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The fracture evolution and the strength characteristics of a jointed rock mass under hydro-mechanical coupling are key issues that affect the safety and stability of underground engineering. In this study, a kind of transparent rock-like resin was adopted to investigate the crack initiation and propagation modes of the 3D flaw under hydro-mechanical coupling. The influences of the water pressure and the flaw dip angle on the fracture modes of the 3D flaw and the strength properties of the specimen were analyzed. The experiment results indicated that under the initiation and propagation modes, the 3D flaw presented two types of modes: the low-water-pressure type and the high-water-pressure type. The increase in the water pressure had a significant promoting effect on the crack initiation and propagation, which changed the overall failure mode of the specimen. With the increase in the flaw dip angle, the critical growth length of the wing crack decreased and the initiation moment of the fin-like crack showed a hysteretic tendency. The influences of the water pressure on the crack initiation stress and failure strength had thresholds. When lower than the threshold, the crack initiation stress increased slightly and the failure strength decreased gradually with the increase in the water pressure. Once the threshold was exceeded, both the crack initiation stress and the failure strength decreased significantly with the increase in the water pressure. With the increase in the flaw dip angle, both the crack initiation stress and the failure strength showed a first decreasing and then increasing tendency. The lowest crack initiation stress and the failure strength were found for the specimen containing the 45° flaw, while the highest were found for the specimen containing the 75° flaw. This study helps to deepen the understanding of the fracture mechanism of the engineering rock mass under hydro-mechanical coupling and has certain theoretical and applied value in engineering design and construction safety.

Mechanical properties of reservoirs and overlying strata in the condition of underground coal gasification (UCG) are vital to the stability of surrounding rock. In this work, sandstone samples of seam roof were collected and a series of experiments including X-ray computed tomography (CT) scanning, X-ray diffraction (XRD) and uniaxial compression tests with synchronous acoustic emission (AE) monitor were conducted to study crack distribution, propagation, minerals composition and failure modes of sandstone samples after different temperature treatments. As the treatment temperature increases, the threshold ratios of crack initiation stress and rupture stress gradually decline. Tensile failure mode of the biplane symmetric is dominant for sandstone samples treated with 25 °C and 200 °C. Single plane shear failure mode is dominant for 400 °C and 600 °C. Nevertheless, for 800 °C and 1000 °C, comprehensive features of the above temperatures are exhibited, resulting in a complex failure mode. 800 °C is the threshold temperature for failure behavior and uniaxial compressive strength (UCS). The primary fracture morphology plays a critical role in crack propagation and failure mode of sandstone samples. When axial stress reaches 96% of peak stress, peak frequency abruptly declines. It discloses the precursory features of the critical failure of sandstones after different temperature treatments. Meanwhile, various forms of water evaporation, the muscovite and kaolinite decomposition as well as phase transformation of quartz due to high temperature affect the mechanical properties and failure modes of sandstone. The results can guide the safety evaluation and roof disaster prevention of UCG.HighlightsThe crack propagation characteristics and failure modes of roof sandstone are investigated in macroscopic and microscopic respects.Sandstone samples with different temperature treatments have various failure behaviors and 800℃ is the threshold temperature.As the treatment temperature increases, the crack initiation stress (30–45% peak stress) and rupture stress (65–80% peak stress) gradually decrease.

Acoustic emission (AE) and particle flow code (PFC3D) are utilized to characterize microcracking nature in mode-I fracture of coal with strong bursting liability. The microcracks are represented by AE moment tensor (MT), which corresponds to displacement discontinuities involving opening/closing and sliding motions. The microcracking source parameters, including shear/tensile type, volume, orientation, motion, and magnitude, are calculated by minimizing the errors between the analytical and measured displacements, with an imposed constraint. Then, the detailed fracture processes, including microcracking source mechanisms and energy dissipation during coal tensile failure, are analyzed using both experimental and numerical approaches. The results show that the time-varying of crack mode angle and decomposed crack volume suggest a mixed-mode mechanisms locally with both normal and tangential displacements for microcracks. However, the orientations of microcracks with bigger magnitude are almost parallel to the direction of the crack propagation path, and their movement directions are mainly along the direction of the maximum principal stress, which is compatible with the expected fracture mechanism of mode-I opening globally. The dissipated energy and crack opening displacement obtained from AE inversion are approximately consistent with results calculated from energy release rate G and opening displacement measured using DIC, respectively. A new AE MT simulation method, based on particle motion, is developed using PFC3D to verify the reliability of the above-mentioned microcracking characterization results. The mechanical properties as well as AE responses (including AE rate, magnitude distribution, spatial location, and MT) from PFC3D numerical simulation are in good agreement with the experimental results, which reinforces the reliability the microcracking characterization results from AE inversion and simulation. Due to the tortuous crack propagation path and the inhomogeneity between grains, the maximum principal stress and resultant displacement coexists both on the normal and tangential of the inclined crack surface. Under the combined action of tensile and shear stress, the normal and tangential motion displacement components along the crack plane are generated, respectively, resulting in mixed-mode microcrack. This research provides a new approach for characterizing the microcracks in loaded rock/coal, which can deepen the understanding of meso-process and mechanism during rock/coal fracture.HighlightsMicrocracks are represented by acoustic emission moment tensor subjected to displacement discontinuity.A new method for acoustic emission simulation is developed using particle flow code to verify test results.Time–space–energy evolution process and focal-mechanism of microcracks are analyzed.Meso-mechanism of mixed-mode microcracks during coal tensile failure is explained.

