Development Of Microcracks Research Articles

The role of thermal cycling in micro-cracking development and flow alteration: Advanced insights from CT and lab studies

The recent interest in geothermal energy, nuclear waste management, and coal gasification has led to an in-depth examination of how granite rock behaves under subterranean reservoir conditions. Our research delves into the intricate microstructural and fluid dynamics of the Australian Harcourt granite. We emphasize its reactions to repeated temperature changes, cycling between 25 and 900 °C over 1–15 cooling episodes. To achieve this, we utilised X-ray computed tomography (CT) to visualize and reconstruct the heat-treated samples. Subsequently, we measured the porosity, identifying both total and directly interconnected spaces within these samples post-thermal treatment. Using avizo 2021.2, we created a pore network model (PNM) based on the connected porosity data to shed light on how fluids move through these heat-affected samples.The data highlighted that total porosity stayed below 0.5% up to 500 °C, with no directly connected porous regions detected. However, beyond this temperature, interconnected porosity began to appear, intensifying with increasing temperatures and more cooling cycles. The PNM allowed us to visualize and quantify specific details like pore and throat dimensions, their volumes, their interconnectedness, and the lengths of their connecting channels. Notably, these properties became more pronounced with escalating temperatures and cooling episodes, largely because of the fractures caused by the heat. Furthermore, we used AVIZO to analyse the absolute permeability of the heat-treated samples, providing a detailed understanding of their permeation characteristics. We also ran a set of rigorous lab tests to assess the impact of both confining pressure (ranging from 10 MPa to 40 MPa) and temperature (from 22 to 250 °C) on the fluid movement in the heat-treated rock. The experiments confirmed that increasing confining pressure non-linearly decreased Darcy's permeability. A rise in temperature further reduced permeability, mainly due to the narrowing of flow channels from mechanical and thermal stresses.Overall, the study provides a holistic understanding of porosity evolution and the subsequent fluid movement within thermally quenched rock samples under various reservoir conditions, providing valuable insights for energy and waste management applications.

Effect of Confining Pressure on the Macro- and Microscopic Mechanisms of Diorite under Triaxial Unloading Conditions

In this study, the response mechanism between macro- and microscales of deep hard-rock diorite is investigated under loading and unloading conditions. Moreover, the statistical theory is combined with particle flow code simulations to establish a correlation between unloading rates observed in laboratory experiments and numerical simulations. Subsequent numerical tests under varying confining pressures are conducted to examine the macroscopic mechanical properties and the evolution of particle velocity, displacement, contact force chain failures, and microcracks in both axial and radial directions of the numerical rock samples during the loading and unloading phases. The findings indicate that the confining pressure strength curve displays an instantaneous fluctuation response during unloading, which intensifies with higher initial confining pressures. This suggests that rock sample damage progresses in multiple stages of expansion and penetration. The study also reveals that with increased initial confining pressure, there is a decrease in particle velocity along the unloading direction and an increase in particle displacement and the number of contact force chain failures, indicating more severe radial expansion of the rock sample. Furthermore, microcracks predominantly accumulate near the unloading surface, and their total number escalates with rising confining pressure, suggesting that higher confining pressures promote the development and expansion of internal microcracks.

Experimental investigation on monitoring the evolution of fatigue microcracks in marine high-strength steel by nonlinear ultrasonic technique

ABSTRACT Due to harsh service conditions, fatigue damage is the main reason for numerous catastrophic accidents of ships and offshore constructions. This paper uses a nonlinear ultrasonic technique to monitor the evolution of fatigue microcracks in marine steel AH36. An experimental system was established and nonlinear ultrasonic inspections were conducted on the specimens with different fatigue degrees. The microstructural changes induced by fatigue loading were also studied by microscopic observation. The experimental results demonstrate that the normalised nonlinear parameters show a closed relation to the evolution of fatigue microcracks in steel AH36. Meanwhile, microscopic observation indicates that the microcrack generation mode and grain boundary cracking are the two mechanisms for microcrack generation. Both the generation and growth of the microcracks contribute to the rise of the nonlinear effect. Consequently, the nonlinear ultrasonic techniques provide a reliable and effective method for monitoring the evolution of fatigue microcracks in marine high-strength steel.

