Microcracks In Zone Research Articles

Rock failure mechanisms based on rheological dynamics

This paper investigates the mechanisms of rock failure related to axial splitting and shear failure due to hoop stresses in cylindrical specimens. The hoop stresses are caused by normal viscous stress. The rheological dynamics theory (RDT) is used, with the mechanical parameters being determined by P- and S-wave velocities. The angle of internal friction is determined by the ratio of Young's modulus and the dynamic modulus, while dynamic viscosity defines cohesion and normal viscous stress. The effect of frequency on cohesion is considered. The initial stress state is defined by the minimum cohesion at the elastic limit when axial splitting can occur. However, as radial cracks grow, the stress state becomes oblique and moves towards the shear plane. The maximum and nonlinear cohesions are defined by the rock parameters under compressive strength when the radial crack depth reaches a critical value. The efficacy and precision of RDT are validated through the presentation of ultrasonic measurements on sandstone and rock specimens sourced from the literature. The results presented in dimensionless diagrams can be utilized in microcrack zones in the absence of lateral pressure in rock masses that have undergone disintegration due to excavation.

Apparent Joint Swarms Formed by the Crack‐Jump Process

ABSTRACTJoint swarms can be important components of fractured reservoirs. They are often explained as damage around faults or related to mechanical differences between layers, although this does not explain the close spacing of the joints. Joint swarms around Bergen (Norway) are described, which are not related to exposed faults and are not influenced by layering or foliation in the Lower Palaeozoic gneisses. We suggest an evolution whereby: (1) a zone of microcracks develops; (2) one microcrack propagates and becomes connected to a source of mineralising fluid; (3) the fracture becomes a microvein, with a higher tensile strength than the microcracked host rock; (4) another microcrack propagates and the cycle is repeated, producing a zone of microveins; (5) the veins are partly weathered out, producing an apparent joint swarm, or the microveins crack at or near the ground‐surface. Joint swarms in exposed analogues may therefore not occur at reservoir depths.

Mechanisms of anisotropic fracture resistance of nacreous structure and design: Essential role of cracking modes of organic interfaces

Nacreous structure holds significant promise for biomimetic design applications due to its excellent mechanical properties. In this paper, the influence of loading direction on the mechanical properties of nacreous structure was investigated to understand its mechanisms of anisotropic fracture resistance. The results revealed significant anisotropy in the mechanical properties of nacre when loaded along different directions (perpendicular to y and z axes). When loaded perpendicular to the top face, nacre exhibits better mechanical properties and greater resistance to fracture compared to loading perpendicular to the side face. This distinction can be attributed to different cracking modes of organic interfaces in nacreous structure, where the nacreous structure loaded perpendicular to the top face exhibits more tortuous crack deflection, larger fracture stepped surface area and microcrack zones. Furthermore, the theoretical analysis indicates that the differences in the volume fraction of organic interfaces and the main crack deflection along organic interfaces are important factors leading to different cracking modes of organic interfaces in nacreous structure. Based on the analysis of the mechanisms of anisotropic fracture resistance, the critical parameters to characterize the anisotropic mechanical properties of the nacreous structure were proposed. The research results provide a reference and basis for future biomimetic application design.

Fracture of bio-cemented sands

Bio-chemical reactions enable the production of biomimetic materials such as sandstones. In the present study, microbiologically-induced calcium carbonate precipitation (MICP) is used to manufacture laboratory-scale specimens for fracture toughness measurement. The mode I and mixed-mode fracture toughnesses are measured as a function of cementation, and are correlated with strength, permeability and porosity. A micromechanical model is developed to predict the dependence of mode I fracture toughness upon the degree of cementation. In addition, the role of the crack tip T-stress in dictating kink angle and toughness is determined for mixed mode loading. At a sufficiently low degree of cementation, the zone of microcracking in the vicinity of the crack tip is sufficiently large for a crack tip K-field to cease to exist and for crack kinking theory to not apply. The interplay between cementation and fracture properties of sedimentary rocks is explained; this understanding underpins a wide range of rock fracture phenomena including hydraulic fracture.

