In the mining process of deep metal mines,diff erent types of rock mass instability failures are caused by strong mining disturbance.It is beneficial to master the fracture mechanism of rock mass in time to effectivel...In the mining process of deep metal mines,diff erent types of rock mass instability failures are caused by strong mining disturbance.It is beneficial to master the fracture mechanism of rock mass in time to effectively prevent and control the ground pressure disasters.Microseismic signals are generated by the propagation and expansion of cracks inside the rock mass that contain plentiful information about the structural changes of rock mass.The ratio of the radiated energy of S and P waves(Es/Ep)of microseismic events can fast and eff ectively calculate the rock fracture mechanism,which is widely used for ground pressure hazard risk assessment.In this paper,this method was used to analyze the fracture mechanism of rock mass around deep stope in Hongtoushan copper mine and Ashele copper mine.Furthermore,the spatial distribution characteristics and proportion changes of microseismic events with diff erent fracture mechanisms along with the mining process were studied.The results show that tensile cracks play a dominant role,accounting for 62%of the total events,during non-shear fracturing of the rock mass caused by the stoping unloading eff ect,while shear cracks occupy 68%of the total events during orebody slip failure.When the physical and mechanical properties of the orebody and rock mass are signifi cantly diff erent,slip failure along their contact zone is prone to occur under blasting disturbance.During deep mining,it is necessary to control the exposed area of the roof by each stoping,especially during the earlier mining stage,to avoid tensile stress concentration.The temporal and spatial variation of tension cracks and shear cracks induced by roof damage obtained in this paper can guide the prevention and control of ground pressure disasters in deep mining eff ectively.展开更多
With the increasing demand for energy,traditional oil resources are facing depletion and insufficient supply.Many countries are rapidly turning to the development of unconventional oil and gas resources.Among them,sha...With the increasing demand for energy,traditional oil resources are facing depletion and insufficient supply.Many countries are rapidly turning to the development of unconventional oil and gas resources.Among them,shale oil and gas reservoirs have become the focus of unconventional oil and gas resources exploration and development.Based on the characteristics of shale oil and gas reservoirs,supercritical CO_(2) fracturing is more conducive to improving oil recovery than other fracturing technologies.In this paper,the mechanism of fracture initiation and propagation of supercritical CO_(2) in shale is analyzed,including viscosity effect,surface tension effect,permeation diffusion effect of supercritical CO_(2),and dissolution-adsorption effect between CO_(2) and shale.The effects of natural factors,such as shale properties,bedding plane and natural fractures,and controllable factors,proppant,temperature,pressure,CO_(2) concentration and injection rate on fracture initiation and propagation are clarified.The methods of supercritical CO_(2) fracturing process,thickener and proppant optimization to improve the efficiency of supercritical CO_(2) fracturing are discussed.In addition,some new technologies of supercritical CO_(2) fracturing are introduced.The challenges and prospects in the current research are also summarized.For example,supercritical CO_(2) is prone to filtration when passing through porous media,and it is difficult to form a stable flow state.Therefore,in order to achieve stable fracturing fluid suspension and effectively support fractu res,it is urge nt to explo re new fracturing fluid additives or improve fracturing fluid formulations combined with the research of new proppants.This paper is of great significance for understanding the behavior mechanism of supercritical CO_(2) in shale and optimizing fracturing technology.展开更多
Conglomerate rock's complex and heterogeneous microstructure significantly affects its mechanical properties,especially under dynamic loading.However,research on their dynamic behavior and fracture mechanisms is l...Conglomerate rock's complex and heterogeneous microstructure significantly affects its mechanical properties,especially under dynamic loading.However,research on their dynamic behavior and fracture mechanisms is limited.Through uniaxial compression tests and split Hopkinson pressure bar(SHPB)impact tests,the dynamic compressive mechanical properties and fracture mechanisms of conglomerate rock were studied.Nanoindentation and high-resolution X-ray computed tomography were employed to analyze the micro-mechanical behavior and internal structure of the conglomerate rock.Results indicate significant differences in mechanical properties between different gravel particles and cementing materials,with initial fractures primarily distributed at the gravel-cement interfaces.The dynamic mechanical properties of conglomerate rocks exhibit a clear strain rate dependency.Based on the stress−strain curves and failure characteristics,the dynamic compressive mechanical behavior can be categorized into two types using a critical strain rate.The dynamic compressive strength,peak strain,and toughness of conglomerate rock increased with the strain rate,with the strength at 54 s−1 being 2.6 times that at 6 s−1.The dynamic compressive fracture mechanism of conglomerate rock is related to the strain rate and microstructure;at low strain rates,gravel distribution is the key factor,whereas at high strain rates,gravel content becomes critical.展开更多
The mechanism of grain fracturing in a zirconia metering nozzle used in the continuous casting process was studied. The phase composition, microstructure, and chemical composition of the residual samples were studied ...The mechanism of grain fracturing in a zirconia metering nozzle used in the continuous casting process was studied. The phase composition, microstructure, and chemical composition of the residual samples were studied using an X-ray fluorescence analyzer, scanning electron microscope, and electron probe. Results revealed that the composition, structure, and mineral phase of the original layer, transition layer, and affected layer of the metering nozzle differed because of stabilizer precipitation and steel slag permeation. The stabilizer MgO formed low-melting phases with steel slag and impure SiO2 on the boundaries（pores） of zirconia grains; consequently, grain fracturing occurred and accelerated damage to the metering nozzle was observed.展开更多
The mainstream method for extracting shale gas involves hydraulic fracturing to create fracture networks.However,as extraction depth increases,notable issues such as rapid production decline,low recovery rates,high wa...The mainstream method for extracting shale gas involves hydraulic fracturing to create fracture networks.However,as extraction depth increases,notable issues such as rapid production decline,low recovery rates,high water consumption,and resource waste become apparent.Identifying new and efficient auxiliary rock-breaking technologies is crucial for overcoming these challenges.The laser,successfully utilized in industrial production,medical treatment,and technological research,offers unique features such as good directionality,coherence,and high energy density,providing novel possibilities for addressing the limitations of existing deep reservoir transformation.This research focuses on a novel laser-assisted rock-breaking technology,with shale featuring different bedding angles as the subject of investigation.The investigation methodically explored how shale responded to thermal fracture at high temperatures when exposed to laser irradiation with different spot diameter.It investigates the spatiotemporal evolution characteristics of the shale temperature field under laser irradiation,the propagation features of cracks on shale surface,and the physicochemical fracture mechanisms.The research yields the following results:(1)The region of thermal influence of the irradiation surface can be divided into three regions based on the change of rise curve of temperature in the shale surface.(2)Based on the scanning electron microscopy(SEM)testing,combined with the macroscopic and microscopic morphological characteristics of shale fracture surfaces,it reveals significantly distinct zoning characteristics in the roughness of the rock sample’s fracture surfaces after laser irradiation.(3)The thermal fracturing process of shale under laser irradiation involves chemical reactions of constituent minerals and stress generated by the thermal expansion of shale oil in the reservoir.(4)The damage and fracture of shale under the irradiation of laser show significant bedding effect,and there are three modes of rock sample failure:Pattern T(thermal failure),Pattern T-B(thermal and bedding synergistic failure),and Pattern B(bedding failure).The research findings presented in this article serve as a foundation and reference for the theory and technology of laser-assisted shale gas extraction.展开更多
