Initial Microcracks Research Articles

An analytical solution of direction evolution of crack growth during progressive failure in brittle rocks

AbstractMicrocrack growth during progressive compressive failure in brittle rocks strongly influences the safety of deep underground engineering. The external shear stress τxy on brittle rocks greatly affects microcrack growth and progressive failure. However, the theoretical mechanism of the growth direction evolution of the newly generated wing crack during progressive failure has rarely been studied. A novel analytical method is proposed to evaluate the shear stress effect on the progressive compressive failure and microcrack growth direction in brittle rocks. This model consists of the wing crack growth model under the principal compressive stresses, the direction correlation of the general stress, the principal stress and the initial microcrack inclination, and the relationship between the wing crack length and strain. The shear stress effect on the relationship between y‐direction stress and wing crack growth and the relationship between y‐direction stress and y‐direction strain are analyzed. The shear stress effect on the wing crack growth direction during the progressive compressive failure is determined. The initial crack angle effect on the y‐direction peak stress and the wing crack growth direction during the progressive compressive failure considering shear stress is also discussed. A crucial conclusion is that the direction of wing crack growth has a U‐shaped variation with the growth of the wing crack. The rationality of the analytical results is verified by an experiment and from numerical results. The study results provide theoretical support for the evaluation of the safety and stability of surrounding rocks in deep underground engineering.

Damage mechanism of rock induced by microcrack evolution: A multi-dimensional perspective

This paper investigated the multi-dimensional and multi-scale damage mechanisms of rocks employing Discrete Element Method (DEM) numerical tests, macroscopic damage models, microscopic damage models, and Acoustic Emission (AE) tests. The study analyzed the correlation between the number of microcracks and AE accumulative ring-down count, established a macroscopic damage evolution equation representing rock volume strain, and explored the mechanisms of microscopic damage evolution. The impact of confining pressure and initial microcracks on rock damage characteristics, the relationship between crack volume strain and macroscopic damage evolution, and the effects of different crack types on microscopic damage evolution were discussed. Key findings include: 1) The DEM incorporating gapped contacts can effectively assess the impact of initial microcrack volume on compaction deformation. Concurrently, the consistency observed between the number of microcracks in DEM and the AE accumulative ring-down count aptly characterizes the failure trend. 2) Macroscopic damage is quantitatively derivable through microcrack enumeration, with the noteworthy observation that microcracks undergo substantial development preceding the volumetric expansion of the rock. Consequently, the evolution of macroscopic damage can be aptly described as a piecewise exponential function relative to rock volume strain. 3) Microcracks manifest in two forms: tension and shear, and the evolution of microscopic damage is intricately linked to the type of microcracks, thus prompting a comprehensive consideration of the microscopic damage induced by different microcrack types. This multi-dimensional exploration enhances our understanding of the dynamic processes underlying rock damage at both macroscopic and microscopic levels.

Study on meso-deformation and failure mechanism of rock mass with micro-cracks under freeze-thaw loading

The long-term freeze-thaw effect leads to the development of rock fractures and strength degradation in cold region engineering construction, which poses a serious challenge to the stability of the project. In this paper, the microscopic model of sandstone freeze-thaw cycles was established using a particle flow code (PFC2D). Through numerical simulation, the variation law of mechanical properties of rock mass with micro-cracks is systematically studied from the aspects of displacement, crack development, strain and strength. The results show that: (i) The freeze-thaw loading displacement is concentrated on both sides of the initial micro-cracks, and the crack development is not controlled by it. When the number of freeze-thaw cycles is greater than 17, the displacement and crack development are significant. The crack increases with the increase of the crack distance ratio. (ii) When the number of freeze-thaw cycles is less than 15, the compressive crack develops at the end of the initial micro-crack, and the development is controlled by it. When the number of freeze-thaw cycles is greater than 21, the crack increases sharply, and the development is no longer controlled by the initial micro-cracks. When the number of freeze-thaw cycles is less than 15, the damage strain shows minimal variation but decreases sharply with the increase of freeze-thaw cycles. (iii) Under each crack distance ratio, the strength changes in three stages: when the number of freeze-thaw cycles is less than 15, the strength changes little, and the strength decreases sharply with the increase of freeze-thaw cycles. With the increase of the crack distance ratio, it increases first and then decreases. And it is more significant when the number of freeze-thaw cycles is greater than 17. The research results can provide theoretical support for the influence and evaluation of rock freeze-thaw action in cold regions.

Study on mechanical properties of interbedded rock masses with microcrack based on thermal-mechanical coupling.

