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The method of backfill in underground mining is important for ground control as well as material recycling and energy efficiency. Even though extensive testing and field studies of backfill have been conducted, less is known about the detailed damage and fracturing that occurs directly at the rock/backfill interface. In this paper, cylindrical specimens containing an inner diameter of backfill and an outer diameter of rock (RB) were tested under triaxial compression. Acoustic emissions (AE) were used throughout testing, and X-ray computed tomography (CT) scanning was conducted before loading was applied and after the specimens had failed. The high-resolution CT images were then converted into point clouds to isolate the fractures and visualize them in three dimensions. The point clouds clearly show that fracturing occurred both in the rock and along with the contact between rock and backfill, while very little fracturing was found to occur in the backfill. Based on the point cloud and AE results, a unique evolution of fracturing is found to occur that includes two stages of shear fracturing in the rock, tensile fracturing along with the rock/backfill interface, and final tensile fracturing in the rock after delamination from the backfill, all of which contributed to the nonlinear stress–strain response. This paper presents a novel approach for investigating the initiation and propagation of 3D fractures in laboratory testing and can offer a useful reference for further studies on the mechanics of bi-material structures. [ABSTRACT FROM AUTHOR]

The scientific foundations for the development of a method for assessing the degree of danger of single crack-like defects found in thin-sheet metal structural elements are being investigated using AE testing using the low-temperature loading method. As a criterion, it is proposed to use the "critical activity" of the AE source, calculated using the equations of fracture mechanics and thermal conductivity. Loading is carried out via cooling the local ring-shaped zone by placing the carbon dioxide with a temperature of –78°C. At the same time, as a result of thermal contraction, tensile stresses arise that load the defect area during AE testing. The resulting temperature field and mechanical stresses are calculated by known theoretical methods using the heat and fracture mechanics equations. The annular appearance of the cooling zone determines the invariance of loading relative to the angle of the crack plane. Fracture mechanics criteria are employed that use the known power-law dependence of the number of acoustic emission acts on the stress intensity factor. Thus, the possibility of using the fracture mechanics criteria to assess the danger of crack-like defects by AE testing using the known power-law dependence of the number of acoustic emission acts on the stress intensity factor is shown. [ABSTRACT FROM AUTHOR]

Large-volume cubic high-pressure apparatus is commonly used to produce synthetic diamond. Due to the high pressure, high temperature and alternative stresses in practical production, cracks often occur in the carbide anvil, thereby resulting in significant economic losses or even casualties. Conventional methods are unsuitable for crack detection of the carbide anvil. This paper is concerned with acoustic emission-based crack detection of carbide anvils, regarded as a pattern recognition problem; this is achieved using a microphone, with methods including sound pulse detection, feature extraction, feature optimization and classifier design. Through analyzing the characteristics of background noise, the cracked sound pulses are separated accurately from the originally continuous signal. Subsequently, three different kinds of features including a zero-crossing rate, sound pressure levels, and linear prediction cepstrum coefficients are presented for characterizing the cracked sound pulses. The original high-dimensional features are adaptively optimized using principal component analysis. A hybrid framework of a support vector machine with k nearest neighbors is designed to recognize the cracked sound pulses. Finally, experiments are conducted in a practical diamond workshop to validate the feasibility and efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

This paper deals with an improved representation of the progression of damage in lime-based masonry under cyclic loads using the Acoustic Emission (AE) technique. Stress-strain relations are a familiar concept in engineering which detail the elastic-plastic behaviour of the material. During AE monitoring, AE activates with the increase of stress and initiation of micro fracture due to local changes in the material structure. The population of AE events registered at an increasing stress levels provides information on the structure's integrity. In the present study, masonry couplets and walls were built with clay bricks in combination with two different mortar types: hybrid lime-cement (LC) and lime hydrate mortar (LH). One set of experiments focuses on masonry couplets and another set of experiments focuses on masonry walls, both under compression. The AE-based damage quantification is validated by a comparative analysis with strain measurements from Linear Variable Differential Transformers (LVDTs). In conclusion, results indicated that at initial stages the AE trend was closely related to axial strain for LC specimens and lateral strain for LH specimens. Regardless of size effects with respect to masonry couplets and walls, there was no significant difference in damage progress. [ABSTRACT FROM AUTHOR]

