It is important to analyze the damage evolution process of surrounding rock under different water content for the stability of engineering rock mass.Based on digital speckle correlation(DSCM),acoustic emission(AE)and ...It is important to analyze the damage evolution process of surrounding rock under different water content for the stability of engineering rock mass.Based on digital speckle correlation(DSCM),acoustic emission(AE)and electromagnetic radiation(EMR),uniaxial hierarchical cyclic loading and unloading tests were carried out on sandstones with different fracture numbers under dry,natural and saturated water content,to explore the fracture propagation,failure precursor characteristics and damage response mechanism under the influence of water content effect.The results show that with the increase of water content,the peak stress and crack initiation stress decrease gradually,and the decreases are 15.28%-21.11%and 17.64%-23.04%,respectively.The peak strain and crack initiation strain increase gradually,and the increases are 19.85%-44.53%and 19.15%-41.94%,respectively.The precracked rock with different water content is mainly characterized by tensile failure at different loading stages.However,with the increase of water content,the proportion of shear cracks gradually increases,while acoustic emission events gradually decrease,the dissipative energy and energy storage limits of the rock under peak load gradually decrease,and the charge signal increases significantly,which is because the lubrication effect of water reduces the friction coefficient between crack surfaces.展开更多
Preexisting cracks inside tight sandstones are one of the most important properties for controlling the mechanical and seepage behaviors.During the cyclic loading process,the rock generally exhibits obvious memorabili...Preexisting cracks inside tight sandstones are one of the most important properties for controlling the mechanical and seepage behaviors.During the cyclic loading process,the rock generally exhibits obvious memorability and irreversible plastic deformation,even in the linear elastic stage.The assessment of the evolution of preexisting cracks under hydrostatic pressure loading and unloading processes is helpful in understanding the mechanism of plastic deformation.In this study,ultrasonic measurements were conducted on two tight sandstone specimens with different bedding orientations subjected to hydrostatic loading and unloading processes.The P-wave velocity was characterized by a similar response with the volumetric strain to the hydrostatic pressure and showed different strain sensitivities at different loading and unloading stages.A numerical model based on the discrete element method(DEM)was proposed to quantitatively clarify the evolution of the crack distribution under different hydrostatic pressures.The numerical model was verified by comparing the evolution of the measured P-wave velocities on two anisotropic specimens.The irreversible plastic deformation that occurred during the hydrostatic unloading stage was mainly due to the permanent closure of plastic-controlled cracks.The closure and reopening of cracks with a small aspect ratio account for the major microstructure evolution during the hydrostatic loading and unloading processes.Such evolution of microcracks is highly dependent on the stress path.The anisotropy of the crack distribution plays an important role in the magnitude and strain sensitivity of the P-wave velocity under stress conditions.The study can provide insight into the microstructure evolution during cyclic loading and unloading processes.展开更多
The rotation correction method of unloading compliance for a compact tensile （CT） specimen in the current J-integral resistance curves or critical J-integral test standards was under suspicion by tracing to its sour...The rotation correction method of unloading compliance for a compact tensile （CT） specimen in the current J-integral resistance curves or critical J-integral test standards was under suspicion by tracing to its source, and a new rotation correction method was proposed. The rotation center of the specimen is close to the crack tip, and the material constitutive relationship has little effect on the rotational radius R. If the formula for rotation correction recommended in current standards is used to measure the crack length of CT specimen, it will lead to larger error. A new formula with better accuracy for the crack length measurement of the CT specimen was presented.展开更多
With the increase of underground engineering construction depth,the phenomenon of surrounding rock sudden failure caused by supporting structure failure occurs frequently.The conventional unloading con-fining pressure...With the increase of underground engineering construction depth,the phenomenon of surrounding rock sudden failure caused by supporting structure failure occurs frequently.The conventional unloading con-fining pressure(CUCP)test cannot simulate the plastic yielding and instantaneous unloading process of supporting structure to rock.Thus,a high stress loading-instantaneous unloading confining pressure(HSL-IUCP)test method was proposed and applied by considering bolt’s fracture under stress.The wall thickness of confining pressure plates and the material of bolts were changed to realize different confin-ing pressure loading stiffness(CPLS)and lateral maximum allowable deformation(LMAD).The superio-rity of HSL-ICPU method is verified compared with CUCP.The rock failure mechanism caused by sudden failure of supporting structure is obtained.The results show that when CPLS increases from 1.35 to 2.33 GN/m,rock’s peak strength and elastic modulus increase by 25.18%and 23.70%,respectively.The fracture characteristics change from tensile failure to tensile-shear mixed failure.When LMAD decreases from 0.40 to 0.16 mm,rock’s residual strength,peak strain,and residual strain decrease by 91.80%,16.94%,and 21.92%,respectively,and post-peak drop modulus increases by 140.47%.The test results obtained by this method are closer to rock’s real mechanical response characteristics compared with CUCP.展开更多
To further investigate the forming mechanism and springback characteristics of strips under multi-square punch forming (MSPF) considering partial-unloading effects, a series of concave form ing tests of strips are con...To further investigate the forming mechanism and springback characteristics of strips under multi-square punch forming (MSPF) considering partial-unloading effects, a series of concave form ing tests of strips are conducted on the MSPF machine. This paper aims to reveal the physical mecha nism of the elastic-plastic deformation in the MSPF process considering the effect of the forming ap proaches, and derive appropriate mathematical interpretations. The theoretical model is firstly estab lished to analyse the concave forming mechanism and springback characteristics of the strip, and its accuracy is then validated by experimental data. The forming history and load evolutions are depicted to explore the required forming capacity through the proposed analytical method. Besides, the paramet ric studies are carried out to discuss their effects on the springback of the strip. The results suggest that the deformation paths of the strip are influenced by the forming approach, and the springback of the strip in convex forming is larger than that in concave forming.展开更多