The mechanical properties of deep rock masses are significantly influenced by temperature and other factors. The effect of temperature on the strength of deep rock masses will pose a serious challenge to deep resource exploitation and engineering construction. In this paper, the thermal-mechanical coupling calculation model is established by particle flow code (PFC2D) to study the uniaxial compression response of rock masses with microcracks after temperature load. The strength of failure, microcracks, and strain was analyzed. The results show that: (i) When the soft rock thickness ratio Hs/H < 0.5, the displacement caused by the applied temperature is concentrated at the structural plane, and the contact force is concentrated at the end of the initial microcrack. When Hs/H ≥ 0.5, the displacement caused by the applied temperature is concentrated on both sides of the initial microcrack, and the contact force is concentrated in the hard rock area. (ii) The number of microcracks decreases with the increase of soft rock thickness under different working conditions. When the soft rock thickness ratio Hs/H < 0.5, the relationship curve between the number of microcracks and the vertical strain shows two stages of change. When Hs/H ≥ 0.5, the relationship curve between the number of cracks and the vertical strain changes shows three stages of change. (iii) When the soft rock thickness ratio Hs/H < 0.5, the failure strength decreases with the increase of soft rock thickness ratio at T = 100°C and 200°C. When T = 300°C and 400°C, the failure strength decreased first and then increased. When Hs/H ≥ 0.5, the failure strength increases with the increase of soft rock thickness at T = 200°C, 300°C, and 400°C. At T = 100°C, the failure strength decreases with the increase of soft rock thickness.

Evolution mechanism and quantitative characterization of initial micro-cracks in marble under triaxial compression

Evolution mechanism and quantitative characterization of initial micro-cracks in marble under triaxial compression

Initial Microcrack Characteristics of Concrete Interfacial Transition Zone and Cement Paste

Initial Microcrack Characteristics of Concrete Interfacial Transition Zone and Cement Paste

Determining the Role of Acicular Ferrite Carbides in Cleavage Fracture Crack Initiation for Two Medium Carbon Microalloyed Steels.

Two medium-carbon microalloyed steels with a predominant acicular ferrite microstructure were investigated in this study in order to determine the initial micro-crack formation mechanism and the role of acicular ferrite structure in cleavage fracture. In order to ensure cleavage fracture, samples were investigated at -196 °C for uniaxial tension and four point bending fracture. Previous investigations have shown that cleavage fracture for steels with a predominant acicular ferrite microstructure has not been initiated by the fracture of coarse TiN particles as in ferrite-pearlite, bainite, or martensitic microalloyed steels. The average maximal thickness of cementite plates measured in this work is 0.798 µm and 0.966 µm, for V and TiV steel, respectively. The corresponding stress values required for their fracture according to Griffith's equation are 1970 MPa and 1791 MPa, respectively. Estimated values of the effective surface energy for the V steel with an average cementite volume fraction of 3.8% range from 40 Jm-2 to 86 Jm-2, and for the TiV steel with an average cementite volume fraction of 18.3% range from 55 Jm-2 to 82 Jm-2. The fracture of coarse cementite plates was found to not to be responsible for the cleavage fracture initiation in case of both steels.

Mechanical behavior and fracture mechanism of red-bed mudstone under varied dry-wet cycling and prefabricated fracture planes with different loading angles

Red-bed mudstone is a common geological material in engineering construction in the Southwest region in China. Its initial fractures are easily influenced by the combined effects of dry-wet cycling and external loads. Therefore, evaluating the mechanical behavior and fracture mechanism of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles is of great significance. In this study, the cracked straight through Brazilian disc (CSTBD) specimens with a prefabricated crack were tested using an RMT testing machine. Multiple-scale analysis methods, including digital image correlation (DIC) and acoustic emission (AE) techniques, 3D scanning, and scanning electron microscopy (SEM), were employed to comprehensively analyze the mechanical evolution and fracture damage mechanism of mudstone under dry-wet cycling. The experimental results indicate that as the loading angle increases, the peak load gradually increases, while an increase in the number of dry-wet cycles leads to a decrease in peak load. At the same time, dry-wet cycling causes the expansion and connection of initial microcracks, forming weak layers that affect the morphology of the main crack and the distribution of secondary cracks. Furthermore, with an increasing number of dry-wet cycles, the acoustic emission count significantly decreases, and the relative high value range of acoustic emission count expands. 3D scanning results show that an increase in the number of dry-wet cycles results in rougher fracture planes, while an increase in loading angle generally increases the roughness of the fracture planes. Additionally, SEM observations reveal that with an increasing number of dry-wet cycles, intragranular fracture patterns and microcrack distribution become more widespread, and tensile and shear fracture patterns are also observed in the center of the fracture plane. These research findings have important practical value for evaluating the stability and reliability of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles.

