The strength of rockmass from two aspects is analyzed.Firstly,the strength of the rockmass is mainly controlled by the critical stress value of rock,and the contribution of joints is to increase the effective stresses...The strength of rockmass from two aspects is analyzed.Firstly,the strength of the rockmass is mainly controlled by the critical stress value of rock,and the contribution of joints is to increase the effective stresses of rock and to decrease the damage strength of rockmass according to the macro damage mechanics of rockmass.Secondly,the strength of rockmass is mainly controlled by the fracture strength of joints.Based on the comprehensive analysis and comparison for the damage strength of rockmass and the fracture strength of joints,a composite damage theory of rockmass may be established.展开更多
Borehole instability in naturally fractured rocks poses significant challenges to drilling.Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure(P...Borehole instability in naturally fractured rocks poses significant challenges to drilling.Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure(P w)and pore pressure(P p)during drilling,which may cause wellbore instability.However,the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses,which may yield significant errors and misleading predictions.In addition,only limited factors were analyzed,and the fracture distribution was oversimplified.In this paper,the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM(distinct element method)and DFN(discrete fracture network)method.It provides estimates of the effect of fracture strength weakening,wellbore pressure,in situ stresses,and sealing efficiency on borehole stability.The results show that mud intrusion and weakening of fracture strength can damage the borehole.This is demonstrated by the large displacement around the borehole,shear displacement on natural fractures,and the generation of fracture at shear limit.Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure,which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations.A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore.Moreover,the effect of sealing natural fractures on maintaining borehole stability is verified in this study,and the increase in sealing efficiency reduces the radial invasion distance of drilling mud.This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations,which can consider the discrete fracture network,mud intrusion,and associated weakening of fracture strength.The information provided by the numerical approach(e.g.displacement around the borehole,shear displacement on fracture,and fracture at shear limit)is helpful for managing wellbore stability and designing wellbore-strengthening operations.展开更多
The effect of T6heat treatment on the fracture strength and reliability of AM60B alloy was studied.The tensile specimens were poured at three different temperatures of670,685and700?C for different holding times of5,10...The effect of T6heat treatment on the fracture strength and reliability of AM60B alloy was studied.The tensile specimens were poured at three different temperatures of670,685and700?C for different holding times of5,10and15min.The fluidity test was also conducted to determine the fluidity length under different pouring temperatures and holding times.According to the results,the optimum pouring temperature and holding time were determined as685?C and10min,respectively.SEM fractography of the tensile specimens reveals that the entrained oxides and oxide-related porosities are the main factors responsible for the reduction of fracture strength under the non-optimal casting conditions.The Weibull statistical approach was used to quantify the scatter of fracture strength in as-cast and heat-treated conditions.For this purpose,T6schedule was applied to the specimens prepared under the optimal casting condition.It is found that,despite minor effect on the average fracture strength,T6heat treatment improves the reliability of castings,where the Weibull modulus is increased by75%.According to the microstructural and fractography observations,this improvement is related to the evolution of more uniform microstructure and the elimination of coarse brittleβ-particles in heat-treated samples.展开更多
Structural integrity procedures were used to demonstrate the fitness for the purpose of engineering components transmitting loads. The prediction of the fracture strength of titanium alloys containing sharp notches th...Structural integrity procedures were used to demonstrate the fitness for the purpose of engineering components transmitting loads. The prediction of the fracture strength of titanium alloys containing sharp notches through the damage model depends on the un-notched strength and the critical length of the damage zone ahead of the notch. In general, the critical length of the damage zone depends on the material, specimen, and size of the sharp notch. Modifications were made in one of the stress fracture criteria known as the average stress criterion for accurate prediction of notched tensile strength of titanium alloy specimen containing sharp notches. To examine the adequacy of these modifications, fracture data of center-cracked titanium alloys with various thicknesses are considered. The notched （fracture） strength estimates are found to be close to the test results. The modified average stress criterion is very simple to predict the notched tensile strength.展开更多
Three-way pipes, T and Y pipes, are very important connecting components in pipeline systems, their strength are related to the safety of pipelines. In the case that crack is not detected in the three-way pipe, ANSYS ...Three-way pipes, T and Y pipes, are very important connecting components in pipeline systems, their strength are related to the safety of pipelines. In the case that crack is not detected in the three-way pipe, ANSYS finite element program version 5.6 is applied to study the stress distribution of the three-way pipe and to obtain the optimum fillet radius in the crotch region of the two pipes. The reasonable intersection angle of the two pipes is also obtained. In the case that a surface crack is detected in the three-way pipe, the maximum stress intensity factor (SIF) near the front of the surface crack is studied.展开更多
