Long-term cyclic train loading can cause settlement and deformation of the roadbed,affecting the normal operation of trains.In order to investigate the strain pattern of reinforced sandy soil under train loading,a ser...Long-term cyclic train loading can cause settlement and deformation of the roadbed,affecting the normal operation of trains.In order to investigate the strain pattern of reinforced sandy soil under train loading,a series of dynamic triaxial tests were carried out using multi-stage loading,focusing on the effects of the number of reinforcement layers,the confining pressure,and the mesh size of the geogrid on the accumulated plastic strain of reinforced sandy soil.Moreover,prediction models were proposed.The test results show that:1)The cumulative plastic strain versus vibration times of the specimens under different reinforcement layers exhibited three stages,namely,the rapid development stage,the rate transformation stage and the stability stage;2)The cumulative plastic strain decreases with increasing the number of reinforcement layers,but the magnitude of the effect of reinforcement on the cumulative plastic strain decreases with increasing the number of reinforcement layers,increasing the perimeter pressure and decreasing the mesh size of the geogrid have similar effects on the cumulative plastic strain pattern as increasing the number of reinforcement layers;3)Combined with the cumulative plastic strain law,a comprehensive model is proposed and the coefficient of determination is greater than 0.99.Furthermore,The cumulative plastic strain evolution law can be effectively predicted.The significance of parameters A,B and C is analyzed in detail.This study can provide theoretical references for further understanding of the deformation characteristics and settlement prediction of railway subgrades.展开更多
Two improvements have been made on the calculation of plastic strain ratio (r value) in BCC monocrystals by considering the differences in the critical shear stresses among the three slip systems and the rotation of c...Two improvements have been made on the calculation of plastic strain ratio (r value) in BCC monocrystals by considering the differences in the critical shear stresses among the three slip systems and the rotation of crystal lattice. It is found that the results calculated by the improved method are more rational.展开更多
The change of width and length of an aluminium-killed(AK)steel sheet was measured in detail at various tensile strain levels.The effects of tensile strain on the conventional strain ratio,rc-value,and the instantaneou...The change of width and length of an aluminium-killed(AK)steel sheet was measured in detail at various tensile strain levels.The effects of tensile strain on the conventional strain ratio,rc-value,and the instantaneous strain ratio,r^(1)-value,were analyzed.It was shown that the strain ratio of the AK steel sheet had obvious strain-dependency.The individual r^(c)-values for all tested directions tended to unity with increasing strain so that the planar anisotropy decreased with increasing strain.The r^(1)-value had overall straidenpendency similar to that of r^(c)-value,though it was overly sensitive to the experimental errors and showed significant variation with strain.In the representation of average strain ratio.the linear regression strain ratio,r^(r)-value,which was obtained by fitting a straight line to the εw vs εt curve,is preferable to the r^(c)-value measured at some specfic extension.展开更多
By using multi-pass straight-line scan strategies,a larger bending angle can be achieved.There is,however,a limited understanding of the variation in bending angle per pass during multi-pass under various process para...By using multi-pass straight-line scan strategies,a larger bending angle can be achieved.There is,however,a limited understanding of the variation in bending angle per pass during multi-pass under various process parameters.In multi-pass laser scanning,the bending angle cannot always linearly increase with scanning passes and this phenomenon can be observed mostly in low heat input.Strain hardening is the common explanation for this phenomenon.However,it could not explain why this bending angle reduction phenomenon occurs at low heat input scanning rather than at high heat input scanning.In this study,this phenomenon is discussed based on strain development in experiments and numerical simulation.The different growing mechanism of plastic strain is analyzed to reveal the effects of laser power and scanning velocity.Furthermore,the opposite bending trend that occurred at larger laser power is discussed in comparison to the plastic strain development.The study shows that relatively larger heat input below 6.7 J/mm can help to avoid the bending angle reduction phenomenon and the opposite bending trend is highly dependent on the larger laser power.For achieving the expected cumulative bending angle in multi-pass laser bending,it is recommended to decrease the scanning velocity at a relatively low laser power level while increase the scanning velocity at a high level of laser power.展开更多
The present work is concerned with the characterization of hardening parameters for an elasto-plastic continuum model, taking into account the memory effect of plastic strain amplitude, in order to predict the hystere...The present work is concerned with the characterization of hardening parameters for an elasto-plastic continuum model, taking into account the memory effect of plastic strain amplitude, in order to predict the hysteretic responses of a ferritic steel. This elasto-plastic three-dimensional model is based on the internal thermodynamic variables which composed of the nonlinear kinematic hardening and isotropic hardening with the plastic strain memorization. The emphasis is put on the determination of strain memory parameters along with other material parameters of the proposed model in order to better simulate the behavior of the material at different strain range. The material parameters are calibrated with the experimental stabilized loops of stress-strain curves available in the literature. The predicted stabilized loops from the simulation with the determined parameters show good agreement with the experimental results signifying the validity of the considered model.展开更多
