Abstract: To investigate the effect of stacking structure on the wettability of coal dust,ten selected coal samples from China's different regions with different metamorphic grades were analyzed by X-ray diffraction. In combination with the dynamic contact angle determination results of the liquid-solid interface,the relationship between microcrystalline parameters and the wettability of coal dust was deduced. The results show that the inorganic mineral types gradually decrease with increasing metamorphic grade,and the more the primary mineral types are,the better the wettability of coal dust is,but the secondary minerals have no significant influence on the wettability of coal dust. For different coal types,the higher the metamorphic grade of coal is,the more the crystalline component is,the bigger the structural unit of aromatic microcrystals is,and the higher the aromatic ring condensation degree will be. While the coals with low-medium metamorphic grades have more amorphous components,which include hydrophilic alkyl side chains of small molecules,oxygen-containing functional groups,etc. With increasing stack degree and spread degree,the plane spacing decreases gradually,and the wettability of coal dust becomes worse. When the stack degree is 18.5 nm,the spread degree is 41.7 nm and the plane spacing is 0.898 nm,the wettability of coal dust will be the best. The closer the X-ray diffraction microcrystalline parameters are to the above combinations,the better the wettability of coal dust is.
Abstract: A ubiquitous jointed rock mass(UJRM) model constructed by combining the FLAC3D software and discrete fracture network(DFN) technique,which takes the features of rock and joints into account,has a clear concept,high efficiency of modeling and calculation,and so on. Firstly,the in-situ survey results of joints were statistically analyzed to obtain their probability distribution parameters,on the basis of which the joints were considered in two scales and their DFN models were determined. Secondly,based on the determined DFN models,the jointed rock mass was modeled and the strength characteristics were researched to get the uniaxial compressive strength(UCS) of engineering rock mass,then the mechanical parameters of jointed rock mass were fitted and determined by the Hoek-Brown and Mohr-Coulomb criteria while considering the max confining stress. Finally,the determined parameters were applied to slope stability analysis,the safety factors of the two kinds of criterions solved by the limit equilibrium method(LEM) and the strength reduction method(SRM) are basically the same,indicating that using the proposed approach to determine the mechanical parameters of jointed rock mass is practicable.
Abstract: Based on blockiness and the matter-element extension theory,this article introduces BET method(classification method based on blockiness and the extension theory) especially suitable for roof stability analysis. The integrity of roof rock was reflected by blockiness in three-dimensional space,and an evaluation system was constructed with blockiness,uniaxial compressive strength,joint condition and groundwater condition as evaluating indicators. The classical domain,the segment and the object element of the model were established,the incompatible problems between these indexes were analyzed,and the index weights were calculated by the extension method. After the correlation degree was comprehensively calculated,the stability classification method and standard were determined. Selecting four test areas of fractured rock mass in Guangxi Tongkeng Mine,an engineering application study of BET classification method was carried out. The results of traditional classification methods,such as RMR method,RSR method and Q method,were respectively compared to that of BET method. It is shown that compared with these traditional classification methods,the index system selected by BET method is more comprehensive and its evaluation method is more objective and scientific.
Abstract: As for the problem that different gypsum types have different influence on the condensation performance of water-rich filling materials,micro experiments,such as X-ray diffraction,scanning electron microscopy and infrared spectroscopy,were used to analyze the composition of the hardened body and investigate the influence of dihydrate gypsum and hemihydrate gypsum on the setting and hardening characteristics of water-rich filling materials. It is found that in a water-rich filling material system of sulphoaluminate cement-gypsum-lime,the gypsum type should be dihydrate gypsum so as to ensure the normal condensation setting and hardening; if hemihydrate gypsum is used as a raw material,the water-rich filling material has not strength after hydrating for 7 d. When dihydrate gypsum is adequate,the generated ettringite crystals are fine needle-like; but when dihydrate gypsum is inadequate,the generated ettringite crystals are short hexagonal columnar. The strength of the water-rich filling material is mainly derived from ettringite generated by the reaction of sulphoaluminate cement gypsum and lime,rather than the hydration products of cement generated by itself such as hydration calcium sulphoaluminate,alumina gel and calcium hydroxide.
