查看更多>>摘要：Bolted joints play a more and more important role in the structure with lighter weight and heavier load.This paper aims to provide an overview of different experimental approaches for the dynamic behavior of structures in the presence of bolted joints,especially the energy dissipation or damping at frictional interfaces.The comprehension of energy dissipation mechanisms due to friction is provided first,while the key parameters and the measurement techniques,such as the excitation force,the preload of the bolt,or the pressure at the interfaces,are briefly introduced.Secondly,the round-robin systems aim to measure the hysteresis parameters of the frictional joints under tangential loads are reviewed,summarizing the basic theory and the strategies to apply the excitation force or acquire the response in different testing systems.Followed by parameter identification strategies for bolted structures,the test rigs with one or more simplified bolted joints are summarized to give an insight into the understanding of typical characteristics of bolted structures,which are affected by the presence of friction.More complex test rigs hosting real-like or actual engineering structures with bolted lap or flange joints are also introduced to show the identification process of the dynamic characteristics of bolted connections employed in specific applications.Based on the review paper,researchers can get the basic knowledge about the experimental systems of the bolted structures,especially several classical round robin systems,such as the Gaul resonator and widely used Brake-Reuß beam system.Readers can take advantage of this background for more creative and effective future studies,make more progress on the study of assembled structures and understand the influence of bolting frictional connections on the dynamic response better.
查看更多>>摘要：Wearable biomechanical energy harvesting devices have received a lot of attention recently,benefiting from the rapid advancement of theories and devices in the field of the micro electromechanical system(MEMS).They not only fulfil the requirements for powering wearable electronic devices but also provide an attractive prospect for powering self-powered flexible electronic devices when wearing.In this article,we provide a review of the theories and devices of biomechanical energy harvesting technology for wearable applications.Three different forms of biomechanical energy harvesting mechanisms,including the piezoelectric effect,electromagnetic effect,and electrostatic effect,are investigated in detail.The fundamental principle of converting other types of energy from the biomechanical environment into electrical energy,as well as the most commonly-used analytical theoretical models,are outlined for each process.Therefore,the features,properties,and applications of energy harvesting devices are summarized.In addition,the coupled multi-effect hybrid energy harvesting devices are listed,showing the various possibilities of biomechanical energy harvesting devices for serving as sources,sensors,and actuators.Finally,we present perspectives on the future trends of biomechanical energy harvesting devices for wearable electronics applications.
查看更多>>摘要：As a crucial part in micro-electromechanical manufacture,local ultra-precision processing of highly ductile copper is expected to be realized by fluid jet polishing(FJP),which widely utilized in optical elements.Since copper exhibits different wear behavior from stiff and brittle material,there is currently no abrasive wear prediction model applicable for copper to investigate the polishing mechanism.This research reveals that the copper material removal is dominated by deformation wear rather than cutting wear through abrasive jet impact experiments and localized wear scars analysis.A three-dimensional gas-liquid-particle triphasic wear model for copper in FJP is developed by considering impact energy and wear mechanism simultaneously.Ultimately,validation assessments at various working pressures and impingement angles achieve the goodness-of-fit up to 0.92-0.97 in quantitative comparison between simulations and experimental measurements,which demonstrate the wear prediction ability of the proposed model.This investigation facilitates a better understanding of copper wear mechanism and provides theoretical guidance for FJP process optimization.
查看更多>>摘要：Controllable friction regulation has drawn much interest in both scientific and industrial fields.And there have been many researches on friction regulation by many physical fields and chemical factors.Photo-sensitive materials are promising because it is relatively easy to change their properties compared with others.Here,a new kind of photo-induced grease/oil switched lubricant is designed and shows great reversibility under ultraviolet(UV)and visible(Vis)irradiation.Its viscosity can change more than 50 times under different irradiation,and the coefficients of friction(COFs)obviously increase under Vis irradiation and decrease under UV irradiation,which performs better than those of the common grease.According to the experimental results,the phenomena are contributed to the break and reconstruction of the three-dimensional network inside the lubricant.With a switchable grease/oil state under different irradiation,this work provides a new principle for designing a smart lubricant with controllable friction regulation.
查看更多>>摘要：Accurately measuring the coefficient of friction(COF)is the fundamental prerequisite of superlubricity research.This study aimed to reduce the COF measurement resolution Δμ of atomic force microscopy(AFM).Based on the theoretical model,a distinctive strategy was adopted to reduce Δμ by optimizing the cantilever's cross-section of the AFM probe,inspired by civil engineering.Δμ can be reduced by decreasing the width of the horizontal side wR and the wall thickness t and increasing the width of the vertical side wH.Moreover,the I-shape demonstrates the highest reduction in Δμ,followed by the U-shape.Considering the processability,the AFM probe with the U-shaped cross-sectional cantilever was investigated further,and the dimensions are 35 μm wR,3.5 μm wH,0.5 μm t,50 μm l(cantilever length),and 23 μm htip(tip height).The finite element analysis results confirm its reliability.After being fabricated and calibrated,the AFM probe achieves the minimal Δμ of 1.9×10-6 under the maximum normal force so far.Additionally,the friction detection capability of the fabricated AFM probe improves by 78 times compared to the commercial tipless-force modulation mode(TL-FM)AFM probe with the conventional solid rectangular cross-sectional cantilever.This study provides a powerful tool for measuring 10-6 COF.
