Cumulative Acoustic Emission Hits Research Articles

Compressive Fracture Behavior and Acoustic Emission Characteristics of Sandstone under Constant Crack Water Pressure

Abstract Engineering rock containing flaws or defects under a large water source is frequently subject to the couple influence of constant crack water pressure and geostress. To investigate the fracture behavior of precracked rock under hydromechanical coupling with constant crack water pressure, compression tests were conducted on red sandstone specimens containing a single crack of different angles using a device to realize the constant crack water pressure during loading, and the failure process of rock specimens was monitored by acoustic emission (AE) technique. The results show that the presence of constant crack water pressure has a significant promotion effect on the development of shear wing cracks, and the promotion effect is influenced by the prefabricated crack angle and water pressure. As the constant crack water pressure increases, the failure mode of the 0° precrack specimen changes from “X”- shear failure to the single oblique shear failure along the shear wing crack direction, the main failure crack of the inclined precracked specimens (precrack angles of 15°, 45°, and 60°) changes from a small acute angle with the prefabricated crack to a direction along the shear wing crack, and irregular cracks occur at the chipped prefabricated crack in the 90° precracked specimen. With an increase in the constant crack water pressure, the average energy for a single hit, cumulative AE energy, and cumulative AE hits decrease, and the proportion of the tensile cracks increases and that of the shear cracks decreases.

Damage evaluation and failure mechanism analysis of axially compressed square concrete-filled steel tubular columns by acoustic emission techniques

Concrete-filled steel tubular (CFST) columns are widely used as major load-bearing components in structures due to their excellent load-bearing properties. However, the presence of steel tube makes it difficult to accurately assess the damage level of concrete by traditional mechanical measurements alone. This paper investigated the efficiency of acoustic emission (AE) technique in assessing the damage characteristics of the concrete in CFST columns. The damage properties of four axially compressed square CFST columns with the widths of 500–1000 mm and the width-to-thickness ratios of 40 and 50 were analyzed through AE signals. According to the AE results, the axial compression progress of specimens could be divided into five stages. Then, through the correlation analysis, the cumulative AE energy, amplitude, and hits were found to be linear related to the applied load. Finally, the damage percentage of each specimen at different stages was discovered to be influenced by width and width-to-thickness ratio.

Experimental investigation of the tensile behavior and acoustic emission characteristics of anisotropic shale under geothermal environment

Shale gas is becoming a top priority for future global energy consumption and climate change mitigation, and deep and ultradeep shale gas extraction cannot overlook the influence of the geothermal environment. This paper experimentally investigated the tensile behavior and acoustic emission (AE) characteristics of anisotropic shale under geothermal environment of 25–200 °C. The results indicate that the tensile strength and absorbed energy of all specimens decreased with increasing temperature, and minimum values occurred in specimens with a layer inclination of 90° at each temperature. The evolution of tensile strength and absorbed energy versus the layer inclination did not vary with increasing temperature, exhibiting an inverted-V shape. Anisotropic failure mode of specimens with layer inclinations of 30°, 45° and 60° was influenced by the increasing temperature, but central tensile fracturing in specimens with layer inclinations of 0° and 90° was temperature-independent. With increasing temperature, the cumulative AE hits of specimens showed a highly obvious step-up trend, and anisotropic crack initiation stress levels obtained via the AE definition significantly varied. Finally, microstructure response, fracture length and fracture location and proportion of fracture types were discussed, and potential applications and challenges of geothermal environment in hydraulic fracturing of deep shale reservoirs were proposed.

