Crack Coalescence Behavior Research Articles

A Three-Dimensional Numerical Investigation of Dynamic Fracture Characteristics of Rock Specimens with Preexisting Surface Flaws

FEM-based numerical investigation was conducted to investigate the fracture characteristics of heterogeneous rocks with preexisting surface flaws under dynamic loadings. Different types of cracks from initiation to coalescence of the specimen with a single flaw were reproduced in numerical tests, which were very highly agreed with experimental results. Numerical testing on specimens with a single flaw demonstrated that the crack behaviors, including its path, length, and cracking velocity, were significantly different on the surface of and inside the rock specimen. In addition, the higher the incident pressure level applied, the longer the cracking distance propagated; the more the AE count is, the larger the AE energy released and the greater the fragmentized. Furthermore, the cutting depth of flaw could significantly affect crack propagation patterns and AE characteristics. When the cutting depth of flaw is equal to the thickness of the specimen, there would be no shell-like crack emerged. Subsequently, testing results of the multi-preexisting flaws indicated that the surface flaws on the specimen made the crack initiation and coalescence behaviors more complicated. The coalescence between flaw tips is not easy, and the coalescence of preexisting flaws needs two or multiple cracks to achieve. The coalescence points occurred not only in the preexisting flaw but also in the trajectory of the wing cracks and antiwing cracks. The accumulative AE energy and AE counts are positively correlated with the amounts of preexisting flaws. Finally, crack evolution under dynamic and static loading was compared. Short tensile cracks are found to be the most common crack type, which are responsible for specimen failure. These findings could give a better explanation on the crack initiation and propagation of rock specimen with preexisting flaw, which is the phenomenon that has not been observed in many dynamic experiments.

Fracturing Behavior Study of Three-Flawed Specimens by Uniaxial Compression and 3D Digital Image Correlation: Sensitivity to Brittleness

Brittleness has significant effects on the fracturing behaviors of preflawed rock-like materials. In this study, uniaxial compressive experiments are conducted on rock-like specimens containing three flaws, and a three-dimensional digital image correlation technique is applied to capture the real-time displacement and strain fields on the rock-like specimens’ surface. The present experimental study focuses on the effects of the brittleness of rock-like materials on crack initiation, propagation, and coalescence behaviors in three flaw-contained specimens. The experimental results show that the crack initiation mode transforms from the tensile mode to the shear mode with a decrease in the brittleness index of rock-like materials. Meanwhile, crack coalescence modes transform from the tensile crack coalescence mode to the mixed tension–shear coalescence mode, then to the shear coalescence mode as the brittleness index of rock-like materials decreases. Moreover, the ratio of the crack initiation stress to the peak stress increases, while the ratio of the crack coalescence stress to the peak stress decreases as the brittleness index decreases. This paper also addresses the influences of the rock bridge angle on the mechanical and cracking behaviors of three flaw-contained specimens. For specimens with the lower brittleness index, the ratio of the crack initiation stress to the peak stress decreases, while the ratio of the crack coalescence stress to the peak stress increases as the rock bridge angle increases. However, for specimens with the higher brittleness index, the ratio of the crack initiation stress to the peak stress and the ratio of the crack coalescence stress to the peak stress decrease with increasing the rock bridge angle. The experimental results provide a better understanding of the sensitivity of mechanical and cracking behaviors of three flaw-contained specimens to the brittleness of rock-like materials under uniaxial compression.

Mechanical and cracking behavior of granite containing two coplanar flaws under conventional triaxial compression

Flaws widely exist in rock mass, which significantly influences the strength and failure behavior of rock. The study of the flaw effect on the mechanical and cracking behavior is important to predict the unstable failure of fractured rock engineering. However, three-dimensional crack propagation behavior of real rock containing preexisting flaws under triaxial compression has been rarely studied. To increase the understanding of crack coalescence behavior, a series of laboratory conventional triaxial compression experiments were carried out on granite specimens containing two coplanar three-dimensional flaws. On the basis of experimental results, the effects of flaw angle and confining pressure on the strength properties of granite specimens were analyzed. As the flaw angle increased from 30° to 60° and the confining pressure increased from 0 to 30 MPa, the triaxial compressive strength and crack damage threshold of granite specimen increased. The cohesion and internal friction angle of preflawed granite specimen increased with increasing flaw angle. And then, X-ray micro-CT scanning technique was used to investigate the internal fracture characteristic of granite specimen. Four typical crack coalescence modes were identified in this experiment, i.e. no coalescence, two types of indirect coalescence, and shear coalescence. Under uniaxial compression, cracks from the tips of the flaws led to the specimen failure under tension, while under higher confining pressure, shear cracks and antiwing cracks were dominant.

