Acoustic Emission Measurement System Research Articles

A low-power AE b-value sensor for structural health monitoring

An increase in the quantity of aging infrastructure has become a major social issue. It is thus necessary to properly maintain and manage aging infrastructure, which incurs increasingly large maintenance costs. Therefore, there is a demand for maintenance and management using low-cost and wide-application technologies such as Internet of Things (IoT). We propose a unique, ultra-low-power acoustic emission b-value sensor for structural health monitoring. Acoustic emission (AE) is an elastic wave generated by crack generation or growth inside a material. The characteristics of AE, such as waveform, source frequency, source location, and velocity, can indicate damage affecting material strength. When focusing on the source frequency, the b-value, which is the slope of AE amplitude-frequency distribution derived from the source frequency, is important and widely used as an indicator of material integrity. The proposed sensor was developed to detect this b-value trend more conveniently than ever and to achieve continuous structural or machinery health monitoring. We pursued two key points: ultra-low power consumption and long-distance wireless transmission, which enabled the AE sensor to be applied as an IoT sensor. The wake-on-AE algorithm, which wakes the sensor when AE waves reach the sensor, enables AE sensing to consume a very low current of 172μA on two dry batteries. Generally, the waveform of AE is very short in duration, such as microseconds to milliseconds. If the sensor did not wake up and take measurements in such a short time, the AE wave would pass, and the sensor would fail to measure AE. Therefore, we developed a unique analog- digital co-circuit technology that uses a peak-hold circuit to extract AE features with less sampling. In addition, sensor edges reduce data rates through statistical analysis and transmit data using low-power wide-area networks. In this paper, we present validation results obtained with the sensor on bearings. We performed validation experiments with faulty and normal bearings, generating AEs with different b-values. The developed sensor, operated with two dry batteries, and the AE measurement systems for verification simultaneously acquired the AE generated by the bearing rotation. As a result, we successfully confirmed that the developed sensor provides almost the same results as conventional AE systems in b-value measurement with ultra-low power consumption.

Acoustic emission monitoring of immersion quenching bath attributes

This paper describes the use of acoustic emission (AE) testing for monitoring immersion quenching bath attributes. Immersion quenching is a heat treatment process used to change the mechanical properties of metallic workpieces. The quenching bath cooling attributes degrade with prolonged use, thus affecting the workpiece mechanical properties. A non-destructive monitoring method of the quenching bath is required. An AE measurement system is proposed for this purpose. Steel specimens are quenched in tap water, deionized water, 5 % NaCl-water solution and 5 % detergent-water solution. AE is measured with four piezoelectric sensors. The influence of quenching bath attributes on specimen cooling is described together with the measured AE signals. AE signal peak amplitude diagrams are evaluated. A reference AE peak amplitude signal is presented with the measurements. The quenching bath attribute assessment method is established by monitoring the amount of outlying AE signals in regard to the reference AE signal. The results confirm that AE monitoring provides a decent way of assessing quenching bath attributes and thus also the quenched specimen mechanical properties.

Acoustic emission monitoring of abrasion and corrosion in double-walled pipelines

At KRONOS TITAN, double-walled pipelines are used in production of Titandioxid. An abrasive medium is transported in the inner pipe at high temperatures. In the outer pipe, cooling water ensures that the process is cooled down. During the process, it is important that no abrasion leads to complete wall erosion and thus to leakage between the inner and outer pipe. During a test campaign lasting several months, a section of such a pipe was instrumented with acoustic emission measurement system on the outer pipe and the abrasion process in the inner pipe was monitored with AE until the removal of the pipe section. Different AE-signal parameters (Events, Hits, Energy and Frequency) were investigated regarding their suitability and are discussed in this contribution. The results of the acoustic emission measurements with regard to the location and progress of damages were then compared and verified with classic NDT test results.