Fracture data of both parent metal and weldment metals from surface cracked tensile plates made of 2219-T87 Al alloy at cryogenic temperatures were correlated using a modified inherent flaw model. Fracture parameters ...Fracture data of both parent metal and weldment metals from surface cracked tensile plates made of 2219-T87 Al alloy at cryogenic temperatures were correlated using a modified inherent flaw model. Fracture parameters to generate the failure assessment diagram were determined for the material. Fracture analysis was carried out considering the ultimate tensile strength value and the fracture data of aluminium base metal and weldment metal generated from center–surface cracked tensile specimens having different thicknesses. The failure assessment diagram of a material generated from tensile fracture plate configuration can be applied to failure pressure estimation of any cracked component, made of the same material.展开更多
Molecular dynamics simulations are implemented to study the mechanical fracture of CoSb_(3) with penetrated nanocracks under the mode-Ⅰ stress.The crack surface and crack front direction are(100)and[001],respectively...Molecular dynamics simulations are implemented to study the mechanical fracture of CoSb_(3) with penetrated nanocracks under the mode-Ⅰ stress.The crack surface and crack front direction are(100)and[001],respectively.It is found that,at a fixed initial crack length,the fracture strength varies with the sample size,but the calculated value of fracture toughness KIC,by employing the classical formula of linear elastic fracture mechanics,maintains constant.When the crack is short in length relative to the sample,the variation of the fracture strength with the initial crack length is well fitted mathematically,and the extrapolation shows rationality even up to the macroscale.More general analyses reveal that,the fracture toughness increases monotonically with increasing the initial crack length until reaching the limit,and the increment is particularly noticeable below 36 nm.Furthermore,different atomic configurations at the crack tip are considered,which show an evident influence on the strength of nano-cracked CoSb_(3).展开更多
In recent times, conventional materials are replaced by metal matrix composites （MMCs） due to their high specific strength and modulus. Strength reliability, one of the key factors restricting wider use of composite...In recent times, conventional materials are replaced by metal matrix composites （MMCs） due to their high specific strength and modulus. Strength reliability, one of the key factors restricting wider use of composite materials in various applications, is commonly characterized by Weibull strength distribution function. In the present work, statistical analysis of the strength data of 15% volume alumina particle （mean size 15 um） reinforced in aluminum alloy （1101 grade alloy） fabricated by stir casting method was carried out using Weibull probability model. Twelve tension tests were performed according to ASTM B577 standards and the test data, the corresponding Weibull distribution was obtained. Finally the reliability of the composite behavior in terms of its fracture strength was presented to ensure the reliability of composites for suitable applications. An important implication of the present study is that the Weibull distribution describes the experimentally measured strength data more appropriately.展开更多
A novel test structure to characterize the fracture strength of MEMS(Micro-electro-Mechanical Systems)thin films is presented.The test structure is comprised of a micro fabricated chevron-shaped thermal actuator and t...A novel test structure to characterize the fracture strength of MEMS(Micro-electro-Mechanical Systems)thin films is presented.The test structure is comprised of a micro fabricated chevron-shaped thermal actuator and test specimen.The test structure is capable of producing large displacement and stresswhile keeping a relatively low temperature gradient across the test specimen.A voltage is applied across the beams of the chevron-shaped actuator,producing thermal expansion force to fracture the test specimen.Actuator deflection is computed based on elastic analysis of structures.To verify the test structure,simulations have been implemented using COMSOL Multiphysics.A 620μmlong,410μm wide,10μm thick test structure produced stress of 7.1 GPawhile the applied voltage is 5 V.The results indicate that the test structure is suitable for in-situ measurement of the fracture strength of MEMS thin films.展开更多
Background The increased use of all-ceramic crown provides a rationale for tooth-colored core. Due to superior mechanical properties, Vita Celay infiltration ceramic developed for crown and bridge works presents the p...Background The increased use of all-ceramic crown provides a rationale for tooth-colored core. Due to superior mechanical properties, Vita Celay infiltration ceramic developed for crown and bridge works presents the potential for fabricating all-ceramic posts and cores in one piece. This study was conducted to compare the fracture strength of endodontically treated teeth which were thereafter given different types of posts and cores and crowns restoration, respectively. The evaluated post-and-core systems are： custom-fabricated Celay all-ceramic post-core, custom cast metal post-core, and prefabricated stainless steel post （Parapost） with and without 2.0 mm dentine ferrule. Methods Sixty freshly extracted human maxillary central incisors were endodontically treated and randomly divided into five groups with 12 samples