It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F=RU and spin W in F -1 are not correct. Errors will arise in plastic deformation rate ...It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F=RU and spin W in F -1 are not correct. Errors will arise in plastic deformation rate if it is directly expressed with amounts of velocity of slips in glide systems such as vn. The geometrical figure of crystal lattices does not change after slips and based on this idea a simple way in mechanics of continuous media to get the plastic deformations rate induced by slips is proposed. Constitutive equations are recommended.展开更多
The paper describes an energy-based constitutive model for sand, which is modified based on the modified plastic strain energy approach, represented by a unique relationship between the modified plastic strain energy ...The paper describes an energy-based constitutive model for sand, which is modified based on the modified plastic strain energy approach, represented by a unique relationship between the modified plastic strain energy and a stress parameter, independent of stress history. The modified plastic strain energy approach was developed based on results from a series of drained plastic strain compression tests along various stress paths on saturated dense Toyoura sand with accurate stress and strain measurements. The proposed model is coupled with an isotropically work-hardening and softening, non-associtated, elasto-plastic material description. The constitutive model concerns the inherent and stress system-induced cross-anisotropic elastic deformation properties of sand. It is capable of simulating the deformation characteristics of stress history and stress path, the effects of pressure level, anisotropic strength and void ratio, and the strain localization.展开更多
It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F = RU and spin TY in (F) over dot F-1 ore not correct. Errors will arise in plastic deformation rare if...It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F = RU and spin TY in (F) over dot F-1 ore not correct. Errors will arise in plastic deformation rare if it is directly expressed with amounts of velocity of slips in glide systems such as (gamma) over dot upsilon circle times n. The geometrical figure of crystal lattices does nor change after slips and based on this idea a simple way in mechanics of continuous media to get the plastic deformations rare induced by slips is proposed. Constitutive equations are recommended.展开更多
A new spinning method to manufacture the cylindrical parts with nano/ultrafine grained structures is proposed, which consists of quenching, power spinning and recrystallization annealing. The microstructural evolution...A new spinning method to manufacture the cylindrical parts with nano/ultrafine grained structures is proposed, which consists of quenching, power spinning and recrystallization annealing. The microstructural evolution during the different process stages and macroforming quality of the spun parts made of ASTM 1020 steel are investigated. The results show that the microstructures of the ferrites and pearlites in the ASTM 1020 steel are transformed to the lath martensites after quenching. The martensite laths obtained by quenching are refined to 87 nm and a small amount of nanoscale deformation twins with an average thickness of 20 nm is generated after performing a 3-pass stagger spinning with 55% thinning ratio of wall thickness, where the equivalent strain required is only 0.92. The equiaxial ferritic grains with an average size of 160 nm and nano-carbides are generated by subsequent recrystallization annealing at 480°C for 30 min. The spun parts with high dimensional precision and low surface roughness are obtained by the forming method developed in this work, combining quenching with 3-pass stagger spinning and recrystallization annealing.展开更多
The Türkiye earthquake sequence on February 6, 2023, was featured by the closely located earthquake doublet of M_(W)7.8 and M_(W)7.5. The consequent strong ground motions are supposed to be able to impose high de...The Türkiye earthquake sequence on February 6, 2023, was featured by the closely located earthquake doublet of M_(W)7.8 and M_(W)7.5. The consequent strong ground motions are supposed to be able to impose high demands on the ultra-low-cycle fatigue performance of metallic dampers in buildings, including the widely used buckling restrained braces. This study evaluates the cumulative plastic deformation(CPD) demands on bucklingrestrained braces(BRBs) in multi-story buildings imposed by the strong ground motions in the 2023 Türkiye earthquake doublet. Thirty-two records of the highest peak ground accelerations were selected from the strong motion database. Among them, eight captured the ground motions during both events, and the rest only captured the shaking of either of the events. The CPD demands on the BRBs in reinforced concrete frames with various fundamental periods, brace-to-frame stiffness ratios, and BRB ductility ratio are calculated by nonlinear time history analyses and are summarized in the form of enveloped spectra of CPD ratios at constant ductility. The results show that the CPD demands on BRBs increase with smaller brace-toframe stiffness ratios and larger BRB ductility ratios. The enveloped CPD demands are several hundreds of times the nominal yield deformation of the BRB, which are much higher than the CPD demands for the calibration tests of BRBs stipulated by AISC 341 in the US.展开更多