Applicability of the Specimens Fabricated by Fused Deposition Modeling for Evaluating Acoustic Emission Measurement Systems

The authors have been conducting studies on standard specimens for the acoustic emission (AE) method that realize the quantitative discussion about the performance of measurement systems, including both instruments and engineers’ skills, in the environment equivalent to the practical situation. In this study, the applicability to standard specimens was examined for those fabricated by the fused deposition modeling type 3D printer. The behavior of AE generation was monitored during the four-point bending test using a desktop-type loading instrument for the specimen laminated toward the longitudinal direction and having a conical surface defect. It was shown that multiple delaminations associating AE signals occur on the extensional line of the cone tip. This fact implies that the intended actual AE events are generated intermittently at specific locations. Hence, it was found that the specimen has potential as standard specimens for the AE method with low cost and convenience of use.

Fibre Bragg Grating Based Acoustic Emission Measurement System for Structural Health Monitoring Applications.

Fiber Bragg grating (FBG)-based acoustic emission (AE) detection and monitoring is considered as a potential and emerging technology for structural health monitoring (SHM) applications. In this paper, an overview of the FBG-based AE monitoring system is presented, and various technologies and methods used for FBG AE interrogation systems are reviewed and discussed. Various commercial FBG AE sensing systems, SHM applications of FBG AE monitoring, and market potential and recent trends are also discussed.

Applicability of the Specimens with Holes Aligned to the Polygonal Line that Cause Acoustic Emission for Evaluating Acoustic Emission Measurement Systems

Applicability of the Specimens with Holes Aligned to the Polygonal Line that Cause Acoustic Emission for Evaluating Acoustic Emission Measurement Systems

Acoustic emission characteristics of a dry sandy soil subjected to drained triaxial compression

Acoustic emission (AE) technique that is capable of diagnosing the failure process of stressed materials has rarely reported its application to sandy soils subjected to triaxial compression. In this paper, drained triaxial compression tests incorporating with a high-performance AE measurement system were conducted for dry sands with different confining stresses and initial relative densities. Generally, an increased confining stress or initial relative density generates more acoustic emissions, while there also exist exceptions due to different failure patterns. A good resemblance between stress–strain and AE hit rate–strain relations was observed, and power functions between the mechanical parameters and AE hit rate were well established regardless of different confining stresses and initial relative densities. Besides, the behavioral state of yield and peak during compression could be also evaluated by AE hit rate, compared with conventional stress–strain determination. Particularly, the peak AE hit rate is found not always synchronous to but fluctuating at around the peak stress depending on different failure patterns, which might provide beneficial insights into the incipient failure of sands. The present good consistencies suggest that AE characteristics could be used as alternative parameters to evaluate and even predict the mechanical behavior of dry sands.

Study on Specimens with Multiple Holes for Standardization of Acoustic Emission Measurement Systems

Study on Specimens with Multiple Holes for Standardization of Acoustic Emission Measurement Systems

Monitoring micro-damage in Lithium-ion battery by analysing acoustic emission signal

Lithium metal battery was evaluated by using acoustic emission (AE) technique. AE measurement system was set for acquiring an AE signal during accelerated charge/discharge cyclic test. Particle cracking, the formation of solid electrolyte interphase (SEI) layer, and cracking of the SEI layer were confirmed by microstructural observation after the cycle test. The charge/discharge process was divided into four stages (i.e., A, B, C, and D) based on the relation between state-of-voltage and damages. AE Signals in each stage were successfully detected and correlated with the damage mechanism. The AE time-domain characteristics (i. e., waveform, amplitude, and duration) in each stage were similar. On the other hand, AE signal could be classified into two distinct groups (i.e., high and low frequency) on basis of the frequency characteristic. Stages containing high-frequency components of 250-300 kHz were related with cracking phenomenon, while low-frequency AE signals in different stage were emitted from SEI layer formation. Based on correlation between frequency-domain characteristic and damage type on the electrode, In-situ evaluation of the damage progression in lithium metal battery by monitoring AE activities for the classified signal is suggested.

