Acoustic Emission Signal Energy Research Articles

Acoustic Emission based determination of delamination initiation in GFRP laminates

Acoustic Emission (AE) has the potential to identify delamination initiation in quasi-static Mode I fracture tests on glass fiber-reinforced polymer-matrix (GFRP) laminates. It had been shown that a combined AE activity and intensity criterion yields critical Mode I delamination resistance values GIC for an AS4/PEEK carbon fiber thermoplastic composite that are comparable to those from data analysis according to ASTM D5528. However, the AE historic index also allows for detection of delamination initiation under Mode I and Mode II loading of carbon fiber reinforced thermoplastics and thermosets. In the present study, these AE criteria for initiation under Mode I, Mode II and Mixed Mode I/II loading of GFRP epoxy laminates are compared with AE signal energy as additional initiation criterion. Initiation under Mode II and Mixed Mode I/II at a ratio of 4:3 was determined with two different set ups for each Mode. For some test configurations noise signals make unambiguous initiation detection difficult. Determination by means of AE energy or AE activity and intensity yields more conservative GC values compared with the AE historic index or criteria defined in the standards, however, with similar repeatability.

Response characteristics of acoustic emission signal and judgment criteria for different fracture modes

Coal mining is going deeper into the ground, resulting in more intense and frequent mine dynamic disasters. To ensure the early warning of dynamic disasters, it is significant to accurately reveal the differences of acoustic emission (AE) signals generated by rock fractures. This paper focuses on studying how AEs average frequency (AF) and rise angle (RA) change in sandstone during loading, and examines the characteristics of waveform amplitude, energy, and frequency distribution of AE signals from different types of fractures. The findings reveal that the percentage of shear fracture gradually increases with increasing load, and the energy activity of the AE signal is significantly enhanced in the high stress stage. The occurrence of the main fracture was preceded by a large number of low-frequency tensile fracture AE signals, accompanied by a small decrease in RA and a small increase in AF. The spectra of different fracture signals were characterized as “low anomalous F” for shear fractures, “vertical F” for low-frequency tensile fractures, and “high anomalous F” for high-frequency tensile fractures. Their energy distribution in the frequency domain was respectively described as “peak-centered distribution”, “pillar-shaped distribution” and “peak-skewed distribution”. The outcomes of the decomposition and reconstruction from the P- and S-waves reveal that the P wave energy (Ep) dominates in the high-frequency tensile fracture AE signal, while the S wave energy (Es) is greater in the low-frequency tensile fracture and the shear fracture AE signals. The ES/EP threshold value for laboratory discrimination of tensile and shear fracture is 3, as the fracture extends and penetrates, the ES/EP value rises rapidly at the appearance of macroscopic fracture. The findings offer theoretical guidance for monitoring and early warning of coal and rock dynamic hazards as well as revealing the morphology of fracture source.

Acoustic emission characteristics of micro-discharges in initial pulsed PEO process on 60% SiCp/Al composite

The initial discharge process of pulsed plasma electrolytic oxidation (PEO) on the 60% SiCp/2009 aluminum metal matrix composite (Al MMC) in silicate solution was monitored by acoustic emission (AE) technique. Parameters and correlations of AE signals on the Al MMC sample and under water were analyzed, and their generation mechanism was discussed. It was found that the peak amplitudes of AE signals and AE hits during the pulse time quickly increased with the increase of micro-discharge intensity, and the absolute energy of AE signals improved several orders of magnitude. Moreover, different from the peak amplitude, duration and rise time, the duration and count had a strong correlation. Elastic stress waves resulted from the microjet of plasma bubble collapse, the inner-surface friction inside discharge channel, the expansion-shrinkage process of plasma bubbles and micro-crack propagation during rapid solidification of melt are sources of AE signals on the Al MMC sample during the pulse time. However, the expansion-shrinkage process of plasma bubbles plays a key role in the generation of underwater AE signals. In the pause time of one pulse period, the bursting and moving of vapor bubbles result in weak AE signals. It is demonstrated that the AE technique can effectively characterize the features of micro-discharges within a pulse period.