Experimental study on uniaxial mechanical properties and crack propagation in sandstone containing a single oval cavity

The geometry and orientation of cavities in a rock mass has a significant influence on the failure mechanism and fracture propagation. The theoretical study of these behaviors is of great importance to enable fundamental understanding of unstable failure in rock mechanics. A very limited number of experimental studies have been conducted for sandstone specimens containing oval cavities. Sensitive parameters include the short axis (b) to long axis (a) ratio. Here, we enhance the present understanding of crack coalescence behavior around an oval cavity by applying uniaxial compression tests to sandstone cores containing manually inserted single oval cavity, combined with photographic and acoustic emission monitoring. The b:a ratio was varied from 1/4 to 1/1, thus the cross-sectional cavity shape evolved from oval to a fully circular opening. The experimental results showed that the stress–strain curves of specimens containing cavities exhibited multiple stress reductions prior to peak strength, which led to a sudden increase in the accumulated AE count-rate. As the b:a ratio was increased, the uniaxial compressive strength, crack initiation stress and peak strain of cavity specimens all decreased linearly. Tensile fractures, shear fractures, and compound shear-tensile fractures were all observed to emanate from the cavities. Compared to the intact specimen, the failure mode of cavity specimens comprised a mixture of tensile and shear cracks, emanating from the tips (or surfaces) of the oval cavity. For specimen with a cavity angle of 45°, when the b:a ratio was below 1/2, no tensile crack from the surface of short axis was initiated, whereas tensile crack was observed clearly after b:a=1/2. This study not only increases our understanding of mechanical failure behavior in fractured rock, but also enhances our knowledge of underground tunneling or roadway stability.

Crack Initiation and Coalescence Behavior of Two Non-parallel Flaws

Crack initiation and coalescence behavior of rock or rock-like specimens containing artificial flaws under uniaxial compression have been subjects of intensive investigation in the past. Most of these investigations however focused on crack initiation and coalescence between two or more parallel flaws. Although there have been few experimental studies on non-parallel flaws, these studies did not address the influence of geometrical factors such as ligament length and ligament angle on the crack initiation and coalescence behavior of non-parallel flaws. To investigate whether the individual geometrical factors have similar effects on the crack initiation and coalescence behavior of both parallel and non-parallel flaws, we conducted uniaxial compression tests to investigate crack cracking and coalescence processes in rock like material containing two non-parallel flaws. The paper presents the influence of individual geometrical factors on the crack initiation process and coalescence pattern of non-parallel flaws. Initiation of primary first cracks from all the tips of the two flaws did not occur simultaneously in all the flaw configurations. The flaw configuration of the non-parallel flaws influences the crack initiation, crack trajectories and coalescence behavior. The crack coalescence pattern changes with an increasing ligament angle from indirect to shear crack or mixed tensile-shear crack to tensile crack coalescence. The chance of direct coalescence is reduced with an increase in ligament length. In conclusion, the crack initiation and coalescence behavior of prismatic rock-like specimens with non-parallel flaws, as influenced by the geometrical factors, are analogous to the cracking and coalescence pattern observed in specimens with parallel flaws.

