Abstract: With the rapid development of e-commerce and network finance involving the Internet, hundreds of millions of online transactions are being carried out on the Internet every moment. Guaranteeing the security of these transactions and realizing the secure storage, exchange, and sharing of massive transaction data have become paramount. Blockchain is a practical technology recently proposed to solve the above problems. Through P2P network technology, distributed ledger technology, asymmetric cryptography, consensus mechanism, and smart contract technology, blockchains can ensure data integrity, nonrepudiation, privacy, consistency, and other security protections. Hence, it has attracted wide attention from academia and industry in recent years. Starting from the design and demand of blockchains, this paper first expounds the basic concepts, features, and typical architecture in the current blockchains. Taking Bitcoin as an example, this paper also explored the various proposed structures and the corresponding mechanisms, including block storage structure and tamper-proof mechanism, transaction structure and scripting language, trader identification mechanism, and efficient network transaction propagation mechanism. Moreover, several current mainstream blockchain consensus algorithms were described according to the categories of proof-mode, Byzantine-type, traditional consensus, and hybrid consensus. In addition, the latest developments in smart contracts were discussed from some aspects, including concepts, organizational structure, the relationship among modules, as well as execution approaches and processes. Finally, the main security challenges faced by blockchains were summarized in order to systematically grasp the developments and trends of blockchain technology.
Abstract: Research on the deformation and failure mechanism of rock slopes in high-altitude cold areas has obtained certain results, but based on the current theory and technology, it is difficult to comprehensively solve problems related to the cold-slope instability mechanisms and disaster prevention and control. As yet, no overall research system for high-altitude slope mining or criteria for slope stability have been established. In this paper, based on an extensive literature review, five measures of the deformation and failure of alpine rock slopes were presented, including the indoor rock mechanics test, simulation of physically similar slopes, multi-field multi-phase coupled numerical simulation, in-situ monitoring of deformation and damage, and the instability mechanism of rock slopes in high-altitude cold areas. After summarizing the research results related to the deformation and failure of alpine rock masses, existing problems were discussed and current research deficiencies were analyzed. The key problems that require urgent solutions in the research of the deformation and failure of alpine rock slopes were summarized. The first problem is the damage mechanism of a rock mass in a high-altitude cold area under mining disturbance conditions. The second problem is the aging characteristics and evaluation methods of the instability of a flow–solid–gas multi-phase multi-field coupled slope under freeze–thaw cycles. The future research direction and development trends in the deformation and failure of rock slopes in cold regions with high altitude were also analyzed. Research should be conducted on the following: (1) the damage degradation mechanism of rock masses with different stress paths coupled with freeze–thaw cycles, (2) the structural plane collapse mechanism and instability of rock slopes in high-altitude cold areas under the condition of blast mining, (3) the dynamic response and disaster occurrence law of jointed rock slopes in high-altitude cold areas under earthquake loading, (4) the mechanism of damage deterioration of jointed rock masses under multi-field and multi-phase coupling conditions, and (5) real-time safety monitoring and early-warning technology regarding the instability of multi-parameter cold resistance of mine slopes in high-altitude cold areas. These five research areas constitute the trends of future research.
Abstract: Shale gas reservoirs are extremely tight, their pores are mainly nano-micron size, and their gas flow resistance is greater than that of conventional gases. Thus, the flow with low-velocity non-Darcy seepage characteristics of diffusion, slippage, and desorption needs to be solved. Moreover, the fractured reservoir has a complicated structure of pores and fractures, which causes the problem of multi-scale flow. The pressure disturbance propagates over time and does not instantaneously reach infinity. Another problem is that the moving boundary pressure disturbance of unstable seepage propagates slowly with time. Based on the above issues, in this paper, the permeability model of fractal distribution and Gaussian distribution was obtained to describe the different fracturing characteristics. Using the method of successive replacements of steady states and considering desorption, diffusion, and slippage, the mathematical model of unstable flowing pressure disturbance in a multistage fractured horizontal well was established. The moving characteristics of the different fractured conditions were compared and analyzed. The research shows that the pressure moving boundary increases with time, and the lower the permeability, the slower the pressure boundary moves. In general, the shale gas reservoir pressure propagates slowly, the natural productivity of the gas well is low, and the velocity of the pressure moving boundary of the matrix reservoir is less than the fractal distribution of the fractured reservoir and less than the Gaussian distribution of the fractured reservoir. Thus, it is necessary to carry out the large-scale fracturing treatment and reasonable control of the fracturing degree to improve the permeability as well as the development effect. When the production time is 6000 days, based on the moving boundary of the fractured horizontal well, the horizontal section length was optimized to 90 m. The optimum well distance of the well with fractal distribution permeability was 318 m, while the well with Gaussian distribution permeability was 252 m. Thus, the fracture treatment scale should be reasonably controlled to achieve optimal production and high yield.
