Parallel Flaws Research Articles

Co-effects of parallel flaws and interface characteristics on the fracture behavior of rock-concrete composite specimens

The fracture behaviour of rock-concrete composite (RCC) containing interfaces and flaws is important for assessing the stability and safety of concrete systems constructed on rock foundations. Uniaxial compression experiments were performed on RCC specimens, and acoustic emission (AE) and digital image correlation (DIC) techniques were adopted to capture the characteristics of AE and crack extension process. The impacts of interface dip angle, concrete strength, and flaw geometry on the coalescence behavior of RCC specimens were analyzed. The findings indicate that four kinds of crack coalescence patterns were recognized: indirect coalescence, left-tip and right-tip cracks connected, coalescence of same-side tip cracks, and no connection. The peak strength displays a reducing trend with increasing the interface dip angle, while the peak strength presents an increasing trend with increasing concrete strength. The overlapping parallel distribution of composite specimens presents a higher peak strength. The crack coalescence mode of overlapping parallel distribution changes from the coalescence of same-side tip cracks to the no-connection mode when interface dip angle is larger. It is worth noting that cracks tend to appear first in the concrete when the strength is relatively low. The AE counts tended to increase as the interface dip angle increased, with the earliest rise in AE counts in the left-stepping distribution and longer duration in the overlapping parallel distribution. Additionally, the energy characteristics of composite specimens correlate closely with the interface dip angle and flaw geometry.

Experimental study of the effect of crack distribution on the failure mechanism of sandstone specimens based on inclination angles and number of parallel flaws

Discontinuous joints are prevalent in engineered rock masses and play a significant role in the stability of the rock mass. This study aims to analyze the impact of the inclination angle and number of prefabricated flaws on the crack evolution and failure pattern of sandstone specimens. Uniaxial compression tests, along with acoustic emission technology and digital image technology, were employed to monitor and analyze the effects. The findings indicate that: (1) With the increase in the flaw inclination angle, the damage mode of the specimen transitions from tensile to compressive-shear failure. The localized high-strain region on the surface of the specimen predicts the propagation path for the formation of macroscopic cracks. (2) When the number of prefabricated flaws is small, the flaws mainly expand through tensile wing cracks. As the number of flaws increases, the inner flaw tip does not produce cracks. Instead, the failure of the entire specimen occurs along the direction of the outer flaw's tensile wing crack, with the inner flaw running through it. (3) The winged tensile crack is the first crack to appear in all rock samples, regardless of the flaw initiation angles. Finally, the stress intensity factor at the crack tip under uniaxial compression conditions, without considering the closure effect, was expressed based on fracture mechanics theory. The crack initiation angle was then calculated. The results of the theoretical calculation of the initiation angle were found to be consistent with the test results. These research findings can serve as theoretical references and provide insights into the failure mechanisms of cracked rocks and the development of disaster control methods in rock engineering.

Experimental Investigation on the Destruction Features and Acoustic Characteristics of a Brittle Rock Sample Containing Both 2D and 3D Preset Flaws

Original fracture structures always present discontinuity in the real rock mass, and many invisible fractures hide inside the rock mass, which may cause serious engineering safety issues. To mimic the true 3D fracture structures through the experimental method, the gypsum rock-like samples containing both 2D through-type and 3D internal-type preset flaws are prepared, and multiple sets of inclination angles of the twin parallel flaws are set in the test. By applying the AE and DIC monitoring technologies during the uniaxial compression tests, the main results are as follows: (1) The flaw inclination angle presents a direct influence on the surface cracks distribution, maximum principal strain field, and the density of secondary failure in the middle rock; (2) AE events initially distribute around the internal 3D preset flaw, while the gradient inclination angle shows a slight impact on the events’ location before reaching the UCS status of samples; (3) mutations in b-values and S values can serve as evidence for predicting local damage, and the final failures quickly form at various scales and energy levels; (4) when the statistical analysis grid is divided sufficiently, the data window width and moving step length have little impact on the evaluation results, while the recommended bin width of event magnitude is 0.5 or 1.0.

