Cracked Chevron Notched Brazilian Disc Research Articles

Mode-I failure and mechanical behavior of sandstone under cyclic loading: Laboratory testing and DEM simulation

To study the mode-I failure and mechanical behavior of rock under cyclic loading, the cracked chevron notched Brazilian disc (CCNBD) of sandstone was employed in this paper, and thus three different upper-limit loads cyclic tests were carried out, including 95 %SUL(static ultimate load) ∼ 35 %SUL, 85 %SUL ∼ 25 %SUL, 75 %SUL ∼ 15 %SUL. The 3D discrete element model of CCNBD specimen was built to study mechanical properties and cracking behavior at a meso-scale. Meanwhile, the effect of cyclic loading on microstructure characteristics and fracture process zone (FPZ) was discussed. The results show that cyclic loading has a weakening effect on fracture toughness, and a modified fracture toughness calculation method was proposed considering the effect of irreversible plastic deformation under cyclic loading. There are obvious stages of microcracking behavior under cyclic loading, specific for the condition of 85 %SUL ∼ 25 %SUL, and the microcrack density is positively correlated with the number of cycles. Meanwhile, the effect of cyclic load on sandstone microstructure contains weak-structure fracture effect and compaction effect, and they exist simultaneously and work together. In addition, cyclic loading has a great influence on the length of FPZ, and the length of FPZ under cyclic loading is greater than that under monotonic loading.

Fracture toughness of schist, amphibolite, and rhyolite from the Sanford Underground Research Facility (SURF), Lead, South Dakota

The Cracked Chevron Notched Brazilian Disc (CCNBD) method was selected for Mode I fracture toughness tests on Poorman schist, Yates amphibolite, and rhyolite dikes from the EGS Collab site at the SURF in Lead, South Dakota. The effects of lithology, anisotropy, and loading rate were investigated. Fracture toughness was greatest in amphibolite, with schist and rhyolite having similar toughness values ({K}_{amphibolite} > {K}_{rhyolite} ≈ {K}_{schist}). The effects of anisotropy on fracture toughness were investigated in the foliated schist samples. Schist samples were prepared in three geometries (divider, arrester, and short transverse) which controlled how the fracture would propagate relative to foliations. The divider geometry was strongest and short transverse geometry was the weakest ({K}_{divider} > {K}_{arrester} > {K}_{short transverse}). Fracture toughness was observed to decrease with decreasing loading rate. Optical and SEM microscopy revealed that for the short transverse geometry, fractures tended to propagate along grain boundaries, whereas in arrester and divider geometries fractures tended to propagate through grains. In foliated samples, the tortuosity of the fracture observed in thin section was greater in arrester and divider geometries than in short transverse geometries.

Experimental and numerical investigations on fracture behaviours of cracked chevron notched Brazilian disc (CCNBD) sandstone specimen under cyclic loading

In this work, laboratory tests and numerical simulations were used to investigate the fracture behaviour of Cracked Chevron Notched Brazilian Disc (CCNBD) sandstone specimens subjected to cyclic loading. A series of fracture tests performed on CCNBD specimens, including static loading and cyclic loading. The cycle upper limit load was set as 0.75SUL (static ultimate load), 0.85SUL, and 0.95SUL, respectively. The fracture surface topography was also reconstructed by a 3D laser scanner to analyse the fracture characteristics. The results show that the phenomenon of ‘stress lags behind strain’ and ‘strain lags behind stress’ can reflect the evolution of hysteretic rings under cyclic loading, and the spacing between hysteretic rings increases with the increase in upper limit load. A weakening effect of the cyclic loading on the mode I fracture toughness can be found, and the more cycles there are, the smaller is the fracture toughness. The threshold value of the upper limit load is approximately 0.75SUL − 0.80SUL in this work. The nonlinear development of the fracture process zone can be divided into three stages: the initial stage, stable stage, and acceleration stage. In addition, a higher fractal dimension reflects an uneven and rougher fracture surface, and the more cycles there are, the rougher is the fracture surface topography. Based on the proposed meso-dilatancy model, the failure reason is confirmed as the larger tangential deformation caused by cyclic loading, and a larger tangential deformation leads to a rougher fracture surface.