Effects of water on mechanical behavior and acoustic emission characteristics of coal in Brazilian tests

An in-depth understanding of how water affects mechanical behavior of coal and fracture failure modes is important for evaluating the stability of underground engineering. In this paper, Brazilian splitting tests are carried out on coal samples with different moisture contents (0%, 1.10%, 2.20%, 3.29% and 4.41%), and acoustic emission (AE) signals are collected simultaneously. The fractal dimension of splitting surfaces is calculated by the cubic covering method to quantitatively describe characteristics of specimens. Combined with two microscopic imaging technologies, i.e. computed tomography (CT) technology and scanning electron microscopy (SEM), the mechanism of crack propagation and strength degradation in coal samples under the effect of water is discussed. Experimental results present that moisture contents change the mechanical strength, AE signals and failure mode of coal samples. As moisture contents increase, the tensile strength of coal samples reduces exponentially, the plastic behavior increases, while the wave velocity increases logarithmically. In the perspective of AE signals, the maximum R value (ratio of cumulative AE hits to cumulative AE energy) increases with the increase of moisture contents, indicating that specimens under higher moisture content conditions can induce more low-energy events, and more complicated crack network. After the splitting failure in coal samples, the fractal dimension of splitting surfaces increases logarithmically with the growth of moisture contents. Based on the macroscopic and mesoscopic characterization results such as AE signals, CT and SEM, it is considered that the existence of water reduces the strength of pre-existing weaknesses (beddings and cleats) in coal samples. As a result, the splitting surface develops along pre-existing weaknesses, which increases the fractal dimension of splitting surfaces and reduces the intensity of AE signals.

Thermal effect on the fracture behavior of granite using acoustic emission and digital image correlation: An experimental investigation

The irreversible deterioration of bearing rocks via the thermal effect is a severe challenge faced by underground engineering. A comprehensive understanding of the fracture behavior of rocks subjected to high-temperature treatments can contribute significantly to engineering infrastructure design and safety. We conducted three-point bending tests on semi-circular bend granite samples exposed to high-temperature treatments. Complementary acoustic emission (AE) monitoring and digital image correlation analyses were used to examine the following: (i) AE timing response characteristics associated with microcracks initiation and propagation; (ii) acquired AE waveform frequency and shape parameters, which are closely related to the microcracking scale and mode; and (iii) fracture parameters, including the fracture process zone (FPZ) and crack opening displacement. The in-depth reasons for changes in the fracture behavior were then investigated. According to the results, the decreasing trend of multifractal index Δα indicates that high-temperature treatments reduce the distribution difference in the value of AE counts and make microcracks with relatively low energy more prone to be generated. Above heating temperature of 600 °C, microcracking activities tend to be low frequency and high amplitude and shear fracturing gradually becomes dominant. Observations show that the cohesive stress in granite is remarkably reduced with the elevated thermal damage, and the accumulated elastic energy becomes easier to dissipate. This promotes FPZ development and makes granite more prone to cracking and opening. Moreover, the evolution of crack-opening displacement of granites before the peak load is highly correlated with cumulative AE hits, with correlation coefficients ranging from 0.922 to 0.997. Finally, it is found that the equivalent crack length should be adopted in the evaluation of fracture toughness, and the fracture toughness of lower-heat-treated granites is more closely linked to the FPZ length.

Performance of Expanded Polystyrene (EPS) sandwiched concrete panels subjected to accelerated corrosion

Expanded Polystyrene (EPS) Sandwiched Concrete Panel is economical and light weight construction material which functions as load bearing as well as insulating member. EPS sandwiched concrete panels are advantageous in rapid construction of houses as well as saving of materials. This experimental study attempts to assess the out-of-plane behavior (four-point flexural bending) and performance of EPS sandwiched concrete panels subjected to varying degrees of corrosion using impressed current corrosion technique. EPS panels with dimensions of 1000 × 600 × 130 mm (Length × Width × Thickness) with welded wire mesh of size 50 × 50 mm as wythe reinforcement were corroded to different levels and tested under a monotonically increasing load. The mode of failure and behavior of corrosion damaged EPS sandwiched concrete panels has been investigated in terms of resultant strength, stiffness, ductility and composite action. The experimental study shows a decrease of 6.98%, 13.93%, 20.62%, and 60.45% in the ultimate flexural load carrying capacity after exposure to 5, 10, 15, and 20 days of accelerated corrosion, respectively. The ductility of panels evaluated using Ductility Factor (DF) shows a decrease up to 85.01 % in highly corroded EPS panel. Along with destructive flexural testing, non-destructive technique of Acoustic Emission (AE) monitoring was also attempted during flexural testing using surface mounted AE sensors to correlate to the performance of corroded EPS panels. AE parameters of cumulative AE hits and their amplitudes could easily relate to the extent of damage experienced with increasing levels of corrosion.

