Crack Damage Threshold Research Articles

Study of Cracking Process in Granite

Behavior of intact brittle rocks during loading is characterized by a micro-cracking process, which occurs due the presence of flaws in their microscopic structure, propagating through the intact rock leading to shear fracture. This fracturing process is of fundamental importance as it affects the mechanical properties of the rock. The purpose of the present study is to investigate the fracture mechanism and specifically detect the crack initiation and crack damage stresses in a granite rock. For this purpose, stress-strain data were recorded and the acoustic emission method was used in uniaxial compression tests. The results from both methods correlate well and show that the crack initiation and crack damage thresholds are about 46% and 78% of the uniaxial compressive strength of the intact rock respectively, slightly higher than the results reported in literature from previous studies in granite.

Crack initiation and damage thresholds in some brittle metamorphic rock types

In this study 60 unconfined compression laboratory tests performed on three brittle metamorphic rock types were analysed in order to evaluate the stress thresholds associated with the onset of stable and unstable microcracking which are known in literature as the crack initiation stress and the crack damage stress, respectively. The crack initiation stress was identified applying the lateral strain response (LSR) methodology while the crack damage stress was detected where total volumetric strain reversal occurs. Although the mineralogical and textural features were quite similar among the investigated rock types, the results of this study showed that the crack initiation and damage stress values were affected by variability. However, when the normalized stress values ci/UCS and cd/UCS were considered, the scatter in results decrease consistently suggesting that the stable and unstable crack growth begin at similar stress levels in the investigated rock types

An experimental investigation on thermal damage and failure mechanical behavior of granite after exposure to different high temperature treatments

A detailed understanding of the thermal damage and failure mechanical behavior of granite at elevated temperatures is a key concern in nuclear waste disposal engineering, underground coal gasification, and heat mining in enhanced geothermal energy. In this research, uniaxial compression tests were first carried out to evaluate the effect of high temperature treatments (200, 300, 400, 500, 600, 700 and 800°C) on the crack damage, strength and deformation failure behavior of a granite. The results demonstrated that, in all cases, the crack damage threshold, the strength and static elastic modulus of granite were increased at 300°C, before decreasing up to our maximum temperature of 800°C. However, the static Poisson’s ratio of granite first decreased at 600°C, and then increased rapidly with the temperature. The crack damage and peak axial strain always showed an increase when the temperature was increased. However, the dynamic elastic modulus decreased with the temperature, whereas the dynamic Poisson’s ratio did not depend on the temperature. The gradual increase of temperature results in a more ductile failure of granite. Next, the thermal damage mechanism of uncompressed granite was analyzed by optical microscopic observation. At T=25–300°C, the mechanisms were favored by the thermal expansion of mineral grains but no microcracks were observed; at T=400–600°C, the mechanisms were contributed by boundary cracks and transgranular cracks in feldspar and quartz grains; and at T=700–800°C, the mechanisms were associated with the coalescence of boundary cracks and transgranular cracks. The internal crack evolution process was then monitored during deformation using acoustic emission (AE) monitoring. The results showed that the cracking process of granite depended on the heat treatment temperature. Finally, the deformation mechanism of failed granite at various temperatures was analyzed using X-ray micro CT. During loading, the uniaxial compression stress direction dominated the more brittle fracture process of granite at T=25–600°C, which led to splitting tensile main cracks induced along the axial stress, and thermal damage determined the larger ductile fracture process of granite at T=700–800°C, which resulted in a more ductile deformation after the peak strength.

Experimental study on deformation, peak strength and crack damage behavior of hollow sandstone under conventional triaxial compression

Based on a series of triaxial compression experiments using hollow sandstone specimens with various hole diameters (d=0, 11, 15 and 26mm), the deformation, peak strength and crack damage behavior of hollow sandstone specimens under different confining pressures are investigated. The experimental results show that the Young's modulus of hollow sandstone only depends on the confining pressure and is not affected by hole diameter in the tested range of d=0–26mm. Two types of methods used to confirm the elastic modulus and Poisson's ratio of rock material are proposed to evaluate the triaxial deformation characteristics of hollow sandstone. The effects of confining pressure and hole diameter on the Poisson's ratio and peak strain of hollow sandstone are analyzed. The peak strength and crack damage parameters of hollow sandstone depend on not only the confining pressure (σ3) but also the hole diameter. Under uniaxial compression, the peak strength and crack damage threshold (σcd) of hollow sandstone are independent of hole diameter, whereas the triaxial compressive strength of hollow sandstone decreases linearly with increasing hole diameter. The peak strength and crack damage threshold of hollow sandstone increase with increasing confining pressure, which can be better described by the nonlinear Hoek-Brown criterion than by the linear Mohr-Coulomb criterion. Furthermore, the sensitivity of the crack damage threshold of hollow sandstone to hole diameter is lower than that of peak strength on hole diameter. The concluding remarks can be used to improve the stability and safety of deep underground engineering.