In this study,a uniaxial cyclic compression test is conducted on coal-rock composite structures under two cyclic loads using MTSE45.104 testing apparatus to investigate the macro-mesoscopic deformation,damage behavior...In this study,a uniaxial cyclic compression test is conducted on coal-rock composite structures under two cyclic loads using MTSE45.104 testing apparatus to investigate the macro-mesoscopic deformation,damage behavior,and energy evolution characteristics of these structures under different cyclic stress disturbances.Three loading and unloading rates(LURs)are tested to examine the damage behaviors and energy-driven characteristics of the composites.The findings reveal that the energy-driven behavior,mechanical properties,and macro-micro degradation characteristics of the composites are significantly influenced by the loading rate.Under the gradual cyclic loading and unloading(CLU)path with a constant lower limit(path I)and the CLU path with variable upper and lower boundaries(path II),an increase in LURs from 0.05 to 0.15 mm/min reduces the average loading time by 32.39%and 48.60%,respectively.Consequently,the total number of cracks in the samples increases by 1.66-fold for path I and 1.41-fold for path II.As LURs further increase,the energy storage limit of samples expands,leading to a higher proportion of transmatrix and shear cracks.Under both cyclic loading conditions,a broader cyclic stress range promotes energy dissipation and the formation of internal fractures.Notably,at higher loading rates,cracks tend to propagate along primary weak surfaces,leading to an increased incidence of intermatrix fractures.This behavior indicates a microscopic feature of the failure mechanisms in composite structures.These results provide a theoretical basis for elucidating the damage and failure characteristics of coal-rock composite structures under cyclic stress disturbances.展开更多
During underground excavation,the surrounding rock mass is subjected to complex cyclic stress,significantly impacting its long-term stability,especially under varying water content conditions where this effect is ampl...During underground excavation,the surrounding rock mass is subjected to complex cyclic stress,significantly impacting its long-term stability,especially under varying water content conditions where this effect is amplified.However,research on the mechanical response mechanisms of surrounding rock mass under such conditions remains inadequate.This study utilized acoustic emission(AE)and resistivity testing to monitor rock fracture changes,revealing the rock’s damage state and characterizing the damage evolution process during uniaxial cyclic loading and unloading.First,a damage variable equation was established based on AE and resistivity parameters,leading to the derivation of a corresponding damage constitutive equation.Uniaxial cyclic loading and unloading tests were then conducted on sandstone samples with varying water contents,continuously monitoring AE signals and resistivity,along with computed tomography scans before and after failure.The predictions from the damage constitutive equation were compared with experimental results.This comparison shows that the proposed damage variable equation effectively characterizes the damage evolution of sandstone during loading and unloading,and that the constitutive equation closely fits the experimental data.This study provides a theoretical basis for monitoring and assessing the responses of surrounding rock mass during underground excavation.展开更多
Multiple filling of gobs will lead to a layered structure of the backfill.To explore the influence of layering structure on the mechanical properties and failure modes of backfill,different backfill specimens were pre...Multiple filling of gobs will lead to a layered structure of the backfill.To explore the influence of layering structure on the mechanical properties and failure modes of backfill,different backfill specimens were prepared with a cement/sand ratio of 1:4,a slurry concentration of 75%,and backfilling times of 1,2,3 and 4,separately.Triaxial cyclic loading and unloading experiments were carried out.The results show that with an increase in backfilling time,the peak strength of backfill decreases as a polynomial function and the peak strain increases as an exponential function.The cyclic load enhances the linear characteristic of backfill deformation.The loading and unloading deformation moduli have a linear negative correlation with the backfilling time.The unloading deformation modulus is always slightly higher than the loading deformation modulus.The failure modes of stratified backfill are mainly characterized by conjugate shear failure at the upper layer and tensile failure across the layer plane,and there is usually no damage in the lower layer away from the loading area.展开更多
The mechanism by which electromagnetic forming(EMF)enhances the formability of metals is unclear owing to the coupling effect of multi-physics fields.In the present work,the associated formability improvement mechanis...The mechanism by which electromagnetic forming(EMF)enhances the formability of metals is unclear owing to the coupling effect of multi-physics fields.In the present work,the associated formability improvement mechanisms were qualitatively categorized and illustrated.This was realized by comparing the formability of fully annealed 2219 aluminum alloy(AA 2219-O)sheet under quasi-static(QS),electromagnetic dynamic(EM),and mechanical dynamic(MD)tensile loadings.It was found that the forming limit of AA 2219-O sheet under EM tensile loading was significantly(45.4%)higher than that under QS tensile loading,and was marginally(3.7%–4.3%)higher than that under MD tensile loading.In addition,the forming limit of AA 2219-O sheet demonstrated a negative dependency on the strain rate within the range of the dynamic tensile tests conducted.The deformation conditions common to EM and MD tensile loadings were responsible for the significant formability improvement compared with QS tensile loading.In particular,the inertial effect was dominant.The different deformation conditions that distinguish EM tensile loading from MD tensile loading resulted in the marginal improvement in formability.This was caused by the absence of a sustaining contact force at the later deformation stage and the lower strain rate.The body force exerted little influence on the formability improvement,and the thermal effect under the two dynamic tensile loadings was negligible.展开更多
A new analytical solution for ground surface settlement induced by deep excavation is proposed based on the elastic half space Melan’s solution,and the analytical model is related to the physical and mechanical prope...A new analytical solution for ground surface settlement induced by deep excavation is proposed based on the elastic half space Melan’s solution,and the analytical model is related to the physical and mechanical properties of soil with the loading and unloading action during excavation process.The change law of earth pressure of the normal consolidation soil after the foundation pit excavation was analyzed,and elastic displacement calculation methods of analytic solution were further established given the influence of excavation and unloading.According to the change of stress state in the excavation process of foundation pit,the planar mechanical analysis model of the foundation excavation problem was established.By combining this model with the physical equations and geometric equations of plane strain problem with consideration of the loading and unloading modulus of soil,constitutive equation of the plane strain problem was also established.The loading and unloading modulus formula was obtained by using the parameter calculation method in Duncan-Chang curve model.The constitutive equation obtained from the model was used to calculate the soil stress state of each point to determine its loading and unloading modulus.Finally,the foundation pit displacement change after excavation was calculated,and thus the soil pressure distribution after retaining structure deformation.The theoretical results calculated by making corresponding programs were applied to engineering practice.By comparing the conventional calculation results with monitoring results,the practicability and feasibility of the calculation model were verified,which should provide a theoretical basis for similar projects.展开更多