The non-plane initiation and propagation mechanism of multiple hydraulic fractures in tight reservoirs considering stress shadow effects

Multi-cluster fracturing in horizontal wells is an important stimulation technology for unconventional tight reservoirs. The interference among multiple simultaneously growing hydraulic fractures results in a non-uniform fluid distribution into fractures and non-planar fracture geometries. In this paper, the competition mechanism between perforations under the stress shadow, the formation mechanism of complex non-plane fractures near the wellbore, and the initiation and propagation mechanism of multiple hydraulic fractures are investigated by using the 3D lattice method. Also, the effects of fracture geometry, fracture spacing and rock mechanics parameters on the stress interaction among fractures are analyzed by the dislocation theory. The results showed the complexity of fracture initiation near the wellbore and the low efficiency of the perforation initiation in the case of spiral perforation, due to a strong interaction between perforations. However, these initial micro-cracks will eventually coalesce to be a macroscopical fracture which perpendicular to the direction of minimum principal stress. The study indicated that stress interaction and fracture propagation is significantly affected by inclined well orientation. The hydraulic fracture is more likely to deflect and propagate non-uniformly with the increasing width of hydraulic fracture, the decreasing spacing between perforation clusters and the increasing rock modulus. When the horizontal principal stress difference is small, it is obvious to cause a large deflection angle and sometimes furcation of fractures. This study can provide a theoretical basis and technical guidance for hydraulic fracturing design in tight reservoirs.

Mechanical impact characteristics of hollow shell granule based on continuous damage theory

Walnut, which is one of specific hollow shell granules, is inevitably subjected to damage deriving from repeated impacts in postharvest and processing. In this regard, the existence of damage was validated through force-deformation profile. The damage model was established and validated, respectively. Effects of walnut size and impact energy on damage behaviors, including damage variable characterizing degree of damage and damage accumulative coefficient describing sensibility to damage, were discussed, respectively. Results indicate that damage model by repeated impacts has been demonstrated to be valid for walnut shell fracture, in a way that is relatively independent of specific impact energy, only varying with walnut size. There exists a threshold size determining fracture mechanisms. Below threshold size, it is due to damage driving initial micro-crack gradually occurs, whereas beyond threshold size, it attributes to losing structural stability. The piecewise function between damage accumulation coefficient and walnut size was established.

Concrete Composites Based on Quaternary Blended Cements with a Reduced Width of Initial Microcracks

This article is devoted to the study of the combined effect of siliceous fly ash (FA), silica fume (SF), and nanosilica (nS) on the cement matrix morphology and size of microcracks occurring in the Interfacial Transition Zone (ITZ) between the coarse aggregate and the cement paste of concrete composites based on ordinary Portland cement (OPC). The manuscript contains analyses of width of microcracks (Wc) occurring in the ITZ area of concretes based on quaternary blended cements and changes in ITZ morphology in the concretes in question. Experiments were planned for four types of concrete. Three of them were composites based on quaternary blended cements (QBC), while the fourth was reference concrete (REF). Based on the observations of the matrices of individual composites, it was found that the REF concrete was characterized by the most heterogeneous structure. However, substitution of part of the cement binder with active pozzolanic additives resulted in a more compact and homogenous structure of the cement matrix in each of the QBC series concretes. Moreover, when analyzing the average Wc values, it should be stated that the modification of the basic structure of the cement matrix present in the REF concrete resulted in a significant reduction of the analyzed parameter in all concretes of the QBC series. For QBC-1, QBC-2, and QBC-3, the Wc values were 0.70 μm, 0.59 μm, and 0.79 μm, respectively, indicating a decrease of 38%, almost 48%, and 30%, respectively, compared with the working condition of concrete without additives. On the basis of the above results, it can therefore be concluded that the proposed modification of the binder composition in the analyzed materials clearly leads to homogenization of the composite structure and limitation of initial internal damages in concrete.

Dynamic Evolution and Quantitative Characterization of Fractures in Coal at the Eastern Edge of Ordos Basin under Axial Loading

Understanding the evolution of pore-fracture networks in coal during loading is of paramount importance for coalbed methane exploration. To shed light on these dynamic changes, this study undertook uniaxial compression experiments on coal samples collected from the eastern edge of the Ordos Basin, complemented by μ-CT scanning to obtain a 3D visualization of the crack network model. The compression process was divided into three stages, namely, micro-crack compaction, linear elasticity, and peak failure. An increase in stress resulted in greater concentration and unevenness in fractal dimensions, illustrating the propagation of initial cleats and micro-cracks in the dominant crack direction and the ensuing process of crack merging. These results provide valuable insights into the internal structure and behavior of coal under stress, informing more efficient strategies for coalbed methane extraction.