Indentation‐induced ductile behavior of glass–ceramics involving layered aluminosilicates

AbstractContact damage in materials is critical in engineering applications because it influences mechanical resistance, such as wear, erosion, and impact failure. Indentation tests were performed using a tungsten carbide ball indenter (Hertzian contact) on the surfaces of glass–ceramics containing hexagonal CaAl2Si2O8 or mica crystals (fluorophlogopite), both of which have a layered structure. The stress–strain relation and the permanent deformation on the surface, as well as the observation of the microcrack zone by X‐ray computed tomography using synchrotron radiation, revealed that the glass–ceramic with hexagonal CaAl2Si2O8 showed ductility similar to the quasi‐plastic behavior previously observed in the mica glass–ceramic. The yield stresses of the glass–ceramics were estimated from the stress deviating from the stress–strain relation assuming complete elastic response between the ball and the sample. The ratio of the yield stress to Young modulus (Y/E) of the glass–ceramic with hexagonal CaAl2Si2O8 was determined to be higher than that of the mica glass–ceramic.

Toughening amplification of microcracks in staggered composites

Biological materials depict an excellent combination of both strength and toughness, which is a vital requirement for most engineering materials. It provides motifs to revolutionize the techniques for producing novel materials with improved properties. However, there is still a lack of deep understanding regarding the toughening mechanisms in biological materials, in order to overcome the conflict between strength and toughness in synthetic materials. This paper establishes a microstructure-based fracture model to reveal the toughening effect of the microcrack zone near the crack tip. Firstly, the stress–strain relationship considering the initiation and saturation of microcracks is derived. Then, a fracture mechanics model incorporating the micro-mechanics of microcracks is established, based on the above nonlinear mechanical response. Finally, the model is used to investigate the influence of microstructural parameters on the fracture toughness considering microcracks. The results show that there are two nondimensional parameters that significantly influence the fracture toughness: the ratio between the gap and overlapped zone, ξ, and λla. In addition, the selection of the microstructural parameters not only needs to consider the balance between stiffness, strength, and toughness, but also achieve the optimal toughness by combining the effects of several different mechanisms.

Theoretical and experimental researches on fracture toughness for bedded shale using the centrally cracked Brazilian disk method with acoustic emission monitoring

To investigate the fracture toughness including pure mode I, combined mode I + II, and pure mode II, the CCBD (centrally cracked Brazilian disk) fracture experiments with acoustic emission (AE) detecting are conducted on the shale specimens with distinct bedded inclination angles (0°, 30°, 60°, and 90°), and this research first introduces the detailed derivations on the three dominating fracture criteria. The investigated results demonstrate that the fracture resistances for the tested CCBD shale are insensitive to the bedded direction and the anisotropy of the fracture toughness is regarded as relatively weak, which is reflected in the smoother and more regular rupturing paths. The improved criteria (GMTS, GMTSN, and GMTSEDF) can provide a reasonable agreement with the measured data by considering the variable damaged radius of microcracking zone. The proposed k-means clustering method succeeds in identifying the specific fracture modes for the tested CCBD bedded shale in conjunction with the KDE (kernel density estimation) method, whereas the conventional AE parameter (RA and AF) method fails to analyze the fracture mechanisms.

Fracture evolution and failure behavior around an opening in brittle jointed rocks subjected to uniaxial compression

To investigate the fracture evolution and failure behavior around an opening in brittle jointed rocks, an experimental study was carried out in which cross-jointed samples with granite cavities were uniaxially loaded while the mechanics, acoustics and optics were simultaneously observed. These results were compared and analyzed to better understand the deformation and cracking process. The results show that the presence of cross joints and the change in included angles play a crucial role in influencing the strength and deformation properties of the samples. The peak strength and elastic modulus depend on the included angle and first decrease and then increase with increasing included angle, reaching the minimum values when β equals 45°. A stress fluctuation point generally corresponds to a large acoustic emission (AE) event and a turning point in the accumulated AE count curve. The good correlation of the three observations provided a reasonable holistic picture of the process zone nucleation and the subsequent cracking processes. The cracking behavior around openings and cross joints is significantly affected by the included angle because of drastic changes in the stress distribution surrounding the defects. The highlighted strain localization zones formed near the joint tip and around the chamber interact with each other and jointly dominate the initiation and propagation mode of new cracks, eventually leading to more complex failure patterns and severe fracture degrees. Moreover, there will also be an interaction between the same set of cross joints, and the tips can be connected, showing the characteristics of intensified deformation in local areas. The unstable failure of a sample is primarily dominated by tensile cracks and locally by shear cracks around the cavern and the joint tips. Microcrack zones are regarded as white patches macroscopically, and microcrack nucleation is considered to be the microformation mechanism of a white patch.

