Plastic behavior of 603 armor steel is studied at strain rates ranging from 0.001 s-1 to 4500 s-1, and temperature from 288 K to 873 K. Emphasis is placed on the effects of temperature, strain rate, and plastic strain...Plastic behavior of 603 armor steel is studied at strain rates ranging from 0.001 s-1 to 4500 s-1, and temperature from 288 K to 873 K. Emphasis is placed on the effects of temperature, strain rate, and plastic strain on flow stress. Based on experimental results, the JC and the KHL models are used to simulate flow stress of this material. By comparing the model prediction and the experimental results of strain rate jump tests, the KHL model is shown to have a better prediction of plastic behavior under complex loading conditions for this material, especially in the dynamic region.展开更多
Some micromechanics-based constitutive models are presented in this study for porous geomaterials.These micro-macro mechanical models focus on the effect of porosity and the inclusions on the macroscopic elastoplastic...Some micromechanics-based constitutive models are presented in this study for porous geomaterials.These micro-macro mechanical models focus on the effect of porosity and the inclusions on the macroscopic elastoplastic behaviors of porous materials. In order to consider the effect of pores and the compressibility of the matrix, some macroscopic criteria are presented firstly for ductile porous medium having one population of pores with different types of matrix(von Mises, Green type, Misese Schleicher and Druckere Prager). Based on different homogenization techniques, these models are extended to the double porous materials with two populations of pores at different scales and a Druckere Prager solid phase at the microscale. Based on these macroscopic criteria, complete constitutive models are formulated and implemented to describe the overall responses of typical porous geomaterials(sandstone,porous chalk and argillite). Comparisons between the numerical predictions and experimental data with different confining pressures or different mineralogical composites show the capabilities of these micromechanics-based models, which take into account the effects of microstructure on the macroscopic behavior and significantly improve the phenomenological ones.展开更多
Biological invasions are a major driver of global biodiversity loss,impacting endemic species,ecosystems,and economies.Although the influence of life history traits on invasive success is well-established,the role of ...Biological invasions are a major driver of global biodiversity loss,impacting endemic species,ecosystems,and economies.Although the influence of life history traits on invasive success is well-established,the role of behavior in the invasive potential of animals is less studied.The common coqui frog,Eleutherodactylus coqui,is a highly successful invader in Hawai'i.We build on previous research characterizing changes in physiology and morphology to explore behavioral variation across the invasive range of coqui in Hawai'i.Coqui have expanded both outward and upward from their initial introduction site,andby comparing frogs from different densities and elevations-we specifically asked how the physiological challenges of high-elevation living interact with the competitive challenge of high-densities at population centers.To investigate whether differences in the field represent local adaptation or behavioral plasticity,we additionally evaluated behavior following acclimation to a shared laboratory environment.Although we identified only subtle behavioral variation among populations in the field,we found that individuals from all populations became less bold,active,and exploratory in the laboratory,converging on a similar behavioral phenotype.Alongside previous work,our results suggest that coqui adjust their behavior to local environmental conditions across their invasive range and that behavioral flexibility may contribute to invasive success.展开更多
The plastic flow behavior of the rotating band material is investigated in this paper. The rotating band material is processed from H96 brass alloy, which is hardened to a much higher yield strength compared to the an...The plastic flow behavior of the rotating band material is investigated in this paper. The rotating band material is processed from H96 brass alloy, which is hardened to a much higher yield strength compared to the annealed one. The dynamically uniaxial compression behavior of the material is tested using the split Hopkinson pressure bar(SHPB) with temperature and strain rate ranging from 297 to 1073 K and500 to 3000 s^(-1), respectively, and a phenomenological plastic flow stress model is developed to describe the mechanical behavior of the material. The material is found to present noticeable temperature sensitivity and weak strain-rate sensitivity. The construction of the plastic flow stress model has two steps. Firstly, three univariate stress functions, taking plastic strain, plastic strain rate and temperature as independent variable, respectively, are proposed by fixing the other two variables. Then, as the three univariate functions describe the special cases of flow stress behavior under various conditions, the principle of stress compatibility is adopted to obtain the complete flow stress function. The numerical results show that the proposed plastic flow stress model is more suitable for the rotating band material than the existing well-known models.展开更多