Effect of gravel on rock failure in glutenite reservoirs under different confining pressures

Due to the existence of gravel, glutenite is heterogeneous and different from fine-grained rocks such as sandstone and shale in structure. To fully understand the effect of gravel on failure mode in glutenite, we performed triaxial compression tests on different glutenites. The results indicate that failure modes of glutenite are complex due to the existence of gravel. Under different confining pressures, three failure modes were observed. The first failure mode, a tensile failure under uniaxial compression, produces multiple tortuous longitudinal cracks. In this failure mode, the interaction between gravels provides the lateral tensile stress for rock splitting. The second failure mode occurs under low and medium confining pressure and produces a crack band composed of micro-cracks around gravels. This failure mode conforms to the Mohr-Coulomb criterion and is generated by shear failure. In this failure mode, shear dilatancy and shear compaction may occur under different confining pressures to produce different crack band types. In the second failure mode, gravel-induced stress concentration produces masses of initial micro-cracks for shear cracking, and gravels deflect the fracture surfaces. As a result, the fracture is characterized by crack bands that are far broader than in fine-grained rocks. The third failure mode requires high confining pressure and produces disconnected cracks around gravels without apparent crack bands. In this failure mode, the gravel rarely breaks, indicating that the formation of these fractures is related to the deformation of the matrix. The third failure mode requires lower confining pressure in glutenite with weak cement and matrix support. Generally, unlike fine-grained rocks, the failure mode of glutenite is not only controlled by confining pressure but also by the gravel. The failure of glutenite is characterized by producing cracks around gravels. These cracks are produced by different mechanisms and distributed in different manners under different confining pressures to form different fracture patterns. Therefore, understanding the rock microstructure and formation stress state is essential in guiding glutenite reservoir development.

Initial damage analysis in bone cement-stem debonding procession of cemented hip arthropsty

The initial interface debonding damage behavior of cemented hip arthroplasty was simulated and investigated through the push-in experiment. A series of nondestructive testing techniques were carried out to detect damage to the stem-cement interface and cement. Results suggested that the initial stem-cement interface debonding and cement damage were induced at femur loading early period. The cement was subjected to a combination action of shear stress and bending from a sinking stem causing a pressure-arch effect resulting in compression creep and micro-crack on the cement successively. The stem-cement interfacial friction prevent stem from sinking due to the internal shear effect caused by the cement-stem interface micro-sliding. The interface debonding procession increased at first and then became stable, which was shown in cement's creep deformation-friction-fracture model. The initial micro-cracks released damage energy which was converted into stress waves. Further, the relationship of bond stress and slippage at different locations caused different degrees of damage belt for the interface and cement. It was concluded that the combination effect of pressure arching and shear lag was the primary reason for cement's initial damage and the matrix defect affected the failure progress.

Coupled thermal-hydraulic simulations of fracturing in granite under high temperature and high pressure treatment via peridynamic

This paper described an improved multi-layer computational method based on fully coupled thermal-mechanical ordinary state-based peridynamics (OSB-PD), which enables simulating fracturing in granite under coupled thermal-hydraulic effects. Four computational layers, i.e., pre-treatment thermal layer (PTL), mechanical loading layer (MLL), interactive thermal layer (ITL), and hydraulic mechanical layer (HML) were included in this method. PTL was used to simulate the heterogeneity of granite materials by creating initial pre-existing microcracks. MLL was utilized to apply borehole pressure, while ITL was designed to generate real-time thermal forces. HML was employed to apply equivalent hydraulic pressure to cracks, and distinguish between thermal-induced and hydraulic-induced damage. Two verification cases, including hydraulic fracturing and sleeve fracturing of granite specimens at different pressurization rates, as well as the steady-state heat conduction of a hollow cylindrical specimen, were simulated to investigate the convergence, capability and accuracy of the numerical method. The proposed method was then applied to the hydraulic fracturing of granite under high temperature and high pressure (HTHP) treatment. The complex interactions among natural pre-existing microcracks, thermal-induced microcracks and hydraulic-induced cracks was properly predicted. The effect of cold water (temperature gradient) on the fracture morphology and breakdown pressure of granite specimens was also discussed. A systematic comparison with the experimental results shed lights to the failure mechanisms of granite specimens subjected to HTHP hydraulic fracturing tests.