Abstract: Finely disseminated iron ores are a type of refractory iron ores that exist in many regions in China, where fine grind is essential to liberate iron minerals from gangue. The flotation process of fine particles is plagued with losses of recovery and selectivity, which are due to the low collision efficiencies of fine particles with bubbles, mechanical/hydraulic entrainment, and high specific surface area. Carrier flotation, which is based on the carrier effect of coarse particles, is one of the effective methods for fine particle flotation. However, scarce information is available in the literature with regard to the " self-carrier” effect and mechanism of coarse hematite particles during flotation, which are necessary and beneficial for the efficient utilization of refractory iron ore resources. In this paper, micro-flotation test, optical microscopy analysis, E-DLVO theory calculations, and particle aggregation kinetics were used to study the self-carrier effect of hematite flotation in the sodium oleate system. Flotation results show that the recovery of coarse hematite (?106 + 45 μm) could be up to 90% when the sodium oleate concentration is over 15 mg·L?1. However, for fine hematite (?18 μm) particles, the flotation recovery and flotation rate are relatively low. The highest recovery of fine-coarse hematite mixtures is obtained when the fine and coarse hematite are approximately equal in mass ratio, thereby indicating that the self-carrier effects are strongest; meanwhile, the improvement of coarse particles for flotation recovery gradually weakens with excessive coarseness in the mixtures. Optical microscopy analysis and E-DLVO theory calculations show that the interaction energies and aggregation tendencies between fine and coarse hematite particles are stronger than those among the fine hematite particles, which might be the main reasons that coarse particles could enhance the flotation performance of fine hematite particles. However, excessive coarse particles could strengthen the grinding/attrition effects during the flotation, thereby possibly weakening the self-carrier effects of coarse particles and resulting in decreased flotation recovery.
Abstract: The mine tailings generated from metallic ore not only occupies a large area of surface resources but also easily causes mud-rock flow and tailings dam failure. Moreover, the existence of a large number of underground voids threatens the safety of underground mining operations and can induce mines earthquake and surface subsidence. The paste filling technology involves thickening the mine tailings into paste and placing the paste in underground voids. The technology has been widely accepted and applied around the world for its advantages in safety, environmental protection, economy, and high efficiency. The dewatering of mine tailings is a prerequisite for the paste filling process. In the paste backfill, after tailings thickening, the concentration increase is limited, the yield stress is increased, and fluidity is reduced with flocculant dosage. The flocculant dosage and thickening synergist work together to further increase unclassified tailings paste concentration and reduce slurry yield stress. The mechanism of the thickening synergist was researched from a microscopic point of view. The results show that the best addition method is to add thickening synergist after tailings settlement with flocculant dosage by settlement and rheological test. The solid mass fraction can be increased by 8.57%?10.13%, and the yield stress can be reduced by 6.68?12.85 Pa. The multi-component thickening synergist can not only reduce unit consumption and cost but also improve the compressive strength of the paste backfill material. The compressive strength of paste backfill material with thickening synergist and cement-tailings mass ratio of 1∶12 is 2.5 MPa at the age of 28 d. The difference is less than 20% compared with the compressive strength of the material with cement-tailings mass ratio of 1∶6 and without thickening synergist. By total organic carbon adsorption test and Zeta potential test, the synergist is found to have functions of adsorption and dispersion. It can destroy the flocculation structure and release the contained water, thereby increasing the tailings concentration and improving the fluidity of tailings particles.
