Cyclic freezing-thawing can lead to fracture development in coal,affecting its mechanical and consumer properties.To study crack formations in coal,an ultrasonic sounding method using shear polarized waves was propose...Cyclic freezing-thawing can lead to fracture development in coal,affecting its mechanical and consumer properties.To study crack formations in coal,an ultrasonic sounding method using shear polarized waves was proposed.Samples of three coal types(anthracite,lignite and hard coal)were tested.The research results show that,in contrast to the shear wave velocity,the shear wave amplitude is extremely sensitive to the formation of new cracks at the early stages of cyclic freezing-thawing.Tests also show an inverse correlation between coal compressive strength and its tendency to form cracks under temperature impacts;shear wave attenuation increases more sharply in high-rank coals after the first freezing cycle.Spectral analysis of the received signals also confirmed significant crack formation in anthracite after the first freeze-thaw cycle.The initial anisotropy was determined,and its decrease with an increase in the number of freeze-thaw cycles was shown.The data obtained forms an experimental basis for the development of new approaches to preserve coal consumer properties during storage and transportation under severe natural and climatic conditions.展开更多
Molecular dynamics simulation of uniaxial tension along [001] has been performed to study the influence of various surface defects on the initiation of plastic deformation and fracture of γ-TiAl single crystals.The r...Molecular dynamics simulation of uniaxial tension along [001] has been performed to study the influence of various surface defects on the initiation of plastic deformation and fracture of γ-TiAl single crystals.The results indicate that brittle fracture occurs in perfect bulk; surfaces and edges will be detrimental to the strength of materials and provide dislocation nucleation site. The defects on surfaces and edges cause further weakening with various effects depending on defect type, size, position and orientation,while the edge dimples are the most influential. For γ-TiAl rods with surface dimples, dislocations nucleate from an edge of the rod when dimples are small, dimple dislocation nucleation occurs only when the dimples are larger than a strain rate dependent critical size. The dislocations nucleated upon [001]tension are super dislocations with Burger vectors 〈011] or 1/2 〈 112] containing four 1/6 〈 112 〉 partials. The effects of surface scratches are orientation and shape sensitive. Scratches parallel to the loading direction have little influence, while sharp ones perpendicular to the loading direction may cause crack and thus should be avoided. This simulation also shows that, any type of surface defect would lower strength,and cause crack in some cases. But some may facilitate dislocation nucleation and improve ductility of TiAl if well controlled.展开更多
The paper is summarizing latest results connected with application of the incubation time approach to problems of dynamic fracture of rock materials. Incubation time based fracture criteria for intact media and media ...The paper is summarizing latest results connected with application of the incubation time approach to problems of dynamic fracture of rock materials. Incubation time based fracture criteria for intact media and media with cracks are discussed. Available experimental data on high rate fracture of different rock materials and incubation time based fracture criteria are used in order to evaluate critical parameters of causing fracture in these materials. Previously discovered possibility to optimize (minimize) energy input for fracture is discussed in connection to industrial rock fracture processes. It is shown that optimal value of momentum associated with critical load in order to initialize fracture in rock media does strongly depend on the incubation time and the impact duration. Existence of optimal load shapes minimizing momentum for a single fracturing impact or a sequence of periodic fracturing impacts is demonstrated.展开更多
In this paper,a semi-discrete model based on peridynamics(PD)for engineered cementitious composites(ECCs)is applied to simulate the fracture behavior of functionally graded ECC(FGECC)beams.This is a new application of...In this paper,a semi-discrete model based on peridynamics(PD)for engineered cementitious composites(ECCs)is applied to simulate the fracture behavior of functionally graded ECC(FGECC)beams.This is a new application of PD in ECC.Prior to simulating the crack behavior,the convergence of the PD model for ECC is discussed and the appropriate horizon size 5 and nonlocal ratio m are obtained,i.e.,S=1.6 mm and m=4.In addition,when the bond strain exceeds the elastic limit,a damage variable is introduced into the model,and the model is validated using a simple numerical algorithm.Finally,the dynamic fracture behavior of a two-dimensional FGECC beam under four-point bending is investigated,and the effect of the initial crack location on the fracture behavior is analyzed.Simulation results show that the initial crack location can affect the crack propagation pattern,thereby enabling one to understand the dynamic fracture behavior of ECC structures and guide the engineering practice.展开更多
Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation.Understanding the anisotropic fracture mechanism of shale under impact loading is vital for...Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation.Understanding the anisotropic fracture mechanism of shale under impact loading is vital for optimizing shock wave fracturing equipment and enhancing shale oil production.In this study,using the well-known notched semi-circular bend(NSCB)sample and the novel double-edge notched flattened Brazilian disc(DNFBD)sample combined with a split Hopkinson pressure bar(SHPB),various dynamic anisotropic fracture properties of Lushan shale,including failure characteristics,fracture toughness,energy dissipation and crack propagation velocity,are comprehensively compared and discussed under mode Ⅰ and mode Ⅱ fracture scenarios.First,using a newly modified fracture criterion considering the strength anisotropy of shale,the DNFBD specimen is predicted to be a robust method for true mode Ⅱ fracture of anisotropic shale rocks.Our experimental results show that the dynamic mode Ⅱ fracture of shale induces a rougher and more complex fracture morphology and performs a higher fracture toughness or fracture energy compared to dynamic mode Ⅰ fracture.The minimal fracture toughness or fracture energy occurs in the Short-transverse orientation,while the maximal ones occur in the Divider orientation.In addition,it is interesting to find that the mode Ⅱ fracture toughness anisotropy index decreases more slowly than that in the mode Ⅰ fracture scenario.These results provide significant insights for understanding the different dynamic fracture mechanisms of anisotropic shale rocks under impact loading and have some beneficial implications for the controllable shock wave fracturing technique.展开更多
The shape of underground chambers in deep mining varies due to their geological environment and intended use,which results in different failure modes under the influence of mining activities.However,the effect of cham...The shape of underground chambers in deep mining varies due to their geological environment and intended use,which results in different failure modes under the influence of mining activities.However,the effect of chamber shape on the mechanism of structural integrity under dynamic load is still unclear.In this paper,granite samples with circular(C),rectangular(R),long ellipse(EL),and short ellipse(ES)holes were prepared.The dynamic mechanical response and cracking mechanism of granite were systematically analyzed using the split Hopkinson pressure bar(SHPB)test system and the hybrid finite and discrete element method(HFDEM).The results indicate that the dynamic strengths of granite with EL and ES represent the maximum and minimum values within the range of close strain rates,respectively.When EL granite is subjected to dynamic load,the axial stress concentration(in the load direction)is weak,and the transverse stress shows relative dispersion,which is the primary reason for its highest dynamic strength.The failure of granite with various holes primarily involves a tensile-shear mixed fracture,with relatively few pure typeⅡcracks.The chamber’s transverse span is the primary factor influencing the distribution range of the fracture area.展开更多
Separated specimens of Ti-6Al-4V alloy were dynamically loaded at a strain rate of 3 900 s-1 using a split Hopkinson pressure bar（SHPB） apparatus.The fracture features of the separated specimens were investigated by...Separated specimens of Ti-6Al-4V alloy were dynamically loaded at a strain rate of 3 900 s-1 using a split Hopkinson pressure bar（SHPB） apparatus.The fracture features of the separated specimens were investigated by a scanning electron microscope.The results show that adiabatic shear failure occurs in the tested specimens,and two typical areas（dimple and smooth areas） with different features are alternatively distributed on the whole fracture surface.The dimple areas originate from voids generation and coalescence,exhibiting ductile fracture characteristics.Simultaneously,ultrafine grains（UFGs） and microcracks among grains are observed on the smooth areas,indicating that the emergence of UFG areas is caused by the propagation of microcracks along grain boundaries and exhibits brittle fracture characteristics.Fracture occurring in adiabatic shear bands is not uniform and ultimate rupture is resulted from ductile and brittle fracture modes.展开更多
The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact...The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose ＂tips＂ are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional ＂stagger ridge＂ structure.展开更多
The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabric...The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabricated and the reflective coating on the surface of the specimen was prepared. Secondly the crack path and the shadow spot patterns during the crack propagation process for the two specimens were recorded by using a multi-spark high speed camera.Thirdly some dynamic parameters of two cement paste specimens including crack onset time the dynamic stress intensity factor and crack growth velocity were determined and analyzed comparatively.This indicates that the glass fibres can improve the fracture resistance and delay fracture time.These results will play an important role in evaluating the dynamic fracture properties of cement paste.展开更多