In-situ rock failures can result from stress changes due to pure loading and/or unloading. Understanding of the damage evolution behavior in brittle rocks during loading and unloading is imperative for the designs of ...In-situ rock failures can result from stress changes due to pure loading and/or unloading. Understanding of the damage evolution behavior in brittle rocks during loading and unloading is imperative for the designs of rock structures. In this paper, we investigate the damage evolution characteristics of a granitic rock during loading and unloading after a series of triaxial experiments performed at different confining pressures. The axial stress-axial strain variations of the tested specimens revealed that the specimens undergoing unloading fail with a lower axial strain compared to the specimens failed purely by loading. Higher confining pressures were observed to exacerbate the difference. Volumetric strain versus axial strain curves indicated that the curves reverse the trend with the beginning of major damage of specimens. We suggest here a new form of equation to describe the secant modulus variation of brittle rocks against the axial stress for the unloading process. Failure mechanisms of tested specimens showed two distinct patterns, namely, specimens under pure loading failed with a single distinct shear fracture while for the unloading case specimens displayed multiple intersecting fractures. In addition, analysis of the evolution of dissipation and elastic energy during deformation of the specimens under loading and unloading conditions showed differentiable characteristics. Moreover, we evaluated the variations of two damage indices defined based on the energy dissipation and secant modulus evolution during deformation and observed that both of them satisfactorily distinguish key stages of damage evolution.展开更多
To investigate the effect of different environmental conditions of GFRP bars in concrete beams with work cracks subjected to sustained loads, the beams were exposed in indoor, freeze/thaw cycles and immersed in alkali...To investigate the effect of different environmental conditions of GFRP bars in concrete beams with work cracks subjected to sustained loads, the beams were exposed in indoor, freeze/thaw cycles and immersed in alkaline solution at elevated temperature. The bars were carefully extracted from the beams and tested in order to evaluate residual tensile properties. The results show that the tensile strength decreased significantly in the highly aggressive conditions but not in the natural conditions. The effect of GFRP bars casting in concrete beams demonstrated approximately 2.5% decrease of tensile strength caused by pore water environment in concrete beams on basis of those of the original bars. The effect of sustained loading plus work cracks demonstrated about 10.5% tensile strength decrease on basis of those of the bars only casted in concrete beams. The effect of environments under sustained loading plus work cracks demonstrated about 17% tensile strength decrease caused by a saturated solution of Ca（OH）2 and 60-2 ℃ tap water （pH=12-13） and about 8% tensile strength decrease caused by freezing and thawing cycle （F/T）, both on basis of those of the bars of the indoor beams only under sustained loading plus work cracks. The results demonstrate the effects of the tensile strengths under different environmental conditions of GFRP bars in concrete beams with work cracks subjected to sustained loads.展开更多
Rocks in underground works usually experience rather complex stress disturbance.For this,their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions.The eff...Rocks in underground works usually experience rather complex stress disturbance.For this,their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions.The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies.However,little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading(FL-CSU)conditions that influence rock failure.In this paper,we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography(CT)scanning technique and the GCTS RTR 2000 rock mechanics system.Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble.The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests,and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests.In FL-CSU tests,as the unloading rate increases,the dissipated energy grows and elastic energy reduces.CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension.It shows that the crack pattern after FL-CSU tests depends on the unloading rate,and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released.The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.展开更多
This paper investigates mechanical behaviours of sandstone during post-peak cyclic loading and unloading subjected to hydromechanical coupling effect, confirming the peak and residual strengths reduction laws of sands...This paper investigates mechanical behaviours of sandstone during post-peak cyclic loading and unloading subjected to hydromechanical coupling effect, confirming the peak and residual strengths reduction laws of sandstone with water pressure, and revealing the influence of water pressure on the upper limit stress and deformation characteristics of sandstone during post-peak cyclic loading and unloading.Regarding the rock strength, the experimental study confirms that the peak strength σ_(p) and residual strength σ_(r) decrease as water pressure P increases. Especially, the normalized strength parameters σ_(p)/σ_(pk) and σ_(r)/σ_(re) was negatively and linearly correlated with the P/σ_(3). Moreover, the Hoek-Brown strength criterion can be applied to describe the relationship between effective peak strength and effective confining stress. During post-peak cyclic loading and unloading, both the upper limit stress σ_(p(i)) and crack damage threshold stress σ_(cd(i)) of each cycle tend to decrease with the increasing cycle number. A hysteresis loop exists among the loading and unloading stress–strain curves, indicating the unloading deformation modulus E_(unload) is larger than the loading deformation modulus E_(load). Based on experimental results,a post-peak strength prediction model related to water pressure and plastic shear strain is established.展开更多
The functional relation between the residual tensile strength of plain concrete and number of cycles was determined. 99 tappered prism specimens of plain concrete were tested under uniaxial tensile fatigue loading. Ba...The functional relation between the residual tensile strength of plain concrete and number of cycles was determined. 99 tappered prism specimens of plain concrete were tested under uniaxial tensile fatigue loading. Based on the probability distribution of the residual tensile strength, the empirical expressions of the residual tensile strength corresponding to the number of cycles were obtained. The residual tensile strength attenuating curves can be used to predict the residual fatigue life of the specimen under variable-amplitude fatigue loading. There is a good correlation between residual tensile strength and residual secant elastic modulus. The relationship between the residual secant elastic modulus and number of cycles was also established.展开更多
The mechanical properties are essentially different when rock material is subjected to loading or unloading conditions. In this study, loading and unloading tests with various confining pressures are conducted to inve...The mechanical properties are essentially different when rock material is subjected to loading or unloading conditions. In this study, loading and unloading tests with various confining pressures are conducted to investigate the mechanical properties of marble material samples taken from the deep diversion tunnels of Jinping II Hydropower Station. The stress-strain relationship, failure characteristics and strength criterion are compared and analyzed based on the experiment results. The results show： in the loading and unloading test, peak strength, lateral strain, axial strain and plastic deformation increase significantly as the confining pressure increases. Lateral strain increased significantly and obvious lateral dilatancy can be observed to the change of confining pressure; The fracture mode is mainly the single shear fracture for the triaxial compression test and post-peak test, angle between the failure surface and the ends of the rock material becomes smaller as the confining pressure increases. Hock-Brown strength criterion reflects the strength characteristics of marble material under two different unloading conditions, and has some supplementary effects to the rock material of mechanical field.展开更多