查看更多>>摘要：Most studies of liquid lubricants were carried out at temperatures below 200 ℃.However,the service temperature of lubricants for aerospace and aeroengine has reached above 300 ℃.In order to investigate the friction mechanism and provide data for high temperature lubrication,the friction and wear properties of chlorophenyl silicone oil(CPSO)-lubricated M50 steel and Si3N4 friction pairs were investigated herein.Ball-on-disk experimental results show that the lubrication performance of CPSO varies significantly with temperature.Below 150 ℃,coefficient of friction(COF)remains at 0.13-0.15 after the short running-in stage(600 s),while the COF in the running-in stage is 0.2-0.3.At 200 ℃and above,the running-in time is much longer(1,200 s),and the initial instantaneous maximum COF can reach 0.5.Under this condition,the COF gradually decreases and finally stabilizes at around 0.16-0.17 afterwards.This phenomenon is mainly due to the different thickness of boundary adsorption film.More importantly,the wear rate of M50 steel increases significantly with the temperature,while the wear rate barely changes at temperatures above 200 ℃.The anti-wear mechanism is explained as tribochemical reactions are more likely to occur between CPSO and steel surface with the increased temperature,generating the FeCl2 protective film on the metal surface.Accordingly,FeCl2 tribochemical film improves the lubrication and anti-wear capacity of the system.At high temperatures(200-350 ℃),FeCl2 film becomes thicker,and the contact region pressure becomes lower due to the larger wear scar size,so the wear rate growth of M50 steel is much smaller compared with that of low temperatures(22-150 ℃).The main findings in this study demonstrate that CPSO lubricant has good anti-wear and lubrication capacity,which is capable of working under temperatures up to 350 ℃.
查看更多>>摘要：Herein,we have prepared SiO2 particles uploaded MXene nanosheets via in-situ hydrolysis of tetraetholothosilicate.Due to the large number of groups at the edges of MXene,SiO2 grows at the edges first,forming MXene@SiO2 composites with a unique core-rim structure.The tribological properties of MXene@SiO2 as lubricating additive in 500 SN are evaluated by SRV-5.The results show that MXene@SiO2 can reduce the friction coefficient of 500 SN from 0.572 to 0.108,the wear volume is reduced by 73.7%,and the load capacity is increased to 800 N.The superior lubricity of MXene@SiO2 is attributed to the synergistic effect of MXene and SiO2.The rolling friction caused by SiO2 not only improves the bearing capacity but also increases the interlayer distance of MXene,avoiding accumulation and making it more prone to interlayer slip.MXene@SiO2 is adsorbed on the friction interface to form a physical adsorption film and isolate the friction pair.In addition,the high temperature and high load induce the tribochemical reaction and form a chemical protection film during in the friction process.Ultimately,the presence of these protective films results in MXene@SiO2 having good lubricating properties.
查看更多>>摘要：A stochastic uncertain tribodynamic model is established for a spur gear pair for the first time.The stochastic uncertainty of pinion rotation speed propagated to lubrication performance is investigated.The probability density function of the minimum lubricant film thickness hmin evolves over time periodically at interval of an engagement process.Correspondence between abrupt increase in meshing force and amplification of hmin uncertainty is found.Robust and reliable lubrication performance can be achieved by suppressing the hmin uncertainty and decreasing the lubrication failure probability.This can be done by increasing lubricant viscosity,and decreasing input torque and uncertainty level of input rotation speed.This work lays a solid foundation for robust and reliability based optimization for tribodynamic gear system.
查看更多>>摘要：Cartilage is well lubricated over a lifetime and this phenomenon is attributed to both of the surface hydration lubrication and the matrix load-bearing capacity.Lubricious hydrogels with a layered structure are designed to mimic cartilage as potential replacements.While many studies have concentrated on improving surface hydration to reduce friction,few have experimentally detected the relationship between load-bearing capacity of hydrogels and their interface friction behavior.In this work,a bilayer hydrogel,serving as a cartilage prototype consisted of a top thick hydrated polymer brush layer and a bottom hydrogel matrix with tunable modulus was designed to investigate this relationship.The coefficient of friction(COF,μ)is defined as the sum of interfacial component(μInt)and deformation/hysteresis component(μHyst).The presence of the top hydration layer effectively dissipates contact stress and reduces the interface interaction(μInt),leading to a stable and low COF.The contribution of mechanical deformation(μHyst)during the sliding shearing process to COF can be significantly reduced by increasing the local mechanical modulus,thereby enhancing the load-bearing capacity.These results show that the strategy of coupling surface hydration layer with a high load-bearing matrix can indeed enhance the lubrication performance of hydrogel cartilage prototypes,and implies a promising routine for designing robust soft matter lubrication system and friction-control devices.
查看更多>>摘要：Inorganic nanoparticles have been proved as powerful lubricant additives at elevated temperature.However,the tribological properties are inevitably impaired due to poor dispersion and insufficient high temperature resistance of organic matter modified nanoparticles.Here,we prepare a self-dispersed molybdenum disulfide quantum dot/graphene crumpled ball(MGCB)comprising molybdenum disulfide quantum dot uniformly interspersed on the wrinkled graphene ball.The crumpled ball composite possesses excellent dispersity in polyalkylene glycol base oil without depending on surface modifiers.Compared with the conventional phosphate esters lubricant,our results indicate MGCB could vastly improve the lubrication performance of polyalkylene glycol with an extremely low concentration(0.05 wt%)at elevated temperature(150 ℃),showing a friction reduction of 47%and a wear reduction of 30%compared with the conventional phosphate esters lubricant(tricresyl phosphate,TCP).This is because crumpled ball potentiates synergistic lubrication effect within the boundary lubrication.Overall,we envision our designed self-dispersed MGCB has significant potential in tribological application at elevated temperature.
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