Crack propagation mechanism in bedded rock with parallel flaws: Insights from moment tensor inversion

Studying defects’ effect on bedded rock mechanical properties and failure mechanisms is essential for slope and underground excavation engineering. Acoustic emission helps analyze microcrack generation in rocks, shedding light on quasi-brittle material failure. However, previous studies have shown that the inhomogeneous geometry of defect-containing specimens complicates the localization of acoustic emission events, and some events may go undetected by sensors. This study uses discrete element modeling to examine bedded rock failure with parallel defects. We differentiate damage between acoustic emission events and macro fractures by assessing forces on failure sources. Our findings are as follows: (1) Bedding influences crack propagation direction but has limited impact on final failure. Cracks merging and penetrating in the bridge area are critical for catastrophic failure of samples. (2) Shear failure accumulates at the bridge area and defect tip, resulting in fragmented macroscopic failure. Tensile failure occurs during nucleation region development, leading to a more regular macroscopic failure. (3) High-magnitude events primarily arise from microcrack merging and connections, while crack propagation struggles to generate high-magnitude events. Most acoustic emission events involve 1∼4 microcracks, regardless of bedding angles or defects. The acoustic emission simulation method based on moment tensor inversion presents a novel approach for quantitatively determining the crack propagation law and failure mechanism of rocks with defects.

Interaction between flaws and failure characteristics of red sandstone containing double flaws under compressive-shear loading

The failure characteristics of the fractured rock mass and the interaction between flaws were significantly affected by the flaw configurations. This study quantitatively investigated the effect of flaw geometries on the interaction between flaws under compressive-shear loading. The evolution of the surface strain field was recorded and analyzed using digital image correlation (DIC) technology, enabling the localization of crack paths and the determination of crack initiation mechanisms. Combined with DIC and scanning electron microscopy, both the macroscopic and microscopic failure characteristics of specimens with double flaws subjected to compressive-shear loading were comprehensively investigated. The results showed that the axial peak force of flawed specimen was sensitive to the flaw configuration, and the axial peak forces of flawed specimens with parallel flaws were greater than those with coplanar ones. The tensile strain was the primary factor dominating the initiation and propagation of cracks from flaw tips, whereas the cracking mechanism of edge cracks was a combination of tensile and shear strains. In addition, two novel parameters, namely independent parameter Ri and reinforced parameter Rr, were proposed to quantitatively characterize the independent effect discovered between coplanar flaws and the reinforced effect between parallel flaws, respectively. Besides, it was observed that the flaw configuration has a significant effect on rock failure characteristics, and three typical crack coalescent modes were found. The findings of this paper could facilitate a better understanding of the interaction between flaws and rock failure characteristics subjected to compressive-shear loading conditions.

Interaction of two offset parallel flaws in geomaterials under unloading conditions

Rock excavation operations (also known as unloading) in high rock slopes, tunnels, and underground engineering cause large and extensive stress relaxation, even accompanied by the generation of tensile stress. Nevertheless, there have been few studies on the theoretical damage mechanism of rock under unloading conditions. In the present work, based on the superposition principle and the classic Kachanov method, we first proposed a theoretical calculation method for the stress intensity factor (SIF) at the tips of two parallel-offset flaws under unloading conditions. Then, a theoretical analysis of the effect of various geometric factors, such as flaw staggered distance, rock bridge length, flaw length and inclination angle, was carried out. The interaction between two offset flaws was captured. The results support the following findings: 1) The smaller the staggered distance or the rock bridge length is, the larger the flaw length, and the interaction between the two flaws becomes more significant. 2) With increasing inclination angle, the tendency of tensile failure becomes more obvious, while that of shear failure weakens. 3) The longer the rock bridge length, the more significantly the flaw interaction is affected by the inclination angle. Finally, discrete element simulations were carried out to verify the theoretical analysis results, and good consistency was found. These research results are helpful for understanding the failure mechanism of fractured rock masses caused by excavation (unloading condition).