Strength Restoration of Cracked Sandstone and Coal under a Uniaxial Compression Test and Correlated Damage Source Location Based on Acoustic Emissions.

Underground rock masses have shown a general trend of natural balance over billions of years of ground movement. Nonetheless, man-made underground constructions disturb this balance and cause rock stability failure. Fractured rock masses are frequently encountered in underground constructions, and this study aims to restore the strength of rock masses that have experienced considerable fracturing under uniaxial compression. Coal and sandstone from a deep-buried coal mine were chosen as experimental subjects; they were crushed by uniaxial compression and then carefully restored by a chemical adhesive called MEYCO 364 with an innovative self-made device. Finally, the restored specimens were crushed once again by uniaxial compression. Axial stress, axial strain, circumferential strain, and volumetric strain data for the entire process were fully captured and are discussed here. An acoustic emission (AE) testing system was adopted to cooperate with the uniaxial compression system to provide better definitions for crack closure thresholds, crack initiation thresholds, crack damage thresholds, and three-dimensional damage source locations in intact and restored specimens. Several remarkable findings were obtained. The restoration effects of coal are considerably better than those of sandstone because the strength recovery coefficient of the former is 1.20, whereas that of the latter is 0.33, which indicates that MEYCO 364 is particularly valid for fractured rocks whose initial intact peak stress is less than that of MEYCO 364. Secondary cracked traces of restored sandstone almost follow the cracked traces of the initial intact sandstone, and the final failure is mainly caused by decoupling between the adhesive and the rock mass. However, cracked traces of restored coal only partially follow the traces of intact coal, with the final failure of the restored coal being caused by both bonding interface decoupling and self-breakage in coal. Three-dimensional damage source locations manifest such that AE events are highly correlated with a strength recovery coefficient; the AE events show a decreasing tendency when the coefficient is larger than 1, and vice versa. This study provides a feasible scheme for the reinforcement of fractured rock masses in underground constructions and reveals an internal mechanism of the crushing process for restored rock masses, which has certain instructive significance.

Radon emission evolution and rock failure

Previous experimental studies showed that the variation of Radon (Rn) emission is closely related to the development of rock deformation: Rn emission has a rapid increase with the growth of cracks and their surface areas (i.e. onset of fracture coalescence point), and reaches the maximum at rock failure. In this study, the stress level at the onset of fracture coalescence λ c is predicted with the renormalization group theory. It is assumed that rock is a strain-softening medium whose strength can be described by Weibull’s distribution. Result shows that λ c depends mainly on the homogeneity index or shape parameter m in the Weibull’s distribution for the rock. Both experimental and analytical results show that this point of rapid increase in Rn emission on the Rn-strain curve corresponds to the critical point on the stress–strain curve; for rock compression, the stress at this point is approximately 70–80 % of the peak strength. Hence a generalized crack damage threshold is proposed in the present study based on the characteristics of Rn emission during the loading process, for recognizing the critical point of rock fracture.

Discrete element modeling on the crack evolution behavior of brittle sandstone containing three fissures under uniaxial compression

Based on experimental results of brittle, intact sandstone under uniaxial compression, the micro-parameters were firstly confirmed by adopting particle flow code $$(\hbox {PFC}^{\mathrm{2D}})$$ . Then, the validation of the simulated models were cross checked with the experimental results of brittle sandstone containing three parallel fissures under uniaxial compression. The simulated results agreed very well with the experimental results, including the peak strength, peak axial strain, and ultimate failure mode. Using the same micro-parameters, the numerical models containing a new geometry of three fissures are constructed to investigate the fissure angle on the fracture mechanical behavior of brittle sandstone under uniaxial compression. The strength and deformation parameters of brittle sandstone containing new three fissures are dependent to the fissure angle. With the increase of the fissure angle, the elastic modulus, the crack damage threshold, and the peak strength of brittle sandstone containing three fissures firstly increase and secondly decrease. But the peak axial strain is nonlinearly related to the fissure angle. In the entire process of deformation, the crack initiation and propagation behavior of brittle sandstone containing three fissures under uniaxial compression are investigated with respect to the fissure angle. Six different crack coalescence modes are identified for brittle sandstone containing three fissures under uniaxial compression. The influence of the fissure angle on the length of crack propagation and crack coalescence stress is evaluated. These investigated conclusions are very important for ensuring the stability and safety of rock engineering with intermittent structures.

