Shear Fracture Research Articles

Investigation on the fracture mechanics characteristics and crack initiation of deep dense shale

The mechanical behavior of shale fractures significantly affects the structure and stability of hydraulic fracturing networks in shale gas reservoirs. Currently, there is insufficient research on the mechanical characteristics of fractures, crack initiation, and the prediction of fracture trajectories in deep dense shale. This study involved conducting direct tension, shear, and three-point bending fracture mechanical experiments on shale samples, complemented by numerical calculations and mechanistic analyses. The results show a significant influence of the bedding plane angle (γ) on the initiation properties and locations of shale fractures. With γ ranging from 0° to 90°, shale fractures transition from alignment with the bedding plane to penetration through it. Fracture initiation locations show a significant degree of randomness when fractures propagate along the bedding plane. The horizontal tensile stress changes with increasing γ, initially increasing and then decreasing as it moves away from the center. The evolution of the horizontal tensile stress field primarily causes alterations in shale fracture paths, reaching its peak value at γ = 45°. Shale initiation fractures can be classified into four types: tensile fractures through the bedding plane, tensile fractures along the bedding plane, shear fractures across the bedding plane, and shear fractures along the bedding plane. Established a criterion called the “maximum stress strength ratio” to determine fracture initiation, which can predict the initiation properties, locations, and paths of shale fractures. When γ is small, fractures mainly propagate along the bedding plane, with a higher likelihood of initiation occurring at the center of the specimen. Conversely, when γ is large, fractures mainly propagate through the bedding plane. The spacing of the bedding planes minimally affects the predicted fracture paths of shale. This study is of significant theoretical importance for reservoir stimulation and stability assessment in deep dense shale gas reservoirs.

Experimental Study on Energy Evolution and Acoustic Emission Characteristics of Fractured Sandstone under Cyclic Loading and Unloading

To investigate the dynamic response of fractured rock under cyclic loading and unloading, a WHY-300/10 microcomputer-controlled electro-hydraulic servo universal testing machine was used to conduct uniaxial cyclic loading and unloading tests. Simultaneously, acoustic emission (AE) and a CCD high-speed camera were employed to monitor the fracturing characteristics of sandstone. The mechanical properties, energy evolution, AE characteristics, and deformation of 45° sandstone were analyzed. The results indicate that as the load cycle level increases, both the elastic modulus and deformation modulus exhibit a “parabolic” increase, with a rapid rise initially and a slower rate of increase later. The damping ratio generally shows a decreasing trend but tends to rise near the peak load. The total energy, elastic energy, dissipated energy, damping energy, and damage energy all follow exponential function increases with the load level. The b-value fluctuates significantly during the stable crack propagation phase, unstable crack propagation phase, and peak phase. When the FR (Felicity ratio > 1), the rock is relatively stable; when the FR (Felicity ratio < 1), the rock gradually extends towards an unstable state. The Felicity ratio can be used as a predictive tool for the precursors of rock failure. Shear fractures dominate during the compaction and peak phases, while tensile fractures dominate during the crack propagation phase, ultimately leading to a failure characterized by tensile fracture. High-speed camera observations revealed that deformation first occurs at the tips of the prefabricated cracks and gradually spreads and deflects toward the ends of the sandstone. This study provides theoretical support for exploring the mechanical behavior and mechanisms of fractured rock under cyclic loading and unloading, and it has significant practical implications.