By introducing a fatigue blunting factor, the cyclic elasto-plastic Hutchinson-Rice-Rosengren （HRR） field near the crack tip under the cyclic loading is modified. And, an average damage per loading-cycle in the cycl...By introducing a fatigue blunting factor, the cyclic elasto-plastic Hutchinson-Rice-Rosengren （HRR） field near the crack tip under the cyclic loading is modified. And, an average damage per loading-cycle in the cyclic plastic deformation region is defined due to Manson-Coffin law. Then, according to the linear damage accumulation theory-Miner law, a new model for predicting the fatigue crack growth （FCG） of the opening mode crack based on the low cycle fatigue （LCF） damage is set up. The step length of crack propagation is assumed to be the size of cyclic plastic zone. It is clear that every parameter of the new model has clearly physical meaning which does not need any human debugging. Based on the LCF test data, the FCG predictions given by the new model are consistent with the FCG test results of Cr2Ni2MoV and X12CrMoWVNbN 10-1-1. What＇s more, referring to the relative researches, the good predictability of the new model is also proved on six kinds of materials.展开更多
It has not been a simple matter to obtain a sound extension of the classical J2 flow theory of plasticity that incorporates a dependence on plastic strain gradients and that is capable of capturing size-dependent beha...It has not been a simple matter to obtain a sound extension of the classical J2 flow theory of plasticity that incorporates a dependence on plastic strain gradients and that is capable of capturing size-dependent behaviour of metals at the micron scale. Two classes of basic extensions of classical J2 theory have been proposed： one with increments in higher order stresses related to increments of strain gradients and the other characterized by the higher order stresses themselves expressed in terms of increments of strain gradients. The theories proposed by Muhlhans and Aifantis in 1991 and Fleck and Hutchinson in 2001 are in the first class, and, as formulated, these do not always satisfy thermodynamic requirements on plastic dissipation. On the other hand, theories of the second class proposed by Gudmundson in 2004 and Gurtin and Anand in 2009 have the physical deficiency that the higher order stress quantities can change discontinuously for bodies subject to arbitrarily small load changes. The present paper lays out this background to the quest for a sound phenomenological extension of the rateindependent J2 flow theory of plasticity to include a de- pendence on gradients of plastic strain. A modification of the Fleck-Hutchinson formulation that ensures its thermo- dynamic integrity is presented and contrasted with a comparable formulation of the second class where in the higher or- der stresses are expressed in terms of the plastic strain rate. Both versions are constructed to reduce to the classical J2 flow theory of plasticity when the gradients can be neglected and to coincide with the simpler and more readily formulated J2 deformation theory of gradient plasticity for deformation histories characterized by proportional straining.展开更多
Abstract For an infinite slab of strain gradient sensitive material subjected to plane-strain tensile loading, compu- tation established and analysis confirmed that passivation of the lateral boundaries at some stage ...Abstract For an infinite slab of strain gradient sensitive material subjected to plane-strain tensile loading, compu- tation established and analysis confirmed that passivation of the lateral boundaries at some stage of loading inhibits plastic deformation upon further loading. This result is not surprising in itself except that, remarkably, if the gradient terms contribute to the dissipation, the plastic deformation is switched off completely and only resumes at a clearly defined higher load, corresponding to a total strain ec, say. The analysis presented in this paper confirms the delay of plastic deformation following passivation and determines the exact manner in which the plastic flow resumes. The plastic strain rate is continuous at the exact point ec of resumption of plastic flow and, for the first small increment Ae = e - ec in the imposed total strain, the corresponding increment in plastic strain, AeP, is proportional to （Ae）2. The constant A in the relation AeP（0） = A（Ae）2, where AeP（0） denotes the plastic strain increment at the centre of the slab, has been determined explicitly; it depends on the hardening modulus of the material. The presence of energetic gradient terms has no effect on the value of ec unless the dissipative terms are absent, in which case passivation reduces the rate of plastic deformation but introduces no delay. This qualitative effect of dissipative gradient terms opens the possibility of experimen- tal discrimination of their presence or absence. The analysisemploys an incremental variational formulation that is likely to find use in other problems.展开更多
The phenomenological flow theory of higher-order strain gradient plasticity proposed by Fleck and Hutchinson(J.Mech.Phys.Solids,2001)and then improved by Fleck and Willis(J.Mech.Phys.Solids,2009)is used to investigate...The phenomenological flow theory of higher-order strain gradient plasticity proposed by Fleck and Hutchinson(J.Mech.Phys.Solids,2001)and then improved by Fleck and Willis(J.Mech.Phys.Solids,2009)is used to investigate the surface-passivation problem and micro-scale plasticity.An extremum principle is stated for the theory involving one material length scale.To solve the initial boundary value problem,a numerical scheme based on the framework of variational constitutive updates is developed for the strain gradient plasticity theory.The main idea is that,in each incremental time step,the value of the effective plastic strain is obtained through the variation of a functional in regard to effective plastic strain,provided the displacement or deformation gradient.Numerical results for elasto-plastic foils under tension and bending,thin wires under torsion,are given by using the minimum principle and the numerical scheme.Implications for the role of dissipative gradient effect are explored for three non-proportional loading conditions:(1)stretch-passivation problem,(2)bending-passivation problem,and(3)torsion-passivation problem.The results indicate that,within the Fleck-Hutchinson-Willis theory,the dissipative length scale controls the strengthening size effect,i.e.the increase of initial yielding strength,while the surface passivation gives rise to an increase of strain hardening rate.展开更多