By measuring and analyzing infrared thermal image of the specimen in static load tensile test process, it was studied that the influence of the undercut defects and double-sided dressing method on the deformation beha...By measuring and analyzing infrared thermal image of the specimen in static load tensile test process, it was studied that the influence of the undercut defects and double-sided dressing method on the deformation behavior of the laser welded joint specimens of TC4 titanium alloy. The results showed that for the unmodified specimens, the yield phenomenon occurs first in the region of the joint, but the undercut value has an effect on the stress and strain of starting to yield phenomenon, and a great effect on the plastic deformation behavior.When the undercut is less than a certain value, the large plastic deformation occurs in the base metal region and the plasticity of the specimen is comparable to that of the base metal, but the larger undercut defect results in a concentrated plastic deformation in the joint region and rapidly failed in this region. But the double-sided dressing specimen is significantly different. The physical yield is no longer concentrated in the joint region, but at the same time occurs in the several regions including joint and the base metal. And the plastic deformation mainly occurs in the base material area, similar to that of the base material.展开更多
In coal mining,rock strata are fractured under cyclic loading and unloading to form fracture channels.Fracture channels are the main flow narrows for gas.Therefore,expounding the flow conductivity of fracture channels...In coal mining,rock strata are fractured under cyclic loading and unloading to form fracture channels.Fracture channels are the main flow narrows for gas.Therefore,expounding the flow conductivity of fracture channels in rocks on fluids is significant for gas flow in rock strata.In this regard,graded incremental cyclic loading and unloading experiments were conducted on sandstones with different initial stress levels.Then,the three-dimensional models for fracture channels in sandstones were established.Finally,the fracture channel percentages were used to reflect the flow conductivity of fracture channels.The study revealed how the particle size distribution of fractured sandstone affects the formation and expansion of fracture channels.It was found that a smaller proportion of large blocks and a higher proportion of small blocks after sandstone fails contribute more to the formation of fracture channels.The proportion of fracture channels in fractured rock can indicate the flow conductivity of those channels.When the proportion of fracture channels varies gently,fluids flow evenly through those channels.However,if the proportion of fracture channels varies significantly,it can greatly affect the flow rate of fluids.The research results contribute to revealing the morphological evolution and flow conductivity of fracture channels in sandstone and then provide a theoretical basis for clarifying the gas flow pattern in the rock strata of coal mines.展开更多
During the constructions of motorways and high-speed railway lines in the Yanji Basin,large amounts of excess mudstones due to the enormous tunnel excavations and slope cuts would be deposited as landfills.Assessing t...During the constructions of motorways and high-speed railway lines in the Yanji Basin,large amounts of excess mudstones due to the enormous tunnel excavations and slope cuts would be deposited as landfills.Assessing the deformation and permeability of Yanji mudstone became important for the design,construction and operation of the landfills.This paper presents an experimental study on the deformation and permeability of Yanji mudstone by carrying out a series of oedometer tests with loading/unloading cycles.The results show that the sample with a lower initial water content exhibited greater swelling deformation after inundation,a lower yield stress,greater deformation and a higher hydraulic conductivity during the loading/unloading cycles.As the number of loading/unloading cycles increased,the yield stress and accumulated plastic deformation increased,while the compression index,rebound index and hydraulic conductivity decreased.The samples became stiffer and their hydromechanical behaviour tended to be stable after three cycles.The compression curves could be divided into pre-yield and post-yield zones.The post-yield zones of compression curves and the rebound curves could be normalized into a unique line,and the pre-yield zones of the compression curves could be described as lines.Basic equations were developed to predict mudstone deformation under cyclic loading and unloading.Additionally,an empirical relationship between the hydraulic conductivity and void ratio was also proposed.The ability of the proposed methods was verified by the overall good agreement between the experimental results and predicted values.展开更多
A reasonable evaluation of unloading deformation characteristics is of great significance for the effective analysis of deformation and stability of surrounding rocks after underground excavation.In this study,the dam...A reasonable evaluation of unloading deformation characteristics is of great significance for the effective analysis of deformation and stability of surrounding rocks after underground excavation.In this study,the damage-controlled cyclic triaxial loading tests were conducted to investigate the pore compaction mechanism and its influences on the unloading deformation behavior of red sandstone,including Young’s modulus,Poisson’s ratio,volumetric strain,and irreversible strain.The experimental results show that the increases of volumetric and irreversible strains of rocks can be attributed to the compaction mechanism,which almost dominates the entire pre-peak deformation process.The unloading deformation consists of the reversible linear and nonlinear strains,and the irreversible strain under the influence of the porous grain structure.The pre-peak Young’s modulus tends to increase and then decrease due to the influence of the unloading irreversible strain.However,it hardly changes with the increasing volumetric strain compaction under the influence of reversible nonlinear strain.Instead,the initial unloading tangent modulus is highly related to the volumetric strain,and clearly reflects the compaction state of red sandstone.Furthermore,both the reversible nonlinear and irreversible unloading deformations are independent of confining pressure.This study is beneficial for the theoretical modeling and prediction of cyclic unloading deformation behavior of red sandstone.展开更多
基金financially supported by National Natural Science Foundation of China(No.52304136)Young Talent of Lifting Engineering for Science and Technology in Shandong,China(No.SDAST2024QTA060)Key Project of Research and Development in Liaocheng(No.2023YD02)。
文摘It is important to analyze the damage evolution process of surrounding rock under different water content for the stability of engineering rock mass.Based on digital speckle correlation(DSCM),acoustic emission(AE)and electromagnetic radiation(EMR),uniaxial hierarchical cyclic loading and unloading tests were carried out on sandstones with different fracture numbers under dry,natural and saturated water content,to explore the fracture propagation,failure precursor characteristics and damage response mechanism under the influence of water content effect.The results show that with the increase of water content,the peak stress and crack initiation stress decrease gradually,and the decreases are 15.28%-21.11%and 17.64%-23.04%,respectively.The peak strain and crack initiation strain increase gradually,and the increases are 19.85%-44.53%and 19.15%-41.94%,respectively.The precracked rock with different water content is mainly characterized by tensile failure at different loading stages.However,with the increase of water content,the proportion of shear cracks gradually increases,while acoustic emission events gradually decrease,the dissipative energy and energy storage limits of the rock under peak load gradually decrease,and the charge signal increases significantly,which is because the lubrication effect of water reduces the friction coefficient between crack surfaces.