Abstract: Water hazards in submarine gold mines pose a great threat to mine production, construction personnel, and mining equipment, and represent one of the natural disasters that occur in mining. To prevent and control accidents, it is critical to quickly and effectively identify water sources. Cracks in the rocks surrounding the roadway in the Sanshandao Gold Mine are a widespread and long-term water gushing phenomenon. The main sources of mine water hazards in mining areas are seawater, Quaternary water, bedrock fissure water, and groundwater. To accurately and quickly identify mine water sources and effectively prevent inrushes of mine water and water-hazard threats, the hydrogeological conditions and chemical composition of water samples from different monitoring points were analyzed and six indicators, i.e., Mg2+, Na++K+, Ca2+, SO42?, Cl?, and HCO3?, were selected as discriminant factors. Based on the analysis principle of the Bayesian algorithm, the Markov chain Monte Carlo (MCMC) approach was introduced into the Bayesian method. A Bayesian discriminant analysis model was then constructed using SPSS Statistics and the MCMC Bayesian method. The posterior distribution estimated by the algorithm is based on water-sample information, which enables the analysis of the mine water source. Based on the water-sample data from a water intake point at the Sanshandao Gold Mine, detailed analysis and verification were performed, and a water-source model for the inrush of mine water was established. An analysis of different water samples was then performed. Through the selection of variables, variables with a strong discriminant ability and high degree of correlation were introduced into the discriminant function to obtain the Bayesian statistical function, thus enabling a discriminatory analysis of the water sources. The accuracy and practicability of the proposed Bayesian mine-water-source identification model were verified. This model has certain significance for guiding future field work and water-hazard prevention and control efforts.
Abstract: Considering the polycrystalline and anisotropic features of rock, its mechanical failure actually involves the generation, propagation, and penetration of internal micro-cracks until an ultimate macro-fracture is achieved. The nucleation and propagation of cracks emits energy outward as elastic waves referred to as acoustic emission (AE). The close relationship between AE signals and the rock fracture mechanism has been demonstrated. Many instability and failure processes in underground engineering are induced by the effects of tensile stress on tunnels and chambers or local damage to the rock structure. Several compression experiments show that the main fracture mode of rock is tensile failure. Thus, investigations of rock AE characteristics under tensile failure and the effects of the rock fabric on crack propagation patterns are of great significance. This study assesses the signal characteristics AE and its relationship with the micro-rupture mechanisms in granite and marble under tensile stress. Herein, an MTS-322 rock mechanical test system was employed to carry out Brazilian split tests, and a scanning electron microscope was employed to carry out micro-morphological analysis of rupture surfaces. According to the trends of RA and AF, the distribution of crack modes-tensile and shear or mixed patterns in both rock types and its fracture strength depend on the rock fabric. By contrast, the evolution process of crack propagation appears to depend on the softening process. Although the rock fracture signals are mainly in the range of 400?499 kHz and 100?199 kHz, the variation trend of peak frequency shows significant differences at different failure stages. At the microtopographic level, granite mainly shows three micro-morphologies, including laminated, stepwise, and smooth planar patterns. Marble is mostly smooth polyhedrals. The signals at 400?499 kHz may be inferred to be mainly generated by fractures in the k-feldspar and marble minerals, while those at 100?199 kHz are mainly produced by discontinuous separation among quartz mineral particles and slipping among mineral particles in the compaction stage.