The fracture mechanisms of coarse-grained heat-affected zone(CGHAZ)for Mg and Mg–Ca deoxidized high-strength low-alloy(HSLA)steels after high heat input welding(HHIW)were investigated based on the microstructures,cra...The fracture mechanisms of coarse-grained heat-affected zone(CGHAZ)for Mg and Mg–Ca deoxidized high-strength low-alloy(HSLA)steels after high heat input welding(HHIW)were investigated based on the microstructures,crack behaviors and mechanical properties.Compared to Mg–Ca steel,the proportion of intergranular acicular ferrites(IAFs)and polygonal ferrites(PFs)in Mg steel increases from 59.97%to 90.16%.The high-angle grain boundaries(HAGBs)and geometrically necessary dislocations density increase from 55.5%and 4.30×10^(14) m^(-2)to 70.4%and 5.48×10^(14) m^(–2),respectively,while effective grain size decreases from 9.46 to 8.12μm.The area fraction of radial zone in Mg steel decreases from 80.8%to 37.7%and cleavage plane is smaller with more curved and finer tearing ridges.The inclusions distributed at the center of cleavage planes and along river lines can serve as crack initiation sites.The zigzag pattern of primary crack propagation path has width of 476μm and the length of secondary cracks remains below 10μm.These cracks are deflected or arrested by IAFs,PFs and HAGBs,and tend to propagate along{110}plane family.These factors contribute to superior overall mechanical properties of Mg steel,especially increasing low-temperature impact toughness from 23 to 175 J.展开更多
The advent of coarse-grain superplasticity has provided a pathway for novel applications in material forming.This article investigated the underlying deformation mechanisms that enabled achieving superplastic elongati...The advent of coarse-grain superplasticity has provided a pathway for novel applications in material forming.This article investigated the underlying deformation mechanisms that enabled achieving superplastic elongation exceeding 230%in a coarse-grained Ni-Co-based superalloy.The deformed microstructure and fractographic characteristics of the alloy were examined utilizing optical microscopy(OM),scanning electron microscopy(SEM),and electron backscatter diffraction(EBSD).The results of the analysis revealed that below 1100℃,the process of dynamic recrystallization(DRX)occurred at a sluggish rate,resulting in low plasticity and the initiation of severe cracks.Complete DRX occurred when the deformation temperature exceeded 1100℃,leading to a more uniformly deformed microstructure,reduced crack initiation,and enhanced ductility demonstrated by elongation to failure surpassing 230%.The augmented occurrence of the DRX facilitated prolonged plastic-forming periods,which delayed fracture propagation and promoted the deformation flow within the alloy,thereby transitioning the fracture behavior from intergranular-brittle at 1050℃to ductile intergranular at 1140℃.At this temperature,the deformation was predominantly governed by the discontinuous-DRX(DDRX)mechanism and grain growth,facilitated by the formation of twin boundaries.展开更多
The effects of accumulative hot rolling followed by solution treatment on the microstructural evolution and fracture behavior of 30CrMo/316L multilayered composites have been investigated.A scanning electron microscop...The effects of accumulative hot rolling followed by solution treatment on the microstructural evolution and fracture behavior of 30CrMo/316L multilayered composites have been investigated.A scanning electron microscope equipped with an electron backscatter diffraction probe,a laser confocal microscope,an electron probe microanalysis,and a universal testing machine were employed to characterize the microstructures and mechanical properties.The results indicate that solution treatment transformed the microstructure of the 30CrMo layer from ferrite to martensite,while the 316L layer remained austenitic but transitioned from the rolled to the recrystallized state.Additionally,solution treatment significantly enhanced the mechanical properties of the composite,leading to an increase in yield strength and ultimate tensile strength to 744 and 1106 MPa,respectively—258 and 276 MPa higher than those of the hot-rolled plate.The enhancement in strength is primarily attributed to the formation of high-strength martensite in the 30CrMo layer.During deformation,the composite interface effectively impeded crack propagation and induced step-like deflection.However,the formation of cross-layer grains facilitated crack nucleation at grain boundaries,leading to rapid crack propagation and instantaneous fracture.Therefore,preventing the formation of cross-layer grains during the heat treatment process is crucial,as their presence weakens the interfacial strengthening effect of the composite plate.This study provides valuable insights for the design and development of multi-layered steels.展开更多
The energy-focusing blast is an innovative and ingenious method to achieve directional fracturing.Understanding its energy regulation mechanism is critical to enhancing its practical effectiveness.This study investiga...The energy-focusing blast is an innovative and ingenious method to achieve directional fracturing.Understanding its energy regulation mechanism is critical to enhancing its practical effectiveness.This study investigates the energy regulation mechanism and explores the medium-filling effects within the energy-focusing blast by employing theoretical analysis,numerical simulations,and model tests.The findings by theoretical and numerical analysis first reveal that two stages of the fracturing and tensile stage govern the directionally crack propagation,in which the explosion energy in the non-energyfocusing direction is suppressed,compressing the borehole wall,while redirected energy produces tensile stress in the energy-focusing direction,driving the formation of directional cracks.The choice of filling medium significantly affects directional cracking due to its impact on energy distribution and regulation,and key properties such as wave impedance and compressibility of the filling medium are critical.Experimental comparisons using air,sand,and water as filling media further disclose the distinct effects of the medium on energy regulation and directional crack growth of the energy-focusing blast.The maximum shaped-energy coefficients for air,sand,and water are 1.30,4.41,and 6.12 in the energy-focusing direction,respectively.Meanwhile,the stress attenuation rate of air,sand,and water increases in that order.The higher wave impedance and lower compressibility of water support efficient and uniform energy propagation,which subtly enhances the tensile actions in the focusing direction and intensifies the overall stress impact of the energy-focusing blast.In addition,the stresses in the non-energyfocusing directions decrease as the angle from the energy-focusing direction increases,while the stresses are relatively uniform for both air and water but noticeably uneven for sand;meanwhile,the fractal dimensions of blasting cracks in the case of air,water,and sand are 1.076,1.068,and 1.112,respectively.Sand as a filling medium leads to increased crack irregularities due to its granularity and heterogeneity.The water medium strikes an optimal balance by promoting the blasting energy transition and optimizing the energy distribution,maintaining the least flatness of the directional crack during energy-focusing blasts.展开更多