Despite their key role as model organisms in many behavioral studies, crustacean decapods have been only slightly touched upon by the recent surge of scientific interest in animal personality. Only seven articles inve...Despite their key role as model organisms in many behavioral studies, crustacean decapods have been only slightly touched upon by the recent surge of scientific interest in animal personality. Only seven articles investigated the issue in a handful of species among hermit crabs, crabs, and crayfish. Obviously, a limited number of publications does not mean that personality is rare in decapods. On the contrary, few studies might be the result of a form of reluctance by behavioral ecologists to deal with such a phenomenon in these and other invertebrates. This reluctance contrasts with the enthusiasm shown in tackling the beha- vioral plasticity issue. Here we discuss the possible theoretical and methodological difficulties raised by applying the animal per-sonality perspective to decapods and analyze implications of personality studies for their ecology, conservation, and welfare. By highlighting gaps in knowledge and directions of future research, our intention is to increase scientific emphasis on the issue.展开更多
Background: The drivers of intraspecific variation in behavioral plasticity are poorly known. A widely held hypothesis is that brain size is positively correlated with behavioral plasticity.Methods: A total of 71 Ches...Background: The drivers of intraspecific variation in behavioral plasticity are poorly known. A widely held hypothesis is that brain size is positively correlated with behavioral plasticity.Methods: A total of 71 Chestnut Thrushes(Turdus rubrocanus) were caught in the wild population. We quantified behavior plasticity of activity of individuals measured in the same cage across two contexts(common and with a novel object stimulation), using a random regression analysis. We then investigated whether head volume(a proxy for brain size) was associated with behavioral plasticity in activity level using Spearman rank-order correlation.Results: We found no significant evidence that activity plasticity was associated with relative head volume. There was no sex difference in head volume or in variance in head volume.Conclusions: We speculate that the absence of an association between brain volume and activity behavior plasticity may result from the inaccuracy of using external skull measurements to estimate brain size, or from a particular part of the brain being responsible for plasticity in activity level.展开更多
Mechanical properties and fracture behavior of Cu-0.84Co 0.23Be alloy after plastic deformation and heat treatment were comparatively investigated. Severe plastic deformation by hot extrusion and cold drawing was adop...Mechanical properties and fracture behavior of Cu-0.84Co 0.23Be alloy after plastic deformation and heat treatment were comparatively investigated. Severe plastic deformation by hot extrusion and cold drawing was adopted to induce large plastic strain of Cu 0.84Co-0.23Be alloy. The tensile strength and elongation are up to 476.6 MPa and 18%, respectively. The fractured surface consists of deep dimples and micro voids. Due to the formation of su- persaturated solid solution on the Cu matrix by solution treatment at 950℃ for 1 h, the tensile strength decreased to 271.9 MPa, while the elongation increased to 42%. The fracture morphology is parabolic dimple. Furthermore, the tensile strength increased significantly to 580.2 MPa after aging at 480 ℃ for 4 h. During the aging process, a large number of precipitates formed and distributed on the Cu matrix. The fracture feature of aged specimens with low elongation （4.6%） exhibits an obvious brittle intergranular fracture. It is confirmed that the mechanical properties and fracture behavior are dominated by the microstrueture characteristics of Cu-0.84Co 0.23Be alloy after plastic de- formation and heat treatment. In addition, the fracture behavior at 450 ℃ of aged Cu-0.84Co 0.23Be alloy was also studied. The tensile strength and elongation are 383.6 MPa and 11.2%, respectively. The fractured morphologies are mainly candy-shaped with partial parabolic dimples and equiaxed dimples. The fracture mode is multi mixed mechanism that brittle intergranular fracture plays a dominant role and ductile fracture is secondary.展开更多
The hot deformation simulation of a ZK60 magnesiuln alloy at different temperatures from 373 to 673 K and different strain rates of 0.1, 0.01 and 0.002 s^-1 was studied by using the Gleebe-1500 simulator. The plastic ...The hot deformation simulation of a ZK60 magnesiuln alloy at different temperatures from 373 to 673 K and different strain rates of 0.1, 0.01 and 0.002 s^-1 was studied by using the Gleebe-1500 simulator. The plastic deformation behavior was measured and the deformation activation energy was calculated. The microstructures of ZK60 magnesium alloy with an addition of neodymium during the deformation process were observed by using Polyvar-MET optical microscope and Tecnai G^2 20 TEM. The results show that the working hardening, the dynamic recovery and the dynamic recrystallization occur during the plastic deformation process at different temperatures and strain rates. The dynamic recrystallization starts when the temperature is over 473 K and the DRX grain size after hot deformation is only 5-10 μm. So the refined grains improve both the tensile strength and the elongation of alloys at room temperature. Neodymium is added into the alloy and a precipitate phase Mg12Nd that impedes the movement of dislocations is formed, which benefits to increasing mechanical properties of ZK60 magnesium alloy.展开更多
