Shear Crack Coalescence Research Articles

Crack Coalescence Behavior of Rock-Like Specimens Containing Two Circular Embedded Flaws

Abstract Experimental research on the growth of internal flaws has rarely been reported due to the fact that it is difficult to cut internal flaws in specimens and cannot capture the initiation and propagation processes of internal flaws through direct observations. This paper proposed a method for creating internal flaws in specimens by utilizing the volatilization of camphor. A series of compression tests were performed on rock-like specimens including two embedded circular flaws, and CT techniques were used to investigate the internal damage behavior of flawed specimens. Experimental results illustrate that the strength and deformation properties of flawed specimens increase nonlinearly with the confining pressure as well as flaw inclination angle. Crack coalescence patterns and failure modes of flawed specimens depend on not only the confining pressure but also the flaw inclination angle. The crack coalescence pattern varies from wing crack coalescence to mixed tension-shear crack coalescence and then to the shear crack coalescence as the crack inclination angle increases. Confining pressure contributes to shear crack growth and has an inhibiting effect on the propagation of tension cracks. For specimens with the same flaw inclination angle, the failure mode changed from tension failure to mixed shear-tension failure or from mixed shear-tension failure to pure shear failure with the increase of confining pressure.

Biaxial Shear Crack Propagation Modes of Rock-Like Specimens with Prefabricated Fissures and Their Strength Characteristics

A biaxial shear test is performed on prefabricated, single-fissure type, cubic rock-like specimens by using the TZW-500 rock direct shear apparatus to study the shear strength characteristics, crack coalescence, and propagation modes of the specimens with different geometric parameters. Results show that the crack coalescence and propagation modes of the rock-like specimens with prefabricated fissures can be divided into four types, namely, single main shear crack coalescence mode, main shear crack coalescence and secondary tensile-shear crack propagation mode, main shear crack coalescence and secondary shear crack propagation mode, and main shear crack coalescence and secondary tensile crack propagation mode. All modes are affected by the dip angle α and length l of the prefabricated fissure. When the dip angle of the prefabricated fissure is α∈[0°, 20°) or (70°, 90°], the cracks center on shear failure, and most shear cracks propagate along one end of the prefabricated fissure. At α∈(30°, 50°), the cracks bear the tensile-shear combined action, and the shear cracks propagate along the two ends of the prefabricated fissure. The peak shear strength of the rock-like specimens with prefabricated fissures is also closely related to the dip angle α and length l of the fissure. With the increase in dip angle α of the prefabricated fissure, the peak shear strength of each rock-like specimen decreases initially then increases, and the peak shear strength curve presents a similar “U” shape. At α∈[30°, 60°], the peak shear strength is within the peak-valley interval. When the length l of the prefabricated fissure is increased, the peak shear strength experiences a gradual reduction. When l > 20 mm, the peak shear strength is greatly influenced by l, but the influence is minimal when l ≥ 20 mm. At the same dip angle α and fissure length of l ≥ 20 mm, the correlation between peak shear strength and fissure width b is low.

Crack coalescence patterns and local strain behaviors near flaw tip for rock-like material containing two flaws subjected to biaxial compression

To understand crack coalescence patterns and local strain behaviors near flaw tip for flawed rock subjected to biaxial compression, a series of biaxial compression tests with a lateral stress of 2.0 MPa were carried out on rock-like specimens containing two flaws. The paper classified crack coalescence patterns, measured the local strain concentration near the flaw tip, and analyzed the stress–strain behavior related to crack initiation, propagation, and coalescence. Some interesting cracking behaviors of flawed specimens subjected to biaxial compression were observed. Seven basic crack types (wing crack, anti-wing crack, oblique and coplanar shear cracks, lateral crack, remote crack, and en echelon cracks) are identified and ten patterns of crack coalescences are observed subjected to biaxial compression. In general, the crack coalescence varies from the shear crack coalescence (S-mode) to the tensile wing crack coalescence (T-mode), and then to mixed shear/tensile crack coalescence (TS-mode), with the increase of the rock bridge ligament angle β, ranging from 25 to 120° subjected to biaxial compression. Crack initiation and coalescence usually can be reflected by a load stress rapid drop, a measured strain jump, or turning near flaw tip subjected to biaxial compression. The measured tensile or compression strains near flaw tip tend to increase almost simultaneously with higher axial stress before crack initiation; however, become very complex after crack initiation, due to the stress relaxation and adjustment near flaw tip induced by the combination of crack development and constant lateral stress. The existence of lateral stress results in many fragments in a shear failure mode with local surface spallings in flawed specimens containing two flaws.

