Search Results (297)

Traditional homogeneous materials often face an inherent trade-off between strength and toughness, restricting their application in high-performance impact protection. Mechanical metamaterials overcome this fundamental limitation by integrating structure and material. The 3D-printed Bouligand laminates (3DPBLs), a type of mechanical metamaterial, are renowned for their exceptional impact resistance. While the 3DPBLs have been proven to provide superior resistance under normal impact, actual service conditions inevitably involve complex, multi-directional loading. We aimed to investigate the 3DPBLs’ oblique impact resistance here. To this purpose, samples of 3DPBLs with varying helical angles (0°, 7°, 15°, 60°, 90°) were fabricated and subjected to low-velocity drop-weight impact tests at impact angles of 0°, 30°, 45°, and 60° to evaluate their damage evolution and energy dissipation. The experimental investigation exhibited distinct temporal evolutions of contact forces, with the 15° helical configuration identified as the optimal design. Further numerical analysis using a finite element model (validated with a deviation < 10%) is conducted to simulate performance under diverse impact angles in order to validate the reasonability of the experimental investigation. Mechanistically, 3DPBLs enhance impact resistance by increasing fracture tortuosity through their periodically rotated layered structure. These findings establish a theoretical foundation for developing high-performance, lightweight, and toughened protective materials. Full article

The microstructure of a high-manganese low-activation austenitic steel after aging for 500 and 1000 h at 700 °C was investigated using transmission and scanning electron microscopy. Two structural states were examined: cold rolling (CR) and high-temperature thermomechanical treatment (HTMT). After CR, aging leads to the precipitation of dispersed M₂₃C₆ carbides (M = Cr, W), primarily along grain and deformation twin boundaries. After HTMT, these particles are mainly localized at grain and low-angle boundaries. With increasing aging time, both the size and volume fraction of the particles increase. In both states, the microtwin and substructure are partially retained after aging. Local regions corresponding to the early stages of recrystallization were identified after both treatments. These regions were associated with intense decomposition of the supersaturated solid solution and the coarsening of carbide particles. The mechanical properties were evaluated by tensile testing at 20, 650, and 700 °C. Aging reduced average ductility after both treatments and at all test temperatures, with this trend persisting with increasing aging time. After CR and aging, a significant scatter in elongation to failure was observed, with minimum values of ≈2–3%. This behavior is attributed to the high density of plate-like M₂₃C₆ carbides at grain and microtwin boundaries. Microcrack formation and intercrystalline fracture features were observed, directly linked to the high density of boundary carbides. These effects were less pronounced in the HTMT condition after aging. In this paper, strategies for suppressing carbide precipitation in high-manganese low-activation austenitic steels via chemical composition and thermomechanical processing optimization are discussed. Full article

To address the challenge of surrounding rock instability in deep-buried tunnels crossing fractured fault zones, this study focuses on the Xigu Tunnel of the Lanzhou–Hezuo Railway. A combination of laboratory triaxial tests, an optimized multi-source advanced geological prediction workflow, and a site-specific parameter-weakened Mohr–Coulomb numerical simulation is employed to systematically reveal the physical–mechanical properties, spatial distribution, and deformation response of fractured rock masses under excavation-induced disturbance. The triaxial test results show that the average peak strength of the surrounding rock reaches 149.04 MPa; however, significant variability is observed among samples, and the failure mode exhibits a typical brittle–shear composite feature. The measured cohesion and internal friction angle are 20.57 MPa and 49.91°, respectively, indicating high intrinsic strength of individual rock blocks. Nevertheless, due to the presence of densely developed joints and crushed structures, the overall mass is loose and highly sensitive to dynamic disturbances such as blasting and excavation, revealing a unique mechanical paradox of high-strength rock blocks with low overall rock mass stability in deep-buried fractured zones. Joint TSP (Tunnel Seismic Prediction Ahead) and ground-penetrating radar (GPR) prediction reveals decreased P-wave velocity, increased Poisson’s ratio, and intensive seismic reflection interfaces; a quantitative index system for identifying the boundaries of narrow deep-buried fractured zones is proposed based on these geophysical characteristics. Combined with geological face mapping, these results confirm the existence of a highly fractured zone approximately 130 m in width, characterized by well-developed joints, heterogeneous mechanical properties, and localized risks of blockfall and groundwater ingress. The developed numerical model, with parameters weakened based on triaxial test and geological prediction data, effectively reproduces the deformation law of the fractured zone, and the simulation results agree well with field monitoring data, with peak displacement concentrated at section DK4 + 595, thus accurately identifying the center of the fractured belt as a key engineering validation result of the integrated technical framework. During construction, based on the identified spatial characteristics of the fractured zone and the proposed targeted support insight, enhanced dynamic monitoring and targeted support measures at the fractured zone center are required to ensure structural safety and long-term stability of the tunnel. This study develops an integrated engineering-oriented technical framework for deep-buried tunnels crossing narrow fractured zones, and provides novel mechanical insights and quantitative identification indices for such complex geological engineering scenarios. Full article