each. Group A： Celay ceramic post-cores restored teeth with 2.0 mm dentine ferrule. Group B： Celay ceramic post-cores restored teeth with no dentine ferrule. Group C： cast metal post-cores restored teeth with 2.0 mm dentine ferrule. Group D： cast metal post-cores restored teeth with no dentine ferrule. Group E： prefabricated post and composite cores restored teeth with 2.0 mm dentine ferrule. All teeth were restored with Celay ceramic crowns. Each specimen was subjected to a load at a 45-degree angle to the long axis on MTS 810 material testing machine until failure, at crosshead speed of 0.02 cm/minute. Analysis of variance followed by the Newman-Keuls pairwise multiple comparison tests were used to compare the results of the groups tested. Results There was a statistically significant difference among the five groups （P〈0.01）. Celay ceramic post-cores restored teeth with 2.0 mm dentine ferrule [（758.35± 119.26） N] and cast metal post-cores restored teeth with 2.0 mm dentine ferrule [（756.63± 166.22） N] had a significantly greater mean fracture strength than the other three groups in which no significant difference was observed. The 2.0 mm dentine ferrule could cause significant fracture resistance alteration of Celay post-core restored teeth. Conclusions When covered with Celay ceramic crowns, Celay post-cores restored teeth with 2.0 mm dentine ferrule and cast metal post-cores restored teeth with 2.0 mm dentine ferrule have similar fracture strength. There was a statistically significant difference between the fracture resistance of Celay post-core restored teeth with and without 2.0 mm dentine ferrule.展开更多
Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However...Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However, laboratory tests of intact rock samples cannot provide information about the strength and deformation behaviors of fractured rock masses that include many fractures of varying sizes, orientations and locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costly and often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numerical predicting using discrete element methods(DEM) is a suitable approach for such modeling because of their advantages of explicit representations of both fractures system geometry and their constitutive behaviors of fractures, besides that of intact rock matrix. In this study, to generically determine the compressive strength of fractured rock masses, a series of numerical experiments were performed on two-dimensional discrete fracture network models based on the realistic geometrical and mechanical data of fracture systems from feld mapping. We used the UDEC code and a numerical servo-controlled program for controlling the progressive compressive loading process to avoid sudden violent failure of the models. The two loading conditions applied are similar to the standard laboratory testing for intact rock samples in order to check possible differences caused by such loading conditions. Numerical results show that the strength of fractured rocks increases with the increasing confning pressure, and that deformation behavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses and strains obtained from these numerical experiments were used to ft the well-known Mohr-Coulomb(MC) and Hoek-Brown(H-B) failure criteria, represented by equivalent material properties defning these two criteria. The results show that both criteria can provide fair estimates of the compressive strengths for all tested numerical models. Parameters of the elastic deformability of fractured models during elastic deformation stages were also evaluated, and represented as equivalent Young’s modulus and Poisson’s ratio as functions of lateral confning pressure. It is the frst time that such systematic numerical predicting for strength of fractured rocks was performed considering different loading conditions, with important fndings for different behaviors of fractured rock masses, compared with testing intact rock samples under similar loading conditions.展开更多
An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1....An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.展开更多
The brittle fracture probability and reliability are obtained in terms of dislocation mechanism of microcrack evolution. The statistical distribution functions and statistical deviations of elongation, strength, plast...The brittle fracture probability and reliability are obtained in terms of dislocation mechanism of microcrack evolution. The statistical distribution functions and statistical deviations of elongation, strength, plastic work, crack extension force, fracture foughness, critical and crack length, can be derived in a unified fashion.展开更多
The Chaoyanggou oil field is a fractured low-permeability reservoir, where the distribution of oil and gas is controlled by the distribution and development of fractures.Based on outcrop, drilling core, thin section a...The Chaoyanggou oil field is a fractured low-permeability reservoir, where the distribution of oil and gas is controlled by the distribution and development of fractures.Based on outcrop, drilling core, thin section and log data, the development characteristics of fractures in this area are described.On this basis, the degree of fracture development was predicted by quantitative analysis of fracture strength and numerical simulation.The result shows that four groups of structural fractures, i.e., in near NS, and EW directions and in due NW and NE directions, were developed in the reservoir, with the nearly NS and EW fractures dominant, which are the along bedding decollement fractures formed by compressive folding action, while low angle shear fractures are related to thrusts.These fractures are mainly formed in the reversed tectonic stage at the end of the Mingshui formation during the Cretaceous period.The degree of fracture development is controlled by such factors as lithology, stratum thickness, faults, folds and depth.The fractures are developed with a clear zonation and are best developed in the northern zone, moderately developed towards the south and poorly developed in the middle zone.These prediction results are in good agreement with interpretation results from logs.展开更多