Coupled thermo-mechanical phase field modeling for shear ductile fracture at high strain rate and temperature

The objective of the current study is to introduce a coupled thermo-mechanical phase field model to elucidate shear ductile fracture under varying strain rates and temperatures. Both the plastic work threshold and fracture energy exhibit dependence on the strain rate and temperature. To explore this in detail, hat-shaped specimens are adopted to systematically investigate shear ductile fracture under distinct strain rates and temperatures by the split Hopkinson pressure bar apparatus. The simulated and experimental strain waveforms are then compared to calibrate and validate the damage parameters of the presented phase field model. The results demonstrate that the numerical simulations are in agreement with the experiments. Moreover, the fracture process of the hat-shaped specimen is also examined. The numerical results furnish a reliable description of the hat-shaped specimen from damage initiation to complete fracture. It's found that the influences of strain rate on shear stress and fracture initiation are less pronounced than that of temperature for U75V steel. In summary, the developed phase field model can reproduce shear ductile failure of the hat-shaped specimen under diverse strain rates and temperatures.

Relationship between Natural Fracture and Structural Style and its Implication for Tight Gas Enrichment: A Case Study of Deep Ahe Formation in the Dibei‐Tuzi Area, Kuqa Depression

AbstractThe deep Lower Jurassic Ahe Formation (J1a) in the Dibei‐Tuzi area of the Kuqa Depression has not been extensively explored because of the complex distribution of fractures. A study was conducted to investigate the relationship between the natural fracture distribution and structural style. The J1a fractures in this area were mainly high‐angle shear fractures. A backward thrust structure (BTS) is favorable for gas migration and accumulation, probably because natural fractures are more developed in the middle and upper parts of a thick competent layer. The opposing thrust structure (OTS) was strongly compressed, and the natural fractures in the middle and lower parts of the thick competent layer around the fault were more intense. The vertical fracture distribution in the thick competent layers of an imbricate‐thrust structure (ITS) differs from that of BTS and OTS. The intensity of the fractures in the ITS anticline is similar to that in the BTS. Fracture density in monoclinic strata in a ITS is controlled by faulting. Overall, the structural style controls the configuration of faults and anticlines, and the stress on the competent layers, which significantly affects deep gas reservoir fractures. The enrichment of deep tight sandstone gas is likely controlled by two closely spaced faults and a fault‐related anticline.

Study on strengthening and toughening Al/CFRP hybrid composites with biomimetic interface

The biomimetic interface of Al/CFRP hybrid composite material was obtained by combining laser surface treatment and hot-pressing process. The experimental results showed that the tensile shear strength and fracture energy of the biomimetic interface were 44.5 MPa and 1652 J/m2, which were 224% and 72.6% higher than those of the untreated interface. The new C-O-Al chemical bond was formed at the biomimetic interface, which replaced the van der Waals force and improved the interface strength. The transition layer of the biomimetic interface can effectively delay the transmission of cracks at the interface. The finite element results showed that the stress distribution of biomimetic interface was uniform and stress concentration was not obvious. The strengthening and toughening effect of the interface is mainly due to the pinning effect of mechanical interlocking of the interface and the behavior of crack deflection of the transition layer.

Experimental study on the evolutive shear fracture behaviour and properties of cemented paste backfill

Shear stress prevalence in in-situ cemented paste backfill (CPB) makes understanding mode-II fracture behaviour and properties vital for the mechanical stability of the backfill structure. This study delves into the effect of cement content on CPB's shear fracture behaviour and properties. Results reveal that higher cement content leads to notable changes in load-displacement characteristics under mode-II loading, including steeper pre-peak slopes, higher peak loads, smaller peak-load displacements, and minimal post-peak resistance. However, increased cement content does not linearly enhance fracture properties. Furthermore, this study proposed predictive models to capture the influence of cement content on the evolutive fracture properties.