Evaluation of damage in GFRP repaired steel fiber reinforced concrete beams using acoustic emission technique

AbstractIn the present study, monitoring of damage initiation and progression in steel fiber reinforced beams (SFRBs) subjected to half cyclic loading under flexure is attempted using acoustic emission (AE) technique. Variation in AE parameters of cumulative AE hits and acoustic energy with damage progression in these beams along with their flexural performance is studied and compared. Further SFRB beams failed to 90% damage level are repaired using GFRP wraps and are subjected to flexural loading and simultaneous AE monitoring. Variation in AE parameters along with load deflection behavior for control SFRB and retrofitted SFRB beams have been discussed and compared. Based on AE and load–deflection data, Damage Index values were calculated and Damage States have been proposed which can help in providing an early warning of failure proximity. Subsequently empirical relationships have been developed between Damage Index and AE parameters using genetic programming (GP) technique and are in excellent coherence with experimental results. It can be concluded from the study that AE analysis is an effective tool for damage monitoring in SFRB and GFRP repaired SFRB beams.

Damage progression in RC beam‐column joints under cyclic loading using acoustic emission technique

AbstractBeam‐column joint (BCJ) is the most critical element of a RC structure and plays an important role in the sound seismic performance of buildings. A number of joint failures have been reported in the literature and it is important to monitor the BCJ regularly to assess their performance so that the damage is predicted much before it reaches a catastrophic level. This study aims to monitor the initiation and progression of damage in BCJ reinforced with and without ductile reinforcement detailing under cyclic loading using a real‐time passive acoustic emission (AE) technique. Results have been analyzed on the basis of AE parameters such as cumulative AE hits, cumulative energy, rise angle, average frequency, and Ib‐value. The Ib‐value analysis has demonstrated good correlation with the experimental observations and it has been established that macroscopic fracture occurs in a RC BCJ during loading. The AE event map successfully gives pictorial representation of the crack development inside the concrete and crack pattern plotted by the AE event maps gives an idea of presence of cracks near the damage area of BCJ. It is also possible to distinguish between tensile and shear crack developing at the joint by using AE data. A linear relationship has been observed between the AE energy and energy dissipated by BCJ to evaluate damage indices in seismically damaged structures. The AE energy gives better representation of quantum of damage than AE hits.

Crack initiation stress of brittle rock with different porosities

Accurately determining the crack initiation stress (CI) is of great importance to evaluate the stability of deep underground openings and the permeability of host rock in the near field. For this paper, uniaxial compression tests and increasing amplitude cyclic stressing tests were performed on four kinds of rocks with different porosities. The stress-strain data and acoustic emission (AE) data of these tests are analyzed to evaluate the applicability and accuracy of the existing methods for determining the CI. The results demonstrate that for marble and Beishan granite with low porosity, the lateral strain (LS) method is the simplest and most direct method to obtain the CILS. For high porosity sandstone, the result of lateral strain response (LSR) method is significantly smaller than that obtained by AE. The cumulative AE hits (CAEH) method heavily depends on the “S-shaped” characteristic of the cumulative AE hits curve, which limits its scope of application. The cumulative AE hits curve slope (CAHS) method, which determines the key inflection point of the cumulative AE hits curve by slope variation to determine the CICAHS, possesses the widest applicability and highest accuracy. The CI/UCS ratios of rocks with similar grain sizes decreases with increasing porosity. For Beishan granite, the average CICAHS is less than that of CILS. They are 0.43 UCS and 0.53 UCS, respectively. According to the residual strain data from the cyclic loading tests, the two crack initiation thresholds of Beishan granite can be clearly identified. These two crack initiation thresholds represent different crack initiation sequences in grain boundaries and different minerals.