Experimental Investigation on Crack Behavior and Stress Thresholds of Sandstone Containing a Square Inclusion under Uniaxial Compression

To study the effect of strength, stiffness and inclination angle of square inclusions on failure characteristics of rock, uniaxial compression tests were carried out on prismatic sandstone containing a square hole with different filling modes and hole angles using a servo-hydraulic loading system. Digital image correlation and acoustic emission techniques were jointly applied to analyze the damage and fracture process, and the crack stress thresholds were determined qualitatively and quantitatively by combining the stress–strain behavior. The results show that the mechanical properties and crack stress thresholds of pre-holed specimens increase with the increase of the strength and stiffness of inclusions, and are affected by the hole angle. Rock failure is mainly caused by secondary crack propagation and shear crack coalescence, eventually forming mixed tensile-shear failure. The crack behavior, especially the crack initiation position, is affected by the filling mode and the hole angle. Interface debonding tends to initiate at the vertical interface, while interface slipping tends to propagate along the inclined interface. Under identical loading conditions, the specimen with 45° hole is more susceptible to crack and damage than that with 0° hole. Notably, inclusions can inhibit the hole deformation and the fracture of rock matrix, especially the sidewall spalling of 0° hole.

Experimental study on the dynamic coalescence of two-crack granite specimens under high loading rate

In this study, the dynamic fracture coalescence of Fangshan granite specimens was studied using the Split Hopkinson Pressure Bar system. Two prefabricated surface cracks with varying inclination angles and inner tip distances were applied to the granite specimens. Two high-speed cameras were set up on two sides of the specimens, one for the side with speckles and the other for crack propagation. According to the crack coalescence mode, the experimental outcome was classified into three types, namely Tensile crack coalescence, Tensile crack and second tension crack coalescence and Tensile crack and second shear crack coalescence. The coalescence mode is mainly a tensile failure, and the change in the inner tip distance of the prefabricated cracks had little effect on the coalescence mode. Digital Image Correlation (DIC) measurement results showed that there were obvious strain concentration areas instead of strain concentration bands in the crack coalescence area. This phenomenon is considerably different from the Brazilian disc tests. The alteration of the inclination angle was expected to have an impact on the coalescence area, thereby leading the coalescence area to change continuously. There was an elliptical enveloping area for crack coalescence. After dynamic equilibrium, the specimen was in a compressed state. The size of the enveloping area was significantly affected by the inclination of the prefabricated cracks.

Numerical Analysis of Fracture Behaviour on Marble Samples Containing Two Flaws

Uniaxial compression tests were conducted on marble specimens containing two flaws. There are coplanar flaws and noncoplanar flaws. The inclination angle and spacing of flaws were considered of the coplanar flaws model, and the step angle and spacing of flaws were considered of the noncoplanar flaws model. Strength failure and crack coalescence behaviour were analysed in the paper. The crack evolution process containing microcrack initiation, coalescence, and failure is focused on the rock bridge coalescence and the extent of the pre‐existing flaws. There are four forms of rock bridge coalescence: tensile crack coalescence, shear crack coalescence, mixed tensile and shear crack coalescence, and no coalescence. Also, there are four forms of the rock failure mode: tensile failure, shear failure, mixed tensile and shear failure, and split fracture. The outer end of the critical stress values were used to compare with the crack initiation strengths, and the crack initiation strengths were slightly larger than the critical stress. In addition, energy dissipation laws were analysed during the model fracturing process. The crack evolution mechanisms around the pre‐existing flaw in the model were revealed by the distribution of microcrack and energy dissipation.

Experimental and Numerical Study on Failure Modes and Shear Strength Parameters of Rock-Like Specimens Containing Two Infilled Flaws

To investigate the influence of infilled flaws on mechanical properties and failure modes of rock masses, seven types of pre-existing infilled two-flaw specimens, which have different flaw inclination angle (α), rock bridge length (L2) and rock bridge inclination angle (β), were made from concrete. The crack coalescence process, failure modes and mechanical parameters of the specimens under triaxial or biaxial compression were studied by lab test and numerical tests, respectively. According to test results, two failure modes of specimen (shear failure, tensile–shear failure) and three rock bridge coalescence modes (tensile crack coalescence, shear crack coalescence, no coalescence) were identified. As the rock bridge length and inclination angle increase, the peak strengths of specimens also increase gradually, while the peak strengths of specimens decrease with flaw inclination angle being increase. The shear strength parameters (cohesion c and internal friction angle φ) of samples show nonlinear changes with various factors (flaw angle, rock bridge length, rock bridge angle). The particle flow code (PFC) was used to simulate the propagation process of microcracks and porosities, stress–strain curves for loading process were also obtained, numerical results are in good agreement with experimental results. The number of cracks and porosities increase rapidly in the post-peak stage, and a significant shear fracture zone was caused by cracks. This study provides a better understanding of peak strength and cracking behaviour of rock mass containing infilled flaws.

Dynamic Fracture Evolution and Mechanical Behavior of Sandstone Containing Noncoplanar Elliptical Flaws under Impact Loading

To investigate the effects of preexisting flaws with different geometries, including flaw inclination angle and ligament angle on dynamic strength, deformation properties, and fracture evolution of rock materials, a series of dynamic impact tests were conducted on green sandstone specimens containing double elliptical flaws using a 75 mm diameter split Hopkinson pressure bar (SHPB) testing device with a high-speed camera recording in real time. The experimental results show that dynamic strength of specimens with different flaw angles is reduced between 5.91% and 39.92% but from 18.50% to 28.44% for specimens with different ligament angles, indicating that the effect of the flaw angle on the dynamic strength is more significant than that of the ligament angle. However, the dynamic deformation properties are influenced greatly by the ligament angle. Macroscale cracks mostly initiate at or near the flaw tips and then propagate in different paths with varying flaw geometries, leading to the ultimate failure in five typical modes based on the crack coalescence. Shear crack coalescence and tensile crack coalescence are identified through both macroscopic fracturing photos taken by the high-speed camera and microscopic surface morphology obtained by the scanning electron microscope (SEM).

