Acoustic Emission Energy Rate Research Articles

Damage assessment of concrete via acoustic emission and mesoscopic damage constitutive model

This paper experimentally investigated the acoustic emission (AE) behavior of concrete specimens. By using the MTS810 servo hydraulic testing system and the multi-channel AE system, a series of specimens graded C40, C50, and C60 were loaded under uniaxial compression. The spatial distribution of the AE events, AE events rate, and AE energy rate were recorded. Additionally, a theoretical mesoscopic damage constitutive (MDC) model, which defines the theoretical mesoscopic damage variable as the ratio of the fractured micro-elements to the total micro-elements, was introduced. The results indicated that the spatial distribution of AE events resembled the shape of failure modes. Moreover, the theoretical mesoscopic damage calculated by the MDC model matched well with the observed experimental damage, represented by the AE events. This association not only related the theoretical mesoscopic damage with experimental macroscopic damage, but also revealed that the macroscopic damage of concrete stemmed from mesoscopic fracture. Additionally, the AE energy was related both to the energy of fractured micro-elements and its derivative in the MDC model.

Critical slowing down phenomenon for predicting the failure of solid rocks and cement mortar materials: Insight from acoustic emission multiparameters

Failure precursors with clear physicomechanical connotations are of great significance in predicting catastrophic instability in major constructions and rock engineering. Here, we conducted uniaxial compression tests on natural solid rocks and cement mortar materials and obtained their acoustic emission (AE) activity characteristics during the damage evolution to explore the feasibility of AE parameters in characterizing the failure precursory characteristics of building materials. Based on the critical slowing down theory, the curves of the variance and autocorrelation coefficient for AE multiparameters were analyzed. The results show that the sudden increase in the variance and autocorrelation coefficient of AE multiparameters can be used as primary and secondary criteria, respectively, for the identification of failure precursors. Their physicomechanical connotations were clear, characterizing the dominance of shear failure modes and the onset of accelerated fracture. The discriminability of failure precursors characterized by the critical slowing down of the AE energy rate, RA value, and RA/AF value was higher than that characterized by the critical slowing down of the cumulative AE ring count and peak frequency. Moreover, we found that the building materials with high brittleness, strength, and heterogeneity present a more significant critical slowing down phenomenon. In the view of early-warning time, the occurrence of precursory features identified by autocorrelation coefficient was earlier than the occurrence of precursory features identified by variance.

Experimental study on the effect of anchors on the creep mechanical properties and acoustic emission characteristics of fractured sandstone under uniaxial compression

This study conducts laboratory tests to investigate the creep and acoustic emission (AE) of unanchored and anchored fractured sandstone samples under uniaxial compression by taking the Triassic fine sandstone collected from the Xiaolangdi Reservoir area as the research object and ensuring the same testing conditions using an RLJW-2000 rock rheometer and a PCI-Ⅱ AE instrument. Moreover, this study identified the effects of anchors on the creep mechanics and AE characteristics of fractured sandstone samples by analyzing the strain, long-term strength, AE event count, AE rate, AE energy rate, and rising angle – average frequency (RA-AF) characteristics of each sample before and after anchoring. According to the study results: (1) Under the same stress level, the anchored sample's instantaneous strain, creep strain, and total strain are smaller than those of the unanchored sample. Anchors exerted the largest inhibitory effect on creep strain, followed by the total strain, while anchors inhibited instantaneous strain to the smallest extent. (2) Compared to the unanchored sample, the anchored sample experienced a significant increase (as high as 50%) in long-term strength. Anchor reinforcement significantly improves the long-term strength of samples. (3) Under the same stress level, the AE event count and cumulative AE event count of the anchored sample are smaller than those of the unanchored sample. Anchors significantly inhibited microcracks initiation, propagation, overlapping, and penetration in samples. (4) The maximum AE rate of the anchored sample is only 69.30% of that of the unanchored sample, whereas the peak AE energy rate of the anchored sample is only 23.21% of that of the unanchored sample. Anchors reduce the AE event count generated and the AE energy released by samples per unit of time, exerting a significant crack-arrest effect on them. (5) The presence of anchors changes the type and number of microcracks in the samples' creep process. Anchors greatly inhibit the generation of high-RA and low-AF AE signals and generate low-RA and high-AF ones. As a result, more tensile microcracks occur in samples under stress levels close to the failure stress level (especially the last stress level), which inhibit the formation of shear microcracks in samples and causes samples to be more prone to macroscopic tensile-shear failure. (6) The microcrack characteristics of samples characterized by RA-AF values are consistent with the macrocrack characteristics of samples. Therefore, RA-AF values can satisfactorily characterize the microcracks' variation characteristics in the sandstone samples' creep process before and after anchoring.