Mechanical behavior of an opening in a jointed rock-like specimen under uniaxial loading: Experimental studies and particle mechanics approach

Generally, in many cases of rock engineering, the openings often constructed in rock-mass containing non-persistent joints. However, comparing with the previous works, few studies investigate the failure or damage due to the crack propagation and coalescence around an opening. Based on the uniaxial compression tests and particle flow code (PFC) the interaction effect of opening and joints on the crack coalescence behavior around an opening are investigated in this study. From the view of experimental and numerical results, strength parameters are mainly effected by joints (inclination and distance). Specifically, the uniaxial compressive strength of jointed specimen (UCSJ) and elastic modulus of jointed specimen (EJ) of specimens decrease for 0°≤α≤45° and increase for α>45°. UCSJ and EJ increases with increasing joint distance (d) for all joint inclination angel (α) values, with the highest and lowest strengths obtained for d=50mm and d=20mm, respectively. The opening has a great influence on the failure mode of jointed specimen. Unlike previous results, in this study, jointed specimens present four new kinds of failure modes: Mode-I (horizontally symmetrical splitting failure); Mode-II (stepped failure at opening sides); Mode-III (failure through a plane); Mode-IV (mixed failure). The strength parameters and failure modes in the numerically simulated and experimental results are in good agreement, and the results are expected to be useful in predicting the stability of an opening in a non-persistently jointed mass.

The modeling of crack propagation and coalescence in rocks under uniaxial compression using the novel conjugated bond-based peridynamics

In this paper, crack propagation and coalescence behaviors in rock specimens containing pre-existing open flaws under uniaxial compression are investigated using the novel conjugated bond-based peridynamics (BB-PD). The novel conjugated BB-PD is an extended BB-PD model, where interacting forces are not only related to normal stretch of bonds, but also related to rotation bond angles of a pair of conjugated bonds. Simple elastic-brittle bond rupture criteria, which are related to the uniaxial tensile strength and uniaxial compressive strength, are implemented into this novel conjugated BB-PD. The novel conjugated BB-PD model is found to be effective for simulating rock fracture problems under compression. Initiation and propagation of cracks in rock specimens with a single flaw are firstly studied. Then, crack initiation, propagation and coalescence of two parallel and un-parallel flaws under uniaxial compression are investigated. Meanwhile, effects of flaw inclination angles and rock bridge angles on crack propagation and coalescence behaviors are analyzed. Finally, characteristics of crack coalescence modes are discussed, which reveals the crack coalescence mechanism of pre-flawed rock specimens under uniaxial compression. Comparing the present numerical results with the previous experimental observations and other numerical results, it can be found that the present numerical results are in good agreement with the experimental observations.

Physical and mechanical behavior of granite containing pre-existing holes after high temperature treatment

To understand the high temperature effects on the mechanical and failure behaviors of rock, uniaxial compression tests were carried out on granite specimens containing three pre-existing holes using a rock testing system. Based on the experimental results, the influences of testing temperature on the physical and mechanical parameters of granite were analyzed in detail. An obvious color change of tested granite occurs from gray at room temperature to reddish after 450°C and to red-brown after 900°C high temperature treatment. The granite volume increases, mass decreases and density decreases with increasing testing temperature. As the temperature increases, the peak strength first increases and then decreases, while the elastic modulus decreases. However, the peak strain changes slightly before 450°C, increases dramatically up 450°C. As the bridge angle increases, the mechanical parameters of granite specimens first decrease and then increase. And then, the crack initiation, propagation and coalescence behavior of granite specimens after high temperature exposure was investigated using an acoustic emission (AE) and photography monitoring technique. The cracking process shows that the propagation of crack from the surface of holes leads to the coalescence between adjacent holes. A large AE count and a stress drop are observed during the crack initiation and propagation. The failure modes can be generally classified into three categories: splitting mode, shear mode and mixed mode and they are closely related to heat treatment temperature and bridge angle. Finally, the mechanism causing the differences in the mechanical parameters observed with increasing temperature was discussed based on the SEM observations.

Experimental study on cracking behaviour of moulded gypsum containing two non-parallel overlapping flaws under uniaxial compression

Failure of rock mass that is subjected to compressive loads occurs from initiation, propagation, and linkage of new cracks from preexisting fissures. Our research investigates the cracking behaviour and coalescence process in a brittle material with two non-parallel overlapping flaws using a high-speed camera. The coalescence tensile crack and tensile wing cracks were the first cracks to occur from the preexisting flaws. The initiation stresses of the primary cracks at the two tips of each flaw were simultaneous and decreased with reduced flaw inclination angle. The following types of coalescence cracks were identified between the flaws: primary tensile coalescence crack, tensile crack linkage, shear crack linkage, mixed tensile-shear crack, and indirect crack coalescence. Coalescence through tensile linkage occurred mostly at pre-peak stress. In contrast, coalescence through shear or mixed tensile-shear cracks occurred at higher stress. Overall, this study indicates that the geometry of preexisting flaws affect crack initiation and coalescence behaviour.

