Dynamic tensile failure is a common phenomenon in deep rock practices,and thus accurately evaluating the dynamic tensile responses of rocks under triaxial pressures is of great significance.The Brazilian disc(BD)test ...Dynamic tensile failure is a common phenomenon in deep rock practices,and thus accurately evaluating the dynamic tensile responses of rocks under triaxial pressures is of great significance.The Brazilian disc(BD)test is the suggested method by the International Society for Rock Mechanics and Rock Engineering(ISRM)for measuring both the static and dynamic tensile strengths of rock-like materials.However,due to the overload phenomenon and the complex preloading conditions,the dynamic tensile strengths of rocks measured by the BD tests tend to be overestimated.To address this issue,the dynamic BD tensile strength(BTS)of Fangshan marble(FM)under different preloading conditions were measured through a triaxial split Hopkinson pressure bar(SHPB).The fracture onset in BD specimen was captured through a strain gage around the disc center.The discrepancy between the traditional tensile strength(TTS,determined by the peak load P_(f) of the BD specimen)and the nominal tensile strength(NTS,obtained from the load P_(i) when the diametral fracture commences in the tested BD specimen)was applied to quantitatively evaluating the overload phenomenon.The Griffith criterion was used to rectify the calculation of the tensile stress at the disc center under triaxial stress states.The results demonstrate that the overload ratio(s)increases with the loading rate(σ)and decreases with the hydrostatic pressure(σ_(s)).The TTS corrected by the Griffith criterion is independent of theσ_(s)due to the overload phenomenon,while the NTS corrected by the Griffith criterion is sensitive to both the andσ.Therefore,it is essential to modify the tensile stress in dynamic confined BD tests using both the overload correction and the Griffith criterion rectification to obtain the accurate dynamic BTS of rocks.展开更多
The Brazilian test is a widely used method for determining the tensile strength of rocks and for calibrating parameters in bondedparticle models（BPMs）. In previous studies, the Brazilian disc has typically been trim...The Brazilian test is a widely used method for determining the tensile strength of rocks and for calibrating parameters in bondedparticle models（BPMs）. In previous studies, the Brazilian disc has typically been trimmed from a compacted rectangular specimen. The present study shows that different tensile strength values are obtained depending on the compressive loading direction. Several measures are proposed to reduce the anisotropy of the disc. The results reveal that the anisotropy of the disc is significantly influenced by the compactibility of the specimen from which it is trimmed. A new method is proposed in which the Brazilian disc is directly generated with a particle boundary, effectively reducing the anisotropy. The stiffness（particle and bond） and strength（bond） of the boundary are set at less than and greater than those of the disc assembly, respectively,which significantly decreases the stress concentration at the boundary contacts and prevents breakage of the boundary particle bonds. This leads to a significant reduction in the anisotropy of the disc and the discreteness of the tensile strength. This method is more suitable for carrying out a realistic Brazilian test for homogeneous rock-like material in the BPM.展开更多
The general contact problem of a disc squeezed between jaws of arbitrary curvature is considered employing Muskhelishvili’s complex potentials.Taking advantage of the general solution introduced,the closed-form expre...The general contact problem of a disc squeezed between jaws of arbitrary curvature is considered employing Muskhelishvili’s complex potentials.Taking advantage of the general solution introduced,the closed-form expressions for the stresses along strategic loci(loaded rim,loaded diameter,disc’s center)are obtained,in terms of the ratio r of the disc’s to the jaw’s curvature.Then,the effect of r(as well as that of the relative stiffness of the disc’s and jaw’s materials dictating the contact arc)on the stress distribution along these loci is explored.It is concluded that,for both smooth contact(zero friction)and contact with friction,the role of the jaw’s curvature is significant not only along the disc-jaw contact arc(as it could be expected),but also all along the loaded diameter.On the other hand,it is indicated that the stress field at the disc’s center is more or less insensitive to the jaw’s curvature assuming that r lies within the range(0,0.67)or in other words within the limits defined by the two standardized suggestions,i.e.that of American Society for Testing and Materials(ASTM)(plane loading platens with r ? 