Laboratory hydraulic fracturing test on large-scale pre-cracked granite specimens

Hydraulic fracturing could be a useful method for controlling the propagation path of cracks near an injection borehole by means of the perforating technique, pre-manufactured cracks, or the natural defects of the rock mass. The crack propagation path is not commonly along a fixed direction. Rather, it is likely to deflect gradually toward the direction of maximum horizontal stress. In order to study the behaviours of crack deflection in the hydraulic fracturing process, granite specimens having a through hole with pre-manufactured cracks on both sides, with a size of 1000 × 1000 × 1000 mm, were used in fracturing tests. The acoustic emission events during the whole fracturing process were recorded by an acoustic emission measurement system. The value of the horizontal stress difference during the tests clearly impacted the crack propagation path during the fracturing process. (1) New cracks were initiated from the tip of the pre-manufactured crack under the different loading conditions used in the tests, and the pre-manufactured crack was able to supply an initiation point for crack propagation. When the horizontal stress difference approached the average tensile strength value of the specimens, the crack propagation path on the surface of the specimen became irregular. (2) The work done during the hydraulic fracturing process was used mostly to overcome the restriction of the horizontal stress. A greater value of horizontal stress difference resulted in greater curvature of the crack propagation path. Additionally, the value of the horizontal stress difference was in proportion with the value of the energy for crack initiation. (3) The main failure model of the cracks generated during the fracturing process was tensile failure. Shear failure, with the high energy release, occurred mostly at the moment of crack initiation, when crack deflection activities were observed. The cracks generated by shear failure during the fracturing process were usually irregular.

Research on the Phase-Shifted Fiber Bragg Grating Spectra under Dynamic Strain Fields

Fiber optic sensing technology has been widely used for acoustic emission (AE) measurement in aerospace and geotechnical engineering due to the advantages of immunity to electro-magnetic interference, high accuracy, and multiplexing capacity. As for the fiber Bragg grating based on AE measurement system, traditional efforts have been made to study the spectra of Fiber Bragg Grating under AE signal. While this paper focused on phase-shifted fiber Bragg grating (PS-FBG) which is a new type fiber Bragg grating, and investigated the spectra of under dynamic strain field generating with an AE signal. A dynamic strain sensing model of PS-FBG was built based on the transfer matrix theory, and a cosine exponential attenuation function was utilized to simulate the dynamic strain field. Then the effects of amplitude, sampling time, exponential attenuation coefficient, frequency and wavelength of AE signal on the spectra of PS-FBG were studied in detail using numerical simulation. The result demonstrate that the spectra of PS-FBG changes periodically with frequency and different sampling time, especially the peak wavelength of the transmission window in the PS-FBG spectrum shifts periodically; and the increase of amplitude mainly contribute to more harmonic peaks in the reflected spectrum. The attenuation coefficient affects the spectrum of PS-FBG within certain range. And the AE wavelength affect the spectrum of PS-FBG when it is between 0.1 to 2 L, beyond the range, the spectrum of PS-FBG has minor variation with the AE wavelength. Finally, dynamic strain experiments were conducted, and the spectrum of PS-FBG under continuously vibration signal with different amplitude and frequency were logged for analysis. The experimental results agree well with simulation, which indicate that under different dynamic strain fields generated by AE signal, the spectra of PS-FBG are different, while the variation of spectra follows certain laws. This paper provides theoretical support for the AE measurement system on the basis of PS-FBG.

Automatic Event Detection in Noisy Environment for Material Process Monitoring by Laser AE Method

Laser acoustic emission (AE) method is a unique in-situ and non-contact nondestructive evaluation (NDE) method. It has a capability to detect signals generated from crack generation and propagation, friction and other physical phenomena in materials even in high temperature environment. However, laser AE system has lower signal-to-noise ratio compared to the conventional AE system using PZT sensors, so it is difficult to apply this method in noisy environment. A novel AE measurement system to detect events in such difficult environments was developed. This system could continuously record all AE waveforms and enable unrestricted post-analyses. Noise reduction filters in frequency domain coupling with a new AE event extraction using multiple threshold values showed a good potential for AE signal processing. This system was successfully applied for crack monitoring of plasma spray deposition process of ceramic coating.