Marine invertebrates that move too slowly to evade unfavorable environmental change may instead exhibit phenotypic plasticity,allowing them to adjust to varying conditions.The orange-footed sea cucumber Cucumaria fron...Marine invertebrates that move too slowly to evade unfavorable environmental change may instead exhibit phenotypic plasticity,allowing them to adjust to varying conditions.The orange-footed sea cucumber Cucumaria frondosa is a slow-moving suspension feeder that is preyed on by the purple sunstar Solaster endeca.The sea cucumber's antipredator behavior involves changing shape and detaching from the substratum,which might increase its probability of being displaced by water motion into an unsuitable environment.We hypothesized that sea cucumbers'antipredator responses would be diminished under stronger hydrodynamic forces,and that behavioral strategies would be flexible so that individuals could adjust to frequent changes in water flows.In a natural orange-footed sea cucumber habitat,individuals lived along a pronounced hydrodynamic gradient,allowing us to measure antipredator behavior under different water flow strengths.We placed purple sunstars in physical contact with sea cucumbers living at various points along the gradient to elicit antipredator responses.We then repeated this procedure in a laboratory mesocosm that generated weak and strong hydrodynamic forces similar to those observed at the field site.Subjects in the mesocosm experiment were tested in both wave conditions to determine if their antipredator behavior would change in response to sudden environmental change,as would be experienced under deteriorating sea conditions.Antipredator responses did not covary with hydrodynamic forces in the field.However,antipredator responses in the mesocosm experiment increased when individuals were transplanted from strong to weak forces and decreased when transplanted from weak to strong forces.Overall,our results indicate environmentally induced plasticity in the antipredator behavior of the orange-footed sea cucumber.展开更多
A visco-elastoplastic damage constitutive model is proposed for simulating non-linear behavior of concrete. Based on traditional plastic theory, the irreversible deformation is simulated in effective stress space. In ...A visco-elastoplastic damage constitutive model is proposed for simulating non-linear behavior of concrete. Based on traditional plastic theory, the irreversible deformation is simulated in effective stress space. In order to reflect different stiffness degradation mechanism of concrete under tensile and compressive loading conditions, both tensile and compressive damage variables are introduced, and then on the basis of energy release rate, the model is firmly derived within the concept of irreversible thermodynamics. The rate-dependent model is considered by introducing viscous regularization into the inelastic strain and damage variable, and combined with an additional elastic condition. Fully implicit backward-Euler algorithm is used to perform constitutive integration. Results of numerical examples using the proposed model agree well with test results for specimens under uniaxial tension and compression, biaxial loading and triaxial loading. Failure processes of single-edge-notched （SEN） beam and double-edge-notched （DEN） specimen are also simulated to further validate the proposed model.展开更多
In this paper, a novel unified plasticity methodology is proposed to allow the coupling of rate-and temperature-sensitivity of engineering alloys as well as the non-linear kinematic hardening behavior often observed d...In this paper, a novel unified plasticity methodology is proposed to allow the coupling of rate-and temperature-sensitivity of engineering alloys as well as the non-linear kinematic hardening behavior often observed during cyclic loading. The proposed methodology is general in the sense that an arbitrary constitutive model may be chosen for the viscoplastic part, as well as the cyclic part. We adapt our model with a physically-motivated viscoplasticity flow rule and a nonlinear kinematic hardening model. In contrast with other unified plasticity models, the simplified theory involves few material parameters that can be readily calibrated from standard mechanical tests. The capabilities of the proposed theory are demonstrated for a hot rolled annealed 304 L stainless steel supplied by Vimetal Peckover. The model is tested with stress–strain curves obtained from standard tensile and cyclic uniaxial tests at various strain amplitudes and strain-rates, and good accuracy of the response is obtained for strains up to 15%, within a temperature range of 293–673 K. We note that the cyclic plasticity model in our adapted theory can be readily enhanced with ratchetting, mean stress relaxation, strain amplitude history, Masing effects or other complex capabilities.展开更多
