Acoustic Emission Amplitude Distribution Research Articles

Cyclic compressive behavior of high-performance hybrid fiber reinforced recycled aggregate concrete: Degradation law and acoustic emission features

To tackle the issues of inadequate mechanical properties of recycled aggregate concrete and its limited application in load-bearing structural components, a high-performance hybrid fiber reinforced recycled aggregate concrete (HP-HFRAC) was developed in this study. Sixteen groups of HP-HFRAC specimens fabricated with varying replacement rates of recycled coarse aggregate and different fiber parameters were subjected to uniaxial compression, as well as acoustic emission (AE) tests, to investigate the cyclic behavior and uncover the underlying mechanisms. The findings demonstrated a strong correlation between the AE signals and the macroscopic phenomenon throughout the whole compression process. The AE amplitude distribution was found to be capable of identifying damage mechanisms, including matrix cracking (40–50 dB), fiber sliding (50–80 dB), and fiber pull-out (more than 80 dB). Additionally, the inclusion of hybrid fibers contributed to mitigating the degradation of stiffness and the accumulation of plastic strain, resulting in the ductile failure of the specimens and significantly enhancing the energy dissipation capacity. Specifically, the peak stress of specimens with 1.5% steel fibers increased by 19% compared to that of specimens without steel fibers, compensating for the strength loss due to the presence of recycled coarse aggregate. Finally, constitutive equations governing the skeleton curves and the unloading and reloading curves of HP-HFRAC subjected to cyclic compression were formulated, which were validated using both the test results and the predicted results of the physics-based data-driven model previously published in our companion paper. The findings of this study might contribute to the development of sustainable high performance recycled aggregate concrete for reliable application in load-bearing components.

A new acoustic emission characteristic parameter can be utilized to evaluate the failure of cemented paste backfill and rock combination

In backfill mining, there is a phenomenon of the joint bearing of cemented paste backfill and rock, so it is essential to study the destabilization damage mechanism of cemented paste backfill and rock combination (BRC). This study aimed to improve the evaluation system for the instability failure of BRC. A new evaluation index, Dr, based on acoustic emission (AE) characteristic parameters was proposed for predicting BRC instability failure caused by the load. This index comprehensively considers the time dependence and interaction of the AE rise time, amplitude, and average frequency during BRC failure. The study conducted uniaxial compression and simultaneous AE experiments for various ratio combinations of BRC to obtain the AE rise time/amplitude (RA) and average frequency (AF) values of BRC. Then, the RA/AF values were numerically calculated with fractal theory to obtain the Dr index, and the intrinsic relationship between Dr and BRC instability failure was further discussed. Finally, the index was validated through comparison with the statistical index b-value based on AE amplitude, and a discrimination criterion for BRC instability failure was proposed. The results show that: After reaching the yield stress, the number of AE events of the rock gradually decreases. In contrast, the number of AE events of the CPB fluctuates in the high-level range. The cement-tailings ratio significantly affects the BRC AE frequency and amplitude distribution. The decrease of the cement-tailings ratio enhanced the high-frequency amplitude signal of CPB, while weakening the medium and high-frequency amplitude signal of rock. The sudden drop of Dr in the near-failure stage can be used as a criterion for BRC failure determination. Compared with the b-value, using Dr as an evaluation index to predict BRC failure can obtain a slightly conservative result. The research provides a new and effective method for evaluating BRC instability failure, which can be used for real-time prediction of BRC instability failure in mining engineering practice.

Acoustic Emission Analysis of Fracture and Size Effect in Cementitious Mortars

The size effect is a phenomenon where the strength and the ductility of a material depend on the size of the structure. Investigating size effects and related crack formation in brittle materials requires advanced monitoring methods. The aim of this research is to experimentally investigate the impact of size effect with the acoustic emission (AE) technique. Brazilian splitting tests with AE monitoring were performed on cement-based mortar cylinders of three sizes. It was found that in addition to the size, the boundary condition affects the final strength. When adopting similar boundary conditions in samples with different sizes, the larger samples had the lowest tensile splitting strength. For the larger samples, initially, there were fewer AE activities. However, there was a surge of high-amplitude AE events near the peak load. This indicates that as size increases, there is a lack of micro-cracking before macro-crack propagation, and the material fails in a more brittle manner. The width of the fracture process zone was quantified with AE and increased with sample size. A further analysis of the AE amplitude distribution demonstrated a change in the distribution in the pre-peak phase for the larger samples and for the smaller samples in the post-peak phase, signifying the brittle to ductile failure transition that occurs as size decreases.