The Effects of Crack Openings on Crack Initiation, Propagation and Coalescence Behavior in Rock-Like Materials Under Uniaxial Compression

This paper experimentally investigates the cracking behavior of rock-like specimens containing artificial open flaws under uniaxial compressive loads. The present experiments mainly focus on the effects of crack openings on crack propagation and coalescence behavior in rock-like materials under uniaxial compression. The real-time crack coalescence processes in the specimens with different crack openings are analyzed. The experimental results show that the crack openings significantly affect the crack initiation stresses and the crack initiation modes. The initiation stresses of wing cracks and coplanar secondary cracks decrease with increasing crack openings. However, the initiation stress of anti-wing cracks increases with increasing crack openings. Moreover, five types of crack coalescence in the specimens containing three pre-existing open flaws under uniaxial compression are observed. The effects of crack openings on the mechanical properties of rock-like materials, which include the complete axial stress–strain curves, peak stresses, peak strains and initiation stresses, are investigated in detail.

Study on multiple surface crack growth and coalescence behaviors

Interaction of multiple surface cracks is studied by experimental and numerical methods. For experiment, a new method to introduce two surface cracks with different sizes is developed. Using this technique, four point bending fatigue tests including coalescence process of two surface cracks are conducted by changing crack sizes of two cracks. Crack growing process is studied by introducing beach marks. Change of crack shapes and coalescence behaviors are observed clearly. Locations of crack coalescence change due to the change of crack sizes. Same problem is simulated by using S-FEM. Two models are simulated. One is Crack coalescence model, and another is Virtual single rack model. Virtual single crack model is based on the proximity rule of JSME maintenance code. Results of both models are compared with those of experiment. Results show the availability of numerical methods to predict coalescence process of two surface cracked specimens. It is also shown that JSME code is useful to simulate coalescence problem.

Real-time ultrasonic experiments and mechanical properties of soil and rock mixture during triaxial deformation

Non-linear mechanical behaviours of soil and rock mixture (SRM) are known to be the result of elasticity mismatch between soil matrix and rock blocks. Yet the rock–soil interactions, rock block interlocking phenomenon and their dependence under triaxial deformation are poorly understood. Here the authors investigate on the mechanical and ultrasonic properties of SRM by analysing the variation of ultrasonic pulse velocity (UPV) during triaxial compression. The results show that rock–soil interaction can be reflected by the variation of UPV during deformation. In this work, the change of UPV mainly depends on the axial stress level, and they present a consistent trend during deformation. With the increasing rock block percentage, both axial stress and UPV take on the fluctuation growth trend; the fluctuation is most obvious at the strain hardening stage. Internal friction angle increases with the increasing of rock block percentage, while the cohesion presents the opposite trend. The ultimate failure modes and crack coalescence behaviour were also evaluated, with the increasing rock blockpercentage, failure mode of SRM experiences such as shear failure and bulging failure.

The role of flaws on crack growth in rock-like material assessed by AE technique

Experiments for the rectangular rock-like samples (made of high-strength gypsum and water–cement ratio is 1) with two parallel pre-existing flaws subjected to uniaxial compression were carried out to further investigate the influence of varying flaw geometries on mechanical properties and crack coalescence behaviors. According to the tests results, eight crack types were characterized on a basis of the mechanisms of crack nucleation, formation and propagation, and seven coalescence modes occurred through the ligament, including S-mode, M1-mode, M2-mode, M3-mode, T1-mode, T2-mode and T3-mode. The AE and photographic monitoring techniques were adopted to further clarify the procedure of the crack coalescence and failure during uniaxial compression tests and in consequence the whole process of crack emergence, growth, coalescence and failure was recorded in real time. The results of AE technique revealed that the characteristics of acoustic emission energy associate with crack coalescence modes, and AE location method can emphasize the moments of crack occurrences and follow the crack growth until final failure. This study put forward better understanding of the fracture and failure mechanism of underground rock engineering, like rock burst.

