Pre-existing Flaws Research Articles

Study on the mechanisms of crack turning in bedded rock

Crack turning is difficult to study by performing physical experiments and field tests due to the multiple and complex cracking processes that occur in bedded rock. Stress concentration around a pre-existing flaw can induce the initiation and propagation of one or two macro-cracks. Hence, crack turning can be observed and analysed. In the present study, the failure of flaw-containing bedded rock is simulated, and the simulation replicates most of the phenomena observed in similar physical experiments. Five types of crack turning at bedding planes are observed during the cracking process. The results reveal that crack turning corresponds to not only tensile cracks but a combination of tensile and shear crack segments. Furthermore, crack turning corresponds to not only the continuous propagation of a single macro-crack but also the coalescence of two macro-cracks. By analysing force and displacement results, some intrinsic mechanisms are also revealed, which is beneficial for the study of bedded rock failure.

Simulation on heterogeneous rocks with a flaw using grain-based discrete-element method

To take into consideration the effect of mineral microstructure on crack growth and mechanical properties of heterogeneous rocks containing a single pre-existing flaw under uniaxial compression, th...

Failure characteristics of coarse and fine sandstone containing two parallel fissures subjected to true triaxial stresses

Failure characteristics of flawed rocks subjected to true triaxial stress are not studied, and the cracking mechanism is not fully understood. Especially, the damage and failure mechanisms incorporating the effect of material properties such as grain diameter are seldom revealed. In this study, true triaxial compression tests are conducted on flaws-contained specimens of coarse and fine sandstone, to explore the coupled effects of the microstructure and the pre-existing flaws on cracking characteristics and further to reveal the cracking mechanism. Acoustic emission (AE) data and the mechanical data are the main experimental output. It is found that cracking characteristics are collectively affected by the microstructure features and the geometrical features of specimen-sized pre-existing flaws. When the confining pressure is relatively large, the mechanical interaction between coarse grains inhibits the specimen’s rupture and contributes to the increase of the strength of coarse sandstone. Due to the complex interaction between grain-scale microcracks, the micro-shear cracking mechanism and the micro-tensile cracking mechanism are presented in the physical process of damage progression in coarse sandstone. However, the microcrack clustering phenomenon in fine sandstone is insignificant, and the interaction between grain-scale microcracks is weak. Accordingly, the micro-shear cracking is seldom generated in fine sandstone.

Peridynamic micro-elastoplastic constitutive model and its application in the failure analysis of rock masses

In the process of compression, rock exhibits post-peak strength, which means that the failure criterion of traditional bond-based peridynamics is not suitable for rock materials. The peridynamic constitutive function is mainly for homogeneous materials and functional materials, which cannot reflect the characteristics of the post-peak strength of the rock. Therefore, based on peridynamic theory, a micro-elastoplastic constitutive model considering the post-peak stage of rock mass under compression loading is proposed, and the loading-unloading path in the post-peak stage of the rock is considered. This model makes up for the disadvantage that the traditional peridynamics model cannot reflect the post-peak strength of rock. Considering that the nonlinearity of the mechanical properties is caused by the heterogeneity of rock materials, the nonuniform discrete method is used in the model, which further improves the accuracy of the numerical simulation. Using the micro-elastoplastic model, the rock specimens were numerically simulated in uniaxial compression tests and uniaxial cyclic load tests, while the flaw propagation of a single pre-existing flaw with different inclination angles under uniaxial compression was analysed. The numerical simulation results are in good agreement with experimental results, which verifies the accuracy and superiority of the new model.

Crack Growth in Rocks with Preexisting Narrow Flaws under Uniaxial Compression

Crack growth in rocks with preexisting flaws is of real concern for many rock engineering applications. The present paper considers the flaw that has a narrow gap that eventually gets closed on loading. Crack growth behavior leads to different failure modes, and corresponding strength characteristics were studied with respect to flaw angle. An experimental study was carried out with gypsum specimens replicating rock, that considered single and double flaws subjected to uniaxial compression. Crack growth and stress behaviors were analyzed and compared. The study was later extended to include numerical analyses using ABAQUS that adopted the extended finite element method with a cohesive zone model to capture crack growth behavior. The numerical model successfully mimicked wing crack propagation similar to the experimental results. The appearance of shear cracks and their growth upon loading was also predicted and crack initiation stress with respect to wing- and shear cracks was investigated. No significant differences were observed for single flaw specimens. However, in the case of multiple flaws with nonparallel configurations, the effect of shielding had a significant impact on coalescence behavior.

