Development Of Microcracks Research Articles

Multiscale modeling of diffuse damage and localized cracking in quasi-brittle materials under compression with a quadratic friction law

The diffuse damage and localized cracking of quasi-brittle materials (i.e., rocks and concretes) under compression can be delineated by a matrix-microcrack system, wherein a solid matrix phase is weakened by a large number of randomly oriented and distributed microcracks, and the macroscopic cracking is formed by a progressive evolution of microcracks. Several homogenization-based multiscale models have been proposed to describe this matrix-microcrack system, but most of them are based on a linear friction law on the microcrack surface, rendering a linear strength criterion. In this paper, we propose a new quadratic friction law within the local multiscale friction-damage (LMFD) model to capture the plastic distortion due to frictional sliding along the rough microcrack surface. Following that, a macroscopic Ottosen-type nonlinear strength criterion is rationally derived with up-scaling friction-damage coupling analysis. An enhanced semi-implicit return mapping (ESRM) algorithm with a substepping scheme is then developed to integrate the complex nonlinear constitutive model. The performance of LMFD model is evaluated compared to a wide range of experimental data on plain concretes, and the robustness of ESRM algorithm is assessed through a series of numerical tests. Subsequently, to effectively describe the localized cracking process, a regularization scheme is proposed by combining the phase-field model with the established LMFD model, and the discretization independent crack localization is numerically verified.

Study on the stressing state features of basalt fibre concrete lining structure under sulphate erosion

Basalt fibre (BF) is an environmentally friendly material with excellent corrosion resistance. When tunnels cross gypsum rock strata, the gypsum rock will dissolve sulphate and erode the supporting structure, thus affecting the safe operation of the tunnels in the long term. Adding BF to the supporting structure of gypsum rock tunnels can improve its resistance to sulphate erosion. This paper took Wuzhishan Tunnel as a case, and first determined the sulphate concentration of the gypsum rock stratigraphic environment at the site; then, analysed the change of mechanical properties of basalt fibre concrete (BFC) before and after sulphate erosion by mechanical tests; finally, examined the deformation and force characteristics of the BFC secondary lining structure by numerical simulation. The results showed that the bridging effect of BF fibres and the stable spatial network structure formed inside the concrete enhanced the mechanical properties of BFC and its ability to resist sulphate erosion. However, due to the agglomeration effect of BF fibres, too much BF would reduce the mechanical properties and erosion resistance of BFC. At a BF content of 6 kg/m³, BFC had the strongest ability to resist sulphate erosion, and the cube compressive strength, axial compressive strength, and modulus of elasticity of BFC were the highest, with 47.8 MPa, 33.6 MPa, and 38.7 GPa, respectively. The dense calcium silicate hydrate (C-S-H) gel, generated near the BF, enhanced the connection between the cementitious and the aggregate, inhibited the development of microcracks within the concrete, and reduced the erosion channels for sulphate ions. The maximum displacement of the BFC secondary lining structure was reduced by 20.15 % compared to plain concrete (PC) after sulphate erosion. The secondary lining structures consisting of both BFC and PC were stressed the most at footing. The secondary lining structure composed of BFC had better deformation resistance and higher strength than PC.

A microscopic approach to brittle creep and time-dependent fracturing of rocks based on stress corrosion model

A brittle creep and time-dependent fracturing process model of rock is established by incorporating the stress corrosion model into discrete element method to analyze the creep behavior and microcrack evolution in brittle rocks at a micro-scale level. Experimental validation of the model is performed, followed by numerical simulations to investigate the creep properties and microcrack evolution in rocks under single-stage loading, multistage loading, and confining pressure, at various constant stress levels. The results demonstrate that as the stress level increases in single-stage creep simulations, the time-to-failure progressively decreases. The growth of microcracks during uniaxial creep occurs in three stages, with tensile microcracks being predominant and the spatial distribution of microcracks becoming more dispersed at higher stress levels. In multi-stage loading-unloading simulations, microcracks continue to form during the unloading stage, indicating cumulative damage resulting from increased axial stress. Additionally, the creep behaviour of rocks under confining pressure is not solely determined by the magnitude of the confining pressure, but is also influenced by the magnitude of the axial stress. The findings contribute to a better understanding of rock deformation and failure processes under different loading conditions, and they can be valuable for applications in rock mechanics and rock engineering.