Acoustic emission, damage and fracture mechanisms of structural steel under mixed-mode loading

Mechanical and acoustic properties, damage kinetics and fracture mechanisms of 30CrMnSiA structural steel under mixed loading modes (I + II) have been investigated. It is shown that the change in loading mode leads to changes in strength and acoustic parameters, microcrack accumulation kinetics and fracture mechanism. The greatest changes in average frequency (AF) and rise angle (RA) of acoustic emission signal with increase in load are observed when the sample is oriented at an angle of 15°, at which the AF decreases linearly depending on the average values of the signal RA. However, the acoustic emission characteristics are not only determined by the loading mode but also by the fracture mechanism: brittle fracture at an angle of 45° is accompanied by increase in many acoustic characteristics including RA and AF parameters and a change in the type of time dependence of the total number of acoustic signals (ΣNAE), bAE – value and cumulative energy of acoustic emission signals (∑EAE). The frequency analysis showed that the average values of the peak frequency and the frequency centroid at rupture exceed the values of these parameters under mixed loading modes. A change in the failure mechanism under mixed loading modes does not affect these acoustic characteristics.Analysis of the kinetics of microcrack accumulation showed that the rate of damage accumulation decreases as the shear component of the load increases.

Investigation of crack propagation in filament wound composite samples of Mode-I by using acoustic emission technique

A common industrial method of fabricating composite structural is filament winding, which is widely used to manufacture axisymmetric components. One of the most prevalent damage mechanisms in these composite structures is delamination, which occurs in various types of loading. In this research, the mechanical behavior and propagation of the initial crack of Mode-I in carbon/epoxy filament wound (FW) composite curve beams have been investigated via the acoustic emission (AE) method. For this purpose, at first, a composite tube was made with considering the artificial interlayer separation, and the beam specimens were cut from the longitudinal direction. At first, the initiation of the failures was determined using the AE method. Then, the growth and spread of interlayer delamination were investigated by the energy of AE signals and the sentry function. By determining the speed of the AE waves and providing a method to filter unwanted signals, the momentary position of the interlayer delamination tip during growth is predicted. The results showed that the fiber bridging phenomenon was due to the penetration of the two regions adjacent of the separation interface, and the fiber bridging length considerably affected the Mode-I delamination fracture toughness. In addition, the fracture toughness increased as the fiber bridging zone enlarged. Finally, good agreement was observed between the fracture toughness values obtained from the AE method and those found in the standard ASTM D5528.

Study the effects of ferrous materials inside wood on the propagation characteristics of acoustic emission signals

ABSTRACT This study compares the propagation characteristics of acoustic emission signals for wood containing ferrous material by implanting ferrous nails into wood specimens to investigate the acoustic emission (AE) detection method of foreign metal objects inside the wood. It obtains specimens for the comparative test by drilling holes and implanting ferrous nails into identical Zelkova schneideriana and Pinus sylvestris var. mongholica specimens. Initially, the AE source is generated by conducting pencil-lead break (PLB) tests on the specimen surfaces according to ASTM standards, and the AE signals are then collected at the specimen surface with a 1 MHz sampling frequency. The effects of ferrous materials on the propagation characteristics of AE signals are examined using FFT and EWT methods for the time–frequency analysis of the original AE signals. Finally, the signal generator produces the 150 kHz pulse train at the PLB location. The AE signal energy is calculated based on the alternating current principle to establish the corresponding AE energy attenuation function. The results showed that the proportion of AE signals in the frequency ranges of (30–60 kHz) and (150–180 kHz) increases significantly when ferrous nails are present in the specimens.

