Based on the moment tensor theory, a mesoscopic simulation method was proposed for acoustic emissions (AE) during Brazilian test in mesoscale. Its rationality was proved by comparative analysis between experimental and calculative results. This method can simultaneously provide the occurrence characteristics of AE events such as time, location and magnitude, and reproduce the spatial evolution law of fracture. It is found that before the peak tensile strength, the ratio and magnitude of AE events are lower, but from the peak tensile strength to residual tensile strength, the ratio and magnitude of AE events are higher. The ratio of AE events shows an extreme distribution with the variation of magnitude. Between the mean and peak value of magnitude, the cumulative ratio of AE events increases in an exponential function with the decrease of magnitude. The number of microcracks in each single AE event similarly increases in an exponential function with the increase of magnitude, but the ratio of AE events exhibits a negative exponential function relationship with the number of microcracks.
Abstract: The effects of two types of organic depressants, guar gum and tannic acid, on the flotation of galena, chalcopyrite and pyrite were investigated using micro-flotation tests, and different addition orders of the two reagents and ethyl xanthate were also studied. When guar gum is added first, its depression effect on sulfides is considerable;but when ethyl xanthate is added first, guar gum has little depression effect on sulfide flotation. Tannic acid depresses pyrite and galena whatever addition order of ethyl xanthate. The interaction between guar gum or tannic acid and ethyl xanthate on the sulfide surface was studied by UV spectroscopy. It is indicated that guar gum has no impact on the adsorption of ethyl xanthate on the sulfide surface, while tannic acid can prevent the adsorption of ethyl xanthate. Infrared spectroscopy analysis suggests that tannic acid adsorbs on the galena surface by chemical action, which is the reason that tannic acid can depress sulfides adsorbing ethyl xanthate.
Abstract: The CO2 corrosion behavior of X70 pipeline steel, including the corrosion product's morphology, three-dimensional surface topography, and corrosion thickness reduction as well as its statistical analysis, was investigated in high temperature and high pres-sure CO2 environment using self-developed loop jet impingement apparatus and computation fluid dynamic (CFD) technique. The relationship between the obtained results and flow regimes under jet impingement was also discussed. It is found that the differences of fluid mass transfer and wall shear stress distributed on the steel surface located at different flow regimes are the main reason for the differences of the corrosion product's morphology, three-dimensional surface topography and corrosion thickness reduction. According to the order of the laminar zone, the wall jet zone and the transition zone, the corrosion product is thinned, degraded and even removed from the steel surface because of the continuous increasing of wall shear stress, which will decrease the mass transfer resistance, accelerate the mass transfer rate, and continuously enhance the corrosion process of the steel. Therefore, in the order of the laminar zone, the wall jet zone and the transition zone, the structure of the corrosion product changes from complete and compact to loose and porous, the three-dimensional morphology of the substrate surface changes from flat to steep, the average roughness and the root mean square as well as the average corrosion thickness reduction and the standard deviation gradually increase. The corrosion rate and the wall shear stress can be correlated properly with the exponential relationship.
Abstract: Metallographic microscopy and intercept method were adopted to investigate the austenitic grain growth behavior of locomotive wheel steel at different heating temperatures and holding time. On the basis of analyzing the effects of heating temperature and holding time on the austenite grain size, a simple kinetic model was used to analyze the austenite grain growth process, and the influence of the evolution of second phase particles on the austenite grain growth was also discussed. It is found that the austenite grain size increases with the rise of heating temperature and the prolonging of holding time, but the influence of heating temperature is more obvious. The kinetic time exponent of austenitic grain growth increases with temperature rise and the value is close to the theoretical value of 1/2. Austenite grain growth and the volume fraction and size variation of second phase particles AlN in the steel have a significant correlation.
