Current Issue : October - December Volume : 2020 Issue Number : 4 Articles : 5 Articles
The current research reports the improvement in surface integrity and tribological\ncharacteristics of steel prepared using a powder metallurgy (PM) by ultrasonic nanocrystal surface\nmodification (UNSM) at 25 and 300................
This paper aims to provide a new approach to predict the friction resistance of slurry pipe jacking. Friction force usually\nconstitutes the main component of jacking force. It can be calculated by multiplying an effective friction coefficient and the normal\nforce acting on the external surface of the pipe. This effective friction coefficient is introduced to reflect the effect of contact state of\npipe soil slurry, highly affected by the effect of lubrication and the interaction of pipe soil slurry. Firstly, by making some\nreasonable assumptions, the analytical formula of the effective friction coefficient is obtained, in which the critical quantity of\ncontact (contact angle or width) is calculated by using the Persson contact model. Then, the analytical formula of normal force of\ncircular pipeline is derived, which needs to determine the vertical soil pressure. To allow for a better prediction, three typical silo\nmodels are introduced and compared. Finally, a method for calculating the friction resistance of slurry pipe jacking is established.\nThe main difference from the existing method is that this method takes into full consideration the influence of lubrication, soil\nproperties (such as internal friction angle, cohesion, and void ratio), and design parameters (such as buried depth, overcut, and\npipe diameter). By using reasonable silo models, the predicted results are in good agreement with the measured values collected\nfrom 10 in situ cases, which proves that the new approach can provide accuracy prediction of friction resistance for slurry pipe\njacking with various soil conditions, and it may help for better future design and less construction costs....
In this paper, a concurrent multiscale simulation strategy coupling atomistic and continuum\nmodels was proposed to investigate the three-dimensional contact responses of aluminum single\ncrystal under both dry and lubricated conditions. The Hertz contact is performed by using both\nthe multiscale and full molecular dynamics (MD) simulations for validation. From the contact area,\nkinetic energy and stress continuity aspects, the multiscale model shows good accuracy. It can also\nsave at least five times the computational time compared with the full MD simulations for the same\ndomain size. Furthermore, the results of lubricated contact show that the lubricant molecules could\neffectively cover the contact surfaces; thereby separating the aluminum surfaces and bearing the\nsupport loads. Moreover, the surface topography could be protected by the thin film formed by the\nlubricant molecules. It has been found that the contact area decreases obviously with increasing the\nmagnitude of load under both dry and lubricated contacts. Besides, a decrease in contact area is\nalso seen when the number of lubricant molecules increases. The present study has confirmed that\nthe dimension of lubricated contacts could be greatly expanded during the simulation using the\nproposed multiscale method without sacrificing too much computational time and accuracy....
Ball burnishing appears to be a very promising approach for reducing surface height,\ngenerating compressive residual stresses and increasing hardness. Ball burnishing treatment was\ncarried out using a Haas CNC Vertical Mill Center VF-1 equipped with the Ecoroll burnishing\nsystem. After burnishing, surface topographies of machined samples and hardness were measured.\nWear tests were conducted using a ball-on-disc tribotester in dry sliding conditions. During tests,\nthe friction force was monitored as a function of time. After tests, wear volumes were determined\non the basis of surface texture measurements. Tests revealed that ball burnishing in most cases\nresulted in minimizing friction and wear of contacting elements....
Experimental studies reveal that the simultaneous addition of zinc dialkyl dithiophosphates\n(ZDDPs) and multi-wall carbon nanotubes (MWCNTs) to a poly-alpha-olefin base oil strongly reduces\nwear. In this paper, it is shown that MWCNTs promote the formation of an anti-wear (AW) layer on\nthe metal surface that is much thicker than what ZDDPs can create as a sole additive. More importantly,\nthe nanotubesâ?? action is indirect, i.e., MWCNTs neither mechanically nor structurally strengthen\nthe AW film. A new mechanism for this effect is also proposed, which is supported by detailed\ntribometer results, friction track 3D-topography measurements, electron diffraction spectroscopy\n(EDS), and Raman spectroscopy. In this mechanism, MWCNTs mediate the transfer of both thermal\nand electric energy released on the metal surface in the friction process. As a result, this energy\npenetrates more deeply into the oil volume, thus extending the spatial range of tribochemical reactions\ninvolving ZDDPs....
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