Experimental study of effect of number of heating–cooling cycles on mode I and mode II fracture toughness of travertine

Fracture toughness is considered a critical index for measuring the fracture strength of cracked rocks, which is sometimes influenced by the heating–cooling process. This process and its repetition change the physical and mechanical properties of rocks. The façade of buildings bears a larger number of heating–cooling cycles over days and seasons. Accordingly, this study simulated the heating–cooling process and its impact on the mode I and mode II fracture toughness of travertine as a widely used rock in the façade of modern buildings by designing a larger number of low-temperature heating–cooling cycles. Previous researches mostly has been studied the effect of high temperatures on physical and mechanical rock properties but in this study, effect of low temperature in a large number of heating–cooling cycle is investigated. Two types of travertine were considered and studied. The mode I and mode II fracture toughness tests were performed on 160 cracked chevron notched Brazilian disc (CCNBD) specimens after applying 0, 1, 4, 8, 16, 32, 64, and 128 heating–cooling cycles. Mechanical properties were measured, and scanning electron microscope (SEM) images were taken to evaluate and match the results of fracture toughness tests. According to the results, mode I and mode II fracture toughness increased after a heating–cooling cycle and then decreased by increasing the number of cycles. The changes in the fracture toughness were consistent with variations of mechanical properties and SEM images with increasing the number of heating–cooling cycles.

Experimental Study of Mode I Fracture Toughness Anisotropy in Granitic Rocks

Granitic rocks can in general be easily split through three mutually orthogonal surfaces, empirically identified in the field1,2. In this regard, the present study intends to evaluate the mode I fracture toughness of a syenogranite, obtained from a quarry in the city of Cachoeiro de Itapemirim in the state of Espirito Santo. Fracture toughness consists of an intrinsic mechanical property of rocks and indicates the resistance to the initiation or propagation of a fracture as well as the amount of energy the rock absorbs until its fracture. In order to evaluate mode I fracture toughness anisotropy, the Cracked Chevron Notched Brazilian Disc (CCNBD) test was conducted, following ISRM3. Twenty-five (25) samples were tested considering four (4) different orientations: short transverse, arrester, divider and a random direction (inclined). In addition, the indirect tensile strength test (Brazilian test) was also conducted, in order to characterize the samples. The test was performed on fifteen (15) samples and in three (3) different orientations: short transverse, arrester and divider. The tests and sample preparation were carried out at the Structural and Materials Laboratory (LEM-DEC), and the physical properties were obtained at the Geotechnical and Environmental Laboratory (LGMA), both located at PUC-Rio. It is possible to observed that the physical properties results obtained in the present study were very similar to the results obtained by Almeida et al.4 and by Jaques5. In the Brazilian test it was possible to observe that all the mean tensile strength values are similar to each other, which indicates that there is little anisotropy in this rock. In the CCNBD test it was possible to verify that the mean fracture toughness values are similar in every direction, which indicates that there is little anisotropy in this rock as well as in the Brazilian test.

Logging prediction and evaluation of fracture toughness for the shales in the Longmaxi Formation, Southern Sichuan Basin

The mode-I fracture toughness is of great significance for evaluating the fracturing ability of shale reservoirs. In this study, the mode-I fracture toughness of the shale in the Lower Silurian Longmaxi Formation, Southern Sichuan Basin was determined with the Cracked Chevron Notched Brazilian Disc (CCNBD). Based on the experimental data, the relationships among the mode-I fracture toughness, the density, the acoustic time and the clay mineral content were analyzed. The shale samples for fracture toughness test from cores were commonly limited, the investigation of fracture toughness using well logs was necessary. Therefore, a prediction model was proposed by correlating the fracture toughness with well logs responses. The results indicate that the fracture toughness of shale samples is from 0.4744 MPa·m 1/2 to 1.0607 MPa·m 1/2 with an average of 0.7817 MPa·m 1/2 , indicating that the anisotropy of fracture toughness of the Longmaxi Formation shale. The clay mineral content and the density have a positive effect on the fracture toughness, whereas the acoustic time plays a negative role on the fracture toughness. The clay mineral content has an important effect on the relationships among fracture toughness, acoustic time and density. The prediction model can provide continuous data of mode-I fracture toughness along the wellbore for field hydraulic fracturing operation, and it has certain guiding significance in the exploration and development of oil and gas reservoirs