Recrystallized grains, less than 200 nm in diameter were observed in heavily shear zones of a high strength low alloy steel and a Ni-based alloy, and Also grain refinement, less than 3 μm in diameter was made in high...Recrystallized grains, less than 200 nm in diameter were observed in heavily shear zones of a high strength low alloy steel and a Ni-based alloy, and Also grain refinement, less than 3 μm in diameter was made in high purity aluminum by ECAE at ambient temperature. The experimental results showed that high strain rate and large deformation could induce dynamic recrystallization.Based on dislocation dynamics and grain orientation change enhanced by plastic deformation,a model for the recrystallization process is developed. The model is used to explain the ultra fine grains which are formed at a temperature still much lower than that for the conventional recrystallization展开更多
This paper presents a separated law of hardening in plasticity with strain gradient effects. The value of the length parameter l contained in this model was estimated from the experimental data for copper.
The microstructures and hardness of pure Al samples subjected to plastic deformation with different tem- peratures and strain rates were investigated. The results showed that the strain-induced grain refinement is sig...The microstructures and hardness of pure Al samples subjected to plastic deformation with different tem- peratures and strain rates were investigated. The results showed that the strain-induced grain refinement is significantly benefited by increasing strain rate and reducing deformation temperature. The saturated size of refined subgrains in Al can be as small as about 240 nm in cryogenic dynamic plastic deformation （DPD）. Grain boundaries of the DPD Al samples are low-angle boundaries due to suppression of dynamic recovery during deformation. Agreement of the measured hardness with the empirical Hall-Petch relation extrapolated from the coarse-grained Al implies that the low-angle boundaries can contribute to strengthening as effective as the conventional grain boundaries.展开更多
The homogenized response of metal matrix composites（MMC） is studied using strain gradient plasticity.The material model employed is a rate independent formulation of energetic strain gradient plasticity at the micro...The homogenized response of metal matrix composites（MMC） is studied using strain gradient plasticity.The material model employed is a rate independent formulation of energetic strain gradient plasticity at the micro scale and conventional rate independent plasticity at the macro scale. Free energy inside the micro structure is included due to the elastic strains and plastic strain gradients. A unit cell containing a circular elastic fiber is analyzed under macroscopic simple shear in addition to transverse and longitudinal loading. The analyses are carried out under generalized plane strain condition. Micro-macro homogenization is performed observing the Hill-Mandel energy condition,and overall loading is considered such that the homogenized higher order terms vanish. The results highlight the intrinsic size-effects as well as the effect of fiber volume fraction on the overall response curves, plastic strain distributions and homogenized yield surfaces under different loading conditions. It is concluded that composites with smaller reinforcement size have larger initial yield surfaces and furthermore,they exhibit more kinematic hardening.展开更多
This paper studies a method for obtaining the stress with plastic deformation by finding the plastic strain on U-bent specimens of austenitic stainless steel that have been subjected to large plastic deformation using...This paper studies a method for obtaining the stress with plastic deformation by finding the plastic strain on U-bent specimens of austenitic stainless steel that have been subjected to large plastic deformation using the EBSD (Electron Backscatter Diffraction) method. The Mises stress calculated on the basis of the KAM of the EBSD shows good agreement with the stress that can be geometrically calculated from the U-bent specimens. In contrast, general methods for measuring residual stress on the basis of elastic strain produce residual stress measurement results that differ specimen by specimen. Thus, for true strain not less than 0.05, stress estimation based on the EBSD method produces better results than other general methods.展开更多
A thorough understanding of the texture evolution of near-αtitanium alloys during the hot metal forming can help obtain an optimal crystallographic texture and material performance.The strain state has an obvious eff...A thorough understanding of the texture evolution of near-αtitanium alloys during the hot metal forming can help obtain an optimal crystallographic texture and material performance.The strain state has an obvious effect on the texture evolution of near-αtitanium alloys during the hot metal forming.In this paper,the texture evolution of a near-αTA15 titanium alloy during the hot metal forming under different strain states were discussed based on the crystal plasticity finite element method.It is found that the basal and prismatic slip systems are regarded as the dominant slip modes due to the similar low critical resolved shear stress during the hot metal forming of the TA15 sheet rotating the lattice around the[1010]and 0001 axis,respectively.Once both of them cannot be activated,the pyramidal-2 slipping occurs rotating the lattice around the[1010]axis.The relationship between the texture evolution and strain state is established.All the(0001)orientations form a band perpendicular to the direction of the first principal strain.The width of the band along the direction of the second principal strain depends on the ratio of the compressive effect to the tensile effect of the second principal strain.This relationship can help control the crystallographic texture and mechanical properties of the titanium alloys component during the hot metal forming.展开更多
基金Sponsored by National Natural Science Foundation of China(Grant No.42077249)Innovation an Entrepreneurship Training Program for College Students of Hefei University of Technology(Grant Nos.S202410359131 and X202410359244).
