| Disc brake has become the primary braking applications by replacing brake drumnowadays. Because it possesses many advantages, such as simple structure, light weight, easyadjustment. Moreover, it can also perform larger braking momental with steady characteristicof rubbing and better wear-resistance. During braking action, brake disc not just enduresbraking friction force and braking pressure, but also undergoes centrifugal force and thermalstress as a key component of disc brake. Therefore, its working condition belongs to typicalthermal-mechanical coupling issue. Shown by some researchs, most kinetic energy isconverted into heat in single stopping, and large thermal gradient and thermal stress wereproduced at rubbing surface. Frequent braking which results in cyclic thermal stress leads tothermal-mechanical failure of brake disc by producing crack initiation and propagation.Currently, improving thermal-mechanical property of brake disc has become one key and hotissue for brake dic researchers. Gray cast iron can satisfied the braking requirement of vehicleand it has been widely applied to car industry, because of high strength, better wear resistance,and good thermal conductivity, meanwhile, the final cost of manufacturing and processing thiskind material is inexpensive. While, nowadays, with the development of high speed and heavyduty of vehicle, brake disc undergoes hard working conditions more and more which acceleratethermal-mechanical failure of brake disc. Traditional methods that addressed to figure out howto improve the fatigue performance of brake disc were mainly focused on adding alloys intowhole matrix material or changing body structure of brke disc. However, few of these methodsconsidered this problem by synthesizing surface morphology, structure, and chemicalcomponent. Therefore, these methods often result worse performance and need highinvestment, and could affect other characteristics of brake disc. Nature has already providedthe most efficient and economical solution to the hardest issue that troubles brake disc in itsthermal-mechanical fatigue performance. Many creatures that possess the most ingeniousmorphology, coupling structure, and economical materials of multi-phase, perform remarkablecapabilities in resisting crack and anti-fatigue properties, such as shell, dragonfly, and lamina.Therefore, novel idea, creative methods, and developed technic could be inspired by studyingcoupling function of surface morphology, structure, and materials of creatures, which could beutilized to improve thermal-mechanical properties of brake disc.In this article, temperature field, stress field and elastic/plastic strain of brake disc werecalculated by thermal-mechanical calculation. With this numerical result, the mechanism ofappearing cracks on rubbing surface of brake disc was cleared and was discussed. Based onfatigue characteristic of lamina and the wings of dragonfly, and other creatures, couplingbionic surfaces of multi-factors were fabricated by laser alloying with considering surface morphology, local structure, and material composition. Through classical test of crackpropagation, the function of bionic coupling surface in resisting crack propagation was verifiedby investigating relationship between crack length and fatigue cycles. With the help of self-retrain equipment, thermal fatigue resistance of bionic coupling surface was investigated, andmechanism of bionic unit in resisting crack initiation and propagation were revealed. Finally,temperature field and stress field of coupling bionic brake disc were calculated by threedimensional finite element analysis, meanwhile, the fracture properties were also calculated.The function of coupling bionic surface in retarding crack propagation and thermal fatigueresistance were verified and were cleared.Results showed that, under initial braking velocity of60km/h,120km/h, and180km/h, themaximum temperatures of brake disc reached129.7℃,241.6℃, and357.9℃, respectively, andthe maximum equivalent stress were122Mpa,245Mpa, and362Mpa at the place wheremaximum temperature appeared. During braking action, there was an anticlockwise distortionaround the normal of cross section in radial direction, and there was an anticlockwise distortionaround radius in circumference direction. While, during cooling action, the distortions changedto opposite clockwise direction. Both the plastic strain and residual tension stress incircumference direction are larger than that in radial direction, which lead to produce radialcrack on brake disc surface.Laser alloying technique was choosen to fabricated strip-type, grid-type, and pin-typebionic units on cast iron surface according to the anti-fatigue parameters of coupling structureof creatures. There were different depths of bionic units including0.5mm,1.0mm, and1.5mm,and angles between bionic unit and crack propagation have90°,60°, and45°. In order to getbest results of producing bionic unit, parameters of laser processing were optimized, and theywere electricity165~175A, pulse width3ms, frequency20Hz, scanning speed1.5mm/s, energyof single pulse18~20J.Strip-type, pin-type, and grid-type of coupling bionic samples were choosen to verifiedtheir mechanical fatigue properties. Results showed that all coupling bionic samples withdifferent kinds of bioni unit had better performance in resisting mechanical crack propagationthan normal samples. Meanwhile, strip-type bionic unit showed best performance of fatigueresistance. Among different distribution of bionic units, fatigue resistance of coupling bionicsamples performed best when the angle between bionic unit and crack propagation is90°.Between0°and90°, the performance gets better as the angle increases. Among different sizesof bionic units, the capability of resisting crack of bionic unit performed better when the size ofbionic unit was enlarged.Strip-type bionic unit was choosen to be verified its thermal fatigue resistance. Duringfatigue test, there were few defects appeared in bionic unit which lead to few cracks initiation.Under cyclic thermal stress, binonic unit performed better in resisting plastic deformationwhich indicate that bionic unit could resist crack initiatiation and propagation. Meanwhile,bionic unit was set as hard phase on the surface of materials, because its nanohardness was much larger than that of cast iron. Therefore, propagation rate and path of crack were modifiedby bionic unit with the result of improving fatigue life.With comparing the results from three dimensional computer simulation, it was found thatcoupling bionic brake disc has better capability of heat dissipation which could reducemaximum temperature and modify temperature field. According to the crack’s dimension ofbrake disc surface, different crack models were established in order to simulate the progress ofcrack propagation. By comparing the results of displacement field, equivalent stress, and Jintegral of crack front, the function of coupling bionic surface in resisting the propagation ofsurface crack, and the function of strengthing surface were verified. The results showed thatcoupling bionic surface can reduce effectively opening displacement of crack, and reduce Jintegral which meant the propagation rate of crack was reduced. |
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