Machine learning approaches to rock fracture mechanics problems: Mode-I fracture toughness determination

The cracked chevron notched Brazilian disc (CCNBD) specimen is a suggested testing method to measure Mode-I fracture toughness of rocks by ISRM, which is widely adopted in the laboratory experiments. However, sizes of CCNBD rock specimens are uncertain in the laboratory experiments, which leads to be inaccurate in measurement of Mode-I fracture toughness of rocks in tests. In this work, four machine learning approaches, including decision regression tree, random regression forest, extra regression tree and fully-connected neural networks (FCNNs) are developed and their feasibility and value are demonstrated through the analysis and predictions of Mode-I fracture toughness of rocks. It can be found that solutions based on the four machine learning approaches can provide the accurate results for predicting Mode-I fracture toughness of rock by in ISRM-suggested CCNBD rock specimens. The random regression forest is more suitable to predict Mode-I fracture toughness of rocks in ISRM-suggested CCNBD rock tests than others. The reliable functionality and rapid development of machine learning solutions are demonstrated that it is a major improvement over the previous analytical and empirical solutions by this example. When analytical and empirical solutions are not available, machine learning approaches overcome the associated limitations, which provides a substantially way to solve rock engineering problems.

A new experimental approach to quantify microfractures in the Fracture Process Zone (FPZ) under various loading conditions

Rock fracturing or breakage is a complex phenomenon that requires advanced and sophisticated techniques to predict fracture initiation and propagation in geological structures under in-situ or induced stresses. In previous studies less attention have been paid to quantify damage in the Fracture Process Zone (FPZ) under various loading conditions. This study presents the results of experimental cracked chevron notched Brazilian disc (CCNBD) tests undertaken to investigate the microfracture evolution at the FPZ for four different rock types: marble, monzonite, tuff and sandstone under static and cyclic (stepped cyclic and continuous cyclic) loadings. This research aims to determine the intensity of fractures by quantifying fracture density under various loading conditions. The Computed Tomography (CT) technique was successfully used to capture the fracture density and FPZ developed within the tested rock specimens. It was found that the width of FPZ is much wider in samples tested under the cyclic loading compared to that of static loading. The main difference between the micro-fractured surfaces refers to intergranular cracks between grain and matrix in cyclic loading, while the smooth and sleek cleavage planes of cracks was observed in static loading condition. Image processing results also showed that by applying cyclic loading, crack propagation is extremely contorted and crack surfaces are very rough compared to that of static loading test. Pixel quantity presented fracture morphology and microfracture quantity within the tested specimens. The ratio of pixels size was found mainly dependent on the loading condition, plastic deformation, grain size, fracture toughness and material stiffness. This study provides insight into the quantifying the fracture intensity and the shape of the FPZ in various rock types tested under the static and cyclic loading.

Precursory Indicator for Mode I Fracture in Brittle Rock through Critical Slowing Down Analysis

The abrupt rock-related hazards, such as landslide, rock burst, and collapse, seriously threaten the safety and service life of engineering works. Precursory information on critical transitions preceding sudden fracture is of great significance in rock mechanics and engineering. This study investigates the critical slowing down feature of acoustic emission (AE) signals and precursory indicators during the mode I fracture process of brittle rock. Cracked chevron notched Brazilian disc (CCNBD) specimens were utilized, accompanied by acoustic emission monitoring. The principle of critical slowing down was introduced to study AE count sequences, and the variance and autocorrelation coefficient versus loading time curves were analyzed. The results show critical slowing down phenomenon exists during mode I rock fracture. The variance and autocorrelation coefficient of AE counts grow significantly prior to rock fracture, and thus, the significant growth of variance and autocorrelation coefficient of AE signals can act as the precursory indicator of rock fracture. Compared to the autocorrelation coefficient, the precursors determined by the variance are more remarkable. The time interval between the precursory indicator using the critical slowing down theory and fracture moment ranges from 2% to 15% of the entire loading time. The findings in this study could facilitate better understandings on the rock fracture process and early-warning technique for rock fracture-related geological disasters.