Morphology of subsurface cracks in glass-ceramics induced by Vickers indentation observed by synchrotron X-ray multiscale tomography

The characterization of subsurface cracks induced by indentation is a challenge for understanding contact damage, impact, wear, erosion, and abrasion of brittle materials, because the crack pattern observable on the surface is only a part of the total crack system. Here we applied synchrotron X-ray multiscale tomography to observe the morphology of subsurface cracks produced by Vickers indentation in a novel CaO–Al2O3–SiO2 glass-ceramic with plate-like crystals forming a house-of-cards microstructure. It revealed a diverse type of crack systems around the semispherical microcrack zone beneath the indent, including a new mode II inclined lateral crack driven by the maximum shear stress. Tomography images provided knowledge on how the heterogeneous microstructure affected the toughening processes such as crack deflection, crack bridging, and microcracking.

Study of the fracturing behaviour of an electrohydraulic shock wave in sandstone under static pressure

To understand the application of electrohydraulic shock waves (EHS) in deep rock formations, the fracturing behaviour of repetitive EHS under a static pressure environment is studied in this paper. The static pressure environment of sandstone is simulated and a load model of the transmission wave of EHS at the liquid–rock interface is proposed. Analysis of the spatial distribution, dense zone and fracture zone of microcracks shows that static pressure has a significant inhibitory effect on the formation, distribution and growth of microcracks under repetitive EHS. Compared with normal pressure, cracks in a static pressure environment are mainly concentrated around the well hole, the microcracks are denser and the extension length is shorter. At the same time, the degree of damage is established to evaluate the overall cumulative damage to the sandstone. The coupling relationship between degree of damage, static pressure, discharge energy and discharge times is fitted. The fitting model shows a behaviour of continuous decelerating growth of microcracks, which can be divided into two phases, an initial growth phase and a slow-growth phase. The increase in static pressure mainly suppresses the initial growth phase, and the discharge energy needs to be increased to promote the initial degree of damage and the rate of accumulation of microcracks in the slow-growth phase. The rate of accumulation of microcracks in the slow-growth phase can be controlled through multi-level adjustment of the discharge energy. This paper provides guidance and simulation methods for rock fracturing applications of EHS in static pressure environments.

Numerical parametric study of Nonlinear Coda Wave Interferometry sensitivity to microcrack size in a multiple scattering medium

This paper reports a numerical study of the sensitivity and applicability of the Nonlinear Coda Wave Interferometry (NCWI) method in a heterogeneous material with a localized microcracked zone. We model the influence of a strong pump wave on the localized microcracked zone as a small average increase in the length of each crack. Further probing of this microcracked zone with a multiply scattered ultrasonic wave induces small changes to the coda-type signal, which are quantified with coda wave interferometry. A parametric sensitivity study of the CWI observables with respect to the changes in crack length is established via numerical simulations of the problem using a 2D spectral element method (SEM2D). The stretching of the signal, proportional to the relative variation in effective velocity, is found to be linearly proportional to the global change in crack length, while the other CWI parameter, the remnant decorrelation coefficient, is found to be quadratically proportional to the crack length change. The NCWI method is shown to be relevant for the detection of different damaged material states in complex solids. The reported numerical results are especially significant in the context of quantitative nondestructive evaluation of micro-damage level of a heterogeneous materials using nonlinear ultrasound signals.

A combined stress/energy-based criterion for mixed-mode fracture of laminated composites considering fiber bridging micromechanics

A combined stress/energy-based (CSE) criterion is developed to predict mixed mode I/II fracture behavior in laminated composites by taking into account the effect of fiber bridging toughening. Following the proposed criterion, first, the absorbed energy resulting from fiber bridging micro-mechanisms is obtained and translated into a defined effective critical distance (i.e., the size of the crack tip micro-cracking zone). Then, the maximum principal stress is evaluated at the calculated effective critical distance around the crack tip to predict the onset of fracture. The CSE criterion is employed, along with other criteria, to predict experimental data reported in the literature on the onset of mixed mode I/II fracture in laminated composites and wood species (i.e. orthotropic composites). A higher correlation is found between the theoretical results predicted by the CSE criterion and the test data, as compared to other criteria. The stress-based formulation of the CSE criterion offers more computational simplicity as compared to the energy-based criteria while providing an acceptable accuracy and taking into account the effects of absorbed energy resulting from the micromechanical fiber bridging in the solution.