Microcrack characteristics of granite under circumferential strain-controlled uniaxial compression test using UDEC-GBM

Rockburst is one of the significant challenges for underground excavation stability. The post-peak behaviour of rock controls the way it fails, and the released energy is potentially involved in the rockburst. However, understanding of both microscopic and macroscopic behaviours of brittle rock in the post-peak stage is limited due to the lack of proper loading methods and quantitative microscopic analysis methods. This study proposes a circumferential strain-controlled loading algorithm in UDEC to control the failure process, which has been verified against previous literature. By employing the improved geometry generation method, the microcrack evolution is quantitatively assessed. Energy analysis reveals that up to 80% of the input energy is stored in the granite prior to complete failure, which could be released if the rock fails at this moment. Conversely, after the complete failure, intragranular cracks account for the majority of energy dissipation, indicating its importance in post-peak stability. A small portion of the peak stored strain energy needs to be extracted to achieve a controlled failure process. Furthermore, it is found that the significant increases in the formation of intergranular shear and low-angled microcracks potentially feature the complete failure of the specimen. The proposed model offers a complementary method to systematically investigate the post-peak behaviour of rock, including the evolution of microcracks and energy.

Experimental investigation on the macro- and micromechanical properties of water-cooled granite at different high temperatures

To investigate the damage mechanisms in granite’s physical and mechanical properties after high-temperature water quenching, this study employed MTS815.04 for uniaxial compression tests on thermally treated specimens, with concurrent acoustic emission monitoring, and utilized nanoindentation for micromechanical analysis. The results show that with increasing temperature, granite's peak strength and elastic modulus decrease, with a sharp decline after 400–500 °C, corresponding to a significant increase in the internal damage, which can be detected by acoustic emission monitoring. Below 500 °C, macroscopic mechanical degradation is due to mineral thermophysical property differences, while above 500 °C, microcrack development is the main deterioration factor. The failure mode shifts from tensile to tensile-shear complex to shear failure, with transition points at 400 °C and 800 °C. The results of this study are of certain reference value for improving the efficiency of extracting thermal energy from dry-hot rocks and providing security guidance for the tunnel restoration process following fire damage.

Study on the influence of temperature on the damage evolution of hot dry rock in the development of geothermal resources

In order to more accurately understand the rock mechanics properties of hot dry rock (HDR) reservoirs under high-temperature conditions and further guide the drilling and construction of HDR wells, conducting research on rock damage evolution under high-temperature conditions plays a guiding role in the efficient development of geothermal resources. To reveal the evolution law of fracture damage of HDR under temperature influence, taking marble as an example, the thermo-mechanical damage and failure mechanism under the conditions of 30 °C, 60 °C, 90 °C, 120 °C and 150 °C were studied using a rock high-temperature rheometer and an acoustic emission (AE) testing system. The numerical simulation results were compared with the indoor experimental results by combining the particulate flow program Particle Flow Code 3D (PFC3D) to verify the effectiveness of the simulation analysis model of thermo-coupled particulate flow at the micro scale. The results show that under high temperature, the marble three-axis compression failure passes through the compaction and elastic deformation stage, plastic deformation stage, ductile failure stage and instability failure stage, and the peak strength and elastic modulus of the marble decrease with the increase of temperature, and the acoustic emission b-value of the sample can judge the damage evolution process of the marble. At the micro scale, high temperature accelerates the force chain transmission between particles, the contact force between marble particles gradually decreases during the heating process, and the number of particle bonding failures increases steadily; the bearing capacity and elastic modulus attenuation of the rock are the result of microcrack evolution, and the force chain void zone and particle displacement field inside the sample are consistent with its macroscopic failure form. The development of microcracks in marble under different test loading conditions is different, and with the increase of test temperature, the total number of cracks inside the sample increases, but the proportion of shear cracks decreases, the activity of particle displacement increases, and the “plastic extension” characteristic of marble becomes more significant. The thermal damage evolution of marble was quantitatively analyzed based on the elastic modulus method and the peak stress method, and the damage evolution law of marble under different temperatures was obtained.