The paper is discussing problems connected with embedment of the incubation time criterion for brittle fracture into finite element computational schemes. Incubation time fracture criterion is reviewed; practical ques...The paper is discussing problems connected with embedment of the incubation time criterion for brittle fracture into finite element computational schemes. Incubation time fracture criterion is reviewed; practical questions of its numerical implementation are extensively discussed. Several examples of how the incubation time fracture criterion can be used as fracture condition in finite element computations are given. The examples include simulations of dynamic crack propagation and arrest, impact crater formation （i.e. fracture in initially intact media）, spall fracture in plates, propagation of cracks in pipelines. Applicability of the approach to model initiation, development and arrest of dynamic fracture is claimed.展开更多
An effective Dynamic Moire method was presented to determine the critical time of crack instable propagation in rock dynamic fracture. Two pieces of grating were installed near the notch of Short Rod specimen to form ...An effective Dynamic Moire method was presented to determine the critical time of crack instable propagation in rock dynamic fracture. Two pieces of grating were installed near the notch of Short Rod specimen to form the Moire fringes, then the COD versus time could be monitored from the movement of the Moire fringes, and finally the critical time could be determined from the velocity of COD. This method was also compared with another one.It could be concluded that the critical time determined by Dynamic Moire method corresponds with that of the Transmitted Wave method at the loading rates from 103 to 104 MPa·m(1/2)·S(-1).展开更多
This study presents a meso-criterion of dynamic fracture, on the basis of stress in integral form In such way the difficulty due to the singularity of stress distribution at the crack tip is overcome. A micro-paramete...This study presents a meso-criterion of dynamic fracture, on the basis of stress in integral form In such way the difficulty due to the singularity of stress distribution at the crack tip is overcome. A micro-parameter, the atom radius, is introduced into the criterion.Meanwhile a characteristic time concept is taken into account for describing the inertia effect of material. The criterion reveals The criterion reveals the effects of loading rate, defect and sample geometry,material constants including the micro-structure parameter.展开更多
The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a nove...The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.展开更多
Accurate 3-D fracture network model for rock mass in dam foundation is of vital importance for stability,grouting and seepage analysis of dam foundation.With the aim of reducing deviation between fracture network mode...Accurate 3-D fracture network model for rock mass in dam foundation is of vital importance for stability,grouting and seepage analysis of dam foundation.With the aim of reducing deviation between fracture network model and measured data,a 3-D fracture network dynamic modeling method based on error analysis was proposed.Firstly,errors of four fracture volume density estimation methods(proposed by ODA,KULATILAKE,MAULDON,and SONG)and that of four fracture size estimation methods(proposed by EINSTEIN,SONG and TONON)were respectively compared,and the optimal methods were determined.Additionally,error index representing the deviation between fracture network model and measured data was established with integrated use of fractal dimension and relative absolute error(RAE).On this basis,the downhill simplex method was used to build the dynamic modeling method,which takes the minimum of error index as objective function and dynamically adjusts the fracture density and size parameters to correct the error index.Finally,the 3-D fracture network model could be obtained which meets the requirements.The proposed method was applied for 3-D fractures simulation in Miao Wei hydropower project in China for feasibility verification and the error index reduced from 2.618 to 0.337.展开更多
To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature condit...To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system.Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves.A high-speed camera was used to check the fracture characteristics of the specimens.The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave.The dynamic force balance is achieved even when the specimens are at a temperature of 400°C.The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25-400°C,and high temperature has a strengthening effect on the dynamic fracture toughness.展开更多
