Large-scale discontinuities can significantly affect the mechanical properties of rock masses.However,the tensile behavior of rock discontinuities is often less investigated.To study the statistical characteristics of...Large-scale discontinuities can significantly affect the mechanical properties of rock masses.However,the tensile behavior of rock discontinuities is often less investigated.To study the statistical characteristics of failure strength and fracture characteristics of rock discontinuities,Brazilian disc tests were conducted on limestone specimens with a single natural discontinuity at different load-discontinuity angles(β).In this study,β=0°andβ=90°correspond to the discontinuity parallel and perpendicular to loading direction,respectively.The results show that Brazilian failure strength(BFS)can reasonably represent the tensile strength of rock with discontinuities,by comparing the BFS and tensile stress in the disc center at peak force.The two-parameter Weibull distribution can capture the statistical BFS characteristics of rock discontinuities parallel to loading direction(β=0°)and at different loaddiscontinuity angles(β≠0°).All specimens with discontinuity at different load-discontinuity angles show more plastic deformational behaviour than intact rock specimen.With increasingβ,the mean BFS of limestone with discontinuity increases before reaching a plateau atβ=45°.The single plane of weakness theory best explains the BFS of fractured limestone withβ.Only a specific segment of preexisting rock discontinuity could affect the fracture process.Whenβ=0°,interfacial cracks and alternative cracks formed.Whenβ≠0°,mixed failure mode with shear and tensile failure occurred,particularly whenβ=30°andβ=60°.The findings can contribute to better understanding the failure and fracture characteristics of rock with discontinuities,particularly the interaction of pre-existing discontinuities with stress-induced fracturing.展开更多
Under strong earthquakes, long-span spatial latticed structures may collapse due to dynamic instability or strength failure. The elasto-plastic dynamic behaviors of three spatial latticed structures, including two dou...Under strong earthquakes, long-span spatial latticed structures may collapse due to dynamic instability or strength failure. The elasto-plastic dynamic behaviors of three spatial latticed structures, including two double-layer cylindrical shells and one spherical shell constructed for the 2008 Olympic Games in Beijing, were quantitatively examined under multi-support excitation （MSE） and uniform support excitation （USE）. In the numerical analyses, several important parameters were investigated such as the peak acceleration and displacement responses at key joints, the number and distribution of plastic members, and the deformation of the shell at the moment of collapse. Analysis results reveal the features and the failure mechanism of the spatial latticed structures under MSE and USE. In both scenarios, the double-layer reticulated shell collapses in the ＂overflow＂ mode, and the collapse is governed by the number of invalid plastic members rather than the total number of plastic members, beginning with damage to some of the local regions near the supports. By comparing the numbers and distributions of the plastic members under MSE to those under USE, it was observed that the plastic members spread more sufficiently and the internal forces are more uniform under MSE, especially in cases of lower apparent velocities in soils. Due to the effects of pseudo-static displacement, the stresses in the members near the supports under MSE are higher than those under USE.展开更多
A discrete element method （DEM） called particle flow code （PFC2D） was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of...A discrete element method （DEM） called particle flow code （PFC2D） was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2D that reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2D was used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows. （1） The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance. （2） The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle. （3） Three major failure modes, i.e., pure tensile failure, shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test. （4） The notch angle roles on crack initiation and and joint angle play important propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.展开更多
The strength of structural loess consists of the shear strength and tensile strength. In this study, the stress path, the failure envelope of principal stress （ Kf line）, and the strength failure envelope of structu...The strength of structural loess consists of the shear strength and tensile strength. In this study, the stress path, the failure envelope of principal stress （ Kf line）, and the strength failure envelope of structurally intact loess and remolded loess were analyzed through three kinds of tests： the tensile strength test, the uniaxial compressive strength test, and the conventional triaxial shear strength test. Then, in order to describe the tensile strength and shear strength of structural loess comprehensively and reasonably, a joint strength formula for structural loess was established. This formula comprehensively considers tensile and