文摘Long-term cyclic train loading can cause settlement and deformation of the roadbed,affecting the normal operation of trains.In order to investigate the strain pattern of reinforced sandy soil under train loading,a series of dynamic triaxial tests were carried out using multi-stage loading,focusing on the effects of the number of reinforcement layers,the confining pressure,and the mesh size of the geogrid on the accumulated plastic strain of reinforced sandy soil.Moreover,prediction models were proposed.The test results show that:1)The cumulative plastic strain versus vibration times of the specimens under different reinforcement layers exhibited three stages,namely,the rapid development stage,the rate transformation stage and the stability stage;2)The cumulative plastic strain decreases with increasing the number of reinforcement layers,but the magnitude of the effect of reinforcement on the cumulative plastic strain decreases with increasing the number of reinforcement layers,increasing the perimeter pressure and decreasing the mesh size of the geogrid have similar effects on the cumulative plastic strain pattern as increasing the number of reinforcement layers;3)Combined with the cumulative plastic strain law,a comprehensive model is proposed and the coefficient of determination is greater than 0.99.Furthermore,The cumulative plastic strain evolution law can be effectively predicted.The significance of parameters A,B and C is analyzed in detail.This study can provide theoretical references for further understanding of the deformation characteristics and settlement prediction of railway subgrades.
文摘Two improvements have been made on the calculation of plastic strain ratio (r value) in BCC monocrystals by considering the differences in the critical shear stresses among the three slip systems and the rotation of crystal lattice. It is found that the results calculated by the improved method are more rational.
基金Supported by the National Natural Science Foundation of China。
文摘The change of width and length of an aluminium-killed(AK)steel sheet was measured in detail at various tensile strain levels.The effects of tensile strain on the conventional strain ratio,rc-value,and the instantaneous strain ratio,r^(1)-value,were analyzed.It was shown that the strain ratio of the AK steel sheet had obvious strain-dependency.The individual r^(c)-values for all tested directions tended to unity with increasing strain so that the planar anisotropy decreased with increasing strain.The r^(1)-value had overall straidenpendency similar to that of r^(c)-value,though it was overly sensitive to the experimental errors and showed significant variation with strain.In the representation of average strain ratio.the linear regression strain ratio,r^(r)-value,which was obtained by fitting a straight line to the εw vs εt curve,is preferable to the r^(c)-value measured at some specfic extension.
基金supported by the National Natural Science Foundation of China(No.61876024)the Natural Science Foundation for higher education of Jiangsu Province(No.21KJA510003&No.21KJB460035)Suzhou Municipal Science and Technology Plan Project(No.SYG202129)。
文摘By using multi-pass straight-line scan strategies,a larger bending angle can be achieved.There is,however,a limited understanding of the variation in bending angle per pass during multi-pass under various process parameters.In multi-pass laser scanning,the bending angle cannot always linearly increase with scanning passes and this phenomenon can be observed mostly in low heat input.Strain hardening is the common explanation for this phenomenon.However,it could not explain why this bending angle reduction phenomenon occurs at low heat input scanning rather than at high heat input scanning.In this study,this phenomenon is discussed based on strain development in experiments and numerical simulation.The different growing mechanism of plastic strain is analyzed to reveal the effects of laser power and scanning velocity.Furthermore,the opposite bending trend that occurred at larger laser power is discussed in comparison to the plastic strain development.The study shows that relatively larger heat input below 6.7 J/mm can help to avoid the bending angle reduction phenomenon and the opposite bending trend is highly dependent on the larger laser power.For achieving the expected cumulative bending angle in multi-pass laser bending,it is recommended to decrease the scanning velocity at a relatively low laser power level while increase the scanning velocity at a high level of laser power.