Influence of boundary conditions on acoustic emission propagation characteristics of Zelkova schneideriana

To study the propagation characteristics of acoustic emission signals in Zelkova schneideriana under different boundary conditions, three types of boundary conditions were generated by applying aluminum plates and sound-absorbing cotton on the surface of Zelkova schneideriana specimens. Firstly, the sudden and continuous acoustic emission (AE) sources were simulated by PLB (pencil–lead break) tests and signal generator on the specimen surface, and the AE signals were collected by 5 sensors equally spaced on the surface of the specimen, and the sampling frequency was set to 500 kHz. Then, the detailed signals of different frequency bands were obtained by wavelet decomposition, and TDOA (the time difference of arrival) and correlation analysis method were used to calculate the time difference of longitudinal wave and surface transverse wave and the corresponding propagation velocity, respectively. Finally, the pulse trains with different energy levels generated by the signal generator were used as AE sources to study the attenuation law of AE signal energy with distance under different boundary conditions. The results show that the boundary changes can lead to a significant increase in the surface transverse wave velocity, and have no significant effect on the longitudinal wave velocity. At the same time, the energy attenuation of surface and longitudinal waves is faster after the aluminum plate and sound-absorbing cotton are affixed, and the distance of longitudinal waves attenuation to 90% is reduced from 186 to 139 mm, and the distance of surface transverse waves propagation is reduced from 312 to 226 mm.

Acoustic emission monitoring of wood materials and timber structures: A critical review

• Different NDE methods applied to wood and timber structures were explained. • Concept of AE and its experimentation was introduced. • Impact of wood characteristics on AE wave (velocity, attenuation) was discussed. • Applications of AE in monitoring of wood materials were critically reviewed. • Opportunities, challenges, and outline of future research were discussed. The growing interest in timber construction and using more wood for civil engineering applications has given highlighted importance of developing non-destructive evaluation (NDE) methods for structural health monitoring and quality control of wooden construction. This study, critically reviews the acoustic emission (AE) method and its applications in the wood and timber industry. Various other NDE methods for wood monitoring such as infrared spectroscopy, stress wave, guided wave propagation, X-ray computed tomography and thermography are also included. The concept and experimentation of AE are explained, and the impact of wood properties on AE signal velocity and energy attenuation is discussed. The state-of-the-art AE monitoring of wood and timber structures is organized into six applications: (1) wood machining monitoring; (2) wood drying; (3) wood fracture; (4) timber structural health monitoring; (5) termite infestation monitoring; and (6) quality control. For each application, the opportunities that the AE method offers for in-situ monitoring or smart assessment of wood-based materials are discussed, and the challenges and direction for future research are critically outlined. Overall, compared with structural health monitoring of other materials, less attention has been paid to data-driven methods and machine learning applied to AE monitoring of wood and timber. In addition, most studies have focused on extracting simple time-domain features, whereas there is a gap in using sophisticated signal processing and feature engineering techniques. Future research should explore the sensor fusion for monitoring full-scale timber buildings and structures and focus on applying AE to large-size structures containing defects. Moreover, the effectiveness of AE methods used for wood composites and mass timber structures should be further studied.

Damage mechanism of Al2O3f/Al2O3 composites by acoustic emission technology

Continuous alumina fiber-reinforced alumina matrix composites (Al2O3f/Al2O3 composites) were produced via sol-gel process, the structure, micromorphology, and matrix properties of the composites were characterized, and then the damage mechanism was investigated using acoustic emission (AE) technology. The results showed that the composites were mainly composed of α-Al2O3. When the load first increased with time in the tensile test, the composites showed low AE signal energy, and cracks started and expanded in Al2O3 matrix. When the load reached its peak and abruptly declined, the number of AE signal events and accumulated energy increased dramatically. The proportion of AE signal events in the Al2O3f/Al2O3 composites was 0.37, and the proportion of accumulated energy was as high as 0.79, indicating that most of the fibers and the matrix were destroyed at this time. During the short-beam shear test, almost no AE signal was generated when the load rise reached the peak, and the crack instability expanded along the interlayer, resulting in delamination failure. After the initial load decreased, numerous high-energy AE signals were generated with increasing specimen deformation, and many fibers in Al2O3f/Al2O3 composites were crushed synchronously with the matrix.