Numerical study on failure behavior of brittle rock specimen containing pre-existing combined flaws under different confining pressure

To understand deeply the fracture mechanism of brittle rock material, the rock specimen containing combined flaws (two square holes and one pre-existing fissure) with seven different fissure angles is carried out the numerical simulation by RFPA2D. Numerically simulated results show a good agreement with the experimental results. The crack coalescence behavior of specimen containing combined flaws under uniaxial compression is summarized, which is closely dependent to fissure angle. The stable propagation of original cracks does not lead to a larger AE event, but the coalescence of new cracks causes a larger AE event. The peak strength of specimen containing combined flaws increases with the confining pressure. According to the linear Mohr–Coulomb criterion, the cohesion and internal friction angle of specimen containing combined flaws are obtained, which is found to take on a distinct nonlinear relation with the fissure angle. The accumulated AE events decreases as the confining pressure increases from 0 to 30MPa, which results mainly from the restraining of higher confining pressures on the initiation and propagation of tensile cracks at the fissure tips and nearby double squares.

Experimental study on mechanical properties of sandstone specimens containing a single hole after high-temperature exposure

The effect of temperature levels and hole diameter on the mechanical behaviour of intact and flawed sandstone specimens containing a single hole was analysed based on uniaxial compression test results. It was found that, with increasing temperature, both the peak strength and elastic modulus first increase gradually and then decrease sharply, reaching maximum values at 400°C. These two mechanical parameters of the flawed specimens were significantly lower than those of the corresponding intact specimens, with the extent of reduction being distinctly related to the hole diameter. Thermal damage of sandstone specimens changed gradually from ‘negative thermal damage’ to positive values with increasing temperature, while the variation of the thermal damage increased gradually with the diameter. The ultimate failure modes and crack coalescence behaviour were also evaluated for the flawed specimens after high temperature, and crack types (tensile crack, hole collapse, far-field crack and surface spalling) based on the crack coalescence mechanism are discussed. The sequence of crack coalescence in a flawed sandstone specimen after high temperature was captured, and its effect on the macroscopic axial stress–strain curve was also investigated.

Mechanical behavior and failure analysis of brittle sandstone specimens containing combined flaws under uniaxial compression

Based on the axial stress-axial strain curves, the effect of fissure angle on the strength and deformation behavior of sandstone specimens containing combined flaws is analyzed. The mechanical parameters of sandstone specimens containing combined flaws are all lower than that of intact specimen, but the reduction extent is distinctly related to the fissure angle. The results of sandstone specimens containing combined flaws are obtained by the acoustic emission, which can be used to monitor the crack initiation and propagation. The ultimate failure mode and crack coalescence behavior are evaluated for brittle sandstone specimens containing combined flaws. Nine different crack types are identified on the basis of their geometry and crack coalescence mechanism (tensile crack, hole collapse, far-field crack and surface spalling) for combined flaws. The photographic monitoring was also adopted for uniaxial compression test in order to confirm the sequence of crack coalescence in brittle sandstone specimens containing combined flaws, which recorded the real-time crack coalescence process during entire deformation. According to the monitored results, the effect of crack coalescence process on the strength and deformation behavior is investigated based on a detailed analysis for brittle sandstone specimens containing combined flaws by using digital photogrammetry.

Strength failure and crack coalescence behavior of sandstone containing single pre-cut fissure under coupled stress, fluid flow and changing chemical environment

In order to study the strength failure and crack coalescence characteristics of cracked rocks, uniaxial compression experiments were conducted on cylindrical sandstone specimens, sampled from Longyou Grottoes of Zhejiang Province, China, with a single pre-cut crack soaking in different chemical solutions. Based on the results of uniaxial compressive test under different chemical solutions and velocities of flow, the effect of strength and deformation characteristics and main modes of crack coalescence for cracked rocks under chemical corrosion were analyzed. The results show that the pH value and velocity of the chemical solutions both have great influence on the sandstone sample’s uniaxial compressive strength and deformation characteristics. Cracked sandstone samples are tension-destructed under uniaxial compression, and the crack propagation directions are consistent with the loading direction. The phenomena of crack initiation, propagation and coalescence of sandstone are well observed. Four different crack types are identified based on the crack propagation mechanism by analyzing the ultimate failure modes of sandstone containing a single pre-cut fissure. The failure process of specimen in air is similar with the specimen under chemical solutions, however, the initial time of crack occuring in specimen under chemical solutions is generally earlier than that in the natural specimen, and the crack propagation and coalescence process of specimen under chemical solutions are longer than those of the natural specimen due to softening of structure of rock caused by hydro-chemical action. Immersion velocity of flow and chemical solutions does not have influence on the ultimate modes of crack coalescence.