The paper addresses the dependence of acoustic emission produced in rolled hafnium GFÉ-1 under tensile deformation on the material's structural state. A correlation has been established between the material structure, the level of mechanical properties and the values of acoustic parameters. Acoustic emission in non-recrystallized hafnium under tensile deformation is recorded only at the stages that precede fracture. Upon recrystallization annealing at temperatures of 1123 and 1373 K acoustic emission occurs at all the stages of tensile deformation. The highest level of acoustic emission activity in hafnium is observed during the transition from elastic to elastic-plastic strain. [ABSTRACT FROM AUTHOR]

This paper deals with two aspects of characterization of the fracture process zone (FPZ) in concrete beams. A comparative study is done between acoustic emission (AE) and scanning electron microscope (SEM) to understand the microlevel aspect of the FPZ in concrete beams. SEM analysis, is mainly based on the preparation and analyzing samples which are considered as being a very important part (poor preparation techniques can lead to erroneous diagnosis of the concrete study). On the other hand, an AE is a localized and quick release of strain energy in a stressed material. The released energy causes stress waves, which propagate through the specimen. These waves can be detected at the specimen surface and analyzed to evaluate the magnitude and the nature of damage. The results obtained from the two experimental studies have shown that there is a small divergence between experimental results (SEM and AE). This may be related to some details during the sample analysis. [ABSTRACT FROM AUTHOR]

The progressive brittle failure of high-stress hard surrounding rock masses caused by layered excavation severely threatens the stability of large underground caverns. We summarize the stress path in the potential failure zone of the Shuangjiangkou underground powerhouse during layered excavation, and then design the multistep unloading confining pressure test and the cyclic loading–unloading test with multistep unloading confining pressure. The deformation, strength, acoustic emission, and failure characteristics of granite, as well as the evolution of critical stress thresholds and mechanical parameters are analyzed based on experimental results under different stress paths. The peak strength of the rock significantly depends on the stress path and the amount of unloading confining pressure. The rock failure process under multistep unloading confining pressure is divided into stable and unstable crack growth stages, with the failure mode shifting from tensile cracks to mixed tensile–shear cracks. The evolution modes of critical stress thresholds, cohesion, and internal friction angle with amount of unloading confining pressure differ significantly from that with the equal plastic strain. The former is nearly linear, while the latter is exponentially nonlinear. The evolution of the deformation modulus, cohesion, and internal friction angle confirms the rationality of the cohesion-weakening–friction-strengthening model with degraded deformation modulus for the stability analysis of surrounding rock masses under layered excavation. The experimental methods for obtaining the evolution of rock mechanical parameters using a single specimen under unloading confining pressure are also discussed. The results imply that the layered excavation-induced nonlinear deterioration of the deformation modulus and strength parameters of high-stress hard surrounding rock masses should be considered in the stability analysis of large underground caverns. Further, the timely compensation of the excavation-induced confining pressure loss is critical to ensure the stability of high-stress hard surrounding rock masses under layered excavation. Highlights: A stress path was designed to reflect the stress evolution characteristics for rock masses of the large underground cavern under layered excavation; The deformation, strength, acoustic emission, and failure characteristics of granite under the layered excavation stress path were analyzed; The evolution of rock mechanical parameters with unloading confining pressure and the equivalent plastic strain were obtained; An experimental method for obtaining the evolution of rock mechanical parameters using a single specimen under unloading confining pressure was discussed. [ABSTRACT FROM AUTHOR]