Fractures occur in nearly all rocks at the Earth’s surface and exert essential control on the mechanical strengths of rock masses and permeability.The fractures strongly impact the stability of geological or man-made...Fractures occur in nearly all rocks at the Earth’s surface and exert essential control on the mechanical strengths of rock masses and permeability.The fractures strongly impact the stability of geological or man-made structures and flow of water and hydrocarbons,CO_(2) and storing waste.For this,the dependence of opening mode fracture spacing(s)on bed thickness(t)in sedimentary basins(reservoirs)is studied in this context.This paper shows that the MichaeliseMenten equation can provide an algebraic expression for the nonlinear s-t relationship.The two parameters have clear geological meanings:a is the maximum fracture spacing which can no longer increase with increasing t,and b is the characteristic bed thickness when s=0.5a.The tensile fracture strength(C)of the brittle beds during the formation of tensile fractures can be estimated from the two parameters.For sandstones of 16 areas reported in the literature,C ranges from 2.7 MPa to 15.7 MPa with a mean value of 8 MPa,which lies reasonably within the range of tensile strengths determined experimentally.This field-based approach by means of MichaeliseMenten equation provides a new method for estimating the tensile fracture strength of rock layers under natural conditions.展开更多
Two linked models have been developed to explore the relationship between the amount of porosity arising in service from both radiolytic oxidation and fast neutron damage that influences both the strength and the forc...Two linked models have been developed to explore the relationship between the amount of porosity arising in service from both radiolytic oxidation and fast neutron damage that influences both the strength and the force-displacement(load-displacement)behaviour and crack propagation in pile grade A graphite used as a nuclear reactor moderator material.Firstly models of the microstructure of the porous graphite for both unirradiated and irradiated graphite are created.These form the input for the second stage,simulating fracture in lattice-type finite element models,which predicts force(load)-displacement and crack propagation paths.Microstructures comprising aligned filler particles,typical of needle coke,in a porous matrix have been explored.The purpose was to isolate the contributions of filler particles and porosity to fracture strength and crack paths and consider their implications for the overall failure of reactor core graphite.展开更多
In this paper a two dimensional (2 D) model of earthquake fault rupturing was presented. It was estabilished on the basis of 1 D spring block model. Using this model, we studied the dynamical plane strain fractur...In this paper a two dimensional (2 D) model of earthquake fault rupturing was presented. It was estabilished on the basis of 1 D spring block model. Using this model, we studied the dynamical plane strain fracture problem, modeled the whole dynamical process of nucleating, expanding and propagating of fracture on a 2 D fault with homogeneous or inhomogeneous rupture strength distribution. Our studies show that under homogeneous prestress condition, the fault will gain enough momentum to tear strong obstacles in their propagating path. The rupturing fronts can also propagate forth around the isolated barriers. It is shown that the stopping conditions for rupturing processes play an important role in modeling whole earthquake process. We also studied the dynamical rupturing problems of the fault on which the rupture strength distribution is inhomogeneous, and modeled the earthquake sequence generated on a 2 D fault with the strength distribution of fractal structure. It possesses some similar features as a seismic sequence in the nature. These features mainly depend on the distribution of rupture strength on the fault plane and the level of initial stress drop. The modeling studies which were established on the basis of experiments and observations provided the physical basis for explaining some statistical rules of seismicity.展开更多
In this study, magnesium matrix composites reinforced with different loading of AlN particles were fabricated by the powder metallurgy technique. The microstructure, bending strength and fracture behavior of the resul...In this study, magnesium matrix composites reinforced with different loading of AlN particles were fabricated by the powder metallurgy technique. The microstructure, bending strength and fracture behavior of the resulting Mg-Al/Al N composites were investigated. It showed that the 5 wt% AlN reinforcements led to the highest densification and bending strength. The total strengthening effect of AlN particles was predicted by considering the contributions of CTE mismatch between the matrix and the particles,load bearing and Hall-Petch mechanism. The results revealed that the increase of dislocation density,the change of Mg17Al12 phase morphology, and the effective load transfer were the major strengthening contributors to the composites. The fracture of the composites altered from plastic to brittle mode with increasing reinforcement content. The regions of clustered particles in the composites were easy to be damaged under external load, and the fracture occurred mainly along grain boundaries.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
