Experimental Study on AE Response and Mechanical Behavior of Red Sandstone with Double Prefabricated Circular Holes Under Uniaxial Compression

Three‐point‐bend testing of a continuous unidirectional glass reinforced unsaturated polyester composite was chosen as a model to study the acoustic emission (AE) response and associated damage events in composite materials. Conventional three‐point‐bend samples were tested over a wide range of span‐to‐depth ratios, with failure modes established by post‐failure scanning electron microscope (SEM) analysis. In addition,in‐situAE monitoring with simultaneous SEM recording was undertaken during three‐point‐bend testing inside an SEM vacuum chamber. These two test methods, using different equipment and sample geometry, were found to yield similar mechanical and AE results. Thein‐situtests enabled the development of the various stages of damage to be observed while the AE response was simultaneously monitored. This enabled the AE signals to be unambiguously assigned to specific damage accumulation mechanisms. It was concluded that the AE amplitude distribution can be divided into three main ranges in this system: 60 to 65 dB, 75 to 85 dB, and larger than 90 dB corresponding to matrix damage, fiber‐matrix debonding, and fiber breakage, respectively. It has been demonstrated that the coupling of AE monitoring with SEM recording of a composite system undergoing mechanical loading is a very powerful technique in the study of damage accumulation in advanced composite materials.

Experimental study on AE response and damage evolution characteristics of frozen sandstone under uniaxial compression

Reinforced concrete beams were tested in flexure, and their acoustic emission (AE) response was recorded. This research was performed to investigate the characteristic AE response that is associated with microcrack development, localized crack propagation, corrosion, and debonding of the reinforcing steel in an attempt to use AE to characterize the source of damage. Concrete beams were prepared to isolate these damage mechanisms by using unreinforced, notched-unreinforced, reinforced, and corroded-reinforced specimens. The AE response was analyzed to obtain key parameters such as the total number and rate of AE events, the amplitude and duration of the events, and the characteristic features of the waveform. Initial analysis of the AE signal has shown that a difference in the AE response can be observed depending on the source of the damage. By plotting the AE signal amplitude versus duration (cross-plot), it can be seen that distributed microcracking is typically characterized by a relatively low amplitude and short duration, whereas debonding cracks have a higher amplitude and longer duration. The Felicity ratio (ratio of the load level at which AE activity begins to occur and the previous loading level) exhibits a favorable correlation with the overall damage level, and the total number of AE events that occur during unloading may provide an effective criterion for estimating the level of corrosion distress in reinforced concrete structures. Based on these results, AE parameter analysis may provide a promising approach for estimating the level of damage and corrosion distress in reinforced concrete structures.

Pillar rheological failure is one of the important reasons to induce earthquake, surface collapse, and water inrush disaster in underground mining engineering. Pillar is generally present under an inclined state and is significantly influenced by combining compression and shear loading. However, many scholars regard the long-term strength of coal or rock mass under pure uniaxial compression loading as the main evolution index of pillar strength, which is not consistent with engineering practice. In this paper, a new inclined uniaxial compression strength (IUCS) test system was developed and then used to carry out the IUCS test and creep test of coal specimens combined with an acoustic emission (AE) technology at various inclination angles (0°, 5°, 10°, 15°, and 20°). The variation of time-dependent deformation, peak strength, long-term strength (LTS), creep fracture model, and AE behavior of the coal with the inclination angels were discussed in detail. The results indicated that the peak strength and LTS of the coal nonlinearly decreased with the inclination angle increasing. The proportion of the peak strength to the LTS had remained constant between 58.5% and 62.9%, which can be considered as the inherent properties of coal rock. The creep failure model of the coal was transformed from tension-splitting failure at the inclination angle of 0° and 5° to tension-shear failure at the inclination angle of 10°-20°, which revealed that the inclination angles were favorable to the initiation and propagation of shear cracks. No matter any inclination angles, AE events can be divided into quiet period, low amplitude rising period, and high amplitude rising period with the periodic mutation of multistage loading points. Moreover, the cumulative AE energy gradually decreases with the increase of the inclination angles, which indicated that the shear stress caused by the specimen inclination can make crack initiation and propagation with less energy absorption. The research results will assist in the long-term strength design and time-dependent stability assessment of the coal pillar.