The former studies indicate that loading rates significantly affect dynamic behavior of brittle materials,for instance,the dynamic compressive and tensile strength increase with loading rates.However,there still are m...The former studies indicate that loading rates significantly affect dynamic behavior of brittle materials,for instance,the dynamic compressive and tensile strength increase with loading rates.However,there still are many unknown or partially unknown aspects.For example,whether loading rates have effect on crack dynamic propagating behavior(propagation toughness,velocity and arrest,etc).To further explore the effect of loading rates on crack dynamic responses,a large-size single-cleavage trapezoidal open(SCTO)specimen was proposed,and impacting tests using the SCTO specimen under drop plate impact were conducted.Crack propagation gauges(CPGs)were employed in measuring impact loads,crack propagation time and velocities.In order to verify the testing result,the corresponding numerical model was established using explicit dynamic software AUTODYN,and the simulation result is basically consistent with the experimental results.The ABAQUS software was used to calculate the dynamic SIFs.The universal function was calculated by fractal method.The experimental-numerical method was employed in determining initiation toughness and propagation toughness.The results indicate that crack propagating velocities,dynamic fracture toughness and energy release rates increase with loading rates;crack delayed initiation time decreases with loading rates.展开更多
The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are rec...The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are recorded by a high speed camera during the dynamic fracture process. Then, the dynamic crack propagation, the stress intensity factor （SIF） and the dynamic fracture toughness of aged PC are obtained through an analysis of the characteristic size of caustic pattern. Moreover, by combining with the investigation of the fracture surface and the energy release rate analysis, the influence of aging time on the dynamic fracture behavior is discussed. Results show that the dynamic fracture toughness and critical energy release rate of PC decreases with aging time for short aging times, whereas they have little change or even increase for longer aging times. Therefore, aging modifies the mechanical properties especially the dynamic fracture properties of PC nonlinearly, not linearly as generally thought of.展开更多
The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also th...The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also they offered required resistance to HIC. However, the combat vehicles used in military operations will be required to operate under a wide range of road conditions ranging from first class to cross country. Structural components in combat vehicles are subjected to dynamic loading with high strain rates during operation. Stress loadings within the vehicle hull of these vehicles are expected to fluctuate considerably and structural cracking especially in welds during the service life of these vehicles can lead to catastrophic failures. Under these conditions, fracture behaviour of high strain rate sensitive structural steels can be better understood by dynamic fracture toughness （K1d）. Hence, an attempt was made to study dynamic fracture toughness of the armour grade quenched and tempered steel and their welds fabricated using LHF consumables. The experimental results indicate that the K1d values of the joints fabricated by shielded metal arc welding （SMAW） are higher than those of the joints fabricated using flux cored arc welding （FCAW） process.展开更多
A novel method was proposed for the evaluation of Mode I dynamic fracture toughness （DFT） under plane stress and small scale yielding conditions for welded joints of stainless steel （SS）, 0Cr18Ni10Ti. In a hybrid ...A novel method was proposed for the evaluation of Mode I dynamic fracture toughness （DFT） under plane stress and small scale yielding conditions for welded joints of stainless steel （SS）, 0Cr18Ni10Ti. In a hybrid experimental-numerical approach, the experiments were carried out on the Hopkinson pressure bar apparatus, and three dimensional （3D） transient numerical simulations were performed by a finite element （FE） computer program. Macroscopical plastic deformation was observed at the loading and supporting points, on the specimens, after the test, which could cause a large error if omitted in the numerical simulation. Therefore, elustic-viscoplustic analysis was performed on the specimen by adopting the Johnson-Cook （J-C） model to describe the rate-dependent plastic flow behavior of the material. The material heterogeneity in the mismatched welded joints, induced by the difference in the base metal （BM） and the weld metal （WM） in yield stress, has also been taken into consideration by using the J-C models separately. Good accordance was obtained between the experimental and the computational results by the present approach. The relationship between plane stress DFT and loading rate was also obtained on the order of 108 MPa.m^1/2.s^-1.展开更多
Phase field description of fracture is a very promising approach for simulating crack initiation, propagation, merging and branching. This method greatly reduces the implementation complexity, compared with discrete d...Phase field description of fracture is a very promising approach for simulating crack initiation, propagation, merging and branching. This method greatly reduces the implementation complexity, compared with discrete descriptions of cracks. In this work, we provide an overview of phase field models for quasistatic and dynamic cases. Afterward, we present useful vectors and matrices for the implementation of this method in two and three dimensions.展开更多
基金Russian Foundation for Basic Research,grant number 18-05-70002.