Very high cycle fatigue behavior （107 --109 cycles） of 304L austenitic stainless steel was studied with ultra- sonic fatigue testing system （20 kHz）. The characteristics of fatigue crack initiation and propagation...Very high cycle fatigue behavior （107 --109 cycles） of 304L austenitic stainless steel was studied with ultra- sonic fatigue testing system （20 kHz）. The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation. It was found that micro-plasticity （slip markings） could be observed on the specimen surface even at very low stress amplitudes. The persistent slip mark- ings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure. By detailed investigation using a scanning electron microscope and an infrared camera, slip markings appeared at the large grains where the fatigue crack initiation site was located. The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack. Finally, the cou- pling relationship among the fatigue crack propagation, appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.展开更多
Based on the minimum principle of acceleration in the elastic-plastic continua under finite def ormation, the dynamic response of an elastic-perfectly plastic pin-ended beam subjected to rectangular impulse loading is...Based on the minimum principle of acceleration in the elastic-plastic continua under finite def ormation, the dynamic response of an elastic-perfectly plastic pin-ended beam subjected to rectangular impulse loading is studied with the help of a numerical approach. The calculated results once again show the anomalous behavior of the beam during its response process, which was previously found in [1]. By carefully analyzing the instantaneous distribution of the bending moment, the membrane force, the curvature and displacement during the response process, it is concluded that the interactive effect between the geometry and materials nonlinearities of the structure is the key reason for leading to the anomalous behavior. This will be helpful for clarifying some misunderstandings in explaining the problem before.展开更多
The frictional resistance and machining quality when cutting carbon fiber reinforced plastics (CFRP) laminates are associated with tribological behavior of tool materials. In the present study, the tribological proper...The frictional resistance and machining quality when cutting carbon fiber reinforced plastics (CFRP) laminates are associated with tribological behavior of tool materials. In the present study, the tribological properties of three types of monolayer microcrystalline diamond (MCD) coatings, nanocrystalline diamond (NCD) coatings and dual-layer MCD/NCD coatings sliding against CFRP are investigated under dry lubricated conditions using the rotational friction tester. The coefficients of friction (COF), wear rate and worn surfaces of the contacted surfaces are analyzed for the MCD-CFRP, NCD-CFRP and MCD/NCD-CFRP contacting pairs. The results show that compared with the monolayer MCD and NCD, the bilayer of MCD/NCD coating displays the lowest COF with the value of ~0.13, it is 42% and 55% of the values for MCD and NCD coatings. Due to the rough surfaces of MCD, the wear debris of CFRP on MCD samples exhibits the plowing effect. While for the NCD and MCD/NCD samples, the wear fragments display the planar shapes. The wear rate of CFRP against MCD is more than twice that of CFRP against NCD, due to the excellent loading capacity. While the wear rate of CFRP against MCD/NCD is about twice than that of CFRP-NCD pairs. The bilayer of MCD/NCD combines the excellent advantages of high hardness of MCD and the smooth surface of NCD. It shows the broad application potential for the bilayer coatings.展开更多
The accuracy and effi ciency of the modelling techniques utilized to model the nonlinear behavior of structural components is a signifi cant issue in earthquake engineering. In this study, the suffi ciency of three di...The accuracy and effi ciency of the modelling techniques utilized to model the nonlinear behavior of structural components is a signifi cant issue in earthquake engineering. In this study, the suffi ciency of three diff erent modelling techniques that can be employed to simulate the structural behavior of columns is investigated. A fi ber-based fi nite length plastic hinge (FB-FLPH) model is calibrated in this study. In order to calibrate the FB-FLPH model, a novel database of the cyclic behavior of hollow steel columns under simultaneous axial and lateral loading cycles with varying amplitudes is used. By employing the FB-FLPH model calibrated in this study, the interaction of the axial force and the bending moment in columns is directly taken into account, and the deterioration in the cyclic behavior of these members is implicitly considered. The superiority of the calibrated FB-FLPH modelling approach is examined compared with the cases in which conventional fi ber-based distributed plasticity and concentrated plasticity models are utilized. The effi ciency of the enumerated modelling techniques is probed when they are implemented to model the columns of a typical special moment frame in order to prove the advantage of the FB-FLPH modelling approach.展开更多
文摘Plastic behavior of 603 armor steel is studied at strain rates ranging from 0.001 s-1 to 4500 s-1, and temperature from 288 K to 873 K. Emphasis is placed on the effects of temperature, strain rate, and plastic strain on flow stress. Based on experimental results, the JC and the KHL models are used to simulate flow stress of this material. By comparing the model prediction and the experimental results of strain rate jump tests, the KHL model is shown to have a better prediction of plastic behavior under complex loading conditions for this material, especially in the dynamic region.