Research on the effect of loading rate on the fracture characteristics of granite using cracked straight through Brazilian disc specimens

The loading rate effect is an important mechanical property affecting the failure mechanism of rock materials. In this paper, the effect of loading rate on the mode I fracture characteristics of granite was studied by using Cracked Straight Through Brazilian Disc (CSTBD) specimen. The acoustic emission (AE) and Digital Image Correlation (DIC) techniques were used to monitor the fracture behavior of the specimen. Due to the nonlinearity of specimen deformation, the tested fracture toughness was corrected by equivalent energy method. The test results show that the equivalent fracture toughness has obvious rate dependency effect. Meanwhile, the crack tip opening displacement increases with the increase of the loading rate at low loading rate, and decreases with the increase of the loading rate when the loading rate reaches a certain value. The AE results show that the relationship between the cumulative energy of AE signals and the loading time can be well fitted by the power-law function, and the power exponent gradually decreases with the increase of loading rate. Moreover, b value decreases with the increase of loading rate, and there is a negative linear correlation between b value and fracture toughness. These indicate that with the increase of loading rate, the cracking process of the specimen gradually changes from that dominated by small-scale failure to that dominated by large-scale failure, and more fracture energy is needed for instability failure of specimens. In addition, the relationship between b value and fracture toughness can provide the possibility for estimating the equivalent fracture toughness of the material by the b value obtained from the AE test.

Investigation of the Scale Factor Impact on the Results of Acoustic Emission Monitoring of the Steel Specimens Tension Process

The research is devoted to steel structure diagnostics by the acoustic emission (AE) method. The existing regulatory documents for AE diagnostics of metals and alloys do not take into account some critical factors, among which one is the scale factor should be highlighted. As a result, this can lead to an unreliable assessment of the danger degree of defects in structures when using standard AE diagnostic criteria. This paper presents a quantitative assessment of the scale factor impact on the AE data during the static tension test of steel specimens to failure. Experimental studies were carried out on flat specimens of various thicknesses with a side notch made of high-quality alloyed steel 30 KhGSA. It was established that AE data changed (rise in the AE signals amplitudes and AE activity) within the increase of specimen thickness. Growth in the recorded AE signals cumulative energy was registered with a greater specimen thickness. Partial correlation dependences of the mean count frequency and cumulative energy of AE signals on the specimen thickness were obtained. It was shown that such an effect occurred due to both a general increase in the deformed metal volume and greater strain intensity during the tension of thick specimens. The obtained dependences may contribute to the development of AE diagnostics of metallic materials which is invariant to the scale factor impact.

STUDY ON THE TIME-FREQUENCY CHARACTERISTICS AND PROPAGATION LAW OF ACOUSTIC EMISSION LONGITUDINAL WAVES IN WOOD GRAIN DIRECTION

To study the propagation law of acoustic emission (AE) longitudinal waves in wood, therelationship among wave velocity, standing wave fundamental frequency and Young's modulus of elasticity was studied, and the energy decay model of AE longitudinal waves along the grain direction was established. Firstly, the propagation velocity of the longitudinal wave was calculated using the time-difference method. Then, the relationship between the wave velocity and Young's modulus of elasticity was analyzed and the method of calculating thelongitudinal wave velocity using the fundamental frequency was proposed. Finally, using different levels' pulse strings as AE sources, the attenuation law of AE signal energy with distance was studied. The results show that the longitudinal wave velocity can be estimated more accurately by using the standing wave fundamental frequency. The influence of Poisson's ratio needs to be considered when calculating the Young's modulus of elasticity by using thelongitudinal wave velocity.

Time-Frequency Characteristics of Acoustic Emission Signals on Water-Bearing Sandstone Specimen Subjected to Conventional Uniaxial Compression

This paper presents an experimental investigation of acoustic emission (AE) time-frequency characteristics of a water-bearing sandstone specimen in a conventional uniaxial compression test. The main achievements are as follows: (1) The violent fluctuation of AE time domain parameters indicates that the water-bearing sandstone specimen is about to be destroyed. This characteristic provides a theoretical basis for predicting the failure of water-bearing rock in engineering practice. (2) In the elastic phase, the AE b value is the lowest but has a sudden increase after falling into the steady crack propagation phase. In the unsteady crack propagation phase, the AE b value is further increased. This characteristic is of great indicative value for predicting the failure of the water-bearing sandstone specimen. (3) The difference of dominant frequency among the three key points is very small, indicating that the crack initiation and propagation of the water-bearing sandstone specimen has a certain stability in the damage and failure process. But, the two-dimensional frequency spectrum structure of AE waveform signals shows that the closer to failure, the more the number of the frequency spectrum structure peaks. (4) The energy of AE signals is mainly concentrated in the first three frequency bands. The closer to failure, the more the energy proportion of the first three frequency bands is reduced; conversely, the energy proportion of the latter five frequency bands is increased, which leads to more complexity of the failure modes of the water-bearing sandstone specimen.