Quasi-static loading rate effects on fracture process zone development of mixed-mode (I-II) fractures in rock-like materials

The development of a fracture process zone (FPZ) (microcrack zone) ahead of a fracture tip is a prominent fracture characteristic of rock-like materials. At present, understanding of the quasi-static loading-rate effect on the FPZ development of a mixed-mode (I-II) fracture remains challenging for rock-like materials. In this paper, the centrally cracked Brazilian disk (CCBD) specimens of artificial rock-like materials were tested to create mixed-mode (I-II) fractures at different quasi-static loading rates (0.02–2 mm/min), and the centrally crack in each specimen is prefabricated at 15° (tensile-shearing mixed-mode), 45° (compressive-shearing mixed-mode) and 0° (pure mode-I) to the loading direction. The digital image correlation (DIC) was employed to identify FPZ, with the discontinuous characteristics of DIC-measured displacement and strain field ahead of the fracture tip. Based on test results, a couple of outcomes were obtained. (1) The geometries of mixed-mode (I-II) fractures changed barely at different quasi-static loading rates. (2) At mixed-mode (I-II) fracture tips, the DIC-measured tensile displacement fields presented remarkable discontinuity; in contrast, the DIC-measured sliding displacement fields were nearly continuous. Consequently, at different quasi-static loading rates, the tensile-shearing and compressive-shearing mixed-mode (I-II) just represents the fracture bearing tensile-shearing and compressive-shearing stresses for rock-like materials. Still, at the mixed-mode (I-II) fracture tip, the deformation in FPZ and the generation of the real fracture surface is tensile. (3) With the loading rate increasing, the FPZ length of mixed-mode (I-II) fracture increased roughly from 5 mm to 17 mm, which is similar to the pure mode-I fracturing (FPZ length: 4.5–17.3 mm). The rate-dependent FPZ length of mixed-mode (I-II) fracture follows a power-law, consistent with the mode-I fracture. (4) The peak load (an index indicating the fracture resistance) of mixed-mode (I-II) and pure mode-I fracturing was strengthened with the increasing loading rate, linearly correlated to the increasing FPZ length. It indicates that rate-dependent FPZ is supposed to strengthen the fracture resistance of mixed-mode (I-II) cracks with loading rates increasing. This research can provide a basis for controlling mixed-mode (I-II) fracturing of rock-like materials in underground excavation projects and geo-energy exploitation, such as the deflecting propagation of hydraulic fractures.

Recent Advances in Nonlinear Fracturing Characteristics of the Hydraulic Fracture in the Deep Reservoir

During hydraulic fracturing in the deep reservoir, the rock surrounding the hydraulic fracture (HF) presents three remarkable nonlinear mechanical behaviors: the fracture process zone (FPZ; microcrack zone) developing at the HF tip, the microcrack band surrounding the HF surface, and the plastic zone growing due to the opening compression of the HF. The above three mechanical behaviors constitute the HF nonlinear fracturing characteristics. However, understanding the HF nonlinear fracturing characteristics is challenging and crucial. In this work, we present our recent published advances in HF nonlinear fracturing. (1) We developed a characterization method for rock FPZ by utilizing and integrating fiber Bragg grating (FBG), digital imaging correlation, and acoustic emission (AE). The integrated measurements showed that FPZ softening followed a linear response. The relationship between FPZ length and crack tip opening displacement was linear, and the relationship between dissipated energy and FPZ length was quadratic. (2) Detailed analysis of the AE energy during cyclic fracturing tests of sandstone revealed that the FPZ propagation rate of subcritical fractures was related to the increasing rate of dissipated energy accumulated from all activated microcracks. Thus, a general law (Zhang’s law) of dl/dN = (1/2CG)l −1(dG D/dN) was proposed and validated. (3) The HF microcrack band of high-temperature (120 °C) granite was identified using AE waveform analyses. The fracture energy was reduced by approximately 75% adjacent to the wellbore (approximately 40% of the fracture length) in the microcrack band. The effective width of the microcrack band was reduced by 40%–56.4% at 120 °C, indicating that high temperatures decreased the effective stimulated volume of HF. (4) We proposed a thermoplastic constitutive model for high-temperature rocks (such as shale) in deep reservoirs. The constitutive relation depends on hydrostatic pressure, stress deviator, and temperature. Parameters characterizing effects of temperature on thermoplasticity were also proposed. The above research advances can provide bases for HF in the deep reservoir.