To determine the influence of bedding planes on pure mode I and mixed-mode I–II dynamic fracture toughness (DFT) and crack propagation characteristics of coal, a modified split Hopkinson pressure bar (SHPB) system is used to test notched semi-circular bend (NSCB) specimens. The DFT is calculated by the finite element code Abaqus. Two strain gauges are used to measure the crack propagation velocity and the crack propagation path is recorded by a high-speed digital camera. The results show that bedding planes have significant influences on the effective DFT and crack propagation characteristics of coal. As the bedding-plane angles increase (meaning the geometric positional relationship of the loading direction and the bedding-plane direction transforms from perpendicular to parallel), the effective DFT and peak force decrease. The bedding planes affect the crack initiation direction, and the crack propagation path is jointly determined by the direction of maximum principal stress and the bedding planes. For pure mode I, the crack propagation velocities rise with the increase of bedding-plane angles. However, for mixed-mode I–II, the largest velocity is at the bedding-plane angle of 45°. Moreover, the largest loading velocities at the linear stage of the loading history all occur at 45° and decrease rapidly thereafter.

The rotating detonation characteristics of high-temperature hydrogen-rich gas were studied. Hydrogen-rich gas was generated by the pre-combustion of hydrogen, and a rotating detonation experiment of hydrogen-rich gas and air was subsequently performed. The auto-initiation of high-temperature hydrogen-rich gas was observed in the experiment, and the influence of pre-detonation tube ignition on the steady propagation of the detonation wave was analyzed. The results show that high-temperature hydrogen-rich gas and air have the ability to spontaneously form rotating detonation waves. The operation of the pre-detonation tube has a significant influence on the propagation mode and propagation velocity of the continuous rotating detonation wave after auto-initiation. The rotational detonation wave formed by the auto-initiation of hydrogen-rich gas and air has a short instability in the propagation process. The propagation velocity of the detonation wave before and after the unstable state is 1345.4 m/s and 1425.3 m/s, respectively, the unstable state is 1345.4 m/s and 1425.3 m/s, respectively.

Natural fractures and laminae are well-developed in continental shale, which greatly affects the fracture propagation and failure mode. Based on the natural fractures and laminae developed in the outcrops of Triassic continental shale from the southern Ordos Basin, China, four different types of shale models are constructed in this research. The CASRock software V1.0 is utilized to conduct numerical simulations to investigate the influence of natural fractures and soft-to-hard laminae on the mechanical behavior of continental shale. The results demonstrate that the uniaxial compressive strength of shale models can improve by up to 34.48% when soft-to-hard laminae are present, but it can drop by up to 18.97% when weak interfaces are present. New fractures are consistently initiated at the ends of natural fractures, with various propagation patterns in different laminae. Fractures in soft laminae usually propagate in an oblique path at an angle β ≈ 20°–30° relative to the direction of compressive stress, manifesting as shear fractures. Fractures in medium-to-hard laminae tend to propagate parallel to compressive stress, primarily featuring tensile fractures. The ultimate fracture morphology becomes more complex as soft, medium, and hard laminae and weak interfaces occur successively. It changes from a nearly linear fracture to an echelon pattern with more secondary fractures and finally a network shape, with a total fracture area increase of up to 270.12%. This study reveals the combined effect of natural fractures, soft-to-hard laminae, and weak interfaces on the fracture propagation and failure model of continental shale, providing support for fracturing optimization based on shale’s authentic structure characteristics.

During tunnel construction and service, the rock surrounding tunnel is often subjected to multiple factors that influence its behaviour, such as dynamic disturbances (explosions, mechanical excavations, etc.) and existing cracks. These factors can readily induce safety issues, such as rock bursts and collapses. To investigate the effect of the loading direction on the failure modes of fractured tunnels, this study numerically investigated the destructive behaviour of tunnel models under the coupling effect of dynamic disturbance loads and external cracks using the finite-difference method (FDM). Additionally, a physical tunnel model with prefabricated cracks was created using green sandstone. A drop weight impact testing device (DWITD) was employed as the dynamic disturbance loading apparatus, while the relative azimuth angle between the tunnel and the cracks was varied. The crack initiation, arrest time, and extension rate were obtained using a crack fracture tester (CFT). The research results indicated that the preexisting cracks propagated continuously and eventually connected with the tunnel on the incident side under impact loads. The failure area of the tunnel was primarily controlled by the loading direction, exhibiting different modes of failure, often occurring at the bottom and arch of the tunnel. New cracks on the transmitted side of the tunnel appeared at different locations for different impact angles. The presence of cracks around the tunnel had a significant impact on the dynamic stress concentration factor (DSCF) of the rock surrounding the tunnel. The findings of this research can provide valuable guidance for tunnel stability analysis and the optimization of support schemes.HighlightsCrack parameter test was applied in crack propagation speed calculation.A large specimen with tunnel was used to calculate rock dynamic fracture toughness.The displacement trend line diagram was used to identify the fracture pattern of the crack under impact.The fracture toughness of rock is calculated by experimental numerical method. Stress wave and fracture mechanics theories were used to explain the interaction mechanism between cracks and tunnel.