Hollow auxetic structures enable lightweight mechanical design by reducing mass while preserving architected deformation. However, hollow auxetic studies focus on LPBF metals. This study presents a manufacturing-constrained design and validation framework for a hollow hybrid re-entrant chiral lattice produced by stereolithography. The unit cell was parameterised by chiral angle, re-entrant strut length, and hollow internal diameter, with drainage features integrated into the CAD model to preserve hollow channels during printing and post-processing. A minimum internal diameter study defined the printable design window. Within these limits, a central composite design coupled with finite element analysis mapped the response surface and identified an optimised geometry of θ = 15°, L = 3.5 mm, and d = 1.68 mm, with a predicted unit-cell negative Poisson’s ratio of about −1.17. Compression testing confirmed that the printed unit cell and 3 × 3 × 3 lattice retained the intended rotation-dominated auxetic deformation mode. At the selected comparison strain, the unit cell showed a negative Poisson’s ratio of −0.68 and the 3 × 3 × 3 lattice showed −0.29. Relative to the solid lattice, the hollow lattice reduced density by 42.4% with only a 3.0% reduction in stiffness, increasing specific stiffness by 68.9% and specific peak strength by 5.2%, but reducing specific energy absorption by 25.6% due to earlier localisation and junction driven fracture. These results provide practical design guidance for manufacturable hollow SLA auxetic lattices, especially for lightweight and stiffness-limited applications where low mass and high specific stiffness are more important than energy absorption. Full article

Hydrogen embrittlement is known to adversely affect the mechanical properties of low-carbon steels used for pipelines and pressure vessels, leading to accelerated crack growth and lowered fracture toughness. To overcome the limitations of surface-based analysis, this study employs X-ray micro-computed tomography (µ-CT) to provide a comprehensive 3D evaluation of the crack evolution. This approach is used to assess hydrogen-assisted crack growth in P355NH compact tension samples from previous fracture mechanical tests and enables a precise quantification of the internal crack path and the crack tip opening angle (CTOA) across the entire specimen thickness as well as the local damage morphology. By integrating these spatial parameters, a deeper understanding of the impact of hydrogen on local fracture mechanisms is achieved, revealing insights that have remained hidden in previous two-dimensional microscopy observations. For instance, µ-CT results clearly demonstrate that the hydrogen-assisted crack propagation is associated with increased void formation and secondary cracking in vicinity of the crack tip. However, it is proposed that the results are superimposed with continuous hydrogen desorption, which implies a need for in situ charging during mechanical loading and an analysis of the hydrogen concentration profile. Both will be the scope of further studies. Full article