Energy evolution and damage model of sandstone under seepage-creep-disturbance loading

The geological environment of deep coal seam is complex, and the failure mechanism of coal seam under the action of seepage pressure and disturbance stress is unclear, which restricts the development of deep ground engineering. In this study, triaxial compression test of sandstone under seepage-creep-disturbance loading (SCD) was carried out to analyze the characteristics of disturbance deformation, creep rate and permeability. The focus was on the energy evolution law of sandstone under SCD, and the damage creep model was constructed based on the energy proportion. The results show that the creep increment of sandstone under disturbed load was significantly higher than that under undisturbed load, and the stress level can promote the expansion and development of microcracks, in which the creep increment under high stress level showed a jump increase. The total energy and elastic energy of sandstone showed a step change with time. The damage factor under disturbance load was defined in the form of energy dissipation ratio, and the corresponding coupling damage factor was determined by considering the seepage factor and creep factor, which can reflect the coupled damage deformation of sandstone under SCD. Based on fractional order theory, a nonlinear damage creep model describing sandstone deformation under SCD was established by connecting the acceleration element in series with the Nishihara model and introducing the coupling damage factor. This study can provide research ideas for water inrush of coal seam rock mass.

Numerical simulations on compression behaviors of the laminated shale based on the digital image technology and the discrete element method

As an unconventional reservoir sedimentary rock, shale contains a series of layers and various microstructures that lead to complex mechanical properties, such as the anisotropy of stiffness and strength. This study is directed towards the anisotropy caused by the microstructures of the shale, employing the 2D particle flow code (PFC2D) to explore stiffness, strength, failure mode, and micro-crack evolution. More realistic microstructures and the calibration of microscopic parameters of the shale are reasonably considered through the computed tomography (CT) images and mineral analysis. The corresponding numerical simulation results are fully compared with the experimental results. In what follows, the sensitivity analysis is conducted on the key microscopic parameters and microstructure characteristics in numerical samples with laminated characteristics. The results show that the influence of microscopic parameters of the parallel bonding model on macroscopic parameters is related to the layering angle and the face type, and the microstructures and initial cracks of numerical samples can considerably affect the macroscopic mechanical behaviors of the laminated samples. Next, the effect of confining pressure on the mechanical properties of layered shale is also discussed based on the numerical results. These findings highlight the potential of this approach for applications in micro-scaled models and calibration of microscopic parameters to probe mechanical behaviors of the laminated rock.

Nucleation, Development and Healing of Micro-Cracks in Shape Memory Polyurethane Subjected to Subsequent Tension Cycles.

Thermoresponsive shape memory polymers (SMPs) have garnered increasing interest for their exceptional ability to retain a temporary shape and recover the original configuration through temperature changes, making them promising in various applications. The SMP shape change and recovery that happen due to a combination of mechanical loading and appropriate temperatures are related to its particular microstructure. The deformation process leads to the formation and growth of micro-cracks in the SMP structure, whereas the subsequent heating over its glass transition temperature Tg leads to the recovery of its original shape and properties. These processes also affect the SMP microstructure. In addition to the observed macroscopic shape recovery, the healing of micro-crazes and micro-cracks that have nucleated and developed during the loading occurs. Therefore, our study delves into the microscopic aspect, specifically addressing the healing of micro-cracks in the cyclic loading process. The proposed research concerns a thermoplastic polyurethane shape memory polymer (PU-SMP) MM4520 with a Tg of 45 °C. The objective of the study is to investigate the effect of the number of tensile loading-unloading cycles and thermal shape recovery on the evolution of the PU-SMP microstructure. To this end, comprehensive research starting from structural characterization of the initial state and at various stages of the PU-SMP mechanical loading was conducted. Dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS) and scanning electron microscopy (SEM) were used. Moreover, the shape memory behavior in the thermomechanical loading program was investigated. The obtained average shape fixity value was 99%, while the shape recovery was 92%, which confirmed good shape memory properties of the PU-SMP. Our findings reveal that even during a single loading-unloading tension cycle, crazes and cracks nucleate on the surface of the PU-SMP specimen, whereas the subsequent temperature-induced shape recovery process carried out at the temperature above Tg enables the healing of micro-cracks. Interestingly, the surface of the specimen after three and five loading-unloading cycles did not exhibit crazes and cracks, although some traces of cracks were visible. The traces disappeared after exposing the material to heating at Tg + 20 °C (65 °C) for 30 min. The crack closure phenomenon during deformation, even without heating over Tg, occurred within three and five subsequent cycles of loading-unloading. Notably, in the case of eight loading-unloading cycles, cracks appeared on the surface of the PU-SMP and were healed only after thermal recovery at the particular temperature over Tg. Upon reaching a critical number of cycles, the proper amount of energy required for crack propagation was attained, resulting in wide-open cracks on the material's surface. It is worth noting that WAXS analysis did not indicate strong signs of typical highly ordered structures in the PU-SMP specimens in their initial state and after the loading history; however, some orientation after the cyclic deformation was observed.

