Planetary bearings are critical components in planetary transmission systems. As transmission systems evolve toward higher speeds, the lubrication of planetary bearings faces significant challenges. In this study, a specialized test rig was developed to visualize the oil flow in a planetary bearing cavity under a stationary carrier condition, a configuration featuring outer ring rotation and central oil supply. The experimental results reveal the oil flow patterns and distribution characteristics within the bearing cavity. Additionally, a dynamic-static fluid domain coupled numerical model was established to simulate the oil-air two-phase flow field in the rotating bearing domain. This model was used to analyze the variations in oil distribution inside the bearing cavity and the oil volume fraction(OVF) on component surfaces under different speeds. The research results indicate that the bearing cavity is fully filled with oil at low speeds. As the speed increases, the oil near the roller end faces on the inner ring side begins to decrease, and an arc-shaped oil film forms along the inner side of the cage. With further speed increase, this oil film expands radially outward. At 9000 r/min, only a small amount of oil remains distributed near the outer ring. Increasing the oil flow rate further enhances the OVF on these surfaces, which can alleviate oil starvation in the lubrication zone between the rollers and the inner ring under highspeed conditions. However, higher flow rates also lead to increased churning losses in the planetary bearing. The research results provide theoretical guidance for the lubrication design of planetary bearings in transmission systems.

Repair of irregular caverns using gas blanket holds broad application prospects, yet implementation methods and effectiveness require elucidation. This study classifies irregular caverns into three types and proposes a novel repair mode. The formation mechanisms and repair laws of irregular caverns are investigated through model experiments and numerical simulation. Typical irregular caverns include edge protrusion, narrow-elongated, and partially soluble types. Controlling gas-brine interface lifting manner and avoiding salt layers with faults or fractures can prevent irregular cavern formation. The repair priorities are protrusions and flat roofs, overall cavern expansion, and under-dissolved sides for different cavern types. Enlarged volume and regularized shape act inversely on hydrogen storage stability performance. During repair of edge protrusion and partially soluble cavern, the gas-brine interface concentrates at deformed sections with small adjustment distances, whereas narrow-elongated cavern features large repair ranges with more uniform interface positioning. This study provides insights for irregular cavern repair and utilization.

Traditional roadside backfill bodies made of high-water-content material (TRBBs) are commonly adopted in gob-side entry retaining (GER). However, their brittle response often results in crack propagation, loss of load-carrying capacity, and instability under roof rotation and mining-induced stress. This study investigates the fracture-resistance mechanism, damage evolution, and field performance of a fiber-reinforced roadside backfill body (FRBB). A fracture-mechanics-based model was developed to quantify the toughening effects of fiber bridging and fiber pull-out, and Mode I fracture toughness was measured using semi-circular bending tests. The optimum polypropylene fiber dosage was 0.3

This study investigates the fatigue behavior of mudstone under cyclic triaxial loading conditions, with a focus on the effects of stress amplitude and confining pressure relevant to underground Compressed Air Energy Storage (CAES) operations. Graded triaxial intermittent fatigue tests (GTIF) were conducted to simulate the stress path of intermittent loading. The results show that the fatigue life of mudstone exhibits a non-monotonic dependence on confining pressure: the longest life occurs at 3 MPa, while higher confining pressures (6-12 MPa) cause a pronounced reduction followed by a plateau, indicating a threshold-like confinement effect. The elastic modulus decreases with increasing stress amplitude but, at a given stress ratio, increases with confining pressure, reflecting a trade-off between stress-induced damage and confinement-enhanced stiffness. Residual strain behavior was found to depend on the stress limit interval pattern: upper limit intervals led to reduced residual strain post-rest, whereas lower limit intervals caused strain reversal due to internal stress relaxation. These trends reveal that the loading history significantly influences deformation recovery and potential damage evolution. The damping ratio decreased with increasing confining pressure but increased with stress amplitude, implying that while confining pressure suppresses internal defect activity, higher stress promotes energy dissipation through microcracking. Dissipated energy showed distinct patterns under different loading modes: it increased with stress ratio but decreased with cycle number, highlighting the fatigue softening effect. Interestingly, confining pressure enhanced energy dissipation in upper-limit loading but reduced it under lower-limit conditions. Overall, these results clarify how confining pressure, stress amplitude, and loading intervals jointly govern fatigue life, residual strain, and energy dissipation in mudstone interlayers, and identify measurable damage indicators that can support safer mechanical design and long-term performance assessment of CAES in layered geological formations.