Machine learning‐based approach for fatigue crack growth prediction using acoustic emission technique

AbstractIn this study, a general machine learning‐based approach is proposed for fatigue crack growth rate (FCGR) prediction using multivariate acoustic emission (AE) online monitoring data. To improve the prediction accuracy, a backpropagation neural network optimized by genetic algorithm (GA‐BPNN) is developed to describe the intricate link between the FCGR and multivariate input data. Several conventional machine learning models and the traditional FCGR prediction method based on the linear relationship between AE energy rate and FCGR are also used to examine the effectiveness of the proposed GA‐BPNN model. The results indicate that the developed GA‐BPNN exhibits higher accuracy and superior adaptability in predicting FCGR from unseen data than other methods. The findings of this study will provide a strategy for developing and optimizing a machine learning solution for FCGR prediction based on AE monitoring data and also aid in determining the most suitable feature for AE monitoring studies.

Study on the effect of different angles on the propagation characteristics of acoustic emission signals in wood.

To explore the propagation characteristics of acoustic emission (AE) signals in wood at different angles. The AE signals at different angles were obtained by changing the angle of incidence by sawing the inclined surfaces at different angles. The Zelkova schneideriana specimen was sawn 5 times with an increment of 15°, and 5 different incidence angles were obtained. AE signals were collected by 5 sensors arranged equally on the surface of the specimen, AE energy and energy attenuation rate were calculated. The reflection signals corresponding to different angles were collected on the uncut specimen by changing the position of the sensors, and the propagation speed of the AE signals at different angles was calculated. The results showed that the kinetic energy provided by the external excitation is small, and the AE energy is mainly provided by the displacement potential energy. With the change of the incidence angle, the AE kinetic energy changes significantly. With the increase of the reflection angle, the speed of the reflected wave also continued to increase, and finally stabilized at 4600 m/s.

Early properties and modeling of cemented superfine tailings backfill containing sodium dodecyl sulfate: Microstructure, mechanics, and acoustics

Primary microcracks exist within brittle materials such as cemented superfine tailings backfill (CSTB), and these microcracks control the damage mechanism and determine the structural strength of the brittle material. The early damage evolution mechanism and the constitutive model of CSTB under the consideration of initial defects are analyzed and discussed from macroscopic and microscopic viewpoints by using acoustic emission (AE), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) techniques. It is demonstrated that AEA inclusion can slow the hydration reaction inside the CSTB, reducing the number of hydration products and hence the density of the CSTB's internal structure. The amount of AEA incorporated was correlated with the elastic modulus, and the initial damage to the CSTB was quantified as a result. As the initial damage increases, the effective contact area inside the CSTB decreases, and the damage form of the CSTB changes from predominantly tensile damage to predominantly flaking areas in patches, accompanied by a weakening of the AE signal. Analyzed from the perspective of the AE energy rate, the incorporation of AEA reduces the energy storage capacity of CSTB, and with the increase in AEA content, it further leads to a sharp decrease in the energy released from CSTB. The early damage constitutive model of CSTB with different initial damage proposed in this paper agrees well with the experimental data and can provide a reference for the design of CSTB in structural filling.