C/C composites with banded structure pyrocarbon were fabricated by fast chemical vapor infiltration（CVI）,with C3H6 as carbon source,N2 as carrier gas,and three-dimensional（3D） 12K PAN-based carbon fabric with high...C/C composites with banded structure pyrocarbon were fabricated by fast chemical vapor infiltration（CVI）,with C3H6 as carbon source,N2 as carrier gas,and three-dimensional（3D） 12K PAN-based carbon fabric with high density of 0.94 g/cm3 as preform.Experimental results indicated that the fracture characteristics of C/C composites were closely related to the frequency of high-temperature treatment（HTT） at the break of CVI process.According to the load？displacement curves,C/C composites showed a pseudoplastic fracture after twice of HTT.After three times of HTT,load？displacement curves tended to be stable with a decreasing bending strength at 177.5 MPa.Delamination failure and intrastratal fiber fracture were observed at the cross-section of C/C composites by scanning electronic microscope.Because the content of pyrocarbon and fibers has a different distribution in layers,the C/C composites show different fracture characteristics at various regions,which leads to good toughness and bending strength.展开更多
Coalbed gas extraction is an important means of exploiting and utilizing gas resources,as well as a means of preventing coal mine disasters.In view of the low gas extraction rate from coalbeds with high gas content an...Coalbed gas extraction is an important means of exploiting and utilizing gas resources,as well as a means of preventing coal mine disasters.In view of the low gas extraction rate from coalbeds with high gas content and low permeability,a method of improving permeability through deep-hole cumulative blasting is applied to develop initial directional fractures using a jet flow.Under the action of the blasting stress wave and detonation gas wedge,the fractures extend over a large range within the coal,thereby improving coalbed permeability.This study focuses on the criteria of cumulative blasting-induced coalbed fracturing based on a literature review of the penetration effect of cumulative blasting.On this basis,we summarize the coal fracturing zone,crack extension process,and the key technologies of charging and hole sealing for cumulative blasting.In addition,the latest research progress in the optimization of field test drilling and blasting parameters for cumulative blasting is introduced.Research findings indicate that the permeability improvement mechanism of cumulative blasting could be further enhanced,and the technology and technical equipment are in urgent need of improvement.Finally,development trends in the cumulative blasting permeability improvement technique are identified.展开更多
In this study,servo-controlled biaxial compression tests were conducted on marble specimens to investigate their failure characteristics and fracturing process.The complete stressestrain curves were obtained,and the t...In this study,servo-controlled biaxial compression tests were conducted on marble specimens to investigate their failure characteristics and fracturing process.The complete stressestrain curves were obtained,and the three-dimensional(3D)features of the failure surfaces were acquired by 3D laser scanning.Acoustic emission(AE)monitoring and moment tensor(MT)analysis were used in combination to better understand the fracturing mechanism of marble under biaxial compression.It was noted that a type of 3D stepwise cracking behaviour occurred on the fracturing surfaces of the examined specimens.The stress dropped multiple times,and a repeated fracturing mode corresponding to the repeated stress drops in the post-peak regime was observed.Three substages,i.e.stress stabilisation,stress decrease and stress increase,were identified for a single fracturing mode.Then quantitative and statistical analyses of the fracturing process at each substage were discussed.Based on the testing results,it was found that at the stress stabilisation substage,the proportion of mixed-mode fractures increased.At the stress decrease substage,the proportion of mixed-mode fractures decreased,and the tensile or shear fractures increased.At the stress increase substage,the proportion of mixed-mode or tensile fractures decreased,and the shear fractures increased.Finally,a conceptual model for the stepwise crack formation was proposed.展开更多
Understanding the temperature dependent deformation behavior of Mg alloys is crucial for their expanding use in the aerospace sector.This study investigates the deformation mechanisms of hot-rolled AZ61 Mg alloy under...Understanding the temperature dependent deformation behavior of Mg alloys is crucial for their expanding use in the aerospace sector.This study investigates the deformation mechanisms of hot-rolled AZ61 Mg alloy under uniaxial tension along rolling direction(RD)and transverse direction(TD)at-50,25,50,and 150℃.Results reveal a transition from high strength with limited elongation at-50℃ to significant softening and maximum ductility at 150℃.TD samples consistently showed 2%-6%higher strength than RD;however,this yield anisotropy diminished at 150℃ due to the shift from twinning to thermally activated slip and recovery.Fractography indicated a change from semi-brittle to fully ductile fracture with increasing temperature.Electron backscattered diffraction(EBSD)analysis confirmed twinning-driven grain refinement at low temperatures,while deformation at high temperatures involved grain elongation along shear zones,enabling greater strain accommodation before material failure.展开更多
The stability of rock slopes is frequently controlled by the initiation and propagation of inherent dominant cracks.This study systematically investigated these processes in valley slopes by combining fracture-mechani...The stability of rock slopes is frequently controlled by the initiation and propagation of inherent dominant cracks.This study systematically investigated these processes in valley slopes by combining fracture-mechanics analysis with transparent soil model tests.An analytical expression for the stress field at the dominant crack tip was derived from the slope stress distribution by superposing the corresponding stress intensity factors(SIFs).The theoretical predictions were then validated against observations from transparent soil model tests.The influences of slope angle(β),crack inclination angle(α),crack position parameter(b),and crack length parameter(h)on crack initiation and propagation were quantified.The results indicated that:(1)cracks at the slope crest tended to propagate in shear mode,and the shear crack initiation angle(θ_(s))was approximately 8°.Cracks at the slope toe might propagate in either tensile or shear mode.(2)θ_(s) at the slope crest increased withβ,b,and l,and decreased withα.The maximum change inθ_(s) induced by the considered parameters was approximately 30°.(3)The tensile crack initiation angle(θ_(t))at the slop toe decreased withβ,α,and l,while the influence of b was comparatively minor.The maximum change inθ_(t) caused by individual parameters ranged approximately from 25°to 60°.Predicted crack propagation modes and directions showed good agreement with experimental results.These findings provide theoretical guidance for stability assessments of valley slopes controlled by dominant crack propagation.展开更多
X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiatio...X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiation, propagation and subsequent fracture of specimen. Results showed that four types of porosi- ties, including gas-shrinkage pore, gas pore, net-shrinkage and island-shrinkage, could be identified according to the formation mechanism and morphology characterization. During tensile deformation, it was shown that the gas-shrinkage pore and net-shrinkage, rather than gas pore or island-shrinkage, were the main sources for crack initiation. In addition, the crack propagated by interconnecting the po- rosities at the cross section with minimum efficient force bearing area. At these locations where externally solidified crystals （ESCs） were present, the crack would propagate along the ESC boundaries in an inter- granular mode, while at these locations without ESCs, the crack would propagate roughly along the direction perpendicular to the tensile stress in a combination of trans-granular and inter-granular modes.展开更多