Experimental Investigation on Frost Heaving Force (FHF) Evolution in Preflawed Rocks Exposed to Cyclic Freeze-Thaw Conditions

This work is aimed at investigating the structural deterioration and the frost heaving force evolution characteristics of flawed rocks using a self-developed frost heaving force (FHF) measurement system. Three kinds of preflawed rocks with different flaw geometry parameters were used to conduct the FHF measurement tests. The testing results reveal five distinguished stages from the frost heaving force evolution curve; they are the inoculation stage, explosive stage, decline to steady stage, recovery stage, and sudden drop stage. In addition, a secondary frost heaving phenomenon is found, and the secondary peak value is lower than the initial peak value. Moreover, the FHF decreases with increasing the F-T cycle number, and its decreasing rate becomes faster at a high F-T cycle. The frost heaving force is affected not only by flaw geometry but also by the lithology. For low-pore hard rock, damage propagates quickly after the occurrence of freeze-thaw damage. It is suggested that the mesoscopic structure of rock affects the water migration and water-ice phase transformation, and rock can be fractured by FHF in the preexisting flaws.

Experimental study of crack evolution in prefabricated double-fissure red sandstone based on acoustic emission location

The development of micro-cracks during damage to rocks affects the propagation of macroscopic cracks on the surface. To explore the crack evolution among pre-existing flaws in rocks, based on the acoustic emission (AE) source location results, the paper analysed the influences of micro-crack development on failure mode. By analyzing AE characteristic parameters, the main controlling factors affecting the development of micro-fractures are determined. The results show that: (1) the development of micro-cracks in rock samples presents a trend of having a high-energy fracture point, the low-energy and small fracture region, the extensive fracture region, and a macroscopic fracture zone. (2) The difference in rock failure mode arises from the influence of rock bridge on the number, location, and extension direction of high-energy fractures, which results in the difference in the formation of main damage zone in rock samples. (3) The total number of micro-cracks, the proportion of shear cracks, and mixed-mode cracks gradually increase with the increase of the rock bridge inclination angle, leading to the gradual evolution of rock samples from simple tensile failure mode to complex crack penetration mode. The variation of the rock bridge inclination angle affects the stress direction of the rock particles at a microscopic level, while it affects the direction of macroscopic crack propagation at a macroscopic level.

Mechanical properties and fracture behavior of flawed granite under dynamic loading

The dynamic mechanical properties and fracture process of rock materials with pre-existing flaws are crucial for explaining the failure mechanisms of deeply buried constructions. To investigate the mechanical properties and fracture behavior of flawed granite subjected to dynamic loading, dynamic experiments were conducted on granite specimens by using a split-Hopkinson pressure bar test system. The effects of strain rate and flaw inclination angle on granite strength and deformation properties were investigated. The white patch initiation, micro-crack propagation, and failure mode were analyzed using a high-speed camera and digital image correlation technology. The fracture mechanism was revealed by quantitatively analyzing the localized strain field and fractal features. The results demonstrated that the dynamic strength and deformation of the flawed specimen compared to an intact specimen had more obvious strain rate effects. The development of white patches determined the direction and path of crack propagation. The fracture process and shear-tensile failure mode were closely correlated with the evolution of the strain field. Furthermore, the fractal characteristics indicated that a larger fractal dimension resulted in a more complex fracture distribution and mixed failure modes. These results provide a reference for the construction design and safety control of deep rock engineering.

