In this paper,Brazilian test was performed on disk samples of analogue materials with defined structural planes.The surface strain evolution process of the disk samples during loading was analyzed via digital image co...In this paper,Brazilian test was performed on disk samples of analogue materials with defined structural planes.The surface strain evolution process of the disk samples during loading was analyzed via digital image correlation.The damage evolution process was explored from a microscopic perspective by combining discrete element numerical simulation technology.The criterion of the failure mode of the disc specimen in the split state was theoretically deduced.The influence of structural surface roughness and loading inclination angle on the stress state at the center of the specimen was explored.The results showed that the failure modes of the samples could be divided into three typical modes as matrix failure,structural plane failure and combination failure.The rough structural plane improves the failure strength of the specimen by limiting its lateral deformation,and the degree of improvement weakens continuously with the increase of the inclination angle of the structural plane.As the inclination angle of the structural plane increases,the main type of microcracks in the structural plane changes from shear microcracks to tensile microcracks.This study contributes to a better understanding of macro-and meso-failure characteristics of rock masses with structural planes under a splitting state.展开更多
Tensile failure(spalling or slabbing)often occurs on the sidewall of deep tunnel,which is closely related to the coupled stress state of deep rock mass under high pre-static load and dynamic disturbance.To reveal the ...Tensile failure(spalling or slabbing)often occurs on the sidewall of deep tunnel,which is closely related to the coupled stress state of deep rock mass under high pre-static load and dynamic disturbance.To reveal the mechanism of rock tensile failure caused by this coupled stress mode,the Brazilian disc tests were carried on red sandstone under high pre-static load induced by dynamic disturbance.Based on the pure static tensile fracture load of red sandstone specimen,two static load levels(80%and 90%of the pure static tensile fracture load)were selected as the initial high pre-static loading state,and then the dynamic disturbance load was applied until the rock specimen was destroyed.The dynamic disturbance loading mode adopted a sinusoidal wave(sine-wave)load,and the loading wave amplitude was 20%and 10%of the pure static tensile fracture load,respectively.The dynamic disturbance frequencies were set to 1,10,20,30,40,and 50 Hz.The results show that the tensile failure strength and peak displacement of red sandstone specimens under coupled load actions are lower than those under pure static tensile load,and both parameters decrease significantly with the increase of dynamic disturbance frequency.With the increase of dynamic disturbance frequency,the decrease range of tensile strength of red sandstone increased from 3.3%to 9.4%when the pre-static load level is 80%.While when the pre-static load level is 90%,the decrease range will increase from 7.4%to 11.6%.This weakening effect of tensile strength shows that the deep surrounding rock is more likely to fail under the coupled load actions of pre-static load and dynamic disturbance.In this tensile failure mechanism of the deep surrounding rock,the stress environment of deep sidewall rock determines that the failure mode of rock is a tensile failure,the pre-static load level dominates the tensile failure strength of surrounding rock,and dynamic disturbance promotes the strength-weakening effect and affects the weakening range.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...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 SAI.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
A new specimen geometry-the double edge-cracked Brazilian disk and a relevant fracture analysis byweight function method are proposed for the investigation of rock fracture caused by compression-shear loading. Notonly...A new specimen geometry-the double edge-cracked Brazilian disk and a relevant fracture analysis byweight function method are proposed for the investigation of rock fracture caused by compression-shear loading. Notonly can the mixed mode fracture with any ratio of KⅠ/KⅡ be achieved, but also the pure mode Ⅱ crack extensioncan be obtained. The combined mode fracture analysis for this geometry shows that diametral compression in the far-field can induce a compression-shear stress state in the singular stress field ahead of crack tips. Experimental investi-gations conducted on marble specimens show that the pure mode Ⅱ crack extension can be obtained when the dimen-sionless crack length a＞0.7 and the inclined crack angle 5°≤ψ≤40°. Normalized mode Ⅰ and mode Ⅱ stress inten-sity factors decrease from -0.45 and 2.47 at ψ=5° to -1.65 and 1.52 at ψ=40°, respectively. The strains at threepoints of specimen are also measured in order to investigate the influence of stress singularity on initial crack exten-sion. The results show that the principal orientations of strain at three points are very stable in the loading process.The derived formulae are quite explicit, and the specimen geometry is easy to fabricate and convenient to achieve thepure mode Ⅱ crack extension. Therefore, it can hopefully be used to obtain mode Ⅱ fracture toughness of rock.展开更多
