelet threshold denoising method was utilized to extract effective information from AE signals associated with the pencil-lead breaks on the surface of asphalt mixture beam by simultaneously considering the wavelet basis functions, decomposition levels and threshold rules. Finally, an effective time difference of arrival (TDOA) estimation method combining noise reduction, wavelet decomposition and cross-correlation processing was established to accurately locate the damage source of asphalt mixtures. The results show that there is less serious attenuation of amplitude and energy of AE parameters and frequency spectrum of AE waveforms within the range of 100 mm in asphalt mixtures. The larger signal-to-noise ratio (SNR) and the smaller root mean square error (RMSE) of denoised AE signals indicate that the threshold denoising method optimized by the Fruit Fly Optimization Algorithm (FOA) is more effective for AE detection of asphalt mixtures. The improved TDOA estimation method could obtain a minimum TDOA value, and the calculated locations of AE events are closer to the actual PLB points.

Crack initiation of granite under uniaxial compression tests: A comparison study

Abstract In this study, a combination of acoustic emission (AE) method (AEM) and wave transmission method (WTM) is used to investigate the behaviors of AE and ultrasonic properties corresponding to initial fracturing in granitic rocks. The relationships of AE characteristics, frequency spectra, and spatial locations with crack initiation (CI) are studied. The anisotropic ultrasonic characteristics, velocity distributions in different ray paths, wave amplitudes, and spectral characters of transmitted waves are investigated. To identify CI stress, damage initiations characterized by strain-based method (SBM), AEM and WTM are compared. For granite samples, it shows that the ratio of CI stress to peak strength estimated by SBM ranges from 0.4 to 0.55, and 0.49–0.6 by WTM, which are higher than that of AEM (0.38–0.46). The CI stress identified by AEM indicates the onset of microcracking, and the combination of AEM and WTM provides an insight into the detection of rock damage initiation and anisotropy.

Investigation of Galling Wear Using Acoustic Emission Frequency Characteristics

In the sheet metal stamping process, during sliding contact between the tool and sheet, it is expected that severe events such as tool wear or fracture on the sheet generate acoustic emission (AE) burst waveforms. Attempts have been made in the literature to correlate the AE burst waveform with the wear mechanisms. However, there is a need for additional studies to understand the frequency characteristics of the AE burst waveform due to the severity and progression of the galling wear. This paper will determine the AE frequency characteristics that can be used to monitor galling wear, independent of the experimental process examined. The AE burst waveforms generated during the stamping and scratch tests are analysed in this paper to understand the change in the AE frequency characteristics with the galling severity. These AE burst waveforms were investigated using the Hilbert Huang Transform (HHT) time-frequency technique, band power, and mean-frequency. Subsequently, these AE frequency features are correlated with the wear behaviour observed via high-resolution profilometer images of the stamped parts and scratch surfaces. Initially, the HHT technique is applied to the AE burst waveform to understand the influence of wear severity in the power distribution over the wide AE frequency range. Later, the AE bandpower feature is used to quantitatively analyse the power in each frequency interval during the unworn and worn tool conditions. Finally, the mean-frequency of AE signal is identified to be able to determine the onset of galling wear. The new knowledge defined in this paper is the AE frequency features and wear measurement feature that can be used to indicate the onset of galling wear, irrespective of the processes examined.

Detection of cracking behaviors in granite with open precut cracks by acoustic emission frequency spectrum analysis

Preexisting cracks are widely distributed in brittle rocks, and these cracks strongly influence the rock failure mechanisms. The cracking process is accompanied by rapid energy release in the form of elastic waves, known as acoustic emission (AE), enabling detection of the cracking behaviors and providing an early warning of sudden failure. Granites with three types of open precut cracks were tested in uniaxial and triaxial compression tests with AE monitoring; the cracking process was recorded by a high-speed camera during the uniaxial compression tests. The peak values of the AE count rate and normalized peak amplitude corresponded well to the peak stress points, while the normalized peak amplitude was more sensitive and effective in revealing the cracking behaviors. The peak frequency can be divided into three bands. Five types of AE waveform signals were identified, according to the value of the normalized peak amplitude and the range of the peak frequency band: low normalized peak amplitude with low peak frequency (LL), high normalized peak amplitude with low peak frequency (LH), low normalized peak amplitude with moderate peak frequency (ML), low normalized peak amplitude with high peak frequency (HL), and high normalized peak amplitude with high peak frequency (HH). A comparison of the high-speed images, b values, and AE frequency spectrum characteristics showed that the LH and HH signals corresponded to macrocracks, while the LL and HL signals corresponded to microcracks. Additionally, LH and ML signals always appeared just before failure, meaning that they can be used as precursor information.