A comprehensive understanding of the failure behavior and mechanism of coal is a prerequisite for dealing with dynamic problems in mining space.In this study,the failure behavior and mechanism of coal under uniaxial d...A comprehensive understanding of the failure behavior and mechanism of coal is a prerequisite for dealing with dynamic problems in mining space.In this study,the failure behavior and mechanism of coal under uniaxial dynamic compressive loads were experimentally and numerically investigated.The experiments were conducted using a split Hopkinson pressure bar(SHPB)system.The results indicated that the typical failure of coal is lateral and axial at lower loading rates and totally smashed at higher loading rates.The further fractography analysis of lateral and axial fracture fragments indicated that the coal failure under dynamic compressive load is caused by tensile brittle fracture.In addition,the typical failure modes of coal under dynamic load were numerically reproduced.The numerical results indicated that the axial fracture is caused directly by the incident compressive stress wave and the lateral fracture is caused by the tensile stress wave reflected from the interface between coal specimen and transmitted bar.Potential application was further conducted to interpret dynamic problems in underground coal mine and it manifested that the lateral and axial fractures of coal constitute the parallel cracks in the coal mass under roof fall and blasting in mining space.展开更多
High energy gas fracturing is a simple approach of applying high pressure gas to stimulate wells by gen- erating several radial cracks without creating any other damages to the wells. In this paper, a numerical algori...High energy gas fracturing is a simple approach of applying high pressure gas to stimulate wells by gen- erating several radial cracks without creating any other damages to the wells. In this paper, a numerical algorithm is proposed to quantitatively simulate propagation of these fractures around a pressurized hole as a quasi-static phenomenon. The gas flow through the cracks is assumed as a one-dimensional transient flow, governed by equations of conservation of mass and momentum. The fractured medium is modeled with the extended finite element method, and the stress intensity factor is calculated by the simple, though sufficiently accurate, displacement ex- trapolation method. To evaluate the proposed algorithm, two field tests are simulated and the unknown parameters are determined through calibration. Sensitivity analyses are performed on the main effective parameters. Considering that the level of uncertainty is very high in these types of engineering problems, the results show a good agreement with the experimental data. They are also consistent with the theory that the final crack length is mainly determined by the gas pressure rather than the initial crack length produced by the stress waves.展开更多
MnFeCoCuNix high-entropy alloys(HEAs)with different Ni contents were fabricated by vacuum induction melting.XRD and SEM−EDS were used to analyze the phase constitution and structure,and the tensile properties of the s...MnFeCoCuNix high-entropy alloys(HEAs)with different Ni contents were fabricated by vacuum induction melting.XRD and SEM−EDS were used to analyze the phase constitution and structure,and the tensile properties of the samples were determined using a universal tensile tester.The results show that the HEAs consist of a dual-phase structure,in which FCC1 phase is rich in Fe and Co,while the FCC2 phase has high contents of Cu and Mn.As Ni content increases,the segregation of Cu decreases,accompanied by the decrease of FCC2 phase.Moreover,the tensile strength of the HEAs increases first and then decreases,and the elongation increases slightly.This is attributed to the combined effect of interface strengthening and solid solution strengthening.The in-situ stretched MnFeCoCuNi0.5 alloy shows obvious neck shrinkage during the tensile fracture process.In the initial deformation stage,the slip lines show different morphologies in the dual-phase structure.However,in the later stage,the surface slip lines become longer and denser due to the redistribution of atoms and the re-separation of the dissolved phase.展开更多
Similar to hydraulic fracturing(HF), the coalescence and fracture of cracks are induced within a rock under the action of an ultrasonic field, known as ultrasonic fracturing(UF). Investigating UF is important in both ...Similar to hydraulic fracturing(HF), the coalescence and fracture of cracks are induced within a rock under the action of an ultrasonic field, known as ultrasonic fracturing(UF). Investigating UF is important in both hard rock drilling and oil and gas recovery. A three-dimensional internal laser-engraved crack(3D-ILC) method was introduced to prefabricate two parallel internal cracks within the samples without any damage to the surface. The samples were subjected to UF. The mechanism of UF was elucidated by analyzing the characteristics of fracture surfaces. The crack propagation path under different ultrasonic parameters was obtained by numerical simulation based on the Paris fatigue model and compared to the experimental results of UF. The results show that the 3D-ILC method is a powerful tool for UF research.Under the action of an ultrasonic field, the fracture surface shows the characteristics of beach marks and contains powder locally, indicating that the UF mechanism includes high-cycle fatigue fracture, shear and friction, and temperature load. The two internal cracks become close under UF. The numerical result obtained by the Paris fatigue model also shows the attraction of the two cracks, consistent with the test results. The 3D-ILC method provides a new tool for the experimental study of UF. Compared to the conventional numerical methods based on the analysis of stress-strain and plastic zone, numerical simulation can be a good alternative method to obtain the crack path under UF.展开更多
Fatigue performance of hot-rolled ribbed-steel bar with the yield strength of 500 MPa （HRB500） was stud- ied with bend-rotating fatigue test at a stress ratio of R = -1. It is determined by staircase method that its...Fatigue performance of hot-rolled ribbed-steel bar with the yield strength of 500 MPa （HRB500） was stud- ied with bend-rotating fatigue test at a stress ratio of R = -1. It is determined by staircase method that its fatigue strength for 107 cycles is 451 MPa, which is higher than that of common carbon structural steel. This should be at- tributed to the fine-grain strengthening resulting from the high content of alloy element V and Thermo-Mechanical Control Process （TMCP）. The S-N curve function is also obtained by nonlinear regression with three parameters power function. The fatigue fractures of the specimen were further analyzed with Scanning Electron Microscopy （SEM） and Energy Disperse Spectroscopy （EDS） to study the fracture mechanism. Taking into account microstruc- ture, hardness and cleanliness of the material, it implies that the fatigue fractures of HRB500 rebar all arise from surface substrates in which many brittle inclusions are contained, and that the fatigue crack propagation is principally based on the mechanism of quasi-cleavage fracture, because of the intracrystalline hard spots leading to stress con- centration and thus to the cracks. Moreover, the transient breaking area exhibits microvoid coalescence of ductile fracture due to the existing abundant inclusions.展开更多
基金the National Key Research and Development Program of China(2017YFC0602904)the National Natural Science Foundation of China(51974059)the Fundamental Research Funds for the Central Universities(N180115010).