Abstract: To study the feasibility of producing automobile steel from medium-manganese TRIP steel by continuous annealing processes, the effects of annealing temperature on the microstructure and mechanical properties of low-carbon medium-manganese steel were investigated on a heat treatment system CCT-AY-II for thin steel sheet at 590 to 710℃. The microstructure and components of the steel during annealing were characterized by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and energy dispersive X-ray spectroscopy. The amount of retained austenite in the steel was determined by X-ray diffraction analysis. The mechanical properties of the steel were investigated by uniaxial tensile testing. Experimental results show that after holding for 3 min, the amount of retained austenite first increases and then decreases with increasing annealing temperature. The elongation and the product of strength and elongation reach their maximum values of 21.3% and 28 GPa·% for the steel annealed at 650℃, respectively, and the tensile strength is 1330 MPa. Ultra-fine grain structures are obtained by recovering for the martensitic matrix and by twinning for retained austenite. It is believed that the high plasticity is provided by the TRIP effect of metastable austenite and ultra-fine grained ferrite or martensite together. A zig-zag shape appears on the true stress-strain curve. The work hardening exponent of the steel is much larger than that of traditional TRIP steel at the relatively stable stage.
Abstract: Taking heat transfer, flow of liquids, and solute diffusion into account, a three-dimensional cellular automata-finite element (3D CAFE) model was built to simulate the solidification microstructure of 430 stainless steel. Based on the model, the change laws of temperature, solidification fraction and grain morphology in the steel ingot were revealed during the solidification process. In the model, Gaussian distribution was employed to describe the relationship between the grain nucleation density and the degree of undercooling, and the KGT model was adopted to describe the dendrite growth. According to the Fe-C-17%Cr equilibrium phase diagram, the solidification path was determined, and the distribution of porosities and shrinkage cavities in the steel ingot was predicted. The temperature history and the final microstructure simulated by the model are in agreement with experimental results.
Abstract: The erosion characteristics of chromium-plated and nitrocarburized gun barrels were studied by erosion simulation test. It is shown that microcracks form in the brittle chromium plate when the chromium-plated barrel is exposed to high pressure and high temperature propellant gases. The cracks extend and end at interfaces between the coating and steel substrate. The propagation of the cracks can lead to brittle delamination of the chromium plate. In the nitrocarburized gun barrel, many wide and deep cracks form in the bore surface. The cracks penetrate through the nitrocarburized layer into the steel substrate, resulting in serious erosion of the steel substrate by combustion flow and causing the barrel failed. Based on experimental results, failure modes are proposed for erosion of gun barrels with the different surface treatments.
Abstract: The high-temperature annealing process of high permeability grain-oriented silicon steel with AlN as an inhibitor was studied by interrupting test. The evolution of Goss texture in this process was analyzed by electron back-scattered diffraction. It is found that the Goss grain size first decreases and then increases with the rise of temperature. Goss texture appears in the orientation distribution function at 800℃, but the intensity is very weak and the deviation angle is more than 10o. The average growth rate of Goss grains is faster than other grains at 900℃. Goss grains grow abnormally from 950 to 1000℃, and the deviation angle ranges from 3o to 6o. Before 1000℃, in comparison with other grains, Goss grains have no size advantage.
Abstract: By field exposure test, atmospheric corrosion tests of AZ31 magnesium alloy were conducted in Xisha Islands for 4 a. The surface and cross-section morphologies of corrosion products as well as the corrosion morphologies of the alloy after removing corrosion products were observed by scanning electron microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction analysis were used to obtain the element content and phase composition of corrosion products. The results indicate that the alloy undergoes severe corrosion. The average corrosion rate is 11.95μm·a-1. Cl- and CO2 play important roles in the corrosion process. The Cl-—containing absorbed electrolyte layers will destroy the oxidation film and induce anodic dissolution of the alloy. While CO2 dissolved in the absorbed electrolyte layers tends to neutralize the alkali formed in the cathodic area and reacts with Mg (OH) 2 to form Mg5(CO3)4(OH)2·xH2O. The surface corrosion products restrict the transport of CO2 and Cl- to the surface of the alloy, so the inner layer of corrosion products is mainly composed of Mg(OH) 2.
