The classical deviatoric hardening models are capable of characterizing the mechanical response of granular materials for a broad range of degrees of compaction.This work finds that it has limitations in accurately pr...The classical deviatoric hardening models are capable of characterizing the mechanical response of granular materials for a broad range of degrees of compaction.This work finds that it has limitations in accurately predicting the volumetric deformation characteristics under a wide range of confining/consolidation pressures.The issue stems from the pressure independent hardening law in the classical deviatoric hardening model.To overcome this problem,we propose a refined deviatoric hardening model in which a pressure-dependent hardening law is developed based on experimental observations.Comparisons between numerical results and laboratory triaxial tests indicate that the improved model succeeds in capturing the volumetric deformation behavior under various confining/consolidation pressure conditions for both dense and loose sands.Furthermore,to examine the importance of the improved deviatoric hardening model,it is combined with the bounding surface plasticity theory to investigate the mechanical response of loose sand under complex cyclic loadings and different initial consolidation pressures.It is proved that the proposed pressure-dependent deviatoric hardening law is capable of predicting the volumetric deformation characteristics to a satisfactory degree and plays an important role in the simulation of complex deformations for granular geomaterials.展开更多
In the present work,the laminar plasma surface hardening method is employed to enhance the service life of metal components fabricated from 65 Mn steel.The mechanical and wear behaviors of the laminar plasma surface h...In the present work,the laminar plasma surface hardening method is employed to enhance the service life of metal components fabricated from 65 Mn steel.The mechanical and wear behaviors of the laminar plasma surface hardened 65 Mn steel were analyzed.The martensite transition transformation of the temperature of the laminar plasma-hardened 65 ferrite Mn steel was determined by a thermal-solid coupling model.Based on the orthogonal experimental results,the optimal hardening parameters were confirmed.The scanning velocity,quenching distance and arc current are 130 mm/min,50 mm and 120 A,respectively.The pearlites and ferrites are transformed into martensites in the hardened zone,while the ratio of martensite in the heataffected zone decreases with the increase in the hardening depth.Compared to the untreated 65Mn steel,the average hardness increases from 220 HV_(0.2)to 920 HV_(0.2)in the hardened zone and the corresponding absorbed power increases from 118.7 J to 175.5 J.At the same time,the average coefficient of friction(COF)decreases from 0.763 to 0.546,and the wear rate decreases from 5.39×10^(-6)mm^(3)/(N·m)to 2.95×10^(-6)mm^(3)/(N·m),indicating that the wear resistance of 65Mn steel could be significantly improved by using laminar surface hardening.With the same hardening parameters,the depth and width of the hardened zone predicted by the thermal-solid coupling model are 1.85 mm and 11.20 mm,respectively,which are in accordance with the experimental results;depth is 1.83 mm and width is 11.15 mm.In addition,the predicted hardness distributions of the simulation model are in accordance with the experimental results.These results indicate that the simulation model could effectively predict the microstructure characteristics of 65 Mn steel.展开更多
This paper investigated an experimental method for bake hardening properties, a technique for deriving the true stress-strain curves after reaching the maximum load, and a constitutive equation considering both work h...This paper investigated an experimental method for bake hardening properties, a technique for deriving the true stress-strain curves after reaching the maximum load, and a constitutive equation considering both work hardening and bake hardening in order to apply the work hardening occurring in the forming process of parts and the bake hardening induced in the baking process to an automotive crash simulation. A general bake hardening test is that a pre-tensioned specimen is baked and then the same specimen is tensioned again without any further treatment. For a bake hardening test of automotive steel with a tensile strength of 1.2 GPa or more, fractures often occur in curvature section outside, an extensometer due to the difference in the material strength caused by non-uniform bake hardening. This causes a problem in that the bake hardening properties cannot be obtained. In this paper, to prevent curvature fracture, tensile specimens were re-machined in the uniformly deformed region of large specimens subjected to pre-strain, and the re-machined specimens with uniform strength in all regions were re-tensioned. In the bake hardening test of ultra-high strength steels with a tensile strength of 1 GPa or more, shear band fractures occur when the pre-strain is large. This makes it impossible to obtain a true stress-strain curve because there is no uniformly deformed region under a tensile test. To overcome this problem, a new method to calculate the true stress-strain curve by comparing experimental results and the load calculated by the local strain obtained from digital images was developed. This method can be applied not only where shear band deformation occurs, but also in necking deformation, and true stress-strain curves for strains up to 2 - 3 times the uniform elongation can be obtained. A new constitutive equation was developed since an appropriate hardening model is required to simultaneously apply the work hardening and the bake hardening to the simulation. For the newly developed model, the user material subroutine of LS-Dyna was configured, and the simulation was performed on the single hat specimens with pre-strain. When both work hardening and bake hardening were considered, there was a significant increase in absorbed energy compared to when only work hardening was considered. This means that both work hardening and bake hardening should be considered in the car crash simulations to enhance the accuracy of the simulation.展开更多