Study of high-speed-impact-induced conoidal fracture of Ti alloy layer in composite armor plate composed of Ti- and Al-alloy layers

In order to understand the mechanism of conoidal fracture damage caused by a high-speed fragment-simulating projectile in titanium alloy layer of a composite armor plate composed of titanium- and aluminum-alloy layers, the ballistic interaction process was successfully simulated based on the Tuler–Butcher and GISSMO coupling failure model. The simulated conoidal fracture morphology was in good agreement with the three-dimensional industrial-computed-tomography image. Further, three main damage zones (zones I, II, and III) were identified besides the crater area, which are located respectively near the crater area, at the back of the target plate, and directly below the crater area. Under the high-speed-impact conditions, in zone II, cracks began to form at the end of the period of crack formation in zone I, but crack formation in zone III started before the end of crack formation in zone II. Further, the damage mechanism differed for different stress states. The microcracks in zone I were formed both by void connection and shear deformation. In the formation of zone I, the stress triaxiality ranged from −2.0 to −1.0, and the shear failure mechanism played a dominant role. The microcracks in zone II showed the combined features of shear deformation and void connection, and during the formation process, the stress triaxiality was between 0 and 0.5 with a mixed failure mode. Further, the microcracks in zone III showed obvious characteristics of void connection caused by local melting. During the zone III formation, the triaxiality was 1.0–1.9, and the ductile fracture mechanism was dominant, which also reflects the phenomenon of spallation.

Microstructural characterization of pressure-induced cracking in melamine/F2311 composites and crack-healing behavior via thermal-pressure aging treatment

The effects of F2311 content on the crack morphology and crack healing behavior of melamine/F2311 composites, a safe polymer-bonded explosive analog, were systematically studied in terms of its microstructure and mechanical properties. Scanning electron microscopy and CT analysis were used to quantitatively characterize the microcrack zone. The results demonstrated that thermal-pressure aging treatment could eliminate cracks and further improve the density and mechanical properties. The density and mechanical properties of the M-10F samples improved by the largest margin, its density increased by 1.9% and its tensile and compressive strength increased by >100%. The improvement of the density and mechanical properties were related to the processing parameters, the initial density, and the binder content. The thermal energy and compressive stress provided to the system during thermal-pressure aging treatment can promote the binder migration at the interface. In addition, small melamine particles that are prone to displacement and rotation may migrate with F2311 to fill cracks. These processes could collectively reduce damage and defects, partially or completely heal cracks, and restore the structure and properties of the material. Crack healing under pressure and temperature indicates that thermal-pressure aging treatment has high potential for damage repair of composites with a thermoplastic binder.

Micro‐computed tomography imaging and probabilistic modelling of rock fracture by freeze–thaw

AbstractA major problem in studies of rock fracture by frost is the paucity of direct observations in space and time of the initiation and growth of microcracks and their transition to macrocracks. Such observations are essential to understand the location, timing and controls of rock fracture by freeze–thaw. The aim of the present work is to image and elucidate the early stages of rock fracture by applying imaging and statistical methods to a frost‐weathering experiment using intact specimens of a limestone (chalk) and sandstone. First, micro‐computed tomography (μ‐CT) is used to visualize rock fracture in three dimensions over the course of 20 freeze–thaw cycles and to estimate transverse strain using a pixel‐based approach. Second, probabilistic correlation functions are applied to quantify the progressive expansion of the fracture phase and associated damage to rock specimens. The method of μ‐CT is demonstrated for visualizing the growth and coalescence of microcracks and their transition to macrocracks. Fracture proceeded faster and to a greater extent in chalk relative to sandstone, and the macrocracks in chalk were mostly concentric and vertical. Both fracture development and positive transverse strain (dilation) accelerated after cycle 15, suggesting that a threshold has been exceeded, after which macrocracks were evident. Of three probabilistic correlation functions applied to the μ‐CT results, the modified lineal‐path function – which measures the continuous connectivity of the fracture phase in a specific direction – reveals that damage was more extensive in the chalk than the sandstone. It also allows a novel approach to define and quantify three zones of microcracking during freeze–thaw cycling of anisotropic rock: (1) the zone of inherent flaws; (2) the zone of active microcracking; and (3) the zone of weak influence during microcracking. The broader significance of this work is that it provides a new approach to investigate mechanistically how frost action damages rock. © 2019 John Wiley & Sons, Ltd.