Features of acoustic emission and micro‐CT of 3D‐printed continuous Kevlar fiber‐reinforced composites during progressive damage under flexural load

AbstractFailure mechanism and its characterization methods of 3D‐printed fiber‐reinforced composites are still needed thorough investigation. In this work, typical 3D‐printed composites reinforced with 10% and 20% continuous Kevlar fiber are prepared, respectively, three point bending progressive damage experiments are carried out, and the damage evolution are real‐time monitored by acoustic emission (AE) technology. When load reaches the maximum value and failure point, microcomputed tomography (micro‐CT) observations are performed for loaded specimens to explore inside damages. Results show that bending strength of Specimen B (20% fiber) increases by 23% relative to Specimen A (10% fiber). Cumulative count and energy of AE signals are higher for Specimen B, and K‐means clustering algorithm is employed for deep analysis of AE signals. Three damage modes including matrix crack, fiber/matrix debonding, and fiber fracture are clustered and validated by micro‐CT, indicating that the combination of AE and micro‐CT is effective for the characterization of damage evolution of composites. Debonding due to poor adhesion between fiber and matrix is the main damage mode of 3D‐printed Kevlar fiber‐reinforced composites, and damages of Specimen B concentrated in the later stage. This work provides failure characterization methods for 3D‐printed fiber‐reinforced composites.

Experimental investigation of damage evolution and failure criterion on hollow cylindrical rock samples with different bore diameters

The roadway direction is often parallel to the direction of the maximum principal stress for the structure arrangement of underground roadway. To study the instability mechanism of roadway under the maximum principal stress, a series of uniaxial compression acoustic emission (AE) experiments were carried out on hollow red sandstone and granite rock samples with different bore diameters. The test results suggest that the oblique shear or X-shaped shear failure occurs in hollow rock samples, and the failure process is accompanied by a sudden jump in accumulative energy of AE signals, at the same time, the ringing count arrived at peak value. The increase in bore diameter haves contributed to the decline of ringing count and accumulative energy. A statistical damage constitutive model of Weibull distribution was established for hollow rock samples. On the basis of optimizing the damage variable, a damage constitutive model based on AE ringing count was obtained, which can better describe the damage state and stress state of rock samples. The AE accumulative energy was selected as the state variable, and the failure criterion model was finished based on the cusp catastrophe theory to estimate the rock sample failure. The judgment results show that the failure moment of rock sample is accurately included in the judgment time interval. This research can provide reference and technical support for the stability control and monitoring of underground engineering roadways.

Three Stages of Composite Specimen Destruction in Static Failure

Abstract The paper presents the results of standard specimen fracture made of anisotropic carbon fiber plastic with an epoxy matrix. Static stepwise loading of the specimen was carried out on an Instron 8801 testing machine to determine the characteristics of ductile fracture G1C in the first mode in accordance with ASTM D5528. During loading, the parameters of acoustic emission (AE) signals, such as AE impulse amplitudes and their energy were synchronously recorded. At the same time, the magnitude of the opening and the growth of the crack initiated by the artificial cut at the end of the specimen were recorded. According to the analysis of the acoustic emission signals, three zones with different G1C behaviour were identified: initial crack propagation, its stationary growth and accelerated fracture of the specimen. The zonal character of the change in the acoustic emission signals made it possible to determine the energy of the acoustic emission signals as diagnostic evidence for the onset of rapid destruction of the specimen. The amplitude of the AE-signals in the zones, however, remained constant. Online monitoring of changes in the energy of acoustic emission signals will prevent the onset of rapid destruction of an object in places of its deformations. The paper does not aim at defining G1C as usual. It presents the investigation of the fracture stages for a composite material using an acoustic emission method.