shear properties. Studies have shown that the tensile strength exhibits a decreasing trend with increasing water content. When the water content is constant, the tensile strength of the structurally intact soil is greater than that ofremolded soil. In the studies, no loss of the originally cured cohesion in the structurally intact soil samples was observed, given that the soil samples did not experience loading disturbance during the uniaxial compressive strength test, meaning there is a high initial structural strength. The results of the conventional triaxial shear strength test show that the water content is correlated with the strength of the structural loess. When the water content is low, the structural properties are strong, and when the water content is high, the structural properties are weak, which means that the water content and the ambient pressure have significant effects on the stress-strain relationship of structural loess. The established joint strength formula of structural loess effectively avoids overestimating the role of soil tensile strength in the traditional theory of Mohr-Coulomb strength.展开更多
Breaking down the entire structure of a material implies severing all the bonds between its atoms either by applying work or by heat transfer. Because bond-breaking is indifferent to either means, there is a kind of e...Breaking down the entire structure of a material implies severing all the bonds between its atoms either by applying work or by heat transfer. Because bond-breaking is indifferent to either means, there is a kind of equivalence between heat energy and strain energy. Based on this equivalence, we assume the existence of a constant maximum storage of energy that includes both the strain energy and the corresponding equivalent heat energy. A temperaturedependent fracture strength model is then developed for ultrahigh temperature ceramics （UHTCs）. Model predictions for UHTCs, HfB2, TiC and ZrB2, are presented and compared with the experimental results. These predictions are found to be largely consistent with experimental results.展开更多
A modified failure criterion is proposed to determine the strength of transversely isotropic rocks. Me-chanical properties of some metamorphic and sedimentary rocks including gneiss, slate, marble, schist, shale, sand...A modified failure criterion is proposed to determine the strength of transversely isotropic rocks. Me-chanical properties of some metamorphic and sedimentary rocks including gneiss, slate, marble, schist, shale, sandstone and limestone, which show transversely isotropic behavior, were taken into consider-ation. Afterward, introduced triaxial rock strength criterion was modified for transversely isotropic rocks. Through modification process an index was obtained that can be considered as a strength reduction parameter due to rock strength anisotropy. Comparison of the parameter with previous anisotropy in-dexes in literature showed reasonable results for the studied rock samples. The modified criterion was compared to modified Hoek-Brown and Ramamurthy criteria for different transversely isotropic rocks. It can be concluded that the modified failure criterion proposed in this study can be used for predicting the strength of transversely isotropic rocks.展开更多
Based on uniaxial compression experimental results on fractured sandstone with grouting and anchorage, we studied the strength and deformation properties, the failure model, crack formation and evolution laws of fract...Based on uniaxial compression experimental results on fractured sandstone with grouting and anchorage, we studied the strength and deformation properties, the failure model, crack formation and evolution laws of fractured sandstone under different conditions of anchorage. The experimental results show that the strength and elastic modulus of fractured sandstone with different fracture angles are significantly lower than those of intact sandstone. Compared with the fractured samples without anchorage,the peak strength, residual strength, peak and ultimate axial strain of fractured sandstone under different anchorage increase by 64.5–320.0%, 62.8–493.0%, and 31.6–181.4%, respectively. The number of bolts and degree of pre-stress has certain effects on the peak strength and failure model of fractured sandstone. The peak strength of fractured sandstone under different anchorage increases to some extent, and the failure model of fractured sandstone also transforms from tensile failure to tensile–shear mixed failure with the number of bolts. The pre-stress can restrain the formation and evolution process of tensile cracks, delay the failure process of fractured sandstone under anchorage and impel the transformation of failure model from brittle failure to plastic failure.展开更多
Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showe...Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showed that the violence of coal specimen failure depends on both the interface friction and width-to-height（W/H） ratio of coal specimen. The mode of failure for a uniaxially loaded coal specimen or a coal pillar is a combination of both shear failure along the interface and compressive failure in the coal. The shear failure along the interface triggered the compressive failure in coal. The compressive failure of a coal specimen or a coal pillar can be controlled by changing its W/H ratio. As the W/H ratio increases, the ultimate strength increases. Hence, with a proper combination of interface friction and the W/H ratio of pillar or coal specimen, the mode of failure will change from sudden violent failure which is brittle failure to non-violent failure which is ductile failure. The main objective of this paper is to determine at what W/H ratio and interface friction the mode of failure changes from violent to non-violent. In this research, coal specimens of W/H ratio ranging from 1 to 10 were uniaxially tested under two interface frictions of 0.1 and 0.25, and the results are presented and discussed.展开更多
The permeability evolution of rock during the progressive failure process is described. In combination with the strength degradation index, the degradation formulas of s and a, which are dependent on the plastic confi...The permeability evolution of rock during the progressive failure process is described. In combination with the strength degradation index, the degradation formulas of s and a, which are dependent on the plastic confining strain component, the material constants of Hock-Brown failure criterion are presented, and a modified elemental scale elastic-brittle-plastic constitutive model of rock is established. The rela- tionship between volumetric strain and permeability through tri-axial compression is investigated. Based on the above, a permeability evolution model is established. The model incorporates confining pressure- dependent degradation of strength, dilatancy and corresponding permeability evolution. The model is implemented in FLAC by the FISH function method. The permeability evolution behavior of rock is inves-tigated during the progressive failure process in a numerical case. The results show that the model is cap- able of reproducing, and allowing visualization of a range of hydro-mechanical responses of rock. The effects of confining pressure on degradation of strength, dilatancy and permeability evolution are also reflected.展开更多
This work aims to investigate local stress distribution, damage evolution and failure of notched composite laminates under in-plane loads. An analytic method containing uniformed boundary equations using a complex var...This work aims to investigate local stress distribution, damage evolution and failure of notched composite laminates under in-plane loads. An analytic method containing uniformed boundary equations using a complex variable approach is developed to present layer-by-layer stresses around the notch. The uniformed boundary equations established in series together with conformal mapping functions are capable of dealing with irregular boundary issues around the notch and at infinity. Stress results are employed to evaluate the damage initiation and propagation of notched composites by progressive damage analysis（PDA）. A user-defined subroutine is developed in the finite element（FE） model based on coupling theories for mixed failure criteria and damage mechanics to efficiently investigate damage evolution as well as failure modes. Carbon/epoxy laminates with a stacking sequence of [45°/0°/ 60°/90°]sare used to investigate surface strains, in-plane load capacity and microstructure of failure zones to provide analytic and FE methods with strong validation. Good agreement is observed between the analytic method, the FE model and experiments in terms of the stress（strain） distributions, damage evaluation and ultimate strength, and the layerby-layer stress components vary according to a combination effect of fiber orientation and loading type, causing diverse failure modes in individuals.展开更多
The mechanical properties of Z-pins reinforced composites adhesively bonded single-lap joints(SLJs)under un-directional tension loading are investigated by experimental and numerical methods.Three kinds of joint confi...The mechanical properties of Z-pins reinforced composites adhesively bonded single-lap joints(SLJs)under un-directional tension loading are investigated by experimental and numerical methods.Three kinds of joint configurations,including SLJs with three/two rows of Z-pins and“I”array of Z-pins,are investigated by tension test.The failure modes and mechanism of reinforced joints with different Z-pins numbers and alignment are analyzed,and the comparison is performed for the failure strengths of no Z-pins and Z-pins reinforced joints.According to experimental results,failure modes of three kinds of joints are all mixed failure.It turns out that the Z-pins are pulled out ultimately.The strength of joints of more Z-pins at the end of the overlap area is relatively bigger for the joint of the same Z-pins numbers.The strength of joints with Z-pins compared with non Z-pins joints is growing at 16%.Finally,the three-dimensional distribution of interfacial stress in the lap zone of three kinds of Z-pins reinforced joints is simulated,and the numerical results are in good agreement with the experimental results.It is effective that the numerical calculation of stress analysis is verified.展开更多