With the increasing consumption of conventional oil and gas reservoirs for gas recovery/production, unconventional reservoirs, such as coalbed methane, shale gas, and gas hydrate, have become very popular in recent times. In this regard, hydraulic fracturing is an effective technique commonly used for enhanced coalbed methane recovery. In previous studies, the fracture morphology was described by comparing the fracture morphology before and after hydraulic fracturing from a macroscopic perspective. Because fracture initiation and subsequent networks of fractures are formed instantly when coal mass is subjected to hydraulic fracturing, it is almost impossible to acquire complete information about fracture initiation by only analyzing the change in hydraulic pressure and fracture morphology. In this paper, a triaxial experimental system was developed to simulate hydraulic fracturing using raw coal and briquette coal samples, respectively. The borehole wall strain observed during hydraulic fracturing was plotted (borehole wall strain curves) and the acoustic emission response was also obtained. In addition, the fracture behaviors during hydraulic fracturing were analyzed. Our results show that the response of coal subjected to hydraulic fracturing can be divided into the following four stages: microcrack formation, fracture initiation, unstable crack propagation, and fracture closure. The borehole wall strain curves effectively reflected the deformation and failure of borehole wall. Acoustic emission response can thus be utilized to identify the orientation of fractures during hydraulic fracturing. The combination of the two methods offers an effective option for clarifying the fracture initiation and instability mechanism near the borehole subjected to hydraulic fracturing. [ABSTRACT FROM AUTHOR]

Summary: The mechanical health of any composite specimen can be monitored by recording the acoustic emissions (AE) from the surface and hence can be used to predict mechanical fracturing of the material. The AE technique, which is a nondestructive and noninvasive technique, is a common method to monitor the mechanical health of a structure/material. The AE data emanated from a specimen under mechanical stress appears to follow power law distribution that can be used to estimate the power exponent (b value). It has been observed that empirical data do not completely follow power law, which in turn makes the estimation to be significantly erroneous. Some attempts have been made by the researchers in the past to filter out the non‐power law parts of the dataset; however, issues such as choice of upper and lower bounds and fixed values of these bounds for different types of datasets remain a challenge for efficient and robust estimation of b value. In the presented work, an attempt has been made to rectify the inherent shortcomings of b value and improved b value (ib value) estimation methodology. The proposed method, which is computationally light to implement, has been verified on some synthetic and experimental datasets. The results show promising improvements in the estimation of b value parameter from the previous reported methods in the literature. [ABSTRACT FROM AUTHOR]

Summary: The cracks of rock seriously threaten the safety of building structures and underground space. Uniaxial compression tests were carried out on prefabricated single‐crack sandstone samples with different angles and lengths. The results show that the existence of a prefabricated single‐crack has a certain influence on the mechanical properties and acoustic emission response characteristics of sandstone samples. A function formula of the effect of uniaxial compression stress on the angle and length of prefabricated crack is established. It is found that there is an obvious moment of acoustic emission energy mutation in the three deformation and failure stages before sandstone failure. The change of angle and length of crack has a great influence on the stress value corresponding to the energy mutation at the stable development stage of the micro‐crack. Meanwhile, the acoustic emission waveform characteristic parameters of sandstone samples under various fracture conditions are analyzed. Thus, the crack propagation mode of the sample with small angle can be judged. The damage variables based on real‐time acoustic emission energy were defined, and the damage constitutive model of rock material with a single fracture under uniaxial compression was established based on real‐time acoustic emission energy. The results show that the new model and method can not only reasonably explain the damage mechanism of prefabricated rock with a single fracture under uniaxial compression but also accurately simulate the full stress‐strain process of rock with a single fracture at different angles and lengths, which has good rationality and feasibility. [ABSTRACT FROM AUTHOR]

In order to explore the evolution characteristics of multi-scale rock-like material failure, we studied the acoustic emission (AE) and electromagnetic radiation (EMR) characteristics of different scale rock-like materials by using the AE-EMR experimental system of coal and rock failure, and the AE and EMR response law of rockburst in mining sites was analyzed. The results show that under uniaxial loading, the stress–strain curve of the specimen has a compaction stage, linear elastic stage, elastic–plastic stage and failure stage. The cumulative AE count, AE energy and stress level of the specimen have an exponential relationship during loading and compression. The cumulative EMR counts of loading and unloading showed a trend of first decreasing and then increasing with the increase in stress level. Electromagnetic radiation and microseismic hypocentral distance show an abnormal change trend when rockburst occurs, and this abnormal phenomenon can be used as a precursor feature signal for rockburst monitoring and early warning. [ABSTRACT FROM AUTHOR]