304 스테인레스 강의 부식 손상 중 발생하는 음향방출신호 분석

음향방출법을 이용하여 304 스테인레스 강의 부식 손상 과정을 평가하였다. 스테인레스 강의 가속부식시험을 수행하면서 음향방출신호를 수집할 수 있는 측정 시스템을 구성하였다. 양극분극시험에서 공식부식(pitting corrosion)이 발생하는 시점 이후부터 음향방출(AE)신호가 검출되기 시작함을 확인하였다. 부식 실험 후 시편 표면을 광학현미경으로 관찰하여 다수의 공식부식이 발생하였음을 검증하였다. 부식 시간의 증가에 따른 AE 누적카운트 증가율과 AE 신호 진폭의 변화는 3단계로 구분되는 특징을 보였다. 이러한 AE 신호 발생 특징을 스테인레스 강의 부식 발생 과정의 단계별 변화와 관련하여 고찰하였다. AE 신호를 이용하여 금속 소재의 부식 손상 정도 및 부식 과정의 평가 가능성을 제시하였다. In this work, corrosion of 304 stainless steel was evaluated by using acoustic emission(AE) technique. AE measurement system was set for detecting acoustic signal during accelerated corrosion test of the specimen. AE signal started to be detected after the time of pitting corrosion initiation was evaluated by anodic polarization curve. Pitting corrosion damage was confirmed by optical microscopic observation of the surface morphology. AE cumulative counts and amplitude according to corrosion time could be divided into three stages. These trends were discussed in relation with changing pitting corrosion mechanism. Feasibilities of AE technique for evaluation of corrosion damage and mechanism were suggested.

Piezonuclear Fission Reactions from Earthquakes and Brittle Rocks Failure: Evidence of Neutron Emission and Non-Radioactive Product Elements

Neutron emission measurements, by means of He3 devices and bubble detectors, were performed during three different kinds of compression tests on brittle rocks: (i) under monotonic displacement control, (ii) under cyclic loading, and (iii) by ultrasonic vibration. The material used for the tests was Luserna stone, with different specimen sizes and shapes, and consequently with different brittleness numbers. Some studies had been already conducted on the different forms of energy emitted during the failure of brittle materials. They are based on the signals captured by acoustic emission measurement systems, or on the detection of electromagnetic charge. On the other hand, piezonuclear neutron emissions from very brittle rock specimens in compression have been discovered only very recently. In this paper, the authors analyse this phenomenon from an experimental point of view. Since the analyzed material contains iron, additional experiments have been performed on steel specimens subjected to tension and compression, observing, also in this case, neutron emissions well distinguishable from the background level. Our conjecture is that piezonuclear reactions involving fission of iron into aluminum, or into magnesium and silicon, should have occurred during compression damage and failure. This hypothesis is confirmed by the direct evidence of Energy Dispersive X-ray Spectroscopy (EDS) tests conducted on the specimens. It is also interesting to emphasize that the anomalous chemical balances of the major events that have affected the geomechanical and geochemical evolution of the Earth’s Crust should be considered as an indirect evidence of the piezonuclear fission reactions considered above.

Peculiar formation mechanism of a plateau stress region during dynamic compressive deformation of porous carbon steel with oriented cylindrical pores

Dynamic and quasi-static compressive deformation of as-cast and normalized porous S15CK carbon steels with cylindrical pores oriented in one direction was investigated at 298 and 77K, using the split Hopkinson pressure bar method and a universal testing machine, combined with an acoustic emission measurement system, to clarify the formation mechanism of a plateau stress region where deformation proceeds with almost no stress increase. Dynamic and quasi-static compressions perpendicular to the orientation of the pores at 298 and 77K do not produce a plateau stress region in the as-cast and normalized porous S15CK, because the localized crack formation and slip deformation that originate from the large concentration of stress around pores promotes densification in the early stage of the stress–strain curves. When the samples undergo dynamic compression parallel to the pore direction at 77K, the matrix becomes brittle, and cracks are easily formed. However, the pores do not easily collapse, because they are oriented along the compressive direction. Therefore, densification occurs at a higher strain level. In addition, the formation of small cracks in the matrix decreases the work hardening rate. As a result, a plateau stress region with high stress amplitude and wide strain range appears, which is independent of the microstructure. This mechanism for the formation of the plateau stress region is completely different from that of metal foams with isotropic pores, which is based on sequential inhomogeneous deformation. As a result, energy absorption 10 times that of commercial aluminum foams is achieved.