Influence of initial microcracks on the dynamic mechanical characteristics of sandstone

Influence of initial microcracks on the dynamic mechanical characteristics of sandstone

In-situ investigation on tensile properties of a novel Ti/Al composite sheet

• The discrepancy of mechanical properties between ti and al layers evidences the slip bands with 45° to tensile direction in Al grains. • Morphology and/or crystallographic orientation changes in al grains of composite achieve a compatible deformation. • Heterogeneous deformation induced stress concentration facilitates the initiation of micro-crack and sequential failure at the interfacial zone. Laminated/bimetal composites can offer the advantage of combining constituent metals, but their heterogeneous microstructures exhibit different mechanical properties during subsequent remanufacturing process than those of the homogeneous counterparts. This study investigated the tensile properties, microstructural evolution and deformation behaviours of a new explosive welded TA1/5083 composite sheet using the in-situ characterisation. It is found that a wavy transition zone forms in the composite sheet, showing gradients in the element diffusion and micro-hardness adjacent to the interface. The slip bands with the tensile direction of 45 ° primarily occur in the 5083 side (rather than the TA1 side) with an increase in deformation. Although the dislocation density of the grains in the 5083 layer increases significantly, these grains are slightly elongated along the tensile direction without a significant orientation change. Three deformation stages and a subsequent crack initiation stage appear during the in-situ tensile process. The initiation of micro-cracks in the interfacial zone results from the internal stress concentration occurring at the bottom of interfacial wave structure. Once initial micro-cracks evolve into the intermetallic compounds (IMCs) of the interfacial zone, the high magnitude of stress concentrations of the TA1 and 5083 layers leads to necking and sequential fractures. This study can provide a reference for the design of high-quality Ti/Al composite components for industry.

Tensile and compressive creep of UHSM with CaO-based expansive agent

To mitigating shrinkage cracking of ultra-high strength mortar (UHSM), the CaO-based expansive agent (CaO-EA) has been considered as an important additive. In this paper, the tensile and compressive creep of UHSM with CaO-EA were studied by the newly designed creep device. Similar creep deformation under tension and compression was observed. The compressive and tensile specific creep of UHSM increase with the increase of EA content. The increasing creep deformation could be attributed to the increasing initial microcracks due to the incorporation of EA. The multiscale shear sliding mechanism (manifested as microcrack propagation) was proposed to explain the tensile and compressive creep behavior. The commonly used creep models, ACI 209, B3 and Gl2000, can be considered in cracking calculation of UHSM with EA due to good prediction accuracy within a few days.

Rolling contact fatigue crack propagation on contact surface and subsurface in mixed mode I+II+III fracture

Rolling contact fatigue (RCF) is the critical damage from rolling bearings constraining the service, performance, and reliability, which is directly caused by fatigue crack propagation. The crack paths on both the contact surface and subsurface were observed after the bearing fatigue test. The three-dimensional geometry of the RCF crack was characterized and reconstructed based on experimental statistics. Finite element analysis was performed to obtain the mixed mode stress intensity factors (SIFs) along the crack fronts, and the crack propagation behavior was numerically studied. Results indicated that the propagation of cracks on the contact surface and subsurface causes the gradual formation of micropits and the sudden spalling of material respectively. Initial micro-cracks with an inclination angle of 15–30° have the highest growth rate, which should be prevented during the bearing raceway manufacturing process. The propagation paths of the RCF cracks can be more accurately predicted by considering the superposition of fracture mode I + II + III in three-dimensional, and mechanisms of crack propagation on contact surface and subsurface were revealed respectively.

Numerical modeling of the opening mode fracturing emanating from deformation localization in layered rocks

It is known that rock fracture includes inelastic straining or damage that should localize at a certain loading stage and result in fracture initiation. The details of this process are not clear, and it is frequently omitted in the models by imposing the initial microcracks (seeds) with certain lengths and orientations. Here we investigate 2-D systems of three layers in finite-difference models. The layers subjected to the horizontal extension are separated by cohesive-frictional interfaces and have contrasted properties typical of sedimentary piles. Fractures are initiated in a more brittle central layer in the vicinity of the interfaces with the adjacent layers. It starts with the initially distributed inelastic straining, which then localizes into narrow bands. The damage within these bands is strongly accelerated, resulting in complete material failure locally. Short initial fractures corresponding to narrow bands of failed material are normal to the least local stress. They then propagate from the interfaces to the layer center with further extension. We carefully investigate the impact of different regularization procedures, the grid geometry, and structure on all stages of the fracture process and define the optimal conditions that can be applied for fracture modeling in different structural and loading configurations.