Abstract: Tailings consolidation discharge can effectively solve the problem of tailings disposal. However, the tailings pile after consolidation is mostly on the surface, and its performance is greatly affected by the natural environment. Freeze-thaw cycles are widespread in northern China. Freeze-thaw has a great influence on the strength, ultrasonic velocity, and electrical resistance characteristics of cemented mass. To explore the damage evolution state and mechanism of the cemented mass of unclassified tailings under freeze-thaw cycle, in this paper, a series of freeze-thaw tests on a cemented mass of unclassified tailings from the Lilou iron mine were performed. Then the cemented mass samples after different runs of freeze-thaw tests were used to conduct uniaxial compressive strength tests, scanning electron microscopy (SEM) test, resistivity test, and ultrasonic wave velocity test. Quantitative analysis of surface crack images of samples was performed using MATLAB-based binarized digital image processing technology, and a test method for joint testing of freeze-thaw cycle damage of cemented mass specimens using electrical resistivity (ER) and ultrasonic pulse velocity (UPV) testing techniques was proposed. The results indicate that the uniaxial compressive strength (UCS) decreases with increase in freeze-thaw cycles. The greatest decline is for the UCS of cemented mass subjected to 0–5 freeze-thaw cycles. The damage of the cemented mass in the freeze-thaw cycle is a gradual accumulation process. The development process of the apparent degradation characteristics of the cemented mass of unclassified tailings is as follows: micro-fracture initiation → fracture extension development → outer layer failure → internal structure failure; the higher the initial strength of the cemented mass, the fewer the number of surface cracks. The internal microstructure changes from dense to loose. The UCS of the cemented mass is positively correlated with the ER and the UPV, following the logarithmic function relationship, and the nondestructive testing models of UCS-ER and UCS-UPV are established. It is shown that the ER and UPV can accurately and comprehensively evaluate the damage state in cemented mass of unclassified tailings.
Abstract: Al?Li alloys have the advantage of low density, high strength and toughness, and low corrosion fatigue rate. This combination of properties has led to their use in aerospace. Al3Li（δ′）phase is one of main strengthening phases of Al?Li alloys. The higher chemical reactivity of Li clearly influences its corrosion behavior. In order to explain the effect of δ′ phase in the Al?Li alloy electrochemical corrosion process, an Al?2Li binary alloy was prepared by vacuum melting. Aging treatment of Al?2Li alloy at 180 ℃ followed by solution treatment were carried out. A potentiodynamic polarization plot of the alloy was tested in a 3.5% (mass fraction) NaCl solution. Phase composition of all samples was determined by X-ray diffraction (XRD). Passive film was formed on this alloy at a passivation potential of ?0.85 V vs SCE. Corrosion resistance of the passive film on the surface of the Al?2Li binary alloy was tested by electrochemical impedance spectroscopy (EIS). Structure of the passive film was analyzed by potentiostatic polarization and the Mott?Schottky (M?S) approach. Results show that the corrosion potentials of Al?2Li alloy initially move toward the positive, then toward negative, along with increasing aging time. EIS spectra of the passive films on the solution treatment and aging have two capacitive impedance arcs; the corrosion mechanism is not changed by the aging treatment. Corrosion resistance of the passive film is, in order from high to low, aged 20 h > solution treatment > aged 40 h > aged 1 h, and is related to the compactness and acceptor concentration of the passive film.
Abstract: To study the multi-objective rush order insertion rescheduling problem under hybrid flow shop with multiple stages and multiple machines, the constraints, such as job lots, sequence-dependent set-up times, and round-trip transportation times, were simultaneously considered. A static optimal scheduling model of initial orders was first established to minimize the maximum order completion time and minimize the total transportation time. The non-dominated sorting genetic algorithm (NSGA)-II algorithm was applied to solve a two-objective optimal problem. Then, for the rush order insertion disturbance factor, the objective to minimize the total machine deviation between the initial scheduling and rescheduling plans was added as a stability index to establish an optimal rush order rescheduling model. The NSGA-III algorithm based on the event-driven rescheduling strategy and order rearrangement strategy was applied to solve a three-objective optimal problem. Finally, a realistic ship pipe parts manufacturing enterprise is regarded as a study case. Two sets of experiments are carried out to explain the motivation of the selected method. The performances of the NSGA-II and NSGA-III algorithms are evaluated by three metrics, including the mean ideal distance, spread of non-dominated solution, and percentage of domination. The results show that the NSGA-II algorithm is more suitable for solving two-objective optimal problem, whereas NSGA-III algorithm performs better in solving three-objective optimal problems. Then, the proposed model and method were applied to 10 rush order insertion cases of the enterprise. All the three objectives were improved according to the compared results obtained by the actual and optimal scheduling. The optimal rate is close to one third, which verifies the feasibility of the proposed model and the effectiveness of the proposed method. The proposed model and method may assist other enterprises that apply make-to-order production mode to reduce the impact of rush order insertion and realize a win-win mechanism between enterprises and customers.