An Investigation on the Fabric Type Dependency of the Crack Damage Thresholds in Brittle Rocks

Fabric-guided micro-fracturing phenomenon in brittle rocks and its effect on crack damage thresholds remains subject to continuing research. The available fabric in rocks can act as a motivator for nucleation and/or extension and interaction of micro-fractures in a preferred orientation, or as a suppressor for growth of micro-cracks in a given direction by different mechanisms such as compliance (stiffness contrast) or preferred orientation of minerals and their boundaries. While anisotropy of brittle rocks in terms of their mechanical strengths can play a significant role in the stability of underground openings, the understanding of the dependency of crack initiation (CI) and crack propagation (CD) thresholds on the available fabric in rocks can improve predictions of the extension and density of micro-fracturing in different directions in the walls of underground openings. To better understand the fabric-guided micro-fracturing phenomenon, and also to study the effect of fabric types available in brittle rocks on their anisotropic behaviour, four types of brittle rocks with different types of fabric are investigated in terms of crack damage anisotropy in this paper. The rocks that are chosen for this study are limestone from the Cobourg Formation, Queenston shale, Olkiluoto mica gneiss and Lac du Bonnet granite. For each rock type, CI and CD thresholds are identified from the unconfined compressive strength testing data. The mechanical behaviour of the four rock types are investigated at each damage stress level and the contributing factors to the isotropic or anisotropic behaviour of the rocks at different crack damage thresholds are discussed.

Experimental investigation on mechanical damage characteristics of sandstone under triaxial cyclic loading

The mechanical damage characteristics of sandstone subjected to cyclic loading is very significant to evaluate the stability and safety of deep excavation damage zones. However to date, there are very few triaxial experimental studies of sandstone under cyclic loading. Moreover, few X-ray micro-computed tomography (micro-CT) observations have been adopted to reveal the damage mechanism of sandstone under triaxial cyclic loading. Therefore, in this research, a series of triaxial cyclic loading tests and X-ray micro-CT observations were conducted to analyse the mechanical damage characteristics of sandstone with respect to different confining pressures. The results indicated that at lower confining pressures, the triaxial strength of sandstone specimens under cyclic loading is higher than that under monotonic loading; whereas at confining pressures above 20 MPa, the triaxial strength of sandstone under cyclic loading is approximately equal to that under monotonic loading. With the increase of cycle number, the crack damage threshold of sandstone first increases, and then significantly decreases and finally remains constant. Based on the damage evolution of irreversible deformation, it appears that the axial damage value of sandstone is all higher than the radial damage value before the peak strength; whereas the radial damage value is higher than the axial damage value after the peak strength. The evolution of Young's modulus and Poisson's ratio of sandstone can be characterized as having four stages: (i) Stage I: material strengthening; (ii) Stage II: material degradation; (iii) Stage III: material failure and (iv) Stage IV: structure slippage. X-ray micro-CT observations demonstrated that the CT scanning surface images of sandstone specimens are consistent with actual surface crack photographs. The analysis of the cross-sections of sandstone supports that the system of crack planes under triaxial cyclic loading is much more complicated than that under triaxial monotonic loading. More axial and lateral tensile cracks were observed in the specimens under cyclic loading than under monotonic loading.

Experimental Study of Mechanical Behavior and X-Ray Micro CT Observations of Sandstone Under Conventional Triaxial Compression