The study systematically investigates the effect of molybdenum (Mo) content (0.70–1.57 wt.%) on the microstructure and mechanical properties of quenched and tempered martensitic steel for ultra-high-strength and -toughness oil well pipes. The results demonstrate that increasing the Mo content substantially enhances the strength of the steel. The yield strength (YS) increases from 1135 MPa to 1233 MPa, the ultimate tensile strength (UTS) rises from 1176 MPa to 1285 MPa, and the elongation after fracture is marginally improved to 19%. However, the low-temperature impact energy (AKV₂) of the steel at −20 °C exhibits a pronounced decrease, from 117 J to 36 J. Mo refines the multi-scale martensitic microstructure, increases the fraction of high-angle grain boundaries (HAGBs) and dislocation density, and promotes the precipitation of three types of carbides. Quantitative analysis indicates that grain refinement strengthening is the predominant factor contributing to the enhancement of steel strength. The decline in the steel’s resistance to low temperatures is attributed to the separation of coarse, blocky M₃C-type carbides at the grain boundaries. This results in the accumulation of stress at these boundaries, leading to a transformation in the steel’s fracture mode from ductile to brittle. Full article

Grain-scale heterogeneity strongly influences hydraulic fracture initiation and trajectory in crystalline rocks, yet its contributions to non-planar growth and the interaction of multiple nearby cracks remain insufficiently quantified. To address this gap, we perform numerical experiments on a model containing two parallel pre-existing cracks using a hydro-mechanical phase-field framework, systematically quantifying how mineral distribution and axial compression govern non-planar hydraulic fracture growth and inter-fracture competition. The results demonstrate that mineral distribution is the primary driver of fracture complexity. Even within the same Voronoi tessellation, redistributing minerals alone yields markedly different trajectories, deflections, branching patterns, and final morphologies. Furthermore, non-planar growth follows a stepwise, energy-threshold-driven mechanism. When cracks penetrate strong grains or undergo large-angle deflections, propagation is impeded, and injection pressure builds up. Once a critical energy threshold is reached, accumulated energy is rapidly released along the path of minimum incremental energy, manifested as abrupt pressure drops and rapid crack advance. Additionally, the two nearby fractures exhibit strong mechanical competition. Despite negligible hydraulic interference in low-permeability granite, early growth of one fracture redistributes stresses and suppresses the driving force of the other, resulting in asymmetric development. Finally, axial compression primarily governs the overall propagation orientation and influences local failure modes but has a limited effect on peak pressure relative to mineral distribution. Full article

Mode II fracture toughness is an important material parameter of rocks, but accurate measurement of this parameter is still a challenge in rock fracture mechanics. This study aims to modify the mode II fracture toughness of red sandstone measured through shear box testing by emphasizing the critical role of crack initiation angle. Experimental tests combining fracture trajectory scanning and digital image correlation reveal distinct fracture mechanisms of red sandstone under varying loading angles: tensile spalling dominates low angles, and shear fractures emerge at medium angles, while tensile fracture initiates from the rock bridge center at high angles. Although shear fracture initiates from the notch tip, its initiation angle deviates from the initial crack plane, invalidating traditional mode II fracture toughness determination methods. A modified Mohr–Coulomb criterion incorporating fracture angle and Mode I stress intensity factor is proposed to correct the significant errors of traditional methods, and this study establishes a refined framework for mode II fracture toughness determination under compression–shear conditions. Full article

Accurately evaluating fracability is crucial for improving shale gas fracturing efficiency. This study proposes a new mechanical deformation modulus to characterize rock fracture modes under coupled effects of stress conditions and mechanical parameters. Combined with tensile strength and fracture toughness, a ternary-index fracability evaluation method is established covering the full process of “fracture initiation–propagation–network formation”. Taking intervals Q1–Q9 of Gulong Shale as the research object, experiments were conducted to classify main intervals into four mechanical models: (1) “low tensile–low toughness–low modulus” (Q2), where fractures crack and grow easily but exhibit small apertures and weak fracture-forming capacity; (2) “low tensile–low toughness–medium modulus” (Q1, Q3, Q6), where fractures crack and grow easily, forming low-angle intersecting fracture networks; (3) “low tensile–low toughness–high modulus” (Q7, Q9), where fractures crack and grow easily, creating large-aperture, high-angle through-going fracture networks; and (4) “high tensile–low toughness–high modulus” (Q4, Q5, Q8), where fractures crack with difficulty but grow easily, developing high-angle through-going shear fractures. The evaluation results are consistent with the actual fracability characteristics of the Gulong Shale. Compared with conventional evaluation methods, the ternary index evaluation method can more clearly reveal the progressive evolution process of fractures from crack to propagation and then to fracture network formation, providing a reliable basis for fracture network prediction and fracturing optimization. Full article