Damage Evolution Characteristics of Non-fractured Regions during the Rock Fracture Process of Cracked Chevron Notched Brazilian Disc Specimens with Different Water Contents

To investigate the influence of water content on crack evolution during the fracture process of the red sandstone rock material, the damage and mechanical properties of cracked chevron notched Brazilian disc red sandstone specimens, with different water contents, were experimentally investigated. Using a P-wave velocity meter, the damage information of red sandstone specimens with different water contents, in the non-fractured region, during the pure mode I and II fracture processes, was quantitatively analyzed. The experimental results show that the damage amount of the non-fractured region, during the pure mode I and II fracture processes, increases linearly with an increase in the internal water content of the red sandstone specimens. Thus, the results from this study can provide certain theoretical basis for studying the evolution mechanism of microcracks in the fracture process of CCNBD specimens with the disparate water contents.

Type I Fracture Toughness Test of Rock-like Materials Based on the Particle Flow Method

In order to investigate the feasibility and reliability of the three-dimensional particle flow method in simulating the type I fracture toughness test, four types of numerical samples were established by particle flow code PFC3D: straight crack three-point bending (SC3PB), edge cracked flattened semicircular disc (ECFSD), cracked chevron notched Brazilian disc (CCNBD), and edge cracked flattened ring (ECFR). Three models with different strength parameters (group A, group B, and group C) were established for each type, in which group A parameters are obtained from the concrete model, group B parameters are applied for simulating marble, and group C parameters are for granite. The type I fracture toughness and the failure form of each model are obtained by conducting the numerical test, and the curves of load versus displacement of loading point are recorded. The numerical test results show that, with the same strength parameter, the maximum difference in test results of each specimen type is 0.39 MPa·m1/2. The KIC of ECFR specimen is 0.13–0.28 MPa·m1/2 smaller than that of CCNBD specimen, and the KIC of ECFSD specimen is slightly higher than that of CCNBD sample. The KIC of SC3PB specimen is 0.06–0.21 MPa·m1/2 smaller than that of the CCNBD sample. When the loading rate is less than 0.01 m/s, the effect of loading rate on fracture toughness can be reduced to less than 0.1 MPa·m1/2.

Experimental and Numerical Investigation of the Effect of Bedding Layer Orientation on Fracture Toughness of Shale Rocks

Fracture toughness is a critical parameter responsible for fracture initiation and propagation. Fracture toughness in shale gas reservoir, however, is highly variable because of its anisotropy and spatial variation of clay content. In this paper, a series of laboratory and numerical experiments are carried out to estimate shale fracture toughness at different bedding plane orientations with respect to loading direction (angle of bedding layer with respect to Cracked Chevron notched Brazilian disc (CCNBD) test samples) and in different brine solutions. The CCNBD test was conducted on 12 cylindrical samples for fracture toughness. Shale samples were prepared at four different angles (0°, 30°, 45° and 90°) relative to the bedding plane. The prepared specimens were saturated in potassium chloride (KCL) solutions of different concentrations. The laboratory results of toughness have shown to be highly variable with respect to both bedding plane and brine concentration and that the sample at the angle of 90° in 4% KCL concentration exhibited the highest fracture toughness. Numerical simulations based on extended finite element method (XFEM) were also carried out to simulate fracture evolution and propagation in CCNBD samples with different bedding planes. The results have shown that the bedding layers caused the fracture path to deflect. The deviation from straight crack path is caused by mixed mode fracture initiation and propagation instead of tension mode. The mixed mode fracture propagation behavior was verified by analyzing fracture propagation path using both laboratory experiments and numerical simulation.