This paper reports a series of triaxial compression experiments and X-ray observations carried out to explore the mechanical behavior and internal damage mechanism of sandstone material. The results show that the Young’s modulus of sandstone increased nonlinearly with increasing confining pressure, but the Poisson’s ratio remained unaffected. The nonlinear Hoek–Brown criterion can better reflect the peak strength properties than the linear Mohr–Coulomb criterion. However, the residual strength of sandstone exhibits a clear linear relationship with the confining pressure, which can be best described by the linear Mohr–Coulomb criterion. The sensitivity of the crack damage threshold on the confining pressure was clearly lower than that for the peak strength. After unaxial and triaxial compression failure, the sandstone specimens were analyzed using a 3D X-ray micro CT scanning system. Based on the horizontal and vertical cross-sections of sandstone specimens, we found that under uniaxial compression and lower confining pressure, the sandstone specimen is dominated mainly by axial splitting tensile cracks; however, under higher confining pressure, the sandstone specimen is mainly dominated by a single shear crack. To quantitatively evaluate the internal damage of sandstone material, crack area and aperture extent for each horizontal cross-section were calculated by analyzing the binarized pictures. The system of crack planes under uniaxial compression is much more complicated than that under triaxial compression, which is also testified by the evolution behavior of crack area and aperture extent. Finally, the fracture mechanism of sandstone during uniaxial and triaxial compression is discussed in detail and simplified models are proposed.

A Comparative Evaluation of Stress–Strain and Acoustic Emission Methods for Quantitative Damage Assessments of Brittle Rock

The purpose of this study is to identify the crack initiation and damage stress thresholds of granite from the Korea atomic energy research institute’s Underground Research Tunnel (KURT). From this, a quantitative damage evolution was inferred using various methods, including the crack volumetric strain, b value, the damage parameter from the moment tensor, and the acoustic emission (AE) energy. Uniaxial compression tests were conducted, during which both the stress–strain and AE activity were recorded simultaneously. The crack initiation threshold was found at a stress level of 0.42–0.53 σc, and the crack damage threshold was identified at 0.62–0.84 σc. The normalized integrity of KURT granite was inferred at each stress level from the damage parameter by assuming that the damage is accumulated beyond the crack initiation stress threshold. The maximum deviation between the crack volumetric strain and the AE method was 16.0 %, which was noted at a stress level of 0.84 σc. The damage parameters of KURT granite derived from a mechanically measured stress–strain relationship (crack volumetric strain) were successfully related and compared to those derived from physically detected acoustic emission waves. From a comprehensive comparison of damage identification and quantification methods, it was finally suggested that damage estimations using the AE energy method are preferred from the perspectives of practical field applicability and the reliability of the obtained damage values.

Research on the Methods of Determination of Characteristic Strength of Brittle Rock Materials in Compression Test

The failure of rock materials in compression test suffers three characteristic stages: crack initiation, crack damage threshold, and macro-failure. There are some methods which can be adopt to identity the characteristic strengths in uniaxil or triaxial compression such as crack strain identifying method, AE activity and longitudinal wave velocity monitoring methods. In this paper, an creative experiment of recording stress-strain data, AE spectrums, and velocity of longitudinal wave is proposed and put into practice. Test results show that the mean initiation strength and damage strength by ratio of UCS are between 0.38 to 0.52 and 0.82 to 0.86 respectively with AE and longitudinal wave velocity methods synthetically. The AE monitoring method and the lateral P-wave or the axial S-wave test is an effective indicative of identifying the damage threshold when subject an axial compression failure.

Geopolymer as well cement and the variation of its mechanical behavior with curing temperature

AbstractAnthropogenic emissions of greenhouse gases are a major problem for all nations, and carbon dioxide (CO2) sequestration is one of the best practical solutions to reduce greenhouse gases. A large amount of CO2 is injected through sequestration wells into injection reservoirs. The injection wells play an important role; well integrity is an important part of any CO2 sequestration projects and well cement is the key to maintaining well integrity. To date, Ordinary Portland cement (OPC)‐based well cement has been used and there are many problems associated with this, including cement degradation, durability issues, and sustenance in acid‐rich environments. Therefore, this paper aims to study geopolymer as a well cement and the variation of its mechanical properties with different curing temperatures. Temperatures from ambient level (23 °C) up to 80 °C were considered, as well cement undergoes a range of temperatures from the ground surface to deep underground with a geothermal gradient of 30 °C/km. Stress‐strain variations and crack propagation stress thresholds were studied using stress‐strain and acoustic emission (AE) methods, and failure strain and orientation were studied using ARAMIS photogrammetry software for samples cured at different curing temperatures. The results show that the optimal curing temperature for higher strength is 60 °C; Young's modulus and Poisson's ratio generally increase with curing temperature. In general, crack closure, crack initiation, and crack damage thresholds increase with curing temperature. ARAMIS image capture showed that failure modes of low‐temperature cured samples are diagonal, whereas elevated‐temperature cured samples start to fail at the top and bottom compression plates. In addition, low‐temperature cured samples exhibit higher strain (6–8%) at failure, while elevated‐temperature cured samples exhibit low strain (0.8–3.5%) at failure. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd

Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression

The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling. Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing. To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures. All specimens were tested at their natural water content with loading approximately normal to the bedding. Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation. The unconfined compression strength of the tested specimens averaged 6.9 MPa. The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress–strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress.