Wavelet and fractal analysis of acoustic emission characteristic of fatigue damage of asphalt mixtures

Fatigue cracking of asphalt materials directly affects the performance and safety of asphalt pavements. It is necessary to identify the materials condition with acoustic emission (AE) as the real time monitoring technique. The objective of this study was to diagnose the damage behavior of asphalt mixtures from the time and frequency characteristics of AE signals. Firstly, the semi-circular bending (SCB) fatigue test and AE detection were conducted to explore the AE response of asphalt mixtures subjected to cyclic loads. Then, the Fast Fourier Transform (FFT) and Continuous Wavelet Transform (CWT) were applied to discuss the dominant frequency characteristics and time–frequency distribution of AE waveforms. Finally, an index (RBD), defined as the ratio of fractal dimension of AE waveforms decomposed by the Discrete Wavelet Transform (DWT), was proposed to evaluate the damage state of asphalt mixtures. The results show that AE energy rate exhibits distinct variation signatures at different damage stages. The average frequency (AF) and rise angle (RA) of AE signals reveal that the coupling effect of tensile and shear cracks promotes the deterioration of asphalt mixtures. Most of the dominant frequencies of AE events generated from cracks are concentrated within the range of 13 to 30 kHz. The discrepancies in the time–frequency distribution of AE events could effectively discriminate the types of damage sources. The proposed index RBD is sensitive to the change of damage state of asphalt mixtures, producing corresponding reductions.

Influence of temperature on the transformation and self-control of energy during sandstone damage: Experimental and theoretical research

The surrounding rock in tunnelling engineering and coal mining will sometimes be exposed to high temperature. Rock failure is the result of energy dissipation, and the study of the evolution and transformation behavior of energy is of great significance for the in-depth understanding of the deformation and failure of rock after high temperature. This study analyzed the intrinsic connection between mechanical properties and acoustic emission (AE) energy under temperature effect. Based on the energy dissipation and release theory, the distribution and transformation of energy was analyzed, and the energy self-incentive and self-inhibition (EII) model was further established. The main findings are that temperature effect affects the confidence interval trend and the value of AE energy rate, which is related to the change in mechanical properties. The ability of sandstone to store elastic strain energy after exposure to high temperature is independent of the mechanical properties. In this study, the ratio of dissipated energy to elastic energy is used to characterize the stable state of the sandstone system, which can be used as the energy indicator of rock failure precursor. During the absorption, storage, and release of energy before the peak stress, there exists self-incentive and self-inhibition of energy (control behavior).

Study on Mechanical and Acoustic Emission Characteristics of the Influence of Rock Bridge Length on Rock Failure

Abstract Rock bridge of jointed rock plays a key role in its instability failure and has great influence on underground engineering. In this paper, the direct shear tests of jointed rocks with different rock bridge lengths are carried out, and the mechanical properties, acoustic emission (AE) characteristics, and fracture mechanism during the failure of rock bridge are studied. The results show that the shear strength increases with the increase of normal stress. When the rock bridge is long, cohesion c plays a leading role to control the failure of jointed rock; on the contrary, the internal friction angle φ plays a leading role. AE count rate, cumulative AE counts, and peak AE energy rate increase with the increase of rock bridge length. With the increase of rock bridge length, peak AE energy rate and AE count rate occur earlier. Considering the influence of rock bridge length, the fracture criterion and crack growth angle of shear fracture are derived based on linear elastic fracture mechanics. The theoretically calculated result is consistent with the experimental result. The research in this paper can provide relevant theoretical basis and useful reference for the instability and failure analysis of jointed rock.