Acoustic Emission Characteristics and Joint Nonlinear Mechanical Response of Rock Masses under Uniaxial Compression

The joint arrangement in rock masses is the critical factor controlling the stability of rock structures in underground geotechnical engineering. In this study, the influence of the joint inclination angle on the mechanical behavior of jointed rock masses under uniaxial compression was investigated. Physical model laboratory experiments were conducted on jointed specimens with a single pre-existing flaw inclined at 0°, 30°, 45°, 60°, and 90° and on intact specimens. The acoustic emission (AE) signals were monitored during the loading process, which revealed that there is a correlation between the AE characteristics and the failure modes of the jointed specimens with different inclination angles. In addition, particle flow code (PFC) modeling was carried out to reproduce the phenomena observed in the physical experiments. According to the numerical results, the AE phenomenon was basically the same as that observed in the physical experiments. The response of the pre-existing joint mainly involved three stages: (I) the closing of the joint; (II) the strength mobilization of the joint; and (III) the reopening of the joint. Moreover, the response of the pre-existing joint was closely related to the joint’s inclination. As the joint inclination angle increased, the strength mobilization stage of the joint gradually shifted from the pre-peak stage of the stress–strain curve to the post-peak stage. In addition, the instantaneous drop in the average joint system aperture (aave) in the specimens with medium and high inclination angles corresponded to a rapid increase in the form of the pulse of the AE activity during the strength mobilization stage.

Fracture evolution in mudstone specimens containing a pre-existing flaw under true triaxial compression

In this study, a novel true triaxial loading device that can be paired with an X-ray computerized tomography (CT) scanning system was developed. Using this test equipment, a series of in-situ CT scans of mudstone specimens containing a pre-existing flaw were performed under two true triaxial stress states, namely, the intermediate principal stress parallel to the pre-existing flaw (IPSPPF) and the minimum principal stress parallel to the pre-existing flaw (MPSPPF). The results showed that the difference between the two lateral deformations of the specimen under the IPSPPF condition was more significant than that under the MPSPPF condition and that the stress level of the specimen entering the damage stage under the IPSPPF condition was less than that under the MPSPPF condition. Under the IPSPPF condition, two wing cracks formed in the damage stage first, and an anti-wing crack and a far-field crack then formed successively in the post-peak stage. The surfaces of these four cracks were all parallel to the front edge of the pre-existing flaw. After failure, the pre-existing flaw in the specimen remained fully open. Conversely, under the MPSPPF condition, a far-field crack appeared first in the damage stage, which stopped propagating after intersecting the pre-existing flaw. Concurrently, a wing-like crack and an anti-wing-like crack formed at the tips of the pre-existing flaw in the damage stage, and a new far-field crack formed in the post-peak stage. The surfaces of these four cracks were all perpendicular to the front edge of the pre-existing flaw. The pre-existing flaw was cut off at multiple locations by these cracks and eventually collapsed in large areas. Under both conditions, the inclination angles of all of the cracks in the cross sections of the specimens with respect to the direction of the intermediate principal stress were approximately 0°.

Experimental investigation of crack propagation behavior and failure characteristics of cement infilled rock

A detailed understanding of the failure characteristics of infilled jointed rock is a key concern in underground engineering and is critical to the design, construction and maintenance of relevant projects. In this paper, uniaxial compression tests were performed to investigate the effect of the joint arrangement (different rock bridge angles) on the strength and crack propagation of red sandstone containing a set of preexisting infilled flaws of the same sizes and angles, while the crack fracturing process was observed by AE and strain monitoring. When the angle between the rock bridge and loading stress was less than 24°, the rock mass failure form was tensile failure with some localized shear failure occurring in the rock bridge damaged by crack penetration, leading to an overall decrease in the carrying capacity. The turning points of the strain and AE signal were clearly observed to be caused by the initiation and propagation of cracks, and the energy consumed by the initiation of shear cracks was greater than that of tensile cracks. The study was further conducted using the fracture mechanics numerical code FRACOD to investigate the crack propagation law and failure mechanism under different loading stress conditions. Simulation results from three 2D similar models showed that the percentage of open fractures decreased, but slipping increased; in addition, the total length of the fractures was greatly reduced with increasing confining stress. These results are important and valuable for understanding the crack mechanism of rock engineering in deep underground mining excavations.

