Cracked Chevron Notched Brazilian Disc Specimens 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.

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.

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.

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.

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.

Loading-rate-dependent progressive fracturing of cracked chevron-notched Brazilian disc specimens in split Hopkinson pressure bar tests

The dynamic progressive fracturing process of the cracked chevron-notched Brazilian disc (CCNBD) specimen, the latest chevron-notched specimen suggested by the International Society for Rock Mechanics (ISRM) for fracture toughness measurements, is numerically assessed via discrete element method. Simulated acoustic emission distributions reveal that the cracking front is rather curved with jags and the critical crack surpasses the location specified by the measuring assumption. Most importantly, even if the dynamic force balance can be achieved by careful pulse shaping, the positions of the critical fracturing profiles remain highly dependent on the dynamic loading rates. Our investigation indicates that unchecked adoption of the quasi-static assumption on the CCNBD specimen in SHPB testing to measure the dynamic fracture toughness of rocks is suspect. The loading-rate-dependent cracking profiles of chevron-notched specimens cannot be ignored in order to accurately determine the dynamic mode I fracture toughness of rocks.

Fracture Toughness Determination of Cracked Chevron Notched Brazilian Disc Rock Specimen via Griffith Energy Criterion Incorporating Realistic Fracture Profiles

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to measure the mode I fracture toughness of rocks, and has been widely adopted in laboratory tests. Nevertheless, a certain discrepancy has been observed in results when compared with those derived from methods using straight through cracked specimens, which might be due to the fact that the fracture profiles of rock specimens cannot match the straight through crack front as assumed in the measuring principle. In this study, the progressive fracturing of the CCNBD specimen is numerically investigated using the discrete element method (DEM), aiming to evaluate the impact of the realistic cracking profiles on the mode I fracture toughness measurements. The obtained results validate the curved fracture fronts throughout the fracture process, as reported in the literature. The fracture toughness is subsequently determined via the proposed G-method originated from Griffith’s energy theory, in which the evolution of the realistic fracture profile as well as the accumulated fracture energy is quantified by DEM simulation. A comparison between the numerical tests and the experimental results derived from both the CCNBD and the semi-circular bend (SCB) specimens verifies that the G-method incorporating realistic fracture profiles can contribute to narrowing down the gap between the fracture toughness values measured via the CCNBD and the SCB method.

Study of fracture toughness and weakening mechanisms in gypsum interlayers in corrosive environments

China's rock salt deposits, which are mostly located in the Yunying area, are typically characterized by their interlayers. In the construction and operation of salt caverns, gypsum interlayers are soaked in brine and oil. To investigate the fracture toughness and weakening mechanisms of gypsum interlayers in corrosive environments, a series of laboratory tests, including cracked chevron notched Brazilian disc (CCNBD) tests and micro computed tomography (MCT) scanning, was performed. Ninety CCNBD specimens and 15 small specimens, with dimensions of 5 mm × 5 mm × 5 mm, were prepared and then soaked in 4 types of liquid and at 3 different temperatures. The results indicate that the fracture toughness of gypsum decreases significantly with increasing temperature when soaked in a given liquid. By contrast, the fracture toughness of gypsum remains approximately identical after treatment with water, half-saturated brine, and saturated brine at the same temperature. With increasing time and temperature, the porosity of the gypsum increases. The rate of porosity change (slope) increases with increasing temperature, but it remains stable with increasing brine concentration. Water and brine have more severe corrosive effects on gypsum than does acidic oil. The weakening of gypsum soaked in water and brine is attributed to the combined effects of water, temperature, and chlorine ions. Notably, the water-temperature effect contributes to a severe weakening of gypsum interlayers, but the chlorine ions have little effect. The mechanism of the weakening of gypsum soaked in acidic oil is chemical reactions and dehydration. Petroleum acid (naphthenic acid) reacts with gypsum to produce calcium naphthenate, which can be dissolved in the oil phase. During this process, the dissolution of calcium naphthenate in the oil phase promotes the forward reaction. Consequently, the study has significance for the evaluation of the leakage risk of storage caverns in bedded salt deposits.