AN INVESTIGATION INTO FRACTURE TOUGHNESS AND MECHANISMS IN ANISOTROPIC ROCKS USING THE ACOUSTIC EMISSION METHOD

Anisotropy in rock is among the important parameters that should be considered in studying fracture toughness and fracture mechanisms. Acoustic emission is one of the methods applied to investigate fracture mechanics and how micro- cracks grow in rock samples. This method can help to identify different parts of stress versus strain curves. Using this method, the current research aims to study the effect of anisotropy angles on fracture toughness and fracture mecha- nisms of rock samples. To study the designated parameters, the Cracked Chevron Notched Brazilian Disc (CCNBD) method was selected. Blocks of slate, as an anisotropic rock, were prepared from the Sanandaj-Sirjan zone in Iran. The sample sizes were 54 mm and 74 mm in diameter and 24 mm and 37 mm in thickness, respectively, and their anisotropy angles were 0°, 15°, 30°, 45°, 60°, 75°, and 90°. The tests were carried out in accordance with the method suggested by the International Society for Rock Mechanics (ISRM). Two acoustic sensors were mounted on the specimens to log acoustic waves and determine the acoustic parameters (rise angle and average frequency) for fracture mechanism analysis. About 10 tests were carried out for each angle of anisotropy. Based on the obtained results, it was inferred that anisotropy affects the strength and fracture toughness of rock samples. By increasing the anisotropy angle from 0° to 30°, the strength and fracture toughness decreased. Exceeding this range of angle, with an increase in the anisotropy angle, the strength and fracture toughness increased as well.

Effects of temperature and confining pressure on the mode I and mode II fracture toughness of cement mortar

The effects of temperature and confining pressure were investigated on the mode I and mode II fracture toughnesses of Cracked Chevron-Notched Brazilian Disc (CCNBD) cement mortar specimens. The specimens were tested at ambient temperature (20 °C), as well as 60, 100 and 150 °C to study the effects of temperature. Furthermore, CCNBD cement mortar specimens were tested under 0, 3, 5, 7, and 10 MPa to investigate the effect of confining pressure on fracture toughness. Cement mortar specimens were used for testing in this study. According to the results, both mode I and mode II fracture toughnesses of the cement mortar increased linearly with increasing the confining pressure. Mode I and mode II fracture toughnesses initially increased by increasing the temperature up to 60 °C but then decreased as the temperature was elevated to the 100–150 °C range.

FRACTURE TOUGHNESS ASSESSMENT OF DEEP WELL ROCK USING CCNBD AND SCB TEST PROTOCOLS

Fracture mechanics is widely deployed in stone fracture modeling, digging with hydraulic, and dynamic fracture techniques for assessing the effects of rock fracture mechanical properties on different hydraulic fracture digging process. To get a better insight about the process, testing the actual rock samples can be extremely imperative. This study, indeed, attempts to evaluate the fracture toughness of actual rock samples using two different test protocols, i.e., Cracked Chevron Notched Brazilian Disc (CCNBD) and Semi-Circular Bend (SCB). The results indicate that similar behavior is observed for the samples under the two tests. However, due to the geometry, created a crack, and the loading conditions of the prepared samples, the fracture occurs for the SCB samples under a less needed force. The results also indicate that the SCB test samples yield more fracture toughness.

Analysing mixed mode (I–II) fracturing of concrete discs including chevron and straight-through notch cracks

Pre-existing cracks in concrete material and discontinuities in concrete structures are seldom subjected to pure tensile loading (mode I); rather, they are subjected to compressive, shear (mode II) or mixed mode loading (mode I-II). Cracked Chevron Notched Brazilian Disc (CCNBD) and Semi-Circular Brazilian (SCB) specimens including chevron notch and straight-through cracks were used in this research to analyse mixed mode fracturing of concrete. Both the experimental and numerical results showed that the crack initiation angle (θ) was found dependent on the crack inclination angle (β). Moreover, CCNBD specimens with chevron notch crack geometry was found more suitable than the SCB geometry with straight-through notch crack to analyse mixed mode fracturing behaviour of plain concrete. However, using CCNBD or SCB specimens straight-through notch cracks may be more useful for testing the fracturing properties of coarse-grained concrete as sharp chevron cracks reduce the possible Fracture Process Zone (FPZ) size in front of the notch crack tip. Further analyses showed that the critical crack length is found as 13 mm (dimensionless notch length (α) = 0.5). Thus, it is understood that the unstable crack propagation starts between the initial chevron notch crack and final chevron notch crack length and it shows all experiments were done accurately. This outcome would be useful for concrete researches to obtain the yield point by using the relative analysis for finding where the unstable crack propagation starts.