The fractured lacustrine carbonate oil reservoir in the Lower submember of Member 4 (Qian-4) of the Qianjiang Formation in the Zhongshi area, Jianghan Basin, represents an important target for hydrocarbon exploration and exhibits substantial exploration and development potential. To clarify the mechanisms by which fractures control reservoir effectiveness, this study integrates core description, thin-section petrography, petrophysical measurements, and geophysical interpretation to systematically characterize matrix properties and fracture development. Results show that the reservoir matrix is dominated by micritic carbonate rocks and grain-dominated carbonate rocks, and overall exhibits low-porosity and ultra-low-permeability characteristics, with an average porosity of 5.19% and permeability generally below 5 mD. Fractures are well developed within the matrix, mainly comprising non-tectonic bedding-parallel fractures and tectonic high-angle fractures. Fracture-related porosity averages 8.42%, and permeability can reach 10–100 mD or higher. The fracture attributes and their spatial distribution are the key controls on hydrocarbon enrichment and deliverability; the occurrence of different fracture types across lithologies and sublayers can significantly enhance reservoir flow capacity. Moreover, natural-fracture characteristics provide critical geological constraints for hydraulic fracturing design and implementation. These findings offer a theoretical basis for fine-scale exploration and development of fractured lacustrine carbonate reservoirs. Full article

To address the high energy consumption and low efficiency in shredding film–stalk mixtures during the resource utilization of cotton-field residues in Xinjiang—issues arising from the large mechanical-property differences among the mixture components—a custom single-support shearing fixture was developed to investigate the effects of residual-film wrapping layers, blade rake angle, sliding-cutting angle, and shearing speed on the F_j^max. Based on a Box–Behnken response surface design combined with analysis of variance and microscopic observations of the shearing process, the results showed that all main-effect factors had extremely significant influences on the F_j^max (p < 0.0001). Their relative contributions followed the following order: number of film wrapping layers > blade rake angle > shearing speed > sliding-cutting angle. Residual-film wrapping markedly increased shear resistance; increasing the sliding-cutting angle effectively reduced the shearing force; and reducing the rake angle facilitated more energy-efficient shredding. Interaction analysis further revealed significant coupling between sliding-cutting angle and shearing speed, rake angle and sliding-cutting angle, and rake angle and shearing speed (p < 0.05). Comparative shearing tests indicated that pure cotton stalks exhibited continuous brittle fracture with relatively stable force–displacement profiles, whereas film–stalk composites showed a sequentially coupled failure mode characterized by “residual-film pre-shearing–primary stalk fracture–secondary film stretching,” leading to multi-peak fluctuations in the force–displacement curves. Based on response surface optimization and mechanistic analysis, a parameter combination of a 35° rake angle, a 4–8° sliding-cutting angle, and medium-to-low shearing speed is recommended for shredding operations. This study elucidates the shearing and fragmentation mechanisms of film–stalk mixtures, provides theoretical guidance for optimizing key structural and operational parameters of post-recovery equipment, and offers important engineering value for promoting farmland residual-film pollution control and agricultural waste resource utilization. Full article

Coalbed methane (CBM) reservoirs are characterized by low permeability and poor methane desorption, which limit recovery rates. To address this, a novel graphite composite thermally conductive proppant is proposed, offering enhanced thermal conductivity and mechanical performance. The composite consists of porous ceramsite as a mechanical scaffold, epoxy resin as an interfacial binder, and graphite as a thermally conductive reinforcement. The effects of graphite content and resin dosage on the composite’s structure, thermal conductivity, suspension stability, surface wettability, and interfacial adhesion are systematically investigated. The results show that an optimized formulation with 20 wt% graphite and 1.0 g epoxy resin achieves a thermal conductivity of 3.8 W/(m·K)—6.3 times that of pure ceramsite—along with an improved thermal response under simulated stimulation, good suspension stability (suspension ratio of 0.53 in 0.2 wt% guar gum solution), a hydrophobic surface (contact angle 74.9°) to mitigate water lockup, and strong interfacial adhesion (125 nN under 2500 nN load) for durable proppant performance. Microscopic analysis confirms the formation of a continuous “resin–graphite–ceramsite” three-phase interface and a percolative thermal conductive network. This study provides a feasible design strategy for high-performance thermally conductive proppants and demonstrates their potential for application in the hydraulic fracturing of unconventional oil and gas reservoirs. Full article