Investigation on acoustic emission characteristics of fault stick-slip under different lateral pressures

To explore the effect of different stress environments on fault-slip rockbursts. Bidirectional shear friction experiments with different lateral pressures were conducted on precracked syenogranites buried at 800 m. The macroscopic statistical parameters (cumulative number of AE events, magnitude and b value) and local characteristic parameters (amplitude and dominant frequency) of acoustic emission during the stick-slip process under different lateral pressures were investigated. In addition, based on fractal theory, the nonlinear characteristics of AE spectrum were analyzed. On this basis, the microscopic mechanism of fault stick-slip was discussed. The results show that the lateral pressure influences the friction strength of the fault and stick-slip motion characteristics. With increasing lateral pressure, the proportion of transgranular shear fractures increases, which leads to an increase of cumulative number of AE events and magnitude. The periodic decrease in the b value is more significant at high lateral pressure. There is a good correlation between a high-magnitude AE event and a stress drop. The AE frequency with phased response characteristics can be used to effectively identify the evolution of fault stick-slip instability at the laboratory scale. A sharp increase in the amplitude of the dominant frequency can be regarded as one of the precursory features of fault stick-slip instability. The AE frequency spectra have multifractal characteristics, that differ among the different stages. The maximum multifractal dimension and spectral width can reflect the difference in energy released during fault stick-slip motion.

Shear performance of headless studs in ultra-high performance concrete bridge deck

Conventional stud connectors were proved to be inconvenient for the later disassembly and replacement in ultra-high performance concrete (UHPC) bridge decks. In this study, the headless studs which is a type of studs with their heads removed were employed as a substitute for conventional stud connectors, aiming to enhance the detachability of the bridge decks on the premise of ensuring the anti-lift bearing capacity of the decks. Three push-out specimens containing the headless studs with a diameter of 13 mm were carried out to study the mechanical properties of headless studs. In addition, the finite element model was developed to reveal the failure mechanism of headless studs in UHPC. Results indicated that the load-slip curve of headless stud connector can be divided into elastic stage, plastic stage and failure stage. The failure mode of headless studs was the shear fracture at the root of stud shank. Headless stud connections under cyclic loading can produce greater plastic deformation. The ultimate shear capacity and shear stiffness of headless studs were lower than those of conventional studs. Additionally, the UHPC slabs configured with headless studs have a small quantity of separation at the steel-UHPC interface, so it is suggested to use headless studs and conventional studs together in UHPC slabs to ensure the necessary anti-lift bearing capacity. Finite element analysis indicated that in the elastic stage, the bearing capacity of the push-specimen was dominated by the material properties of the headless stud. After entering the plastic stage, the bearing capacity of the push-specimen was mainly controlled by the UHPC below the headless stud.

Outcome determinants for coronal shear fractures of the distal humerus.

Coronal shear fractures of the distal humerus involving the capitellum and trochlea are rare injuries. Internal fixation with headless compression screws provides a stable construct facilitating early mobilisation. Our study aimed to identify the key determinants of both radiological and functional outcomes of patients with distal humerus coronal shear fractures treated with internal fixation. A retrospective analysis of 61 patients with distal humerus coronal shear fractures who were treated surgically was done. Demographics, fracture morphology, time to surgery, operative details such as surgical approach and implant used, quality of reduction, time to union, and associated complications from hospital records. Radiological outcomes were assessed using plain radiographs, and the functional outcomes were by Oxford Elbow Score (OES) and Mayo Elbow Performance Index (MEPI). Patients with anatomical reduction of the fracture had better functional outcomes and range of motion. The presence of posterior comminution of capitellum resulted in poorer outcomes (p = 0.03). Delayed presentation did not alter the outcome when the anatomical reduction was achieved. Myositis ossificans was noted in nine patients and non-union in five patients. Two patients developed avascular necrosis of the capitellum and arthritis of the elbow joint. Anatomical reduction and posterior comminution are the two key determinants of the functional outcome in these coronal shear fractures of the distal humerus. Early mobilisation following a stable fixation is crucial in achieving a good outcome.