Under strong shocks,long-span spatial-latticed structures may collapse due to dynamic instability or strength failure.The elasto-plastic dynamic behaviors of three spatiallatticed structures,including two double-layer...Under strong shocks,long-span spatial-latticed structures may collapse due to dynamic instability or strength failure.The elasto-plastic dynamic behaviors of three spatiallatticed structures,including two double-layer cylindrical shells and a spheri-cal shell used for the 2008 Olympic Games in Beijing,were quantitatively examined under multi-support excitation(MSE) and uniform support excitation(USE).Numerical analyses described several important parameters such as the peak acceleration and displacement responses at key joints,the number and distribution of plastic elements,and the deformation of the shell at the moment of collapse.Results of the analysis revealed the features and the failure mechanism of the spatial-latticed structures under MSE and USE.In both scenarios,the double-layer reticulated shell collapsed in the "overflow" mode,collapse was govrned by the number of invalid plastic elements rather than the total number of plastic elements,and the collapse of the structure began with damage to certain local regions near the supports.By comparing the numbers and distributions of the plastic members under MSE to those under USE,it was observed that the plastic members spread more sufficiently and the internal forces were more uniform under MSE,especially for lower apparent velocities in soils.Due to the effects of pseudo-static displacement,the stresses in members near supports under MSE were higher than those under USE.These regions are prone to failure during earthquakes and deserve special attention in the seismic design of reticulated structures.展开更多
Discontinuities or structural planes are widely distributed in natural rock masses and significantly influence their geo-mechanical and geometric properties.Herein,a batch of rock samples,each with a single structural...Discontinuities or structural planes are widely distributed in natural rock masses and significantly influence their geo-mechanical and geometric properties.Herein,a batch of rock samples,each with a single structural plane,is created using a 3D printer equipped with two robotic manipulators.One of the manipulators is connected via a nozzle to a concrete pumping truck,which can extrude brittle rock-like material to form layered intact rock masses;the rock-like material is mainly composed of cement,silica fume,sand and water.The other manipulator features a knife,which can carve discontinuities onto each layer of the printed model.By means of this system,rock masses with discontinuous joints are formed,and the failure strengths of rock masses with different joints are demonstrated via uniaxial compression tests and direct shear tests.The results thereof obtained via digital image correlation technology show that discontinuities lower the strength of the rock mass models significantly.With the increase of the angle between the fracture and horizontal plane,the uniaxial compressive strength first decreases,and then increases.During shear testing,the shear strength of the rock mass models increases with the surface roughness of the preset joint.These test results indicate that the influence of artificial joints on the mechanical properties of the models is consistent with that of natural rock mass joints.Using digital modeling and 3D printing technology,cracks hidden in a rock mass can be reproduced.展开更多
基金financially supported by the Hong Kong Polytechnic University(Project No.1-ZVJW)the Program for Guangdong Introducing Innovative and Entrepreneurial Teams,China(Grant No.2019ZT08G315)。
文摘Large-scale discontinuities can significantly affect the mechanical properties of rock masses.However,the tensile behavior of rock discontinuities is often less investigated.To study the statistical characteristics of failure strength and fracture characteristics of rock discontinuities,Brazilian disc tests were conducted on limestone specimens with a single natural discontinuity at different load-discontinuity angles(β).In this study,β=0°andβ=90°correspond to the discontinuity parallel and perpendicular to loading direction,respectively.The results show that Brazilian failure strength(BFS)can reasonably represent the tensile strength of rock with discontinuities,by comparing the BFS and tensile stress in the disc center at peak force.The two-parameter Weibull distribution can capture the statistical BFS characteristics of rock discontinuities parallel to loading direction(β=0°)and at different loaddiscontinuity angles(β≠0°).All specimens with discontinuity at different load-discontinuity angles show more plastic deformational behaviour than intact rock specimen.With increasingβ,the mean BFS of limestone with discontinuity increases before reaching a plateau atβ=45°.The single plane of weakness theory best explains the BFS of fractured limestone withβ.Only a specific segment of preexisting rock discontinuity could affect the fracture process.Whenβ=0°,interfacial cracks and alternative cracks formed.Whenβ≠0°,mixed failure mode with shear and tensile failure occurred,particularly whenβ=30°andβ=60°.The findings can contribute to better understanding the failure and fracture characteristics of rock with discontinuities,particularly the interaction of pre-existing discontinuities with stress-induced fracturing.