基金supported by the National Natural Science Foundation of China(Grant No.U2244215)the Knowledge Innovation Program of Wuhan-Basic Research(Grant No.2022010801010159)the Major Project of Inner Mongolia Science and Technology(Grant No.2021ZD0034).
文摘Preexisting cracks inside tight sandstones are one of the most important properties for controlling the mechanical and seepage behaviors.During the cyclic loading process,the rock generally exhibits obvious memorability and irreversible plastic deformation,even in the linear elastic stage.The assessment of the evolution of preexisting cracks under hydrostatic pressure loading and unloading processes is helpful in understanding the mechanism of plastic deformation.In this study,ultrasonic measurements were conducted on two tight sandstone specimens with different bedding orientations subjected to hydrostatic loading and unloading processes.The P-wave velocity was characterized by a similar response with the volumetric strain to the hydrostatic pressure and showed different strain sensitivities at different loading and unloading stages.A numerical model based on the discrete element method(DEM)was proposed to quantitatively clarify the evolution of the crack distribution under different hydrostatic pressures.The numerical model was verified by comparing the evolution of the measured P-wave velocities on two anisotropic specimens.The irreversible plastic deformation that occurred during the hydrostatic unloading stage was mainly due to the permanent closure of plastic-controlled cracks.The closure and reopening of cracks with a small aspect ratio account for the major microstructure evolution during the hydrostatic loading and unloading processes.Such evolution of microcracks is highly dependent on the stress path.The anisotropy of the crack distribution plays an important role in the magnitude and strain sensitivity of the P-wave velocity under stress conditions.The study can provide insight into the microstructure evolution during cyclic loading and unloading processes.
基金supported by the National Natural Science Foundation of China (No. 11072205)
文摘The rotation correction method of unloading compliance for a compact tensile （CT） specimen in the current J-integral resistance curves or critical J-integral test standards was under suspicion by tracing to its source, and a new rotation correction method was proposed. The rotation center of the specimen is close to the crack tip, and the material constitutive relationship has little effect on the rotational radius R. If the formula for rotation correction recommended in current standards is used to measure the crack length of CT specimen, it will lead to larger error. A new formula with better accuracy for the crack length measurement of the CT specimen was presented.
基金the National Natural Science Foundation of China(Nos.52374218,52174122 and 52374094)Outstanding Youth Fund of Shandong Natural Science Foundation(No.ZR2022YQ49)Taishan Scholar Project in Shandong Province(Nos.tspd20210313 and tsqn202211150).
文摘With the increase of underground engineering construction depth,the phenomenon of surrounding rock sudden failure caused by supporting structure failure occurs frequently.The conventional unloading con-fining pressure(CUCP)test cannot simulate the plastic yielding and instantaneous unloading process of supporting structure to rock.Thus,a high stress loading-instantaneous unloading confining pressure(HSL-IUCP)test method was proposed and applied by considering bolt’s fracture under stress.The wall thickness of confining pressure plates and the material of bolts were changed to realize different confin-ing pressure loading stiffness(CPLS)and lateral maximum allowable deformation(LMAD).The superio-rity of HSL-ICPU method is verified compared with CUCP.The rock failure mechanism caused by sudden failure of supporting structure is obtained.The results show that when CPLS increases from 1.35 to 2.33 GN/m,rock’s peak strength and elastic modulus increase by 25.18%and 23.70%,respectively.The fracture characteristics change from tensile failure to tensile-shear mixed failure.When LMAD decreases from 0.40 to 0.16 mm,rock’s residual strength,peak strain,and residual strain decrease by 91.80%,16.94%,and 21.92%,respectively,and post-peak drop modulus increases by 140.47%.The test results obtained by this method are closer to rock’s real mechanical response characteristics compared with CUCP.
文摘To further investigate the forming mechanism and springback characteristics of strips under multi-square punch forming (MSPF) considering partial-unloading effects, a series of concave form ing tests of strips are conducted on the MSPF machine. This paper aims to reveal the physical mecha nism of the elastic-plastic deformation in the MSPF process considering the effect of the forming ap proaches, and derive appropriate mathematical interpretations. The theoretical model is firstly estab lished to analyse the concave forming mechanism and springback characteristics of the strip, and its accuracy is then validated by experimental data. The forming history and load evolutions are depicted to explore the required forming capacity through the proposed analytical method. Besides, the paramet ric studies are carried out to discuss their effects on the springback of the strip. The results suggest that the deformation paths of the strip are influenced by the forming approach, and the springback of the strip in convex forming is larger than that in concave forming.