文摘In the mining process of deep metal mines,diff erent types of rock mass instability failures are caused by strong mining disturbance.It is beneficial to master the fracture mechanism of rock mass in time to effectively prevent and control the ground pressure disasters.Microseismic signals are generated by the propagation and expansion of cracks inside the rock mass that contain plentiful information about the structural changes of rock mass.The ratio of the radiated energy of S and P waves(Es/Ep)of microseismic events can fast and eff ectively calculate the rock fracture mechanism,which is widely used for ground pressure hazard risk assessment.In this paper,this method was used to analyze the fracture mechanism of rock mass around deep stope in Hongtoushan copper mine and Ashele copper mine.Furthermore,the spatial distribution characteristics and proportion changes of microseismic events with diff erent fracture mechanisms along with the mining process were studied.The results show that tensile cracks play a dominant role,accounting for 62%of the total events,during non-shear fracturing of the rock mass caused by the stoping unloading eff ect,while shear cracks occupy 68%of the total events during orebody slip failure.When the physical and mechanical properties of the orebody and rock mass are signifi cantly diff erent,slip failure along their contact zone is prone to occur under blasting disturbance.During deep mining,it is necessary to control the exposed area of the roof by each stoping,especially during the earlier mining stage,to avoid tensile stress concentration.The temporal and spatial variation of tension cracks and shear cracks induced by roof damage obtained in this paper can guide the prevention and control of ground pressure disasters in deep mining eff ectively.
文摘With the increasing demand for energy,traditional oil resources are facing depletion and insufficient supply.Many countries are rapidly turning to the development of unconventional oil and gas resources.Among them,shale oil and gas reservoirs have become the focus of unconventional oil and gas resources exploration and development.Based on the characteristics of shale oil and gas reservoirs,supercritical CO_(2) fracturing is more conducive to improving oil recovery than other fracturing technologies.In this paper,the mechanism of fracture initiation and propagation of supercritical CO_(2) in shale is analyzed,including viscosity effect,surface tension effect,permeation diffusion effect of supercritical CO_(2),and dissolution-adsorption effect between CO_(2) and shale.The effects of natural factors,such as shale properties,bedding plane and natural fractures,and controllable factors,proppant,temperature,pressure,CO_(2) concentration and injection rate on fracture initiation and propagation are clarified.The methods of supercritical CO_(2) fracturing process,thickener and proppant optimization to improve the efficiency of supercritical CO_(2) fracturing are discussed.In addition,some new technologies of supercritical CO_(2) fracturing are introduced.The challenges and prospects in the current research are also summarized.For example,supercritical CO_(2) is prone to filtration when passing through porous media,and it is difficult to form a stable flow state.Therefore,in order to achieve stable fracturing fluid suspension and effectively support fractu res,it is urge nt to explo re new fracturing fluid additives or improve fracturing fluid formulations combined with the research of new proppants.This paper is of great significance for understanding the behavior mechanism of supercritical CO_(2) in shale and optimizing fracturing technology.
基金Project(51978674)supported by the National Natural Science Foundation of China。
文摘Conglomerate rock's complex and heterogeneous microstructure significantly affects its mechanical properties,especially under dynamic loading.However,research on their dynamic behavior and fracture mechanisms is limited.Through uniaxial compression tests and split Hopkinson pressure bar(SHPB)impact tests,the dynamic compressive mechanical properties and fracture mechanisms of conglomerate rock were studied.Nanoindentation and high-resolution X-ray computed tomography were employed to analyze the micro-mechanical behavior and internal structure of the conglomerate rock.Results indicate significant differences in mechanical properties between different gravel particles and cementing materials,with initial fractures primarily distributed at the gravel-cement interfaces.The dynamic mechanical properties of conglomerate rocks exhibit a clear strain rate dependency.Based on the stress−strain curves and failure characteristics,the dynamic compressive mechanical behavior can be categorized into two types using a critical strain rate.The dynamic compressive strength,peak strain,and toughness of conglomerate rock increased with the strain rate,with the strength at 54 s−1 being 2.6 times that at 6 s−1.The dynamic compressive fracture mechanism of conglomerate rock is related to the strain rate and microstructure;at low strain rates,gravel distribution is the key factor,whereas at high strain rates,gravel content becomes critical.
基金funded by the National Natural Science Foundation of China(51372193)the Natural Science Basic Research Plan in the Shaanxi Province of China(2014JM6224)
文摘The mechanism of grain fracturing in a zirconia metering nozzle used in the continuous casting process was studied. The phase composition, microstructure, and chemical composition of the residual samples were studied using an X-ray fluorescence analyzer, scanning electron microscope, and electron probe. Results revealed that the composition, structure, and mineral phase of the original layer, transition layer, and affected layer of the metering nozzle differed because of stabilizer precipitation and steel slag permeation. The stabilizer MgO formed low-melting phases with steel slag and impure SiO2 on the boundaries（pores） of zirconia grains; consequently, grain fracturing occurred and accelerated damage to the metering nozzle was observed.
基金supported by the Sichuan Science and Technology Program of China(2024NSFSC0097,2023NSFSC0004)the National Natural Science Foundation of China(42377143,52225403).
文摘The mainstream method for extracting shale gas involves hydraulic fracturing to create fracture networks.However,as extraction depth increases,notable issues such as rapid production decline,low recovery rates,high water consumption,and resource waste become apparent.Identifying new and efficient auxiliary rock-breaking technologies is crucial for overcoming these challenges.The laser,successfully utilized in industrial production,medical treatment,and technological research,offers unique features such as good directionality,coherence,and high energy density,providing novel possibilities for addressing the limitations of existing deep reservoir transformation.This research focuses on a novel laser-assisted rock-breaking technology,with shale featuring different bedding angles as the subject of investigation.The investigation methodically explored how shale responded to thermal fracture at high temperatures when exposed to laser irradiation with different spot diameter.It investigates the spatiotemporal evolution characteristics of the shale temperature field under laser irradiation,the propagation features of cracks on shale surface,and the physicochemical fracture mechanisms.The research yields the following results:(1)The region of thermal influence of the irradiation surface can be divided into three regions based on the change of rise curve of temperature in the shale surface.(2)Based on the scanning electron microscopy(SEM)testing,combined with the macroscopic and microscopic morphological characteristics of shale fracture surfaces,it reveals significantly distinct zoning characteristics in the roughness of the rock sample’s fracture surfaces after laser irradiation.(3)The thermal fracturing process of shale under laser irradiation involves chemical reactions of constituent minerals and stress generated by the thermal expansion of shale oil in the reservoir.(4)The damage and fracture of shale under the irradiation of laser show significant bedding effect,and there are three modes of rock sample failure:Pattern T(thermal failure),Pattern T-B(thermal and bedding synergistic failure),and Pattern B(bedding failure).The research findings presented in this article serve as a foundation and reference for the theory and technology of laser-assisted shale gas extraction.