The effect of flaw geometry and infilling on crack initiation, propagation, and coalescence of a parallel flaw pair in marble

A series of uniaxial compression tests were conducted on unfilled or filled marble specimens with a parallel flaw pair to study the effect of the flaw geometry and infilling on the crack coalescence pattern. Special attention was paid to the crack initiation, propagation, and coalescence processes, which were traced by the CCD camera and digital image correlation (DIC) technique. The experimental results have shown that the appearance of the white patch and strain localization band precedes the initiation of macroscopic crack. The tensile crack evolves from a narrow strain band or white patches with clear boundaries, while the shear crack evolves from a wider strain band or white patch with blurry boundaries. The high-strength fillers suppress the initiation and propagation of the tensile cracks (i.e., T1, T2, and T3) and promote the appearance of the coplanar shear cracks and mixed tensile-shear cracks (i.e., M1, M2, and S1). The variation in the infilling condition leads to the difference in the fundamental crack types, hence resulting in different coalescence patterns for a given flaw geometry. For small rock bridge angles (i.e., β = 30°, 45°), no coalescence and indirect coalescence tends to occur between the open and gypsum-filled flaws, while shear coalescence is prone to occur between the cement- and resin-filled flaws. The coalescence patterns are dominated by the flaw geometry for medium rock bridge angles (i.e., β = 60°, 90°), but the crack coalescence stress and peak strength of specimens are greatly affected by the infilling conditions. For large rock bridge angle (i.e., β = 120°), the increase in the strength of the filler tends to generate straighter wing cracks for the tensile coalescence at the rock bridge and promotes the initiation of mixed tensile-shear cracks for additional coalescence outside the rock bridge.

Experimental and Numerical Investigation on Crack Propagation and Coalescence in Rock-Like Specimens with Fluid-Infiltrated Parallel Flaws

Experimental and Numerical Investigation on Crack Propagation and Coalescence in Rock-Like Specimens with Fluid-Infiltrated Parallel Flaws

Energy Dissipation of Rock with Different Parallel Flaw Inclinations under Dynamic and Static Combined Loading

Deep surrounding rocks are highly statically stressed before mining (excavating) and will inevitably experience disturbances from unloading, mining, stress adjustment or their combinations during mechanical or blasting excavation, which actually suffer from a typical coupled static-dynamic stress. A split Hopkinson pressure bar was used to carry out dynamic-static loading test on rock specimens with different fracture angles. The results show that the change law of energy utilization efficiency is similar to the energy absorption rate that they increase first and then decrease with the increasing of axial pressure. The elastic energy of specimens would also increase first and then decrease with the increasing of axial pressure, while the plastic energy generally decrease overall. Both the energy utilization efficiency and energy absorption rate increase with the growth of dynamic compressive strength under impact loading, which indicate that the energy dissipation exhibits a positive with the dynamic strength. The energy absorption density and energy utilization efficiency gradually increase linearly with the increasing of the average strain rate, while the relationship between energy utilization efficiency and incident energy basically follows the exponential function increasing law. The rock burst of pre-flawed rock is related to the static load level under dynamic-static loading, it occurs obviously under the action of medium energy when the axial pressure is high. Based on the energy dissipation theory, the damage variable model was further established, the damage variable can reasonably describe the damage evolution of crack granite under dynamic-static loading.