A zinc oxide-modified hydroxyapatite-based cement favored sealing ability in endodontically treated teeth

ObjectivesTo evaluate the effectiveness of different endodontic canal sealers for dentin permeability reduction and to determine the viscoelastic performance of root dentin after their application. MethodsCervical, medial and apical root dentin surfaces were treated with two experimental hydroxyapatite-based cements, containing sodium hydroxide (calcypatite) or zinc oxide (oxipatite); an epoxy resin- based canal sealer, AH Plus; and gutta-percha. Root dentin was evaluated for fluid filtration. Field emission scanning electron microscopy, energy dispersive analysis, AFM, Young’s modulus and Nano-DMA analysis were also performed, at the inner and outer zones of dentin. ResultsDentin treated with oxipatite showed the lowest microleakage among groups with hermetically sealed tubules and zinc-based salt formations. Samples treated with oxipatite showed the highest Ei at the cervical dentin third among groups, at 6 m of storage. Oxipatite promoted the highest complex modulus and tan delta values at the inner zone of both cervical and medial root dentin. Calcypatite favored the lowest tan delta outcomes at the inner zone of apical dentin at 6 m. ConclusionsSpecimens treated with oxipatite showed the highest sealing ability, based on the highest Young’s modulus and dentin mineralization, achieved by closing dentinal tubules, voids and pores that reinforced the inner zone of root dentin. The homogeneity of viscoelastic properties among the different root dentin thirds favored the energy dissipation without creating zones of stress concentration and micro-cracking which would have challenge micropermeability. Thereby, among the tested materials oxipatite is proposed as canal filling material and sealer in endodontics. Clinical significanceOxipatite could be considered a good candidate for root canal filling material and sealer due to its improved long-term sealing ability and to the advanced remineralization, and so to the enhanced energy dissipation achieved at the inner zone of the radicular dentin.

Detection and Characterization of Randomly Distributed Micro-cracks in Elastic Solids by One-Way Collinear Mixing Method

Numerical simulations on the propagation of bulk waves in elastic solids with randomly distributed micro-cracks by the one-way collinear mixing method are performed to investigate the behavior of the resonant wave. The results show that the resonant wave can be generated due to the effect of micro-cracks, and the position and the size of micro-crack zone can be detected by the time signal of the resonant wave. Based on statistical analysis from numerous results of random micro-crack models, we find that the acoustic nonlinear parameter increases linearly with the micro-crack density and the size of micro-crack zone, which is also related to the frequency of resonant wave and friction coefficient of micro-crack surfaces. This study numerically reveals that the one way mixing method can be employed to quantitatively identify and characterize micro-cracks in elastic solids.

Generation Mechanism of Nonlinear Rayleigh Surface Waves for Randomly Distributed Surface Micro-Cracks.

This paper investigates the propagation of Rayleigh surface waves in structures with randomly distributed surface micro-cracks using numerical simulations. The results revealed a significant ultrasonic nonlinear effect caused by the surface micro-cracks, which is mainly represented by a second harmonic with even more distinct third/quadruple harmonics. Based on statistical analysis from the numerous results of random micro-crack models, it is clearly found that the acoustic nonlinear parameter increases linearly with micro-crack density, the proportion of surface cracks, the size of micro-crack zone, and the excitation frequency. This study theoretically reveals that nonlinear Rayleigh surface waves are feasible for use in quantitatively identifying the physical characteristics of surface micro-cracks in structures.