Effects of binder type and dosage on the mode I fracture toughness of cemented paste backfill-related structures

The mode I fracture toughness, which is an indication of the amount of stress required to propagate a preexisting flaw, is usually used to evaluate the resistance of interlaminar structure to tensile stress. Considering that the cemented paste backfill (CPB) includes various interfaces (e.g. early-age CPB/advanced-age CPB interface and CPB/Rock interface), a thorough understanding of the magnitude/order of the mode I fracture toughness of different CPB-related structures is critical in stability assessment. However, only limited researches have been dedicated to the fracture toughness of CPB so far, and none of them have addressed the effects of different binders on the development of the mode I fracture toughness. Hence, many semi-circular samples prepared with various binders (e.g., Portland cement type I (PCI), a mixture of PCI and Slag (PCI/Slag) or fly ash (PCI/FA), with a ratio of 25/75, 50/50, 75/25) are prepared and subjected to the semi-circular bending test after being cured for certain periods. The findings reveal that the binder has an observable influence on the mode I fracture toughness of different interface structures, and despite the binder types, CPB depicts the highest mode I fracture toughness in comparison with early-age CPB/advanced-age CPB interface and CPB/Rock interface. Furthermore, the samples prepared with PCI/Slag generally experience higher mode I fracture toughness as well. The results can contribute to the cost-effective design and the safety assessment of CPB structures.

Strain localization and cracking behavior of sandstone with two gypsum-infilled parallel flaws

The infilled flaws within rocks significantly influence the mechanical behavior of non-through flawed rocks. To investigate the influence of infilling on strain localization in the pre-cracking, cracking and cracking coalesce stages, experimental and numerical investigations were conducted on sandstone containing two gypsum-infilled parallel flaws, but of different flaw geometric configurations. The full-field strain evolution and progressive cracking process are analyzed using the acoustic-optical-mechanical methods, while the stress evolution and stress transfer by infilling were studied by the particle flow code modeling. The results show that the major principal strain of flawed sandstone with infilling changes from the diffused ellipse to highly localized linear strip with the increase of loading level, while the shear strain keeps a diffused ellipse shape invariable. The shear strain localization of infillings with a large flaw inclination initiates at a lower loading level than that of small one. The peak strength and Young’s modulus of flawed sandstone show a first decrease, then increase and finally decrease trend with the ligament angle increasing from 0° to 150°, and both achieve their minimums at 60°. Ten types of cracks were observed, and four patterns of crack coalescence were identified. By comparison with sandstone of open flaws, the flaw infilling simplifies the geometric pattern of produced crack networks, which is quantitatively characterized by fractal dimension analysis. The numerical modeling shows that the infilling enlarges the compressive bond force zone and decreases the tensile bond force zone in the surrounding areas of pre-existing flaw, thus increasing the reinforcement coefficient. Moreover, the flaw geometric configuration has a significant influence on the reinforcement coefficient. It is also found that the increment of reinforcement coefficient defined by crack initiation stress is greater than those defined by either crack coalescence or peak stresses.

Underlying Mechanisms of Crack Initiation for Granitic Rocks Containing a Single Pre-existing Flaw: Insights From Digital Image Correlation (DIC) Analysis

Determination on underlying mechanisms of crack initiation is of vital importance to understand the failure processes of geomaterials in practical engineering. In this study, uniaxial compression experiments of granitic samples containing a single pre-existing flaw were conducted and the failure processes were recorded by using the high-speed camera. To quantitatively determine the crack initiation mechanism, a novel method was first proposed based on digital image correlation (DIC) analysis and then its validity was confirmed. By utilizing this method, three types of cracks with different initiation mechanisms were identified and the effect of flaw inclination angle on crack initiation mechanisms was discussed from the viewpoint of theoretical analysis. With the increase of inclination angles, wing cracks change from mixed mode I/II cracks to mode I cracks, while anti-wing cracks have no evident changes and are dominated by mode II cracks. Under compressive pressure, the upper and bottom surfaces of pre-existing flaw deform to each other and the distributions of full-field tangential stress around flaw are different, which might induce the variation of crack initiation mechanisms with regard to the inclination angle.