Compression techniques such as shot peening, laser shock peening, and water jet peening are commonly employed to induce residual compressive stresses in mechanical components. These residual stresses play a crucial role in preventing the initiation and propagation of cracks. An innovative method known as the ElectroMagnetic pulse Peening (EMP) process utilizes magnetic forces to introduce residual compressive stresses in mechanical components. The EMP process shares similarities with the ElectroMagnetic Forming (EMF) process, which has been extensively studied through numerical and experimental investigations. Existing numerical studies predominantly feature axisymmetric 2D simulations, with limited availability of 3D simulations due to numerical constraints regarding computing time and resources. Since the EMP process shares similarities with EMF, similar challenges arise with respect to computational resources and time. This paper presents an innovative approach for the 3D simulation of residual stresses induced by the EMP process, based on efficient 2D axisymmetric calculations of the electromagnetic fields. The main objective of this approach is to simulate the mechanical impact of electromagnetic pulses applied by sweeping a surface, in order to analyze the stress distribution in the overlapping regions. First, the 2D model used to simulate electromagnetic phenomena is presented, and the 2D-to-3D transfer technique developed is detailed for computing residual stresses in 3D. Subsequently, the validity of this approach is established through a comparative study between 2D and 3D mechanical results for a single electromagnetic pulse. Finally, a multiple-pulse simulation is conducted to investigate the effect of overlapping treatment regions on an AA6061 aluminum alloy. The outcomes of this study are discussed in terms of the residual stresses at the subsurface.

Reconsolidated salt, formed from crushed halite under compaction, is a promising buffer and sealing material for deep geological repositories of high-level radioactive waste (HLW) because of its low permeability and self-healing properties. This study investigated the gas permeability behavior of reconsolidated salt with varying porosities under different confining pressures and inlet gas pressures using nitrogen gas. Based on nuclear magnetic resonance (NMR) technology, the pore structure of reconsolidated salt specimens with different porosities was tested and imaged. The experimental results demonstrate that gas permeability decreases with increasing gas and confining pressures, with gas pressure having a more pronounced effect. The observed permeability-pressure relationship is attributed primarily to the Klinkenberg effect, with gas slippage along pore walls enhancing the measured permeability under low-pressure conditions. Using the Klinkenberg correction, the absolute permeability values of reconsolidated salt were derived, reaching as low as 10-19 m2 in low-porosity samples. These values are significantly lower than the apparent gas permeability, indicating excellent sealing performance comparable to or superior to that of bentonite. A logarithmic relationship between the absolute permeability and confining pressure was established, providing a quantitative basis for permeability prediction under repository stress conditions. NMR imaging results indicate that with decreasing porosity, the connectivity between pores also gradually diminishes. Additionally, the slip factor was found to increase with increasing confining pressure, underscoring the evolving influence of pore geometry on gas transport mechanisms. Permeability of reconsolidated granular salt decreases with porosity following a power-law relationship, and the healing supports its sealing effectiveness. This study provides essential data and theoretical insights for evaluating the long-term sealing performance of reconsolidated salt in salt-based HLW repositories.

This study investigates the influence of surface roughness on contact mechanics, addressing the limitations of existing models that often rely on idealized and symmetric asperity shapes. We introduce a generalized representation of asperity geometries, including non-symmetric profiles, to better capture the diversity of surface characteristics encountered in real-world applications.By applying this parametric asperity model, we perform numerical simulations to analyze the impact of different parameters on contact behavior, effectively identifying various interaction regimes. The analysis is based on the assumption of elastic contact and focuses on two-dimensional roughness profiles characterized by surfaces that remain invariant along the axis orthogonal to the rolling direction. This approach effectively simulates geometries that display sufficient invariance along this axis, thereby representing realistic asperities in the form of streaks.Our theoretical framework quantifies the resulting analytical pressure distribution as a function of both geometric and mechanical parameters of the generalized asperities. By accommodating non-symmetric asperity geometries, our approach enhances model accuracy while significantly reducing computational time and resource requirements compared to traditional numerical methods.