An experimental and theoretical comparison of CCNBD and CCNSCB specimens for determining mode I fracture toughness of rocks

AbstractThe cracked chevron notched Brazilian disc (CCNBD) specimen, suggested by the International Society for Rock Mechanics for testing mode I fracture toughness of rocks, usually yields rather conservative toughness measurements, and the reasons have not been fully explored. In this study, the CCNBD method is compared with the cracked chevron notched semicircular bending (CCNSCB) method in the fracture process zone (FPZ) and its influence on the fracture toughness measurement. Theoretical analysis reveals that the FPZ is longer in the CCNBD specimen than in the CCNSCB specimen using a relatively large support span, the toughness measurement using the former is affected more seriously by the presence of FPZ, and thus the CCNBD method is usually, more or less, conservative compared with the CCNSCB method. These inferences are further validated by experimental results, which indicate that the CCNBD test indeed produces much lower fracture toughness values and even the results of 75‐mm radius CCNBD specimens are still lower than those of 25‐mm radius CCNSCB specimens. Consequently, due to smaller FPZ, the CCNSCB specimen with a relatively large span is more likely to produce comparably accurate or representative toughness value, and it may be more suitable than the CCNBD specimen for the engineering applications that require more representative or less conservative fracture toughness.

Experimental and numerical investigation of cracked chevron notched Brazilian disc specimen for fracture toughness testing of rock

AbstractThe cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to quantify mode I fracture toughness (KIc) of rock, and it has also been applied to mode II fracture toughness (KIIc) testing in some research on the basis of some assumptions about the crack growth process in the specimen. However, the KIc value measured using the CCNBD specimen is usually conservative, and the assumptions made in the mode II test are rarely assessed. In this study, both laboratory experiments and numerical modeling are performed to study the modes I and II CCNBD tests, and an acoustic emission technique is used to monitor the fracture processes of the specimens. A large fracture process zone and a length of subcritical crack growth are found to be key factors affecting the KIc measurement using the CCNBD specimen. For the mode II CCNBD test, the crack growth process is actually quite different from the assumptions often made for determining the fracture toughness. The experimental and numerical results call for more attention on the realistic crack growth processes in rock fracture toughness specimens.

Using acoustic emissions to enhance fracture toughness calculations for CCNBD marble specimens

Rock fracture mechanics has been widely applied to blasting, hydraulic fracturing, mechanical fragmentation, rock slope analysis, geophysics, earthquake mechanics and many other science and technology fields. Development of failure in brittle materials is associated with microcracks, which release energy in the form of elastic waves called acoustic emissions. In the present study, acoustic emission (AE) measurements were carried out during cracked chevron notched Brazilian disc (CCNBD) tests on Nestos marble specimens. The fracture toughness of different modes of loading (mode-I and –II) is calculated and the results are discussed in conjunction with the AE parameters.

The Influence of Specimen Type on Tensile Fracture Toughness of Rock Materials

Up to now, several methods have been proposed to determine the mode I fracture toughness of rocks. In this research, different cylindrical and disc shape samples, namely: chevron bend (CB), short rod (SR), cracked chevron notched Brazilian disc (CCNBD), and semi-circular bend (SCB) specimens were considered for investigating mode I fracture behavior of a marble rock. It is shown experimentally that the fracture toughness values of the tested rock material obtained from different test specimens are not consistent. Indeed, depending on the geometry and loading type of the specimen, noticeable discrepancies can be observed for the fracture toughness of a same rock material. The difference between the experimental mode I fracture resistance results is related to the magnitude and sign of T-stress that is dependent on the geometry and loading configuration of the specimen. For the chevron-notched samples, the critical value of T-stress corresponding to the critical crack length was determined using the finite element method. The CCNBD and SR specimens had the most negative and positive T-stress values, respectively. The dependency of mode I fracture resistance to the T-stress was shown using the extended maximum tangential strain (EMTSN) criterion and the obtained experimental rock fracture toughness data were predicted successfully with this criterion.