Empirical analysis of unloading creep damage mechanism of salt rock at 110 °C

Underground energy storage in salt caverns involves operating in high-temperature conditions within deep strata, leading to unloading creep in the surrounding salt rock during cavity construction and natural gas extraction. Therefore, understanding the deformation and damage evolution of salt rock in such conditions is vital for ensuring safe construction and prolonged operation of deep salt cavern gas storage. The present study involves graded unloading creep tests and synchronous acoustic emission tests on the salt rock at 110 °C. The moment tensor inversion theory is employed to analyze focal mechanisms and microscopic crack propagation modes based on the test results. The findings reveal that under constant deviatoric stress, decreasing confining pressure exponentially increases the duration of the deceleration creep stage in salt rock. The creep increment and axial steady-state creep rate exhibited a U-shape trend. Instantaneous strain during stress change mainly involves elastic-plastic strain, where the instantaneous strain decreases first and then increases, exhibiting hardening characteristics in the middle process. The evolution of microcracks in salt rock can be categorized into three stages: compaction stage, stable propagation stage, and accelerated propagation stage. The acoustic emission energy, b-value, and fractal dimension of each stage are different. In unloading creep tests, the peak frequency in acoustic emission signal presents zonal distribution in low-frequency and high-frequency regions. It is found that as the confining pressure decreases, the peak frequency of acoustic emission signal and the proportion of high-frequency signal decrease, and the b-value and fractal dimension exhibit an upward-fluctuation-downward trend. The rupture slip angle of the acoustic emission source can effectively characterize the creep slip behavior in salt rock crystals. The slip gradually shifts from vertical to horizontal, while the strike angle and dip angle of rupture orientation are minimally affected by unloading. The obtained results can provide a reference for the safe construction and the design of the operation pressure for ultra-deep salt cavern gas storage.

Mechanical anisotropy associated with beddings in shale under Brazilian test conditions: Insights from acoustic emission statistics

A comprehensive understanding of shale’s bedding anisotropy is crucial for shale-related engineering activities, such as hydraulic fracturing, drilling and underground excavation. In this study, seven Brazilian tests were conducted on shale samples at different bedding orientations with respect to the loading direction (0°, 45° and 90°) and the disc end face (0°, 45° and 90°). An acoustic emission (AE) system was employed to capture the evolution of damage and the temporal-spatial distribution of microcracks under splitting-tensile stress. The results show that the Brazilian tensile strength decreases with increasing bedding inclination with respect to the disc end face, while it increases with the angle between bedding and loading directions. Increasing the bedding inclination with respect to the end face facilitates the reduction in b value and enhances the shale’s resistance to microcrack growth during the loading process. Misalignment between the bedding orientation and the end face suppresses the growth of mixed tensile-shear microcracks, while reducing the bedding angle relative to the loading direction is beneficial for creating mixed tensile-shear and tensile cracks. The observed microscopic failure characteristics are attributed to the competing effects of bedding activation and breakage of shale matrix at different bedding inclinations. The temporal-spatial distribution of microcracks, characterized by AE statistics including the correlation dimension and spatial correlation length, illustrates that the fractal evolution of microcracks is independent of bedding anisotropy, whereas the spatial distribution shows a stronger correlation. The evolution features of correlation dimension and spatial correlation length could be potentially used as precursors for shale splitting failure. These findings may be useful for predicting rock mass instability and analyzing the causes of catastrophic rupture.