Compressed air energy storage (CAES) in abandoned coal mine roadways is a promising large-scale technology for supporting renewable energy integration. However, assessing the long-term stability of surrounding rocks under cyclic pressure loading remains challenging because conventional numerical simulations are computationally expensive and can only cover limited operating conditions. In this study, a FLAC3D model incorporating the Burgers creep model was developed using geological data from an actual coal mine. Based on an L25(55) orthogonal design, 25 operating scenarios were simulated over a 30-year period, considering burial depth, pressure limits, and operating frequency as key variables. An XGBoost regression model was then trained to predict long-term deformation of the roadway roof, sidewalls, and floor. The results showed that deformation and plastic failure increased with greater burial depth and longer low-pressure duration, whereas a higher operating frequency reduced cumulative deformation under the studied conditions. The XGBoost model achieved excellent predictive accuracy, with the coefficient of determination (R2) exceeding 0.99 for roof subsidence prediction. SHAP analysis identified the lower-pressure limit and burial depth as the two most influential factors controlling roadway stability. The proposed data-driven framework provides an efficient approach for rapid stability evaluation and operational optimization of CAES systems in abandoned coal mine roadways.

Salt rock is widely recognized as an ideal host medium for underground energy storage and radioactive-waste disposal. Nevertheless, the presence of fractures within salt cavern gas storage can markedly compromise the integrity of the surrounding rock mass. Focusing on the mechanisms of fracture evolution and mechanical response in salt rock, this study conducted uniaxial compressive strength (UCS) tests on cubic salt specimens containing prefabricated flaws at various inclinations. Acoustic Emission (AE) monitoring and Digital Image Correlation (DIC) were integrated to capture the damage development and crack evolution in real time. The results show that: (1) Flaw inclination exerts a significant effect on both the UCS and the crack-propagation path of salt rock, exhibiting a clear and reproducible angle effect. (2) DIC effectively captures orientation-dependent crack trajectories, revealing the geometric complexity of surface crack evolution during failure. (3) According to the RA-AF criterion, the tensile-event fraction ranks 90 degrees > 0 degrees > 45 degrees > 30 degrees > 60 degrees, consistent with the DIC observations and energy-index analysis. (4) The fractal dimension D increases over time, indicating progressive network densification and multiscale damage amplification. (5) The spatiotemporal distribution of AE events exhibits a damage-localization trend consistent with the evolution of the DIC principal strain field, confirming the complementary capabilities of the two monitoring techniques. The results provide a solid experimental basis for the quantitative evaluation of inclination effects and for the stability assessment of salt cavern storage, offering practical guidance for cavern design and long-term performance appraisal.

The long-term mechanical stability of surrounding rock in compressed air energy storage (CAES) salt caverns is crucial for the safe and efficient operation of underground energy systems. In this study, the deformation behavior of salt rock was investigated through a combined approach that involved long-term laboratory creep–fatigue tests and engineering-scale numerical simulations. Mechanical experiments were carried out at various cyclic stress levels and loading rates to replicate the creep–fatigue loading conditions that are encountered during CAES operations. The results indicate that both the magnitude and the frequency of cyclic loading significantly influence the time-dependent deformation of salt rock: higher stress levels accelerate damage, whereas lower loading rates lead to increased plastic strain. On the basis of the geological conditions of a planned CAES facility, numerical simulations were conducted using FLAC3D. The model incorporates the Norton creep law to simulate the evolution of the surrounding rock with different numbers of operational cycles and gas pressures. The operating pressure and frequency significantly affect the deformation and plastic zone distribution of the surrounding rock in salt cavern reservoirs. Higher operational frequencies and minimum gas pressures result in reduced deformation and improved cavern stability. For the first time, a direct qualitative analysis was conducted to compare the laboratory experiments and numerical simulation results. Comparative analysis reveals that the experimental and simulation results are generally consistent. These findings offer new insights into the mechanical response of salt cavern-surrounding rock and establish a foundation for predicting the long-term performance of CAES systems.