Experimental investigation on fracture coalescence behavior of red sandstone containing two unparallel fissures under uniaxial compression

Natural rock usually contains a lot of flaws, such as fissures, joints, weak surfaces and faults, which have a significant effect on the strength, deformability and crack coalescence behaviors of rock material [1–5]. In order to improve the understanding of fracture mechanism of rock engineering containing intermittent structures, a lot of experimental studies have been performed for all kinds of rock materials or rock-like materials containing two fissures [6–10]. Wong and Chau [6] investigated the crack propagation and coalescence on rock-like materials containing two inclined open or closed fissures, which obtained three main modes of crack coalescence in two fissured specimens under uniaxial compression. Feng et al. [7] carried out a number of experimental studies to explore the mechanism of multi-crack interaction in limestone specimens (the size of the specimen was 15 30 3 mm) with two fissures of different geometries under the coupled uniaxial compressive stress and chemical solutions with different ionic concentrations and pH values. Yang et al. [8] made uniaxial compression tests for cylindrical marble specimens with different pre-existing fissures, which analyzed the effect of fissure geometry on strength and deformation failure behaviors of brittle marble material. Wong and Einstein [9] summarized the effects of the flaw angle, ligament angle and length on the cracking processes and coalescence patterns of Carrara marble specimens containing two open fissures and used a high speed camera to observe the specimens at a macroscopic scale. Yang [10] performed an experimental study on the influence of coplanar fissure angle on the strength, deformation

Cracking processes in Barre granite: fracture process zones and crack coalescence

This paper presents a comprehensive study of the cracking and coalescence behavior of granite specimens with pre-existing flaw pairs. Uniaxial compressions tests were conducted on Barre granite with pre-existing flaw pairs of varying inclination angles $$(\upbeta )$$ , bridging angles $$(\alpha )$$ and ligament lengths (L). The cracking processes were recorded using a high speed camera to capture crack initiation and determine the mode (tensile or shear) of cracking. Visible fracture process zones of grain lightening, referred to as “white patching”, were also observed. White patching corresponded to fracture process zones that developed before visible cracks appeared. Cracks were typically preceded by a corresponding linear white patching. Diffusive area white patching was also observed near locations where surface spalling eventually occurred. Shear cracks occurred less often when compared to other brittle materials such as gypsum and marble and tensile cracks were typically much more jagged in shape (saw-toothed) due to the larger size and higher strength mineral grains of granite. Crack coalescence behavior trended from indirect to direct shear and combined shear-tensile to direct tensile coalescence as the flaw pair bridging angle $$(\alpha )$$ or flaw angle $$(\upbeta )$$ increased. As the ligament length (L) between flaws increased, more indirect coalescence was observed. As expected, due to the increased occurrence of tensile cracking in granite, more indirect tensile coalescence was observed in granite compared to other materials previously studied.

PS23 オーステナイト系ステンレス鋼の高温高純水環境における応力腐食割れ発生挙動評価

In this paper, constant tensile load tests under simulated primary water environment in boiling water reactor are carried out for sensitized stainless steel type 304 to evaluate influence of applied load and loading time on crack initiation and coalescence behavior by SCC. As a result, many stress corrosion cracks initiated is observed on the smooth surface of specimen, and coalescence of neighboring cracks is also observed. The number of cracks and crack length increase with increasing loading time and applied load. The crack length ranges from 0.02mm to 3.0mm. The crack length which is less than 0.1mm is normal-distributed, and the value of mean and variance of normally distribution are about 50μm and 17μm, respectively.