Numerical Observation of Three-Dimensional Wing Cracking of Cracked Chevron Notched Brazilian Disc Rock Specimen Subjected to Mixed Mode Loading

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by International Society for Rock Mechanics for measuring mode I fracture toughness of rocks. Subsequently, this specimen geometry has been widely extended to conduct mixed mode fracture tests on rocks as well. A straight through crack front during the fracturing process upon the root of the chevron notch is assumed in the testing principle, but has never been thoroughly evaluated before. In this study, for the first time, the progressive rock fracture mechanism of the CCNBD rock specimen under mixed mode loading is numerically simulated. Specimens under representative mixed mode loading angles are modelled; and the assumption of the straight through crack front growth is critically assessed. The results show that not only the notch tip but also the saw-cut chevron notch cracks during the experiments, yielding a prominent twisted front, far from being straight. The crack front never grows up to the root of the notch ligament and the straight through crack front assumption is never satisfied in the realistic rock fracture progress of this chevron notched specimen subjected to mixed mode loads. In contrast, the fracture progress features typical three-dimensional wing cracking towards the loading ends. The numerically observed progressive fracture mechanism reveals that the measuring principle of mixed mode fracture tests employing CCNBD specimens is significantly violated and the measures of both modes I and II fracture toughness are uncertain.

Rock fracture toughness study using cracked chevron notched Brazilian disc specimen under pure modes I and II loading – A statistical approach

Fracture toughness of a white marble is studied experimentally using several cracked chevron notched Brazilian disc (CCNBD) specimens under pure mode I and pure mode II loading. Even in the presence of natural scatters in the test data, it was observed that the average mode II fracture toughness KIIc was considerably larger than that of mode I fracture toughness KIc such that the mean fracture toughness ratio (KIIc/KIc) was about 2. Using the generalized maximum tangential stress theory, the obtained mode II test results were estimated in terms of mode I fracture toughness data. The enhanced KIIc value in the CCNBD specimen could be related to the influence of very large negative T-stress value that exists in the mode II CCNBD specimens. The statistical analyses of test data were performed successfully to predict the Weibull parameters of mode II results in terms of mode I Weibull parameters.

The damage mechanism of rock fatigue and its relationship to the fracture toughness of rocks

This study presents the results of laboratory diametrical compression tests performed on Brisbane tuff disc specimens to investigate their mode-I fracture toughness response to static and cyclic loading, as a function of the applied load. Both the static and cyclic loading tests were carried out on Cracked Chevron Notched Brazilian Disc (CCNBD) rock specimens. Two different types of cyclic loading were applied: (a) cyclic loading with constant mean level and constant amplitude, termed sinusoidal cyclic loading and (b) cyclic loading with increasing mean level and constant amplitude, termed increasing cyclic loading. The fracture toughness response to cyclic loading was found to be different from that under static loading in terms of the ultimate load and the damage mechanisms in front of the chevron crack. A maximum reduction of the static fracture toughness (KIC) of 46% was obtained for the highest amplitude increasing cyclic loading test. Conversely, for sinusoidal cyclic loading, a maximum reduction of the static KIC of 29% was obtained. Detailed scanning electron microscope (SEM) examinations revealed that both loading methods cause fatigue in the CCNBD specimens. When compared with static rupture, the main difference with the cyclically loaded specimens was that intergranular cracks were formed due to particle breakage under cyclic loading, SEM images showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix because of both intergranular fracturing and transgranular fracturing.