Experimental research of the AE responses and fracture evolution characteristics for sand-paraffin similar material

Strain localization peculiarities and distribution of acoustic emission sources in rock samples tested by uniaxial compression and exposed to electric pulses

Q235 steel is widely used in engineering and construction. Therefore, it is important to identify the damage mechanism and the acoustic emission (AE) response of the material to ensure the safety of structures. In this study, an AE monitor system and an in situ tensile test with an optical microscope were used to investigate the AE response and insight into the damage process of Q235 steel. The surface of the specimen was polished and etched before the test in order to improve the quality of micrographs. Two kinds of AE responses, namely a burst and a continuous signal, were recorded by the AE monitor system during the test. Based on the in situ test, it was observed that the damage of Q235 steel was induced by the crystal slip and the inclusion fracture. Since the crystal slip was an ongoing process, continuous AE signals were produced, while burst AE signals were possibly produced by the inclusion fracture which occurred suddenly with released higher energy. In addition, a great number of AE signals with high amplitude were observed during the yielding stage and then the number and amplitude decreased.

Acoustic emission responses and damage estimation of coal with carbon fiber-reinforced polymer confinement under uniaxial compression

The adsorption, permeability properties, and damage evolution of internal cracks are often not considered in simulation tests for coal and gas outbursts. Therefore, in previous studies of similar materials, the materials used may not be similar in the properties mentioned above. To solve these issues, the adsorption, permeability, mechanical, and acoustic emission (AE) response characteristics of a soft coal solid–gas coupling similar material (SCSCSM) were studied in an orthogonal experiment where gas adsorption, permeability, uniaxial compression, and AE tests were performed. The analysis of the experimental data revealed that the SCSCSM has mechanical, gas adsorption and permeability properties similar to those of soft coal. The mass fraction of humic acid sodium solution (HASS) was the main parameter, affecting physical and mechanical properties. As the mass fraction of HASS increased, the adsorption volume and permeability varied logarithmically and exponentially, respectively. Meanwhile, the failure pattern transitioned from extrusion damage to wedge splitting and brittle shear failure. AE response from the beginning to the peak stress can be divided into a slow increase stage and a rapid increase stage. A damage parameter experienced an initial rise, then a rapid decline, then a quick rise, and finally a slow increase. Through segmentation fitting, expressions of damage evolution mechanisms for each section were obtained. Through the study of the SCSCSM's internal gas storage, transport, and mechanical properties, AE response characteristics, and multiscale evaluation of mechanical response, this study has laid a foundation for the subsequent development of solid–gas coupling similar simulation materials and physical simulation experiments of coal and gas outburst for soft coal seams.

Online monitoring of the lubrication and friction conditions in internal combustion engines can provide valuable information and thereby enables optimal maintenance actions to be undertaken to ensure safe and efficient operations. Acoustic emission (AE) has attracted significant attention in condition monitoring due to its high sensitivity to light defects on sliding surfaces. However, limited understanding of the AE mechanisms in fluid-lubricated conjunctions, such as piston rings and cylinder liners, confines the development of AE-based lubrication monitoring techniques. Therefore, this study focuses on developing new AE models and effective AE signal process methods in order to achieve accurate online lubrication monitoring. Based on the existing AE model for asperity–asperity collision (AAC), a new model for fluid–asperity shearing (FAS)-induced AE is proposed that will explain AE responses from the tribological conjunction of the piston ring and cylinder. These two AE models can then jointly demonstrate AE responses from the lubrication conjunction of engine ring–liner. In particular, FAS allows the observable AE responses in the middle of engine strokes to be characterised in association with engine speeds and lubricant viscosity. However, these AE components are relatively weak and noisy compared to others, with movements such as valve taring, fuel injection and combustions. To accurately extract these weaker AE’s for lubricant monitoring, an optimised wavelet packet transform (WPT) analysis is applied to the raw AE data from a running engine. This results in four distinctive narrow band indicators to describe the AE amplitude in the middle of an engine power stroke. Experimental evaluation shows the linear increasing trend of AE indicator with engine speeds allows a full separation of two baseline engine lubricants (CD-10W30 and CD-15W40), previously unused over a wide range of speeds. Moreover, the used oil can also be diagnosed by using the nonlinear and unstable behaviours of the indicator at various speeds. This model has demonstrated the high performance of using AE signals processed with the optimised WPT spectrum in monitoring the lubrication conditions between the ring and liner in IC engines.

