Fretting Fatigue Behavior Of 6061 Aluminum Alloy Under Cyclic Contact Pressure

The phenomenon known as fretting fatigue occurs at the contact surfaces of two close-contacting components that have an oscillatory relative slip of small amplitude,and the relative slip is the consequence of cyclic loading of one or both of the components.The relative motion between contact surfaces could accelerate the forming and expanding of surface cracks and eventually induces that the components failure earlier than that under the condition of plain fatigue.This phenomenon occurs widely in industrial fields,such as,aeronautical industry,nuclear industry,railway transport,motor industry,chemical industry,electric power and so on,thus it is called the “cancer”.The process of fretting fatigue is very complex and contains a large number of parameters,most of which affect each other.Contact pressure is considered as one of the most parameters and large studies have been done in terms of the effect of contact pressure on fretting fatigue behavior under the condition of constant contact pressure.In fact,the contact pressure of the process of fretting fatigue in the industrial scene is constantly changing.At present,however,the study on effect of changing contact pressure on fretting fatigue behavior has not been reported.Thus,a new fretting fatigue test system based on the general fatigue testing machine is developed in this study,the contact pressure of which could be controlled in real time.Furthermore,6061 Aluminum alloy has high strength,low density,good formability and weld-ability and other advantages and is widely used in the manufacture of the aircraft body and auto parts.In this study,a series of fretting fatigue tests of 6061 aluminum alloy under constant and cyclic contact pressure were investigated.In addition,the effects of mean contact pressure,contact pressure amplitude and maximum contact pressure on fretting fatigue behavior of 6061 aluminum alloy are also discussed.Major experimental results are as follows:(1)Shear stress concentration at the junctions of stick region and slip region on contact surfaces of 6061 aluminum alloy specimens induces initiation of fretting fatigue cracks and fretting fatigue life is significantly lower than plain fatigue life at the same axial cyclic stress.(2)Under the conditions of constant contact pressure,fretting fatigue life of 6061 aluminum alloy reduces rapidly first with increasing of the contact pressure,then increases slowly and reaches a minimum at the contact pressure of 2.0 kN which is considered as the critical contact pressure.(3)Under the condition of cyclic contact pressure,the variable behavior of fretting fatigue life of 6061 aluminum alloy with increasing of the mean contact pressure is determined by the cyclic contact pressure amplitude.When cyclic contact pressure amplitude is low,the fretting fatigue life reduces rapidly first and then increases slowly with increasing of the mean contact pressure.While cyclic contact pressure amplitude is high,the fretting fatigue life constantly increases with increasing of the mean contact pressure.In addition,the critical mean contact pressure under the condition of cyclic contact pressure also exists and is lower than the critical contact pressure under the condition of constant contact pressure.(4)When the mean contact pressure is lower 2.0 kN,the fretting fatigue life under the condition of cyclic contact pressure is significantly lower than that under the condition of constant contact pressure at the same mean contact pressure.while the mean contact pressure is higher 2.0 kN,the fretting fatigue life under the condition of cyclic contact pressure is not lower than that under the condition of constant contact pressure.Furthermore,mean contact pressure effect is not significant when maximum contact pressure is used as correlated parameter.(5)Over consideration of the fretting fatigue test and the finite element analysis result,it can be concluded that the increasing of contact pressure is detrimental to fretting fatigue life due to the increasing of maximum shear stress under the conditions of low contact pressures,while the increasing of contact pressure to fretting fatigue life due to the increasing of the width of stick regions on both sides of contact surfaces under the conditions of low contact pressures.