Thermal-damage effects on fracturing evolution of granite under compression-shear loading

Shear-dominant instability disasters in high-temperature conditions pose a significant challenge for deep underground rock projects, necessitating a thorough understanding of rock fracture evolution and the impacts of thermal damage. To investigate how thermal damage influences shear fracturing behavior and geophysical responses, experiments were conducted on thermally-treated granite under compression-shear loading conditions. The detailed failure process was meticulously monitored using integrated acoustic emission (AE), electric potential (EP), and digital image correlation (DIC) techniques. The effects of thermal treatment on the patterns of AEs, EPs, and DIC strain fields during the loading process were analyzed. Micromechanics related to the influence of thermal treatment on mechanical properties and geophysical responses were explored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). By defining damage variables through multiparameter analysis using AE, EP, and DIC, the study delved into predictive insights on the failure process of thermal-damage granite. The results indicate that the physical properties of granite undergo significant changes as the treatment temperature increases, with the peak stress gradually decreasing and the plastic characteristics increasing. These changes can be attributed to thermal damage, which causes internal water escape, expansion of mineral particles, and crack formation. Additionally, increasing the thermal treatment temperature leads to a decrease in both AE and EP intensity, as well as a reduction in the occurrence of high-magnitude AE events, while the cumulative AE count rapidly increases during the plastic stage. Notably, thermal treatment induces significant alterations in mineral composition, intergranular fractures, and the formation of a gradual zigzag crack path, resulting in the intensified development of shear microcracks. This observation is supported by the varying of AE characteristic parameter AF/RA and the analysis of DIC displacement decomposition. Moreover, the three defined damage variables exhibit accelerated growth prior to catastrophic failure, with the order of precursor occurrence observed as AE, DIC, and EP. However, the occurrence of the precursor time for these three geophysical signals becomes earlier with increasing treatment temperature. These research findings shed light on the mechanism of compression-shear failure of thermal-damage rock and provide valuable references for predicting precursors of the shear-dominant instability disasters such as induced earthquakes.

Three-dimensional characteristic and evolution of creep cavity and microcrack of HR3C austenitic heat resistant steel after long-term creep at 650 °C

Three-dimensional characteristics of creep cavities and microcracks in HR3C austenitic steel after creep at 650 °C were investigated using high resolution X-ray computed tomography (CT) and three-dimensional atomic probe (3DAP). The variation of cavity volume fraction is related to the increasing fraction of cavities undergoing growth and coalescence. Irregular lamellar or “W” shaped microcrack along the grain boundary were observed by the CT and the number large sized microcracks decreased significantly with decreasing stress levels. 3DAP results show that the concentration of P elements was high at the M23C6/γ interface at the early stage of creep. With the increase of creep time, M23C6/γ interface have been found to not only contain high concentration of P element, but also be enriched with Si. The coarsening of the carbides at the grain boundary and the enrichment of P and Si elements at the interface creates the conditions for the nucleation of creep cavities and microcracks under stress. Thus, stress has an tremendous influence on the distribution of creep cavities and the propagation path of microcracks. Additionally, with the decrease of stress, the creep damage mode changes from intergranular/transgranular mixed cracks to creep cavity.