Acoustic emission is high frequency elastic waves produced in a plate under load by the rapid release of strain energy during crack propagation. These waves propagated through the plate material. Knowledge of how the plate is vibrated by acoustic emission resulting from crack growth is an important parameter for crack locating. In this paper vibration behavior of Al–Mg plate due to acoustic signals is modeled. The displacement field was used to derive the governing motion equations of the first-order shear deformation theory for plate by means of Hamilton’s principle. The displacement field was written based as double Fourier series. A MATLAB code according Runge–Kutta numerical method was generated to solve the motion equations of plate. To check the theoretical results, experiments were carried out with four sensors installed on a plate of 5056 Al–Mg with dimensions of 1 m ∗ 1 m in a rectangular array. An impact was generated by an artificial AE source such as Hsu–Nielsen method of pencil lead breaking (PLB) at any position of the plate. Resonance frequencies of the used AE sensors are employed to investigate the vibration behavior of plate due to acoustic emission. In the next step, Fast Fourier Transform was taken from theoretical and experimental results. Comparing the obtained results showed that the frequency which carried the most amount of energy is approximately the same which confirm performance of the developed analytical model. [ABSTRACT FROM AUTHOR]

Raw coal was used as the study object in this paper to identify the evolution characteristics of acoustic emission (AE) during the dilation process of gas-containing coal. The coal specimens were stored in gas seal devices filled with gas at different pressures (0, 0.5, 1.0, and 1.5 MPa) for 24 h prior to testing. Then, the specimens were tested in a rock-testing machine, and the deformation and crack fracture patterns were recorded by using strain gauges and an AE system. The axial and volumetric strains-stress curves were analyzed in relation to the AE and the failure mode. Results show that as gas pressure increases, the uniaxial compression strength and elasticity modulus of gas-containing coal decreases, whereas the Poisson's ratio increases. In all the coal specimens, the dilation initiation stress decreases, and the dilation degree increases. During the dilation process, before the loaded coal specimens reach peak stress, and as the load increases, the changes in the specimens and in the AE energy parameter of specimens can be divided into four phases: crack closure deformation, elastic deformation, stable crack propagation, and unstable crack propagation (dilation process). Across the four phases, the AE energy increases evidently during crack closure and elastic deformation but decreases during stable crack propagation. As the gas pressure increases, the AE signal frequency increases from 4.5 KHz to 8.1 KHz during the dilation process. Thus, the gas presence in coal specimens exerts a significant influence on the closure of sample cracks and dilation damage. [ABSTRACT FROM AUTHOR]

Instability failure in rock mass engineering is closely related to expansion of joint fissures. In this study, uniaxial compression tests and acoustic emission (AE) measurements were carried out simultaneously on specimens of soft rock-like material with different fracture angles and connectivity values to better understand their influence on the deformation and failure of the material. The stress–strain curve and AE signal of fractured soft rock-like material are similar to those of intact soft rock-like; specifically, they exhibit a compaction, elastic deformation, stable fracture development, and unstable fracture development. The main differences between fractured and intact material occur during post-peak failure stage. Under the combined influence of fracture angle and connectivity, the uniaxial compressive strength of fractured soft rock-like material ( f cu ′ ) is lower than that of the intact soft rock-like material (fcu), and can be described by the relationship f cu ′ = f cu · α , where α is the strength reduction coefficient, fitted as α = 0.8228 + 0.00411 x - 0.00789 y . In this equation, x is the fracture angle ( ∘ ) and y is the fracture connectivity (%). Under uniaxial compression, the main types of secondary cracks were wing cracks and secondary coplanar cracks. The specimen with a fracture angle of 30° mainly underwent tensile failure under loading, whereas those with fracture angles of 45° and 60°mainly experienced shear failure under high-connectivity conditions (45%). [ABSTRACT FROM AUTHOR]