基金financial support by the National Natural Science Foundation of China(No.52474361)the Independent Research Project of State Key Laboratory of Advanced Special Steel,Shanghai Key Laboratory of Advanced Ferrometallurgy,Shanghai University(No.SKLASS 2023-Z01)the Science and Technology Commission of Shanghai Municipality(No.19DZ2270200).
文摘The fracture mechanisms of coarse-grained heat-affected zone(CGHAZ)for Mg and Mg–Ca deoxidized high-strength low-alloy(HSLA)steels after high heat input welding(HHIW)were investigated based on the microstructures,crack behaviors and mechanical properties.Compared to Mg–Ca steel,the proportion of intergranular acicular ferrites(IAFs)and polygonal ferrites(PFs)in Mg steel increases from 59.97%to 90.16%.The high-angle grain boundaries(HAGBs)and geometrically necessary dislocations density increase from 55.5%and 4.30×10^(14) m^(-2)to 70.4%and 5.48×10^(14) m^(–2),respectively,while effective grain size decreases from 9.46 to 8.12μm.The area fraction of radial zone in Mg steel decreases from 80.8%to 37.7%and cleavage plane is smaller with more curved and finer tearing ridges.The inclusions distributed at the center of cleavage planes and along river lines can serve as crack initiation sites.The zigzag pattern of primary crack propagation path has width of 476μm and the length of secondary cracks remains below 10μm.These cracks are deflected or arrested by IAFs,PFs and HAGBs,and tend to propagate along{110}plane family.These factors contribute to superior overall mechanical properties of Mg steel,especially increasing low-temperature impact toughness from 23 to 175 J.
基金financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDC0140000)the National Science and Technology Major Project(No.J2019-VI-0006-0120)+3 种基金the Science and Technology Major Project of Liaoning Province(No.2024JH1/11700037)the Youth Innovation Promotion Association,CAS(No.2023202)the Natural Science Foundation Project of Liaoning Province(No.2023-MS-024)the National Science and Technology Major Project(No.2024ZD0600600).
文摘The advent of coarse-grain superplasticity has provided a pathway for novel applications in material forming.This article investigated the underlying deformation mechanisms that enabled achieving superplastic elongation exceeding 230%in a coarse-grained Ni-Co-based superalloy.The deformed microstructure and fractographic characteristics of the alloy were examined utilizing optical microscopy(OM),scanning electron microscopy(SEM),and electron backscatter diffraction(EBSD).The results of the analysis revealed that below 1100℃,the process of dynamic recrystallization(DRX)occurred at a sluggish rate,resulting in low plasticity and the initiation of severe cracks.Complete DRX occurred when the deformation temperature exceeded 1100℃,leading to a more uniformly deformed microstructure,reduced crack initiation,and enhanced ductility demonstrated by elongation to failure surpassing 230%.The augmented occurrence of the DRX facilitated prolonged plastic-forming periods,which delayed fracture propagation and promoted the deformation flow within the alloy,thereby transitioning the fracture behavior from intergranular-brittle at 1050℃to ductile intergranular at 1140℃.At this temperature,the deformation was predominantly governed by the discontinuous-DRX(DDRX)mechanism and grain growth,facilitated by the formation of twin boundaries.
基金supported by the National Key Research and Development Program of China(No.2018YFA0707304).
文摘The effects of accumulative hot rolling followed by solution treatment on the microstructural evolution and fracture behavior of 30CrMo/316L multilayered composites have been investigated.A scanning electron microscope equipped with an electron backscatter diffraction probe,a laser confocal microscope,an electron probe microanalysis,and a universal testing machine were employed to characterize the microstructures and mechanical properties.The results indicate that solution treatment transformed the microstructure of the 30CrMo layer from ferrite to martensite,while the 316L layer remained austenitic but transitioned from the rolled to the recrystallized state.Additionally,solution treatment significantly enhanced the mechanical properties of the composite,leading to an increase in yield strength and ultimate tensile strength to 744 and 1106 MPa,respectively—258 and 276 MPa higher than those of the hot-rolled plate.The enhancement in strength is primarily attributed to the formation of high-strength martensite in the 30CrMo layer.During deformation,the composite interface effectively impeded crack propagation and induced step-like deflection.However,the formation of cross-layer grains facilitated crack nucleation at grain boundaries,leading to rapid crack propagation and instantaneous fracture.Therefore,preventing the formation of cross-layer grains during the heat treatment process is crucial,as their presence weakens the interfacial strengthening effect of the composite plate.This study provides valuable insights for the design and development of multi-layered steels.
基金supported by the National Natural Science Foundation of China(No.51904188)the China Postdoctoral Science Foundation Funded Project(No.2024M763564)the Key Laboratory of Rock Mechanics and Geohazards of Zhejiang Province(No.ZJRMG-2022-03)。
文摘The energy-focusing blast is an innovative and ingenious method to achieve directional fracturing.Understanding its energy regulation mechanism is critical to enhancing its practical effectiveness.This study investigates the energy regulation mechanism and explores the medium-filling effects within the energy-focusing blast by employing theoretical analysis,numerical simulations,and model tests.The findings by theoretical and numerical analysis first reveal that two stages of the fracturing and tensile stage govern the directionally crack propagation,in which the explosion energy in the non-energyfocusing direction is suppressed,compressing the borehole wall,while redirected energy produces tensile stress in the energy-focusing direction,driving the formation of directional cracks.The choice of filling medium significantly affects directional cracking due to its impact on energy distribution and regulation,and key properties such as wave impedance and compressibility of the filling medium are critical.Experimental comparisons using air,sand,and water as filling media further disclose the distinct effects of the medium on energy regulation and directional crack growth of the energy-focusing blast.The maximum shaped-energy coefficients for air,sand,and water are 1.30,4.41,and 6.12 in the energy-focusing direction,respectively.Meanwhile,the stress attenuation rate of air,sand,and water increases in that order.The higher wave impedance and lower compressibility of water support efficient and uniform energy propagation,which subtly enhances the tensile actions in the focusing direction and intensifies the overall stress impact of the energy-focusing blast.In addition,the stresses in the non-energyfocusing directions decrease as the angle from the energy-focusing direction increases,while the stresses are relatively uniform for both air and water but noticeably uneven for sand;meanwhile,the fractal dimensions of blasting cracks in the case of air,water,and sand are 1.076,1.068,and 1.112,respectively.Sand as a filling medium leads to increased crack irregularities due to its granularity and heterogeneity.The water medium strikes an optimal balance by promoting the blasting energy transition and optimizing the energy distribution,maintaining the least flatness of the directional crack during energy-focusing blasts.