Effect of Heat Treatment on Mechanical Properties and Corrosion Response of HVOF Sprayed High Entropy Alloy Coatings

Surface and sub-surface related degradation of steels can be minimized using suitable surface coatings. High entropy alloys (HEA) are prominent and emerging materials among many coating materials. The current study investigates the effect of heat treatment of HEA coating on mechanical, metallurgical, and corrosion properties. The HEA coatings on SS304 steel were deposited using a High-Velocity Oxy-Fuel (HVOF) thermal spray process. The developed coatings were furnace heat treated at 700 °C, 900 °C, and 1100 °C, respectively, and their performance was benchmarked with the as-sprayed coatings. The metallurgical, mechanical, and microstructural analyses were performed using X-ray diffraction (XRD), Nanoindentation, Scratch test, and Field Emission Scanning Electron Microscope (FESEM) techniques. The corrosion response of the as sprayed and heat-treated coatings were recorded using a Potentiostat. The results indicated that as-sprayed coatings consisted of a single-phase BCC solid solution; however, the single-phase changed to a dual dual-phase system after heat treatment (BCC+FCC). The 900 °C heat-treated HEA coating exhibited superior mechanical and corrosion properties. But those characteristics started diminishing when the heat treatment temperature exceeded 900 °C. The introduction of the new FCC phase softened the coating, thereby leading to the evolution of microcracks in the coating. These micro-cracks acted as channels for electrolyte diffusion and further corroded the coatings. The current study surmised that HVOF-sprayed HEA coating should not be heat treated at above 900 °C.

Enhancing chloride ion permeability resistance of recycled aggregate concrete through internal curing with glazed hollow beads

This study explores the effects of glazed hollow beads (GHB) dosage and additional water amount on the capillary negative pressure, mechanical properties, initial resistivity, and electric flux in conditions where recycled aggregate concrete (RAC) is subject to cracking. The chloride ion (Cl-) diffusion coefficient for RAC cured internally was evaluated using the resistivity method, which also looked at the relationship between the diffusion coefficient, GHB dosage, and additional water amount. This analysis advances our understanding of the functional mechanisms of pre-soaked GHB in RAC. The research reveals that incorporating pre-soaked GHB enhances RAC in two primary ways. Firstly, pre-soaked GHB acts as an internal curing agent that stimulates the formation of hydration products, thereby filling internal voids and mitigating micro-crack development. Secondly, it functions as an adsorbent. The differential in relative humidity and internal pressure between the unsaturated GHB and the cement stone matrix promotes the sequestration of corrosive agents in cracked RAC. Additionally, an internal curing impact factor has been introduced, forming the basis of a new model to calculate the Cl- diffusion coefficient that accounts for both GHB dosage and additional water intake. The established quantitative relationships between resistivity, electric flux, and Cl- diffusion coefficient in GHB-treated RAC (GHB-RAC) provide a methodological framework for quickly evaluating the durability of GHB-RAC against Cl- intrusion in practical engineering contexts.

Microcrack and Porosity Development in Sealed Cement Mortars Measured with Micro-Computed Tomography.

For the first time, this paper explores the role of hydration kinetics on microcrack development in cement mortars using the μ-CT technique with a resolution of 2.2 µm. Three binders were tested: fine-grained ordinary Portland cement (OPC) with Blaine fineness of 391 m2/kg, coarse-grained OPC made from the same clinker with Blaine fineness of 273 m2/kg, and H-cement as a representative of the alkali-activated binder. It was found that most microcracks have a width in the range of 5-10 µm, increasing their occurrence with the progress of sealed hydration. While H-cement and coarse-grained OPC showed a comparable number of microcracks, fine-grained OPC exhibited more than twice the number of microcracks. In this sense, high hydration kinetics induce more microcracks, promoting later coalescence into visible cracks and disintegration of concrete at the end. Therefore, durable concrete with minimum microcracks should be derived from slow hydration kinetics or alkali-activation processes.