Frequency Domain Identification of Acoustic Emission Events of Wood Fracture and Variable Moisture Content

Abstract The effect of moisture content on acoustic emission (AE) during wood damage was studied and the degree of damage was evaluated accordingly. In this article, the Pinus massoniana test-pieces of three different moisture content states, which are ovendried, air-dried, and water-saturated, were used to study the AE frequency characteristics of wood during bending failure. The original AE signal was subjected to wavelet denoising, and the denoised AE signal was classified into DAE (deformation AE) and FAE (fracture AE) according to the frequency distribution characteristics. Then, the instantaneous frequency of the AE signal was obtained by Hilbert transform, and the density of the two types of AE events was separately counted to evaluate the damage degree of the wood. The test results show that the AE signal in the process of wood damage and fracture was composed of two frequency components. The DAE signal frequency was concentrated at 30 to 60 kHz, the FAE signal was concentrated at 160 to 170 kHz, and the moisture content had no significant effect on the frequency distribution of AE events. In addition, the moisture content caused a change in the fracture toughness of the wood, so that the AE event density characteristics of the three damage processes were different. In particular, the ovendried test-piece showed obvious brittle fracture, and the AE event density level was slightly changed. Studies have shown that the change of AE event density based on instantaneous frequency can objectively evaluate and predict the degree of wood damage and fracture.

STRUCTURE AND PARAMETERS OF PROPELLER UNMANNED AERIAL VEHICLES' SOUND FIELDS

When solving the actual task of detecting an unmanned aerial vehicle (UAV) by its acoustic emission (AE), representing a potential threat to various areas of human activity, it becomes necessary to distinguish its signal from all other acoustic noises. For this purpose, considerable attention is paid in the literature to theoretical and experimental studies of the structure and parameters of sound fields generated by UAVs. The basic properties of the AE generated by aircraft propellers rotations are experimentally determined in this work. The influence of piloting conditions on the shape and characteristics of the emission spectrum is analyzed. The affection of sound attenuation in the atmosphere on the spatial spectrum variability, causing degradation of its high-frequency region, is shown. The effective AE frequency band of UAVs is studied under various conditions. Emission patterns characterizing the spatial directivity of the UAV AE are presented and analyzed.

Evaluation of the acoustic emission monitoring method for stress corrosion cracking on aboveground storage tank floor steel

Stress corrosion cracking (SCC), which is one of the main threats to the structural integrity of aboveground storage tanks (ASTs), has recently attracted much attention. In this paper, the SCC of the storage tank floor was studied by the acoustic emission (AE) and electrochemical methods. Throughout the whole SCC process, the AE signal characteristic parameters, cumulating features, and correlations were analyzed, and the signal time-frequency local features were extracted via wavelet packet analysis based on the MATLAB program. A good correlation of the instantaneous variations between the AE activity and the corrosion behavior was found. Microcrack creeping and macrocrack propagation were the most important AE sources in SCC; these sources can be recognized by the AE parameters and the AE frequency spectrum. The results in this paper will be beneficial for the diagnosis of corrosion severity and the recognition of the corrosion sources by AE online inspection and will further provide an assessment of the AST integrity by means of a risk-based inspection technique.

Learning acoustic emission signatures from a nanoindentation-based lithography process: Towards rapid microstructure characterization

Abstract We present an approach for rapid identification of the salient microstructural phases present on a metallic workpiece surface via nanoindentation-based lithography process. We employ a machine learning approach to connect the time-frequency patterns of the corresponding acoustic emission (AE) signals with the underlying microstructural phases. Results show that the AE frequencies in the range of 0.3–1 kHz and 30–50 kHz can discriminate between the microdynamics of the lithography process arising from different microstructural compositions and thereby predict these microstructural phases with accuracies exceeding 95%. We also draw physical interpretations of our “black-box” machine learning model and demonstrate that the physical insights into the underlying AE signals allow us to identify novel patterns and possible microstructural anomalies.

Study on main Frequency precursor characteristics of Acoustic Emission from Deep buried Dali Rock explosion

Rock burst disaster events have become one of the most common disasters in the field of underground engineering, and it is of great practical significance to study the prediction and early warning signs of rock burst. Acoustic emission waveforms generated during a rock burst event, which are often characterized by parameter analysis, that analysis of the waveform spectrum can reflect the characteristics of rock damage and failure. Based on a single-axis strain-type rock burst test of deep buried marble, characteristics of the frequency spectrum, as well as the time-domain dominant frequency of acoustic emissions during the entire rock burst process, were analyzed. We find that the acoustic emission spectrum characteristics also have their own characteristics at different stages of rock burst. In general, The main frequency amplitude value presents an increasing trend, the peak shape of the acoustic emission spectrum shifts between unimodal and multimodal, and the secondary frequency typically appears in the plastic deformation stage. In the high-frequency domain (more than 200 kHz) of the initial loading of stress redistribution, the elastic phase of the energy build-up shows a relatively quiet period, the main frequency distribution of plastic deformation stage shows increasing tendency. Before and after the rock burst event, the dense area of the main frequency migrates from the high-frequency domain to the low-frequency domain, the main frequency in the high frequency field appears dense distribution on the eve of rock burst. These changes were distinguished as characteristic criteria for impending rock burst and can therefore be used for the prediction and early warning of rock burst events.