The cracked chevron notched Brazilian disc method (CCNBD) recommended by the International Society for Rock Mechanics is a major method for measuring the rock fracture toughness. In the rock fracture mechanics, the micro-crack evolution behavior during Mode I fracture is of great theoretical significance. To fully understand the micro-crack evolution mechanism and fracture process zone evolution information of non-uniform sandstone, numerical simulation of CCNBD specimen under Mode I fracture condition was carried out using PFC3D. Combined with the box fractal dimension theory, the sandstone particle size and mineral composition were considered, and the non-uniform bonded particle model was built. By monitoring the evolution of micro-cracks in the interior and boundary of the cluster, the micro-crack evolution process of the numerical model was obtained. The results showed that the micro-fracture of CCNBD specimen during the cracking process was dominated by weak cementation plane and the contact of interstitial filling particles, while the micro-crack generated between detritus particles mainly concentrated in the CCNBD specimen with the peak stress range of 80–100% and the post-peak failure stage. Combined with the stress field analysis, the Mode I fracture experiment of the bonding between diagenetic mineral particles at the crack tip of CCNBD specimen was dominated by tension, while the bonding between interstitial filling particles is dominated by compression. The micro-crack evolution mechanism of the CCNBD specimen during Mode I fracture was obtained by experiments and numerical simulations. This study is expected to improve the understanding of the unstable fracture mechanism of the fractured rock mass. [ABSTRACT FROM AUTHOR]

Abstract: The methods proposed by Elices and co-workers [1–3] and by Hu and Wittmann [4] are commonly used to determine the size-independent specific fracture energy (GF ) of concrete by correcting the size-dependent specific fracture energy (Gf ) measured by the RILEM work-of-fracture method. In the boundary effect model of Hu and Wittmann [4], the change in the local fracture energy (gf ) is approximated by a bilinear function, whereas the method of Elices et al. [1] consists in determining the non-measured work-of-fracture by adjusting the tail of the P-δ curve that corresponds to the final part of the test. Acoustic emission (AE) experiments on notched specimens (Muralidhara et al. [5,6]) have revealed that under loading the AE events follow approximately a tri-linear distribution; initially the number of events increases almost linearly reaching an extended plateau when the number of events remains nearly constant and eventually the number reduces as the crack approaches the back stress free boundary of the specimen, reminiscent of the development of R-curve in a finite size specimen. This paper exploits this observation and proposes a tri-linear model for the determination of the size-independent specific fracture energy for three different concrete mixes ranging in compressive strength from 57 to 122MPa. Remarkably, it is found that the resulting size-independent specific fracture energy GF determined by this tri-linear model and by the bi-linear model of Hu and Wittmann [4] is very nearly the same and independent of the size of the specimen. [Copyright &y& Elsevier]

Abstract: Material characterization is usually based on the stress–strain constitutive laws directly derived from the load–displacement relations of uniaxial compression tests. This approach, which implies a uniform distribution of deformation along the specimen axis and an energy dissipation within a volume, does not permit to correctly describe the mechanical behavior and the damage evolution by varying the structural size and slenderness. In this paper, three different approaches are used to evaluate the physical dimension of the damage domain in disordered materials subjected to uniaxial compression. The energetic and the statistical methods are based on the acoustic emission monitoring technique, whereas the renormalization group procedure is based on the assumption of anomalous physical dimensions for the material properties. The three approaches agree very well in the determination of the damage domain fractal dimension. [Copyright &y& Elsevier]

Abstract: The boundary effect or the size effect on the fracture properties of concrete has been studied assuming a bi-linear model for the distribution of local fracture energy concept. Boundary effect is observed not only near the back boundary but also near the notch tip, where a fictitious boundary seems to exist, separating the linear and non-linear fracture zones. In this paper a tri-linear function is assumed for the local fracture energy distribution along the ligament and expressions relating RILEM fracture energy and the size-independent fracture energy are developed. Transition ligament length measurements based on the acoustic emission (AE) histogram of events are used to obtain size-independent fracture energy. Length of the fracture process zone is identified in the AE histogram and compared with the value obtained from softening beam model. There seems to be a good agreement between the results. [Copyright &y& Elsevier]

Abstract: This paper addresses a new method based on the combination of mechanical behavior and acoustic emission (AE) information of composite materials during mode I delamination. The method is based on a special purpose function, called sentry function, which is defined as the logarithm of the ratio between mechanical energy and acoustic energy (f =Ln(Es /Ea )). The sentry function is used to study the delamination process and to evaluate the delamination fracture toughness in mode I. The relationship between cumulative fracture toughness energy release rate (G I) and the integral of the sentry function during crack propagation showed a transition point with two sensitive regions below and above it. This behavior can be followed to obtain the critical strain energy release rate value (G I c ). Results obtained by means of the sentry function are compared with results obtained by a methodology proposed by other authors. [ABSTRACT FROM AUTHOR]