New b-value parameter for quantitatively monitoring the structural health of carbon fiber-reinforced composites

Carbon fiber-reinforced composites have excellent mechanical properties but are vulnerable to the accumulation of small cracks. This study used acoustic emission (AE) signals to monitor and quantify composite damage. We subjected carbon fiber/epoxy specimens to cyclic loading (10 cycles/step, up to 700 MPa of tensile stress). We obtained AE signals over a wide frequency range that can be used to monitor failure modes. The conventional Ib-value decreased with increasing damage and shifted upward as the propagation distance increased. We introduced a new parameter, the composite b-value (Cb-value), to analyze AE amplitude distribution at the crack origin. It was demonstrated that the Cb-value is independent of sensor location, thus providing accurate information on the overall damage. Finally, a standard value based on the Cb–value was suggested as an index of the damage level in composite materials; this is a useful indicator of structural health.

Micro-cracking pattern recognition of hybrid CNTs/GNPs cement pastes under three-point bending loading using acoustic emission technique

The generation of microcracks has an important influence on the behaviour of concrete structures. In this study, the acoustic emission (AE) technique was used to investigate the fracture phenomena and micro-cracking behavior of hybrid carbon nanotubes (CNTs, the 1-D allotrope of carbon atoms) and graphene nanoplatelets (GNPs, 2D monolayer of sp2-hybridized carbon atoms), cement composites under three-point bending loading. In addition, dispersion level by ultraviolet visible spectroscopy (UV–Vis), mechanical performance, and micro-structure properties by scanning electron microscopy (SEM) of hybrid cement composites were studied. Different typical AE parameters like AE hits, rise time, frequency and amplitude distribution, and b-value were used for the AE analysis. The cracking pattern and failure mechanism of the specimens were observed into three stages: initial elastic stage, pre-fracture stage, and post-fracture stage. The conclusions were drawn as follows: (1) through the UV–Vis spectrometry, it is found that GNPs act as dispersion agents for CNTs; (2) hybrid specimens experienced large deformation with the increasing wt.% of CNTs + GNPs during post fracture phase, thus showing more ductile behavior as compared to the control sample; (3) based on the average frequency (AF) and RA value (rise time divided by the amplitude), the addition of CNTs + GNPs content in cement composites shifted the failure mode from shear to tensile; (4) the AE parameters are feasible to analyze and differentiate the cracking pattern between micro and macrocracks; (5) the field emission scanning electron microscopy (FESEM) image evidence that hybrid CNTs + GNPs particles are well dispersed within cement composites. Based on the obtained results, it was concluded that the acoustic emission technique is a favorable approach to monitor and identify the microcracking pattern in brittle concrete structures.

Acoustic emission fractal characteristics and mechanical damage mechanism of cemented paste backfill prepared with tantalum niobium mine tailings

The motivation for this work was to investigate the relationship between the fractal characteristics of the acoustic emission (AE) correlation and the mechanical damage of different ratios of tantalum–niobium tailings CPB materials. The AE parameters of the tailings CPB materials under uniaxial compression were obtained by an AE test. On this basis, the fractal dimensions of the AE parameters were derived, and the change rule of the fractal parameters during the entire specimen damage was obtained by analysis. The results showed that the minimum value of the fractal dimension was generally within a stress-level range of 70% to 90%. While approaching the peak value of the stress, the amplitude fractal dimension displayed an increasing trend. The AE amplitude, event rate, energy rate, and spatial distribution exhibited fractal characteristics. In the investigation of the AE fractal, the fractal dimension of the amplitude was more concentrated, which exhibited that it was more accurate to determine the failure of a cemented paste backfill (CPB). Therefore, the study focused on using the amplitude fractal dimension value to monitor the failure of the CPB. This feature provides a reference for the control of the stability of the surrounding rock by backfilling the goaf with CPB.