文摘Some micromechanics-based constitutive models are presented in this study for porous geomaterials.These micro-macro mechanical models focus on the effect of porosity and the inclusions on the macroscopic elastoplastic behaviors of porous materials. In order to consider the effect of pores and the compressibility of the matrix, some macroscopic criteria are presented firstly for ductile porous medium having one population of pores with different types of matrix(von Mises, Green type, Misese Schleicher and Druckere Prager). Based on different homogenization techniques, these models are extended to the double porous materials with two populations of pores at different scales and a Druckere Prager solid phase at the microscale. Based on these macroscopic criteria, complete constitutive models are formulated and implemented to describe the overall responses of typical porous geomaterials(sandstone,porous chalk and argillite). Comparisons between the numerical predictions and experimental data with different confining pressures or different mineralogical composites show the capabilities of these micromechanics-based models, which take into account the effects of microstructure on the macroscopic behavior and significantly improve the phenomenological ones.
基金supported by the United States Department of Agriculture National Institute of Food and Agriculture Hatch project 1026333(ILLU-875-984 to K.M.S)a University of Ilinois Graduate College Master's Fellowship(to K.M.S)+2 种基金a University of Illinois Graduate College Travel Award(to K.M.S)Ilinois State Toll Highway Authority funding(to D.E and A.L.C)University of Illinois Laboratory Start-up funds(to E.K.F).
文摘Biological invasions are a major driver of global biodiversity loss,impacting endemic species,ecosystems,and economies.Although the influence of life history traits on invasive success is well-established,the role of behavior in the invasive potential of animals is less studied.The common coqui frog,Eleutherodactylus coqui,is a highly successful invader in Hawai'i.We build on previous research characterizing changes in physiology and morphology to explore behavioral variation across the invasive range of coqui in Hawai'i.Coqui have expanded both outward and upward from their initial introduction site,andby comparing frogs from different densities and elevations-we specifically asked how the physiological challenges of high-elevation living interact with the competitive challenge of high-densities at population centers.To investigate whether differences in the field represent local adaptation or behavioral plasticity,we additionally evaluated behavior following acclimation to a shared laboratory environment.Although we identified only subtle behavioral variation among populations in the field,we found that individuals from all populations became less bold,active,and exploratory in the laboratory,converging on a similar behavioral phenotype.Alongside previous work,our results suggest that coqui adjust their behavior to local environmental conditions across their invasive range and that behavioral flexibility may contribute to invasive success.