Evaluation of cracking due to dynamic temperature fluctuation during plasma spraying process by laser AE method

Evaluation of cracks in the ceramic coatings is very important to insure the reliability of the coating itself and the coated materials, parts and whole systems. Laser acoustic emission (AE) method was applied to enable a non-contact and in-situ monitoring of cracks in the ceramic coating immediately after plasma spraying. In this study, originally developed Continuous Wave Memory (CWM) was used as an AE measurement system to enable the continuous waveform recording during plasma spraying process. AE signals due to cracks could be successfully distinguished from large noise components such as mechanical vibration and laser scattering during the process. The AE events during plasma spraying revealed correlation with the dynamic temperature fluctuation on the surface of the specimen which was generated by the scanning of the plasma torch. Furthermore, the larger temperature fluctuation brought higher active AE signals.

Neutron emissions in brittle rocks during compression tests: Monotonic vs. cyclic loading

Neutron emission measurements, by means of 3He devices and bubble detectors, were performed during two different kinds of compression tests on brittle rocks: under monotonic and cyclic loading. The material used for the tests was Green Luserna Granite, with different specimen sizes and shapes, and consequently with different brittleness numbers. Since the analyzed material contains iron, our conjecture is that piezonuclear reactions involving fission of iron into aluminum, or into magnesium and silicon, should have occurred during compression damage and failure. Some studies have been already conducted on the different forms of energy emitted during the failure of brittle materials. They are based on the signals captured by acoustic emission measurement systems, or on the detection of electromagnetic charge. On the other hand, piezonuclear neutron emissions from very brittle rock specimens in compression have been discovered only very recently. In this paper, the authors analyse this phenomenon from an experimental point of view.

Energy Emissions from Failure Phenomena: Mechanical, Electromagnetic, Nuclear

Characterizing the nature of the different forms of energy emitted during compressive failure of brittle materials is an open and debated argument in the scientific literature. Some research has been already conducted on this subject in the scientific community based on the signals captured by the acoustic emission measurement systems. On the other hand, there are not many studies yet about the emission of electromagnetic charge, and for the first time we are talking about piezonuclear neutron emissions from very brittle failure of rocks specimens in compression. The authors analyze these three different emissions from an experimental point of view.

Crack Monitoring during Plasma Spraying of Ceramic Coatings by Non-Contact Acoustic Emission Method

Atmospheric plasma spraying (APS) is an effective process to make ceramic coatings. However, several types of cracks are inherently present in the coatings and among them, delamination cracks within the coatings or at the interface with the underlying layer are believed to reduce their durability and reliability. In this study, non-contact laser acoustic emission (AE) method and original AE measurement system successfully detected the cracks during APS in real time, which was very difficult because of noise from the APS system. AE waveform was sampled and recorded continuously during the testing time. AE events due to delamination cracks were successfully detected from noisy waveform by this system. Influences of the scanning speed and the power of the plasma jet to the delamination were also confirmed.

Acquisition and Analysis of Continuous Acoustic Emission Waveform for Classification of Damage Sources in Ceramic Fiber Mat

Waveforms of acoustic emission (AE) events come close and sometimes overlap each other when AE activity is very high. Conventional AE measurement systems which handle discrete AE events are not suitable for this situation because miss-detection of AE event occurs frequently. A new AE measurement system named as Continuous Wave Memory (CWM) was developed to solve this problem by recording the AE waveforms continuously to hard disks for several hours throughout the testing time. This new system enabled multiple analysis of one waveform with different filtering parameters. Short time Fourier transform (STFT) gave the time-frequency-magnitude characteristic of continuous AE waveforms and useful information for evaluation of degradation of materials. In this study, the degradation of ceramic fiber mat during cyclic compression test and the effect of binder-addition were evaluated by this new system. STFT results clearly showed the classification of degradation of the mat; breakage of fibers was the main source in the early compression cycles and sporadic friction between fibers became the main source of AE in the later compression cycles. The effect of organic binder to prevent the degradation of the mat was also estimated. It was observed that the friction signal disappeared and the breakage signal weakened in the binder-added specimens.