Reducing friction and wear is of great significance for improving the performance of mechanical systems and extending the service life of materials.Understanding the physical origin of friction is essential for designing solid lubricating materials and reducing friction and wear.First-principles calculations based on density functional theory can provide the frictional potential energy surface of solid interfaces,which is a key physical quantity for characterizing friction.They could also reveal the physical origin of fluctuations in the sliding potential energy surface through charge density evolution.Thereby,the understanding of friction was advanced to the electronic scale.However,it remained difficult to evaluate and quantify the advantages and limitations of different methods for characterizing charge density evolution in friction.No unified meth-od universally applicable to all materials had yet been established. In this review,recent methods for calculating the frictional properties of solid interfaces based on density functional theory were systemati-cally summarized.Four methods for quantifying charge density evolution during friction were discussed,including differential charge density along the direction normal to the interface,charge density evolution at the interface center,and their improved methods.The correlations be-tween these methods and fluctuations in the frictional potential energy surface,as well as their physical origins,were analyzed.The limitations of existing methods were summarized.Future research directions were prospected.
Aims:In arid sandy areas,while the sand-fixing vegetation system is crucial to maintaining ecological security,artificial vegetation gradually often faces the risk of degradation with the increase of the growth period.Recently,it has become an urgent theoretical and technical challenge to maintain the relative stability of sand-fixing vegetation systems,as well as promote their natural regeneration and succession.To understand the ecological traits of seeds from sand-fixing vegetation areas with different establishment years,we collected vegetation samples from the north side of the Baotou-Lanzhou Railway at the southeastern edge of the Tengger Desert.Our objectives are to determine the characteristics of the seed rain,soil seed bank,and aboveground vegetation,along with interrelationships among them to characterize species growth and community structures in this specific environment.The overall goal is to fully understand the mechanisms of underlying vegetation renewal,succession,and stability maintenance in the sand-fixing vegetation area on the southeastern edge of the Tengger Desert,thereby providing a theoretical basis for the sustainable management of the sand-fixing vegetation system. Methods:On the northern edge of the Baotou-Lanzhou Railway,we selected three types of sand-fixing plots with different establishment years(15-20 years,30-40 years,50-60 years)for a 2-year field study.We collected seed trap and soil seed bank samples annually from these plots.The collected seeds were germinated indoors,and vegetation surveys were conducted each year.The similarity of species composition between aboveground vegetation,seed rain,and soil seed bank were analyzed using the Sørensen similarity index(SI)and non-metric multidimensional scaling(NMDS)ordination methods.Additionally,we quantitatively analyzed the relative contributions of seed source availability versus dispersal limitation to the formation of the soil seed bank. Results:Our results indicated that the seed rain composition in sand-fixing vegetation areas with different establishment years was dominated by annual herbs,with their density dynamics showing a bimodal distribution pattern in July and November,controlled by the phenology of seed dispersal.The highest seed rain density was observed in sand-fixing vegetation established for 30-40 years.The soil seed bank composition in these areas was primarily composed of annual herbs,exhibiting characteristics of both transient and persistent seed banks,with a tendency towards persistent seed banks.However,seeds of Caragana korshinskii showed typical transient soil seed bank characteristics.The highest soil seed bank density was found in the surface soil layer and in areas established for 30-40 years.Aboveground vegetation in sand-fixing areas generally exhibited a simple structure.The similarity between mobile dune areas and areas established for 50-60 years was higher than that between areas established for the 15-20 years and those established for 30-40 years.Furthermore,the similarity between soil seed banks and aboveground vegetation was higher than that between seed rain and aboveground vegetation.Regarding the similarity in monthly species composition,the highest similarity within groups was found in the soil seed bank,followed by aboveground vegetation.Additionally,our species composition similarity results between aboveground vegetation,seed rain,and soil seed bank,showed no significant differences using both SI and NMDS ordination methods.Both methods revealed that similarities between aboveground vegetation,seed rain,and soil seed banks tend to increase with sand-fixing duration. Conclusion:Persistent soil seed banks serve as a potential driving force for long-term vegetation renewal,while seed rain enhances the potential for vegetation regeneration.In this study,we determined that sand-fixing vegetation areas in the southeastern edge of the Tengger Desert,as well as the occurrence of seed rain,the composition of the soil seed bank,and the characteristics of aboveground vegetation,exhibit significant differences in species and habitat.Additionally,we found that as the duration of sand-fixing increases,the degree of connection among the three components(seed rain,soil seed bank,and aboveground vegetation)also increases,tending to form a local soil seed bank supplementation mechanism that supports the cyclical renewal and maintenance processes of aboveground vegetation.