Cracking behavior of concrete/rock bi-material specimens containing a parallel flaw pair under compression

Crack propagation and coalescence of pre-existing flaws in concrete/rock bi-material are critical for stability assessments of rock/concrete structures. A hybrid finite-discrete element method (FDEM) was used to simulate the cracking behavior of the concrete/rock bi-material containing a parallel flaw pair under compression. The model was validated by physical tests. The results indicate that the crack evolution does not occur synchronously in the concrete and rock layers. The cracks in the different material layers penetrate the interface and coalesce at lower interface dip angles, whereas an interface crack always forms at higher interface dip angles, preventing crack coalescence in different material layers. A threshold of the strength ratio exists and determines whether the cracks in different material layers coalesce. If the strength ratio is lower than the threshold, the crack propagations are limited to one material due to the interface crack. If the strength ratio exceeds the threshold, the cracks in the different materials penetrate the interface and coalesce. Two critical values of the interface reduction coefficient exist for a given interface dip angle. The cracking pattern of the bi-material can be classified into cracking along the interface, local cracking in the different materials, and crack coalescence in different materials.

Dynamic mechanical responses and failure characteristics of fractured rocks with hydrostatic confining pressures: An experimental study

A precise understanding of the dynamic mechanical responses and failure characteristics of fractured rock with hydrostatic confinements is of paramount importance for the safety of deep underground engineering construction. In the present investigation, a modified split Hopkinson pressure bar (SHPB) apparatus was used to carry out the tri-axial dynamic tests on sandstone specimens with multiple parallel flaws under different strain rates in a range of 100–200 s−1 and four hydrostatic confining pressures varying from 5.7 to 22.7 MPa. The tested results indicate that the dynamic strength, elastic modulus and peak strain are all positively correlated with hydrostatic confining pressure. As the flaw intensity increases, both the dynamic strength and elastic modulus commonly decrease, and the dynamic peak strain decreases first and then slightly increases. Meanwhile, the energy utilization efficiency slightly declines with rising hydrostatic confining pressure, and features positive correlation to the flaw intensity but is not sensitive to the strain rate. The flaw intensity and strain rate both promote the energy dissipation density of the fractured rocks. By the post-mortem examination, ten crack categories are classified for fractured rock specimens subjected to dynamic loading with hydrostatic confinement, and the coalescence of such cracks causes different final failure patterns. Generally, with increasing flaw intensity, the failure pattern of fractured rock changes from shear dominant to shear-tensile mixed failure. Under higher hydrostatic confining pressure, the fractured rock usually exhibits conjugated X-shape failure patterns which are quite different from the specimens with lower confining pressure.

A field-enriched finite element method for brittle fracture in rocks subjected to mixed mode loading

• The field-enriched finite element method (FE-FEM) is proposed to simulate the crack propagation and coalescence of rocks. • The mixed mode fracture behaviors of different disc-type configurations are investigated. • The effects of the different geometry and loading conditions on fracture parameters, crack initiation and propagation path are analyzed. In this paper, a field-enriched finite element method is proposed to simulate the crack propagation and coalescence of brittle rocks subjected to mixed mode loading. The basic theory is derived, and numerical implementation is achieved. The mixed fracture characteristics and fracture parameters of brittle rocks are studied, and the disc-type specimens are taken as sample configurations. Firstly, a centrally cracked circular disc (CCCD) is used to verify the correctness and accuracy of the numerical model in dealing with mixed fracture problems. The field distribution and crack propagation path are also analyzed. The final failure mode is compared with the experimental observation and previous numerical result. Then, three semi-circular bending (SCB) configurations are employed to investigate the effect of different factors on the fracture patterns under mixed mode loading. The numerical results are qualitatively displayed, and the influence of each controlling factor on the crack initiation angle is also quantitatively analyzed. Finally, Brazilian disk specimens with two and three parallel flaws are used to simulate the fracture process. The present numerical results are in good agreement with those obtained by the other numerical methods, which indicates that the proposed numerical method has ability to simulate cracks propagation and coalescence.