Confinement of rock bolts by the surrounding rock formation has long been recognized as a positive contributor to the pull-out behavior,yet only a few experimental works and analytical models have been reported,most o...Confinement of rock bolts by the surrounding rock formation has long been recognized as a positive contributor to the pull-out behavior,yet only a few experimental works and analytical models have been reported,most of which are based on the global rock bolt response evaluated in pull-out tests.This paper presents a laboratory experimental setup aiming to capture the rock formation effect,while using distributed fiber optic sensing to quantify the effect of the confinement and the reinforcement pull-out behavior on a more local level.It is shown that the behavior along the sample itself varies,with certain points exhibiting stress drops with crack formation.Some edge effects related to the kinematic freedom of the grout to dilate are also observed.Regardless,it was found that the mid-level response is quite similar to the average response along the sample.The ability to characterize the variation of the response along the sample is one of the many advantages high-resolution fiber optic sensing allows in such investigations.The paper also offers a plasticity-based hardening load transfer function,representing a"slice"of the anchor.The paper describes in detail the development of the model and the calibration/determination of its parameters.The suggested model captures well the coupled behavior in which the pull-out process leads to an increase in the confining stress due to dilative behavior.展开更多
In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy...In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy aged at 200℃, 250℃ and 300℃ are β’’(DO19) → β’(BCO) → β(FCC), β’’(DO19) → β’(BCO) → β_(1)(FCC) → β(FCC) and β(FCC), respectively. The streaks sequences of the alloy aged at 200℃, 250℃ and 300℃ are SF, SF → 14H-LPSO and SF → 14H-LPSO, respectively. For the alloy aged at 200℃ and 250℃, the increase in hardness with increasing aging time is contributed from the increase in precipitate volume fraction and the transformation from β’’ to β’ phase with basal → prismatic and spherical → spindle-like precipitate changes. The decrease in hardness after the peak-aging stage is attributed to the appearance of micro-sized β precipitates. Because of the smaller size of precipitates and the triangular arrangement of β’ precipitate, the hardness of the alloy aged at 200℃ is higher than that aged at 250℃. For the alloy aged at 300℃, the appearance of only micro-sized β precipitate and its coarsening with increasing aging time leads to the lowest hardness and an overall decrease in hardness with the aging time.展开更多
Microstructure,texture evolution and strain hardening behaviour of the Mg-1Y and Mg-1Zn(wt%)alloys were investigated under room temperature compression.Microstructural characterization was performed by optical microsc...Microstructure,texture evolution and strain hardening behaviour of the Mg-1Y and Mg-1Zn(wt%)alloys were investigated under room temperature compression.Microstructural characterization was performed by optical microscopy,scanning electron microscopy,electron back scattered diffraction and transmission electron microscopy.The experimental results show that Mg-1Zn alloy exhibits conventional three-stage strain hardening curves,while Mg-1Y alloy exhibits novel six-stage strain hardening curves.For Mg-1Y alloy,rare earth texture leads to weak tensile twinning activity in compression and consequently results in a moderate evolution to<0001>texture.Moreover,inefficient tensile twinning activity and weak slip-twinning interaction give rise to excellent ductility and high hardening capacity but low strain hardening rate.For Mg-1Zn alloy,basal texture leads to pronounced tensile twinning activity in compression and consequently results in rapid evolution to<0001>texture.The intense tensile twinning activity and strong slip-twinning interaction lead to high strain hardening rate but poor ductility and low hardening capacity.展开更多
In this research, the effect of precipitation hardening on the tribological behavior of the ZK60Gd/SiC composite was studied. For this purpose, ZK60Gd alloy containing with 5 and 10 wt% SiC were produced with stir cas...In this research, the effect of precipitation hardening on the tribological behavior of the ZK60Gd/SiC composite was studied. For this purpose, ZK60Gd alloy containing with 5 and 10 wt% SiC were produced with stir casting method. The microstructure characterization of the samples showed the wide distributions of Mg_(7)Zn_(3) and Gd(Mg_(0.5)Zn_(0.5)) precipitates were formed during casting. The results of hardness measurement after precipitation hardening at different temperatures showed that the hardness peck was obtained at 175 ℃. The wear tests with different loads(10, 40, 60, 90, and 120 N) and velocities(0.1, 0.3, 0.6, and 0.9 m/s) were performed on the as-cast and heat treated sample at 125, 175, and 225 for 12 h. Between the different precipitation hardening conditions, the precipitation hardened samples at 175 ℃ had the highest hardness values and least wear rate. The sample containing 10% reinforcement had the least wear rate between the unreinforced alloy and the composites. The results showed that abrasive, adhesive, delamination, MML, and fatigue wear mechanisms were the dominant wear mechanisms for the composite samples. In contrast, the dominant wear mechanism for the unreinforced samples was abrasive, adhesive,delamination, MML, and plastic deformation.展开更多
The interactions between a plate-like precipitate and two twin boundaries(TBs)({1012},{1121}) in magnesium alloys are studied using molecular dynamics(MD) simulations. The precipitate is not sheared by {1012} TB, but ...The interactions between a plate-like precipitate and two twin boundaries(TBs)({1012},{1121}) in magnesium alloys are studied using molecular dynamics(MD) simulations. The precipitate is not sheared by {1012} TB, but sheared by {1121} TB. Shearing on the(110) plane is the predominant deformation mode in the sheared precipitate. Then, the blocking effects of precipitates with different sizes are studied for {1121} twinning. All the precipitates show a blocking effect on {1121} twinning although they are sheared, while the blocking effects of precipitates with different sizes are different. The blocking effect increases significantly with the increasing precipitate length(in-plane size along TB) and thickness, whereas changes weakly as the precipitate width changes. Based on the revealed interaction mechanisms, a critical twin shear is calculated theoretically by the Eshelby solutions to determine which TB is able to shear the precipitate. In addition, an analytical hardening model of sheared precipitates is proposed by analyzing the force equilibrium during TB-precipitate interactions. This model indicates that the blocking effect depends solely on the area fraction of the precipitate cross-section, and shows good agreement with the current MD simulations. Finally, the blocking effects of plate-like precipitates on the {1012} twinning(non-sheared precipitate), {1121} twinning(sheared precipitate) and basal dislocations(non-sheared precipitate) are compared together. Results show that the blocking effect on {1121} twinning is stronger than that on {1012} twinning, while the effect on basal dislocations is weakest. The precipitate-TB interaction mechanisms and precipitation hardening models revealed in this work are of great significance for improving the mechanical property of magnesium alloys by designing microstructure.展开更多