In-situ X-ray micro-CT quantitative analysis and modelling the damage evolution in granite rock

Micro-cracking of rock is important mechanism in underground engineering and subsurface energy extraction. Understanding and modeling microcrack evolution and coalescence, and the eventual macroscopic fracture propagation is still a challenge. A micro-mechanically motivated fracture model is developed based on observation and quantitative analysis of in-situ X-CT uniaxial compression test on brittle intact rock with the resolution of 11.98 μm. We performed uniaxial compression test on a micro-granite and acquired real-time X-CT images of crack development under different loadings to investigate cracking pattern, evolution of the crack network and geometric characteristics of the cracks. The experimental results show that the nucleation of isolated microcracks is the main damage pattern before failure. The evolution of damage is subtle under moderate loading but grows stably in density until the unstable coalescence of microcracks, followed by macro-scale failure. Moreover, the damage process is initially dominated by the development of inclined cracks; when failure is approached, the development of relatively more parallel cracks gradually become predominant, indicating the damage transition nature from shear mechanism to tensile mechanism. Based on the X-CT analysis, an improved physics-informed micro-mechanical sliding model is proposed. Compared with previous sliding models, the crack-crack interaction is improved so that this model can well characterize the abrupt failure process. The total surface area of cracks is used to quantify the damage evolution and a relative calibration method is presented. The simulation results show the stress–strain curve and damage evolution are consistent with the experimental results.

Damage evolution in asphalt mixtures based on in-situ CT scanning

A novel approach utilizing in-situ Computed Tomography (CT) scanning and digital image processing techniques was developed to overcome the limitations of traditional CT image analysis in studying microcrack evolution in asphalt mixtures. Two types of graded asphalt mixtures (continuous gradation/AC and intermittent gradation/SMA) were scanned in-situ to capture internal structural changes. Digital image processing distinguished primary voids from cracks and extracted crack evolution. Multivariate regression analysis linked crack parameters to compressive strain. The study found that crack evolution during uniaxial compression in asphalt mixtures can be divided into densification, linear elastic deformation, and nonlinear failure stages. As load increases, the number, volume, and void percentage occupied by cracks rise, peak, and then decrease due to merging and engulfing after the ultimate load. The skeletal structure of cracks was obtained, and a damage evolution equation was established, linking crack parameters to damage evolution. Crack tortuosity was used to characterize the resistance effect of different gradations on crack propagation. This methodology provides new insights and technical support for studying damage evolution and understanding the failure mechanisms of asphalt mixtures.

A coupled chemo-transport-mechanical damage model for the mineral transformation in cementitious materials under a sulfate-rich environment

The present study proposes a coupled chemo-transport-mechanical damage model integrating thermodynamics, fluid-solid transformation, and mechanical damage modeling to predict the physicochemical characteristics of cementitious materials under a sulfate-rich environment. For verifying the proposed model, calcium sulfoaluminate (CSA) cements with various molar ratios of calcium sulfate over ye'elimite (m-values) were adopted for a case study. The thermodynamics model for the phase assemblage of CSA cements with various m-values under a sulfate-rich environment is compared with the experimental outcomes. The model parameters in the fluid-solid transformation and mechanical damage models are fitted through the available experimental data in the literature. Mineral compositions, crystallization pressure, effective elastic modulus, delayed ettringite formation, and microcracks of CSA cement pastes were modeled by a proposed coupled chemo-transport-mechanical damage model, with model parameters fitted to experimental data, thereby assessing the applicability of the proposed model. The predicted results show that an increase in m-value of the samples increases the rate of sulfate ion penetration and crystallization pressures. Moreover, DEF, microcrack evolution, and mechanical degradation can be attributed to the amount of ye'elimite and hydrated monosulfate of the samples.