Abstract: The microstructure and inclusions in rare earth treated C-Mn steel were studied,the size distribution of the nuclei inducing acicular ferrite nucleation was subjected to statistical analysis,and the formation and growth processes of rare earth inclusions were discussed through quenching the treated samples at different temperatures. It is found that after a small amount of rare earth is added into the C-Mn steel,the inclusion composition changes from Mn S + aluminosilicate to La2O2S + LaAlO3+ MnS + aluminosilicates,the inclusion size decreases,and the microstructure converts from martensite + bainite to ferrite side plate + acicular ferrite+ massive ferrite. The size of effective inclusions inducing acicular ferrite nucleation is mainly concentrated in 1-4 μm in the rare earth treated C-Mn steel,they primarily form in liquid steel,but little in the cooling and solidification processes. The rare earth inclusions are easy to coalesce and grow up at molten temperature and in the cooling and solidification processes,and Mn S can precipitate around the residual rare earth inclusion particles.
Abstract: The austenite grain growth behavior of KT5331 steel and the effect of precipitates on the austenite grain growth were studied by isothermal austenization at 950-1280℃. The results show that the austenite grain growth process can be divided into three stages. Due to the pinning effect of precipitates containing W and Nb,the austenite grains grow slowly below 1075℃. Most of the precipitates dissolve above 1075℃,leading to the austenite grains coarsened. When the temperature rises to 1225℃,δ-ferrite precipitates and results in austenite grain refinement with increasing temperature. The austenite grain growth models below and above the coarsening temperature 1075℃ were established by numerical fitting.
Abstract: Friction stir processing was applied to cool-rolled DC04 IF steel plates with a thickness of 3 mm,and the microstructure and mechanical properties of the processed zone were investigated. A defect-free specimen with a shiny and smooth surface is fabricated at a tool rotation rate of 950 r·min-1 and a travel speed of 60 mm·min-1 using additional forced cooling technology after processing.A fine equiaxed grain structure can be obtained in the processed zone,which causes a higher hardness of HV 135.6,1.4 times as large as the base metal at the center of the processed zone,and the highest hardness of HV 312.8 in the surface. The ultimate tensile strength of processed samples with and without the surface fine-grained layer increases by 50.9% and 47.6% compared to that of the base material,respectively. The both fracture surfaces of samples before and after friction stir processing exhibit dimple ductile fracture characterization. Grain refinement is considered to be the main reason for improving the ultimate tensile strength.
Abstract: The effect of banded structure on the fatigue properties of DG20 Mn and 35 Cr Mo micro-alloyed steels was investigated by using axial tensile fatigue tests at a stress ratio of 0.1. The results show that secondary banded structure has no significant effect on the axial tensile properties of the tested steels and the axial tensile fatigue performance of 35 CrMo steel,but decreases the axial tensile fatigue performance of DG20 Mn steel. Under the high cyclic tensile stress,multi-source damage cracks in the steels initiated by secondary banded structure,which changes the fracture mode,is the major factor of the decrease in fatigue life of the steels.
Abstract: The acoustic emission signals of fatigue crack propagation in Q345R,which is the typical representative of common steel for pressure vessels,were investigated in detail to construct a relationship between fatigue and acoustic emission. The results show that there are three distinct stages of acoustic emission during fatigue crack propagation in Q345R. Cumulative count and cumulative energy can characterize the whole fatigue crack propagation well. The transition from the 2nd stage to the 3rd stage of acoustic emission advances that of linear elastic fracture mechanics,indicating that acoustic emission is more sensitive to the unstable fracture stage of fatigue crack propagation. The relations of count rate and energy rate to fatigue crack propagation rate of Q345R are established respectively for predicting the remaining life of Q345R.
