The Modeling And Application Of Elasto-plastic Contact For Two Rough Surfaces

This thesis mainly adopts the finite element method(FEM)to investigate some basic problem in contact mechanics,and then applies the theoretical research into the emergency brake process of elevator safety gear.Main work of this thesis is summarized as follows:Firstly,focuses on quasi-static contact problem in contact mechanics,this thesis proposed a new quasi-static criterion which based on the fluctuation of strain energy.During the loading stage,if the curve of strain energy versus loading displacement is monotonously increasing and its first derivative is smooth without bulging or multi-peak behavior,then the dynamic effects in the system can be negligible.By the evaluation of two widely used model in contact mechanics,the sliding interaction between two hemispherical asperities and Hertz contact,it is found that the proposed criterion is highly effective.Secondly,a three-dimensional(3D)model of two identical elastic-plastic hemispherical asperities in contact was developed which characterizes the initial contact offset with polar angle ? and azimuthal angle ?.The FEM simulations show that the adhesive COF is mainly dependent on interfacial shear strength.However,the plowing COF is determined by effective contact interference,which reflects the combined effects of ? and ?.Moreover,the maximum elastic rebound force during the unloading phase is mainly dependent on polar angle.In addition,the computation efficiency for the models in this section was improved greatly by using the proposed quasi-static criterion.Thirdly,numerical investigation of normal contact between two rough surfaces(two-rough-model)and its equivalent single rough surface with rigid flat(single-rough-model),proposed by Greenwood and Tripp(GT model),was done and extended to sliding contact.By means of Gaussian rough surface,filtering method and FEM,the two-rough-model and the equivalent single-rough-model were perfectly realized.Then the contact parameters and field variables of two model during normal and tangential sliding contact were compared.Lastly,the proposed approach of rough surface construction was applied in studying the emergency brake process of safety gear.The two rough surfaces of friction pad and rail was constructed by fractal function,and correspondingly,the fully coupled thermo-mechanical sliding contact model was developed.And then the contact area,frictional force and COF as well as some field variables like contact pressure,temperature,stress and plastic strain were discussed.