Analysis of acoustic emission characteristics of ice under three point bending

Acoustic emission (AE) is one of the promising techniques to understand the damage mechanism within a material and structural health monitoring non-invasively. The transient elastic energy released during the microscopic failure within a material is termed as acoustic emission. AE signals carry information about the mechanism and intensity of damage within a material. In this study, AE signatures were recorded during the mechanical loading of notched ice beams subjected to three point bending tests at three different strain rates i.e. 1 × 10−5 s−1, 1 × 10−4 s−1 and 1 × 10−3 s−1. Various AE characteristics such as hits, amplitude, counts, rise time, energy release rate etc. were analysed in relation to the loading behaviour of ice beams. The AE amplitude distribution data was analysed to estimate b-value and its temporal variation throughout loading for all the three strain rate experiments. A decreasing trend in b-value with increasing bending stress was observed in all the three tests. Characterization of dominant failure mode was also attempted through the analysis of RA parameter and average frequency (AF) which indicates tensile mode as the dominant mode of fracture during three-point bending.

Study on acoustic emission characteristics of sandstone under different fracture modes

To investigate the characteristics of the sandstones under two fracture modes which are tensile fracture and shear fracture, we proposed three-point bending test and a modified shear test. Meanwhile, numerical simulations using particle flow code (PFC) and acoustic emission (AE) analysis are performed to obtain their differences. The AE hits and amplitude analyses prove that no matter which mode, they experience obvious three stages. The AE hit curve and amplitude are lower in the first stage. Then, the curve goes up steadily and the amplitude value becomes relatively large in the second stage. Entering into the third stage, the curve increases quickly to its peak value and a lot of AE signals with higher amplitude are generated. Moreover, AE b-value analysis is also used to make crack magnitude assessment. It was obvious that the distribution of b-value curve can be divided into three stages, which are just corresponding to the division of AE hits and amplitude distributions. The b-values are diverse and fluctuating largely in stage I and then show small fluctuations and have increasing trends in the whole process in stage II whereas finally exhibit sharp down to the minimum points at the stage III. The average b-values for tensile fracture tests are in the range of 0.877–1.09 whereas for shear fracture tests are within the range of 0.815–0.876. Comparing to tensile fracture, when shear fracture occurs in the rock sample, the AE energy releases faster and the proportion of large magnitude cracks is larger. The research results in this article could make more instructive and applied value to in situ non-destructive monitoring of fissure development for engineering rock mass.

Differences in the acoustic emission characteristics of rock salt compared with granite and marble during the damage evolution process

Rock salt cavities for energy storage are important elements of energy supply management and sustainability. The mechanical properties of other common rocks have already been adequately investigated, but a systematic characterization of the differences in acoustic emission (AE) characteristics among rock salt and other common rocks is needed. In this study, the AE characteristics of the full-regime uniaxial compression of rock salt and common rocks such as granite and marble were determined. A damage variable based on AE parameters and a methodology for its determination were established, including an analysis of the characteristics of the AE time–space evolution, the AE amplitude distribution and the damage evolution based on AE parameters. Clear differences were observed in the AE characteristics of rock salt compared with granite and marble during the damage evolution process. The AE activity and energy release of rock salt with coarse grains and nonuniform structures decreased gradually with increasing stress level. The AE activity of rock salt appeared in a continuous, group-occurring form. The AE spatial distribution of rock salt was relatively uniform, without a definite rupture surface. The AE amplitude distribution varied slowly, and the proportion of AE events with small amplitudes increased gradually during compression in rock salt. Moreover, the damage in rock salt predominantly occured during the pre-peak period, resulting in a large damage variable of 0.9 at peak stress. By contrast, the damage variables of granite and marble are only 0.5 and 0.1, respectively, at peak stress. Rock salt exhibited less damage when approaching failure, resulting in a gentler and steadier process. And the plastic characteristics of rock salt are relatively obvious. These properties, to some extent, ensure the safety of underground storage engineering when rock salt is chosen as the medium.