Abstract: The essential work of fracture (EWF) concept has become very popular to characterize the plane stress toughness of ductile polymers and related systems. The widespread use of the EWF is due to the simple specimens’ preparation, easy testing and simple data reduction procedure. Though the EWF method is usually used for mode-I type loading, it has been successfully adopted for mode-II and mode-III type deformations, too. Moreover, attempts have also been made to deduce plane strain toughness values from EWF tests. This paper critically reviews the application of the EWF to polymers, polymer blends and composites. The literature survey covers all major aspects of testing and related data reduction methods, and lists the EWF results achieved on different polymer systems. The latter are classified according to their synthesis/production and modifications. Special attention was paid to disclose the correlations between EWF and other fracture mechanics parameters, and to trace the EWF response to molecular and morphological parameters of the tested polymers. [ABSTRACT FROM AUTHOR]

Abstract: Fracture in a planar randomly ordered fiber network subjected to approximately homogenous macroscopic stress and strain field is considered. A theory describing material degradation on a macroscopic scale is derived via Griffith’s energy balance for an internal fractured area in the network assuming the active fracture process on the microscopic level is fiber–fiber bond breakage. Attention is confined to a purely mechanical theory assuming isothermal processes and the theory relies on equations commonly used in theories of statistical physics. In the theory, a bond breaking driving force is stated to be equal to the elastic strain energy density of a non-fractured network. A debond fraction can be coupled to a linearly decrease of the network’s macroscopic stiffness. The rate of the fracture processes is determined by the network’s inherent properties (bond and fiber density, bond strength, etc.). During the loading process, until onset of localization, the bond breaks occur at randomly distributed locations spread over the fiber network and the theory estimate material degradation on a macroscopic level. When localization takes place, the fracture process changes from a two-dimensional randomly distributed process to a one-dimensional process and other theories have to be included to describe post-localization behavior. An approximately in-plane isotropic low-density paper is used in tensile experiments while monitoring acoustic emission activity to evaluate the theory. The experimentally obtained results support the theory surprisingly well. [Copyright &y& Elsevier]

We present on this paper an experimental study of the follow-up of the damage of two laminate composites manufactured by RTM during their loadings in 3 points bending in static and cyclic tests. These materials are composed E glass fibres and epoxy resin. The mechanical tests are carried out on a standard hydraulic machine INSTRON 8516 that can be used in static and fatigue tests. The machine is interfaced whith a detected computer for controlling and data acquisition. The evolution of the damage is followed by acoustic emission using the system EPA NCV-2 connected to the testing machine, then confirmed by optical microscopic observations. The results obtained show that the curves of mechanical behaviour are strictly related to the observed damages. In the case of cyclic bending, the increase in the number of cycles involves a modulus reduction for the two materials that in loading as well as in unloading. The Acoustic emission allows to locate the mechanisms of damage and rupture caused by the loading of these materials. [ABSTRACT FROM AUTHOR]

The replacement of ferrous metals with lighter non-ferrous ones, in particular with aluminum and its alloys, is of great importance for reducing the specific material consumption of products. Modification of aluminum alloy D16 with combined electro spark coating VK8 + Cu is considered. Based on the method of finite element analysis of the Nastran software complex, an optimal coating continuity was determined at the level of 55 - 65%, which provides efficient workability of the coating via reducing the residual stresses in the base and tangent stresses in the plane of adhesive contact, optimization of the coating continuity, distribution of contact loads, and formation of optimal surface geometry. The results of modeling the stress-strain state of the coating-base at a coating continuity of 60% and a normal load of 600 N indicate a 30 MPa increase in equivalent stresses in a unit element of the coating and a decrease of this parameter by 100 MPa in the base as compared with an unmodified D16 surface, indicating the localization of normal stresses mainly in the combined coating. It was experimentally established that at a combined coating continuity of 60%, reduction of D16 wear by 2 times and decrease in the average power of acoustic emission by 1.33 times are provided, which testifies to the efficient structural adaptability of the coating-base under friction. The mechanisms of increasing the wear resistance of the VK8 + Cu coating according to the rheological-kinetic model, which reflects the correlation between processes of fracture and deformation under friction, are considered. It is determined that the high wear resistance of the combined coating is due to the combination of rheological properties of hard alloy VK8 with a fracture toughness of 13.2 MPa·m1/2 and plastic copper material with a fracture toughness of 100 MPa·m1/2, which contributes to the efficient relaxation of stresses under friction. [ABSTRACT FROM AUTHOR]