Assessment of bond-slip behavior of hybrid fiber reinforced engineered cementitious composites (ECC) and deformed rebar via AE monitoring

This paper investigates the efficiency of acoustic emission (AE) technique for monitoring and assessing the bond-slip behavior and damage characteristics of deformed rebar embedded in hybrid fiber reinforced engineered cementitious composites (ECC). A series of pull-out tests were conducted on 48 prism samples with variable rebar diameters (14 mm, 20 mm, and 25 mm), embedded length (3d, 4d and 5d), and cover thickness (2.5d, 4d, 5d and 6d). Two AE sensors, attached on ECC surface and rebar surface respectively, were utilized to critically monitor the AE activity emitted from the ECC matrix and the interface between ECC and rebar throughout the tests. The bond-slip behavior of ECC-rebar presented a more ductile performance than conventional concrete (CC)-rebar because of fiber bridging effect. The damage classification and damage characteristics were clarified by measuring AE activity. The debonding process of ECC-rebar could be divided into four main stages based on the slope changes of cumulative AE hits, and further refined into several substages based on the sudden rises of AE energy. Besides AE cumulative parameters, the Ib-values calculated from AE signals of ECC matrix and ECC-rebar interface were analyzed and compared, which behaved more effectively in identifying the development of cracks and distinguishing the maximum bond stress. Also, the AE intensity analysis was implemented to identify the micro-mechanism transformation during different bonding stages. Moreover, the AE intensity results of CC-rebar obtained from previous research and ECC-rebar in this study were compared.

Investigation on the mechanical behavior and failure characteristics of fully grouted bolts under tension

ABSTRACT A better understanding of the mechanical behavior and failure characteristics of fully grouted bolts under tension is of great significance to ensure the safe production of coal mines. For this purpose, a pull-out experiment of anchorage specimens was conducted and a numerical simulation was conducted based on the experimental results. The experimental and numerical results were analyzed in detail to study the bearing characteristics of fully grouted bolts. The results indicated the following: (1) The bearing characteristics of the anchorage interface could be divided into four stages. The local strain of the anchorage interface was positively correlated with the change trend of the pull-out load. (2) The acoustic emission (AE) activity showed a tendency to first increase and then decrease during the pull-out process. The cumulative AE energy was the largest in the microcrack development stage, while the cumulative AE counts and AE hits were the largest in the shear failure stage. The AE peak frequency presented obvious divisional centralized distribution characteristics, and the AE amplitudes were mainly in the range of 50–80 dB. The cumulative AE events were positively correlated with the damage degree of the anchorage interface and the duration of AE events increased significantly during the microcrack development stage. (3) There are four main stages in the pull-out bearing process of fully grouted bolts. The failure process of the fully grouted bolts was progressive. The increase of the bonding length not only increased the non-uniformity of the shear stress of the anchorage interface, but it also affected the characteristics of the pull-out load-displacement characteristics. With the increase of the bonding length, the peak pull-out load first increased rapidly and then increased at a relatively low but steady rate. The research results may provide guidance for the support design of fully grouted bolts.

Monitoring the flexural performance of GFRP repaired corroded reinforced concrete beams using passive acoustic emission technique

AbstractThe focus of the research is to evaluate the flexural behavior and performance of glass fiber reinforced polymer (GFRP) repaired reinforced concrete (RC) beams corroded to different levels using acoustic emission (AE) techniques. A significant improvement in the flexural characteristics of the GFRP repaired corroded beams is observed with a change in failure mode from brittle to ductile. The benefits of GFRP repair in corroded RC beams are further clearly demonstrated by AE monitoring parameters of cumulative AE hits and their amplitudes. A significant improvement in the number of AE hits and the reappearance of AE cracking phases after repair of corroded beams indicate the efficiency of AE technique to monitor repaired structures also. A pictorial representation of the development of microcracks beneath the surface of concrete and GFRP to the formation of macrocracks is well demonstrated in an AE event plot. The assessment of local damage in RC beams using AE techniques subjected to simultaneous loading and corrosion and their further repair would facilitate in‐situ nondestructive evaluation of actual RC structures in future for detecting damage beneath the surface even after FRP repair.