基金Project (50802115) supported by the National Natural Science Foundation of ChinaProject (2011CB605801) supported by the National Basic Research Program of China
文摘C/C composites with banded structure pyrocarbon were fabricated by fast chemical vapor infiltration（CVI）,with C3H6 as carbon source,N2 as carrier gas,and three-dimensional（3D） 12K PAN-based carbon fabric with high density of 0.94 g/cm3 as preform.Experimental results indicated that the fracture characteristics of C/C composites were closely related to the frequency of high-temperature treatment（HTT） at the break of CVI process.According to the load？displacement curves,C/C composites showed a pseudoplastic fracture after twice of HTT.After three times of HTT,load？displacement curves tended to be stable with a decreasing bending strength at 177.5 MPa.Delamination failure and intrastratal fiber fracture were observed at the cross-section of C/C composites by scanning electronic microscope.Because the content of pyrocarbon and fibers has a different distribution in layers,the C/C composites show different fracture characteristics at various regions,which leads to good toughness and bending strength.
基金The project was supported by the National Science Foundation of China(41430640,U1704242).
文摘Coalbed gas extraction is an important means of exploiting and utilizing gas resources,as well as a means of preventing coal mine disasters.In view of the low gas extraction rate from coalbeds with high gas content and low permeability,a method of improving permeability through deep-hole cumulative blasting is applied to develop initial directional fractures using a jet flow.Under the action of the blasting stress wave and detonation gas wedge,the fractures extend over a large range within the coal,thereby improving coalbed permeability.This study focuses on the criteria of cumulative blasting-induced coalbed fracturing based on a literature review of the penetration effect of cumulative blasting.On this basis,we summarize the coal fracturing zone,crack extension process,and the key technologies of charging and hole sealing for cumulative blasting.In addition,the latest research progress in the optimization of field test drilling and blasting parameters for cumulative blasting is introduced.Research findings indicate that the permeability improvement mechanism of cumulative blasting could be further enhanced,and the technology and technical equipment are in urgent need of improvement.Finally,development trends in the cumulative blasting permeability improvement technique are identified.
基金financial support received from the Key Research Program of Frontier Sciences,Chinese Academy of Sciences(CAS)(Grant No.QYZDJ-SSW-DQC016)。
文摘In this study,servo-controlled biaxial compression tests were conducted on marble specimens to investigate their failure characteristics and fracturing process.The complete stressestrain curves were obtained,and the three-dimensional(3D)features of the failure surfaces were acquired by 3D laser scanning.Acoustic emission(AE)monitoring and moment tensor(MT)analysis were used in combination to better understand the fracturing mechanism of marble under biaxial compression.It was noted that a type of 3D stepwise cracking behaviour occurred on the fracturing surfaces of the examined specimens.The stress dropped multiple times,and a repeated fracturing mode corresponding to the repeated stress drops in the post-peak regime was observed.Three substages,i.e.stress stabilisation,stress decrease and stress increase,were identified for a single fracturing mode.Then quantitative and statistical analyses of the fracturing process at each substage were discussed.Based on the testing results,it was found that at the stress stabilisation substage,the proportion of mixed-mode fractures increased.At the stress decrease substage,the proportion of mixed-mode fractures decreased,and the tensile or shear fractures increased.At the stress increase substage,the proportion of mixed-mode or tensile fractures decreased,and the shear fractures increased.Finally,a conceptual model for the stepwise crack formation was proposed.
基金supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP)the Ministry of Trade,Industry&Energy(MOTIE)of the Republic of Korea Program(No.RS-2025-02603127,Innovation Research Center for Zero-carbon Fuel Gas Turbine Design,Manufacture,and Safety)。
文摘Understanding the temperature dependent deformation behavior of Mg alloys is crucial for their expanding use in the aerospace sector.This study investigates the deformation mechanisms of hot-rolled AZ61 Mg alloy under uniaxial tension along rolling direction(RD)and transverse direction(TD)at-50,25,50,and 150℃.Results reveal a transition from high strength with limited elongation at-50℃ to significant softening and maximum ductility at 150℃.TD samples consistently showed 2%-6%higher strength than RD;however,this yield anisotropy diminished at 150℃ due to the shift from twinning to thermally activated slip and recovery.Fractography indicated a change from semi-brittle to fully ductile fracture with increasing temperature.Electron backscattered diffraction(EBSD)analysis confirmed twinning-driven grain refinement at low temperatures,while deformation at high temperatures involved grain elongation along shear zones,enabling greater strain accommodation before material failure.
基金financially supported by the National Nature Science Foundation of China(Nos.52379110 and 42207222)the Key Technologies for Accurate Diagnosis and Intelligent Prevention and Control of Slope Hazards in Open Pit Mines,181 Major R&D projects of Metallurgical Corporation of China Ltd。
文摘The stability of rock slopes is frequently controlled by the initiation and propagation of inherent dominant cracks.This study systematically investigated these processes in valley slopes by combining fracture-mechanics analysis with transparent soil model tests.An analytical expression for the stress field at the dominant crack tip was derived from the slope stress distribution by superposing the corresponding stress intensity factors(SIFs).The theoretical predictions were then validated against observations from transparent soil model tests.The influences of slope angle(β),crack inclination angle(α),crack position parameter(b),and crack length parameter(h)on crack initiation and propagation were quantified.The results indicated that:(1)cracks at the slope crest tended to propagate in shear mode,and the shear crack initiation angle(θ_(s))was approximately 8°.Cracks at the slope toe might propagate in either tensile or shear mode.(2)θ_(s) at the slope crest increased withβ,b,and l,and decreased withα.The maximum change inθ_(s) induced by the considered parameters was approximately 30°.(3)The tensile crack initiation angle(θ_(t))at the slop toe decreased withβ,α,and l,while the influence of b was comparatively minor.The maximum change inθ_(t) caused by individual parameters ranged approximately from 25°to 60°.Predicted crack propagation modes and directions showed good agreement with experimental results.These findings provide theoretical guidance for stability assessments of valley slopes controlled by dominant crack propagation.
基金the National Natural Science Foundation of China (No.51275269)the Tsinghua University Initiative Scientific Research Program (No.20121087918)the National Science and Technology Major Project of the Ministry of Science and Technology of the People’s Republic of China (No.2012ZX04012011) for financial support
文摘X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiation, propagation and subsequent fracture of specimen. Results showed that four types of porosi- ties, including gas-shrinkage pore, gas pore, net-shrinkage and island-shrinkage, could be identified according to the formation mechanism and morphology characterization. During tensile deformation, it was shown that the gas-shrinkage pore and net-shrinkage, rather than gas pore or island-shrinkage, were the main sources for crack initiation. In addition, the crack propagated by interconnecting the po- rosities at the cross section with minimum efficient force bearing area. At these locations where externally solidified crystals （ESCs） were present, the crack would propagate along the ESC boundaries in an inter- granular mode, while at these locations without ESCs, the crack would propagate roughly along the direction perpendicular to the tensile stress in a combination of trans-granular and inter-granular modes.
