A Dual-scale Simulation On Fretting Fatigue With Surface To Surface Contact Of Ultrafine Grained Aluminum Alloy

Under the background of increasingly severe energy and environmental problems,as a lightweight material,ultra-fine grained aluminum alloy has rapidly entered various fields such as aviation,automobile,machinery manufacturing and so on;As one of the traditional means to realize the strengthening and toughening of materials,grain refinement has shown great potential in improving the service life of materials.With the wide application of ultra-fine grained aluminum alloy in some practical engineering fields,such as bolts,riveted joints,keyway couplings and bearings,the fretting fatigue failure caused by a relative motion with minimal amplitude between the contact surfaces of mechanical components will greatly reduce the service life of components.Therefore,in order to ensure the service safety of aluminum alloy,it is very necessary and urgent to understand and master its fretting fatigue mechanical behavior and crack initiation characteristics.Based on the structural characteristics of fretting fatigue test specimen,two different finite element models,solid model and equivalent model,are established to simulate and analyze the displacement,deformation,stress state and contact state of contact surface under fretting fatigue condition,The critical interface theory is used to predict the initiation location,initial direction and initiation life of fretting fatigue crack.The main research work and achievements of this thesis are as follows:1.In order to simulate fretting fatigue efficiently and accurately,two sets of fretting fatigue geometric models were established by using the two scale analysis method of Chaboche viscoplasticity theory and crystal plasticity theory(polycrystalline aggregate sub model),through the identification of constitutive parameters based on the experimental data of ultra-fine grained aluminum alloy under cyclic loading and the control of mesh accuracy based on Ciaveraella contact theory,a finite element model which can describe the micro motion contact mechanical response of ultra-fine grained aluminum alloy is developed.2.The numerical simulation results based on Chaboche viscoplastic theory show that the stresses and equivalent plastic strains in X and Y directions on the micro-contact surface increase with the increase of normal load,and their peaks appear at the contact edge.The presence of a mixture of adhesion and slip in the contact area is the cause of severe fretting wear under high normal loads.The analysis of stress triaxiality shows that the values of stress triaxiality in the contact area and in most areas under contact are close to 0,which is consistent with the elongated dimples observed at the fatigue fracture and the shear tear morphology.The numerical simulation results based on crystal plasticity theory(polycrystalline aggregate sub-model)show that with the increase of normal load,the micro-strain inhomogeneity of surface grains becomes more serious,and the distribution of accumulated shear strain in polycrystals presents a "ring shape".The grains at the contact front edge mainly bear the main plastic deformation and stress concentration effects.The stress state in the grains between the contact area and the opening area is significantly different.The maximum distribution of the Generalized Schmidt Factor becomes more dispersed with increasing normal load,which is one of the reasons for the phenomenon of "Polarization" in the distribution of surface grain deformation and stress.3.Based on the critical interface theory,the characteristics of crack initiation in surface contact fretting fatigue of ultra-fine grained aluminum alloy are studied.The prediction results of macro and micro theoretical models show that the surface contact fretting fatigue crack is easy to initiate at the contact front edge,and under different normal loads,the crack initiation angle is 40 °? 50 °;The prediction results of FIP are in accordance with the experimental results,and the predicted crack initiation life under higher normal load is more conservative.It is proved that the prediction of fretting fatigue crack initiation based on critical interface theory is reliable and reasonable.