The study of the force chains and acoustic emission response laws of rock masses are of major significance for the prediction and prevention of stope dynamic disasters. In the present study, the response law of the force chains and acoustic emissions of rock specimens were examined using numerical simulation and laboratory testing based on the theory of moment tensor, along with the macro–mesoscopic damage characteristics and the evolution laws of force chains and acoustic emissions. The research results indicated that under axial stress conditions, the strong chains inside rock material were distributed in “root-like” shapes, and the direction of the strong chains had been controlled by the directions of the stress loading. The number of internal force chains and their intensities during the loading of the rock material were found to be negatively correlated. The root causes of the formations and expansions of the fissures in the rock material lies were found to be related to the fractures and instability of the strong chains on both sides of the fracture surfaces. It was determined in this study that the corresponding acoustic emission responses of the examined rock specimens under the conditions of uniaxial compression could be divided into five phases as follows: a weak phase; stable phase; stable enhancement phase; sharp enhancement phase; and peak phase. The phases indicated the spatial distribution shapes and intensities of the acoustic emissions during each stage. The force chains, fissures, and acoustic emissions in the rock material were found to be synchronous in time, spatially homogeneous, and consistent in intensity. The results of this study revealed that the mechanical nature of the fissures and acoustic emissions in the rock specimens were caused by the transferences, transformations, fracturing, and reconstructions of the internal force chains of the material after external force had been exerted. The energy released as a result of the instability of the force chains had led to instability deviations of the coal rock particles, which had subsequently caused damages to the force chains. As the rock material became increasingly damaged, meso-fissures had been formed. Also, part of the unabsorbed energy was released to form acoustic emission phenomena.

Many laboratory experiments on crack propagation under uniaxial loading and biaxial loading have been conducted in the past using transparent materials such as resin, polymethyl methacrylate (PMMA), etc. However, propagation behaviors of three-dimensional (3D) cracks in rock or rock-like materials under tri-axial loading are often considerably different. In this study, a series of true tri-axial loading tests on the rock-like material with two semi-ellipse pre-existing cracks were performed in laboratory to investigate the acoustic emission (AE) characteristics and propagation characteristics of 3D crack groups influenced by intermediate principal stress. Compared with previous experiments under uniaxial loading and biaxial loading, the tests under true tri-axial loading showed that shear cracks, anti-wing cracks and secondary cracks were the main failure mechanisms, and the initiation and propagation of tensile cracks were limited. Shear cracks propagated in the direction parallel to pre-existing crack plane. With the increase of intermediate principal stress, the critical stress of crack initiation increased gradually, and secondary shear cracks may no longer coalesce in the rock bridge. Crack aperture decreased with the increase of intermediate principal stress, and the failure is dominated by shear fracturing. There are two stages of fracture development: stable propagation stage and unstable failure stage. The AE events occurred in a zone parallel to pre-existing crack plane, and the AE zone increased gradually with the increase of intermediate principal stress, eventually forming obvious shear rupture planes. This shows that shear cracks initiated and propagated in the pre-existing crack direction, forming a shear rupture plane inside the specimens. The paths of fracturing inside the specimens were observed using the Computerized Tomography (CT) scanning and reconstruction.

Experimental investigation of the fracture and damage evolution characteristics of flawed coal based on electric potential and acoustic emission parameter analyses

Damage accumulation in continuous unidirectional glass reinforced composites was studied by acoustic emission (AE) monitoring during three‐point‐bend loading. Results are presented for four composites monitored during quasi‐static break loading, and one composite also monitored during cyclic fatigue and static creep loading. AE response was correlated with the mechanical (stress‐strain) response and with visual observation of damage events to study the details of the damage accumulation process. Results show that the failure process is characterized by the sequential occurrence of three distinct damage mechanisms. Specifically, the failure process initiated with cohesive matrix damage, propagated with interfacial debonding, and ended with fiber breakage very near catastrophic failure. The same sequential damage process occurred in all four composites and all three test procedures examined. Results also demonstrate that AE analysis, in combination with mechanical testing and microscopic observation, is a valuable tool in understanding damage accumulation in composites.

Due to long-term coupling of ground stress and water pressure during operation processes, lining concrete will experience different degrees of cracking and spalling, which seriously affect the safety of tunnels during service. In this paper, lining concrete seepage tests under different confining pressures are performed; the mechanical properties and acoustic emission (AE) responses during the loading process and the relationship between load-induced plastic deformation and lining concrete specimen permeability are studied. The plastic internal variable definition is introduced to characterize concrete specimen plastic deformation and is calculated after unloading. Different initial stress states and stress levels during loading and unloading processes affect pore and fissure development in lining concrete. The peak intensity increases with increasing confining pressures, and deformation parameter variation has elastoplastic coupling characteristics. AE parameters and axial stress show good agreement. The AE signal of the samples shows a transition from the Kaiser effect to the Felicity effect before the peak, indicating that AE can be an effective means of monitoring internal sample damage. A power function relationship can represent the relationship between the initial permeability and confining pressure, and the permeability evolution model of concrete samples in the plastic phase is established after the initial yielding.