文摘The present work is concerned with the characterization of hardening parameters for an elasto-plastic continuum model, taking into account the memory effect of plastic strain amplitude, in order to predict the hysteretic responses of a ferritic steel. This elasto-plastic three-dimensional model is based on the internal thermodynamic variables which composed of the nonlinear kinematic hardening and isotropic hardening with the plastic strain memorization. The emphasis is put on the determination of strain memory parameters along with other material parameters of the proposed model in order to better simulate the behavior of the material at different strain range. The material parameters are calibrated with the experimental stabilized loops of stress-strain curves available in the literature. The predicted stabilized loops from the simulation with the determined parameters show good agreement with the experimental results signifying the validity of the considered model.
文摘It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F=RU and spin W in F -1 are not correct. Errors will arise in plastic deformation rate if it is directly expressed with amounts of velocity of slips in glide systems such as vn. The geometrical figure of crystal lattices does not change after slips and based on this idea a simple way in mechanics of continuous media to get the plastic deformations rate induced by slips is proposed. Constitutive equations are recommended.
基金The project supported by the Association of International Education of Japan
文摘The paper describes an energy-based constitutive model for sand, which is modified based on the modified plastic strain energy approach, represented by a unique relationship between the modified plastic strain energy and a stress parameter, independent of stress history. The modified plastic strain energy approach was developed based on results from a series of drained plastic strain compression tests along various stress paths on saturated dense Toyoura sand with accurate stress and strain measurements. The proposed model is coupled with an isotropically work-hardening and softening, non-associtated, elasto-plastic material description. The constitutive model concerns the inherent and stress system-induced cross-anisotropic elastic deformation properties of sand. It is capable of simulating the deformation characteristics of stress history and stress path, the effects of pressure level, anisotropic strength and void ratio, and the strain localization.
文摘It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F = RU and spin TY in (F) over dot F-1 ore not correct. Errors will arise in plastic deformation rare if it is directly expressed with amounts of velocity of slips in glide systems such as (gamma) over dot upsilon circle times n. The geometrical figure of crystal lattices does nor change after slips and based on this idea a simple way in mechanics of continuous media to get the plastic deformations rare induced by slips is proposed. Constitutive equations are recommended.
基金supported by National Natural Science Foundation of China(Grant No.51075153)Natural Science Foundation of Guangdong Province(Grant No.10151040301000000)+1 种基金Key Laboratory of Precision Equipment and Manufacturing Technology of Guangdong Province(Grant No.PEMT1202)the EU FP7 Marie Curie International Research Staff Exchange Scheme(IRSES)Mat Pro Future Project(Grant No.318968)
文摘A new spinning method to manufacture the cylindrical parts with nano/ultrafine grained structures is proposed, which consists of quenching, power spinning and recrystallization annealing. The microstructural evolution during the different process stages and macroforming quality of the spun parts made of ASTM 1020 steel are investigated. The results show that the microstructures of the ferrites and pearlites in the ASTM 1020 steel are transformed to the lath martensites after quenching. The martensite laths obtained by quenching are refined to 87 nm and a small amount of nanoscale deformation twins with an average thickness of 20 nm is generated after performing a 3-pass stagger spinning with 55% thinning ratio of wall thickness, where the equivalent strain required is only 0.92. The equiaxial ferritic grains with an average size of 160 nm and nano-carbides are generated by subsequent recrystallization annealing at 480°C for 30 min. The spun parts with high dimensional precision and low surface roughness are obtained by the forming method developed in this work, combining quenching with 3-pass stagger spinning and recrystallization annealing.
基金supported by the Natural Science Foundation of China [grant number 52122811]。
文摘The Türkiye earthquake sequence on February 6, 2023, was featured by the closely located earthquake doublet of M_(W)7.8 and M_(W)7.5. The consequent strong ground motions are supposed to be able to impose high demands on the ultra-low-cycle fatigue performance of metallic dampers in buildings, including the widely used buckling restrained braces. This study evaluates the cumulative plastic deformation(CPD) demands on bucklingrestrained braces(BRBs) in multi-story buildings imposed by the strong ground motions in the 2023 Türkiye earthquake doublet. Thirty-two records of the highest peak ground accelerations were selected from the strong motion database. Among them, eight captured the ground motions during both events, and the rest only captured the shaking of either of the events. The CPD demands on the BRBs in reinforced concrete frames with various fundamental periods, brace-to-frame stiffness ratios, and BRB ductility ratio are calculated by nonlinear time history analyses and are summarized in the form of enveloped spectra of CPD ratios at constant ductility. The results show that the CPD demands on BRBs increase with smaller brace-toframe stiffness ratios and larger BRB ductility ratios. The enveloped CPD demands are several hundreds of times the nominal yield deformation of the BRB, which are much higher than the CPD demands for the calibration tests of BRBs stipulated by AISC 341 in the US.