The influence of blast furnace slag on the mechanical properties and mesoscopic damage mechanisms of recycled aggregate concrete compared to natural aggregate concrete at different ages

To improve the mechanical property defects caused by recycled aggregate (RCA) in recycled aggregate concrete (RAC) as well as the effect of blast furnace slag (BFS) addition on the mechanical properties and microscopic damage mechanisms of natural aggregate concrete (NAC) and RAC at long ages. In this paper, the effects of different curing ages (T = 7d, 14d, 28d, 56d, 90d, and 150d) and blast furnace slag contents on the mechanical properties and microscopic damage mechanisms under uniaxial compression of NAC and RAC were investigated, where the cement substitution rate of BFS was 0 % and 35 % (water-cement ratio of 0.46, water-reducing agent dosage of 0.25 %), respectively. Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were used to study the internal pore changes and the generation of hydration products in the specimens, and acoustic emission (AE) showed the microcrack development process. The results showed that the peak stress and modulus of elasticity of the specimens gradually increased with increasing age, the peak strain gradually decreased, and the internal porosity gradually decreased. It is worth noting that the addition of BFS caused the mechanical properties of RAC to decrease at 28 days but enhanced the mechanical properties of RAC at 56–120 days. This was attributed to the fact that the properties of BFS were not yet activated in the early stage, whereas it was activated in the later stage under an alkaline environment and reacted with CH to generate more C–S–H hydration products, which enhanced the interfacial transition zone and reduced the porosity. This highlights the potential of BFS to improve the performance of RCA over long periods of time. In contrast, BFS improved the mechanical properties of NAC throughout the age period and more significantly after 56 days. AE results showed that the peak ring counts lagged behind the peak stresses. Finally, combined with the statistical damage principal model, the fitting was performed by Matlab to analyze the microscopic damage mechanism of the specimen during uniaxial compression. It was found that the changes in the four characteristic parameters (εa, εb, εh and H) followed a certain regularity, reflecting the regular changes in fracture damage and yield damage. This revealed the intrinsic linkage between mesoscopic damage mechanisms and macroscopic mechanical properties. It was demonstrated that it was feasible to speculate on the law of the stress-strain curve at the macroscopic level by using the law of change of the four parameters (εa, εb, εh and H) at the microscopic level.

Effect of size and anisotropy on fracture process zone of coal: An experiment and numerical simulation

The size and properties of the fracture process zone (FPZ) have a significant impact on the fracture toughness of materials. Researching and understanding the characteristics of FPZ contribute to optimizing material performance and enhancing the reliability of engineering structures. In this study, the newly proposed modified semi circular bending (SCB) test method was used to carry out the pure mode I fracture test of coal samples with four sizes and four bedding angles, the development process of FPZ was obtained by using digital image correlation method, and the cohesive zone model considering the bedding plane was established. The influence of size and anisotropy on the development and size of FPZ was studied, and the influence mechanism of FPZ on the size effect and anisotropy of fracture toughness was revealed. The results show that the FPZ length of coal has obvious size effect and anisotropy. The FPZ length increases with the increasing specimen size, and increases first and then decreases with the increasing bedding angle. The FPZ relative length decreases with increasing specimen size, indicating that as the specimen size increases, the influence of specimen boundaries on PFZ gradually decreases, and the size effect of fracture toughness also gradually weakens. The competition between microcrack development driven by principal stress and microcrack development driven by bedding plane results in anisotropy in the development process of FPZ. The anisotropy of FPZ development process results in the anisotropy of FPZ length, fracture toughness and crack propagation path. The research findings can provide valuable guidance for the design of hydraulic fracturing in coal seams.

Development and Characterization of the Shale Stratum Well Wall Stabilized with Nanosomal Sealing Agent.

The development of micro cracks in shale formations can easily lead to wellbore instability caused by liquid phase invasion. In order to effectively seal the shale micropores, the surface treatment of nano-SiO2 particles was developed using the silicane coupling agent A-1891. Then, the temperature-sensitive polypenic acrylamide polymer was modified onto the surface of the nanoprocal particle through reaction to obtain the nanosomal blocking agent ASN. The infrared spectrum shows that there are chemical bonds between the generated polymer chains, rather than simple physical composites, indicating the successful synthesis of the temperature-responsive nanosealing agent ASN. The particle size analysis showed that the synthesized nanoparticles in ASN have a uniform particle size distribution and display no agglomeration phenomenon. Applying ASN as a sealing agent in drilling fluid effectively fills the nanoscale micropores and microcracks in shale, making shale denser and significantly improving the wellbore stability of shale formations. In addition, it has good temperature resistance, can adapt to reservoirs at different temperatures, is non-toxic and environmentally friendly, and has good prospects for stable applications in shale formation wellbore.