The results of a theoretical study on the influence of strength of interphase boundaries in metal-ceramic composite on macroscopical characteristics of composite response such as strength, deformation capacity, fractu...The results of a theoretical study on the influence of strength of interphase boundaries in metal-ceramic composite on macroscopical characteristics of composite response such as strength, deformation capacity, fracture energy and fracture pattern are presented. The study was conducted by means of computer-aided simulation by means of movable cellular automaton method taking account of a developed ＂mesoscopical＂ structural model of particle-reinforced composite. The strength of interphase boundaries is found to be a key structural factor determining not only the strength properties of metal-ceramic composite, but also the pattern and rate of fracture. The principles for achievement of the high-strength values of particle/binder interfaces in the metal-ceramic composition due to the formation of the wide transition zones （areas of variable chemical composition） at the interphase boundaries are discussed. Simulation results confirm that such transition zones provide a change in fracture mechanism and make the achievement of a high-strength and a high deformation capacity of metal-ceramic composite possible.展开更多
文摘The strength of rockmass from two aspects is analyzed.Firstly,the strength of the rockmass is mainly controlled by the critical stress value of rock,and the contribution of joints is to increase the effective stresses of rock and to decrease the damage strength of rockmass according to the macro damage mechanics of rockmass.Secondly,the strength of rockmass is mainly controlled by the fracture strength of joints.Based on the comprehensive analysis and comparison for the damage strength of rockmass and the fracture strength of joints,a composite damage theory of rockmass may be established.
基金financially supported by National Natural Science Foundation of China(Grant Nos.52074312 and 52211530097)CNPC Science and Technology Innovation Foundation(Grant No.2021DQ02-0505).
文摘Borehole instability in naturally fractured rocks poses significant challenges to drilling.Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure(P w)and pore pressure(P p)during drilling,which may cause wellbore instability.However,the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses,which may yield significant errors and misleading predictions.In addition,only limited factors were analyzed,and the fracture distribution was oversimplified.In this paper,the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM(distinct element method)and DFN(discrete fracture network)method.It provides estimates of the effect of fracture strength weakening,wellbore pressure,in situ stresses,and sealing efficiency on borehole stability.The results show that mud intrusion and weakening of fracture strength can damage the borehole.This is demonstrated by the large displacement around the borehole,shear displacement on natural fractures,and the generation of fracture at shear limit.Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure,which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations.A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore.Moreover,the effect of sealing natural fractures on maintaining borehole stability is verified in this study,and the increase in sealing efficiency reduces the radial invasion distance of drilling mud.This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations,which can consider the discrete fracture network,mud intrusion,and associated weakening of fracture strength.The information provided by the numerical approach(e.g.displacement around the borehole,shear displacement on fracture,and fracture at shear limit)is helpful for managing wellbore stability and designing wellbore-strengthening operations.
文摘The effect of T6heat treatment on the fracture strength and reliability of AM60B alloy was studied.The tensile specimens were poured at three different temperatures of670,685and700?C for different holding times of5,10and15min.The fluidity test was also conducted to determine the fluidity length under different pouring temperatures and holding times.According to the results,the optimum pouring temperature and holding time were determined as685?C and10min,respectively.SEM fractography of the tensile specimens reveals that the entrained oxides and oxide-related porosities are the main factors responsible for the reduction of fracture strength under the non-optimal casting conditions.The Weibull statistical approach was used to quantify the scatter of fracture strength in as-cast and heat-treated conditions.For this purpose,T6schedule was applied to the specimens prepared under the optimal casting condition.It is found that,despite minor effect on the average fracture strength,T6heat treatment improves the reliability of castings,where the Weibull modulus is increased by75%.According to the microstructural and fractography observations,this improvement is related to the evolution of more uniform microstructure and the elimination of coarse brittleβ-particles in heat-treated samples.
文摘Structural integrity procedures were used to demonstrate the fitness for the purpose of engineering components transmitting loads. The prediction of the fracture strength of titanium alloys containing sharp notches through the damage model depends on the un-notched strength and the critical length of the damage zone ahead of the notch. In general, the critical length of the damage zone depends on the material, specimen, and size of the sharp notch. Modifications were made in one of the stress fracture criteria known as the average stress criterion for accurate prediction of notched tensile strength of titanium alloy specimen containing sharp notches. To examine the adequacy of these modifications, fracture data of center-cracked titanium alloys with various thicknesses are considered. The notched （fracture） strength estimates are found to be close to the test results. The modified average stress criterion is very simple to predict the notched tensile strength.