The mechanical properties of magnesium alloy AZ31 were investigated experimentally with visco-plastic self-consistent modeling. Tension,compression and plane strain compression(PSC) tests were performed along 3 direct...The mechanical properties of magnesium alloy AZ31 were investigated experimentally with visco-plastic self-consistent modeling. Tension,compression and plane strain compression(PSC) tests were performed along 3 directions of a hot rolled plate, and the material parameters input in the model were fitted with the uniaxial stress-strain curves. The critical resolved shear stress(CRSS) for tension twinning was modeled with a modified Voce hardening law first decreasing, and then increasing with strain, that could reproduce better the flow stress for twin-predominant deformation. Such CRSS evolution may better model twin nucleation, propagation and growth. Firstly simulations were carried out assuming latent hardening coefficients for slip by other slip systems equal to self-hardening. Then different heterogeneous latent hardening were used, whose values were based on dislocation dynamics simulations from the literature. This study shows that equal self and latent hardening can reproduce the stress strain curves and plastic anisotropy as well as heterogeneous mode on mode latent hardening.Discrepancies between simulations and experimental results from PSC are explained by an under-estimation of twinning for some PSC strain paths.展开更多
In recent years, the place occupied by the various manifestations of cyber-crime in companies has been considerable. Indeed, due to the rapid evolution of telecommunications technologies, companies, regardless of thei...In recent years, the place occupied by the various manifestations of cyber-crime in companies has been considerable. Indeed, due to the rapid evolution of telecommunications technologies, companies, regardless of their size or sector of activity, are now the target of advanced persistent threats. The Work 2035 study also revealed that cyber crimes (such as critical infrastructure hacks) and massive data breaches are major sources of concern. Thus, it is important for organizations to guarantee a minimum level of security to avoid potential attacks that can cause paralysis of systems, loss of sensitive data, exposure to blackmail, damage to reputation or even a commercial harm. To do this, among other means, hardening is used, the main objective of which is to reduce the attack surface within a company. The execution of the hardening configurations as well as the verification of these are carried out on the servers and network equipment with the aim of reducing the number of openings present by keeping only those which are necessary for proper operation. However, nowadays, in many companies, these tasks are done manually. As a result, the execution and verification of hardening configurations are very often subject to potential errors but also highly consuming human and financial resources. The problem is that it is essential for operators to maintain an optimal level of security while minimizing costs, hence the interest in automating hardening processes and verifying the hardening of servers and network equipment. It is in this logic that we propose within the framework of this work the reinforcement of the security of the information systems (IS) by the automation of the mechanisms of hardening. In our work, we have, on the one hand, set up a hardening procedure in accordance with international security standards for servers, routers and switches and, on the other hand, designed and produced a functional application which makes it possible to: 1) Realise the configuration of the hardening;2) Verify them;3) Correct the non conformities;4) Write and send by mail a verification report for the configurations;5) And finally update the procedures of hardening. Our web application thus created allows in less than fifteen (15) minutes actions that previously took at least five (5) hours of time. This allows supervised network operators to save time and money, but also to improve their security standards in line with international standards.展开更多
The flow stress behavior of ZK60 alloy at elevated temperature was investigated. The strain hardening and dynamic recrystallization of the alloy were modeled by Kocks-Meching model and Avrami equation, respectively. A...The flow stress behavior of ZK60 alloy at elevated temperature was investigated. The strain hardening and dynamic recrystallization of the alloy were modeled by Kocks-Meching model and Avrami equation, respectively. A new constitutive equation during hot deformation was constructed to predict the flow stress considering the dynamic recrystallization. The results show that the flow stress curves predicted by the proposed equation have high correlation coefficients with the experimental data, which confirms that the developed model is accurate and effective to establish the flow stress equation of ZK60 magnesium alloy during hot deformation. Microstructure observation shows that dynamic recovery occurs in the initial stage of hot deformation. However, the microstructure turns to recrvstallization structure as the strain increases.展开更多
The effects of the deformation temperature and the strain rate on the hot deformation behavior of pure copper were investigated based on compression tests. The expressions of strain hardening rate, dynamic recrystalli...The effects of the deformation temperature and the strain rate on the hot deformation behavior of pure copper were investigated based on compression tests. The expressions of strain hardening rate, dynamic recrystallization critical stress, saturated stress, dynamic recovery volume fraction and dynamic recrystallization volume fraction were determined. According to the processing map, the instability regions occur in regions of 400?450 °C, 0.001?0.05 s?1 and 450?750 °C, 0.05?1 s?1. The deformation mechanism in the stability region is dynamic recrystallization. The flow stress was predicted. The results also show that the true stress–true strain curves predicted by the extracted model are in good agreement with the experimental results.展开更多
The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al?Mg?Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) anal...The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al?Mg?Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.展开更多