Field observations and interpretation of extensional fracture in hard rock surrounding deep underground openings

Extensional fracturing often occurs in hard rock masses during excavation at depths, for example, >1000 m below the ground surface. Surface-parallel fractures are created in the surrounding rock mass, which is typically subjected to stresses parallel to the free rock surfaces after excavation. These are called extensional fractures because the strains perpendicular to the fracture planes are extensional and the opposite surfaces of each fracture tend to separate from each other as soon as the fracture is created. These fractures predominantly propagate parallel to the maximum principal stress σ1 in the surrounding rock mass. This study analyses extensional fractures observed during excavations in cut-and-fill mining stopes in a deep metal mine. This analysis explores the process of extensional fracturing during excavation in an undisturbed rock mass. In general, intensive spalling occurred on the roof surfaces immediately after the excavation of the undisturbed rock mass. This spalling terminated after a certain depth of rock failure, while burst sounds intermediately emitted from the surrounding rock mass, indicating that rock fracturing was ongoing at depth. In the subsequent cutting slices, the spacing between the extensional fractures decreased with increasing mine-out space in the stope. An extensional fracturing criterion was proposed based on microscopic observations of microcrack development in the rock in response to applied stress. The crack initiation and extensional fracturing processes are associated with two critical extensional strains which are related to the secondary stress state in the position. In areas close to the free rock surface where σ3 = 0, the stress for crack initiation is (σ1+σ2) = 0.4σc, whereas the stress for extensional fracturing is (σ1+σ2) = 0.8σc.

Cracking process and microstructural characteristics of granite under heating–cooling alternations

Studying the damage correlation mechanism of rocks subjected to heating–cooling alternations is of important significance for deeply understanding the permeability increase mechanism of reservoirs during heat extraction from hot dry rocks and assessing the reservoir stability. By carrying out the uniaxial compression test, Brazilian splitting test, nuclear magnetic resonance test, and scanning electron microscopy test on granite after heating–cooling alternations, the evolution laws of physical and mechanical properties as well as the microstructures of granite were studied. In addition, the multi-scale response characteristics and the damage correlation mechanism of granite after heating–cooling alternations were analyzed. Research results show that, when the heat treatment temperature rises to 600 °C, the physical and mechanical parameters, including the wave velocity, strength, and elastic modulus, reduce abruptly. The heat treatment temperature exerts more significant influences on the physical and mechanical properties of the granite than the heating–cooling alternation cycles. Both the temperature and alternation cycles are beneficial to the microcrack development and propagation, while the temperature more significantly affects the development degree of microcracks. Under the external load, the microscopic defects in the granite after heating–cooling alternations guide the propagation of macrocracks and cause more serious damage to the microstructures in the granite, exhibiting more complex failure modes. The results provide a theoretical basis for fracturing technology and stability evaluation of high-temperature reservoirs.

Crystallographic texture and multiscale boundaries mediated creep anisotropy in additively manufactured Ni-based Hastelloy C276 superalloy

The microstructure and high-temperature creep mechanisms of Ni-based Hastelloy C276 superalloy fabricated using wire and arc-based directed energy deposition were investigated systematically and innovatively. The microstructural investigation revealed that the as-fabricated samples comprise γ-Ni matrix and topologically close-packed (TCP) P phase precipitates. The γ matrix subgrains and grains are spread over multiple highly textured columnar dendrites, with a majority of γ <001> crystallographic orientations closely aligned along the deposition direction. Moreover, the interdendritic regions exhibit severe Mo segregation, P phase particles, and dislocation bands. Creep tests were conducted on miniature samples under various temperature and stress conditions, loaded either in the deposition direction (DD) or travel direction (TD). DD samples exhibit lower minimum strain rates, greater strains-to-failure, and longer creep rupture lifetimes than TD samples, indicating significant creep anisotropy. Dislocation creep was identified as the primary creep mechanism for both DD and TD conditions. During creep, dynamic precipitation of TCP phases occurred in the interdendritic regions, resulting in varying creep resistance between interdendritic and dendritic core regions. Isostress and isostrain models, considering both crystallographic texture and precipitation strengthening, reasonably predicted the observed creep anisotropy during the secondary creep stage. Additionally, variations in the Schmid factor led to significant deformation incompatibility among dendrites in TD samples. Dislocation accumulation in TD sample interdendritic regions promoted new grain nucleation, triggering dynamic recrystallisation, facilitating grain boundary sliding, and accelerating tertiary creep. Furthermore, TCP phase particles in the interdendritic regions contributed to microcrack development, further accelerating creep fracture, especially in the TD condition.