The rheological mechanical properties of the surroundings rock of salt caverns under triaxial stress state are essential for precisely assessing geological stability of salt caverns storage. Triaxial stepwise creep loading tests were carried out to investigate the creep deformations and long-term strength properties of salt rocks under triaxial stress states. The impact of varying deviatoric stresses ( σ _d ) and confining pressures ( σ _c ) on the creep behavior of the surroundings rocks of salt cavern was analyzed. The findings demonstrated that the proportions of creep deformations to total deformations for salt rocks were associated with σ _d , but not with σ _c . As the σ _d reaches 33.4 MPa, the percentage stabilizes between 80.7 σ _d increases, while it decreased exponentially as the σ _c increased. The sensitivity of salt caverns surrounding rock creep to σ _d was greater than its sensitivity to σ _c . The long-term strength of salt rock was defined as the inflection point, which initially decreased progressively and approached a constant value over time, determined by the isochronous stress-strains curve. The time-dependent strength of salt rocks under triaxial stress states could be effectively described using an exponential empirical formulation. At low σ _c , plastic deformations were primarily influenced by crack propagation, whereas at high σ _c , it was governed by dislocation mechanisms. As the σ _c increase, the salt rocks become “softer,” and the failures characteristics transitioned from brittle to ductile. The strength degradation of salt rocks under triaxial stress states could be attributed to a progressive transition in its strength mechanism—from cohesion-dominated behavior to one increasingly governed by frictional resistance associated with the internal friction angle. This process revealed the intrinsic mechanism of the time-dependents evolution of salt rocks strength.

As a critical surrounding rock medium for large-scale underground energy storage, salt rock faces stability challenges induced by creep during long-term operation. To elucidate the microscopic deformation mechanism of salt rock in complex geological environments, this study employed molecular dynamics (MD) simulations to construct a polycrystalline NaCl atomic model conforming to Voronoi tessellation characteristics. The compressive and tensile creep behaviors were systematically investigated within the temperature range of 300 K to 360 K under varying stress levels. The results reveal that the creep of salt rock exhibits significant thermomechanical dependence and tension-compression asymmetry. Microscopic mechanism analysis indicates that compressive creep is dominated by diffusion creep and grain boundary sliding in low-stress regimes, transitioning to dislocation glide and climb control in high-stress regimes. Conversely, tensile creep is governed by grain boundary behavior throughout the process, primarily driven by grain boundary damage and decohesion. Analysis of energy and structural evolution reveals that compressive creep leads to a monotonic accumulation of potential energy through lattice distortion and dislocation pile-up. In contrast, tensile creep releases energy through the nucleation of microcracks at grain boundaries, exhibiting stress relaxation characteristics. Furthermore, under low-temperature and low-stress conditions, the proportion of FCC lattice structures gradually recovers over time, indicating a partial structural re-ordering of the lattice structure. The study confirms that grain boundaries are sensitive zones for the deformation and failure of polycrystalline salt rock; compression induces hardening via dislocation pile-up at grain boundaries, whereas tension leads to premature grain boundary decohesion and fracture. This study elucidates the creep damage mechanisms of polycrystalline salt rock at the atomic scale, providing atomic-scale insights into the underlying deformation and damage processes relevant to salt cavern gas storage facilities.