The center cracked Brazilian disk subjected to diametral compressive stress uniformly distributed along parts of its cylindrical surface is used to investigate combined mode fracture of brittle material. A fracture a...The center cracked Brazilian disk subjected to diametral compressive stress uniformly distributed along parts of its cylindrical surface is used to investigate combined mode fracture of brittle material. A fracture analysis is made of this specimen configuration. Explicit formulae for mode Ⅰ and mode Ⅱ stress intensity factor calculation are derived based on boundary integral equation method and related numerical solution given by Atkinson. The proposed formulae are valid in wide range of crack length a/R . This configuration can avoid splitting along load line usually occuring in Brazilian test and permit one to achieve easily pure mode Ⅱ crack growth (crack coplanar extension) and any combination of K Ⅰ and K Ⅱ by a simple alignment of crack orientation with respect to load line. SIF values from the present calculation and finite element solution are also given for comparison.展开更多
岩石-混凝土界面是工程结构的薄弱环节,对结构整体的强度和稳定性有重要影响。为反映岩-混界面天然粗糙状态,基于内聚力模型(cohesive zone model,简称CZM),建立了具有随机生成粗糙界面的岩石-混凝土复合巴西圆盘试件数值模型,通过不同...岩石-混凝土界面是工程结构的薄弱环节,对结构整体的强度和稳定性有重要影响。为反映岩-混界面天然粗糙状态,基于内聚力模型(cohesive zone model,简称CZM),建立了具有随机生成粗糙界面的岩石-混凝土复合巴西圆盘试件数值模型,通过不同加载角度下的巴西劈裂物理试验验证了该方法的可靠性,并探究了界面粗糙度、加载角度对试件峰值荷载和破坏特征的影响。结果表明:不同加载角度下,试件存在3种典型破坏模式:界面黏结破坏、复合破坏、双材料拉伸开裂破坏;加载角度对试件力学行为的影响以70°为界,加载角度小于70°时影响显著,大于70°后影响不显著;界面粗糙度的影响随加载角度的不同有较大差异,当加载角度在15°~65°范围内,提高界面粗糙度可显著提高试件峰值荷载,增强岩-混结构的承载能力;界面处应力状态的差异决定了试件破坏模式的不同,但粗糙的界面可以增强混凝土与岩石之间的黏结和互锁效应,对试件破坏模式产生影响。研究结果将加深对岩石-混凝土界面破坏机制的认识,对工程建设具有指导意义。展开更多
[Objective]The work is devoted to the study of irreversible deformation of artificial samples subjected to a set of standard experiments,with an aim to study their mechanical properties.The principal idea of the study...[Objective]The work is devoted to the study of irreversible deformation of artificial samples subjected to a set of standard experiments,with an aim to study their mechanical properties.The principal idea of the study is related to the preparation of an artificial material with an established constitutive behavior model.The existence of such a well-described material provides future opportunities to conduct controllable experiments on various mechanical processes in rock-like material for further development and validation of theoretical models used in rock mechanics.[Methods]A set of artificial samples was prepared for careful assessment through a number of loading tests.Experimental work was carried out to determine the rheological properties under conditions of triaxial compression tests and uniaxial tension.Triaxial loading tests are completed for 9 samples with varying radial stress levels(0-5 MPa).The samples are loaded up to the yield point with control of radial and volumetric strain.The experimental results,which contain the obtained interrelationships between axial and radial stresses and strains,are analyzed using the Drucker-Prager yield surface.Material hardening is taken into account through the non-associated plastic flow law with the cap model.Numerical modeling of sample loading is performed through the finite difference method.Mathematical model parameters are adjusted to minimize the discrepancy between numerical modeling results and experimental data.The design of a series of experimental studies necessary to determine all the parameters of the model has been studied.[Results]It is shown that the formulated mathematical model allows to reliably reproduce the inelastic behavior of the studied material,and it can be used to solve a set of applied problems in continuum mechanics,the problem of numerical simulation of hydraulic fracture growth in an elastoplastic medium in particular.It was found that for the entire range of applied lateral loads(0-5 MPa),the elastic limit varied from 2 to 4 MPa,after which the material began to behave plastically.It was also determined that at lateral loads≥3 MPa,compaction began to appear in the material beyond the yield point.Judging by the dependence of volumetric strains under a lateral load equal to 1.4 MPa,compaction should begin to appear even at lateral loads lower than 3 MPa.[Conclusion]Taking the plastic behavior of the material into account is necessary when moving on to modeling the hydraulic fracturing process in such a material,and the resultant plasticity parameters for the model material can be used for numerical modeling of elastoplastic deformation of the rock under consideration,including processes such as hydraulic fracture growth in a poroelastoplastic medium.[Significance]The suggested procedure to interpret results of experimental studies can be used for further numerical modeling of mechanical processes in rock masses with inelastic strain accumulation.This opportunity can increase the reliability of geomechanical models used for the optimization of hydrocarbon fields development.展开更多