0)and that of International Society for Rock Mechanics(ISRM)(curved jaws with r ? 0.67).The upper limit of this range is a kind of compromise between the need to make the stress field at the disc’s center independent of the boundary conditions while keeping at the same time the contact angle large enough to reduce the stress concentration and the risk for premature fracture initiation far from the disc’s center.For jaws with radius of curvature exceeded by that suggested by ISRM,the stress field at the disc’s center is significantly influenced.Especially for jaws with radius approaching that of the disc,the stress field at the disc’s center is dramatically distorted rendering Hondros’ formula inapplicable and the test results erroneous.展开更多
To reveal stress distribution and crack propagation of Brazilian discs under impact loads, dynamic tests were conducted with SHPB (split Hopkinson pressure bar) device. Stress states of specimens were monitored with s...To reveal stress distribution and crack propagation of Brazilian discs under impact loads, dynamic tests were conducted with SHPB (split Hopkinson pressure bar) device. Stress states of specimens were monitored with strain gauges on specimen surface and SHPB bars. The failure process of specimen was recorded by ultra speed camera FASTCAM SA1.1 (675 000 fps). Stress histories from strain gauges offer comprehensive information to evaluate the stress equilibrium of specimen in time and space. When a slowly rising load (with loading rates less than 1 200 N/s for d 50 mm bar) is applied, there is usually good stress equilibrium in specimen. The stress distribution after equilibrium is similar to its static counterpart. And the first crack initiates at the disc center and propagates along the load direction. But with the front of incident wave becoming steep, it is hard for specimens to get to stress equilibrium. The first crack may appear anywhere on the specimen together with multiple randomly distributed secondary cracks. For a valid dynamic Brazil test with stress equilibrium, the specimen will break into two halves neatly. While for tests with stress disequilibrium, missing strap may be found when broken halves of specimens are put together. For those specimens broken up neatly at center but having missing wedges at the loading areas, it is usually subjected to local buckling from SHPB bars.展开更多
Both experimental and numerical approaches were used to study dynamic failure properties and patterns of flattened Brazilian discs,containing two prefabricated cracks intersected at a varying angle.Mechanism of crack ...Both experimental and numerical approaches were used to study dynamic failure properties and patterns of flattened Brazilian discs,containing two prefabricated cracks intersected at a varying angle.Mechanism of crack initiation,propagation,and cut-through were scrutinized and influences of the intersection angle on specimen strength and acoustic emission performance were also studied.All primary cracks initialize near the middle or the tip points of the upper prefabricated crack,and they continue to develop along the load direction and finally cut through the specimen.The secondary cracks could be observed in directions almost horizontal or parallel to the directions of prefabricated cracks.Furthermore,it is found that stress intensity factor reaches its maximum for specimen with intersection angle of 0 degree.展开更多
Split Hopkinson pressure bar(SHPB)has been used to study the dynamic failure pattern of flattened mortar Brazilian disc under impact load.Each disc contains several prefabricated cracks paralleled to each other.Dynami...Split Hopkinson pressure bar(SHPB)has been used to study the dynamic failure pattern of flattened mortar Brazilian disc under impact load.Each disc contains several prefabricated cracks paralleled to each other.Dynamic FEM has also been adopted to simulate such failure behavior.The mechanism of crack initiation,propagation and cut-through have been scrutinized with both experimental and numerical approaches.Influence of the number of the prefabricated cracks on the specimen strength and acoustic emission(AE)performance can be observed and studied.The results show that the strength decreases and AE counts increases,when the number of the prefabricated cracks increases.展开更多