Inhibiting phase conversion and improving cyclic stability of Ni-rich layered oxide by high-valence element concentration gradient doping

Nickel (Ni)-rich layered oxide cathodes are believed to be one of the crucial materials for the development of high-energy density power batteries. However, accompanied by the Ni content of layered cathodes increases, it encounters some awkward issues such as sensitivity to moisture, side reactions, and gas production. Herein, by using a high-valency elements Tellurium (Te) doping strategy, we successfully design and fabricate the layered oxide cathode material Li[(Ni0.90Co0.10)0.99Te0.01]O2 (1.0 Te-NC90) with a high capacity of 231.36 mAh g−1 at 0.1C, which has high cycling stability of 95.01 % after 100 cycles at 0.5C, good thermal stability of 205 °C and high Li+ diffusion rate of 8.12 × 10−10 cm2 s−1. It has been found that the (003) interplanar spacing of 1.0 Te-NC90 cathode will increase from 0.472 nm to 0.491 nm due to the concentration gradient Te-doping strategy, which can promote Li+ diffusion. Besides, it can refine the grain structure and transform the primary particles from bulk-grained to rod-grained morphology, and these elongated and closely packed particles radiate from the center to the surface with a spoke-like arrangement, which effectively dissipates lattice strain generated during deeply charging states and suppresses the abrupt lattice transformation associated with the H2 → H3 phase, thereby avoiding the development of microcracks during cycling. In the meantime, robust Te-O bonds can keep the material lattice stable to prevent oxygen loss and TM ion movement. Therefore, this study reveals the significant role of trace Te-doping in enhancing crystal structure and electrochemical stability through microstructural engineering for the control of primary particle morphology.

A review on gas permeability of polymer matrix composites for cryogenic applications

Fiber-reinforced polymer matrix composites are increasingly considered for lightweight cryogenic pressure vessels due to their excellent combination of tailorability, specific mechanical properties, and relatively low coefficients of thermal expansion. However, significant challenges must be overcome to fully utilize PMCs for cryogenic fuel tanks in terms of transverse microcracking and subsequent permeation of cryogenic fuel. Gas permeation and microcrack densities of cryogenically cycled composites are highly influenced by their layup, ply thickness, load case, and manufacturing defects like voids and resin rich zones. There has been a significant amount of research on measuring gas permeation of composites fatigued under pure thermal or uniaxial thermo-mechanical stresses. However, results demonstrate that the gas permeability should be measured under biaxial thermo-mechanical stresses to properly gauge the leakage characteristics of damaged composites. This paper summarizes the results from over a hundred papers on the key parameters that influence the gas permeability of composites, appropriate testing methods to cycle composites for permeability measurement, methods to limit the evolution of transverse microcracks, and materials traditionally used for the fabrication of all-composite cryogenic fuel tanks. Thin plies and nanofiller-toughening of the matrix have been shown to provide significant improvements in transverse microcrack suppression within cryogenically cycled composites.

Damage deterioration mechanism and damage constitutive modelling of marble after cyclic impact loading

Rock mass in tunnel, underground chamber and other underground engineering is inevitably affected by multiple blasting impact, rock mass cracking and failure are often affected by these damaged rock mass, so it is meaningful to study the rock mass affected by dynamic damage. To explore the influence of cyclic impact on the internal structure of marble, NMR (Nuclear Magnetic Resonance) technique was adopted to test the marble. It was found that the impact would rise the porosity of marble, and the pore size distribution would also change greatly. Then uniaxial compression experiments were conducted on core samples. The results show that the impact will cause significant damage to the marble, and the mechanical properties of the marble will be obviously degraded. With the increase of impact times, the peak strength and tangential deformation modulus tend to decrease. The impact effect promotes the initiation and development of rock microcracks, pushes forward the development of the compaction stage, and increases the strain at the full compaction point. In addition, the damage constitutive model with the combination of different disturbances and loads was constructed in stages according to the principle of statistics, and the experimental results were used to verify the damage constitutive model. The crack closure and failure of the samples subjected to different impact times were well reflected in the model, and the practical significance of the parameters in the model was analyzed. The failure mode of the sample subjected to different impact times is discussed and verified by acoustic emission technology.