The deep formations (burial depth: 3500–4000 m) in the northern Luzhou Block boast favorable geological conditions for shale gas accumulation. However, field development is hindered by the frequent casing deformation of shale gas wells and significant variations in single-well productivity. These issues severely restrict the efficient development of shale gas resources. Existing studies mainly focus on the identification and optimization of geo-engineering dual sweet spots, but few have established a systematic and comprehensive evaluation system from the perspective of engineering risk prevention and control. Based on traditional research on geo-engineering dual sweet spots, this study integrates engineering risk factors. It innovatively establishes a geo-engineering dual sweet spot evaluation system that incorporates engineering risks. Four key evaluation indicators for shale matrix geo-engineering sweet spots are selected: the continuous thickness of a Class I reservoir, the structural location, the fault scale, and natural fracture characteristics. Accordingly, shale matrix geo-engineering sweet spots are classified into three categories: Class I-A Area, Class I-B Area, and Class II Area. Meanwhile, three key indicators affecting fault slip—the angle between fractures and the maximum horizontal in situ stress direction, fracture dip angle, and friction coefficient—are optimized to establish the fault slip risk evaluation criteria. Combined with the distribution characteristics of slip faults, the engineering risks are divided into three levels: high, medium, and low. Finally, by coupling the geo-engineering sweet spots of a shale matrix with engineering risk zones, the geo-engineering sweet spots of shale reservoirs in the study area are classified into four categories (I, II, III, IV). Full article

This paper presents a breakthrough in activating the skin effect at conventional broaching speeds (1–8 m/min) by using laser defocus gradient modification to induce surface embrittlement in martensitic stainless steel Z10C13. Through controlled defocusing, a 50 μm gradient remelting layer was created, which features ultrafine grains (0.8 μm) and a high-density geometrically necessary dislocation (GND) zone (ρGND = 2.27 μm⁻³). The quasi-cleavage fracture was triggered via dislocation pinning by non-oriented low-angle grain boundaries (28.4% LAGBs). Multiscale characterization confirms that this microstructural transformation enhances surface hardness by 12.95% (reaching 31.4 HRC), reduces cutting force by 34.07%, and improves surface roughness by 63.74% (Sz = 28.80 μm). Simultaneously, a parallel crack-deflection mechanism restricts subsurface damage propagation, resulting in a crack-free subsurface zone. These results demonstrate the effectiveness of the embrittlement–toughening dichotomy for precision machining of difficult-to-cut materials under low-speed constraints. Full article

A comprehensive investigation was conducted into the microstructural evolution, high-cycle fatigue properties, and corresponding fatigue fracture mechanism of the simulated coarse-grained heat-affected zone (CGHAZ) in low-carbon microalloyed steel with different Nb contents. The results demonstrated that an increase in Nb content led to a higher density of both low-angle and high-angle grain boundaries (LAGBs and HAGBs), a reduction in the mean equivalent diameter (MED), and a refinement of the prior austenite grains (PAGs) in the CGHAZs. The crack initiation lifetimes accounted for over 97% of the total fatigue life in the CGHAZs, thereby establishing it as the dominant mechanism governing fatigue failure. The fatigue strength of the simulated CGHAZs exhibited a continuous increase from 212.6 MPa to 231.9 MPa as the Nb content was increased from 0.018 wt.% to 0.055 wt.%. The augmentation of Nb content has been demonstrated to be a successful strategy for enhancing the CGHAZ fatigue strength of low-carbon microalloyed steels. Full article