Acoustic Response and Micro-Damage Mechanism of Fiber Composite Materials under Mode-II Delamination**Supported by the Natural Science Foundation of Hebei Province under Grant No E2012201084, and the National University Students' Innovative Training Program under Grant No 201410075004.

Realizing the accurate characterization for the dynamic damage process is a great challenge. Here we carry out testing simultaneously for dynamic monitoring and acoustic emission (AE) statistical analysis towards fiber composites under mode-II delamination damage. The load curve, AE relative energy, amplitude distribution, and amplitude spectrum are obtained and the delamination damage mechanism of the composites is investigated by the microscopic observation of a fractured specimen. The results show that the micro-damage accumulation around the crack tip region has a great effect on the evolutionary process of delamination. AE characteristics and amplitude spectrum represent the damage and the physical mechanism originating from the hierarchical microstructure. Our finding provides a novel and feasible strategy to simultaneously evaluate the dynamic response and micro-damage mechanism for fiber composites.

Failure of compression molded all-polyolefin composites studied by acoustic emission

This paper is aimed at studying the failure behavior of polyolefin-based self-reinforced polymer composites (SRPCs) via acoustic emission (AE). Three matrix materials (ethylene octene copolymer (EOC), polypropylene-based ther- moplastic elastomer (ePP), random polypropylene copolymer (rPP), and three kinds of reinforcing structures of PP homopolymer (unidirectional (UD), cross-ply (CP) and woven fabric (WF)) were used. SRPCs were produced by compres- sion molding using the film-stacking method. The composites were characterized by mechanical tests combined with in situ assessment of the burst-type AE events. The results showed that rPP matrix and UD reinforcement produced the greatest reinforcement, with a tensile strength more than six times as high as that of the matrix and a Young's modulus nearly dou- bled compared to the neat matrix. The number of the detected AE events increased with increasing Young's modulus of the applied matrices being associated with reduced sound damping. The AE amplitude distributions shows that failure of the SRPC structure produces AE signals in a broad amplitude range, but the highest detected amplitude range can be clearly linked to fiber fractures.

Acoustic emission behaviors and damage mechanisms of adhesively bonded single-lap composite joints with adhesive defects

Acoustic emission (AE) technology is widely applied for monitoring the damage of materials and structures. In this work, tensile tests were carried out to investigate the shear properties of adhesively bonded single-lap composite joints with adhesive defects. AE technology was used to evaluate the damage progress in real time. Besides, linear AE source location was also performed to study the damage initiation and propagation. Results show that the shear strength of adhesively bonded single-lap composite joints reduces as a result of the defects in the adhesive layers. Adhesion failure is found to be the dominant failure mode and most of the damage initiates and propagates at the edge of the adhesive layers and aggregates around the defects in the adhesive layer. Furthermore, the AE relative energy, amplitude distribution, cumulative hits and duration time are related to the damage process. The linear AE source location results are in consistent with the initiation and propagation of shear damage. For the adhesive joints with defects, AE signals with the amplitude from 60 to 80 dB increase at the evolution stage. It is suggested that the dynamic characteristics of AE signals can be effectively used to describe the damage mechanisms of adhesive bonded structure for composites.

Effect of Porosity on Mechanical Property and Acoustic Emission Signals of Composite Materials

In order to achieve the effect of porosity on the mechanical property and acoustic emission (AE) signals of composite materials with mode I delamination, unidirectional tensile tests and real-time AE monitoring of high property and low property composites were conducted. The results show that the porosity of composite materials made a detrimental effect on the mechanical properties. As the load increases to an inflection point, the slope of the curve becomes smaller. And the slope of the high porosity specimen is lower than that of the other one. With the increase of the porosity of composite material, the failure load and tensile strength decrease. AE monitoring data show that different porosity materials lead to different evolution process of the delamination. And the corresponding AE amplitude distribution, relative energy signals are also different. Different from high porosity composites, low porosity composites generated unstable expansion at the inflection point of the load curve in the stretching process. And the highest amplitude and relative energy were obtained at the inflection point.