文摘Three-way pipes, T and Y pipes, are very important connecting components in pipeline systems, their strength are related to the safety of pipelines. In the case that crack is not detected in the three-way pipe, ANSYS finite element program version 5.6 is applied to study the stress distribution of the three-way pipe and to obtain the optimum fillet radius in the crotch region of the two pipes. The reasonable intersection angle of the two pipes is also obtained. In the case that a surface crack is detected in the three-way pipe, the maximum stress intensity factor (SIF) near the front of the surface crack is studied.
文摘Fracture data of both parent metal and weldment metals from surface cracked tensile plates made of 2219-T87 Al alloy at cryogenic temperatures were correlated using a modified inherent flaw model. Fracture parameters to generate the failure assessment diagram were determined for the material. Fracture analysis was carried out considering the ultimate tensile strength value and the fracture data of aluminium base metal and weldment metal generated from center–surface cracked tensile specimens having different thicknesses. The failure assessment diagram of a material generated from tensile fracture plate configuration can be applied to failure pressure estimation of any cracked component, made of the same material.
基金funding was provided by the National Natural Science Foundation of China(No.51972253).
文摘Molecular dynamics simulations are implemented to study the mechanical fracture of CoSb_(3) with penetrated nanocracks under the mode-Ⅰ stress.The crack surface and crack front direction are(100)and[001],respectively.It is found that,at a fixed initial crack length,the fracture strength varies with the sample size,but the calculated value of fracture toughness KIC,by employing the classical formula of linear elastic fracture mechanics,maintains constant.When the crack is short in length relative to the sample,the variation of the fracture strength with the initial crack length is well fitted mathematically,and the extrapolation shows rationality even up to the macroscale.More general analyses reveal that,the fracture toughness increases monotonically with increasing the initial crack length until reaching the limit,and the increment is particularly noticeable below 36 nm.Furthermore,different atomic configurations at the crack tip are considered,which show an evident influence on the strength of nano-cracked CoSb_(3).
文摘In recent times, conventional materials are replaced by metal matrix composites （MMCs） due to their high specific strength and modulus. Strength reliability, one of the key factors restricting wider use of composite materials in various applications, is commonly characterized by Weibull strength distribution function. In the present work, statistical analysis of the strength data of 15% volume alumina particle （mean size 15 um） reinforced in aluminum alloy （1101 grade alloy） fabricated by stir casting method was carried out using Weibull probability model. Twelve tension tests were performed according to ASTM B577 standards and the test data, the corresponding Weibull distribution was obtained. Finally the reliability of the composite behavior in terms of its fracture strength was presented to ensure the reliability of composites for suitable applications. An important implication of the present study is that the Weibull distribution describes the experimentally measured strength data more appropriately.
基金supported by the National High Technology Program of P. R. China under Grant No. 2015AA042604
文摘A novel test structure to characterize the fracture strength of MEMS(Micro-electro-Mechanical Systems)thin films is presented.The test structure is comprised of a micro fabricated chevron-shaped thermal actuator and test specimen.The test structure is capable of producing large displacement and stresswhile keeping a relatively low temperature gradient across the test specimen.A voltage is applied across the beams of the chevron-shaped actuator,producing thermal expansion force to fracture the test specimen.Actuator deflection is computed based on elastic analysis of structures.To verify the test structure,simulations have been implemented using COMSOL Multiphysics.A 620μmlong,410μm wide,10μm thick test structure produced stress of 7.1 GPawhile the applied voltage is 5 V.The results indicate that the test structure is suitable for in-situ measurement of the fracture strength of MEMS thin films.
文摘Background The increased use of all-ceramic crown provides a rationale for tooth-colored core. Due to superior mechanical properties, Vita Celay infiltration ceramic developed for crown and bridge works presents the potential for fabricating all-ceramic posts and cores in one piece. This study was conducted to compare the fracture strength of endodontically treated teeth which were thereafter given different types of posts and cores and crowns restoration, respectively. The evaluated post-and-core systems are： custom-fabricated Celay all-ceramic post-core, custom cast metal post-core, and prefabricated stainless steel post （Parapost） with and without 2.0 mm dentine ferrule. Methods Sixty freshly extracted human maxillary central incisors were endodontically treated and randomly divided into five groups with 12 samples each. Group A： Celay ceramic post-cores restored teeth with 2.0 mm dentine ferrule. Group B： Celay ceramic post-cores restored teeth with no dentine ferrule. Group C： cast metal post-cores restored teeth with 2.0 mm dentine ferrule. Group D： cast metal post-cores restored teeth with no dentine ferrule. Group E： prefabricated post and composite cores restored teeth with 2.0 mm dentine ferrule. All teeth were restored with Celay ceramic crowns. Each specimen was subjected to a load at a 45-degree angle to the long axis on MTS 810 material testing machine until failure, at crosshead speed of 0.02 cm/minute. Analysis of variance followed by the Newman-Keuls pairwise multiple comparison tests were used to compare the results of the groups tested. Results There was a statistically significant difference among the five groups （P〈0.01）. Celay ceramic post-cores restored teeth with 2.0 mm dentine ferrule [（758.35± 119.26） N] and cast metal post-cores restored teeth with 2.0 mm dentine ferrule [（756.63± 166.22） N] had a significantly greater mean fracture strength than the other three groups in which no significant difference was observed. The 2.0 mm dentine ferrule could cause significant fracture resistance alteration of Celay post-core restored teeth. Conclusions When covered with Celay ceramic crowns, Celay post-cores restored teeth with 2.0 mm dentine ferrule and cast metal post-cores restored teeth with 2.0 mm dentine ferrule have similar fracture strength. There was a statistically significant difference between the fracture resistance of Celay post-core restored teeth with and without 2.0 mm dentine ferrule.