Three-Dimensional Spatial Microscopic Characteristics and Developmental Influencing Factors of Tight Gas Layers in Hangjinqi Prospect Area, Ordos Basin, China

The unconventional tight oil and gas resources in the Xinzhao East belt of the Hangjinqi Prospect area in the Ordos Basin of China are abundant. However, the reservoir’s internal storage space is complex, and the microscopic pore throat structural features are not well recognized, which has led to some trouble in the deployment of oil and gas exploration. To reveal the microscopic characteristics of the dense sandstone gas layer in the first member of the Lower Stone Box Formation of the D-well Zone in the Xinzhao East belt of the Hangjinqi Prospect area, a three-dimensional space digital core was built, and the stored set spatial data were extracted, based on rock sheet and coring data and X-CT scanning technology. Quartz grain size was segmented and analyzed based on an adaptive approach. The microscopic characteristics of the gas layer in the studied section and the factors influencing its development were studied, combining the use of a field emission scanning electron microscope, helium porosimeter, and gas permeability meter. We found that in the studied section, the porosity is relatively high, the pore throat size is large, and the pore permeability correlation is good. The reservoir space, which consists of intergranular pores, intragranular pores, and microcracks at the grain edges in the study area, is characterized by a complex distribution pattern. Within the gas layer, isolated pores are connected by microcracks to form a network of reservoir spaces, which increases the pore throat size, enhances the connectivity of the pore throat, and makes the microscopic characteristics of the reservoir space better. The first member of the Lower Stone Box Formation could be an advantageous reservoir. Hole–throat connectivity is poor because of the gas layer having underdeveloped primary pores, the blockage of pores by unstable minerals (kaolinite, etc.), and poorly connected pore throats based on insoluble mud cementation. The high content of quartz brittle minerals and the development of natural microcracks within the gas formation are favorable conditions for fracking development. The quartz grain size within the gas layer is positively correlated with the pore throat size, which suggests that the quartz grain size somewhat influences the microscopic characteristics of the reservoir space. This comprehensive study shows that the methodology of the study is more advantageous than traditional methods in the fine and three-dimensional spatial characterization of the microstructure of dense sandstone reservoirs. The research results of this paper have certain guiding significance for further reservoir evaluation and advantageous reservoir prediction in the Hangjinqi Prospect area in the Ordos Basin. We also provide the basis for the subsequent efficient development of the gas reservoir.

Laboratory testing on cracking characteristics and improvement mechanism of coral mud

In recent years, the development and construction of island reefs have been flourishing. Due to the remoteness of island reefs from the mainland, the scarcity of building materials, and the high transportation costs, it is imperative to use local marine resources, and the potential value and status of coral mud on island reefs, which is formed by the remains of corals and other biological entities, is becoming increasingly prominent. Utilization and optimization of natural resources on island reefs have become a brand-new research direction and challenge. This article mainly focuses on the development of a new type of green engineering material, coral mud, for use in building surface layers. Thickness effects, PVA fiber (vinylon staple fiber) modification, and HPMC (Hydroxypropyl Methyl Cellulose) adhesive modification are taken into consideration. Through laboratory tests and image processing technology, fractal theory, and electron microscopy experiments, the macro-meso-microscopic multi-scale cracking rules of the coral mud surface layer and the optimization modification rules of PVA fibers and HPMC adhesives are revealed. The results demonstrate that the performance of the coral mud surface layer is superior to that of the kaolin surface layer, and the 10 mm thickness performs better than the 5 mm and 20 mm thicknesses. As the thickness of the coral mud surface layer increases, the contact between coral mud particles becomes denser, the scale of surface micro-cracks decreases, and the number of micro-pores decreases. PVA fibers can effectively inhibit the further development of macro and micro cracks and play a good bridging role. There is a bonding and adhesion relationship between coral mud and PVA fibers, and they have a good synergistic effect in inhibiting macro and mesoscopic cracks. With the increase in HPMC adhesive content, the number of micro-cracks and the scale of micro-cracks decrease accordingly, and the structure and performance of the coral mud surface layer are further improved. Overall, PVA fibers are more effective than HPMC adhesives in inhibiting the cracking of the coral mud surface layer. This provides valuable guidance for the development and application of coral mud in wall surface materials.

