Study On Bending Fretting Fatigue Damages Of DZ2 Axle Steel

The bending fretting fatigue usually results in premature failure of mechanical parts and significant reduction of their service life.Therefore,the bending fretting fatigue causes many potential safety hazards in various industrial fields and even major catastrophic accidents,which seriously threatens the safety of the human and affects the economic benefits of enterprises.The axle is a key component in the high-speed railway system,and the bending fretting fatigue mode is one of its operating modes.It is therefore very important to carry out a study on the bending fretting fatigue of the axle steel of the high-speed train independently developed in China.The research results can reveal the damage mechanism of the bending fretting fatigue for the axle steel,improve the theoretical research in the fretting fatigue field,and provide a theoretical data support and a reference for practical engineering applications.In this theses,the materials for the axle of a free gauge high-speed train in China(DZ2 axle steels)were studied.The bending fretting fatigue tests under different test parameters were carried out on the self-designed bending fretting fatigue tests platform,and the S～N curves and the bending fretting fatigue life curves under different normal loads were established.Furthermore,the detailed microscopic analysis on the damage area of all tested samples was performed by combining the optical microscope(OM),scanning electron microscope(SEM),white light interferometer(WLI),energy spectrum analyzer(EDS),electron probe micro-analyzer(EPMA)and X-ray photoelectron spectrometer(XPS).The bending fretting fatigue damage mechanism of DZ2 axle steel material was then revealed.The main conclusions are shown as following:(1)With the increase of bending fatigue stress,the bending fretting fatigue life of DZ2 axle steels decreases firstly,then increases and finally decreases.And the fretting running regime firstly runs in the partial slip regime(PSR),then gradually changes into the mixed fretting regime(MFR),and finally transits to the gross slip regime(SR).(2)Among three fretting running regimes of the S～N curves,the bending fretting fatigue life of the DZ2 axle steels is relatively shortest in the mixed fretting regime.In this regime,the micro-cracks are prone to initiate and propagate,and the damage degree is the most serious.Hence,the mixed fretting regime should be avoided as much as possible in practical engineering applications.(3)The shape of the damaged area of the contact surface looks like an irregular ellipse.The damage degree at the load-end is the highest,followed by that at the fixed end,with the lightest in the center of the contact area.All fatigue cracks in damaged area are originated in the sub-surface(about 60-100 μm deep)near the load-end of the damaged area.(4)As the normal load increases,the bending fretting fatigue life of the DZ2 axle steels firstly decreases,then increases,and finally tends to stable.The effect of the normal load on the bending fretting fatigue life gradually decreases.