A comparative study on the crack development in rock-like specimens containing unfilled and filled flaws

Rock masses consist of various discontinuities that significantly affect the crack development patterns which dominates the ultimate failure of geostructures. The interaction of the pre-existing flaws with each other and with the newly formed cracks is complicated and demands a comprehensive investigation. This paper couples the 3D printing technology with the digital image correlation (DIC) and the bonded particle model (BPM) to study failure of rock-like specimens with pre-existing flaws. Systematic flaws configurations are considered including single, coplanar, partially overlapped and fully overlapped arrangements. The flaws are considered to be unfilled and filled with a weak material. The DIC strain and BPM displacement vectors analysis indicate the strong effects of the filling material on the deformation behaviour of the 3D printed specimens. The failure pattern of the single filled flaws is transformation from compressive failure (0°) to shear failure (15°–60°) and to tensile failure (75°–90°). However, in the coplanar flaws failure is transformation from compressive (0°) to mixed-mode compressive shear (15°-30°), then to shear (45°), to mixed-mode tensile-shear (60°–75°) and then to pure tensile (90°). However, the partially coplanar filled flaws all (except 0° which is compressive failure) exhibit mixed-mode failure in the order of compressive-shear (15°–30°) and transformation to the tensile-shear (45°–90°). BPM displacement vectors revealed five crack types in the specimens by the analysis of the relative movement of the vectors in some critical locations such as flaws tips, rock bridge and coalescence zone. Moreover, a new coalescence type was identified in the 15° flaw (either unfilled or filed) which is named Type X and is a mixed-mode shear tensile crack with a coplanar secondary shear crack. Furthermore, it is observed that the peak load of the filled specimens are much higher than that of the unfilled ones because the filling material requires extra energy to fracture and thus the filled specimens can carry larger load before fail.

Cracking behaviours of rock‐like materials containing three preexisting flaws after high‐temperature treatments

AbstractThe mechanical properties and cracking behaviours of rock‐like materials containing three preexisting flaws after high‐temperature treatments are studied. The temperature variations of 25°C, 70°C and 140°C are chosen to explore the influence of temperature, and the ligament angle variations of 75°, 90° and 105° are chosen to explore the effect of preexisting flaws. The crack initiation and coalescence modes and specimen failure after high‐temperature treatments are recorded by a high‐speed dynamic analysis system. Seven modes of crack initiation and ten types of crack coalescence are recorded in the rock‐like specimens. With an increase in temperature and ligament angle, the mechanical properties such as peak strength, peak strain and crack initiation stress change regularly. Furthermore, the experimental results demonstrate that the temperature has a significant impact on the complete stress–strain curves and cracking behaviours of flawed rock‐like materials.

Fracture Propagation of Rock like Material with a Fluid-Infiltrated Pre-existing Flaw Under Uniaxial Compression

Crack propagation can gradually reduce the strength of the rock and eventually result in rock failure. Coupling effect of stress and seepage in fracture could accelerate the rock failure process. In this work, a set of water sealing device is developed to apply different fluid pressures in the pre-existing fracture in specimens made of rock-like material. We have carried out uniaxial compression tests on specimens at different pre-existing flaw dip angles (30°, 45°, and 60°) coupled with fluid pressures in the fracture. Through laboratory experiments and numerical simulations, we find that without fluid pressure in the pre-existing flaw, wing cracks and secondary cracks appear at the pre-existing flaw tips. With the increase of the fluid pressure in the flaw, the propagation of secondary cracks is restrained, no secondary cracks appear at the flaw tips. The increase of fluid pressure accelerates the wing crack propagation, inhibits the secondary cracks, and causes the specimen to undergo tensile failure. Compared with the specimen without the fluid pressure in the flaw, the fluid pressure in the flaw promotes wing crack initiation and propagation, and causes the initiation stress of the wing cracks and the peak strength of the specimens to decrease gradually. With or without fluid pressure in the fracture with the increase of the flaw dip angle, the initiation stress of wing cracks and peak strength of the specimen first decrease and then increase. When the pre-existing flaw dip angle is 45°, the peak strength and the initiation stress are the lowest.