The aim of the present work is to develop a model for simulating double-peak precipitation hardening kinetics in Al-Zn-Mg alloy with the simultaneous formation of different types of precipitates at elevated temperatur...The aim of the present work is to develop a model for simulating double-peak precipitation hardening kinetics in Al-Zn-Mg alloy with the simultaneous formation of different types of precipitates at elevated temperatures based on the modified Langer-Schwartz approach. The double aging peaks are present in the long time age-hardening curves of Al-Zn-Mg alloys. The physically-based model, while taking explicitly into account nucleation, growth, coarsening of the new phase precipitations and two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing）, was used for the analysis of precipitates evolution and precipitation hardening during aging of Al-Zn-Mg alloy. Model predictions were compared with the measurements of Al-Zn-Mg alloy. The systematic and quantitative results show that the predicted hardness profiles of double peaks via adding a shape dependent parameter in the growth equation for growth and coarsening generally agree well with the measured ones. Two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing） were considered operating simultaneously in view of the particle size-distribution. The transition from shearing to bypassing strengthening mechanism was found to occur at rather early stage of the particle growth. The bypassing was found to be the prevailing strengthening mechanism in the investigated alloys.展开更多
The mathematical model of the grinding temperature is established. The grinding temperature and the cooling rate are measured in the grind-hardening process of 40Cr steel under different conditions. Moreover, the grin...The mathematical model of the grinding temperature is established. The grinding temperature and the cooling rate are measured in the grind-hardening process of 40Cr steel under different conditions. Moreover, the grind-hardening effects are investigated. Experimental results show that the calculated temperatures are comparatively close to the measured ones, and the required temperature and cooling rate can be achieved. Furthermore, the microstructure of the hardened zone is similar to that obtained through the high-frequency induction technique. The average hardness of the entirely hardened zone is HV670 and the thickness of the hardened layer is adjacent to 1.3 mm. It indicates that the hardening mechanism induced by the grinding heat and high-frequency heating is identical. Finally, the fine needlelike martensite is obtained.展开更多
The analysis of plane strain elastic-plastic bending of a linear strain hardening curved beam with a narrow rectangular cross section subjected to couples at its end is conducted based on a unified yield criterion. Th...The analysis of plane strain elastic-plastic bending of a linear strain hardening curved beam with a narrow rectangular cross section subjected to couples at its end is conducted based on a unified yield criterion. The solutions for the mechanical properties of plane strain bending are derived, which are adapted for various kinds of non-strength differential materials and can be degenerated to those based on the Tresca, von Mises, and twin-shear yield criteria. The dependences of the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane on different yield criteria and Poisson’s ratios are discussed. The results show that the influences of different yield criteria and Poisson’s ratio on the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane of the curved beam are significant. Once the value of bis obtained by experiments, the yield criterion and the corresponding solution for the materials of interest are then determined.展开更多
The precipitation hardening behavior in dilute Al-Yb alloys upon annealing at different temperatures was investigated to shed light on the mechanism of micro-alloying element in aluminum alloys. When aging at differen...The precipitation hardening behavior in dilute Al-Yb alloys upon annealing at different temperatures was investigated to shed light on the mechanism of micro-alloying element in aluminum alloys. When aging at different temperatures, the samples showed their corresponding peak hardness in the range of 400-416 MPa due to the precipitation of Al3Yb with L 12 crystal structure. The coarsening kinetics of the Al3Yb precipitates obeyed the LSW theory, which indicated that the coarsening process was controlled by the diffusion of Yb. The coherence between Al3Yb particles and matrix was maintained until the particle size reached 11 nm. When the particle size increased to about 2 nm, the shearing mechanism started to change to Orowan mechanism.展开更多
Ageing hardening,microstructure and mechanical properties of Mg-xY-1.5LPC-0.4Zr（x=0,2,4,6）alloys（LPC represents La-based rare earth metal）were investigated.It was found that the age hardening was enhanced,the grai...Ageing hardening,microstructure and mechanical properties of Mg-xY-1.5LPC-0.4Zr（x=0,2,4,6）alloys（LPC represents La-based rare earth metal）were investigated.It was found that the age hardening was enhanced,the grains became finer and the tensile strength was improved with the increase of Y content in Mg-Y-1.5LPC-0.4Zr alloy.The results show that the formed precipitates responsible for age hardening change from fine hexagonal-shaped equilibrium Mg12RE phase to metastableβ′phase with orthorhombic-bc crystal structure when Y is added into Mg-1.5LPC-0.4Zr alloy,and the volume fraction of precipitate phases also increases.The cubic-shapedβ-Mg24Y5 precipitate phases were also observed at grain boundaries in Mg-6Y-1.5LPC-0.4Zr alloy. The distribution of prismatic shapedβ′phases and cubic shapedβ-Mg24Y5 precipitate phases in Mg matrix may account for the remarkable enhancement of tensile strength of Mg-Y-LPC-Zr alloy.The Mg-6Y-1.5LPC-0.4Zr alloy exhibits maximum tensile strength at peak-aged hardness,and the values are 250 MPa at room temperature and 210 MPa at 250°C.展开更多
Aim To determine numerically the field characteristics in the vied at the tip of a place crack growing steadily in a power-law hardening material. Meteods. Methods on the Euler mode and small-scale yield assumption, t...Aim To determine numerically the field characteristics in the vied at the tip of a place crack growing steadily in a power-law hardening material. Meteods. Methods on the Euler mode and small-scale yield assumption, the numerical results were given by nonlinear finite element analysis. Results The numerical results of the shape of the active plastic sone, the angular distribution of stresseses and Clack tip opening displacement (CTOD) in the vicinity at the hp of the steadily groWing CraCk are determined. Conclusion The comparison between the numerical results given by the present wort and those given by analytic asymptotic analysis shows that the present work reached a very high accuracy.展开更多
A discussion of several kinematic hardening rules based on nonproportional cyclic experiments of 42CrMo steel is presented. They include Prager, Ziegler, Chaboche, Mroz and Tseng Lee hardenin...A discussion of several kinematic hardening rules based on nonproportional cyclic experiments of 42CrMo steel is presented. They include Prager, Ziegler, Chaboche, Mroz and Tseng Lee hardening rules. It shows that Mroz and Tseng Lee rule related to a two surface model has the latent potentiality to describe the nonproportional cyclic hardening behaviors, and a simple two surface model is presented.展开更多
基金the funding support from Basic Science Center Program for Multiphase Media Evolution in Hypergravity of the National Natural Science Foundation of China(Grant No.51988101).