Fabrication and bending performance of ultra-low-density all-composite honeycomb sandwich shell based on stretching process

A sandwich shell with an ultra-low-density honeycomb core was designed and manufactured from a carbon-fibre prepreg via a stretching process. Subsequently, the bending bearing limits of the sandwich shell corresponding to five possible failure modes (shear buckling, shear fracture, intracellular dimpling, face fracture, and core debonding) were obtained through theoretical predictions. Failure mechanism maps were generated to reflect the possibility of each failure mode intuitively. In addition, three-point bending tests were performed on specimens with different shell thicknesses to acquire as many of these typical failure modes as possible. Moreover, the competition mechanism between different failure modes was revealed. The research results provide theoretical guidance for parameter optimisation and applications in advanced engineering fields for ultra-low-density carbon-fibre honeycomb sandwich shells.

The low-cycle fatigue behavior and an entropy-based life prediction model for Nickel-based single crystal superalloy across an extensive temperature range

Nickel-based single-crystal superalloys (Ni-SX) are prominently utilized in the fabrication of turbine blade materials for advanced aero-engines, owing to their commendable material characteristics. This paper delves into the low cycle fatigue (LCF) characteristics of a second-generation Nickel-based single-crystal superalloy across a temperature spectrum ranging from room temperature to 1000 °C. During this procedure, there is an escalation in the trend towards deformation homogenization, leading to a transition in fracture mode from shear fracture to normal fracture. A novel approach is introduced in the form of an entropy-based fatigue life prediction model, grounded in the correlation between cumulative plastic strain and entropy increase rate. Through a comparative examination with experimental data, the model demonstrates proficiency in precisely forecasting the fatigue life of Ni-SX under diverse strain amplitudes and temperature conditions.

Experimental study on the physical and mechanical properties of carbonatite rocks under high confining pressure after thermal treatment

AbstractOil and gas exploration studies have been increasingly moving deeper into the earth. The rocks in deep and ultra‐deep reservoirs are exposed to a complex environment of high temperatures and large geo‐stresses. The Tarim oilfield in the Xinjiang Uygur Autonomous Region (Xinjiang for short), China, has achieved a breakthrough in the exploration of deep hydrocarbon reservoirs at a depth of over 9000 m. The mechanical properties of deep rocks are significantly different from those of shallow rocks. In this study, triaxial compression tests were conducted on heat‐treated carbonatite rocks to explore the evolution of the mechanical properties of carbonatite rocks under high confining pressure after thermal treatment. The rocks for the tests were collected from reservoirs in the Tarim oilfield, Xinjiang, China. The experiments were performed at confining pressures ranging from atmospheric to 120 MPa and temperatures ranging from 25 to 500°C. The results show that the critical confining pressure of the brittle–ductile transition increases with increasing temperature. Young's modulus is negatively correlated with the temperature and positively correlated with the confining pressure. As the confining pressure increases, the failure mode of the specimens gradually transforms from shear fracture failure into “V”‐type failure and finally into bulging failure (multiple shear fractures). With increasing temperature, the failure angle tends to decrease. In addition, an improved version of the Mohr‒Coulomb strength criterion with a temperature‐dependent power function was proposed to describe the failure strength of carbonatite rocks after exposure to high temperature and high confining pressure. The surface of the strength envelope of this criterion is temperature dependent, which could reflect the strength evolution of rock under high confining pressures after thermal treatment. Compared with other strength criteria, this criterion is more capable of replicating physical processes.

A novel fracture criterion for elastic‐brittle cracked body under compression and shear conditions

AbstractWhen the elastic‐brittle cracked body is subjected to compressive‐shear loading, not only does the phenomenon of wing crack initiation and propagation occur, but there is also a notable incidence of secondary crack initiation and propagation due to shear stress. Moreover, the shear fracture toughness (KIIc) predicted by the fracture criterion in the framework of classical fracture mechanics is usually smaller than the tensile fracture toughness (KIc), which is inconsistent with the actual observations. Therefore, this research firstly establishes a mechanical model of an elastic‐brittle cracked body under biaxial compression considering three kinds of crack parameters and materials' mechanical properties. Then, the stress field characteristics at the crack tip are analyzed. Next, a compression‐shear mixed fracture criterion is proposed. Finally, the effects of the lateral pressure coefficient (λ), critical plastic zone size (rc), friction coefficient (f), and deformation parameters of the crack surface on the failure modes at the crack tip are investigated.