基金Project(2023YFC3009003) supported by the National Key R&D Program of ChinaProjects(52130409, 52121003, 52374249, 52204220) supported by the National Natural Science Foundation of ChinaProject(2024JCCXAQ01) supported by the Fundamental Research Funds for the Central Universities,China。
文摘In this study,a uniaxial cyclic compression test is conducted on coal-rock composite structures under two cyclic loads using MTSE45.104 testing apparatus to investigate the macro-mesoscopic deformation,damage behavior,and energy evolution characteristics of these structures under different cyclic stress disturbances.Three loading and unloading rates(LURs)are tested to examine the damage behaviors and energy-driven characteristics of the composites.The findings reveal that the energy-driven behavior,mechanical properties,and macro-micro degradation characteristics of the composites are significantly influenced by the loading rate.Under the gradual cyclic loading and unloading(CLU)path with a constant lower limit(path I)and the CLU path with variable upper and lower boundaries(path II),an increase in LURs from 0.05 to 0.15 mm/min reduces the average loading time by 32.39%and 48.60%,respectively.Consequently,the total number of cracks in the samples increases by 1.66-fold for path I and 1.41-fold for path II.As LURs further increase,the energy storage limit of samples expands,leading to a higher proportion of transmatrix and shear cracks.Under both cyclic loading conditions,a broader cyclic stress range promotes energy dissipation and the formation of internal fractures.Notably,at higher loading rates,cracks tend to propagate along primary weak surfaces,leading to an increased incidence of intermatrix fractures.This behavior indicates a microscopic feature of the failure mechanisms in composite structures.These results provide a theoretical basis for elucidating the damage and failure characteristics of coal-rock composite structures under cyclic stress disturbances.
基金Projects(52279117,52325905)supported by the National Natural Science Foundation of ChinaProject(DJ-HXGG-2023-16)supported by the Technology Project of PowerChinaProject(SKLGME-JBGS2401)supported by the State Key Laboratory of Geomechanics and Geotechnical Engineering,China。
文摘During underground excavation,the surrounding rock mass is subjected to complex cyclic stress,significantly impacting its long-term stability,especially under varying water content conditions where this effect is amplified.However,research on the mechanical response mechanisms of surrounding rock mass under such conditions remains inadequate.This study utilized acoustic emission(AE)and resistivity testing to monitor rock fracture changes,revealing the rock’s damage state and characterizing the damage evolution process during uniaxial cyclic loading and unloading.First,a damage variable equation was established based on AE and resistivity parameters,leading to the derivation of a corresponding damage constitutive equation.Uniaxial cyclic loading and unloading tests were then conducted on sandstone samples with varying water contents,continuously monitoring AE signals and resistivity,along with computed tomography scans before and after failure.The predictions from the damage constitutive equation were compared with experimental results.This comparison shows that the proposed damage variable equation effectively characterizes the damage evolution of sandstone during loading and unloading,and that the constitutive equation closely fits the experimental data.This study provides a theoretical basis for monitoring and assessing the responses of surrounding rock mass during underground excavation.
基金the National Natural Science Foundation of China(No.51374033)the Key Projects of the National Key Research and Development Program(No.YS2017YFSF040004).
文摘Multiple filling of gobs will lead to a layered structure of the backfill.To explore the influence of layering structure on the mechanical properties and failure modes of backfill,different backfill specimens were prepared with a cement/sand ratio of 1:4,a slurry concentration of 75%,and backfilling times of 1,2,3 and 4,separately.Triaxial cyclic loading and unloading experiments were carried out.The results show that with an increase in backfilling time,the peak strength of backfill decreases as a polynomial function and the peak strain increases as an exponential function.The cyclic load enhances the linear characteristic of backfill deformation.The loading and unloading deformation moduli have a linear negative correlation with the backfilling time.The unloading deformation modulus is always slightly higher than the loading deformation modulus.The failure modes of stratified backfill are mainly characterized by conjugate shear failure at the upper layer and tensile failure across the layer plane,and there is usually no damage in the lower layer away from the loading area.
基金financially supported by the National Natural Science Foundation of China(Nos.51575206 and 51705169)the Innovation Funds for Aerospace Science and Technology from China Aerospace Science and Technology Corporation(No.CASC150704)+1 种基金the Science Fund of State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body(No.31615006)the Fundamental Research Funds for the Central University(No.2016YXZD055)。
文摘The mechanism by which electromagnetic forming(EMF)enhances the formability of metals is unclear owing to the coupling effect of multi-physics fields.In the present work,the associated formability improvement mechanisms were qualitatively categorized and illustrated.This was realized by comparing the formability of fully annealed 2219 aluminum alloy(AA 2219-O)sheet under quasi-static(QS),electromagnetic dynamic(EM),and mechanical dynamic(MD)tensile loadings.It was found that the forming limit of AA 2219-O sheet under EM tensile loading was significantly(45.4%)higher than that under QS tensile loading,and was marginally(3.7%–4.3%)higher than that under MD tensile loading.In addition,the forming limit of AA 2219-O sheet demonstrated a negative dependency on the strain rate within the range of the dynamic tensile tests conducted.The deformation conditions common to EM and MD tensile loadings were responsible for the significant formability improvement compared with QS tensile loading.In particular,the inertial effect was dominant.The different deformation conditions that distinguish EM tensile loading from MD tensile loading resulted in the marginal improvement in formability.This was caused by the absence of a sustaining contact force at the later deformation stage and the lower strain rate.The body force exerted little influence on the formability improvement,and the thermal effect under the two dynamic tensile loadings was negligible.
基金Project(41672290)supported by the National Natural Science Foundation of ChinaProject(2016J01189)supported by the Natural Science foundation of Fujian Province,China
文摘A new analytical solution for ground surface settlement induced by deep excavation is proposed based on the elastic half space Melan’s solution,and the analytical model is related to the physical and mechanical properties of soil with the loading and unloading action during excavation process.The change law of earth pressure of the normal consolidation soil after the foundation pit excavation was analyzed,and elastic displacement calculation methods of analytic solution were further established given the influence of excavation and unloading.According to the change of stress state in the excavation process of foundation pit,the planar mechanical analysis model of the foundation excavation problem was established.By combining this model with the physical equations and geometric equations of plane strain problem with consideration of the loading and unloading modulus of soil,constitutive equation of the plane strain problem was also established.The loading and unloading modulus formula was obtained by using the parameter calculation method in Duncan-Chang curve model.The constitutive equation obtained from the model was used to calculate the soil stress state of each point to determine its loading and unloading modulus.Finally,the foundation pit displacement change after excavation was calculated,and thus the soil pressure distribution after retaining structure deformation.The theoretical results calculated by making corresponding programs were applied to engineering practice.By comparing the conventional calculation results with monitoring results,the practicability and feasibility of the calculation model were verified,which should provide a theoretical basis for similar projects.