Crack propagation process and acoustic emission characteristics of rock-like specimens with double parallel flaws under uniaxial compression

To further investigate the macroscopic failure law of fractured rock mass, we have performed investigations on the initiation mechanism of microcracks in fractured rock mass from the perspective of micromechanics. The failure of fractured rock mass is essentially due to the accumulation of energy between rock particles to a certain extent, which triggers a series of micro-failure events. In this paper, the theory of moment tensor is introduced to simulate acoustic emission events of rock-like specimens with pre-existing double parallel flaws during the failure process, and the mechanism of micro-failure events and their influence on macro-failure are investigated in depth from the perspective of meso-mechanics. Consequently, a set of macro-micro criteria is established to determine and distinguish the properties and types of cracks. In addition, it is also found that the stress state of rock particles determines the type and propagation direction of cracks. By analyzing the changes in the stress field in the vicinity of cracks, it is feasible to effectively analyze the initiation mechanism of different types of cracks and the trend of further propagation.

Experimental Evaluation of Multiple Frost Heaving Parameters for Preflawed Granite in Beizhan Iron Mining, Xinjiang, China

Ice-driven mechanical weathering in cold regions is considered a main factor impacting the stability of rock mass. In this work, the response surface method (RSM) was employed to evaluate and optimize the multiple frost heaving parameters to seek the maximum frost heaving force (FHF), in combination with experimental modeling based on a specially designed frost heaving force measurement system. Three kinds of rocks were prepared with parallel flaws in it having different flaw width, length, and cementation type, and these factors were used to fit an optimal response of the maximum FHF. The experimental results reveal five distinguished stages from the frost heaving force curve, and they are inoculation stage, explosive stage, decline to steady stage, recovery stage, and sudden drop stage. The sensitivity analysis reveals the influential order of the considered factors to peak FHF, which is the rock lithology, flaw width, flaw cement type, and flaw length. For low-porosity hard rock, increasing flaw width, flaw length, and flaw cement strength can improve the probability of frost heaving failure. It is suggested that rock lithology determines the water migration ability and influences the water-ice phase transformation a lot.

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.

The crack coalescence mode and physical field evolutionary characteristics of a brittle material containing two 3-D parallel embedded flaws

This research presents the crack coalescence mode and physical field evolution characteristics of a brittle material containing two 3-D parallel embedded flaws. A numerical model based on the flat-joint model is established and validated by laboratory experiments. The crack coalescence modes of two 3-D parallel flaws with different geometries are summarized. The physical field evolution characteristics during the cracking process are investigated. The mechanisms of wing crack initiation and propagation are effectively reflected by the physical fields. Moreover, the impacts of the interactions between the two flaws on the crack coalescence mode and physical fields are analysed. The simulation results indicate that the flat-joint model is appropriate for simulating the cracking process of two 3-D flaws in a brittle material. The flaw dislocation O can reflect the interaction strength. A strong interaction promotes the earlier fracture of the rock bridge, which also significantly weakens the mechanical properties of the specimen. In general, the mechanism of wing crack initiation is tensile, and stable propagation occurs by mixed tension and shear. However, strong interactions between flaws have significant impacts on the physical fields within the rock bridge and concentrate the tensile and shear forces, which may induce wing crack initiation as mixed tensile and shear cracks.

Influence of different concealment conditions of parallel double flaws on mechanical properties and failure characteristics of brittle rock under uniaxial compression

Previous 2D studies and tests have mainly focused on the discontinuity of fracture structures along the flaw-inclination direction, and there has been little research into the discontinuity or concealment characteristics of flaws along the flaw-strike direction. To this end, hard brittle rock-like gypsum specimens containing two parallel flaws with three types of flaw-concealment conditions (with proportions of 3D internal flaws of 0%, 50%, and 100%) were prepared. Uniaxial compression tests, accompanied by digital image correlation and acoustic emission monitoring, revealed the following. 1) The concealment conditions of the parallel flaws mainly influenced the uniaxial compression strength of the samples, while the existence of fracture structures was the main factor affecting their elastic modulus. 2) The flaw-concealment conditions had a significant effect on the distributions of the external and internal secondary failure structures. 3) When the proportion of 3D internal flaws was 50%, local failures occurred around the internal flaws and remained hidden, never showing on the outside surface of the specimen; when the proportion of 3D internal flaws was 100%, this kind of 3D sample had greater strength, a faster failure process, more severe damage behaviors, and a more concealed process of destruction than the corresponding 2D sample. 4) The higher strength and concealed failure process of samples containing 3D internal flaws explains why the conclusions of previous 2D research have been found to be not applicable to real 3D conditions.