基金the support from National Natural Science Foundation of China (Grant Nos. 11702137 and U2141246)。
文摘The plastic flow behavior of the rotating band material is investigated in this paper. The rotating band material is processed from H96 brass alloy, which is hardened to a much higher yield strength compared to the annealed one. The dynamically uniaxial compression behavior of the material is tested using the split Hopkinson pressure bar(SHPB) with temperature and strain rate ranging from 297 to 1073 K and500 to 3000 s^(-1), respectively, and a phenomenological plastic flow stress model is developed to describe the mechanical behavior of the material. The material is found to present noticeable temperature sensitivity and weak strain-rate sensitivity. The construction of the plastic flow stress model has two steps. Firstly, three univariate stress functions, taking plastic strain, plastic strain rate and temperature as independent variable, respectively, are proposed by fixing the other two variables. Then, as the three univariate functions describe the special cases of flow stress behavior under various conditions, the principle of stress compatibility is adopted to obtain the complete flow stress function. The numerical results show that the proposed plastic flow stress model is more suitable for the rotating band material than the existing well-known models.
文摘Despite their key role as model organisms in many behavioral studies, crustacean decapods have been only slightly touched upon by the recent surge of scientific interest in animal personality. Only seven articles investigated the issue in a handful of species among hermit crabs, crabs, and crayfish. Obviously, a limited number of publications does not mean that personality is rare in decapods. On the contrary, few studies might be the result of a form of reluctance by behavioral ecologists to deal with such a phenomenon in these and other invertebrates. This reluctance contrasts with the enthusiasm shown in tackling the beha- vioral plasticity issue. Here we discuss the possible theoretical and methodological difficulties raised by applying the animal per-sonality perspective to decapods and analyze implications of personality studies for their ecology, conservation, and welfare. By highlighting gaps in knowledge and directions of future research, our intention is to increase scientific emphasis on the issue.
基金supported by the National Natural Science Foundation of China (Grant No.31472012)
文摘Background: The drivers of intraspecific variation in behavioral plasticity are poorly known. A widely held hypothesis is that brain size is positively correlated with behavioral plasticity.Methods: A total of 71 Chestnut Thrushes(Turdus rubrocanus) were caught in the wild population. We quantified behavior plasticity of activity of individuals measured in the same cage across two contexts(common and with a novel object stimulation), using a random regression analysis. We then investigated whether head volume(a proxy for brain size) was associated with behavioral plasticity in activity level using Spearman rank-order correlation.Results: We found no significant evidence that activity plasticity was associated with relative head volume. There was no sex difference in head volume or in variance in head volume.Conclusions: We speculate that the absence of an association between brain volume and activity behavior plasticity may result from the inaccuracy of using external skull measurements to estimate brain size, or from a particular part of the brain being responsible for plasticity in activity level.
基金Sponsored by National Key Research and Development Program of China(2016YFB0301401)State Key Program of National Natural Science Foundation of China(U1502274)+1 种基金Innovation Scientists and Technicians Troop Construction Projects of Henan Province of China(C20150014)Program for Innovation Research Team(in Science and Technology)in University of Henan Province of China(14IRTSTHN007)
文摘Mechanical properties and fracture behavior of Cu-0.84Co 0.23Be alloy after plastic deformation and heat treatment were comparatively investigated. Severe plastic deformation by hot extrusion and cold drawing was adopted to induce large plastic strain of Cu 0.84Co-0.23Be alloy. The tensile strength and elongation are up to 476.6 MPa and 18%, respectively. The fractured surface consists of deep dimples and micro voids. Due to the formation of su- persaturated solid solution on the Cu matrix by solution treatment at 950℃ for 1 h, the tensile strength decreased to 271.9 MPa, while the elongation increased to 42%. The fracture morphology is parabolic dimple. Furthermore, the tensile strength increased significantly to 580.2 MPa after aging at 480 ℃ for 4 h. During the aging process, a large number of precipitates formed and distributed on the Cu matrix. The fracture feature of aged specimens with low elongation （4.6%） exhibits an obvious brittle intergranular fracture. It is confirmed that the mechanical properties and fracture behavior are dominated by the microstrueture characteristics of Cu-0.84Co 0.23Be alloy after plastic de- formation and heat treatment. In addition, the fracture behavior at 450 ℃ of aged Cu-0.84Co 0.23Be alloy was also studied. The tensile strength and elongation are 383.6 MPa and 11.2%, respectively. The fractured morphologies are mainly candy-shaped with partial parabolic dimples and equiaxed dimples. The fracture mode is multi mixed mechanism that brittle intergranular fracture plays a dominant role and ductile fracture is secondary.