基金Project(52274148)supported by the National Natural Science Foundation of ChinaProject(2022XJLJ01)supported by the Fundamental Research Funds for the Central Universities,China。
文摘In this paper,Brazilian test was performed on disk samples of analogue materials with defined structural planes.The surface strain evolution process of the disk samples during loading was analyzed via digital image correlation.The damage evolution process was explored from a microscopic perspective by combining discrete element numerical simulation technology.The criterion of the failure mode of the disc specimen in the split state was theoretically deduced.The influence of structural surface roughness and loading inclination angle on the stress state at the center of the specimen was explored.The results showed that the failure modes of the samples could be divided into three typical modes as matrix failure,structural plane failure and combination failure.The rough structural plane improves the failure strength of the specimen by limiting its lateral deformation,and the degree of improvement weakens continuously with the increase of the inclination angle of the structural plane.As the inclination angle of the structural plane increases,the main type of microcracks in the structural plane changes from shear microcracks to tensile microcracks.This study contributes to a better understanding of macro-and meso-failure characteristics of rock masses with structural planes under a splitting state.
基金Projects(42077244,41877272,41472269)supported by the National Natural Science Foundation of ChinaProject(2242020R10023)supported by the Fundamental Research Funds for the Central Universities of Southeast University,China。
文摘Tensile failure(spalling or slabbing)often occurs on the sidewall of deep tunnel,which is closely related to the coupled stress state of deep rock mass under high pre-static load and dynamic disturbance.To reveal the mechanism of rock tensile failure caused by this coupled stress mode,the Brazilian disc tests were carried on red sandstone under high pre-static load induced by dynamic disturbance.Based on the pure static tensile fracture load of red sandstone specimen,two static load levels(80%and 90%of the pure static tensile fracture load)were selected as the initial high pre-static loading state,and then the dynamic disturbance load was applied until the rock specimen was destroyed.The dynamic disturbance loading mode adopted a sinusoidal wave(sine-wave)load,and the loading wave amplitude was 20%and 10%of the pure static tensile fracture load,respectively.The dynamic disturbance frequencies were set to 1,10,20,30,40,and 50 Hz.The results show that the tensile failure strength and peak displacement of red sandstone specimens under coupled load actions are lower than those under pure static tensile load,and both parameters decrease significantly with the increase of dynamic disturbance frequency.With the increase of dynamic disturbance frequency,the decrease range of tensile strength of red sandstone increased from 3.3%to 9.4%when the pre-static load level is 80%.While when the pre-static load level is 90%,the decrease range will increase from 7.4%to 11.6%.This weakening effect of tensile strength shows that the deep surrounding rock is more likely to fail under the coupled load actions of pre-static load and dynamic disturbance.In this tensile failure mechanism of the deep surrounding rock,the stress environment of deep sidewall rock determines that the failure mode of rock is a tensile failure,the pre-static load level dominates the tensile failure strength of surrounding rock,and dynamic disturbance promotes the strength-weakening effect and affects the weakening range.
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 SAI.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Project (50274074) supported by the National Natural Science Foundation of China
文摘A new specimen geometry-the double edge-cracked Brazilian disk and a relevant fracture analysis byweight function method are proposed for the investigation of rock fracture caused by compression-shear loading. Notonly can the mixed mode fracture with any ratio of KⅠ/KⅡ be achieved, but also the pure mode Ⅱ crack extensioncan be obtained. The combined mode fracture analysis for this geometry shows that diametral compression in the far-field can induce a compression-shear stress state in the singular stress field ahead of crack tips. Experimental investi-gations conducted on marble specimens show that the pure mode Ⅱ crack extension can be obtained when the dimen-sionless crack length a＞0.7 and the inclined crack angle 5°≤ψ≤40°. Normalized mode Ⅰ and mode Ⅱ stress inten-sity factors decrease from -0.45 and 2.47 at ψ=5° to -1.65 and 1.52 at ψ=40°, respectively. The strains at threepoints of specimen are also measured in order to investigate the influence of stress singularity on initial crack exten-sion. The results show that the principal orientations of strain at three points are very stable in the loading process.The derived formulae are quite explicit, and the specimen geometry is easy to fabricate and convenient to achieve thepure mode Ⅱ crack extension. Therefore, it can hopefully be used to obtain mode Ⅱ fracture toughness of rock.