This work investigates how hardness gradients produced by case carburising modify short-crack behaviour and crack-initiation-site selection in M50NiL bearing steel under ultrasonic very-high-cycle fatigue (VHCF). Specimens containing a single controlled surface defect were prepared in three representative case-depth conditions (core, transition layer and surface layer) obtained by varying the machining allowance prior to carburising. Ultrasonic fatigue tests combined with SEM and ACOM-ASTAR fractography show that crack initiation remains surface-controlled at the defect for the core and transition-layer conditions up to the longest lives investigated, whereas a delayed transition to subsurface initiation occurs in the heavily case-carburised surface-layer condition beyond 108 cycles, producing SNDFCO-type initiation accompanied by a ne granular area (FGA). Crack-growth measurements reveal that, once initiation is delayed beyond 105 cycles, the subsequent propagation time becomes comparatively insensitive to the preceding incubation period and instead tracks the material state, with an order-ofmagnitude dierence in early-stage growth rate between core and surfacelayer conditions. To address the lack of models for microstructural evolution under VHCF, we introduce a Cycle-Based Dynamic Recrystallization (CBDRX) model, in which the local microstructural evolution is described by a dynamic-recrystallisation-type renement variable and an evolving grainsize scale, formulated directly in cycle count. The FGA is represented as an Eulerian isovalue front whose advance is slaved to the renement kinetics, enabling a cycle-based estimate of the critical incubation life for transition to sh-eye growth through a fracture-mechanics criterion.

Salt rock is a critical medium for underground energy storage (e.g., oil, natural gas, and CO2). However, it faces significant challenges under dynamic loading conditions, such as blasting and seismic events, threatening the stability and safety of salt caverns. The dynamic fracture behavior and failure mechanisms of salt rock under high-strain-rate loading remain inadequately understood, particularly concerning fracture propagation patterns and energy dissipation characteristics. This study investigates salt rock, employing a Split Hopkinson Pressure Bar (SHPB) dynamic impact test system integrated with high-speed photography and digital image correlation (DIC) techniques. The mechanical properties and fracture propagation behavior of salt rock under impact loading are systematically analyzed. Based on fractal dimension theory, particle size distribution analysis of fractured salt rock is conducted, and scanning electron microscopy (SEM) is employed to examine the crushed fragments. The results show that: (1) Salt rock material is a strain-rate-sensitive material, the dynamic peak strength of salt rock is positively correlated with strain rate under impact loading, and the fragmentation pattern shifts from coarse particles to finer grains as the strain rate escalates. (2) Salt rock exhibits distinct fracture propagation stages under dynamic loading, with an increasing fragment fractal dimension with strain rate. (3) Within the strain rate range of 68.39 s_ 1-83.51 s_ 1, the dynamic compressive strength of salt rock is lower than the static compressive strength. (4) As the impact pressure increases beyond the threshold of 0.3 MPa, the failure characteristics of salt rock exhibit pronounced "avalanche" dynamic behavior. These findings are highly significant for assessing geological engineering disaster risks under extreme loading conditions.

Creep behaviour in rocks is a typical mechanical property that is directly linked to the stability of underground engineering. The deformations and rate of rocks creep are not only influenced by time but also by the loading and unloading history. To more accurately predict creep mechanical behaviour of salt rocks, the rocks hardening is described by introducing a state variable. A new three-dimensional creep constitutive model of salt rocks was established to describe the loading and unloading history effect of the rheological properties. In this paper, salt rocks creep tests under various loading and unloading histories were conducted to investigate how different loading routes affect the creep behaviour of salt rocks. The effects of the model state variables were analysed through different indicators. An example verification was carried out with the results of plastic deformation tests performed at different loading paths. The findings indicated the creep rate of stepped loading and the stepped unloading under the same stress level were significantly affected by the loading history. The proposed constitutive model can accurately fit the creep test curves of different loading paths, indicate that it can provided a prediction of the historical effect of the creep behaviour of salt rocks. Different parameters affect the different phases of the creep curve. The parameter k primarily affects the overall shape of the creep curves. Parameters m and c primarily influence the steady-state creep length and creep rates, excluding the initial cycle.