文摘The classical deviatoric hardening models are capable of characterizing the mechanical response of granular materials for a broad range of degrees of compaction.This work finds that it has limitations in accurately predicting the volumetric deformation characteristics under a wide range of confining/consolidation pressures.The issue stems from the pressure independent hardening law in the classical deviatoric hardening model.To overcome this problem,we propose a refined deviatoric hardening model in which a pressure-dependent hardening law is developed based on experimental observations.Comparisons between numerical results and laboratory triaxial tests indicate that the improved model succeeds in capturing the volumetric deformation behavior under various confining/consolidation pressure conditions for both dense and loose sands.Furthermore,to examine the importance of the improved deviatoric hardening model,it is combined with the bounding surface plasticity theory to investigate the mechanical response of loose sand under complex cyclic loadings and different initial consolidation pressures.It is proved that the proposed pressure-dependent deviatoric hardening law is capable of predicting the volumetric deformation characteristics to a satisfactory degree and plays an important role in the simulation of complex deformations for granular geomaterials.
基金appreciate the support of the Key Laboratory of Mechanical Structure Optimization&Material Application Technology of Luzhou(No.SCHYZSA-2022-02)the Scientific Research and Innovation Team Program of Sichuan University of Science and Technology(No.SUSE652A004)+1 种基金the Key Laboratory of Intelligent Manufacturing of Construction Machinery Project(No.IMCM202103)the Panzhihua Key Laboratory of Advanced Manufacturing Technology Open Fund Project(No.2022XJZD01)。
文摘In the present work,the laminar plasma surface hardening method is employed to enhance the service life of metal components fabricated from 65 Mn steel.The mechanical and wear behaviors of the laminar plasma surface hardened 65 Mn steel were analyzed.The martensite transition transformation of the temperature of the laminar plasma-hardened 65 ferrite Mn steel was determined by a thermal-solid coupling model.Based on the orthogonal experimental results,the optimal hardening parameters were confirmed.The scanning velocity,quenching distance and arc current are 130 mm/min,50 mm and 120 A,respectively.The pearlites and ferrites are transformed into martensites in the hardened zone,while the ratio of martensite in the heataffected zone decreases with the increase in the hardening depth.Compared to the untreated 65Mn steel,the average hardness increases from 220 HV_(0.2)to 920 HV_(0.2)in the hardened zone and the corresponding absorbed power increases from 118.7 J to 175.5 J.At the same time,the average coefficient of friction(COF)decreases from 0.763 to 0.546,and the wear rate decreases from 5.39×10^(-6)mm^(3)/(N·m)to 2.95×10^(-6)mm^(3)/(N·m),indicating that the wear resistance of 65Mn steel could be significantly improved by using laminar surface hardening.With the same hardening parameters,the depth and width of the hardened zone predicted by the thermal-solid coupling model are 1.85 mm and 11.20 mm,respectively,which are in accordance with the experimental results;depth is 1.83 mm and width is 11.15 mm.In addition,the predicted hardness distributions of the simulation model are in accordance with the experimental results.These results indicate that the simulation model could effectively predict the microstructure characteristics of 65 Mn steel.
文摘This paper investigated an experimental method for bake hardening properties, a technique for deriving the true stress-strain curves after reaching the maximum load, and a constitutive equation considering both work hardening and bake hardening in order to apply the work hardening occurring in the forming process of parts and the bake hardening induced in the baking process to an automotive crash simulation. A general bake hardening test is that a pre-tensioned specimen is baked and then the same specimen is tensioned again without any further treatment. For a bake hardening test of automotive steel with a tensile strength of 1.2 GPa or more, fractures often occur in curvature section outside, an extensometer due to the difference in the material strength caused by non-uniform bake hardening. This causes a problem in that the bake hardening properties cannot be obtained. In this paper, to prevent curvature fracture, tensile specimens were re-machined in the uniformly deformed region of large specimens subjected to pre-strain, and the re-machined specimens with uniform strength in all regions were re-tensioned. In the bake hardening test of ultra-high strength steels with a tensile strength of 1 GPa or more, shear band fractures occur when the pre-strain is large. This makes it impossible to obtain a true stress-strain curve because there is no uniformly deformed region under a tensile test. To overcome this problem, a new method to calculate the true stress-strain curve by comparing experimental results and the load calculated by the local strain obtained from digital images was developed. This method can be applied not only where shear band deformation occurs, but also in necking deformation, and true stress-strain curves for strains up to 2 - 3 times the uniform elongation can be obtained. A new constitutive equation was developed since an appropriate hardening model is required to simultaneously apply the work hardening and the bake hardening to the simulation. For the newly developed model, the user material subroutine of LS-Dyna was configured, and the simulation was performed on the single hat specimens with pre-strain. When both work hardening and bake hardening were considered, there was a significant increase in absorbed energy compared to when only work hardening was considered. This means that both work hardening and bake hardening should be considered in the car crash simulations to enhance the accuracy of the simulation.
基金funding support from the Israeli Ministry of Housing and Construction(Grant No.2028286).
文摘Confinement of rock bolts by the surrounding rock formation has long been recognized as a positive contributor to the pull-out behavior,yet only a few experimental works and analytical models have been reported,most of which are based on the global rock bolt response evaluated in pull-out tests.This paper presents a laboratory experimental setup aiming to capture the rock formation effect,while using distributed fiber optic sensing to quantify the effect of the confinement and the reinforcement pull-out behavior on a more local level.It is shown that the behavior along the sample itself varies,with certain points exhibiting stress drops with crack formation.Some edge effects related to the kinematic freedom of the grout to dilate are also observed.Regardless,it was found that the mid-level response is quite similar to the average response along the sample.The ability to characterize the variation of the response along the sample is one of the many advantages high-resolution fiber optic sensing allows in such investigations.The paper also offers a plasticity-based hardening load transfer function,representing a"slice"of the anchor.The paper describes in detail the development of the model and the calibration/determination of its parameters.The suggested model captures well the coupled behavior in which the pull-out process leads to an increase in the confining stress due to dilative behavior.