Abstract: Based on carbon steel surface corrosion,a study was conducted on multifunction acid water-based surface tolerant coatings. The focus was placed on the effect of antirust components and preparation processes on the coating properties. The barrier property was evaluated and compared with imported counterparts. The results show that when the treatment level of the substrate material surface is St1,the adhesive force of the coatings rises at the beginning and then goes downward as the content of the antirust components keeps increasing. The chelating agent contains active groups which stabilize the substrate surface corrosion,resulting in the adhesion performance of the coatings increasing. The compound rust conversion agent improves the coating adhesive force further by translating the corroded substrate surface complexation. The best preparation process needs to mix at low speed and add the compound rust agent at the end of the steps. The adhesive forces of the coatings on the substrate material surfaces treated at St1 and St3 levels show no difference,and they are larger than 5 MPa. The corrosion resisting properties,resistant to salt water for 1200 h,salt spray for1000 h and ultraviolet aging for 2000 h,are much higher than those of similar imported water-based rust primers. The coatings are suitable for the maintenance of complex steel components in atmospheric environment.
Abstract: The corrosion behavior of hot air solder level printed circuit boards(PCB-HASL) and electroless nickel immersion gold printed circuit boards(PCB-ENIG) in salt spray test was studied by electrochemical impedance spectroscopy. The corrosion product morphology,composition and coating failure mechanism were observed by stereo microscopy,scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results show that PCB-HASL and PCB-ENIG suffer severe corrosion in high salt spray environment. Localized corrosion occurs first in the Sn plating layer,and then almost entire surface corrosion occurs,similar to uniform corrosion. PCB-ENIG mainly generates microporous corrosion. In the PCB-HASL corrosion process,Cl- promotes the corrosion of the Sn layer,then a large amount of corrosion product gradually forms and covers the Sn surface,it makes the corrosion rate reduce.In the PCB-ENIG corrosion process,a thin liquid layer containing Cl- electrolyte forms in micropores,and galvanic coupling between gold and nickel constitutes,which accelerates Ni corrosion. The Cu base directly exposes in the adverse environment after Ni suffers serious corrosion,finally leading to electronic component failure.
Abstract: A standard heat treatment(1105℃/4 h/(oil cooling) + 650℃/24 h/(air cooling) + 760℃/16 h/(air cooling)) and a subsequent long term exposure at temperatures of 720℃ for 0-5000 h and 800℃ for 0-1000 h were proposed to investigate the microstructural evolution of γ' precipitates in GH720 Li alloy. The size,morphology and quantity of the primary γ' phase have no significant variation at 720℃. However,at the same temperature,obvious grain coarsening and the grain size inhomogeneity of secondary γ'particles appear with the aging time up to 200 h and 500 h,respectively. The descending slope of hardness decreases first and when the aging time exceeds 500 h it flattens. At 800℃,the primary γ' phase coarsens after 500 h of aging,while both obvious grain coarsening and the grain size inhomogeneity of secondary γ' particles happen after 100 h of long term exposure. The descending slope of hardness decreases first,and unlike the results at 720℃,it is still downward when the aging time exceeds 500 h. The coarsening behavior is controlled by lattice diffusion. There shows a linear relationship between the average radius of the γ' phase and the cube root of aging time,which accords with the L-S-W(Lifshiz-Slyozov-Wanger) theory.
Abstract: WC-12 Co coatings were prepared on copper matrix composites by high velocity oxygen-fuel flame(HVOF) thermal spraying. Their microstructure,surface and cross-section hardness,composition and content of phases,and tribological behavior were studied. The results show that the coatings appear uniform microstructure and high hardness. During the wear,two kinds of tribofilms form. The most suitable condition for the coatings is 300-500 r·min-1 and 2-3 N. The wear rate increases with increasing load. While the rotating speed increases,the wear rate presents a trend of first decreasing and then increasing. The fitting equations between wear rate and sliding speed and between wear rate and normal load are quadratic and exponential,respectively.
Abstract: A layer of AlTiCrN coating was prepared on the YT14 hard alloy cutter surface by cathodic arc ion plating(CAIP) with Ti,Al and Cr targets. The morphologies,chemical elements and phases of the coating surface and interface were observed by means of scanning electron microscopy(SEM),energy dispersive spectrometer(EDS) and X-ray diffraction(XRD),respectively. The diffusion mechanism of chemical elements at the interface was analyzed by line scan and plane scan. The bonding strength of the interface was characterized by scratch test,and the binding mechanism was discussed. The results show that phases in the coating mainly are composed of AlN,CrN and TiN,and the crystal plane of(111) has a strong preferred orientation. The concentrations of Al,Ti,Cr and N in the coating are higher than those in the substrate,showing a step-like distribution at the interface. C atoms in the substrate diffuse into the TiN,AlN and CrN lattices to form an obvious diffusion layer. The interface is a mechanical and diffusion form,mainly combined with adsorption bonding,diffusion bonding and chemical combination. The coating bears three stages in scratch process,i. e.,elastic deformation,plastic deformation and coating stripping,and the interface bonding strength is 59.2 N.