Failure behaviors of fractured granite during loading and unloading under high confining pressure based on acoustic emission multi-parameter analysis

To investigate failure and mechanical behaviors of fractured rocks during loading and unloading under conditions of high ground stress, a series of loading and unloading tests were conducted on granite containing prefabricated fractures under high confining pressure. Results indicate that the evolution of acoustic emission (AE) events of fractured granite during loading and unloading under high confining pressure is strongly correlated with the deformation and failure and the energy evolution process in the rock, the peak values of AE cumulative energy and AE cumulative count rate in the pre-peak deformation stage decrease first and then increase with the dip angle (the minimum value is reached at 60°), when the dip angle is 0° to 60°, the number of AE events increases abruptly due to closure of the prefabricated fracture in the early stage of loading(only about 30 % of the main fracture time). After the fractured granite enters the unstable crack propagation stage, the crack tips are subject to a dynamic cycle of stress concentration, crack propagation, and stress release, accompanied by significant fluctuations in the energy rate before and after the point of peak AE energy rate. Therefore, the cracks propagate in an intermittent or a stepped manner. Under the unloading path, the process of crack development, gathering, and coalescence in the granite happens earlier, and there are more high-energy AE events in the failure process. The value of the brittleness index Bd is larger under the unloading path under high confining pressure, which reflects more significant brittle failure characteristics. The research provides a basis for judging failure precursors of rocks using AE characteristics in the failure process.

Investigation of the crack and acoustic emission behavior evolution of red sandstone subjected to water

To explore the influence of water on the crack and acoustic emission (AE) behavior evolution of rock specimen during the deformation and failure, uniaxial compression tests with variable water contents (dry, natural, and saturated state) on red sandstone specimens were systematically conducted in the laboratory. Based on real-time AE monitoring technology and scanning electron microscope (SEM) technology, the crack evolution mechanism in the rock specimen during the deformation and failure was studied. The results indicate that the peak strength of red sandstone in the natural and saturated state is 26.13% and 41.75% lower than that of red sandstone in the dry state. With the increase of water content, the dominant failure pattern of rock specimen changes from tension failure to shear failure. According to the variable characteristics of the AE events rate, the deformation and failure process of rock specimens with the change of water contents can be divided into four stress stages. Moreover, the relationships among the AE events rate, peak frequency, water content and different stress stages were discussed. Approaching to the peak stress, there is an obvious trend of relative decrease of the Ib-value and the phenomenon of “sharp increase” and “sharp drop” in the average AE energy rate. The above-mentioned variable characteristics of AE parameters can be used as a precursor for the instability and failure of rock specimens with different water contents.

Exploring the Effect of Asperity Order on Mechanical Character of Joint Specimen from the Perspective of Damage

The joint morphology is multiscale. The effect of each asperity order on the mechanical properties of joints is different. The shear mechanical properties of joint specimens are related to its surface damage characteristics. At present, there are still few studies on the effect of roughness on the shearing mechanical properties of joint from the perspective of damage of each asperity order. In this paper, the standard roughness profile was chosen as initial morphology. The standard roughness profile was decomposed into waviness and unevenness by the method combine the ensemble empirical mode decomposition (EEMD) and the cut-off criterion. Then, the joint specimen which contains waviness and unevenness and the specimen which only contains waviness were prepared by the 3D engraving technology. The 40 sets of joint specimens with different asperity order were subjected to direct shear tests under different normal stresses. Based on the 3D scanning technology and ICP iterative method, the damaged area and the damage volume were calculated. Based on the damage volume data and the acoustic emission (AE) data, the effect of asperity order to the joint mechanical behaviour was studied. The results indicate that (1) under low normal stress, the unevenness plays a control role in the failure mode of the joint specimen. Under low normal stress, the joint surface containing only waviness exhibits slip failure, and the joint surface with unevenness exhibits shear failure. With the increase of the normal stress, the failure mode of the specimen containing only waviness changes from slip failure to shear failure; (2) the unevenness controls the damage degree of the joint specimen. The damaged area, damage volume, AE energy rate, and accumulative AE energy of the joint specimen with unevenness are larger than those of the specimen with only waviness, and this difference increases with the normal stress increase; (3) the difference between the joint specimen with unevenness and specimen with only waviness mainly exists in the prepeak nonlinear stage and the postpeak softening stage. The characteristic parameters of acoustic emission generated in the postpeak softening stage of the joint specimen with unevenness are greater than those of the specimen with only waviness. This phenomenon can be used to explain the stress drop difference at the postpeak softening stage; (4) the AE b value can be used to evaluate the damage of joint specimens. Analysing the damage difference of each asperity order under different normal stresses is of great significance to the analysis of the influence of the morphology of the joint surface on the mechanical properties of the joint.