Abstract: By field exposure test, atmospheric corrosion tests of AZ31 magnesium alloy were conducted in Xisha Islands for 4 a. The surface and cross-section morphologies of corrosion products as well as the corrosion morphologies of the alloy after removing corro-sion products were observed by scanning electron microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction analysis were used to obtain the element content and phase composition of corrosion products. The results indicate that the alloy undergoes severe corrosion. The average corrosion rate is 11.95μm·a-1. Cl- and CO2 play important roles in the corrosion process. The Cl-—containing absorbed electrolyte layers will destroy the oxidation film and induce anodic dissolution of the alloy. While CO2 dissolved in the absorbed electrolyte layers tends to neutralize the alkali formed in the cathodic area and reacts with Mg (OH) 2 to form Mg5(CO3)4(OH)2·xH2O. The surface corrosion products restrict the transport of CO2 and Cl- to the surface of the alloy, so the inner layer of corrosion products is mainly composed of Mg(OH) 2.
Abstract: The sintering process of gold nanoparticles was investigated using molecular dynamics simulation, and the detailed crystal structure transformation and neck growth mechanisms were identified and analyzed for different particle sizes. It is found that the coalescence of two nanoparticles experiences two stages, which are the initial rapid neck formation and stable neck growth. Simulation results show that different particle sizes lead to different neck growth mechanisms. When the particle size is 4 nm, the major neck growth mechanisms are grain-boundary/dislocation slid, surface diffusion, and viscous flow. For the particles with the size of about 6 nm, the major neck growth mechanisms are grain-boundary diffusion, surface diffusion, and viscous flow. But when the particle size is 9 nm, the major neck growth mechanisms are grain-boundary diffusion and surface diffusion. During the sintering process, the fcc structure of the particles transforms to amorphous structure gradually. Besides, the hcp crystal structure forms in the small sized particles because of grain-boundary slid or viscous flow.
Abstract: To exploit the application of chambersite, original chambersite powders after mineral beneficiation were ball-milled into powders with the average particle size less than 10μm by using a high energy ball mill. The milled powders were processed into disk-shaped samples by spark plasma sintering (SPS). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to characterize the structure, shape and size of the samples, and a controlled atmosphere micro friction tester was used to investigate the friction properties of the samples. It is shown that the samples belong to orthorhombic Mn3B7O13Cl, the friction coefficient is from 0.2 to 0.6, and the wear loss is about 1×10-9 cm3·N-1·m-1, which let chambersite have a good prospect of application in the field of friction materials packing.
Abstract: By using a simulation device of CO2 circulation purification in a closed environment, CO2 circulation purification experiments were carried out with three kinds of solid chemical absorbents, LiOH, Ca(OH) 2 and soda lime. Experimental results show that all the absorbents can absorb CO2, and there are the optimal space velocity values for the three absorbents, which are 110400, 38700, and 40500 h-1, respectively. Under these space velocity value conditions, the reaction time is the shortest and the reaction rate is the fastest for absorbing the CO2 volume fraction of 2% to 0.03%. The relationships between reaction rate and CO2 mass concentration as well as the maximum reaction rate order of the three absorbents under experimental conditions were obtained by function fitting and mathematical analysis. Further analysis indicates that the CO2 absorption rates of the three absorbents under the optimal space velocity value conditions are able to meet the requirements of relevant standards for actual CO2 purification in closed environments, while the CO2 volume fraction would fluctuate around certain intermediate values in theory.
Abstract: To determine a suitable turbulence model for the numerical simulation of supersonic jet flow fields, the differences and application scopes of five turbulence models were analyzed theoretically first, and then supersonic jet flow fields with different Mach numbers were numerically simulated using these five turbulence models. The numerical simulation results were comparatively analyzed with the measured and theoretical values. It is shown that compared with other models, the shear stress transmission k-ω (SST k-ω) model has a relative higher accuracy in calculating the supersonic jet flow fields through modifying the transport equation. In the simulation of flow fields inside and outside the nozzle, the simulation result of the SST k-ω model is more in agreement with the measured and the theoretical values. Among the five turbulence models, the SST k-ω model is most suitable for the numerical simulation of the supersonic jet flow fields.
Abstract: To study the running deviation problem of strips during production in a continuous annealing furnace, a roll-strip dynamic simulation model was built using finite element software ABAQUS to quantitatively calculate the effect of double-taper roller shape on the running deviation of strips in the continuous annealing process. The transverse compressive stress and the waved surface of strips were also analyzed. Computation results show that double-taper roller shape has obvious inhibition to the running deviation of strips. The running deviation of strips gradually reduces with the increases of total roller diameter variation, roller diameter variation's ratio and tension, as well as with the decrease in length of the roller's flat section, but the maximum transverse compressive stress increases gradually, which increases the probability of waved surface occurrence. According to the above results, a calculate model of running deviation was established, which can provide a theoretical basis for further optimizing double-taper roller shape and tension setting in the heating furnace section, in combination with the theory of waved surfaces.