According to the results evaluated by researchers for mode I rock fracture toughness measurement, a series of comparison tests with different types Brazilian discs were conducted in order to search for the simplest ge...According to the results evaluated by researchers for mode I rock fracture toughness measurement, a series of comparison tests with different types Brazilian discs were conducted in order to search for the simplest geometry specimens by which accurate, comparable and consistent mode I rock fracture toughness could obtain.展开更多
Cracked straight-through Brazilian disc(CSTBD) samples prepared using two rock materials were used for thermal treatment from room temperature to 700℃. Uniaxial splitting experiments were performed using an automatic...Cracked straight-through Brazilian disc(CSTBD) samples prepared using two rock materials were used for thermal treatment from room temperature to 700℃. Uniaxial splitting experiments were performed using an automatic electro-hydraulic servo press to study the evolution laws of physical and fracture properties of different deep rock materials under high-temperature geological conditions. The fracture characteristics were measured using an industrial camera and digital image correlation technology to analyze the effect of high temperature on fracture properties and failure modes of the CSTBD samples after different thermal treatments. The micro-damage properties of green sandstone and granite materials were obtained using a scanning electron microscope(SEM). The following conclusions were drawn from the test results:(1) With the increasing temperature, the fracture characteristics of green sandstone and granite change from brittle fracture to plasticity fracture, the longitudinal wave velocity of granite decreases sharply at 600℃, and the damage factor reaches 0.8748 at 700℃.(2) The fracture toughness of green sandstone and granite decreases with increasing temperature;however, the decreasing range of granite is larger than that of green sandstone.(3) As the temperature increases, the fracture morphologies of green sandstone and granite materials become rougher, whereas thermal damage cracks of granite and intergranular fractures inside sandstone as well as pores of sandstone increase.(4) The crack tip opening displacement and peak strain corresponding to peak load increase with the temperature.展开更多
通过脆性岩石试样巴西圆盘试验研究了疲劳效应对脆性岩石断裂韧度KIC的影响,阐释了脆性岩石的疲劳损伤机制,首次展示了脆性岩石破坏前宏观裂纹的张开和闭合行为。巴西圆盘试验结果表明,循环荷载作用下,脆性岩样的KIC降低了35%,巴西劈裂...通过脆性岩石试样巴西圆盘试验研究了疲劳效应对脆性岩石断裂韧度KIC的影响,阐释了脆性岩石的疲劳损伤机制,首次展示了脆性岩石破坏前宏观裂纹的张开和闭合行为。巴西圆盘试验结果表明,循环荷载作用下,脆性岩样的KIC降低了35%,巴西劈裂拉伸强度降低了30%。通过高速相机数小时的观测并记录到了岩石力学领域从未观测到的过程,即脆性岩样破坏前,疲劳裂纹在正弦荷载作用下弹性张合。扫描电镜和计算机断层扫描结果显示,巴西圆盘和V形切槽巴西圆盘试样的破坏是由断裂过程区(fracture process zone,简称FPZ)引起的,FPZ中包含许多异于循环荷载作用下单条宏观裂纹的微裂纹,切槽裂纹尖端FPZ的形成导致未破坏脆性岩样中可视疲劳裂纹弹性张合。室内试验和数值计算结果表明,切槽裂纹倾角为60°时获得最大FPZ(即FPZmax),这表明最大FPZ的形成可能与Ⅰ-Ⅱ型(拉伸和剪切)组合加载模式有关。展开更多
基金We acknowledge the funding support from the National Natural Science Foundation of China(Grant Nos.42141010,51879184 and 12172253).
文摘Dynamic tensile failure is a common phenomenon in deep rock practices,and thus accurately evaluating the dynamic tensile responses of rocks under triaxial pressures is of great significance.The Brazilian disc(BD)test is the suggested method by the International Society for Rock Mechanics and Rock Engineering(ISRM)for measuring both the static and dynamic tensile strengths of rock-like materials.However,due to the overload phenomenon and the complex preloading conditions,the dynamic tensile strengths of rocks measured by the BD tests tend to be overestimated.To address this issue,the dynamic BD tensile strength(BTS)of Fangshan marble(FM)under different preloading conditions were measured through a triaxial split Hopkinson pressure bar(SHPB).The fracture onset in BD specimen was captured through a strain gage around the disc center.The discrepancy between the traditional tensile strength(TTS,determined by the peak load P_(f) of the BD specimen)and the nominal tensile strength(NTS,obtained from the load P_(i) when the diametral fracture commences in the tested BD specimen)was applied to quantitatively evaluating the overload phenomenon.The Griffith criterion was used to rectify the calculation of the tensile stress at the disc center under triaxial stress states.The results demonstrate that the overload ratio(s)increases with the loading rate(σ)and decreases with the hydrostatic pressure(σ_(s)).The TTS corrected by the Griffith criterion is independent of theσ_(s)due to the overload phenomenon,while the NTS corrected by the Griffith criterion is sensitive to both the andσ.Therefore,it is essential to modify the tensile stress in dynamic confined BD tests using both the overload correction and the Griffith criterion rectification to obtain the accurate dynamic BTS of rocks.