Understanding the fracture behavior of brittle and ductile multi-flawed rocks by uniaxial loading by digital image correlation

Fracturing behavior of ductile multi-flawed rock mass is very different from those of brittle multi-flawed rock mass. However, there is a very vague distinction of the fracture mechanism of ductile multi-flawed rock mass from that of brittle multi-flawed rock mass due to the absence of quality experimental data. To evaluate the fracturing behavior and further reveal the failure mechanism of brittle and ductile multi-flawed rock masses, a comparison study on brittle and ductile laboratory rock-like specimens that contain nine pre-existing flaws subjected to uniaxial compression is conducted. The advanced optical technique, Digital Image Correlation (DIC), is adopted to capture the real-time displacement and strain fields on the surface of the rock-like specimens. The fractal dimension estimation method is introduced to quantitatively track the fracture process. The fracture process, the crack coalescence mode and the fracturing mechanism are obtained as well as the stress-strain response. For ductile multi-flawed rock-like specimens, four crack initiation modes and nine types of crack coalescence are observed. For brittle multi-flawed rock like specimens, four crack initiation modes and eight types of crack coalescence are observed. Comparison of fracturing behavior indicates that crack initiation mode transforms from shear crack to tensile crack as the brittleness index increases; the coalescence mode transforms from shear crack coalescence to the coalescence of tensile crack and shear crack; then to the tensile crack coalescence as the brittleness index increases. Observations on the ultimate failure modes indicate that the failure mode transforms from shear failure to the mixed tension-shear failure, then to splitting failure as the brittleness index increases. Most macrocracks in brittle multi-flawed rock-like specimens develop from crack coalescence occurring in a “columnar” pattern, while most macrocracks in ductile multi-flawed rock-like specimens develop from shear cracks. Moreover, the effects of the brittleness index on the complete stress-strain curve, the crack initiation stress and the peak strength are also investigated in detail.

Failure mechanical and acoustic behavior of brine saturated-sandstone containing two pre-existing flaws under different confining pressures

To better understand the mechanical, acoustic and failure behaviors of sandstone in deep saline aquifers, conventional triaxial compression tests were carried out on sandstone specimens containing two pre-existing three-dimensional (3D) flaws. First, pre-flawed sandstone specimens were saturated under NaCl solution with a salinity of 20 wt%. Then both dry and brine-saturated sandstone specimens were loaded under triaxial compression. As the flaw angle increased, the peak strength and crack damage stress of the dry and saturated pre-flawed sandstone specimens increased. The peak strength and crack damage stress of dry specimens increased linearly, while those of brine-saturated specimens increased nonlinearly with confining pressure. During triaxial loading, P-wave and AE signals were monitored in real-time for the sandstone specimens. The evolution of P-wave velocity and AE events can be characterized as having five stages based on the internal damage of sandstone specimens. In general, the accumulated AE count can be summarized as follows: very few or no AE counts, increases linearly, increases exponentially and increases stably. The P-wave velocity was also observed as: increases rapidly, increases stably, decreases unstably and keeps constant. Finally, by using X-ray micro CT observations, two crack coalescence modes between the two pre-existing flaws were identified, i.e., shear crack coalescence in the ligament region and indirect coalescence outside the ligament region. The second coalescence mode was only observed in the sandstone specimen with a flaw angle of 45° under low confining pressures. The internal opaque cracks were clearly displayed in the reconstruction images as a curved surface.

Study on the Interaction of Collinear Cracks and Wing Cracks and Cracking Behavior of Rock under Uniaxial Compression

This paper investigates the crack interaction, initiation, and propagation rules of rock‐like materials containing two collinear cracks. Based on the Kachanov method, the formulations for stress intensity factors (SIFs) of two collinear cracks and two winged cracks are derived, respectively. The influences of bridge ligament and crack length on the crack interaction are analyzed theoretically. The results show that the propagation of a long crack is independent of crack interaction when d ≥ a2 and the same rule applies for a short crack when d ≥ a1. With the growth of wing cracks, the SIF of wings first remarkably decreases and then it tends toward a steady value. Subsequently, the propagation of collinear cracks and cracking processes under uniaxial compression are analyzed experimentally and numerically. Both the experimental results and simulation results demonstrate that shear cracks tend to initiate and propagate at higher inclination angle. The crack coalescence is affected by the inclination angle of bridge ligament. For increasing the inclination angle, the crack coalescence varies from wing crack failure to shear crack coalescence. As bridge ligament increases, the crack coalescence varies from shear crack coalescence to shear‐wing crack coalescence and then to wing crack failure.