Mechanical, microstructural, and textural evaluation of aluminum-MWCNT composites manufactured via accumulative roll bonding at ambient condition

In the present study, aluminum-matrix composites reinforced by multi-walled carbon nanotubes (MWCNTs) were manufactured using the accumulative roll bonding (ARB) process. In the first step, MWCNTs were dispersed between two pieces of Al sheets that were then stacked together and rolled. In the second step, the above procedure was repeated for up to 12 cycles without adding MWCNTs. The microstructure and fracture surface of the composites were studied during various ARB cycles using optical microscopy (OM) and scanning electron microscopy (SEM). Also, the mechanical properties of the composites were measured employing tensile and Vickers hardness tests. More uniform dispersion of MWCNTs was found in the microstructure of the aluminum matrix after 12 ARB cycles. With an increase in the number of ARB cycles, the hardness of the composites was enhanced. The results indicated that after 11 cycles, both the elongation and tensile strength improved and strong interfacial bonding formed between MWCNTs and the Al matrix. However, the development of micro-cracks in the composite resulted in a decrease in elongation and tensile strength at the final cycle. SEM investigations of fracture surfaces revealed that the fracture mode of the composites changed from ductile to brittle in the final ARB cycle, while that of monolithic aluminum remained ductile for all ARB cycles. Texture analysis, also confirmed a transition from the initial recrystallized (Cube and Goss) texture to the ideal rolling texture, mainly with Copper and Dillamore components.

Characteristic stress variation and microcrack evolution of granite subjected to uniaxial compression using acoustic emission methods

The size of mineral grain has a significant impact on the initiation and propagation of microcracks within rocks. In this study, fine-, medium-, and coarse-grained granites were used to investigate microcrack evolution and characteristic stress under uniaxial compression using the acoustic emission (AE), digital image correlation (DIC), and nuclear magnetic resonance (NMR) measurements. The experimental results show that the characteristic stress of each granite decreased considerably with increasing grain sizes. The inflection points of the b-value occurred earlier with an increase in grain sizes, indicating that the larger grains promote the generation and propagation of microcracks. The distribution characteristics of the average frequency (AF) and the ratio of rise time to amplitude (RA) indicate that the proportion of shear microcracks increases with increasing grain size. The NMR results indicate that the porosity and the proportion of large pores increased with increasing grain size, which may intensify the microcrack evolution. Moreover, analysis of the DIC and AE event rates suggests that the high-displacement regions could serve as a criterion for the degree of microcrack propagation. The study found that granites with larger grains had a higher proportion of high-displacement regions, which can lead to larger-scale cracking or even spalling. These findings are not only beneficial to understand the pattern of microcrack evolution with different grain sizes, but also provide guidance for rock monitoring and instability assessment.