基金supports for this work,provided by the Natural Science Foundation of Anhui Province(No.1908085QE187,1808085ME161)the Open Research Program of Key Laboratory of Safety and High-efficiency Coal Mining(No.JYBSYS2019202)the Open Research Program of State Key Laboratory Cultivation Base for Gas Geology and Gas Control(No.WS2019B09)are gratefully acknowledged.
文摘A comprehensive understanding of the failure behavior and mechanism of coal is a prerequisite for dealing with dynamic problems in mining space.In this study,the failure behavior and mechanism of coal under uniaxial dynamic compressive loads were experimentally and numerically investigated.The experiments were conducted using a split Hopkinson pressure bar(SHPB)system.The results indicated that the typical failure of coal is lateral and axial at lower loading rates and totally smashed at higher loading rates.The further fractography analysis of lateral and axial fracture fragments indicated that the coal failure under dynamic compressive load is caused by tensile brittle fracture.In addition,the typical failure modes of coal under dynamic load were numerically reproduced.The numerical results indicated that the axial fracture is caused directly by the incident compressive stress wave and the lateral fracture is caused by the tensile stress wave reflected from the interface between coal specimen and transmitted bar.Potential application was further conducted to interpret dynamic problems in underground coal mine and it manifested that the lateral and axial fractures of coal constitute the parallel cracks in the coal mass under roof fall and blasting in mining space.
基金support of Iran National Science Foundation is also gratefully appreciated
文摘High energy gas fracturing is a simple approach of applying high pressure gas to stimulate wells by gen- erating several radial cracks without creating any other damages to the wells. In this paper, a numerical algorithm is proposed to quantitatively simulate propagation of these fractures around a pressurized hole as a quasi-static phenomenon. The gas flow through the cracks is assumed as a one-dimensional transient flow, governed by equations of conservation of mass and momentum. The fractured medium is modeled with the extended finite element method, and the stress intensity factor is calculated by the simple, though sufficiently accurate, displacement ex- trapolation method. To evaluate the proposed algorithm, two field tests are simulated and the unknown parameters are determined through calibration. Sensitivity analyses are performed on the main effective parameters. Considering that the level of uncertainty is very high in these types of engineering problems, the results show a good agreement with the experimental data. They are also consistent with the theory that the final crack length is mainly determined by the gas pressure rather than the initial crack length produced by the stress waves.
基金The authors are grateful for the financial supports from the Jiangsu Provincial Science and Technology Plan Project,China(BE2018753/KJ185629)the National Natural Science Foundation of China(51571118)the 2020 Extracurricular Academic Research Fund for College Students of Nanjing University of Science and Technology,China.Zong-han XIE acknowledges the support of the Australian Research Council Discovery Projects.
文摘MnFeCoCuNix high-entropy alloys(HEAs)with different Ni contents were fabricated by vacuum induction melting.XRD and SEM−EDS were used to analyze the phase constitution and structure,and the tensile properties of the samples were determined using a universal tensile tester.The results show that the HEAs consist of a dual-phase structure,in which FCC1 phase is rich in Fe and Co,while the FCC2 phase has high contents of Cu and Mn.As Ni content increases,the segregation of Cu decreases,accompanied by the decrease of FCC2 phase.Moreover,the tensile strength of the HEAs increases first and then decreases,and the elongation increases slightly.This is attributed to the combined effect of interface strengthening and solid solution strengthening.The in-situ stretched MnFeCoCuNi0.5 alloy shows obvious neck shrinkage during the tensile fracture process.In the initial deformation stage,the slip lines show different morphologies in the dual-phase structure.However,in the later stage,the surface slip lines become longer and denser due to the redistribution of atoms and the re-separation of the dissolved phase.
基金supported by the National Natural Science Foundation of China (Grant Nos. 52104125, U1765204 and 51739008)
文摘Similar to hydraulic fracturing(HF), the coalescence and fracture of cracks are induced within a rock under the action of an ultrasonic field, known as ultrasonic fracturing(UF). Investigating UF is important in both hard rock drilling and oil and gas recovery. A three-dimensional internal laser-engraved crack(3D-ILC) method was introduced to prefabricate two parallel internal cracks within the samples without any damage to the surface. The samples were subjected to UF. The mechanism of UF was elucidated by analyzing the characteristics of fracture surfaces. The crack propagation path under different ultrasonic parameters was obtained by numerical simulation based on the Paris fatigue model and compared to the experimental results of UF. The results show that the 3D-ILC method is a powerful tool for UF research.Under the action of an ultrasonic field, the fracture surface shows the characteristics of beach marks and contains powder locally, indicating that the UF mechanism includes high-cycle fatigue fracture, shear and friction, and temperature load. The two internal cracks become close under UF. The numerical result obtained by the Paris fatigue model also shows the attraction of the two cracks, consistent with the test results. The 3D-ILC method provides a new tool for the experimental study of UF. Compared to the conventional numerical methods based on the analysis of stress-strain and plastic zone, numerical simulation can be a good alternative method to obtain the crack path under UF.
基金Item Sponsored by Fundamental Research Funds for the Central Universities of China(FRF-TP-15-062A3)
文摘Fatigue performance of hot-rolled ribbed-steel bar with the yield strength of 500 MPa （HRB500） was stud- ied with bend-rotating fatigue test at a stress ratio of R = -1. It is determined by staircase method that its fatigue strength for 107 cycles is 451 MPa, which is higher than that of common carbon structural steel. This should be at- tributed to the fine-grain strengthening resulting from the high content of alloy element V and Thermo-Mechanical Control Process （TMCP）. The S-N curve function is also obtained by nonlinear regression with three parameters power function. The fatigue fractures of the specimen were further analyzed with Scanning Electron Microscopy （SEM） and Energy Disperse Spectroscopy （EDS） to study the fracture mechanism. Taking into account microstruc- ture, hardness and cleanliness of the material, it implies that the fatigue fractures of HRB500 rebar all arise from surface substrates in which many brittle inclusions are contained, and that the fatigue crack propagation is principally based on the mechanism of quasi-cleavage fracture, because of the intracrystalline hard spots leading to stress con- centration and thus to the cracks. Moreover, the transient breaking area exhibits microvoid coalescence of ductile fracture due to the existing abundant inclusions.