文摘Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However, laboratory tests of intact rock samples cannot provide information about the strength and deformation behaviors of fractured rock masses that include many fractures of varying sizes, orientations and locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costly and often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numerical predicting using discrete element methods(DEM) is a suitable approach for such modeling because of their advantages of explicit representations of both fractures system geometry and their constitutive behaviors of fractures, besides that of intact rock matrix. In this study, to generically determine the compressive strength of fractured rock masses, a series of numerical experiments were performed on two-dimensional discrete fracture network models based on the realistic geometrical and mechanical data of fracture systems from feld mapping. We used the UDEC code and a numerical servo-controlled program for controlling the progressive compressive loading process to avoid sudden violent failure of the models. The two loading conditions applied are similar to the standard laboratory testing for intact rock samples in order to check possible differences caused by such loading conditions. Numerical results show that the strength of fractured rocks increases with the increasing confning pressure, and that deformation behavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses and strains obtained from these numerical experiments were used to ft the well-known Mohr-Coulomb(MC) and Hoek-Brown(H-B) failure criteria, represented by equivalent material properties defning these two criteria. The results show that both criteria can provide fair estimates of the compressive strengths for all tested numerical models. Parameters of the elastic deformability of fractured models during elastic deformation stages were also evaluated, and represented as equivalent Young’s modulus and Poisson’s ratio as functions of lateral confning pressure. It is the frst time that such systematic numerical predicting for strength of fractured rocks was performed considering different loading conditions, with important fndings for different behaviors of fractured rock masses, compared with testing intact rock samples under similar loading conditions.
基金supported by the 111 Project (B07050)the National Natural Science Foundation of China (10932008)
文摘An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.
文摘The brittle fracture probability and reliability are obtained in terms of dislocation mechanism of microcrack evolution. The statistical distribution functions and statistical deviations of elongation, strength, plastic work, crack extension force, fracture foughness, critical and crack length, can be derived in a unified fashion.
基金Project 40772086 supported by the National Natural Science Foundation of China
文摘The Chaoyanggou oil field is a fractured low-permeability reservoir, where the distribution of oil and gas is controlled by the distribution and development of fractures.Based on outcrop, drilling core, thin section and log data, the development characteristics of fractures in this area are described.On this basis, the degree of fracture development was predicted by quantitative analysis of fracture strength and numerical simulation.The result shows that four groups of structural fractures, i.e., in near NS, and EW directions and in due NW and NE directions, were developed in the reservoir, with the nearly NS and EW fractures dominant, which are the along bedding decollement fractures formed by compressive folding action, while low angle shear fractures are related to thrusts.These fractures are mainly formed in the reversed tectonic stage at the end of the Mingshui formation during the Cretaceous period.The degree of fracture development is controlled by such factors as lithology, stratum thickness, faults, folds and depth.The fractures are developed with a clear zonation and are best developed in the northern zone, moderately developed towards the south and poorly developed in the middle zone.These prediction results are in good agreement with interpretation results from logs.
基金The author thanks the Natural Sciences and Engineering Research Council of Canada for a discovery grant(Grant No.06408),Dr.Le Li for drawing the figures,and Dr.A.I.Chemenda for discussion.Three anonymous reviewers and the editors are sincerely thanked for their critical comments and helpful suggestions.