Study on the influence of pore water pressure on shear mechanical properties and fracture surface morphology of sandstone

To further investigate the weakening effect of pore water pressure on intact rock mechanics properties and characteristics of fracture surface after failure, direct shear tests of sandstone were conducted under different pore pressure. A 3D scanner was employed to digitize the morphology of the post-shear fracture surface. The variogram function was applied to quantify the anisotropic characteristics of post-shear fracture surface. The relationship between deformation during shear failure of intact rock and quantitative parameters of fracture surface after shear failure was initially established. It can be found that amplitudes of the sinusoidal surface determine the maximum value of variogram, and period affect lag distance that reach the maximum value of variogram. Test results revealed that the increase of pore pressure has obvious weakening effect on shear strength and deformation of rock. Moreover, the increase of pore pressure makes the shear fracture surface flatter. It can be obtained that both Sillmax and Rangemax are positively related to shear strain, but negatively related to normal strain.

Wellbore instability in naturally fractured formations: Experimental study and 3D numerical investigation

Wellbore instability due to natural fractures is a significant issue impeding drilling operations in shale formations. However, most of the existing literature on wellbore stability analysis focuses on the coupled effects of fluid flow and deformation, and the impact of three-dimensional natural fracture characteristics on wellbore instability is not yet closely examined. This paper carried out a series of experiments to analyze the shale sample's fracture characteristics and shear strength. A fully coupled 3D hydro-mechanical model was then developed using the distinct element method (DEM) to investigate the wellbore instability. Finally, a comprehensive parametric study was performed to analyze the effects of characteristics of fractures (e.g., distribution density, fracture roughness and associated shear strength, dip angle, and seepage time) on wellbore stability under different in-situ stress states. The results show that the shear strength of shale increases with the increase in normal pressure (4 MPa, 6 MPa, and 8 MPa); the shear strength increases by 64.9% when the roughness is 0.325 mm (the largest increase among the three roughness cases). The maximum displacement around the wellbore is 1.3–1.5 times greater under the presence of weak planes than under no weak planes. Fracture roughness poses an important impact on wellbore stability because larger fracture roughness is associated with higher fracture shear strength. As the dip angle of the weak plane increases, the number of yield zones (shear failure) and maximum wellbore displacement increases and then decreases, and the peak occurs at the dip angle of 45°. The seepage time of drilling fluid is positively correlated with the number of yield zones around the wellbore, and the damage rate of drilling fluid to wellbore stability gradually decreases with the seepage time.

Desirable Strong and Tough Adhesive Inspired by Dragonfly Wings and Plant Cell Walls.

The production of wood-based panels has a significant demand for mechanically strong and flexible biomass adhesives, serving as alternatives to nonrenewable and toxic formaldehyde-based adhesives. Nonetheless, plywood usually exhibits brittle fracture due to the inherent trade-off between rigidity and toughness, and it is susceptible to damage and deformation defects in production applications. Herein, inspired by the microstructure of dragonfly wings and the cross-linking structure of plant cell walls, a soybean meal (SM) adhesive with great strength and toughness was developed. The strategy was combined with a multiple assembly system based on the tannic acid (TA) stripping/modification of molybdenum disulfide (MoS2@TA) hybrids, phenylboronic acid/quaternary ammonium doubly functionalized chitosan (QCP), and SM. Motivated by the microstructure of dragonfly wings, MoS2@TA was tightly bonded with the SM framework through Schiff base and strong hydrogen bonding to dissipate stress energy through crack deflection, bridging, and immobilization. QCP imitated borate chemistry in plant cell walls to optimize interfacial interactions within the adhesive by borate ester bonds, boron-nitrogen coordination bonds, and electrostatic interactions and dissipate energy through sacrificial bonding. The shear strength and fracture toughness of the SM/QCP/MoS2@TA adhesive were 1.58 MPa and 0.87 J, respectively, which were 409.7% and 866.7% higher than those of the pure SM adhesive. In addition, MoS2@TA and QCP gave the adhesive good mildew resistance, durability, weatherability, and fire resistance. This bioinspired design strategy offers a viable and sustainable approach for creating multifunctional strong and tough biobased materials.