基金financially supported by the Key R&D program of Shanxi Province (International Cooperation) (No.201903D421036)the Natural Science Foundation of Shanxi Province (No.201901D111176)+5 种基金the Joint Funds of the National Natural Science Foundation of China (Grant No.U20A20230)the Bureau of Science,Technology and Industry for National Defense of China (No.WDZC2019JJ006)the National Natural Science Foundation of China (Grant No.52075501)Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No.201802072)the National Defense Basic Scientific Research Program (No.JCKY2018408B003)the XX Supporting Scientific Research Project (No.xxxx-2019-021)。
文摘In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy aged at 200℃, 250℃ and 300℃ are β’’(DO19) → β’(BCO) → β(FCC), β’’(DO19) → β’(BCO) → β_(1)(FCC) → β(FCC) and β(FCC), respectively. The streaks sequences of the alloy aged at 200℃, 250℃ and 300℃ are SF, SF → 14H-LPSO and SF → 14H-LPSO, respectively. For the alloy aged at 200℃ and 250℃, the increase in hardness with increasing aging time is contributed from the increase in precipitate volume fraction and the transformation from β’’ to β’ phase with basal → prismatic and spherical → spindle-like precipitate changes. The decrease in hardness after the peak-aging stage is attributed to the appearance of micro-sized β precipitates. Because of the smaller size of precipitates and the triangular arrangement of β’ precipitate, the hardness of the alloy aged at 200℃ is higher than that aged at 250℃. For the alloy aged at 300℃, the appearance of only micro-sized β precipitate and its coarsening with increasing aging time leads to the lowest hardness and an overall decrease in hardness with the aging time.
基金Funded by the Beijing Municipal Natural Science Foundation (No.2202004)the National Natural Science Foundation of China (No.51801048)the Basic Research Fund for Newly Enrolled Teachers and the Fund for Distinguished Young Scholars of China Academy of Space Technology。
文摘Microstructure,texture evolution and strain hardening behaviour of the Mg-1Y and Mg-1Zn(wt%)alloys were investigated under room temperature compression.Microstructural characterization was performed by optical microscopy,scanning electron microscopy,electron back scattered diffraction and transmission electron microscopy.The experimental results show that Mg-1Zn alloy exhibits conventional three-stage strain hardening curves,while Mg-1Y alloy exhibits novel six-stage strain hardening curves.For Mg-1Y alloy,rare earth texture leads to weak tensile twinning activity in compression and consequently results in a moderate evolution to<0001>texture.Moreover,inefficient tensile twinning activity and weak slip-twinning interaction give rise to excellent ductility and high hardening capacity but low strain hardening rate.For Mg-1Zn alloy,basal texture leads to pronounced tensile twinning activity in compression and consequently results in rapid evolution to<0001>texture.The intense tensile twinning activity and strong slip-twinning interaction lead to high strain hardening rate but poor ductility and low hardening capacity.
文摘In this research, the effect of precipitation hardening on the tribological behavior of the ZK60Gd/SiC composite was studied. For this purpose, ZK60Gd alloy containing with 5 and 10 wt% SiC were produced with stir casting method. The microstructure characterization of the samples showed the wide distributions of Mg_(7)Zn_(3) and Gd(Mg_(0.5)Zn_(0.5)) precipitates were formed during casting. The results of hardness measurement after precipitation hardening at different temperatures showed that the hardness peck was obtained at 175 ℃. The wear tests with different loads(10, 40, 60, 90, and 120 N) and velocities(0.1, 0.3, 0.6, and 0.9 m/s) were performed on the as-cast and heat treated sample at 125, 175, and 225 for 12 h. Between the different precipitation hardening conditions, the precipitation hardened samples at 175 ℃ had the highest hardness values and least wear rate. The sample containing 10% reinforcement had the least wear rate between the unreinforced alloy and the composites. The results showed that abrasive, adhesive, delamination, MML, and fatigue wear mechanisms were the dominant wear mechanisms for the composite samples. In contrast, the dominant wear mechanism for the unreinforced samples was abrasive, adhesive,delamination, MML, and plastic deformation.
基金financial support from National Natural Science Foundation of China (12072211)Sichuan Province Science and Technology Project (2020JDJQ0029)。
文摘The interactions between a plate-like precipitate and two twin boundaries(TBs)({1012},{1121}) in magnesium alloys are studied using molecular dynamics(MD) simulations. The precipitate is not sheared by {1012} TB, but sheared by {1121} TB. Shearing on the(110) plane is the predominant deformation mode in the sheared precipitate. Then, the blocking effects of precipitates with different sizes are studied for {1121} twinning. All the precipitates show a blocking effect on {1121} twinning although they are sheared, while the blocking effects of precipitates with different sizes are different. The blocking effect increases significantly with the increasing precipitate length(in-plane size along TB) and thickness, whereas changes weakly as the precipitate width changes. Based on the revealed interaction mechanisms, a critical twin shear is calculated theoretically by the Eshelby solutions to determine which TB is able to shear the precipitate. In addition, an analytical hardening model of sheared precipitates is proposed by analyzing the force equilibrium during TB-precipitate interactions. This model indicates that the blocking effect depends solely on the area fraction of the precipitate cross-section, and shows good agreement with the current MD simulations. Finally, the blocking effects of plate-like precipitates on the {1012} twinning(non-sheared precipitate), {1121} twinning(sheared precipitate) and basal dislocations(non-sheared precipitate) are compared together. Results show that the blocking effect on {1121} twinning is stronger than that on {1012} twinning, while the effect on basal dislocations is weakest. The precipitate-TB interaction mechanisms and precipitation hardening models revealed in this work are of great significance for improving the mechanical property of magnesium alloys by designing microstructure.