Experimental and numerical investigations on crack development in 3D printed rock-like specimens with pre-existing flaws

Rock masses contain various discontinuities such as flaws, cracks, join sets and more, each significantly affect the crack initiation, propagation and coalescence patterns of new cracks which dominates the ultimate failure of geostructures. The interaction of the pre-existing flaws with each other and with the newly formed cracks is complicated which demands a comprehensive look into the phenomenon. This paper focuses on the applicability of the 3D printing technology coupled with the digital image correlation (DIC) and the bonded particle model (BPM) in replicating real behaviour of natural rocks containing pre-existing flaws. Systematic flaws configurations are considered including single, coplanar, partially overlapped and fully overlapped arrangements to enable a comprehensive coalescence analysis. The results show that the BPM is capable of precisely modelling the crack initiation location, type (tensile, shear or mixed-mode) and coalescence type detected by the DIC. Moreover, it is shown that the 3D printed specimens can reproduce the previously identified cracks and coalescence types. By the analysis of the displacement fields, a new coalescence type is detected and introduced and named as type X, which is a mix of wing crack and quasi coplanar secondary shear crack. The analysis of the displacement vectors revealed five types of cracks: shear opposite slip crack, tensile opposite crack, tensile compliant crack, shear compliant crack, and mixed tensile-shear crack. Furthermore, the comparisons made between the peak loads of the experiments and the simulations show a god agreement in terms of peak load magnitude and flaws inclination angle dependency.

On the effect of stress amplitude on fracture and energy evolution of pre-flawed granite under uniaxial increasing-amplitude fatigue loads

Rock mass containing discontinues is often subjected to fatigue loads in many mining and civil engineering. The stress amplitude (SA) as an important factor impacts rock fracture evolution during fatigue loading condition. Existing effects to investigate the impacts of SA on rock fatigue behaviors were mainly on constant SA loading condition, the influences of incremental stress amplitude (ISA) for rock subjected to cyclic uniaxial increasing-amplitude conditions is not well understood. In this work, uniaxial increasing-amplitude fatigue loads and post-test 3D CT scanning experiments were conducted on granite containing two pre-existing flaws. Our experimental results reveal the impact of four applied ISA (i.e., 5, 10, 20 and 30 MPa) on rock fracture evolution, energy conversion and crack network pattern at rock bridge segment. The damage accumulation and volumetric deformation increase with the increase of applied ISA. The accelerated damage evolution stage is different for sample with different ISA, and it starts to appear the earliest for a sample subjected to ISA of 30 MPa. Energy conversion analysis reveals that the dissipated energy used to drive the crack propagation is relatively large for a sample with higher ISA. Post-test 3D CT visualization reveals a most striking finding that the crack network pattern becomes complicated as the applied ISA increases. It is suggested that crack coalescence between the flaws is easy to be formed for rock subjected to low ISA, rock bridge segment fracturing is stress amplitude dependent. The testing results are expected to improve the understanding of the influence of stress amplitude on the fatigue behaviors of pre-flawed rock when rock mass is subjected to increasing-amplitude stress disturbance.

Mechanical properties and failure behavior of rock with different flaw inclinations under coupled static and dynamic loads

The deep fissured rock mass is affected by coupled effects of initial ground stress and external dynamic disturbance. In order to study the effect of internal flaw on pre-stressed rock mechanical responses and failure behavior under impact loading, intact granite specimens and specimens with different flaw inclinations are tested by a modified split Hopkinson pressure bar (SHPB) and digital image correlation (DIC) method. The results show that peak strain and dynamic strength of intact specimens and specimens with different flaw angles (α) decrease with the increase of axial static pressure. The 90° flaw has weak reduction effect on peak strain, dynamic strength and combined strength, while 45° and 0° flaws have remarkable reduction effect. Specimens with 90° flaw are suffered combined shear and tensile failure under middle and low axial static pre-stresses, and suffered shear failure under high axial static pre-stresses. Specimens with 45° and 0° flaws are suffered oblique shear failure caused by pre-existing flaw under different axial static pre-stresses. Besides, based on digital image correlation method, it is found that micro-cracks before formation of macro fractures (include shear and tensile fractures) belong to tensile cracks. Tensile and shear strain localizations at pre-existing flaw tip for specimen with 45° and 0° flaws are produced much earlier than that at other positions.