Characteristics of Weathering Reservoirs and Differences in Fracture Formation in the Weathering Crust of the Pre-Cenozoic Basement of Lishui Sag, East China Sea Basin, China

Characteristics of Weathering Reservoirs and Differences in Fracture Formation in the Weathering Crust of the Pre-Cenozoic Basement of Lishui Sag, East China Sea Basin, China

Holistic Characterization of MgO-Al2O3, MgO-CaZrO3, and Y2O3-ZrO2 Ceramic Composites for Aerospace Propulsion Systems

Aerospace propulsion systems are among the driving forces for the development of advanced ceramics with increased performance efficiency in severe operation conditions. The conducted research focused on the mechanical (Young’s and shear moduli, flexural strength, hardness, and fracture toughness), thermal (thermal conductivity and coefficient of thermal expansion), and electric (dielectric properties) characterization of MgO-Al2O3, MgO-CaZrO3, and stable YSZ ceramic composites. The experimental results, considering structural and functional traits, underscore the importance of a holistic understanding of the multifunctionality of advanced ceramics to fulfill propulsion system requirements, the limits of which have not yet been fully explored.

Numerical study on dynamic cracking characteristic and mechanism of quasi-brittle materials under impulsive loading

This study focuses on investigating the dynamic cracking behaviors of quasi-brittle materials under impulsive loading using the extended non-ordinary state-based peridynamic (NOSB-PD) model. This extended model incorporates two stress-based failure criteria: Mohr-Coulomb and maximum tensile stress criteria, to represent shear and tensile fractures of quasi-brittle materials, respectively. By analyzing the evolution characteristics of the maximum principal stress obtained from the NOSB-PD model, the fracture mechanisms of quasi-brittle materials subjected to impulsive loading are explored. The research reveals that tensile failure predominantly governs the initiation and propagation processes of quasi-brittle materials under impulsive loading. The stress concentration effect, transfer effect, and dissipation effect observed in the maximum principal stress fields provide insights into the cracking mechanism during the failure processes of such materials. To validate the findings, the crack initiation modes and crack propagation paths obtained from the NOSB-PD model are compared with those obtained from the existing experimental and numerical approaches. The results demonstrate that the extended NOSB-PD model is effective in addressing fracture problems associated with quasi-brittle materials under impulsive loading conditions.

Ductile fracture prediction of 7A62 high-strength aluminum alloy under a wide range of stress states

The modified Gurson-Tvergaard-Needleman (GTN) model is employed to predict the ductile fracture of 7A62 high-strength aluminum alloy under a wide range of stress states. Mechanical tests were conducted on specimens with different stress states within the range of −0.33 to 1.35 stress triaxiality, including tension, notched tension, compression, and shear. The results indicate that at high stress triaxialities (0.8 ∼ 1.35), the fracture mechanism is intergranular ductile fracture. Under moderate stress triaxialities (0.33 ∼ 0.8), the fracture mechanism involves a combination of intergranular ductile fracture, void growth, and shear fracture. At low and negative stress triaxialities (−0.33 ∼ 0.33), plastic instability occurs due to uneven stress distribution, leading to shear fracture. Fractography analysis reveals that the fractures occurring under tensile stress are associated with enriched Mn particles of approximately 200 nm. The modified GTN model accurately predicts the load-displacement response, and the fracture paths under various stress states exhibit good consistency with experimental results. This study provides reference for failure prediction in the engineering application of high-strength aluminum alloys.