Abstract: The momentum wheel is a key device used for orbiting satellite attitude control. Its controlling accuracy is strongly influenced by the cage stability of the ball bearings in momentum wheels. The more stable the cage, the smaller the friction moment of the bearing, and the higher the control accuracy of the momentum wheel. In this study, an ADAMS multi-body dynamic model of ball bearings was built. In this model, the collision and friction were considered, which exist in the components of the ball bearing, that is, the balls, the rings, and the cage. The whirl behavior of the cage of the ball bearing used in the momentum wheel was analyzed under variable working conditions, and the cage stability was quantitatively analyzed. The effects of starting acceleration, axial load, and gravity field on the cage stability were discussed. The results show that an increase in the starting acceleration of the ball bearings can shorten the starting time, and the guiding effect of the guiding face on the cage is enhanced. Moreover, the cage is more stable when the speed of the ball bearing is higher. However, the greater starting acceleration can increase the friction moment of the ball bearing, which can shorten the service life. Under the premise of satisfying the cage stability, a smaller starting acceleration should be used as far as possible to prevent the larger friction moment. An increased axial load causes a strong collision of the cage and balls and increases the cage whirling state. The friction moment of the ball bearing increases when the axial load increases, which can lead to the generation of the wear and heat of bearing. In addition, an increase of the axial load of the ball bearing aggravates the collision of the balls and cage and increases the whirling state of the cage, and this reduces the cage stability. The collision of the cage and ring increases without gravity, causing an increase in the cage whirl.
Abstract: The modern rolling industry has improved product quality, and the technical requirements of high accuracy and high dynamic performance have made the issue of rolling mill vibration more prominent. Rolling mill system instability seriously affects the quality of the product, reduces the accuracy of the product, and even causes serious damage to the rolling mill equipment. During hot rolling process, friction is of great importance to vibration and stability of the rolling mill. There is a difference in the friction coefficient between the upper rolling interface and lower rolling interface. Considering the asymmetric friction coefficient, a chatter model was established by combing the rolling process model and the vertical?horizontal?torsional structure model of a hot rolling mill to study the relationship between friction coefficient asymmetry and stability of the rolling mill system. According to the mathematical model, the friction coefficient stability domain of a rolling mill system is determined by the application of stability criterion. And it shows that the influence of the asymmetric friction coefficient on the stability domain is significant. Due to the different degrees of asymmetry, the system is divided into stable domain, horizontal instability domain, and horizontal?torsional instability domain. As the asymmetry in terms of the friction coefficient becomes considerable, it would occur various vibration modes. Through a field test of a hot rolling mill, the vibration signal of the rolling mill system was obtained, which verified the correctness and validity of the simulation analysis results. The degree of asymmetry in the friction coefficient is the same when rolling the container plate and the Q235 plate, but the deformation resistance of the system is different. The system falls into the horizontal instability domain when the container plate is rolled, displaying clearly horizontal vibration. However, the system falls into the stable domain when the Q235 plate is rolled, and the system shows no obvious vibration.