基金Project(2006BAE04B02-3)supported by the National Key Program of 11th Five-Year Plan of China
文摘The hot deformation simulation of a ZK60 magnesiuln alloy at different temperatures from 373 to 673 K and different strain rates of 0.1, 0.01 and 0.002 s^-1 was studied by using the Gleebe-1500 simulator. The plastic deformation behavior was measured and the deformation activation energy was calculated. The microstructures of ZK60 magnesium alloy with an addition of neodymium during the deformation process were observed by using Polyvar-MET optical microscope and Tecnai G^2 20 TEM. The results show that the working hardening, the dynamic recovery and the dynamic recrystallization occur during the plastic deformation process at different temperatures and strain rates. The dynamic recrystallization starts when the temperature is over 473 K and the DRX grain size after hot deformation is only 5-10 μm. So the refined grains improve both the tensile strength and the elongation of alloys at room temperature. Neodymium is added into the alloy and a precipitate phase Mg12Nd that impedes the movement of dislocations is formed, which benefits to increasing mechanical properties of ZK60 magnesium alloy.
文摘Marine invertebrates that move too slowly to evade unfavorable environmental change may instead exhibit phenotypic plasticity,allowing them to adjust to varying conditions.The orange-footed sea cucumber Cucumaria frondosa is a slow-moving suspension feeder that is preyed on by the purple sunstar Solaster endeca.The sea cucumber's antipredator behavior involves changing shape and detaching from the substratum,which might increase its probability of being displaced by water motion into an unsuitable environment.We hypothesized that sea cucumbers'antipredator responses would be diminished under stronger hydrodynamic forces,and that behavioral strategies would be flexible so that individuals could adjust to frequent changes in water flows.In a natural orange-footed sea cucumber habitat,individuals lived along a pronounced hydrodynamic gradient,allowing us to measure antipredator behavior under different water flow strengths.We placed purple sunstars in physical contact with sea cucumbers living at various points along the gradient to elicit antipredator responses.We then repeated this procedure in a laboratory mesocosm that generated weak and strong hydrodynamic forces similar to those observed at the field site.Subjects in the mesocosm experiment were tested in both wave conditions to determine if their antipredator behavior would change in response to sudden environmental change,as would be experienced under deteriorating sea conditions.Antipredator responses did not covary with hydrodynamic forces in the field.However,antipredator responses in the mesocosm experiment increased when individuals were transplanted from strong to weak forces and decreased when transplanted from weak to strong forces.Overall,our results indicate environmentally induced plasticity in the antipredator behavior of the orange-footed sea cucumber.
基金supported by the Chinese-Germany Science Foundation (No. GZ566)the National Natural Science Foundation of China (No. 51138001)
文摘A visco-elastoplastic damage constitutive model is proposed for simulating non-linear behavior of concrete. Based on traditional plastic theory, the irreversible deformation is simulated in effective stress space. In order to reflect different stiffness degradation mechanism of concrete under tensile and compressive loading conditions, both tensile and compressive damage variables are introduced, and then on the basis of energy release rate, the model is firmly derived within the concept of irreversible thermodynamics. The rate-dependent model is considered by introducing viscous regularization into the inelastic strain and damage variable, and combined with an additional elastic condition. Fully implicit backward-Euler algorithm is used to perform constitutive integration. Results of numerical examples using the proposed model agree well with test results for specimens under uniaxial tension and compression, biaxial loading and triaxial loading. Failure processes of single-edge-notched （SEN） beam and double-edge-notched （DEN） specimen are also simulated to further validate the proposed model.
文摘In this paper, a novel unified plasticity methodology is proposed to allow the coupling of rate-and temperature-sensitivity of engineering alloys as well as the non-linear kinematic hardening behavior often observed during cyclic loading. The proposed methodology is general in the sense that an arbitrary constitutive model may be chosen for the viscoplastic part, as well as the cyclic part. We adapt our model with a physically-motivated viscoplasticity flow rule and a nonlinear kinematic hardening model. In contrast with other unified plasticity models, the simplified theory involves few material parameters that can be readily calibrated from standard mechanical tests. The capabilities of the proposed theory are demonstrated for a hot rolled annealed 304 L stainless steel supplied by Vimetal Peckover. The model is tested with stress–strain curves obtained from standard tensile and cyclic uniaxial tests at various strain amplitudes and strain-rates, and good accuracy of the response is obtained for strains up to 15%, within a temperature range of 293–673 K. We note that the cyclic plasticity model in our adapted theory can be readily enhanced with ratchetting, mean stress relaxation, strain amplitude history, Masing effects or other complex capabilities.