Recalibration and Clarification of the Formula Applied to the ISRM-Suggested CCNBD Specimens for Testing Rock Fracture Toughness

The cracked chevron-notched Brazilian disc (CCNBD) was proposed by the International Society for Rock Mechanics (ISRM) to test the mode I (opening mode) fracture toughness of rock. The test method has been vigorously discussed and debated, despite being the subject of intensive research for decades. The minimum (critical) dimensionless stress intensity factors affiliated with the formula for calculating the fracture toughness using CCNBD specimens with different geometric parameters remain elusive and complex. The matter cannot be resolved by simply replacing the diameter in the original formula with the radius, as claimed by several authors. In this paper, the formula is fundamentally improved, as wide-ranging minimum dimensionless stress intensity factors pertaining to diversified CCNBD geometries are recalibrated by three-dimensional finite element analysis, and an expression with tabulated coefficients is obtained through curve-fitting the data obtained from the numerical calibration. The present results are shown to be more accurate than those in the literature. Furthermore, the importance of the reasonability of the results is highlighted; a comprehensive comparison of different values shows that the upper bounds of minimum stress intensity factors are violated by the above claim. The confusion resulting from the claim is, thus, clarified conclusively.

Measuring fracture toughness of crystalline marbles under modes I and II and mixed mode I–II loading conditions using CCNBD and HCCD specimens

Fracture toughness of three crystalline Calcite marbles, which are different only in grain size and distribution, is determined under modes I and II and mixed mode I–II loading conditions using Cracked Chevron-Notched Brazilian Disc (CCNBD) and Hollow Centre Cracked Disc (HCCD) specimens. The results show that mode I fracture toughness ( K I C ) is correlated negatively with grain size. For each marble, HCCD yields lower values of fracture toughness, compared with CCNBD. This difference is negligible under mode I loading condition; while it becomes larger as loading condition transits from mode I to mode II. Measured values of P-wave velocity ( V P ), Brazilian tensile strength ( σ tB ) and Schmidt hammer hardness are in direct relation with K I C of the marbles. The obtained results are compared with three fracture criteria, in which the Minimum Strain Energy Density (MSED) criterion has provided better correlations with different critical combinations of modes I and II Stress Intensity Factors.

Mode I and Mode II Fracture Toughness Testing for a Coarse Grain Marble

A series of fracture toughness tests have been conducted on a type of coarse grain marble. The tests were carried out either in pure mode I or in pure mode II conditions by using the cracked chevron notched Brazilian disc (CCNBD) specimen. A total number of 44 CCNBD specimens were tested, half in pure mode I and the rest in pure mode II, to obtain reliable values for mode I and mode II fracture toughness (KIc and KIIc) of the tested marble. The average value of KIc for this marble was approximately about 1.12 MPa m showing a good agreement with those reported for similar coarse grain marbles. The mode II fracture toughness was found to be 2.25 MPa m in average which is approximately twice the mode I fracture toughness. However, the conventional fracture criteria suggest that the mode I fracture toughness should be higher than the mode II fracture toughness, (KIc>KIIc). According to these criteria, the ratio of KIIc / KIc is a figure typically between 0.63 and 0.96. It is shown in this paper that enhanced mode II fracture toughness of CCNBD specimen could be due to the effects of highly negative T-stress when the specimen is subjected to mode II. It is also shown that an improved prediction for the ratio KIIc / KIc can be achieved when the effect of T-stress is taken into account.

The use of the cracked Brazilian disc geometry for rock fracture investigations

The cracked Brazilian disc geometry is receiving attention for rock fracture mechanics research due to its many advantages over other conventional geometrical configurations. A thorough study of the geometry is vitally important at this stage. A superimposition technique is presented to accurately solve the general cracked straight through Brazilian disc (CSTBD) fracture problem using the dislocation method and also the complex stress function method. The solution is valid for any crack inclination angle and any crack length. The solution for the cracked chevron notched Brazilian disc (CCNBD) is then discussed. The theoretical evaluation for the Mode I cracked Brazilian disc fracture problem, a special case of the general problem, has been numerically calibrated by both finite element and boundary element methods, and a simple and accurate method to calculate the Mode I critical (minimum) dimensionless stress intensity factor for the CCNBD specimen is given. The valid geometrical range and valid minimum specimen size for the CCNBD geometry is presented to allow a valid Mode I fracture toughness value to be determined. The theoretical and numerical solutions for the stress intensity factor and the valid geometrical range are also substantiated by extensive experimental validation by comparing the results with those obtained by the ISRM-suggested CB and SR methods.