Abstract: In order to investigate the heat and the mass transfer during pressure swing adsorption (PSA) for oxygen production and improve oxygen production efficiency, a gas-solid two-phase pressure swing adsorption model was established for the π-shaped centripetal radial flow adsorber (CP-π RFA). The energy model, the adsorption heat, and the particle diameter were comparatively studied using this model. The results show that the maximum temperature in the adsorbent bed during pressurization with air (PR) and high-pressure feed (AD) processes for the single-phase model are 309.19 K and 313.63 K, respectively. The highest oxygen mole fractions in the adsorbent bed during PR step and AD step using the single-phase model are 55.66% and 62.65%, respectively. Under the same operating conditions, the maximum temperature in the adsorbent bed during the PR and AD steps for the two-phase model are 302.27 K and 305.29 K, respectively. The highest oxygen mole fractions in the adsorbent bed during PR step and AD step using the two-phase model are 57.51% and 66.02%, respectively. For no-adsorption heat, the maximum temperatures are 293.5 K and 293.9 K, respectively, and the highest oxygen mole fractions in the adsorbent bed during the PR step and AD step with no-adsorption heat are 59.25% and 72.18%, respectively. However, the maximum temperature in the bed during the two steps with adsorption heat are 302.3 K and 305.3 K, respectively, and the highest oxygen mole fractions are 57.51% and 66.02%, respectively. As the particle diameter increases, the highest oxygen mole fraction of the outlet would decrease, while the oxygen flow rate and recovery would increase. The adsorbent with a particle diameter of 1.6 mm is the best size. The laws of the heat and the mass transfer in the adsorber can provide an important technical reference for CP-π RFA in the PSA for oxygen production.
Abstract: The gaze tracking technology is widely used in many fields, and it has a broad application prospect in the field of human-computer interaction. The technology is based on the eye characteristic parameters and the gaze parameters, and it estimates the direction of sight and placement of sight based on the eye model. Therefore, accurately locating the pupil position is important in the gaze tracking technology, and it directly affects the accuracy of the gaze tracking result. Presently, there are numerous algorithms used in eye detection; however, most of them are characterized by some problems, such as the low accuracy of locating the pupil position, high detection error, and slow operation speed; thus, they cannot meet the accuracy requirements of locating the pupil position. To solve these problems, in this study, a concept of pupil localization method from rough to precise was adopted, and a high-accuracy pupil localization method based on image processing was proposed. In this method, first, the improved maximal between-cluster variance algorithm used the histogram of the pupil region to adaptively segment region to roughly locate the pupil region. Then the pupil edge points can be accurately located by the gradient of the pupil grayscale. Finally, a sub-pixel localization method was adopted on the basis of the pixel level edge points of the pupil to locate the sub-pixel level edge points of pupil more accurately, and the center position of the pupil was accurately determined by the method of ellipse fitting. In addition, an equidistance pupil compensation method was proposed in this paper for the situation of pupil occlusion. Several experimental results show that the algorithm is robust to locate the position of pupil occlusion and that it can achieve accurate pupil localization.
Abstract: Long-term neck flexion posture is a common awkward posture resulting from long-term head-down work, long-term looking at a computer screen, and long-term playing with a mobile phone. Fatigue and chronic injury of cervical muscles are easily caused by long-term bowing of the head. Long-term bowing of the head to play with a mobile phone causes injury to the cervical spine. Long-term neck muscle contraction is an important cause of fatigue and chronic injury of the neck muscles and tissues. Therefore, it is of significance to analyze the changes of muscle activity during long-term neck muscle contraction and to determine the time threshold of neck muscle fatigue to reduce the damage caused by neck muscle fatigue. To quantitatively evaluate the effect of long-term head-down playing with a mobile phone on cervical spine fatigue, 20 healthy subjects were selected and kept the head-down angle between 40° and 60° for 3 h. On the basis of the analysis of cervical spine muscle architecture and anthropometry, the surface electromyography (sEMG) of the sternocleidomastoid, cervical gripper, and shoulder trapezius muscles was recorded. The original sEMG data were processed by filtering, rectifying, and amplitude standardization. The EMG values every 60 s were integrated, and their mean power frequency (MPF) was calculated. Results show that the fluctuation of the integral EMG is regular and the decrease after the initial increase indicates that the muscle is in the fatigue state. The MPF values of different muscles have obvious differences, which determine the duration of fatigue tolerance of the muscle. Moreover, the MPF does not exhibit a simple linear relationship during the entire bowing process. The results also show that negative MPF accumulation can be used to assess neck muscle fatigue. The sternocleidomastoid muscle is in the fatigue state in 20 min, whereas the cervical gripper and shoulder trapezius muscles are temporarily fatigued in approximately 20 min and in the final fatigue state in 75–100 min. Therefore, it is suggested that the duration of continuous bowing should not exceed 20 min.
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