Recycled polyester fiber reinforcing red mud-improved volcanic ash as a sustainable construction material

Recycling waste for soil improvement is a cost-effective strategy to mitigate the energy consumption and carbon emission of traditional stabilization materials (cement, lime, etc.). This paper proposes an eco-friendly geotechnical improvement using recycled polyester fiber (RPF) and red mud to synergistically reinforce volcanic ash. For red mud -reinforced volcanic ash (RV) with five different RPF contents, a series of mechanical tests, water stability tests, microstructure tests and environmental impact tests were performed. Results show that RPF can significantly improve the mechanical performance and water stability of RV. The reinforcement increases with increasing RPF content, and the optimum contents of RPF is 0.9%. For specimens with optimum RPF incorporation, Unconfined compressive strength (UCS), splitting tensile strength are increased by 122%, 180%, respectively. It also allows saturated soil to still maintain higher UCS and the soil does not completely disintegrate in submerged environment. The pH value and toxicity leaching indicators of specimens both satisfy the specification limits and therefore the material has no environmental hazard. TGA, FT-IR and SEM-EDS indicate that the reinforcement mechanism of RPF on RV derives mainly from fiber-matrix interactions, including interface bonding, bridging effect and limiting micro-cracks development. RPF reinforced RV presents favorable engineering performance as a sustainable construction material and can contribute to the win-win objective of environmental conservation and waste recycling. This low-carbon and sustainable soil improvement technology can serve and guide the design of geotechnical engineering in volcanic areas.

Investigation of Rock Mechanical Properties under Liquid Nitrogen Environment

In order to promote sustainable energy development and reduce the impact of fossil fuels on the environment, it is crucial to strengthen the development and utilization of clean and renewable geothermal energy. Liquid nitrogen fracturing, as an emerging waterless fracturing technology, has outstanding advantages in rock fracturing effect and thermal exchange ability with hot dry rock and is more environmentally friendly. In order to evaluate the influence of liquid nitrogen on the mechanical properties, acoustic emission characteristics, and cross-sectional crack propagation characteristics of granite at different initial temperatures, this paper carried out three-point bending tests and acoustic emission detection on granite treated by high-temperature heating and liquid nitrogen cooling. Finally, based on the cross-sectional scanning test, the expansion characteristics of microcracks in granite were analyzed. The results show that the higher the initial temperature of granite, the stronger the cold impact of liquid nitrogen on granite, and the faster the rock’s mechanical performance declines. The acoustic emission ringing count is closely related to the development of microcracks in granite, and as the initial temperature of granite increases, the more ringing counts there are, indicating that the huge temperature difference induces more microcracks inside the rock. In addition, the cold impact of liquid nitrogen can effectively promote the fracturing of granite. After liquid nitrogen treatment, the fractal dimension of the granite cross-section increases, the shape of the cross-section becomes rough, and many micropores appear. This study can provide a scientific basis for the engineering application of liquid nitrogen fracturing technology.

Macro- and micromechanical characteristics and energy evolution law of micro/nano carbon fibre grouting samples

This study comprehensively explores the macro- and micromechanical attributes and energy evolution dynamics of micro/nano carbon fiber (CF) specimens. The investigation centers on a graded sand gravel grouting specimen, aiming to determine the optimal CF content through uniaxial compression tests conducted on samples containing 0.0 %, 0.5 %, 1.0 %, and 1.5 % CF. Utilizing CT and PFC2D simulation analyses, we establish the interrelation between micro- and macromechanics. The findings reveal similar failure modes across CF samples with varying contents, predominantly marked by shear failure in the principal crack of the inclined section, accompanied by proximal secondary cracks. The post-peak stress–strain curves exhibit notable divergences. Specifically, with increasing CF content, the total strain energy, elastic strain energy, and dissipated energy of the CF samples initially decline, subsequently rise, and then diminish once more. Furthermore, the 1.0%CF sample demonstrates heightened dissipated energy, uniaxial compressive strength, and elastic modulus in contrast to the 0%CF sample, showcasing superior macroscopic mechanical properties and enhanced toughness. A staged microcrack evolution unfolds, delineating defect initiation, pore evolution, crack development, and crack penetration stages. The non-uniform distribution of CF and sand gravel profoundly influences the propagation trajectory of minute cracks. Additionally, the failure modes and energy evolution trends from both models closely mirror experimental outcomes. The reinforcing characteristics and crack-resistant attributes of CF usher the transition from brittle failure (0%CF) to ductile failure (1.0%CF). PFC simulations highlight hysteresis in the model's damage and failure, underscoring CF's role in restraining grouting sample failure and shifting it from brittle (0%CF) to ductile (1.0%CF) failure, thus accentuating CF's pivotal role in impeding crack propagation. These findings furnish invaluable theoretical insights for bolstering grouting reinforcement in fractured rock masses.