In order to enhance the tribological and anti-corrosion performance of copper alloy bearing bushes in methanol fuel leakage environ-ments,an epoxy resin(EP)coating filled with Ti3AlC2 and B4C was designed and prepared on the surface of CuPb24Sn by liquid spraying method.The corrosion resistance of the composite coating was characterized by electrochemical impedance spectroscopy in a formic acid and methanol mixed solution with a volume ratio of 1∶9,while its temperature rise tribological performance and wear mechanisms under complex working conditions(formic acid+methanol+lubricating oil)were investigated through high-frequency reciprocating friction-wear tests.Re-sults showed that the Ti3AlC2-and B4C-filled EP coating exhibited excellent corrosion resistance.Under the formic acid+methanol+lubrica-ting oil condition,the coating's friction coefficient decreased with rising temperature,whereas its wear rate increased.At 75 ℃,the wear scar became wider than at 25 ℃,with evident ploughing effects observed on the worn surface.Materials Studio software was used for simulation cal-culation,and it was found that the decrease in shear strength of epoxy resin with increasing temperature was the main reason for the change in wear rate of coating.
The contact network dropper works in a harsh environment,and suffers from the impact effect of pantographs during running of trains,which may lead to faults such as slack and broken of the dropper wire and broken of the current-carrying ring.Due to the low intelligence and poor accuracy of the dropper fault detection network,an improved fully convolutional one-stage(FCOS)object detection network was proposed to improve the detection capability of the dropper condition.Firstly,by adjusting the parameter α in the network focus loss function,the problem of positive and negative sample imbalance in the network training process was eliminated.Secondly,the generalized intersection over union(GIoU)calculation was introduced to enhance the network's ability to recognize the relative spatial positions of the prediction box and the bounding box during the regression calculation.Finally,the improved network was used to detect the status of dropper pictures.The detection speed was 150 sheets per millisecond,and the MAP of different status detection was 0.951 2.Through the simulation comparison with other object detection networks,it was proved that the improved FCOS network had advantages in both detection time and accuracy,and could identify the state of dropper accurately.
Foreign object encroachment is the presence of persons or items in the contact network and railroad track area, which can result in accidents, electrical failures, and train damage on the railroad. By keeping an eye on the lookout, seeing the intrusion of a foreign object, and braking quickly, train drivers can lessen the risks brought on by foreign object invasion. The question of how to maintain and monitor the continuous attention level of train drivers has been a hot issue, because of the monotonous environment of railroad operating makes the alertness decline during threat perception. Based on the theory of situational awareness, this study posits that training drivers in foreign object encroachment experience schemas can enhance their sustained attention levels, leading to improved hazard perception sensitivity. The hypothesis is subsequently verified in different route types and road types. We utilized a set of 24 videos depicting railroad traffic scenarios, generated by a driving simulator, to examine the sustained attention levels and hazard perception capabilities of train drivers. The participants in our study were divided into two groups: one group consisted of train drivers with recent experience of foreign object intrusion ( n = 37), and the other group comprised train drivers without such recent experience ( n = 35). The study's findings suggest that schemas have an impact on hazard sensitivity by influencing the extent of sustained attention. This effect is further moderated by the pseudoneglect effect and prioritization of task goals. The results provide confirmation that the situational awareness theory holds valuable insights for enhancing the training of train drivers in hazard perception and sustaining attention. And it also addresses a previously existing gap in the literature regarding the factors affecting train drivers' hazard perception. These findings can be instrumental in shaping training programs aimed at safety awareness, as well as the design and implementation of alertness monitoring systems for train drivers.