Fracture process and reliability of concrete made from high grade recycled aggregate using acoustic emission technique under compression

The increasing amount of waste concrete makes desirable collection of high quality of recycled aggregate from waste concrete to be reused for construction. This research used high grade recycled coarse aggregate (RCA) created using pulsed power technology to make concrete specimens. Concrete created from natural aggregate was also prepared to compare the properties of concrete made using pulsed power recycled aggregate. Established acoustic emission (AE) parameter analyses which are AE hit, relationship between RA value and average frequency, and b-value of AE amplitude distribution were applied to analyze the concrete fracture behavior. In addition, AE Weibull analysis was also proposed to evaluate the reliability of the concrete. A set of AE measurement testing was applied to the concrete specimens during compression loading. At the age of 28 days, compressive strength reaches 35.4 MPa and Young’s modulus is 23.6 GPa. The results indicate that the fracture process and reliability of concrete made using pulsed power RCA is similar to that of natural coarse aggregate concrete suggesting that both concrete have equivalent characteristic under compression. Furthermore, the good agreement results shared by AE Weibull analysis with those of other analyses suggesting this method can also be employed as one parameter to determine the condition of concrete.

Phenomenological Model of Corrosion Process in Reinforced Concrete Identified by Acoustic Emission

The concrete reinforcing steel bar (reinforcing bar) corrosion process is phenomenologically modeled as three phase corrosion loss. Reinforcing bar has corrosion initiated at the first phase. Under aerobic conditions at the second phase, there is a decreased rate of corrosion loss. Due to corrosion products, reinforcing bar expansion nucleates concrete cracking at the third phase of anaerobic corrosion. For such a corrosion process to be identified as reinforced concrete corrosion loss, continuous acoustic emission (AE) monitoring was conducted in a cyclic wet and dry test and an accelerated corrosion test. There was characteristic observation of two high AE activity periods. The slope gradient of AE amplitude distribution and AE waveform parameters were investigated to elucidate these two activities. Cracking mechanisms were classified as the other-type cracks than a tensile crack and AE events were found to be of small amplitudes at the first period of high activity. That ferrous ions vanished at some surface regions was demonstrated by scanning electron microscope examination of the reinforcing bar surface, although no corrosion products were visually observed on the reinforcing bar surface. The corrosion's onset in the reinforcing bar results in the first high AE activity is suggested by this, corresponding to the phenomenological model's first phase. Corrosion products were visually observed on the surface of the reinforcing bar following the second high AE activity period. It suggested that in concrete around the reinforcing bar, there was nucleation of cracks. Reasonably, AE event amplitudes were found to be fairly large and AE sources were classified as tensile cracks. Concrete cracking is associated with these AE events due to corrosion product expansion that could be observed at the model's third phase. Thus, the corrosion loss phenomenological model agrees remarkably with reinforced concrete AE generating behaviors. Reinforced concrete corrosion time could be identified, by AE activity monitoring, at reinforcing bar corrosion initiation and also at the nucleation of cracks in concrete. There is earlier detection of even the latter AE activity than through other nondestructive evaluation techniques.

Corrosion Process in Reinforced Concrete Identified by Acoustic Emission

Continuous acoustic emission (AE) monitoring in reinforced concrete (RC) was conducted to investigate the corrosion process. In experiments of an accelerated corrosion test and a cyclic wet-dry test, two periods of high AE activities were observed. These AE sources are classified by AE indices of the RA value and the average frequency and the b-value of AE amplitude distribution. At the 1st period, generation of small shear-type cracks is identified. From ingress of chloride ions analyzed, chloride concentration at the cover thickness of the reinforcing steel (rebar) was just over the lower-bound threshold for the initiation of corrosion. Although rebars were removed from the specimen, no corrosion products were observed. The surface of rebar was then examined by the scanning electron micrograph (SEM). The results showed that ferrous ions on the rebar surface disappeared, suggesting that the initiation of corrosion is associated with small AE events of the shear type. Approaching the 2nd period, large-scale tensile cracks were identified by AE. Chloride concentration at the cover-thickness was higher than the threshold level prescribed in the codes. Once rebars were removed, corrosion products were visually observed. This implies that the 2nd AE activities correspond to tensile cracks, which obviously result from concrete cracking due to expansion of corrosive products. These results show that the corrosion process of rebars is identified at the onset of rebar corrosion and at the nucleation of concrete cracking by continuous AE monitoring.