文摘Cyclic freezing-thawing can lead to fracture development in coal,affecting its mechanical and consumer properties.To study crack formations in coal,an ultrasonic sounding method using shear polarized waves was proposed.Samples of three coal types(anthracite,lignite and hard coal)were tested.The research results show that,in contrast to the shear wave velocity,the shear wave amplitude is extremely sensitive to the formation of new cracks at the early stages of cyclic freezing-thawing.Tests also show an inverse correlation between coal compressive strength and its tendency to form cracks under temperature impacts;shear wave attenuation increases more sharply in high-rank coals after the first freezing cycle.Spectral analysis of the received signals also confirmed significant crack formation in anthracite after the first freeze-thaw cycle.The initial anisotropy was determined,and its decrease with an increase in the number of freeze-thaw cycles was shown.The data obtained forms an experimental basis for the development of new approaches to preserve coal consumer properties during storage and transportation under severe natural and climatic conditions.
基金supported by the National Basic Research Program of China (No.2011CB606404)the National Natural Science Foundation of China (No.51171195)the GM Research Project
文摘Molecular dynamics simulation of uniaxial tension along [001] has been performed to study the influence of various surface defects on the initiation of plastic deformation and fracture of γ-TiAl single crystals.The results indicate that brittle fracture occurs in perfect bulk; surfaces and edges will be detrimental to the strength of materials and provide dislocation nucleation site. The defects on surfaces and edges cause further weakening with various effects depending on defect type, size, position and orientation,while the edge dimples are the most influential. For γ-TiAl rods with surface dimples, dislocations nucleate from an edge of the rod when dimples are small, dimple dislocation nucleation occurs only when the dimples are larger than a strain rate dependent critical size. The dislocations nucleated upon [001]tension are super dislocations with Burger vectors 〈011] or 1/2 〈 112] containing four 1/6 〈 112 〉 partials. The effects of surface scratches are orientation and shape sensitive. Scratches parallel to the loading direction have little influence, while sharp ones perpendicular to the loading direction may cause crack and thus should be avoided. This simulation also shows that, any type of surface defect would lower strength,and cause crack in some cases. But some may facilitate dislocation nucleation and improve ductility of TiAl if well controlled.
基金supported by RFBR Research (Grant Nos. 10-01-00810-a, 11-01-00491-a and 10-01-91154-GFEN_a)Russian Federation State Contracts and Academic Programs of the Russian Academy of Sciences
文摘The paper is summarizing latest results connected with application of the incubation time approach to problems of dynamic fracture of rock materials. Incubation time based fracture criteria for intact media and media with cracks are discussed. Available experimental data on high rate fracture of different rock materials and incubation time based fracture criteria are used in order to evaluate critical parameters of causing fracture in these materials. Previously discovered possibility to optimize (minimize) energy input for fracture is discussed in connection to industrial rock fracture processes. It is shown that optimal value of momentum associated with critical load in order to initialize fracture in rock media does strongly depend on the incubation time and the impact duration. Existence of optimal load shapes minimizing momentum for a single fracturing impact or a sequence of periodic fracturing impacts is demonstrated.
基金the Natural Science Foundation of China(Nos.11872339,11472248)the Natural Science Foundation of Henan Province(No.182300410221).
文摘In this paper,a semi-discrete model based on peridynamics(PD)for engineered cementitious composites(ECCs)is applied to simulate the fracture behavior of functionally graded ECC(FGECC)beams.This is a new application of PD in ECC.Prior to simulating the crack behavior,the convergence of the PD model for ECC is discussed and the appropriate horizon size 5 and nonlocal ratio m are obtained,i.e.,S=1.6 mm and m=4.In addition,when the bond strain exceeds the elastic limit,a damage variable is introduced into the model,and the model is validated using a simple numerical algorithm.Finally,the dynamic fracture behavior of a two-dimensional FGECC beam under four-point bending is investigated,and the effect of the initial crack location on the fracture behavior is analyzed.Simulation results show that the initial crack location can affect the crack propagation pattern,thereby enabling one to understand the dynamic fracture behavior of ECC structures and guide the engineering practice.