基金Item Sponsored by National Natural Science Foundation of China(10925211,11172188)Fundamental Research Funds from the Central Universities of China(2012SCU04A05)
文摘Very high cycle fatigue behavior （107 --109 cycles） of 304L austenitic stainless steel was studied with ultra- sonic fatigue testing system （20 kHz）. The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation. It was found that micro-plasticity （slip markings） could be observed on the specimen surface even at very low stress amplitudes. The persistent slip mark- ings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure. By detailed investigation using a scanning electron microscope and an infrared camera, slip markings appeared at the large grains where the fatigue crack initiation site was located. The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack. Finally, the cou- pling relationship among the fatigue crack propagation, appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.
基金the National Natural Science Foundation of China.
文摘Based on the minimum principle of acceleration in the elastic-plastic continua under finite def ormation, the dynamic response of an elastic-perfectly plastic pin-ended beam subjected to rectangular impulse loading is studied with the help of a numerical approach. The calculated results once again show the anomalous behavior of the beam during its response process, which was previously found in [1]. By carefully analyzing the instantaneous distribution of the bending moment, the membrane force, the curvature and displacement during the response process, it is concluded that the interactive effect between the geometry and materials nonlinearities of the structure is the key reason for leading to the anomalous behavior. This will be helpful for clarifying some misunderstandings in explaining the problem before.
文摘The frictional resistance and machining quality when cutting carbon fiber reinforced plastics (CFRP) laminates are associated with tribological behavior of tool materials. In the present study, the tribological properties of three types of monolayer microcrystalline diamond (MCD) coatings, nanocrystalline diamond (NCD) coatings and dual-layer MCD/NCD coatings sliding against CFRP are investigated under dry lubricated conditions using the rotational friction tester. The coefficients of friction (COF), wear rate and worn surfaces of the contacted surfaces are analyzed for the MCD-CFRP, NCD-CFRP and MCD/NCD-CFRP contacting pairs. The results show that compared with the monolayer MCD and NCD, the bilayer of MCD/NCD coating displays the lowest COF with the value of ~0.13, it is 42% and 55% of the values for MCD and NCD coatings. Due to the rough surfaces of MCD, the wear debris of CFRP on MCD samples exhibits the plowing effect. While for the NCD and MCD/NCD samples, the wear fragments display the planar shapes. The wear rate of CFRP against MCD is more than twice that of CFRP against NCD, due to the excellent loading capacity. While the wear rate of CFRP against MCD/NCD is about twice than that of CFRP-NCD pairs. The bilayer of MCD/NCD combines the excellent advantages of high hardness of MCD and the smooth surface of NCD. It shows the broad application potential for the bilayer coatings.
文摘The accuracy and effi ciency of the modelling techniques utilized to model the nonlinear behavior of structural components is a signifi cant issue in earthquake engineering. In this study, the suffi ciency of three diff erent modelling techniques that can be employed to simulate the structural behavior of columns is investigated. A fi ber-based fi nite length plastic hinge (FB-FLPH) model is calibrated in this study. In order to calibrate the FB-FLPH model, a novel database of the cyclic behavior of hollow steel columns under simultaneous axial and lateral loading cycles with varying amplitudes is used. By employing the FB-FLPH model calibrated in this study, the interaction of the axial force and the bending moment in columns is directly taken into account, and the deterioration in the cyclic behavior of these members is implicitly considered. The superiority of the calibrated FB-FLPH modelling approach is examined compared with the cases in which conventional fi ber-based distributed plasticity and concentrated plasticity models are utilized. The effi ciency of the enumerated modelling techniques is probed when they are implemented to model the columns of a typical special moment frame in order to prove the advantage of the FB-FLPH modelling approach.