Acoustic Emission Monitoring of RC Beams Corroded to Different Levels Under Flexural Loading

The focus of the research is to evaluate the behavior and performance of large-sized reinforced concrete (RC) beams corroded to different levels using local acoustic emission (AE) technique under flexural loading. With increase in the level of corrosion, the ultimate load-carrying capacity and midspan deflections are drastically reduced, which is indicative of increasing brittleness and decreasing ductility in the corroded beams. AE monitoring of corroded RC beams under flexural loading clearly demonstrated various failure progression stages right from initiation of cracking to their ultimate failure. As the level of corrosion increases, a significant drop in cumulative AE hits is observed. AE activity depicted as AE phases is delayed and diminished due to the attenuation of the captured AE signals caused by cracks developed due to corrosion. AE event plots give a pictorial representation of actual development and progression of micro- and macrocracking inside concrete as the corroded beams are subjected to flexural loading. The assessment of damage in RC beams subjected to simultaneous corrosion and loading would facilitate in situ nondestructive evaluation of actual RC structures in the future.

Mechanical Properties and Fracture Behavior of Crumb Rubber Basalt Fiber Concrete Based on Acoustic Emission Technology.

Basalt fiber and crumb rubber, as excellent road material modifiers, have great advantages in improving the mechanical properties and fracture behavior of concrete. Acoustic emission (AE) is a nondestructive testing and real-time monitoring technique used to characterize the fracture behavior of concrete specimens. The object of this paper is to investigate the effects of crumb rubber replacement rate, basalt fiber content and water–binder ratio on the mechanical properties and fracture behavior of crumb rubber basalt fiber concrete (CRBFC) based on orthogonal test. The fracture behavior of a CRBFC specimen under three-point flexural conditions was monitored by AE technology and the relative cumulative hit (RCH) was defined to characterize the internal damage degree of CRBFC. The experimental results showed that, considering the mechanical strength and fracture damage behavior of CRBFC, the optimal crumb rubber replacement rate, basalt fiber content and water–binder ratio are 10%, 2 kg/m3 and 0.46, respectively. In addition, it was found that AE parameters can effectively characterize the fracture behavior of CRBFC. The fracture stages of CRBFC can be divided according to the cumulative AE hits and counts. AE amplitude value can be used as an early warning of CRBFC specimen fracture. Moreover, the fracture mode can be identified by RA and average frequency (AF) values variation during the loading process.

Predicting compressive stress‒strain curves of structural adobe cubes based on Acoustic Emission (AE) hits and Weibull distribution