In-situ estimation of defect volume from parameters of acoustic emission signals

By using two methods of nondestructive testing, i.e., acoustic emission (AE) measurements and X-ray computed microtomography (CT), an experimental study of defect accumulation during a uniaxial compression of a natural heterogeneous material was carried out. A joint analysis of the AE and CT data revealed a correspondence between energy characteristics of the acoustic emission accompanying defect formation and volume of defects. It is shown that the dependence of the total energy of AE signals on the defect volume is linear, which is consistent with the phenomenological dependences for earthquake focuses obtained earlier. The linear dependence was used to estimate the average defect size. It is shown that, regardless of the assumed defect shape, its average linear size does not exceed 100 μm.

Оценка предельной чувствительности акустико-эмиссионного контроля

Knowledge of the ultimate sensitivity of the acoustic emission (AE) method is useful in studies of the mechanisms of plastic deformation of the structure, the formation and development of micro-, meso- and macro-cracks, as well as continuous processes, such as the outflow of liquids and gases, friction and a number of others. Analysis of literary data on assessment of limit sensitivity at identification of AE sources was carried out. It has been shown that when using standard resonant piezoelectric transducers with a frequency bandwidth of 30 ± 10 kHz, the ultimate sensitivity is the fraction of the nanometer in the displacement of the surface of the object and the unit micron of the size of the microcrack during its formation and hopping development. When testing industrial facilities, in many cases the level of external noise is significantly higher, the detection of defects decreases and amounts to a fraction of a millimeter. However, an increase in the energy and amplitude of AE signals as the defect develops in the vast majority of cases leads to the fact that when a crack reaches dimensions that begin to threaten the strength of the monitored object, AE signals are reliably detected by the equipment. Knowledge of the maximum sensitivity of the AE method makes it possible to compare it with other NDT methods by this parameter.

Study of Strength and Deformation Evolution in Raw and Briquette Coal Samples under Uniaxial Compression via Monitoring Their Acoustic Emission Characteristics

Briquette coals with different cement contents are frequently used to study the coal body’s properties. In this study, the deformation and strength of briquette coal samples with 0, 5, 10, and 20% cement contents were experimentally and theoretically investigated using the acoustic emission (AE) characteristics monitored during the uniaxial compression tests. The results show that the uniaxial compression process of raw coal and briquette coal samples can be subdivided into compaction, elastic, plastic (yield), and brittle failure stages. With an increase in cement content, briquette coal samples undergo the elastic and plastic stages, and their postpeak stress drop rate gradually grows, and their plastic deformation is followed by brittle failure. The uniaxial compressive strength and elastic modulus of briquette coal samples show a linearly increasing relationship with cement content, while their Poisson’s ratio decreases gradually. During the uniaxial compression, raw coal and briquette coal samples produce the AE signals. The overall AE signal of briquette coal samples is relatively low, and there are no obvious AE events in raw coal samples. The uniaxial compressive strength, elastic modulus, and Poisson’s ratio of briquette coal samples with a 20% cement content and their AE signal cumulative amplitude, count, and energy values are very close to the corresponding parameters of raw coal samples. Therefore, they can be used for simulating raw coal samples in laboratory tests.

Структурно-энергетические закономерности накопления повреждений при деформировании гетерогенного материала

An experimental research of the defect accumulation under uniaxial loading of a natural heterogeneous material is carried out in this work. Defect formation process has been controlled using two non-destructive techniques (methods) such as acoustic emission (AE) and X-ray computed microtomography (CT). The combined use of these methods made it possible to find out correspondence between energy characteristics of AE accompanying the defect formation and the volume of these defects. It was found that total defect volume is lineary dependent on total AE signal energy. This fact corresponds to previously discovered phenomenological relationships for the sources of tectonic earthquakes. Mean defect size has been estimated based on obtained linear dependence. It was shown that mean linear size of defect does not exceed 100μm regardless of the assumptions about the defect shape.