A Study On The Effects Of Residual Stress On Fretting Fatigue Of The Engine Block

Fretting is caused by small amplitude oscillatory motion between two contacting surfaces,which are nominal static.Two type of damage mechanism,i.e.,fretting fatigue and fretting wear,coexist during fretting.Fretting fatigue leads to more severe damage than fretting wear does.In the engine of vehicles,the engine block is the typical component.Residual stress affects fretting fatigue behavior of the engine block.Main researches in this paper are listed as follows:(1)The extended finite element equation,which includes residual stress and cohesive traction,is derived,and corresponding program is written based on the user-defined subroutine UEL of the finite element software ABAQUS.This program can be used to investigate the effects of residual stress on fretting fatigue crack initiation and crack growth.(2)The effects of residual stress on fretting fatigue are researched using the extended finite element method(XFEM)combined with the cyclic cohesive zone model(CCZM).It is shown that residual stress influences fretting fatigue crack growth path and enhances fretting fatigue life.Increasing bulk stress will partially offset the beneficial effects of residual stress on fretting fatigue life.The magnitude of load and residual stress have little effect on damage evolution law on condition that life is normalized.Increasing tangential force does not significantly change the crack initiation location and the crack growth orientation.However,As tangential force is increased,fretting fatigue life decreases.In addition,the decreasing extent of crack initiation life and total fatigue life due to residual stress declines with the increase of tangential force.Increasing normal force can increase the depth of knee point.However,normal force has little effect on fretting fatigue life.(3)The mechanical response of the engine block is studied under the condition of the practical loading case using the finite element method(FEM).It is shown that in the surface of engine block which contacts with the vicinity of the main bearing cap fillet,contact pressure,shear stress,and compressive stress that is parallel to contact surface and perpendicular to the direction of the axis of the main bearing hole fluctuate.The fluctuation is more obvious for the engine block without residual stress.(4)Based on the displacement field in the boundary of the concerned region obtained by FEM,the effects of residual stress on fretting fatigue is studied using XFEM combined with CCZM.It shows that for the engine block without and with residual stress,both crack initiation are located in the surface of engine block,which contacts with the vicinity of the left main bearing cap fillet.Once residual stress is included in the engine block,the crack initiation angle and the depth of knee point increase.However,residual stress has little on the growth orientation of normal crack.In addition,fretting fatigue life of engine block increases when residual stress is included.The method,XFEM including residual stress and cohesive traction,can not only be applied to the engine block,but also can be used in other engine components subjected to fretting fatigue damage.