Acoustic emission based mechanical behaviors of Beishan granite under conventional triaxial compression and hydro-mechanical coupling tests

Beishan area is now considered as a potential site of high-level radioactive waste (HLW) repository. In order to understand the mechanical behavior of Beishan granite, conventional triaxial compression (CTC) and hydro-mechanical (HM) coupling tests are conducted on Beishan granite. An acoustic emission (AE) monitoring system is employed to estimate the rock damage evolution during the tests. The testing results show that the compressibility of the rock specimens increases and expansibility decreases with increasing confining pressure under CTC tests, resulting in failure pattern transforms from splitting failure to shearing failure. Under HM coupling, the compressibility decreases and expansibility increases with increasing pore pressure, and the micro cracks can keep relatively equilibrium state due to the existence of fluid. Based on AE hit counts, the crack closure stress σcc, crack initiation stress σci and crack damage stress σcd are 12.3%, 48.7% and 81.1% of the peak stress σc under CTC tests, respectively. Under HM coupling, the crack closure stress σcc is vanished and the crack initiation stress σci is higher than that of CTC tests. In this sense, pore pressure can facilitate crack development once the crack damage stress σcd is reached. The fluctuation intensities of AE amplitude and frequency centroid (FC) increase during the loading process. The amplitude fluctuation range increases and FC decreases after peak during CTC tests. However, the amplitude fluctuation range reaches its maximum at peak stress under HM coupling tests, and FC decreases. Moreover, simplified models of failure process for crystalline rocks under compression and HM coupling are proposed.

Use of Acoustic Emissions to detect change in contact mechanisms caused by tool wear in abrasive belt grinding process

Abrasive belt tools are widely used for finishing processes, where the abrasive grains on the belt tool serve as the cutting edge to remove materials. The interaction between abrasive grain and the material surface might result in three contact mechanisms, i.e. rubbing, ploughing and cutting, where their nature are not fully understood. On the other hand, the performance of a coated abrasive belt tool is highly affected by the grain wear. A single grain scratch test with different abrasive grain wear conditions is conducted to explore the three contact mechanisms. Through scratch experiments of prismatic Aluminium Oxide (A12O3) grain on Aluminium 6061 workpiece, Acoustic Emission (AE) frequency signatures that correspond to the three mechanisms are examined. Dominant frequencies and energy signatures occupied by the three contact mechanisms are analysed using Short-Time Fourier Transform (STFT). The energy content of the dominant frequency signatures revealed that the cutting mechanism is more predominant on belt tool with new grains, which gradually becomes less significant as the grain wears. A similar trend is also observed in ploughing and rubbing modes with respect to the wear flat level of the belt tool. The general conclusion suggests that the intensity of contact mechanisms changes according to the condition of the abrasive grain, i.e. tool wear, and can be correlated with AE sensor data.

Fatigue Analysis of FRP Strengthened Masonry by Acoustic Emission Monitoring

The safety work of historical and monumental building heritage requires the use of innovative materials compatible with the high architectural value. The Fiber Reinforced Polymers (FRP) represent a valid alternative to traditional ones, and the carbon fiber sheets are very light and easy to glue to the masonry structures. However, the durability of the application of FRP sheets is still uncertain in the long time behavior, especially with regard to cyclic fatigue loads such as seismic ones. In this work an experimental analysis on a set of strengthened masonry walls under fatigue tests (loading and freezing-thawing test) has been carried out in order to evaluate creep effects. During cyclic tests in the laboratory it was possible to monitor the damage pattern through the acoustic emission (AE) technique. The AE monitoring is useful to estimate the amount of energy released from fracture propagation in the adherence surface between masonry and FRP sheet. The different phases of damage evolution are recognized through the analysis of AE data over time. In particular, the time dependence of AE counting number is useful to indicate the beginning of the unstable damage growth and predicts the possible failure of the specimens at the 80% of the test duration. Furthermore, a sudden decay in the AE frequency is detected during the last phase of the fatigue tests. These results illustrate the applicability and the advantages of AE technique for the monitoring of long-term damage growth in strengthened masonry.