Abstract: The common shape defect of warping deformation, which is caused by the uneven vertical elongation of strips in the thickness direction, was studied by applying symplectic elasticity method. The relationship between the warping value and the distribution of vertical elongation was obtained through establishing and solving a mechanical model of warping deformation. Based on the correctness of the mechanical model and calculated results which were verified by general finite element method, the influence laws of factors on warping deformation were gotten by deduction and calculation. It is shown that the warping value increases linearly with the increasing elongation difference between the top and bottom surfaces and the increasing ratio of the transverse to vertical elongation difference, decreases with increasing strip thickness, but increases quadratically with increasing strip width. Finally, the phenomenon of C-warping converting to L-warping was studied by using the symplectic elasticity model of warping deformation, a mechanical model of C-warping converting to L-warping was established, and the formula for this converting was obtained, which was verified by field experiment.
Abstract: A general pseudo-rigid-body model was built for multi-layered lamina emergent mechanisms (LEMs). Based on the analogous linear coil spring model of lamina emergent torsional (LET) joints, a general theoretical formula was derived to describe the relationship between the input load and the output angular displacement of LEMs. The main structural parameters influencing the output were analyzed, and their influence trends were studied in the same load case. According to analysis results, a set of initial values was designed to the structural parameters for an example of the mechanism. The effects of the length and width of LET joint torsional hinges on the angular displacement and error of the mechanism were discussed by theoretical calculations and finite element simulation.
Abstract: A state observer with preview information for linear discrete-time systems was studied and applied to preview control systems. To design the state observer, an augmented error system with previewable desired tracking and disturbance signals was derived first, which leads to obtaining an optimal preview controller. While in designing the state observer, previewable desired tracking and disturbance signals were fully utilized through reformulation of the output equation. The state observer is a full-order dimensional observer with respect to the original system;whereas it is a reduced-order observer with respect to the augmented error system. Numerical simulation verifies the effectiveness of the state observer.
Abstract: A business cycle model with anticipation and delay was constructed by introducing anticipated capital stock into the investment decision function in the present period and considering investment delay in the capital accumulation process. This system's stability was investigated by using related theories of differential dynamical systems first. Then investment delay or the prediction time of capital stock was taken as a bifurcation parameter, and the conditions of Hopf bifurcation caused by the parameter were discussed. Last, some numerical simulations were carried out to confirm the obtained conclusions. It is shown that capital stock anticipation and investment delay together constitute the induced factors of business cycle. Economic fluctuations caused by investment delay can be dampened to some extend if the government makes investment decision based on short-term reasonable forecast on capital stock.
Abstract: Based on the application of long-span space truss structures, a new type of triple-steel tube buckling-restrained brace was proposed by improving and redesigning current triple-steel tube buckling-restrained braces. The mechanical properties of the brace were analyzed in consideration of initial imperfection. According to this analysis, 4 groups of braces were designed and the effect of strength ratio on the mechanical properties, including the stress state of connection parts, hysteretic behavior, and the failure mode of the core tube, were studied under tension-compression cyclic loading using the ABAQUS software. It is shown that the new structure configuration is feasible and the design method is reasonable. Strength ratio is the key parameter affecting the mechanical properties, and within a reasonable range, the hysteretic performance is well. Under the axial cyclic loading, the effect of inner and outer bushing constraints is obvious, the failure mode of the core tube is multiple-wave small buckling, and the deformation is stably. So the new triple steel tube buckling-restrained brace meets the requirements of design.
Monthly, started in 1955 Supervising institution：Ministry of Education Sponsoring Institution：University of Science and Technology Beijing Editorial office：Editorial Department of Chinese Journal of Engineering Publisher：Science Press Chairperson：Ren-shu Yang Editor-in-Chief：Ai-xiang Wu ISSN 2095-9389CN 2095-9389