Influence of Two Cross-Flaws Geometry on the Strength and Crack Coalescence of Rock-Like Material Specimens under Uniaxial Compression

Cross-flaws are very common in natural rocks. To date, the understanding of the failure process of rocks with cross-flaws is very limited. In this research, we study the influence of the cross-flaws geometry on the rock strength and coalescence modes with rock-like specimens. In this work, three groups of specimens with pre-existing cross-flaws are investigated: two aligned cross-flaws, two step cross-flaws, and two collinear primary flaws. The crack propagation and strength of specimens containing two parallel flaws are also studied to compare the results with those of specimens with cross-flaws. The results demonstrate that the cross-flaw geometry influences the rock bridge coalescence patterns in rock-like specimens. Specimens with two aligned cross-flaws and two step cross-flaws coalesce with tensile cracks in the rock bridge areas, while specimens with two collinear primary flaws coalesce with shear cracks. Specimens containing two cross-flaws may have a higher uniaxial compressive strength than specimens containing two parallel flaws, and the number of specimens with a higher uniaxial compressive strength is influenced by the cross-flaws geometry in the different groups. The cross-flaws geometry influences the strength of the specimens. In this research, specimens in the group containing two aligned cross-flaws have the highest mean uniaxial compressive strength, and specimens in the group with two step cross-flaws have the lowest strength.

Dynamic fracturing process of fissured rock under abrupt unloading condition: A numerical study

Unloading can cause many geotechnical disasters, e.g., the rock burst. The underlying mechanism of rock failure is the propagation and coalescence of cracks. To get insight into rock unloading failure, the discretized virtual internal bond (DVIB) model in conjunction with element partition method (EPM) is used to simulate the dynamic fracturing process under unloading conditions. At first, the samples with the single flaw are simulated under unloading conditions. It is found that the fracturing pattern exhibits the tensile-shear transition with the initial stress level increasing. When the flaw inclination is closer to 45 degree or the flaw is longer, shear cracks are more likely to occur. Then, the unloading failure process of rock with multiple flaws are simulated. It is found that the inclination of the parallel flaws has remarkable influence on the unloading failure of rock. When the inclination angle is about 45 degree, the rock is easier to fail. For the rock with multiple randomized flaws, the amount of flaws has slight influence on acoustic emission counts. However, the configuration of randomized flaws can significantly affect the fracture process and failure patterns. These findings can improve understanding of rock burst and provide valuable references for the prediction of unloading failure of rock.

Experimental Study for the Effect of Fillings on Properties of Brittle Rock Containing Two Parallel Flaws

In the underground engineering, rock masses containing flaws are quite sensitive to the fillings, yet the studies on the mechanical properties of the flawed rock under different curing time of the filling slurry are rather limited. To study the effect of filling slurry on the mechanical properties of the flawed rock mass, uniaxial compression tests are conducted on the "backfill–rock" composite sample with different proportions of filling materials under different curing time. The results show that both compressive strength and elastic modulus of the flawed sample containing two parallel flaws with fillings present obvious improvement compared with those of the flawed sample without fillings, and the enhancement extent is 20.52% and 6.35% on average respectively. Both the peak strength and elastic modulus of the samples with fillings increase gradually with the increasing of curing time or the cement–sand ratio. The filling slurry enhances the ability of the flawed sample with fillings to absorb strain energy. Compared with the non-fillings sample, the flawed samples with fillings increase the total input energy and elastic strain energy by 54.52% and 63.45% on average, respectively. As the increasing of curing time or the cement sand ratio, the total input energy and elastic strain energy increases gradually. The filling slurry effectively improves the performance of flawed granite.