• The cracks propagation behavior of infilled jointed rock with different arrangements. • Monitoring stress, acoustic emission and strain parameters during fracturing process. • Using the fracture mechanics numerical code FRACOD. • The crack propagation law under different loading stress conditions. A detailed understanding of the failure characteristics of infilled jointed rock is a key concern in underground engineering and is critical to the design, construction and maintenance of relevant projects. In this paper, uniaxial compression tests were performed to investigate the effect of the joint arrangement (different rock bridge angles) on the strength and crack propagation of red sandstone containing a set of preexisting infilled flaws of the same sizes and angles, while the crack fracturing process was observed by AE and strain monitoring. When the angle between the rock bridge and loading stress was less than 24°, the rock mass failure form was tensile failure with some localized shear failure occurring in the rock bridge damaged by crack penetration, leading to an overall decrease in the carrying capacity. The turning points of the strain and AE signal were clearly observed to be caused by the initiation and propagation of cracks, and the energy consumed by the initiation of shear cracks was greater than that of tensile cracks. The study was further conducted using the fracture mechanics numerical code FRACOD to investigate the crack propagation law and failure mechanism under different loading stress conditions. Simulation results from three 2D similar models showed that the percentage of open fractures decreased, but slipping increased; in addition, the total length of the fractures was greatly reduced with increasing confining stress. These results are important and valuable for understanding the crack mechanism of rock engineering in deep underground mining excavations. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the applicability of acoustic emission (AE) technique to evaluate delamination crack in glass/epoxy composite laminates under quasi-static and fatigue loading. To this aim, double cantilever beam specimens were subjected to mode I quasi-static and fatigue loading conditions and the generated AE signals were recorded during the tests. By analyzing the mechanical and AE results, an analytical correlation between the AE energy with the released strain energy and the crack growth was established. It was found that there is a 3rd degree polynomial correlation between the crack growth and the cumulative AE energy. Using this correlation the delamination crack growth was predicted under both the static and fatigue loading conditions. The predicted crack growth values was were in a good agreement with the visually recorded data during the tests. The results indicated that the proposed AE-based method has good applicability to evaluate the delamination crack growth under quasi-static and fatigue loading conditions, especially when the crack is embedded within the structure and could not be seen visually. [ABSTRACT FROM AUTHOR]

Fracture in quasi-brittle materials like concrete is mainly governed by two processes: energy dissipation and discontinuous displacement fields or discrete cracks. In this study we discuss the simultaneous development of both types of fracture. Energy dissipation and crack openings are analyzed during the fracture development by acoustic emission and digital image correlation (DIC) respectively. DIC results are processed using nonlinear damage modelling and lattice element modelling to reveal the microcracking zone. It is observed that overall failure mechanism is governed by three processes (1) high energy dissipation phase during which energy release rate increases (2) continuum-discontinuum transitional phase where energy dissipation rate attains its maximum value, and (3) discontinuum phase where energy dissipation rate drops and crack openings start increasing. [ABSTRACT FROM AUTHOR]

The article describes a procedure proposed for solving inverse problems on identification of flat specimen fracture by tension using in situ displacements and acoustic emission. Weak associated area with concentration of pores is modeled by a mathematical cut with lengthwise averaged normal tensile stresses. In places with high concentration of pores, Young's modulus is 'degraded,' i.e. strains arise here and cause shear stresses. The latter are calculated by successive approximations of solutions of direct problems by specimen contour displacements and data on acoustic emission. In principle, under consideration are inverse problems of the continuum mechanics. [ABSTRACT FROM AUTHOR]