基金National Natural Science Foundation of China (51871032, 52071039 and 51671040)the 111 Project (B16007) of the Ministry of Education。
文摘The mechanical properties of magnesium alloy AZ31 were investigated experimentally with visco-plastic self-consistent modeling. Tension,compression and plane strain compression(PSC) tests were performed along 3 directions of a hot rolled plate, and the material parameters input in the model were fitted with the uniaxial stress-strain curves. The critical resolved shear stress(CRSS) for tension twinning was modeled with a modified Voce hardening law first decreasing, and then increasing with strain, that could reproduce better the flow stress for twin-predominant deformation. Such CRSS evolution may better model twin nucleation, propagation and growth. Firstly simulations were carried out assuming latent hardening coefficients for slip by other slip systems equal to self-hardening. Then different heterogeneous latent hardening were used, whose values were based on dislocation dynamics simulations from the literature. This study shows that equal self and latent hardening can reproduce the stress strain curves and plastic anisotropy as well as heterogeneous mode on mode latent hardening.Discrepancies between simulations and experimental results from PSC are explained by an under-estimation of twinning for some PSC strain paths.
文摘In recent years, the place occupied by the various manifestations of cyber-crime in companies has been considerable. Indeed, due to the rapid evolution of telecommunications technologies, companies, regardless of their size or sector of activity, are now the target of advanced persistent threats. The Work 2035 study also revealed that cyber crimes (such as critical infrastructure hacks) and massive data breaches are major sources of concern. Thus, it is important for organizations to guarantee a minimum level of security to avoid potential attacks that can cause paralysis of systems, loss of sensitive data, exposure to blackmail, damage to reputation or even a commercial harm. To do this, among other means, hardening is used, the main objective of which is to reduce the attack surface within a company. The execution of the hardening configurations as well as the verification of these are carried out on the servers and network equipment with the aim of reducing the number of openings present by keeping only those which are necessary for proper operation. However, nowadays, in many companies, these tasks are done manually. As a result, the execution and verification of hardening configurations are very often subject to potential errors but also highly consuming human and financial resources. The problem is that it is essential for operators to maintain an optimal level of security while minimizing costs, hence the interest in automating hardening processes and verifying the hardening of servers and network equipment. It is in this logic that we propose within the framework of this work the reinforcement of the security of the information systems (IS) by the automation of the mechanisms of hardening. In our work, we have, on the one hand, set up a hardening procedure in accordance with international security standards for servers, routers and switches and, on the other hand, designed and produced a functional application which makes it possible to: 1) Realise the configuration of the hardening;2) Verify them;3) Correct the non conformities;4) Write and send by mail a verification report for the configurations;5) And finally update the procedures of hardening. Our web application thus created allows in less than fifteen (15) minutes actions that previously took at least five (5) hours of time. This allows supervised network operators to save time and money, but also to improve their security standards in line with international standards.
文摘The flow stress behavior of ZK60 alloy at elevated temperature was investigated. The strain hardening and dynamic recrystallization of the alloy were modeled by Kocks-Meching model and Avrami equation, respectively. A new constitutive equation during hot deformation was constructed to predict the flow stress considering the dynamic recrystallization. The results show that the flow stress curves predicted by the proposed equation have high correlation coefficients with the experimental data, which confirms that the developed model is accurate and effective to establish the flow stress equation of ZK60 magnesium alloy during hot deformation. Microstructure observation shows that dynamic recovery occurs in the initial stage of hot deformation. However, the microstructure turns to recrvstallization structure as the strain increases.
基金Project(cstc2015jcyj BX0115)supported by the Chongqing Research Program of Basic Research and Frontier Technology,China
文摘The effects of the deformation temperature and the strain rate on the hot deformation behavior of pure copper were investigated based on compression tests. The expressions of strain hardening rate, dynamic recrystallization critical stress, saturated stress, dynamic recovery volume fraction and dynamic recrystallization volume fraction were determined. According to the processing map, the instability regions occur in regions of 400?450 °C, 0.001?0.05 s?1 and 450?750 °C, 0.05?1 s?1. The deformation mechanism in the stability region is dynamic recrystallization. The flow stress was predicted. The results also show that the true stress–true strain curves predicted by the extracted model are in good agreement with the experimental results.
基金Project(2014DFA51270)supported by the International Science and Technology Cooperation Program of ChinaProject(51421001)supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China
文摘The influences of high temperature pre-straining (HT-PS) on the natural aging and bake hardening of Al?Mg?Si alloys were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. The results show that pre-straining at 170 °C immediately after quenching can effectively resolve the rather high T4 temper hardness caused by the conventional room temperature (RT) pre-straining treatment, and give a better bake hardening response (BHR) after paint-bake cycle. HT-PS 7% at 170 °C for 10 min is chosen as the optimum process as it provides lower T4 temper hardness and better BHR. The simultaneous introduction of dislocations and Cluster (2) can significantly suppress the natural aging and promote the precipitation of β″ phase, and reduce the effects of deformation hardening by dynamic recovery.