文摘Under strong earthquakes, long-span spatial latticed structures may collapse due to dynamic instability or strength failure. The elasto-plastic dynamic behaviors of three spatial latticed structures, including two double-layer cylindrical shells and one spherical shell constructed for the 2008 Olympic Games in Beijing, were quantitatively examined under multi-support excitation （MSE） and uniform support excitation （USE）. In the numerical analyses, several important parameters were investigated such as the peak acceleration and displacement responses at key joints, the number and distribution of plastic members, and the deformation of the shell at the moment of collapse. Analysis results reveal the features and the failure mechanism of the spatial latticed structures under MSE and USE. In both scenarios, the double-layer reticulated shell collapses in the ＂overflow＂ mode, and the collapse is governed by the number of invalid plastic members rather than the total number of plastic members, beginning with damage to some of the local regions near the supports. By comparing the numbers and distributions of the plastic members under MSE to those under USE, it was observed that the plastic members spread more sufficiently and the internal forces are more uniform under MSE, especially in cases of lower apparent velocities in soils. Due to the effects of pseudo-static displacement, the stresses in the members near the supports under MSE are higher than those under USE.
基金supported by the National Basic Research Programof China(2014CB046905)the Fundamental Research Funds for the Central Universities(China University of Mining and Technology)(2014YC10)
文摘A discrete element method （DEM） called particle flow code （PFC2D） was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2D that reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2D was used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows. （1） The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance. （2） The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle. （3） Three major failure modes, i.e., pure tensile failure, shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test. （4） The notch angle roles on crack initiation and and joint angle play important propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.
基金supported by the National Natural Science Foundation of China(Grant No.11072193)the Fundamental Research Funds for the Central Universities(Grant No.2013G1502009)the China Postdoctoral Science Foundation(Grant No.20100481354)
文摘The strength of structural loess consists of the shear strength and tensile strength. In this study, the stress path, the failure envelope of principal stress （ Kf line）, and the strength failure envelope of structurally intact loess and remolded loess were analyzed through three kinds of tests： the tensile strength test, the uniaxial compressive strength test, and the conventional triaxial shear strength test. Then, in order to describe the tensile strength and shear strength of structural loess comprehensively and reasonably, a joint strength formula for structural loess was established. This formula comprehensively considers tensile and shear properties. Studies have shown that the tensile strength exhibits a decreasing trend with increasing water content. When the water content is constant, the tensile strength of the structurally intact soil is greater than that ofremolded soil. In the studies, no loss of the originally cured cohesion in the structurally intact soil samples was observed, given that the soil samples did not experience loading disturbance during the uniaxial compressive strength test, meaning there is a high initial structural strength. The results of the conventional triaxial shear strength test show that the water content is correlated with the strength of the structural loess. When the water content is low, the structural properties are strong, and when the water content is high, the structural properties are weak, which means that the water content and the ambient pressure have significant effects on the stress-strain relationship of structural loess. The established joint strength formula of structural loess effectively avoids overestimating the role of soil tensile strength in the traditional theory of Mohr-Coulomb strength.
基金The project was supported by the National Natural Science Foundation of China （90505015 and 10702035）.
文摘Breaking down the entire structure of a material implies severing all the bonds between its atoms either by applying work or by heat transfer. Because bond-breaking is indifferent to either means, there is a kind of equivalence between heat energy and strain energy. Based on this equivalence, we assume the existence of a constant maximum storage of energy that includes both the strain energy and the corresponding equivalent heat energy. A temperaturedependent fracture strength model is then developed for ultrahigh temperature ceramics （UHTCs）. Model predictions for UHTCs, HfB2, TiC and ZrB2, are presented and compared with the experimental results. These predictions are found to be largely consistent with experimental results.
文摘A modified failure criterion is proposed to determine the strength of transversely isotropic rocks. Me-chanical properties of some metamorphic and sedimentary rocks including gneiss, slate, marble, schist, shale, sandstone and limestone, which show transversely isotropic behavior, were taken into consider-ation. Afterward, introduced triaxial rock strength criterion was modified for transversely isotropic rocks. Through modification process an index was obtained that can be considered as a strength reduction parameter due to rock strength anisotropy. Comparison of the parameter with previous anisotropy in-dexes in literature showed reasonable results for the studied rock samples. The modified criterion was compared to modified Hoek-Brown and Ramamurthy criteria for different transversely isotropic rocks. It can be concluded that the modified failure criterion proposed in this study can be used for predicting the strength of transversely isotropic rocks.