文摘Fractures occur in nearly all rocks at the Earth’s surface and exert essential control on the mechanical strengths of rock masses and permeability.The fractures strongly impact the stability of geological or man-made structures and flow of water and hydrocarbons,CO_(2) and storing waste.For this,the dependence of opening mode fracture spacing(s)on bed thickness(t)in sedimentary basins(reservoirs)is studied in this context.This paper shows that the MichaeliseMenten equation can provide an algebraic expression for the nonlinear s-t relationship.The two parameters have clear geological meanings:a is the maximum fracture spacing which can no longer increase with increasing t,and b is the characteristic bed thickness when s=0.5a.The tensile fracture strength(C)of the brittle beds during the formation of tensile fractures can be estimated from the two parameters.For sandstones of 16 areas reported in the literature,C ranges from 2.7 MPa to 15.7 MPa with a mean value of 8 MPa,which lies reasonably within the range of tensile strengths determined experimentally.This field-based approach by means of MichaeliseMenten equation provides a new method for estimating the tensile fracture strength of rock layers under natural conditions.
文摘Two linked models have been developed to explore the relationship between the amount of porosity arising in service from both radiolytic oxidation and fast neutron damage that influences both the strength and the force-displacement(load-displacement)behaviour and crack propagation in pile grade A graphite used as a nuclear reactor moderator material.Firstly models of the microstructure of the porous graphite for both unirradiated and irradiated graphite are created.These form the input for the second stage,simulating fracture in lattice-type finite element models,which predicts force(load)-displacement and crack propagation paths.Microstructures comprising aligned filler particles,typical of needle coke,in a porous matrix have been explored.The purpose was to isolate the contributions of filler particles and porosity to fracture strength and crack paths and consider their implications for the overall failure of reactor core graphite.
文摘In this paper a two dimensional (2 D) model of earthquake fault rupturing was presented. It was estabilished on the basis of 1 D spring block model. Using this model, we studied the dynamical plane strain fracture problem, modeled the whole dynamical process of nucleating, expanding and propagating of fracture on a 2 D fault with homogeneous or inhomogeneous rupture strength distribution. Our studies show that under homogeneous prestress condition, the fault will gain enough momentum to tear strong obstacles in their propagating path. The rupturing fronts can also propagate forth around the isolated barriers. It is shown that the stopping conditions for rupturing processes play an important role in modeling whole earthquake process. We also studied the dynamical rupturing problems of the fault on which the rupture strength distribution is inhomogeneous, and modeled the earthquake sequence generated on a 2 D fault with the strength distribution of fractal structure. It possesses some similar features as a seismic sequence in the nature. These features mainly depend on the distribution of rupture strength on the fault plane and the level of initial stress drop. The modeling studies which were established on the basis of experiments and observations provided the physical basis for explaining some statistical rules of seismicity.
基金financially supported by the State Key Laboratory for Mechanical Behavior of Materials (No. 20151712)
文摘In this study, magnesium matrix composites reinforced with different loading of AlN particles were fabricated by the powder metallurgy technique. The microstructure, bending strength and fracture behavior of the resulting Mg-Al/Al N composites were investigated. It showed that the 5 wt% AlN reinforcements led to the highest densification and bending strength. The total strengthening effect of AlN particles was predicted by considering the contributions of CTE mismatch between the matrix and the particles,load bearing and Hall-Petch mechanism. The results revealed that the increase of dislocation density,the change of Mg17Al12 phase morphology, and the effective load transfer were the major strengthening contributors to the composites. The fracture of the composites altered from plastic to brittle mode with increasing reinforcement content. The regions of clustered particles in the composites were easy to be damaged under external load, and the fracture occurred mainly along grain boundaries.
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The investigation has been carried out within the SB RAS Program Ⅲ.20.2 for Basic Researchat partial financial support of the RFBR Grant No.11-08-12069-ofi-m-2011+1 种基金the Project No.5 of the Belarus NASSB RAS Program for Joint Basic Research
文摘The results of a theoretical study on the influence of strength of interphase boundaries in metal-ceramic composite on macroscopical characteristics of composite response such as strength, deformation capacity, fracture energy and fracture pattern are presented. The study was conducted by means of computer-aided simulation by means of movable cellular automaton method taking account of a developed ＂mesoscopical＂ structural model of particle-reinforced composite. The strength of interphase boundaries is found to be a key structural factor determining not only the strength properties of metal-ceramic composite, but also the pattern and rate of fracture. The principles for achievement of the high-strength values of particle/binder interfaces in the metal-ceramic composition due to the formation of the wide transition zones （areas of variable chemical composition） at the interphase boundaries are discussed. Simulation results confirm that such transition zones provide a change in fracture mechanism and make the achievement of a high-strength and a high deformation capacity of metal-ceramic composite possible.