Fatigue failure is invariably the most crucial failure mode for metallic structural components. Most microstructural strategies for enhancing fatigue resistance are effective in suppressing either crack initiation or propagation, but often do not work for both synergistically. Here, we demonstrate that this challenge can be overcome by architecting a gradient structure featuring a surface layer of nacre-like nanolaminates followed by multi-variant twinned structure in pure titanium. The polarized accommodation of highly regulated grain boundaries in the nanolaminated layer to cyclic loading enhances the structural stability against lamellar thickening and microstructure softening, thereby delaying surface roughening and thus crack nucleation. The decohesion of the nanolaminated grains along horizonal high-angle grain boundaries gives rise to an extraordinarily high frequency (≈1.7 × 10 3 times per mm) of fatigue crack deflection, effectively reducing fatigue crack propagation rate (by 2 orders of magnitude lower than the homogeneous coarse-grained counterpart). These intriguing features of the surface nanolaminates, along with the various toughening mechanisms activated in the subsurface twinned structure, result in a fatigue resistance that significantly exceeds those of the homogeneous and gradient structures with equiaxed grains. Our work on architecting the surface nanolaminates in gradient structure provides a scalable and sustainable strategy for designing more fatigue-resistant alloys. Most strategies to improve fatigue resistance address either crack initiation or growth. Here, the authors design a gradient-structured Ti with nacre-like surface nanolaminates that increase fatigue performance by suppressing both stages of cracking

A high degree of alteration causes swelling, disintegration and argillization of granite and thus weakens its physical and mechanical properties. Very few experimental studies, however, have been conducted to quantitatively characterize the strong-to-weak transition of the mechanical behaviours of granite affected by varying degrees of alteration. A new miniature tensile instrument system, which can visualize the failure process and simultaneously record the stress–strain curves, was utilized to test the tensile strength of dog-bone sliced samples of altered granite. Meanwhile, an improved quantitative method was proposed to characterize the alteration degree. The results demonstrate that the tensile strength of granite decreases significantly with increasing alteration rate, while the strain at the peak strength and fracture angle increase. Additionally, the orientation of weaker crystals and the mineral size distribution significantly affect fracture propagation. But the effect will be weakened as the alteration rate increases. On the basis of fracture angle, tensile peak strength, strain at peak strength, and microscopy images of crack initiation and propagation, three major classes of fractures are identified from the test results: (1) extension fracture at a low alteration rate: crack initiates and propagates along biotite-brittle mineral interfaces; (2) shear fracture at a high alteration rate: intragranular crack initiates within altered feldspar and propagates along the interior of altered mineral grains; (3) hybrid fracture at a moderate alteration rate: the above two failure phenomena coexist. A simple model that considers the fracture process of altered granite was proposed to explain the transition from extension fracture to shear fracture.HighlightsA new visual miniature tensile instrument is utilized to capture the crack initiation and propagation.An improved quantitative method is proposed to characterize the alteration degree of granite.As alteration degree increases, the tensile strength of granite significantly decreases, whereas both the strain and the fracture angle increase.The transition from extension fracture to shear fracture of altered granite is detected.

Normal stress (σn) under compression-shear conditions significantly affects crack propagation and failure modes of specimens containing non-persistent joints (NPJs). Crack propagation and failure modes of specimens vary for different σn. In the present study, we adopt a strength-based local maximum stress criterion (SLMS) to describe tensile and shear cracks formed during shear and model crack propagation processes via the finite element method. Two classical cases are performed to validate the SLMS criterion for describing tensile and shear cracks under shear conditions. After that, crack initiation, propagation, and coalescence processes are modeled for specimens containing a pair of horizontal NPJs in coplanar and non-coplanar cases at different σn. Variations of crack propagation and failure modes of specimens containing coplanar and non-coplanar NPJs are investigated for σn ranging from low to high. Effects of rock bridge inclination angle (β) on crack propagation and failure mode of specimens containing non-coplanar NPJs are investigated. Also, the effects of σn on critical shear load (τsc) for crack initiation in specimens containing coplanar and non-coplanar NPJs are examined. Results indicate that failure modes of specimens with coplanar NPJs transform from tensile to mixed tensile-shear, then to shear, and finally to pure shear as σn increases. The increase of β promotes the appearance of tensile failure of specimens containing coplanar NPJs under high σn while suppressing the appearance of shear failure. The modeling results provide a reference for understanding crack propagation and failure modes in rocks containing NPJs under compression-shear conditions.