Destress blasting is a widely adopted technique for mitigating rockburst hazards in deep underground excavations. However, its effectiveness is closely linked to the mechanical degradation characteristics of the rock mass after failure, which are often overlooked in design practices. This study presents a numerical assessment of how residual rock mass properties influence the performance of destress blasting, focusing specifically on residual cohesion, residual friction angle, and critical plastic strain. Three models were developed using the 3DEC code to represent (1) a conventional excavation without boreholes, (2) excavation with relief holes but no blasting, and (3) excavation with relief holes subjected to dynamic loading. Each model was tested under three sets of residual parameters to simulate varying degrees of post-failure degradation. Simulation results show that boreholes without dynamic loading do not induce failure or stress relief, regardless of residual strength conditions. In contrast, when dynamic loading is applied, the extent and continuity of plastic zones, as well as the magnitude of stress redistribution, are significantly influenced by the residual parameters. Lower values of critical plastic strain result in greater post-yield stress reduction in the rock mass and lead to more pronounced stress relief near the excavation face. Additionally, the destressing effects are spatially non-uniform, with greater reductions observed near the center of the drift face where blast influence is strongest. These findings highlight the critical role of post-peak rock behavior in determining the effectiveness of destress blasting. Considering the residual mechanical properties of the rock mass after blast-induced damage is essential for optimizing blast design and enhancing excavation safety in deep mining operations.

The safety of geotechnical engineering in cold regions depends largely on the stability of the supporting structure. Owing to the advantages of less dust, small rebound during operation, and enhanced strength of the sprayed layer, wet shotcrete has gradually been employed to support the roadway with broken surrounding rocks. The curing temperature of wet shotcrete decreases in cold regions, seriously affecting the mechanical characteristics of the wet shotcrete sprayed layer. However, the impact of temperature has not been fully considered in current mine engineering practice. Therefore, this study investigated the factors affecting the strength of wet shotcrete when applying mine wet shotcrete technology in cold regions. The results showed that the strength development of the sprayed layer of wet shotcrete roadway support in cold regions was significantly correlated with temperature. The microstructure of wet shotcrete is primarily influenced by temperature, as the hydration reaction of wet shotcrete is sensitive to temperature changes, ultimately influencing the microstructure of the shotcrete. Furthermore, the strength of the interface between the initial pore fissures and aggregate transition zone directly affects the macroscopic mechanical properties of wet shotcrete. Controlling the water-cement ratio (W/C) stands as a pivotal factor in improving concrete compactness and mitigating crack development. By testing the specimens after spray plate processing and field engineering application research, optimization measures for wet shotcrete support in cold regions were proposed, effectively improving the structural strength of the sprayed layer. After curing for 28 d, the spray layer improved to 32.9 MPa (increased by about 40 %), and the support repair rate was reduced by 85 %, achieving a good construction effect. The findings of this study can guide the construction of wet shotcrete under similar engineering conditions.

This paper proposes a numerical approach to investigate the hysteretic friction phenomenon occurring in sliding contact of viscoelastic materials on rough surfaces. This formulation is implemented in both transient and steady-state regime assuming periodic contact: the rough surface is reduced to a representative unit-cell, which reduces computational effort as the contact is computed within a window of fixed dimensions. This work first investigates on the parameters that drive friction and confirms that the frequency ratio and the amplitude-to-wavelength ratio are of primary influence. This is particularly stressed when considering multi-scale surfaces with different levels of roughness. The transient formulation is then employed to investigate the evolution of the friction coefficient over time. We show that depending on contact conditions, friction can exhibit unusual non-monotonic behaviour, at times exceeding the steady-state asymptotic value.

Pitch bearings are components used in variable-pitch rotors to connect the hub and the blade, while allowing the blade to rotate around its long axis. Most of the studies concerning pitch bearings focus on four-point contact bearings that are widely used in large size wind turbines. However, very few studies consider duplex pairs of angular contact ball bearings that can be found in variable-pitch propellers or small-sized wind turbines. An efficient five degrees of freedom calculation method is proposed to study the load distribution in such bearings while considering the specific design of the blade-hub assembly. The external loads due to aerodynamic and centrifugal effects are prescribed on the blade, and then distributed between the bearings supporting the blade. The governing equation of the blade bending is considered, to further solve the initially statically indeterminate problem. The performance of three bearing arrangements (back-to-back, face-to-face and tandem) is compared in terms of load distribution, with the tandem arrangement being very specific to variable-pitch rotor application, where two angular bearings mounted in the same direction are loaded by the centrifugal force. Additionally, a novel method is proposed to predict the risk of ellipse truncation for each arrangement.