Generalized crack initiation and crack damage stress thresholds of brittle rock masses near underground excavations

The rock mass failure process is characterized by several distinct deformation stages which include crack initiation, crack propagation and coalescence. It is important to know the stress levels associated with these deformation stages for engineering design and practice. Extensive theoretical, experimental and numerical studies on the failure process of intact rocks exist. It is generally understood that crack initiation starts at 0.3 to 0.5 times the peak uniaxial compressive stress. In confined conditions, the constant-deviatoric stress criterion was found to describe the crack initiation stress level. Here, generalized crack initiation and crack damage thresholds of rock masses are proposed. The crack initiation threshold is defined by σ1−σ3=A σcm and the crack damage threshold is defined by σ1−σ3=B σcm for jointed rock masses, where A and B are material constants and σcm is the uniaxial compressive strength of the rock masses. For a massive rock mass without joints, σcm is equal to σcd, the long-term uniaxial strength of intact rock. After examining data from intact rocks and jointed rock masses, it was found that for massive to moderately jointed rock masses, the material constants A and B are in the range of 0.4 to 0.5, 0.8 to 0.9, respectively, and for moderately to highly jointed rock masses, A and B are in the range of 0.5 to 0.6, 0.9 to 1.0, respectively. The generalized crack initiation and crack damage thresholds, when combined with simple linear elastic stress analysis, assist in assessing the rock mass integrity in low confinement conditions, greatly reducing the effort needed to obtain the required material constants for engineering design of underground excavations.

Effects of sample disturbance on the stress-induced microfracturing characteristics of brittle rock

The effects of sampling disturbance on the laboratory-derived mechanical properties of brittle rock were measured on cored samples of Lac du Bonnet granite taken from three different in situ stress domains at the Underground Research Laboratory of Atomic Energy of Canada Limited. A variety of independent measurements and scanning electron microscope observations demonstrate that stress-induced sampling disturbance increased with increasing in situ stresses. The degree of damage was reflected in laboratory measurements of acoustic velocity and elastic stiffness. Examination of the stress-induced microfracturing characteristics during uniaxial compression of the samples revealed that the degree of sampling disturbance had only minor effects on the stress levels at which new cracks were generated (i.e., the crack initiation stress threshold). Crack-coalescence and crack-damage thresholds, on the other hand, significantly decreased with increased sampling disturbance. The presence of numerous stress-relief cracks in the samples retrieved from the highest in situ stress domains was seen to weaken the rock by providing an increased number of planes of weakness for active cracks to propagate along. A 36% strength decrease was seen in samples retrieved from the highest in situ stress domain (sigma1 - sigma3 approximate 40 MPa) as compared with those taken from the lowest in situ stress domain (sigma1 - sigma3 approximate 10 MPa).Key words: sample disturbance, brittle fracture, crack initiation, crack propagation, material properties, rock failure.

Effects of sample disturbance on the stress-induced microfracturing characteristics of brittle rock

The effects of sampling disturbance on the laboratory-derived mechanical properties of brittle rock were measured on cored samples of Lac du Bonnet granite taken from three different in situ stress domains at the Underground Research Laboratory of Atomic Energy of Canada Limited. A variety of independent measurements and scanning electron microscope observations demonstrate that stress-induced sampling disturbance increased with increasing in situ stresses. The degree of damage was reflected in laboratory measurements of acoustic velocity and elastic stiffness. Examination of the stress-induced microfracturing characteristics during uniaxial compression of the samples revealed that the degree of sampling disturbance had only minor effects on the stress levels at which new cracks were generated (i.e., the crack initiation stress threshold). Crack-coalescence and crack-damage thresholds, on the other hand, significantly decreased with increased sampling disturbance. The presence of numerous stress-relief cracks in the samples retrieved from the highest in situ stress domains was seen to weaken the rock by providing an increased number of planes of weakness for active cracks to propagate along. A 36% strength decrease was seen in samples retrieved from the highest in situ stress domain (sigma1 - sigma3 approximate 40 MPa) as compared with those taken from the lowest in situ stress domain (sigma1 - sigma3 approximate 10 MPa).Key words: sample disturbance, brittle fracture, crack initiation, crack propagation, material properties, rock failure.