基金Projects(51774187,51324744,51374129)supported by the National Natural Science Foundation of ChinaProject(71380100006)supported by the Innovation Foundation of Doctoral Student in Hunan Province,China+1 种基金Project(2017SK2280)supported by the Key Research and Development Program of Hunan Provincial Science and Technology Department,ChinaProject(17A184)supported by the Key Research Foundation of Education Bureau of Hunan Province,China
文摘In-situ rock failures can result from stress changes due to pure loading and/or unloading. Understanding of the damage evolution behavior in brittle rocks during loading and unloading is imperative for the designs of rock structures. In this paper, we investigate the damage evolution characteristics of a granitic rock during loading and unloading after a series of triaxial experiments performed at different confining pressures. The axial stress-axial strain variations of the tested specimens revealed that the specimens undergoing unloading fail with a lower axial strain compared to the specimens failed purely by loading. Higher confining pressures were observed to exacerbate the difference. Volumetric strain versus axial strain curves indicated that the curves reverse the trend with the beginning of major damage of specimens. We suggest here a new form of equation to describe the secant modulus variation of brittle rocks against the axial stress for the unloading process. Failure mechanisms of tested specimens showed two distinct patterns, namely, specimens under pure loading failed with a single distinct shear fracture while for the unloading case specimens displayed multiple intersecting fractures. In addition, analysis of the evolution of dissipation and elastic energy during deformation of the specimens under loading and unloading conditions showed differentiable characteristics. Moreover, we evaluated the variations of two damage indices defined based on the energy dissipation and secant modulus evolution during deformation and observed that both of them satisfactorily distinguish key stages of damage evolution.
基金Funded Partly by the National Natural Science Foundation of China(No.51178361)
文摘To investigate the effect of different environmental conditions of GFRP bars in concrete beams with work cracks subjected to sustained loads, the beams were exposed in indoor, freeze/thaw cycles and immersed in alkaline solution at elevated temperature. The bars were carefully extracted from the beams and tested in order to evaluate residual tensile properties. The results show that the tensile strength decreased significantly in the highly aggressive conditions but not in the natural conditions. The effect of GFRP bars casting in concrete beams demonstrated approximately 2.5% decrease of tensile strength caused by pore water environment in concrete beams on basis of those of the original bars. The effect of sustained loading plus work cracks demonstrated about 10.5% tensile strength decrease on basis of those of the bars only casted in concrete beams. The effect of environments under sustained loading plus work cracks demonstrated about 17% tensile strength decrease caused by a saturated solution of Ca（OH）2 and 60-2 ℃ tap water （pH=12-13） and about 8% tensile strength decrease caused by freezing and thawing cycle （F/T）, both on basis of those of the bars of the indoor beams only under sustained loading plus work cracks. The results demonstrate the effects of the tensile strengths under different environmental conditions of GFRP bars in concrete beams with work cracks subjected to sustained loads.
基金The authors would like to thank the editors and the anonymous reviewers for their helpful and constructive comments.This study was supported by National Key Technologies Research&Development Program(Grant No.2018YFC0808402)State Key Laboratory for GeoMechanics and Deep Underground Engineering,China University of Mining and Technology(Grant No.SKLGDUEK1824)the Fundamental Research Funds for the Central Universities(Grant No.FRF-TP-20-004A2).
文摘Rocks in underground works usually experience rather complex stress disturbance.For this,their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions.The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies.However,little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading(FL-CSU)conditions that influence rock failure.In this paper,we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography(CT)scanning technique and the GCTS RTR 2000 rock mechanics system.Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble.The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests,and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests.In FL-CSU tests,as the unloading rate increases,the dissipated energy grows and elastic energy reduces.CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension.It shows that the crack pattern after FL-CSU tests depends on the unloading rate,and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released.The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.
基金supported by the National Natural Science Foundation of China(Nos.52274118 and 52274145)the Construction Project of Chenzhou National Sustainable Development Agenda Innovation Demonstration Zone(No.2021sfQ18).
文摘This paper investigates mechanical behaviours of sandstone during post-peak cyclic loading and unloading subjected to hydromechanical coupling effect, confirming the peak and residual strengths reduction laws of sandstone with water pressure, and revealing the influence of water pressure on the upper limit stress and deformation characteristics of sandstone during post-peak cyclic loading and unloading.Regarding the rock strength, the experimental study confirms that the peak strength σ_(p) and residual strength σ_(r) decrease as water pressure P increases. Especially, the normalized strength parameters σ_(p)/σ_(pk) and σ_(r)/σ_(re) was negatively and linearly correlated with the P/σ_(3). Moreover, the Hoek-Brown strength criterion can be applied to describe the relationship between effective peak strength and effective confining stress. During post-peak cyclic loading and unloading, both the upper limit stress σ_(p(i)) and crack damage threshold stress σ_(cd(i)) of each cycle tend to decrease with the increasing cycle number. A hysteresis loop exists among the loading and unloading stress–strain curves, indicating the unloading deformation modulus E_(unload) is larger than the loading deformation modulus E_(load). Based on experimental results,a post-peak strength prediction model related to water pressure and plastic shear strain is established.
基金the Doctoral Authorization Point Foundation of China(No.30300078)
文摘The functional relation between the residual tensile strength of plain concrete and number of cycles was determined. 99 tappered prism specimens of plain concrete were tested under uniaxial tensile fatigue loading. Based on the probability distribution of the residual tensile strength, the empirical expressions of the residual tensile strength corresponding to the number of cycles were obtained. The residual tensile strength attenuating curves can be used to predict the residual fatigue life of the specimen under variable-amplitude fatigue loading. There is a good correlation between residual tensile strength and residual secant elastic modulus. The relationship between the residual secant elastic modulus and number of cycles was also established.