文摘The center cracked Brazilian disk subjected to diametral compressive stress uniformly distributed along parts of its cylindrical surface is used to investigate combined mode fracture of brittle material. A fracture analysis is made of this specimen configuration. Explicit formulae for mode Ⅰ and mode Ⅱ stress intensity factor calculation are derived based on boundary integral equation method and related numerical solution given by Atkinson. The proposed formulae are valid in wide range of crack length a/R . This configuration can avoid splitting along load line usually occuring in Brazilian test and permit one to achieve easily pure mode Ⅱ crack growth (crack coplanar extension) and any combination of K Ⅰ and K Ⅱ by a simple alignment of crack orientation with respect to load line. SIF values from the present calculation and finite element solution are also given for comparison.
文摘岩石-混凝土界面是工程结构的薄弱环节,对结构整体的强度和稳定性有重要影响。为反映岩-混界面天然粗糙状态,基于内聚力模型(cohesive zone model,简称CZM),建立了具有随机生成粗糙界面的岩石-混凝土复合巴西圆盘试件数值模型,通过不同加载角度下的巴西劈裂物理试验验证了该方法的可靠性,并探究了界面粗糙度、加载角度对试件峰值荷载和破坏特征的影响。结果表明:不同加载角度下,试件存在3种典型破坏模式:界面黏结破坏、复合破坏、双材料拉伸开裂破坏;加载角度对试件力学行为的影响以70°为界,加载角度小于70°时影响显著,大于70°后影响不显著;界面粗糙度的影响随加载角度的不同有较大差异,当加载角度在15°~65°范围内,提高界面粗糙度可显著提高试件峰值荷载,增强岩-混结构的承载能力;界面处应力状态的差异决定了试件破坏模式的不同,但粗糙的界面可以增强混凝土与岩石之间的黏结和互锁效应,对试件破坏模式产生影响。研究结果将加深对岩石-混凝土界面破坏机制的认识,对工程建设具有指导意义。
文摘[Objective]The work is devoted to the study of irreversible deformation of artificial samples subjected to a set of standard experiments,with an aim to study their mechanical properties.The principal idea of the study is related to the preparation of an artificial material with an established constitutive behavior model.The existence of such a well-described material provides future opportunities to conduct controllable experiments on various mechanical processes in rock-like material for further development and validation of theoretical models used in rock mechanics.[Methods]A set of artificial samples was prepared for careful assessment through a number of loading tests.Experimental work was carried out to determine the rheological properties under conditions of triaxial compression tests and uniaxial tension.Triaxial loading tests are completed for 9 samples with varying radial stress levels(0-5 MPa).The samples are loaded up to the yield point with control of radial and volumetric strain.The experimental results,which contain the obtained interrelationships between axial and radial stresses and strains,are analyzed using the Drucker-Prager yield surface.Material hardening is taken into account through the non-associated plastic flow law with the cap model.Numerical modeling of sample loading is performed through the finite difference method.Mathematical model parameters are adjusted to minimize the discrepancy between numerical modeling results and experimental data.The design of a series of experimental studies necessary to determine all the parameters of the model has been studied.[Results]It is shown that the formulated mathematical model allows to reliably reproduce the inelastic behavior of the studied material,and it can be used to solve a set of applied problems in continuum mechanics,the problem of numerical simulation of hydraulic fracture growth in an elastoplastic medium in particular.It was found that for the entire range of applied lateral loads(0-5 MPa),the elastic limit varied from 2 to 4 MPa,after which the material began to behave plastically.It was also determined that at lateral loads≥3 MPa,compaction began to appear in the material beyond the yield point.Judging by the dependence of volumetric strains under a lateral load equal to 1.4 MPa,compaction should begin to appear even at lateral loads lower than 3 MPa.[Conclusion]Taking the plastic behavior of the material into account is necessary when moving on to modeling the hydraulic fracturing process in such a material,and the resultant plasticity parameters for the model material can be used for numerical modeling of elastoplastic deformation of the rock under consideration,including processes such as hydraulic fracture growth in a poroelastoplastic medium.[Significance]The suggested procedure to interpret results of experimental studies can be used for further numerical modeling of mechanical processes in rock masses with inelastic strain accumulation.This opportunity can increase the reliability of geomechanical models used for the optimization of hydrocarbon fields development.