Acoustic Emission and Fractal Precursory Characteristics of Coals with Different Bursting Liabilities in Loading Failure Process

Aimed at investigating the differentiation of acoustic emission (AE) signals and fractal precursory characteristics between strong, weak, and no bursting liability coals under uniaxial compression, as well as improving the accuracy of rockburst monitoring and early warning by AE techniques, we experimentally studied the evolution law and differences of AE ring count rate, energy rate, and correlation dimension between different loaded bursting liability coals by the YAW4306 electric mechanical test system and CTA-1 AE monitor. Our experimental results indicated that the AE count and energy of coal samples with different bursting liabilities showed a similar evolution law of “sharp increase-calm-sharp increase” before their main rupture. The active points of AE signals emitted from coal with strong, weak, and no bursting liability appeared at about 85∼90%, 75∼78%, and 51∼55% of the peak stress, respectively. The stronger the bursting liability of coal, the shorter the duration of main rupture and postpeak failure stage, and the greater the AE energy rate in the main rupture. The AE counts of different coals had obvious fractal characteristics, and the AE correlation dimension values of strong and weak bursting liability coal samples presented the phenomenon of “fluctuating rise to a peak value-sharp drop-continuous decrease,” which can be used as a precursory information of coal failure.

A MICROSTRUCTURE-BASED MECHANISTIC MODEL FOR BONE SAWING: PART 2 - ACOUSTIC ENERGY RATE PREDICTIONS

Abstract Part-2 of this paper is focused on modeling the acoustic emission (AE) energy rate as a function of the specific cortical bone microstructures (viz., osteon, interstitial matrix, lamellar bone, and woven bone) and the depth-of-cut encountered by the bone sawtooth. First, the AE signal characteristics from the sawing experiments (in Part-1) are related to the pure haversian and pure plexiform regions of the cut. Using the cutting force predictions from Part-1 as input, the AE energy rate is then modeled in terms of the energies dissipated in the shearing and ploughing zones encountered by the rounded cutting edge. For this calculation, the rounded edge geometry of the sawtooth is modeled as a combination of (i) shear-based cutting from a negative rake cutting tool; and (ii) ploughing deformation from a round-nose indenter. The spread seen in the AE energy rate is captured by modeling the variations in sawed surface height profile, tool cutting edge geometry, and porosity of the bone. The model calibration and validation protocols are similar to those used in Part-1. The validated AE model is useful for process planning both in terms of its ability to predict AE energy rate trends and cutting force variations, based on the differences in the underlying bone microstructures.

Micro- and macrocracking behaviors in granite and molded gypsum containing a single flaw