基金Financial support for this work, provided by the National Natural Science Foundation of China (Nos. 50774082, 50804046 and 51109209)
文摘Based on uniaxial compression experimental results on fractured sandstone with grouting and anchorage, we studied the strength and deformation properties, the failure model, crack formation and evolution laws of fractured sandstone under different conditions of anchorage. The experimental results show that the strength and elastic modulus of fractured sandstone with different fracture angles are significantly lower than those of intact sandstone. Compared with the fractured samples without anchorage,the peak strength, residual strength, peak and ultimate axial strain of fractured sandstone under different anchorage increase by 64.5–320.0%, 62.8–493.0%, and 31.6–181.4%, respectively. The number of bolts and degree of pre-stress has certain effects on the peak strength and failure model of fractured sandstone. The peak strength of fractured sandstone under different anchorage increases to some extent, and the failure model of fractured sandstone also transforms from tensile failure to tensile–shear mixed failure with the number of bolts. The pre-stress can restrain the formation and evolution process of tensile cracks, delay the failure process of fractured sandstone under anchorage and impel the transformation of failure model from brittle failure to plastic failure.
基金sponsored by Coal and Energy Research Bureau and CDC-NIOSH under Grant No.R01OH009532
文摘Bumps in coal mines have been recognized as a major hazard for many years. These sudden and violent failures around mine openings have compromised safety, ventilation and access to mine workings.Previous studies showed that the violence of coal specimen failure depends on both the interface friction and width-to-height（W/H） ratio of coal specimen. The mode of failure for a uniaxially loaded coal specimen or a coal pillar is a combination of both shear failure along the interface and compressive failure in the coal. The shear failure along the interface triggered the compressive failure in coal. The compressive failure of a coal specimen or a coal pillar can be controlled by changing its W/H ratio. As the W/H ratio increases, the ultimate strength increases. Hence, with a proper combination of interface friction and the W/H ratio of pillar or coal specimen, the mode of failure will change from sudden violent failure which is brittle failure to non-violent failure which is ductile failure. The main objective of this paper is to determine at what W/H ratio and interface friction the mode of failure changes from violent to non-violent. In this research, coal specimens of W/H ratio ranging from 1 to 10 were uniaxially tested under two interface frictions of 0.1 and 0.25, and the results are presented and discussed.
基金the National Natural Science Foundation of China (Nos.51274079,51274110 and 51574139)the Natural Science Foundation of Hebei Province (No.E2013208148)
文摘The permeability evolution of rock during the progressive failure process is described. In combination with the strength degradation index, the degradation formulas of s and a, which are dependent on the plastic confining strain component, the material constants of Hock-Brown failure criterion are presented, and a modified elemental scale elastic-brittle-plastic constitutive model of rock is established. The rela- tionship between volumetric strain and permeability through tri-axial compression is investigated. Based on the above, a permeability evolution model is established. The model incorporates confining pressure- dependent degradation of strength, dilatancy and corresponding permeability evolution. The model is implemented in FLAC by the FISH function method. The permeability evolution behavior of rock is inves-tigated during the progressive failure process in a numerical case. The results show that the model is cap- able of reproducing, and allowing visualization of a range of hydro-mechanical responses of rock. The effects of confining pressure on degradation of strength, dilatancy and permeability evolution are also reflected.
基金sponsored by the National Natural Science Foundation of China, with three different programs (No. 51275410, No. 51305349 and No. 51305352) that supports the present work financially
文摘This work aims to investigate local stress distribution, damage evolution and failure of notched composite laminates under in-plane loads. An analytic method containing uniformed boundary equations using a complex variable approach is developed to present layer-by-layer stresses around the notch. The uniformed boundary equations established in series together with conformal mapping functions are capable of dealing with irregular boundary issues around the notch and at infinity. Stress results are employed to evaluate the damage initiation and propagation of notched composites by progressive damage analysis（PDA）. A user-defined subroutine is developed in the finite element（FE） model based on coupling theories for mixed failure criteria and damage mechanics to efficiently investigate damage evolution as well as failure modes. Carbon/epoxy laminates with a stacking sequence of [45°/0°/ 60°/90°]sare used to investigate surface strains, in-plane load capacity and microstructure of failure zones to provide analytic and FE methods with strong validation. Good agreement is observed between the analytic method, the FE model and experiments in terms of the stress（strain） distributions, damage evaluation and ultimate strength, and the layerby-layer stress components vary according to a combination effect of fiber orientation and loading type, causing diverse failure modes in individuals.