Microstructural Refinement and Surface Modification of Ni-Rich Cathodes for High-Performance Li-Ion Battery

Li[Ni1–x–y–zCoxMnyAlz]O2 (NCMA) cathodes have attracted public attention owing to their improved durability by leveraging the advantages of NCM and NCA cathodes. The combination of Mn and Al in a NCMA cathode effectively stabilizes the host layered structure to increase its resistance to microcracking; consequently, NCMA cathodes outperform both NCM and NCA cathodes with the same Ni contents, in terms of cycle life, without sacrificing energy density. However, as the Ni contents of the cathodes exceed 90%, a simple, compositionally engineered NCMA cathode lacks the chemical and mechanical stability to suppress surface degradation and the development of permanent microcracks due to the high incidence of highly reactive Ni4+ species on its surface.1,2 Considering the limitations of NCMA cathodes, a combination strategy involving microstructural engineering to inhibit microcracking, by enabling the efficient dissipation of local strain,3 and the modification of the exposed cathode surface, to protect it against deleterious electrolyte attack, is expected to afford superior Ni-rich layered cathodes.In this study, we improved the cycling stability of a conventional NCMA cathode with a Ni content of 93% (a NCMA93 cathode) using a combination strategy involving microstructural engineering and surface modification. To this end, we prepared a Sb-doped NCMA93 (denoted as Sb-NCMA93) cathode; Sb was incorporated to refine its microstructure and radially align and elongate its primary particles. Furthermore, we generated a protective F layer on the cathode surface through a chemical reaction between residual lithium compounds and a NH4F coating agent. To comprehensively investigate the synergetic effect of this combination strategy, conventional and Sb-doped NCMA93 cathodes with and without F coatings were synthesized and characterized and their fundamental properties and electrochemical performances compared. Post-mortem analyses were performed to elucidate the origins of the remarkable long-term cycling stability of the optimized cathodes. Reference s : [1] H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources, 2013, 233, 121.[2] H.-H. Ryu, K.-J. Park, C. S. Yoon, Y.-K. Sun, Chem. Mater. 2018, 30, 1155.[3] U.-H. Kim, H.-H. Ryu, J.-H. Kim, R. Mücke, P. Kaghazchi, C. S. Yoon, Y.-K. Sun, Adv. Energy Mater. 2019, 9, 1803902.

Study on reinforcing mechanism of carbon nanotubes on tensile behavior of gangue cemented rockfill

Gangue cemented rockfill (GCR) plays a key role in underground reinforcement projects and construction waste recycling. However, the tensile performance restricted by cementitious materials limits its wider application. Hence, in this study, carbon nanotubes (CNTs) was adopted to reinforce the tensile behavior of GCR samples, since CNTs own excellent mechanical properties and promising microstructure modification in cement-based materials. The test results demonstrate that only 0.007 wt% CNTs could significantly improve tensile strength up to 47.7% and ductility failure property. Through the characterization of acoustic emission (AE), digital image correlation (DIC) and SEM techniques, we found that the microscopic improvement of CNTs could reduce the intensity of failure events, promote the distribution of strain field, and weaken the stress concentration. AE counts of single failure events dropped over 80% and cumulative energy decreased up to 76.4%. Besides, the fractal dimension of macro strain field rose by 7.45∼15.5%, which further indicates the more complex strain distribution. The established CNTs pull-out model, based on above results, further revealed that bridging CNTs provided resistance, consumed fatal work during the pull-out process and slowed down the development of microcracks. This research broadens our understanding of the improvement mechanism of CNTs on the failure mode of GCR materials, which is of great significance for CNTs application and the production of eco-friendly construction materials.