The effect of preexisting defects on fracture behaviors is significant for the stability analysis of rock engineering. To study the effect of preexisting flaws on the micro- and macrocracking behaviors of rocks, photographic monitoring together with strain field analysis, passive acoustic emission (AE) and active ultrasonic testing were carried out on gypsum and granite samples containing a single flaw. Based on coupled analysis of the acousto-optic-mechanical characteristics, it is suggested to identify micro- and macrocrack initiation by AE monitoring rather than stress–strain behaviors, photographic observation and P-wave measurement. The micro- and macrocrack initiation stresses generally increase with the flaw angle. The microcrack initiation stress ranging from 0.22σc to 0.62σc (peak strength) coincides with the onset of AE events, and the macrocrack initiation stress ranging from 0.60σc to 0.85σc coincides well with a dramatic increase in AE energy for flawed granite. The crack types are classified into nine different categories. Macrocrack coalescence is observed before reaching σc, and violent rock failure occurs seconds after σc is achieved for flawed granites, which differs from that process for gypsum samples. The cracking process of flawed granites experiences three typical stages and is highly dependent on the flaw angle. A stress drop accompanied by a significant increase in the AE energy rate appears several times. With an increasing flaw angle, the count of stress drop and the cumulative AE energy decrease, while the maximum value of the AE energy rate and the average hit energy increase gradually. The energy and stress release reduces the violent failure degree of rock. The AE energy correlated with the cracking magnitude changes more violently than the AE event before peak failure. P-wave attenuation increases sharply after macrocrack initiation. A sharp reduction in velocity is always accompanied by a significant increase in the AE rate. Compared with the global cracking damage of rock characterized by AE characteristics, the fracture identified by P-wave velocity is localized.

Diagnosis of damage evolution process for asphalt mixtures using pattern recognition with acoustic emission signals

The fractures in asphalt materials evolving from microcracks directly affect the performance of asphalt pavements. Acoustic Emission (AE) technique plays a crucial role in dynamically monitoring the internal structural changes of stressed materials. This study attempted to characterize the fracture mechanism of asphalt mixtures by identifying the characteristics of recoded AE data. Firstly, the three-point bending tests and AE tests were simultaneously conducted to investigate the damage evolution process of asphalt mixtures through the time characteristics of AE parameters. Secondly, the cusp catastrophe model based on AE energy rate was employed to explore the mutational characteristics of AE process and to evaluate the damage state of asphalt mixtures. Finally, the k-means method was applied to distinguish the patterns of AE signals associating with the specific damage behaviors of asphalt mixtures during the flexural failure process. The results show that abrupt change of AE energy rate and cumulative AE energy could reflect the evolutionary characteristics of damage in asphalt mixtures. The mutational analysis of AE energy rate reveals that the damage state of asphalt mixtures subjected to the continuous external forces would change between instability and stability, which could be served as a complementary approach for further understanding the periodic variation characteristics of damage evolution process based on cumulative AE energy. The k-means method could effectively classify the recorded AE signals of asphalt mixtures in three-point bending tests into three distinctive clusters. The potential mechanisms for the clustered signals corresponding to different damage behaviors of asphalt mixtures could be characterized by the parametric characteristics and the wavelet power spectrum of AE signals.

Acoustic Emission Characteristics and Energy Evolution of Red Sandstone Samples under Cyclic Loading and Unloading

To explore the characteristics of rock deformation and failure under cyclic loading and unloading, the MTS815 rock mechanics test system and acoustic emission (AE) signal acquisition system were used to perform cyclic loading and unloading tests on red sandstone samples. The results showed that, compared with the uniaxial compression test, cyclic loading and unloading had a certain strengthening effect on the strength of the samples. The plastic deformation of the rock samples increased as the number of cycles increased. Based on AE signals, the cracking mode classification was analyzed on the basis of the average frequency and the rise angle of the waveforms. It was observed that the Felicity ratio gradually decreased with the increase in the stress level, which showed a cumulative damage effect. From the perspective of energy, the obvious increase of AE energy rate was mainly concentrated in the early and late stages of uniaxial compression, while the significant increase of dissipated energy rate occurred in the late stage of uniaxial compression. During the cyclic loading and unloading, most of the work done by external forces in the compaction stage and the elastic stage was converted into elastic strain energy, and dissipated energy began to gradually increase in the stage of stable fracture development. In addition, it was found that the damage evolution of the rock samples changed from slow to fast, and the dissipated energy ratio increased when failure was approaching.