To analyze the deformation of solid bodies, it was proposed to use simultaneously three in situ methods of data registration, including loading diagram (macroscale level), correlation of digital images (mesoscale level), and acoustic emission (microscale level). The complex for multiscale in situ investigation of deformation and fracture processes, the operating principle of which is based on interconnection of duration of the characteristic stages of change in the informative parameters and their ratios with the leading scale level, was implemented. By the example of stretching of the samples of aluminum alloy D16 with cut of different depth, the characteristic stages of change in the derivative of the load applied, AE activity, and intensity of shear deformation on the degree of deformation were revealed. It was shown that the characteristic stages marked on graphs of changes in all informative parameters on time agree well between each other and refer to elastic deformation, parabolic strengthening, and crack propagation. [ABSTRACT FROM AUTHOR]

The damage accumulation in a rock mass is conventionally monitored using rock specimens that are subjected to gradually growing compression for acoustic emission recording. The research problem is to establish the relationship between the stress-strain state of a test specimen and the failure initiation, that is, the geometry of accumulated pores. When solving direct problems, it is found that the specimen failure onset depends on boundary conditions created under plates of a compressive device. The authors consider the cases of the damage accumulation and failure initiation in the article. [ABSTRACT FROM AUTHOR]

New time-dependent Benioff strain (TDBS) release diagrams were analyzed for acoustic emission during various loading tests and for electromagnetic (EM) radiation emanating during compression and, tension, which end in failure. TDBS diagrams are Benioff diagrams that are built consecutively, each time using a greater number of events (acoustic or EM emissions) using the last event as if it were associated with the 'actual failure'. An examination of such TDBS diagrams shows that at a certain time point (this time point is denoted by the term 'alarm' time), a comparatively short interval prior to actual collapse, their decreasing part is broken by a positive 'bulge'. This 'bulge' is quantified and an algorithm proposed for its assessment. Using the alarm time and other parameters of the failure process (fall, bulge size and escalation factors, bulge slope and slope fall time), a criterion for estimating the time of the actual collapse is developed and shown to agree well with laboratory experimental results. [ABSTRACT FROM AUTHOR]

Fracture-induced physical phenomena allow a real-time monitoring of damage evolution in rocks induced by mechanical loading. Some of these phenomena, e.g. plastic deformation, changes in permeability and electric resistivity, provide an overall estimate of damage as a function of stress. Others, such as acoustic emission or electromagnetic emission, allow an exact location of cracks in space and time. Being nondestructive, monitoring techniques based on fracture-induced physical phenomena, are a basis for prediction of earthquakes and rockbursts in underground mines. They are also crucial for a fundamental understanding of rock fracture mechanisms and for developing constitutive models of rock behaviour. Memory effects take place in rocks subjected to cyclic loading with the peak stress (strain) value increasing from cycle to cycle. The effects are represented by certain characteristic changes in rock properties and parameters that are observed when the stress (strain) reaches its previous peak value. Potential applications of memory effects range from stress measurement techniques to earthquake prediction to damage surface scanning. Essential experimental and theoretical results on fracture-induced physical phenomena and memory effects in rocks are considered in the article. Possible applications as well as not-yet-clear points are discussed. [ABSTRACT FROM AUTHOR]

We study the acoustic emission produced by micro-cracks using a two-dimensional disordered lattice model of dynamic fracture, which allows to relate the acoustic response to the internal damage of the sample. We find that the distributions of acoustic energy bursts decays as a power law in agreement with experimental observations. The scaling exponents measured in the present dynamic model can related to those obtained in the quasi-static random fuse model. [ABSTRACT FROM AUTHOR]

The cracking behaviour of Cr 3 C 2 –25(Ni20Cr) and WC–20Cr 3 C 2 –7Ni thermally sprayed coatings during tensile load in 3-point bending tests was studied by Acoustic Emission (AE) monitoring and microstructure post-analysis. The AE monitoring reveals a superior resistance against cracking in the WC–Cr 3 C 2 –Ni coatings compared to Cr 3 C 2 –NiCr. The incorporation of tungsten carbides beneficially affects the residual stress state of the coatings and has an impact on the detailed fracture mode. The results hold for both as-sprayed as well as ground and polished coatings. [ABSTRACT FROM AUTHOR]