Abstract: The formation mechanism and influencing factors of fold defects at the medium plate edge were studied by detecting and analyzing edge folding samples. It is found that the plate edge folding phenomenon results from the transformation of slab lateral side to plate surface during the broadside rolling process. The microstructure observed at the fold defects is composed of iron oxide and a decarburization layer formed at high temperature in the plate surface before rolling. Finite element numerical simulation of the rolling process confirms that the folding defects are mainly caused by the transformation of lateral side. The influences of slab defects,rolling procedures,broadening passes and reduction on the plate fold defects were obtained by rolling experiments. Experimental results show that rolling spread leads to the fold edge defects,which do not depend on the quality of the slab. In the process of rolling,the folding of lateral side transfers to the plate surface and changes to be edge seam defects. The distance of folding to the edge increases with the increment of broadside rolling passes and reduction.
Abstract: For the obvious neutral layer offset of a large cross-section bar in the two-roll straightening process,which has large impact on the roller type and the bar straightness accuracy,a neutral layer offset model was established based on the three-point bending and elastic-plastic theory. Combined with room temperature tensile test and bending test,the neutral layer migration regularity was studied in the bar straightening process. It is found that the influences of the bending radius and mechanical properties of the bar on the neutral layer offset are obvious. The smaller the bending radius is,the larger the neutral layer offset is. The neutral layer offset for low strength and strong plastic materials is larger. The reliability and applicability of the model are verified by bending test.
Abstract: A high-precision calibration device was established for self-developed free frame wire high temperature strain gauges,a feasible calibration method was introduced,and some important parameters which affect the measuring results were determined. The change curves of sensitivity coefficient,thermal output,zero-drift and creep with temperature were obtained according to the calibration method,and a compensation model was proposed for strain measurement. Verification test results prove that the calibration method is feasible and correct and can be promoted to other forms of high temperature strain gauge parameter measurements.
Abstract: In view of the problem that current context-aware recommender systems take users and systems as the main research direction and do not consider actual transportation network location characteristics,this article introduces a geographic information recommender system optimized by transportation network data. The system combines with transportation network data of geographic information in the context-aware recommender system to optimize the recommended results. Experimental results show that the recommended quality get improved significantly.
Abstract: The method of receiving the antenna array vector was proposed under the sparse angle domain and Doppler domain.The direction-of-arrival(DOA) estimation problem in the Doppler frequency domain was discussed to extract the relatively large impulse response at the band's effective range. The L2,0 estimation algorithm was used in the recovery process of DOA estimation,which took full advantage of the inherent sparse characteristics of DOA under the architecture of multi-antenna spectrum sensing. As for the unavoidable noise problem in the testing environment,the mean square error of DOA estimation was also analyzed for different noise distributions. Simulation results show that the computational complexity of the L2,0-DOA estimation algorithm in the frequency domain is low and the accuracy is high. Besides,the proposed method is not sensitive to noise and has high robustness.
Abstract: Individual risks in evacuation under toxic gas diffusion conditions were quantified on the basis of the human vulnerability model. The alert communication and people transition laws were used to determine individual risks at different deployment ratios of alert receiving devices. A bi-objective mathematical model of deployment for alert receiving devices around natural gas wells in operation was built by the graph theory and multi-objective optimization method,and based on the greedy algorithm a practical algorithm was introduced to solve a better non-inferior solution of the model. Finally,taking the emergency warning project of a montanic gas well as an example,the border of deployment sub-regions and the corresponding deployment proportion were achieved by program computation. The results show that,for the same risk mitigation goals,this method has a better economy than the average deployment method in reducing deployment costs and relieving road congestion.
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