Purpose The purpose of this paper is to provide a method to predict the situation of a loaded element in the compressive stress curve to prevent failure of crucial elements in load-bearing masonry walls and to propose a material model to simulate a compressive element successfully in Abaqus software to study the structural safety by using non-linear finite element analysis. Design/methodology/approach A Weibull distribution function was rewritten to relate between failure probability function and axial strain during uniaxial compressive loading. Weibull distribution parameters (shape and scale parameters) were defined by detected acoustic emission (AE) events with a linear regression. It was shown that the shape parameter of Weibull distribution was able to illustrate the effects of the added fibers on increasing or decreasing the specimens’ brittleness. Since both Weibull function and compressive stress are functions of compressive strain, a relation between compressive stress and normalized cumulative AE hits was calculated when the compressive strain was available. By suggested procedures, it was possible to monitor pretested plain or random distributed short fibers reinforced adobe elements (with AE sensor and strain detector) in a masonry building under uniaxial compression loading to predict the situation of element in the compressive stress‒strain curve, hence predicting the time to element collapse by an AE sensor and a strain detector. In the predicted compressive stress‒strain curve, the peak stress and its corresponding strain, the stress and strain point with maximum elastic modulus and the maximum elastic modulus were predicted successfully. With a proposed material model, it was illustrated that the needed parameters for simulating a specimen in Abaqus software with concrete damage plasticity were peak stress and its corresponding strain, the stress and strain point with maximum elastic modulus and the maximum elastic modulus. Findings The AE cumulative hits versus strain plots corresponding to the stress‒strain curves can be divided into four stages: inactivity period, discontinuous growth period, continuous growth period and constant period, which can predict the densifying, linear, non-linear and residual stress part of the stress‒strain relationship. By supposing that the relation between cumulative AE hits and compressive strain complies with a Weibull distribution function, a linear analysis was conducted to calibrate the parameters of Weibull distribution by AE cumulative hits for predicting the failure probability as a function of compressive strain. Parameters of m and θ were able to predict the brittleness of the plain and tire fibers reinforced adobe elements successfully. The calibrated failure probability function showed sufficient representation of the cumulative AE hit curve. A mathematical model for the stress–strain relationship prediction of the specimens after detecting the first AE hit was developed by the relationship between compressive stress versus the Weibull failure probability function, which was validated against the experimental data and gave good predictions for both plain and short fibers reinforced adobe specimens. Then, the authors were able to monitor and predict the situation of an element in the compressive stress‒strain curve, hence predicting the time to its collapse for pretested plain or random distributed short fibers reinforced adobe (with AE sensor and strain detector) in a masonry building under uniaxial compression loading by an AE sensor and a strain detector. The proposed model was successfully able to predict the main mechanical properties of different adobe specimens which are necessary for material modeling with concrete damage plasticity in Abaqus. These properties include peak compressive strength and its corresponding axial strain, the compressive strength and its corresponding axial strain at the point with maximum compressive Young’s modulus and the maximum compressive Young’s modulus. Research limitations/implications The authors were not able to decide about the effects of the specimens’ shape, as only cubic specimens were chosen; by testing different shape and different size specimens, the authors would be able to generalize the results. Practical implications The paper includes implications for monitoring techniques and predicting the time to the collapse of pretested elements (with AE sensor and strain detector) in a masonry structure. Originality/value This paper proposes a new method to monitor and predict the situation of a loaded element in the compressive stress‒strain curve, hence predicting the time to its collapse for pretested plain or random distributed short fibers reinforced adobe (with AE sensor and strain detector) in a masonry building under uniaxial compression load by an AE sensor and a strain detector.

Experimental investigation of concrete fracture behavior with different loading rates based on acoustic emission

To study the effect of loading rate on the fracture behavior, three-point bending fracture tests of concrete with loading rates of 0.0001 mm/s, 0.001 mm/s, 0.01 mm/s and 0.1 mm/s were carried out, respectively. And acoustic emission (AE) technology was adopted for real-time monitoring. The results show that the unable toughness of concrete has obvious rate effect. Meanwhile, there are two obvious inflection points in the curve of cumulative AE hits and the cumulative ringing count with the change of time, one of which may represent the starting point of concrete boundary effect. The number of AE events can represent the crack width of concrete fracture. It is found that with the increase of loading rate, the crack width and ductility of concrete decrease, while the quantity of shear crack of concrete increases. By comparing the fracture energy of concrete with the AE cumulative energy, both of them increase with the increase of loading rate, indicating that the AE cumulative energy can represent the change of fracture energy under different loading rates. Finally, the failure forms under different loading rates are analyzed by the change of b value. Based on the above research, AE technology can be used to study the fracture failure under low loading rate.