基金This work was supported by Natural Science Talents Program of Lingnan Normal University(No.ZL2021011).
文摘The mechanical properties of Z-pins reinforced composites adhesively bonded single-lap joints(SLJs)under un-directional tension loading are investigated by experimental and numerical methods.Three kinds of joint configurations,including SLJs with three/two rows of Z-pins and“I”array of Z-pins,are investigated by tension test.The failure modes and mechanism of reinforced joints with different Z-pins numbers and alignment are analyzed,and the comparison is performed for the failure strengths of no Z-pins and Z-pins reinforced joints.According to experimental results,failure modes of three kinds of joints are all mixed failure.It turns out that the Z-pins are pulled out ultimately.The strength of joints of more Z-pins at the end of the overlap area is relatively bigger for the joint of the same Z-pins numbers.The strength of joints with Z-pins compared with non Z-pins joints is growing at 16%.Finally,the three-dimensional distribution of interfacial stress in the lap zone of three kinds of Z-pins reinforced joints is simulated,and the numerical results are in good agreement with the experimental results.It is effective that the numerical calculation of stress analysis is verified.
文摘Under strong shocks,long-span spatial-latticed structures may collapse due to dynamic instability or strength failure.The elasto-plastic dynamic behaviors of three spatiallatticed structures,including two double-layer cylindrical shells and a spheri-cal shell used for the 2008 Olympic Games in Beijing,were quantitatively examined under multi-support excitation(MSE) and uniform support excitation(USE).Numerical analyses described several important parameters such as the peak acceleration and displacement responses at key joints,the number and distribution of plastic elements,and the deformation of the shell at the moment of collapse.Results of the analysis revealed the features and the failure mechanism of the spatial-latticed structures under MSE and USE.In both scenarios,the double-layer reticulated shell collapsed in the "overflow" mode,collapse was govrned by the number of invalid plastic elements rather than the total number of plastic elements,and the collapse of the structure began with damage to certain local regions near the supports.By comparing the numbers and distributions of the plastic members under MSE to those under USE,it was observed that the plastic members spread more sufficiently and the internal forces were more uniform under MSE,especially for lower apparent velocities in soils.Due to the effects of pseudo-static displacement,the stresses in members near supports under MSE were higher than those under USE.These regions are prone to failure during earthquakes and deserve special attention in the seismic design of reticulated structures.
基金the National Natural Science Foundation of China(Grant No.51627812,No.51878241 and No.U1965204).
文摘Discontinuities or structural planes are widely distributed in natural rock masses and significantly influence their geo-mechanical and geometric properties.Herein,a batch of rock samples,each with a single structural plane,is created using a 3D printer equipped with two robotic manipulators.One of the manipulators is connected via a nozzle to a concrete pumping truck,which can extrude brittle rock-like material to form layered intact rock masses;the rock-like material is mainly composed of cement,silica fume,sand and water.The other manipulator features a knife,which can carve discontinuities onto each layer of the printed model.By means of this system,rock masses with discontinuous joints are formed,and the failure strengths of rock masses with different joints are demonstrated via uniaxial compression tests and direct shear tests.The results thereof obtained via digital image correlation technology show that discontinuities lower the strength of the rock mass models significantly.With the increase of the angle between the fracture and horizontal plane,the uniaxial compressive strength first decreases,and then increases.During shear testing,the shear strength of the rock mass models increases with the surface roughness of the preset joint.These test results indicate that the influence of artificial joints on the mechanical properties of the models is consistent with that of natural rock mass joints.Using digital modeling and 3D printing technology,cracks hidden in a rock mass can be reproduced.