基金supported by the National Natural Science Foundation of China(Grant No.12302500)the National Key Research and Development Program of China(Grant No.2020YFA0710503)Postdoctoral Fellowship Program(Grade B)of China Postdoctoral Science Foundation(Grant No.GBZ20230022).
文摘Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation.Understanding the anisotropic fracture mechanism of shale under impact loading is vital for optimizing shock wave fracturing equipment and enhancing shale oil production.In this study,using the well-known notched semi-circular bend(NSCB)sample and the novel double-edge notched flattened Brazilian disc(DNFBD)sample combined with a split Hopkinson pressure bar(SHPB),various dynamic anisotropic fracture properties of Lushan shale,including failure characteristics,fracture toughness,energy dissipation and crack propagation velocity,are comprehensively compared and discussed under mode Ⅰ and mode Ⅱ fracture scenarios.First,using a newly modified fracture criterion considering the strength anisotropy of shale,the DNFBD specimen is predicted to be a robust method for true mode Ⅱ fracture of anisotropic shale rocks.Our experimental results show that the dynamic mode Ⅱ fracture of shale induces a rougher and more complex fracture morphology and performs a higher fracture toughness or fracture energy compared to dynamic mode Ⅰ fracture.The minimal fracture toughness or fracture energy occurs in the Short-transverse orientation,while the maximal ones occur in the Divider orientation.In addition,it is interesting to find that the mode Ⅱ fracture toughness anisotropy index decreases more slowly than that in the mode Ⅰ fracture scenario.These results provide significant insights for understanding the different dynamic fracture mechanisms of anisotropic shale rocks under impact loading and have some beneficial implications for the controllable shock wave fracturing technique.
基金Project(52409128)supported by the National Natural Science Foundation of ChinaProject(SDGZK2425)supported by the Opening Fund of State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering,China。
文摘The shape of underground chambers in deep mining varies due to their geological environment and intended use,which results in different failure modes under the influence of mining activities.However,the effect of chamber shape on the mechanism of structural integrity under dynamic load is still unclear.In this paper,granite samples with circular(C),rectangular(R),long ellipse(EL),and short ellipse(ES)holes were prepared.The dynamic mechanical response and cracking mechanism of granite were systematically analyzed using the split Hopkinson pressure bar(SHPB)test system and the hybrid finite and discrete element method(HFDEM).The results indicate that the dynamic strengths of granite with EL and ES represent the maximum and minimum values within the range of close strain rates,respectively.When EL granite is subjected to dynamic load,the axial stress concentration(in the load direction)is weak,and the transverse stress shows relative dispersion,which is the primary reason for its highest dynamic strength.The failure of granite with various holes primarily involves a tensile-shear mixed fracture,with relatively few pure typeⅡcracks.The chamber’s transverse span is the primary factor influencing the distribution range of the fracture area.
文摘Separated specimens of Ti-6Al-4V alloy were dynamically loaded at a strain rate of 3 900 s-1 using a split Hopkinson pressure bar（SHPB） apparatus.The fracture features of the separated specimens were investigated by a scanning electron microscope.The results show that adiabatic shear failure occurs in the tested specimens,and two typical areas（dimple and smooth areas） with different features are alternatively distributed on the whole fracture surface.The dimple areas originate from voids generation and coalescence,exhibiting ductile fracture characteristics.Simultaneously,ultrafine grains（UFGs） and microcracks among grains are observed on the smooth areas,indicating that the emergence of UFG areas is caused by the propagation of microcracks along grain boundaries and exhibits brittle fracture characteristics.Fracture occurring in adiabatic shear bands is not uniform and ultimate rupture is resulted from ductile and brittle fracture modes.
基金Project (51001014) supported by the Young Scientists Fund of the National Natural Science Foundation of China
文摘The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose ＂tips＂ are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional ＂stagger ridge＂ structure.