基金Project(51021063)supported by the Creative Research Group of the National Natural Science Foundation of ChinaProject(50831007)supported by the National Natural Science Foundation of China+1 种基金Project(2011CB610401)supported by the National Basic Research Program of ChinaProject(12C1142)supported by the Education Department of Hunan Province,China
文摘The aim of the present work is to develop a model for simulating double-peak precipitation hardening kinetics in Al-Zn-Mg alloy with the simultaneous formation of different types of precipitates at elevated temperatures based on the modified Langer-Schwartz approach. The double aging peaks are present in the long time age-hardening curves of Al-Zn-Mg alloys. The physically-based model, while taking explicitly into account nucleation, growth, coarsening of the new phase precipitations and two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing）, was used for the analysis of precipitates evolution and precipitation hardening during aging of Al-Zn-Mg alloy. Model predictions were compared with the measurements of Al-Zn-Mg alloy. The systematic and quantitative results show that the predicted hardness profiles of double peaks via adding a shape dependent parameter in the growth equation for growth and coarsening generally agree well with the measured ones. Two strengthening mechanisms associated with particle-dislocation interaction （shearing and bypassing） were considered operating simultaneously in view of the particle size-distribution. The transition from shearing to bypassing strengthening mechanism was found to occur at rather early stage of the particle growth. The bypassing was found to be the prevailing strengthening mechanism in the investigated alloys.
文摘The mathematical model of the grinding temperature is established. The grinding temperature and the cooling rate are measured in the grind-hardening process of 40Cr steel under different conditions. Moreover, the grind-hardening effects are investigated. Experimental results show that the calculated temperatures are comparatively close to the measured ones, and the required temperature and cooling rate can be achieved. Furthermore, the microstructure of the hardened zone is similar to that obtained through the high-frequency induction technique. The average hardness of the entirely hardened zone is HV670 and the thickness of the hardened layer is adjacent to 1.3 mm. It indicates that the hardening mechanism induced by the grinding heat and high-frequency heating is identical. Finally, the fine needlelike martensite is obtained.
基金The Project of the Ministry of Housing and Urban-Rural Development(No.2014-K4-010)
文摘The analysis of plane strain elastic-plastic bending of a linear strain hardening curved beam with a narrow rectangular cross section subjected to couples at its end is conducted based on a unified yield criterion. The solutions for the mechanical properties of plane strain bending are derived, which are adapted for various kinds of non-strength differential materials and can be degenerated to those based on the Tresca, von Mises, and twin-shear yield criteria. The dependences of the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane on different yield criteria and Poisson’s ratios are discussed. The results show that the influences of different yield criteria and Poisson’s ratio on the two critical bending moments, the radii of the interfaces between the elastic and plastic regions and the radial displacements of the points at the symmetrical plane of the curved beam are significant. Once the value of bis obtained by experiments, the yield criterion and the corresponding solution for the materials of interest are then determined.
基金Project(2013AA031301)supported by Hi-tech Research and Development Program of ChinaProject(S2013ZR0611)supported by the National International Scientific and Technological Cooperation Program of China
文摘The precipitation hardening behavior in dilute Al-Yb alloys upon annealing at different temperatures was investigated to shed light on the mechanism of micro-alloying element in aluminum alloys. When aging at different temperatures, the samples showed their corresponding peak hardness in the range of 400-416 MPa due to the precipitation of Al3Yb with L 12 crystal structure. The coarsening kinetics of the Al3Yb precipitates obeyed the LSW theory, which indicated that the coarsening process was controlled by the diffusion of Yb. The coherence between Al3Yb particles and matrix was maintained until the particle size reached 11 nm. When the particle size increased to about 2 nm, the shearing mechanism started to change to Orowan mechanism.
基金Project(2010A6100153)supported by Natural Science Foundation of Ningbo,China
文摘Ageing hardening,microstructure and mechanical properties of Mg-xY-1.5LPC-0.4Zr（x=0,2,4,6）alloys（LPC represents La-based rare earth metal）were investigated.It was found that the age hardening was enhanced,the grains became finer and the tensile strength was improved with the increase of Y content in Mg-Y-1.5LPC-0.4Zr alloy.The results show that the formed precipitates responsible for age hardening change from fine hexagonal-shaped equilibrium Mg12RE phase to metastableβ′phase with orthorhombic-bc crystal structure when Y is added into Mg-1.5LPC-0.4Zr alloy,and the volume fraction of precipitate phases also increases.The cubic-shapedβ-Mg24Y5 precipitate phases were also observed at grain boundaries in Mg-6Y-1.5LPC-0.4Zr alloy. The distribution of prismatic shapedβ′phases and cubic shapedβ-Mg24Y5 precipitate phases in Mg matrix may account for the remarkable enhancement of tensile strength of Mg-Y-LPC-Zr alloy.The Mg-6Y-1.5LPC-0.4Zr alloy exhibits maximum tensile strength at peak-aged hardness,and the values are 250 MPa at room temperature and 210 MPa at 250°C.
文摘Aim To determine numerically the field characteristics in the vied at the tip of a place crack growing steadily in a power-law hardening material. Meteods. Methods on the Euler mode and small-scale yield assumption, the numerical results were given by nonlinear finite element analysis. Results The numerical results of the shape of the active plastic sone, the angular distribution of stresseses and Clack tip opening displacement (CTOD) in the vicinity at the hp of the steadily groWing CraCk are determined. Conclusion The comparison between the numerical results given by the present wort and those given by analytic asymptotic analysis shows that the present work reached a very high accuracy.
文摘A discussion of several kinematic hardening rules based on nonproportional cyclic experiments of 42CrMo steel is presented. They include Prager, Ziegler, Chaboche, Mroz and Tseng Lee hardening rules. It shows that Mroz and Tseng Lee rule related to a two surface model has the latent potentiality to describe the nonproportional cyclic hardening behaviors, and a simple two surface model is presented.