基金Supported by National Natural Science Foundation of China(No.50974100)WHUT(NO.125106002)
文摘The mechanical properties are essentially different when rock material is subjected to loading or unloading conditions. In this study, loading and unloading tests with various confining pressures are conducted to investigate the mechanical properties of marble material samples taken from the deep diversion tunnels of Jinping II Hydropower Station. The stress-strain relationship, failure characteristics and strength criterion are compared and analyzed based on the experiment results. The results show： in the loading and unloading test, peak strength, lateral strain, axial strain and plastic deformation increase significantly as the confining pressure increases. Lateral strain increased significantly and obvious lateral dilatancy can be observed to the change of confining pressure; The fracture mode is mainly the single shear fracture for the triaxial compression test and post-peak test, angle between the failure surface and the ends of the rock material becomes smaller as the confining pressure increases. Hock-Brown strength criterion reflects the strength characteristics of marble material under two different unloading conditions, and has some supplementary effects to the rock material of mechanical field.
基金Project was supported by National Defense Basic Scientific Research Program of China (JCKY2016205A001)。
文摘By measuring and analyzing infrared thermal image of the specimen in static load tensile test process, it was studied that the influence of the undercut defects and double-sided dressing method on the deformation behavior of the laser welded joint specimens of TC4 titanium alloy. The results showed that for the unmodified specimens, the yield phenomenon occurs first in the region of the joint, but the undercut value has an effect on the stress and strain of starting to yield phenomenon, and a great effect on the plastic deformation behavior.When the undercut is less than a certain value, the large plastic deformation occurs in the base metal region and the plasticity of the specimen is comparable to that of the base metal, but the larger undercut defect results in a concentrated plastic deformation in the joint region and rapidly failed in this region. But the double-sided dressing specimen is significantly different. The physical yield is no longer concentrated in the joint region, but at the same time occurs in the several regions including joint and the base metal. And the plastic deformation mainly occurs in the base material area, similar to that of the base material.
基金This work was financially supported by the National Natural Science Foundation of China(No.52074041)the Chongqing Talent Program(No.cstc2022ycjh-bgzxm0077)the Postgraduate Research and Innovation Foundation of Chongqing,China(No.CYS23060).
文摘In coal mining,rock strata are fractured under cyclic loading and unloading to form fracture channels.Fracture channels are the main flow narrows for gas.Therefore,expounding the flow conductivity of fracture channels in rocks on fluids is significant for gas flow in rock strata.In this regard,graded incremental cyclic loading and unloading experiments were conducted on sandstones with different initial stress levels.Then,the three-dimensional models for fracture channels in sandstones were established.Finally,the fracture channel percentages were used to reflect the flow conductivity of fracture channels.The study revealed how the particle size distribution of fractured sandstone affects the formation and expansion of fracture channels.It was found that a smaller proportion of large blocks and a higher proportion of small blocks after sandstone fails contribute more to the formation of fracture channels.The proportion of fracture channels in fractured rock can indicate the flow conductivity of those channels.When the proportion of fracture channels varies gently,fluids flow evenly through those channels.However,if the proportion of fracture channels varies significantly,it can greatly affect the flow rate of fluids.The research results contribute to revealing the morphological evolution and flow conductivity of fracture channels in sandstone and then provide a theoretical basis for clarifying the gas flow pattern in the rock strata of coal mines.
基金financially supported by the National Natural Science Foundation of China (Grant No. 41430634)the State Key Laboratory of Geomechanics and Geotechnical Engineering (Grant No. Y11002Q02)
文摘During the constructions of motorways and high-speed railway lines in the Yanji Basin,large amounts of excess mudstones due to the enormous tunnel excavations and slope cuts would be deposited as landfills.Assessing the deformation and permeability of Yanji mudstone became important for the design,construction and operation of the landfills.This paper presents an experimental study on the deformation and permeability of Yanji mudstone by carrying out a series of oedometer tests with loading/unloading cycles.The results show that the sample with a lower initial water content exhibited greater swelling deformation after inundation,a lower yield stress,greater deformation and a higher hydraulic conductivity during the loading/unloading cycles.As the number of loading/unloading cycles increased,the yield stress and accumulated plastic deformation increased,while the compression index,rebound index and hydraulic conductivity decreased.The samples became stiffer and their hydromechanical behaviour tended to be stable after three cycles.The compression curves could be divided into pre-yield and post-yield zones.The post-yield zones of compression curves and the rebound curves could be normalized into a unique line,and the pre-yield zones of the compression curves could be described as lines.Basic equations were developed to predict mudstone deformation under cyclic loading and unloading.Additionally,an empirical relationship between the hydraulic conductivity and void ratio was also proposed.The ability of the proposed methods was verified by the overall good agreement between the experimental results and predicted values.
基金supported by the National Natural Science Foundation of China(Grant No.52109135)the Key R&D Projects of Sichuan Province,China(Grant No.2022YFSY0007)the Postdoctoral Research Foundation of China(Grant No.2019M653402).
文摘A reasonable evaluation of unloading deformation characteristics is of great significance for the effective analysis of deformation and stability of surrounding rocks after underground excavation.In this study,the damage-controlled cyclic triaxial loading tests were conducted to investigate the pore compaction mechanism and its influences on the unloading deformation behavior of red sandstone,including Young’s modulus,Poisson’s ratio,volumetric strain,and irreversible strain.The experimental results show that the increases of volumetric and irreversible strains of rocks can be attributed to the compaction mechanism,which almost dominates the entire pre-peak deformation process.The unloading deformation consists of the reversible linear and nonlinear strains,and the irreversible strain under the influence of the porous grain structure.The pre-peak Young’s modulus tends to increase and then decrease due to the influence of the unloading irreversible strain.However,it hardly changes with the increasing volumetric strain compaction under the influence of reversible nonlinear strain.Instead,the initial unloading tangent modulus is highly related to the volumetric strain,and clearly reflects the compaction state of red sandstone.Furthermore,both the reversible nonlinear and irreversible unloading deformations are independent of confining pressure.This study is beneficial for the theoretical modeling and prediction of cyclic unloading deformation behavior of red sandstone.