基金The Ph.D.Programs Foundation of Ministry of Education of China(No.20120023120020)the National Natural Science Foundation of China(No.51404273)
文摘The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabricated and the reflective coating on the surface of the specimen was prepared. Secondly the crack path and the shadow spot patterns during the crack propagation process for the two specimens were recorded by using a multi-spark high speed camera.Thirdly some dynamic parameters of two cement paste specimens including crack onset time the dynamic stress intensity factor and crack growth velocity were determined and analyzed comparatively.This indicates that the glass fibres can improve the fracture resistance and delay fracture time.These results will play an important role in evaluating the dynamic fracture properties of cement paste.
基金supported by RFBR research (10-01-00810-a,11-01-00491-a,10-01-91154-GFEN a),Russian Federation State contracts and academic programs of the Russian Academy of Sciences
文摘The paper is discussing problems connected with embedment of the incubation time criterion for brittle fracture into finite element computational schemes. Incubation time fracture criterion is reviewed; practical questions of its numerical implementation are extensively discussed. Several examples of how the incubation time fracture criterion can be used as fracture condition in finite element computations are given. The examples include simulations of dynamic crack propagation and arrest, impact crater formation （i.e. fracture in initially intact media）, spall fracture in plates, propagation of cracks in pipelines. Applicability of the approach to model initiation, development and arrest of dynamic fracture is claimed.
文摘An effective Dynamic Moire method was presented to determine the critical time of crack instable propagation in rock dynamic fracture. Two pieces of grating were installed near the notch of Short Rod specimen to form the Moire fringes, then the COD versus time could be monitored from the movement of the Moire fringes, and finally the critical time could be determined from the velocity of COD. This method was also compared with another one.It could be concluded that the critical time determined by Dynamic Moire method corresponds with that of the Transmitted Wave method at the loading rates from 103 to 104 MPa·m(1/2)·S(-1).
文摘This study presents a meso-criterion of dynamic fracture, on the basis of stress in integral form In such way the difficulty due to the singularity of stress distribution at the crack tip is overcome. A micro-parameter, the atom radius, is introduced into the criterion.Meanwhile a characteristic time concept is taken into account for describing the inertia effect of material. The criterion reveals The criterion reveals the effects of loading rate, defect and sample geometry,material constants including the micro-structure parameter.
文摘The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.
基金Project(51321065)supported by the Innovative Research Groups of the National Natural Science Foundation of ChinaProject(2013CB035904)supported by the National Basic Research Program of China(973 Program)Project(51439005)supported by the National Natural Science Foundation of China
文摘Accurate 3-D fracture network model for rock mass in dam foundation is of vital importance for stability,grouting and seepage analysis of dam foundation.With the aim of reducing deviation between fracture network model and measured data,a 3-D fracture network dynamic modeling method based on error analysis was proposed.Firstly,errors of four fracture volume density estimation methods(proposed by ODA,KULATILAKE,MAULDON,and SONG)and that of four fracture size estimation methods(proposed by EINSTEIN,SONG and TONON)were respectively compared,and the optimal methods were determined.Additionally,error index representing the deviation between fracture network model and measured data was established with integrated use of fractal dimension and relative absolute error(RAE).On this basis,the downhill simplex method was used to build the dynamic modeling method,which takes the minimum of error index as objective function and dynamically adjusts the fracture density and size parameters to correct the error index.Finally,the 3-D fracture network model could be obtained which meets the requirements.The proposed method was applied for 3-D fractures simulation in Miao Wei hydropower project in China for feasibility verification and the error index reduced from 2.618 to 0.337.
基金support from the National Natural Science Foundation of China(No.41972283)。
文摘To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system.Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves.A high-speed camera was used to check the fracture characteristics of the specimens.The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave.The dynamic force balance is achieved even when the specimens are at a temperature of 400°C.The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25-400°C,and high temperature has a strengthening effect on the dynamic fracture toughness.