Microscopic processes of shearing fracture of old wood, examined using the acoustic emission technique

We examined the process of microscopic fracturing peculiar to old wood, based on the generation characteristics of acoustic emission (AE) events and fracture surface analysis. The shearing tests of old wood obtained from construction-derived lumber and new wood within 3 years after lumbering were performed in accordance with the Japanese Industrial Standards (JIS Z 2101-1994). The species of wood used in this study was Japanese red pine. The old wood had been used as a beam in a building for 270 years. The number of the occurrences of AEs at low load levels was larger in the old wood than in the new wood. As a result of analyzing the AE amplitude distributions, we found that the period in which AEs with small amplitudes were frequently generated was longer in the old wood than in the new wood. Also, the fracture surfaces after the final rupture under scanning electron microscope showed more uneven and complicated surfaces in the old wood. Based on the above findings, we presume that during the shearing test the old wood underwent a relatively long and stable progress of microcracking before the final fracture.

Estimation of Rock In-Situ Stress by Acoustic and Electromagnetic Emission

The Kaiser Effect in acoustic emission is often used for an estimation of the stress to which rocks have been subjected. However, there are cases in which the Kaiser Effect is not clear, since the noises due to the contact and/or the stick slip between the pre-induced fracture surfaces are measured during the reloading process. In such cases, estimation of previous stress is difficult by the conventional method which is based on the acoustic emission activity observed under reloading process. In the tests for the Kaiser Effect on rocks, therefore, the noises must be eliminated from the acoustic emission generated from newly created cracks during the second loading process. Such techniques as analysis of the difference between the acoustic emission activity observed in the first and second reloading and the analysis of the change in the slope of the acoustic emission amplitude distribution have been proposed. In this paper we present a new method by which the maximum previous stress in rocks can be directly estimated without any post signal analysis. In the new method, simultaneous measurement of acoustic and electromagnetic emission during loading test of rock sample is employed. The electromagnetic emission in the deformation of rock sample generates only when the fresh surfaces due to cracking are created in the material, and the source of electromagnetic emission is the electrification between the fresh crack surfaces. This paper describes the simultaneous measurement of acoustic and electromagnetic emission useful for estimating the rock in-situ stress.

Use of the acoustic emission method in detecting the fracture process in specimens made of composite materials

Experimental results on the localization of acoustic emission (AE) signals obtained in stresscycling tests of specimens made from the Organit-10T composite material (CM) are considered. The technique developed for collecting prompt information on the changes in the shape of the AE amplitude distribution is analyzed. This distribution characterizes the flow of arriving signals and allows one to determine the beginning of destruction for CM specimens.

Evaluation on Fracture Characteristics of TiN Ceramics Film with Scratch Testing. 1st Report, Aplication of Fractal Dimensions of AE Amplitude Distrubution.

Many of the ceramics coating materials are in use under the hard working conditions on the ground that these materials have the excellent characteristics. Therefore, it is expected for producing the superior coating materials to establish a methodology for evaluating the mechanical properties of these materials. In this report, 9 kinds of TiN film 5 μm thick that were deposited on the different parent materials under the different N2 partial pressure were scratched and a method for evaluating the properties of the coating materials with Acoustic Emission (AE) is described. Its contents are follows : (1) The scratch testing apparatus that can detect the emitted AE signals from the TiN film during the testing was produced. (2) The relationship between the character of the detected AE signals and the fracture types of film (i.e. crack, chipping and flaking) were investigated. (3) It was became clear that the fractal dimension m led from AE amplitude distribution shows more than 3 when the fracture type is crack, the value shows between 1 and 3 when the type is chipping, the value shows less than 1 when the type is flaking. From these experimental results, it was found that the strength and the tight-adhesiveness of thin film are evaluated by observing the fractal dimensions of AE amplitude distribution during the scratch testing.