基金Support provided by the National Basic Research Program of China (2015CB258500, 2015CB058102, 2014CB046904)
文摘The Brazilian test is a widely used method for determining the tensile strength of rocks and for calibrating parameters in bondedparticle models（BPMs）. In previous studies, the Brazilian disc has typically been trimmed from a compacted rectangular specimen. The present study shows that different tensile strength values are obtained depending on the compressive loading direction. Several measures are proposed to reduce the anisotropy of the disc. The results reveal that the anisotropy of the disc is significantly influenced by the compactibility of the specimen from which it is trimmed. A new method is proposed in which the Brazilian disc is directly generated with a particle boundary, effectively reducing the anisotropy. The stiffness（particle and bond） and strength（bond） of the boundary are set at less than and greater than those of the disc assembly, respectively,which significantly decreases the stress concentration at the boundary contacts and prevents breakage of the boundary particle bonds. This leads to a significant reduction in the anisotropy of the disc and the discreteness of the tensile strength. This method is more suitable for carrying out a realistic Brazilian test for homogeneous rock-like material in the BPM.
基金co-financed by the EU(European Social Fund-ESF)Greek National Funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework(NSRF)-ResearchFunding Program:THALES:Reinforcement of the interdisciplinary and/or inter-institutional research and innovation
文摘The general contact problem of a disc squeezed between jaws of arbitrary curvature is considered employing Muskhelishvili’s complex potentials.Taking advantage of the general solution introduced,the closed-form expressions for the stresses along strategic loci(loaded rim,loaded diameter,disc’s center)are obtained,in terms of the ratio r of the disc’s to the jaw’s curvature.Then,the effect of r(as well as that of the relative stiffness of the disc’s and jaw’s materials dictating the contact arc)on the stress distribution along these loci is explored.It is concluded that,for both smooth contact(zero friction)and contact with friction,the role of the jaw’s curvature is significant not only along the disc-jaw contact arc(as it could be expected),but also all along the loaded diameter.On the other hand,it is indicated that the stress field at the disc’s center is more or less insensitive to the jaw’s curvature assuming that r lies within the range(0,0.67)or in other words within the limits defined by the two standardized suggestions,i.e.that of American Society for Testing and Materials(ASTM)(plane loading platens with r ? 0)and that of International Society for Rock Mechanics(ISRM)(curved jaws with r ? 0.67).The upper limit of this range is a kind of compromise between the need to make the stress field at the disc’s center independent of the boundary conditions while keeping at the same time the contact angle large enough to reduce the stress concentration and the risk for premature fracture initiation far from the disc’s center.For jaws with radius of curvature exceeded by that suggested by ISRM,the stress field at the disc’s center is significantly influenced.Especially for jaws with radius approaching that of the disc,the stress field at the disc’s center is dramatically distorted rendering Hondros’ formula inapplicable and the test results erroneous.
基金Projects(50904079, 51274254, 50934006) supported by the National Natural Science Foundation of ChinaProject(2010CB732004) supported by the National Basic Research Program of ChinaProject(NCET-11-0528) supported by Program for New Century Excellent Talents in University of China
文摘To reveal stress distribution and crack propagation of Brazilian discs under impact loads, dynamic tests were conducted with SHPB (split Hopkinson pressure bar) device. Stress states of specimens were monitored with strain gauges on specimen surface and SHPB bars. The failure process of specimen was recorded by ultra speed camera FASTCAM SA1.1 (675 000 fps). Stress histories from strain gauges offer comprehensive information to evaluate the stress equilibrium of specimen in time and space. When a slowly rising load (with loading rates less than 1 200 N/s for d 50 mm bar) is applied, there is usually good stress equilibrium in specimen. The stress distribution after equilibrium is similar to its static counterpart. And the first crack initiates at the disc center and propagates along the load direction. But with the front of incident wave becoming steep, it is hard for specimens to get to stress equilibrium. The first crack may appear anywhere on the specimen together with multiple randomly distributed secondary cracks. For a valid dynamic Brazil test with stress equilibrium, the specimen will break into two halves neatly. While for tests with stress disequilibrium, missing strap may be found when broken halves of specimens are put together. For those specimens broken up neatly at center but having missing wedges at the loading areas, it is usually subjected to local buckling from SHPB bars.