基金co-supported by National Natural Science Foundation of China (No. 11072205)College Students' National Innovation Foundation of China (No. 101061323)
文摘By introducing a fatigue blunting factor, the cyclic elasto-plastic Hutchinson-Rice-Rosengren （HRR） field near the crack tip under the cyclic loading is modified. And, an average damage per loading-cycle in the cyclic plastic deformation region is defined due to Manson-Coffin law. Then, according to the linear damage accumulation theory-Miner law, a new model for predicting the fatigue crack growth （FCG） of the opening mode crack based on the low cycle fatigue （LCF） damage is set up. The step length of crack propagation is assumed to be the size of cyclic plastic zone. It is clear that every parameter of the new model has clearly physical meaning which does not need any human debugging. Based on the LCF test data, the FCG predictions given by the new model are consistent with the FCG test results of Cr2Ni2MoV and X12CrMoWVNbN 10-1-1. What＇s more, referring to the relative researches, the good predictability of the new model is also proved on six kinds of materials.
文摘It has not been a simple matter to obtain a sound extension of the classical J2 flow theory of plasticity that incorporates a dependence on plastic strain gradients and that is capable of capturing size-dependent behaviour of metals at the micron scale. Two classes of basic extensions of classical J2 theory have been proposed： one with increments in higher order stresses related to increments of strain gradients and the other characterized by the higher order stresses themselves expressed in terms of increments of strain gradients. The theories proposed by Muhlhans and Aifantis in 1991 and Fleck and Hutchinson in 2001 are in the first class, and, as formulated, these do not always satisfy thermodynamic requirements on plastic dissipation. On the other hand, theories of the second class proposed by Gudmundson in 2004 and Gurtin and Anand in 2009 have the physical deficiency that the higher order stress quantities can change discontinuously for bodies subject to arbitrarily small load changes. The present paper lays out this background to the quest for a sound phenomenological extension of the rateindependent J2 flow theory of plasticity to include a de- pendence on gradients of plastic strain. A modification of the Fleck-Hutchinson formulation that ensures its thermo- dynamic integrity is presented and contrasted with a comparable formulation of the second class where in the higher or- der stresses are expressed in terms of the plastic strain rate. Both versions are constructed to reduce to the classical J2 flow theory of plasticity when the gradients can be neglected and to coincide with the simpler and more readily formulated J2 deformation theory of gradient plasticity for deformation histories characterized by proportional straining.
文摘Abstract For an infinite slab of strain gradient sensitive material subjected to plane-strain tensile loading, compu- tation established and analysis confirmed that passivation of the lateral boundaries at some stage of loading inhibits plastic deformation upon further loading. This result is not surprising in itself except that, remarkably, if the gradient terms contribute to the dissipation, the plastic deformation is switched off completely and only resumes at a clearly defined higher load, corresponding to a total strain ec, say. The analysis presented in this paper confirms the delay of plastic deformation following passivation and determines the exact manner in which the plastic flow resumes. The plastic strain rate is continuous at the exact point ec of resumption of plastic flow and, for the first small increment Ae = e - ec in the imposed total strain, the corresponding increment in plastic strain, AeP, is proportional to （Ae）2. The constant A in the relation AeP（0） = A（Ae）2, where AeP（0） denotes the plastic strain increment at the centre of the slab, has been determined explicitly; it depends on the hardening modulus of the material. The presence of energetic gradient terms has no effect on the value of ec unless the dissipative terms are absent, in which case passivation reduces the rate of plastic deformation but introduces no delay. This qualitative effect of dissipative gradient terms opens the possibility of experimen- tal discrimination of their presence or absence. The analysisemploys an incremental variational formulation that is likely to find use in other problems.
基金supported by the National Natural Science Foundation of China(Grants 11702103 and 11972168)the Young Elite Scientist Sponsorship Program by CAST(Grant 2016QNRC001)the Fundamental Research Funds for the Central Universities(Grant HUST 2018KFYYXJJ008)。
文摘The phenomenological flow theory of higher-order strain gradient plasticity proposed by Fleck and Hutchinson(J.Mech.Phys.Solids,2001)and then improved by Fleck and Willis(J.Mech.Phys.Solids,2009)is used to investigate the surface-passivation problem and micro-scale plasticity.An extremum principle is stated for the theory involving one material length scale.To solve the initial boundary value problem,a numerical scheme based on the framework of variational constitutive updates is developed for the strain gradient plasticity theory.The main idea is that,in each incremental time step,the value of the effective plastic strain is obtained through the variation of a functional in regard to effective plastic strain,provided the displacement or deformation gradient.Numerical results for elasto-plastic foils under tension and bending,thin wires under torsion,are given by using the minimum principle and the numerical scheme.Implications for the role of dissipative gradient effect are explored for three non-proportional loading conditions:(1)stretch-passivation problem,(2)bending-passivation problem,and(3)torsion-passivation problem.The results indicate that,within the Fleck-Hutchinson-Willis theory,the dissipative length scale controls the strengthening size effect,i.e.the increase of initial yielding strength,while the surface passivation gives rise to an increase of strain hardening rate.