基金Projects(11672194,U19A2098)supported by the National Natural Science Foundation of ChinaProject(2018SCU12047)supported by Fundamental Research Funds for the Central Universities,ChinaProject(2018JZ0036)supported by the Project of Science and Technology of Sichuan Province,China。
文摘The former studies indicate that loading rates significantly affect dynamic behavior of brittle materials,for instance,the dynamic compressive and tensile strength increase with loading rates.However,there still are many unknown or partially unknown aspects.For example,whether loading rates have effect on crack dynamic propagating behavior(propagation toughness,velocity and arrest,etc).To further explore the effect of loading rates on crack dynamic responses,a large-size single-cleavage trapezoidal open(SCTO)specimen was proposed,and impacting tests using the SCTO specimen under drop plate impact were conducted.Crack propagation gauges(CPGs)were employed in measuring impact loads,crack propagation time and velocities.In order to verify the testing result,the corresponding numerical model was established using explicit dynamic software AUTODYN,and the simulation result is basically consistent with the experimental results.The ABAQUS software was used to calculate the dynamic SIFs.The universal function was calculated by fractal method.The experimental-numerical method was employed in determining initiation toughness and propagation toughness.The results indicate that crack propagating velocities,dynamic fracture toughness and energy release rates increase with loading rates;crack delayed initiation time decreases with loading rates.
基金supported by the National Basic Research Program of China(973 Program)(No.2010CB731503)
文摘The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are recorded by a high speed camera during the dynamic fracture process. Then, the dynamic crack propagation, the stress intensity factor （SIF） and the dynamic fracture toughness of aged PC are obtained through an analysis of the characteristic size of caustic pattern. Moreover, by combining with the investigation of the fracture surface and the energy release rate analysis, the influence of aging time on the dynamic fracture behavior is discussed. Results show that the dynamic fracture toughness and critical energy release rate of PC decreases with aging time for short aging times, whereas they have little change or even increase for longer aging times. Therefore, aging modifies the mechanical properties especially the dynamic fracture properties of PC nonlinearly, not linearly as generally thought of.
基金Armament Research Board (ARMREB),New Delhi for funding this project work (Project No MAA/03/41)
文摘The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also they offered required resistance to HIC. However, the combat vehicles used in military operations will be required to operate under a wide range of road conditions ranging from first class to cross country. Structural components in combat vehicles are subjected to dynamic loading with high strain rates during operation. Stress loadings within the vehicle hull of these vehicles are expected to fluctuate considerably and structural cracking especially in welds during the service life of these vehicles can lead to catastrophic failures. Under these conditions, fracture behaviour of high strain rate sensitive structural steels can be better understood by dynamic fracture toughness （K1d）. Hence, an attempt was made to study dynamic fracture toughness of the armour grade quenched and tempered steel and their welds fabricated using LHF consumables. The experimental results indicate that the K1d values of the joints fabricated by shielded metal arc welding （SMAW） are higher than those of the joints fabricated using flux cored arc welding （FCAW） process.
基金111 project(No.B07050)the National Natural Science Foundation of China(No.90405016).
文摘A novel method was proposed for the evaluation of Mode I dynamic fracture toughness （DFT） under plane stress and small scale yielding conditions for welded joints of stainless steel （SS）, 0Cr18Ni10Ti. In a hybrid experimental-numerical approach, the experiments were carried out on the Hopkinson pressure bar apparatus, and three dimensional （3D） transient numerical simulations were performed by a finite element （FE） computer program. Macroscopical plastic deformation was observed at the loading and supporting points, on the specimens, after the test, which could cause a large error if omitted in the numerical simulation. Therefore, elustic-viscoplustic analysis was performed on the specimen by adopting the Johnson-Cook （J-C） model to describe the rate-dependent plastic flow behavior of the material. The material heterogeneity in the mismatched welded joints, induced by the difference in the base metal （BM） and the weld metal （WM） in yield stress, has also been taken into consideration by using the J-C models separately. Good accordance was obtained between the experimental and the computational results by the present approach. The relationship between plane stress DFT and loading rate was also obtained on the order of 108 MPa.m^1/2.s^-1.
基金the National Natural Science Foundation of China(No.11402146)the Young 1000 Talent Program of China
文摘Phase field description of fracture is a very promising approach for simulating crack initiation, propagation, merging and branching. This method greatly reduces the implementation complexity, compared with discrete descriptions of cracks. In this work, we provide an overview of phase field models for quasistatic and dynamic cases. Afterward, we present useful vectors and matrices for the implementation of this method in two and three dimensions.