基金Funded by the National Natural Science Foundation of China(Nos.51878190 and 51678170)。
文摘Both experimental and numerical approaches were used to study dynamic failure properties and patterns of flattened Brazilian discs,containing two prefabricated cracks intersected at a varying angle.Mechanism of crack initiation,propagation,and cut-through were scrutinized and influences of the intersection angle on specimen strength and acoustic emission performance were also studied.All primary cracks initialize near the middle or the tip points of the upper prefabricated crack,and they continue to develop along the load direction and finally cut through the specimen.The secondary cracks could be observed in directions almost horizontal or parallel to the directions of prefabricated cracks.Furthermore,it is found that stress intensity factor reaches its maximum for specimen with intersection angle of 0 degree.
基金Funded by the National Natural Science Foundation of China(Nos.51678170,51878190)。
文摘Split Hopkinson pressure bar(SHPB)has been used to study the dynamic failure pattern of flattened mortar Brazilian disc under impact load.Each disc contains several prefabricated cracks paralleled to each other.Dynamic FEM has also been adopted to simulate such failure behavior.The mechanism of crack initiation,propagation and cut-through have been scrutinized with both experimental and numerical approaches.Influence of the number of the prefabricated cracks on the specimen strength and acoustic emission(AE)performance can be observed and studied.The results show that the strength decreases and AE counts increases,when the number of the prefabricated cracks increases.
文摘According to the results evaluated by researchers for mode I rock fracture toughness measurement, a series of comparison tests with different types Brazilian discs were conducted in order to search for the simplest geometry specimens by which accurate, comparable and consistent mode I rock fracture toughness could obtain.
基金funding support from the Sichuan Science and Technology Program (Grant No. 2021YJ0511)the State Key Laboratory for Geo-Mechanics and Deep Underground Engineering,China University of Mining&Technology (Grant No.SKLGDUEK2111)the Key Laboratory of Rock Mechanics and Geohazards of Zhejiang Province (Grant No. ZJRMG-2020-01)。
文摘Cracked straight-through Brazilian disc(CSTBD) samples prepared using two rock materials were used for thermal treatment from room temperature to 700℃. Uniaxial splitting experiments were performed using an automatic electro-hydraulic servo press to study the evolution laws of physical and fracture properties of different deep rock materials under high-temperature geological conditions. The fracture characteristics were measured using an industrial camera and digital image correlation technology to analyze the effect of high temperature on fracture properties and failure modes of the CSTBD samples after different thermal treatments. The micro-damage properties of green sandstone and granite materials were obtained using a scanning electron microscope(SEM). The following conclusions were drawn from the test results:(1) With the increasing temperature, the fracture characteristics of green sandstone and granite change from brittle fracture to plasticity fracture, the longitudinal wave velocity of granite decreases sharply at 600℃, and the damage factor reaches 0.8748 at 700℃.(2) The fracture toughness of green sandstone and granite decreases with increasing temperature;however, the decreasing range of granite is larger than that of green sandstone.(3) As the temperature increases, the fracture morphologies of green sandstone and granite materials become rougher, whereas thermal damage cracks of granite and intergranular fractures inside sandstone as well as pores of sandstone increase.(4) The crack tip opening displacement and peak strain corresponding to peak load increase with the temperature.
文摘通过脆性岩石试样巴西圆盘试验研究了疲劳效应对脆性岩石断裂韧度KIC的影响,阐释了脆性岩石的疲劳损伤机制,首次展示了脆性岩石破坏前宏观裂纹的张开和闭合行为。巴西圆盘试验结果表明,循环荷载作用下,脆性岩样的KIC降低了35%,巴西劈裂拉伸强度降低了30%。通过高速相机数小时的观测并记录到了岩石力学领域从未观测到的过程,即脆性岩样破坏前,疲劳裂纹在正弦荷载作用下弹性张合。扫描电镜和计算机断层扫描结果显示,巴西圆盘和V形切槽巴西圆盘试样的破坏是由断裂过程区(fracture process zone,简称FPZ)引起的,FPZ中包含许多异于循环荷载作用下单条宏观裂纹的微裂纹,切槽裂纹尖端FPZ的形成导致未破坏脆性岩样中可视疲劳裂纹弹性张合。室内试验和数值计算结果表明,切槽裂纹倾角为60°时获得最大FPZ(即FPZmax),这表明最大FPZ的形成可能与Ⅰ-Ⅱ型(拉伸和剪切)组合加载模式有关。