文摘Recrystallized grains, less than 200 nm in diameter were observed in heavily shear zones of a high strength low alloy steel and a Ni-based alloy, and Also grain refinement, less than 3 μm in diameter was made in high purity aluminum by ECAE at ambient temperature. The experimental results showed that high strain rate and large deformation could induce dynamic recrystallization.Based on dislocation dynamics and grain orientation change enhanced by plastic deformation,a model for the recrystallization process is developed. The model is used to explain the ultra fine grains which are formed at a temperature still much lower than that for the conventional recrystallization
基金The project supported by the National Natural Science Foundation of China
文摘This paper presents a separated law of hardening in plasticity with strain gradient effects. The value of the length parameter l contained in this model was estimated from the experimental data for copper.
基金support from the National Natural Science Foundation of China (Grants Nos. 50971122, 50431010,50621091, 50890171)Shenyang Science & Technology Project (No. 1071107-1-00)the Ministry of Scienceand Technology of China (2005CB623604)
文摘The microstructures and hardness of pure Al samples subjected to plastic deformation with different tem- peratures and strain rates were investigated. The results showed that the strain-induced grain refinement is significantly benefited by increasing strain rate and reducing deformation temperature. The saturated size of refined subgrains in Al can be as small as about 240 nm in cryogenic dynamic plastic deformation （DPD）. Grain boundaries of the DPD Al samples are low-angle boundaries due to suppression of dynamic recovery during deformation. Agreement of the measured hardness with the empirical Hall-Petch relation extrapolated from the coarse-grained Al implies that the low-angle boundaries can contribute to strengthening as effective as the conventional grain boundaries.
基金supported by the Danish Research Council for Technology and Production Sciences in a project entitled Plasticity Across the Scales
文摘The homogenized response of metal matrix composites（MMC） is studied using strain gradient plasticity.The material model employed is a rate independent formulation of energetic strain gradient plasticity at the micro scale and conventional rate independent plasticity at the macro scale. Free energy inside the micro structure is included due to the elastic strains and plastic strain gradients. A unit cell containing a circular elastic fiber is analyzed under macroscopic simple shear in addition to transverse and longitudinal loading. The analyses are carried out under generalized plane strain condition. Micro-macro homogenization is performed observing the Hill-Mandel energy condition,and overall loading is considered such that the homogenized higher order terms vanish. The results highlight the intrinsic size-effects as well as the effect of fiber volume fraction on the overall response curves, plastic strain distributions and homogenized yield surfaces under different loading conditions. It is concluded that composites with smaller reinforcement size have larger initial yield surfaces and furthermore,they exhibit more kinematic hardening.
文摘This paper studies a method for obtaining the stress with plastic deformation by finding the plastic strain on U-bent specimens of austenitic stainless steel that have been subjected to large plastic deformation using the EBSD (Electron Backscatter Diffraction) method. The Mises stress calculated on the basis of the KAM of the EBSD shows good agreement with the stress that can be geometrically calculated from the U-bent specimens. In contrast, general methods for measuring residual stress on the basis of elastic strain produce residual stress measurement results that differ specimen by specimen. Thus, for true strain not less than 0.05, stress estimation based on the EBSD method produces better results than other general methods.
基金financially supported by the National Natural Science Foundation of China(No.51401065).
文摘A thorough understanding of the texture evolution of near-αtitanium alloys during the hot metal forming can help obtain an optimal crystallographic texture and material performance.The strain state has an obvious effect on the texture evolution of near-αtitanium alloys during the hot metal forming.In this paper,the texture evolution of a near-αTA15 titanium alloy during the hot metal forming under different strain states were discussed based on the crystal plasticity finite element method.It is found that the basal and prismatic slip systems are regarded as the dominant slip modes due to the similar low critical resolved shear stress during the hot metal forming of the TA15 sheet rotating the lattice around the[1010]and 0001 axis,respectively.Once both of them cannot be activated,the pyramidal-2 slipping occurs rotating the lattice around the[1010]axis.The relationship between the texture evolution and strain state is established.All the(0001)orientations form a band perpendicular to the direction of the first principal strain.The width of the band along the direction of the second principal strain depends on the ratio of the compressive effect to the tensile effect of the second principal strain.This relationship can help control the crystallographic texture and mechanical properties of the titanium alloys component during the hot metal forming.