Structural health assessment of prefabricated concrete elements using Acoustic Emission: Towards an optimized damage sensing tool

Abstract The design feasibility and structural integrity of prefabricated construction elements is assessed (concrete cantilever slab carrying thermal insulation break reinforced with steel/basalt rebars and ultra-high-performance concrete) by performing incremental bending tests. The developed damage mechanisms govern failure, therefore their identification appears crucial: concrete cracking, rebars-concrete interface debonding, rebars slippage, concrete crush under compression. However, the complex fracture cannot be effectively characterized by measuring the deterioration of the mechanical properties. In this study, Acoustic Emission (AE) is applied to highlight internal damage progress. It is found based on cumulative AE hits distribution under incremental load and considering the Kaiser and Felicity effects, damage initiation is identified. The damage severity is quantified using Load/Calm ratios. AE sources localization identifies the moment at which the rebars plastically deform; debond from concrete; slip and when concrete cracks and crushes under compression. The damage mechanisms are discriminated based on acoustic signal features distribution as damage evolves (specifically, average frequency and the ratio of rise time over amplitude). It is shown that AE appears an essential and accurate monitoring method that fully characterizes the damage on complex structures. The promising results open new horizons towards the design of fully autonomous prefabricated elements carrying a smart on-line AE sensing tool.

Study on Bending Damage Constitutive Model and Mechanical Properties of Limestone Based on Acoustic Emission

To reveal the mechanical characteristics and damage evolution mechanism of limestone in the bending process, the cumulative acoustic emission (AE) hits were used to define the damage variable, and the rock microbody hypothesis and the Weibull distribution function were applied to further improve the damage variable. Meanwhile, the bending damage constitutive model of limestone under three‐point bending was developed based on the Lemaitre strain equivalence principle and the continuum damage theory. Then, the three‐point bending test with acoustic emission monitoring was carried out to verify the rationality and validity of the model. Results showed that the modified damage variable D had an exponential distribution with the strain ε, and the damage was mainly concentrated in the macrocrack propagation stage. Moreover, the bending neutral layer moved towards the compressive zone in the bending damage process. The bending neutral layer, furthermore, moved slowly a small distance at the initial stage of bending fracture but moved fast a long distance at the end stage of bending fracture. In addition, the bending damage constitutive model could be quantitatively expressed by the cumulative AE hits Np, the stress σ, the strain ε, and Young’s modulus E. The theoretical stress‐strain model curves agreed well with experimental results, which demonstrated that the proposed model could capture the damage evolution of limestone reasonably in the bending process.

Investigation of Damage Processes of Mortar in Different Aggregate Size by Acoustic Emission Characteristics

In order to study acoustic emission (AE) characteristics of damage process of mortar in different aggregate size, mortar specimens were tested under axial compression in the experiment, the whole damage evolution processes were monitored by full-digital AE acquisition system. The effects of mortar in different aggregate size on damage processes and failure mode were conducted by cumulative AE energy, amplitude, peak frequency and the ratio N/E (hits/energy) respectively. The ratio of cumulative AE hits of each frequency band and amplitude band to the whole cumulative AE hits in different stress level were counted, and the difference and evolution process of microcracks of mortar in different aggregate size in the failure process were revealed. The results show that cumulative AE energy increase with the increase of aggregate size in mortar. Distribution of cumulative AE hits of each amplitude band versus the stress in different aggregate size are the same, and AE hits of each amplitude band increase sharply suggests that the specimen will be destroyed. Besides, AE hits focus on low-frequency band at low stress level, while all peak frequency bands are active at high stress level, and the aggregate size of mortar have significant influence on cumulative hits of each peak frequency band. The ratio N/E decreased with the increase of aggregate size, and which increased linearly with the increase of the stress.

Prediction of Fatigue Crack Growth in Gas Turbine Engine Blades Using Acoustic Emission.

Fatigue failure is the main type of failure that occurs in gas turbine engine blades and an online monitoring method for detecting fatigue cracks in blades is urgently needed. Therefore, in this present study, we propose the use of acoustic emission (AE) monitoring for the online identification of the blade status. Experiments on fatigue crack propagation based on the AE monitoring of gas turbine engine blades and TC11 titanium alloy plates were conducted. The relationship between the cumulative AE hits and the fatigue crack length was established, before a method of using the AE parameters to determine the crack propagation stage was proposed. A method for predicting the degree of crack propagation and residual fatigue life based on the AE energy was obtained. The results provide a new method for the online monitoring of cracks in the gas turbine engine blade.