Study of the effect of loading modes on the acoustic emission fractal and damage characteristics of cemented paste backfill

To investigate the relationship between damage variables and fractal dimensions during the deformation and destruction of tantalum–niobium tailing cemented paste backfill (CPB), uniaxial compression and Brazilian split tests were conducted on tantalum–niobium tailing CPB. The changes in the damage variables and fractal dimensions of the CPB during the damage process were investigated using a damage model based on the acoustic emission (AE) ringing count rate and the calculation of the AE energy rate fractal dimension. During the deformation and destruction of the CPB, the fractal dimensions of the CPB gradually decreased, and the damage variables gradually increased; moreover, the stages of reduction in the fractal dimensions corresponded to the stages of increase in the damage variables. The fractal dimensions of the CPB before deformation and failure did not decrease significantly and gradually tended to converge to a certain value. Under different loading modes, the fractal dimensions of the CPB before deformation and failure were different. The damage variability and energy emitted during the initial loading phase of deformation damage were lower than those during the later phase. Before the damage of the CPB, both the damage variables and energy emitted increased rapidly. For both uniaxial compression and Brazilian split loading, the fractal dimensions of the AE of the CPB gradually approaching a certain value is an important precursor of the overall macroscopic destruction of the CPB. Based on this feature, it can be used to predict and analyze the stability of the tantalum-niobium mine tailing CPB in the underground support roof.

On acoustic emission characteristics, initiation crack intensity, and damage evolution of cement-paste backfill under uniaxial compression

The objectives of this study are as follows: To establish the relationship between the acoustic-emission (AE) parameters and mechanical-failure mechanism of tantalum-niobium tailings cemented paste backfill (CPB), to better understand the damage-evolution trend of CPB, to investigate the damage-evolution process of the CPB and AE time-effect mechanism, using an AE monitoring system combined with the stress–strain relationship, and to experimental study the damage-evolution and AE characteristics of tantalum–niobium tailing CPB under uniaxial compression. First, we discussed the crack-initiation and damage strengths, and then analyzed the AE characteristics of the CPB. Then, we proposed a damage variable based on the modified AE energy rate. We developed a CPB damage model based on the AE characteristics under uniaxial compression. The damage-evolution curve and equation of the CPB were obtained. Our results show: that the AE information reflects the damage to the CPB, which is closely related to the evolution of the internal microcracks and initiation, propagation, and penetration of new microcracks. The AE characteristics of the CPB can better describe its deformation and damage-evolution trend. The rationality of the damage model based on the AE characteristics is verified. The evolution process of the damaged CPB under uniaxial compression can be divided into three stages: initial damage stage, stable evolution of the damage and damage development stage, accelerated damages stage. The deformation to destruction of the CPB can be regarded as a gradual process. The entire process consists of the initiation and development of deformation and damage to the occurrence of macrocracks, and finally to failure.

Effect of stress condition on the creep behaviour of Beishan granite

In China, a deep granite formation in Beishan area is currently considered as a potential host rock for high-level radioactive waste repositories. Creep tests were conducted by two modes, namely single step loading mode on several samples and multi-step loading mode on single sample. According to the experiment results, the granite specimens failed almost at the same axial strain and volumetric strain at a given confining pressure, despite of the differences in deviatoric stress, stress ratio or strain ratio. The increase in stress condition (deviatoric stress, stress ratio, or strain ratio) could accelerate the cracking process and creep strain rate, and consequently resulted in a shorter time to failure. The stress effect on the cracking process was also confirmed by the acoustic emission (AE) energy rate. Significant increases in AE energy rate and the percentage of AE events with high energy could be observed with the increase of deviatoric stress. It was indicated from the results that, the stress ratio and strain ratio were more reasonable when discussing the influence of stress condition, perhaps because they were defined by considering both the applied stress level and the mechanical properties of rock material.