| Fretting fatigue damage usually occurs in fastening components in the fields of aerospace,automotive,machinery,rail transportation,electric power and other fields,accelerating cracks initiation and propagation in the fretting damage region and shortening greatly the service life of components.Torsional fretting fatigue is one of the tipical forms of fretting fatigue,scholars have not extensively studied about torsional fretting fatigue at home and abroad,but the torsional fretting fatigue phenomenon exists in the important components,such as the axle and motor shaft in the rail transportation,7075 aluminum alloy can show the damage mechanism because of its preferable damage sensitivity.The torsional fretting fatigue test and the conventional torsional fatigue test of 7075 aluminum alloy were carried out on the indepently designed fretting fatigue test rig based on the multi-axis fatigue testing machine.The damage characteristics of the torsional fretting fatigue and failure mechanisms of the conventional torsional fatigue and the torsional fretting fatigue were obtained based on the analytical equipments such as 3-D optical microscope,stereomicroscope,scanning electron microscopy,electron probe microanalysis.The main conclusions are as follows.(1)The conventional torsional fatigue life of 7075 aluminum alloy decreased with the increase of torsional fatigue stress,while the torsional fretting fatigue life showed a trend of decreasing first,then increasing and finally decreasing again.Namely,there was a characteristic of“?”curve about S-N curve of torsional fretting fatigue.(2)The fracture type of the conventional torsion fatigue specimens was transverse shear fracture,while the fracture type of the fretting torsional fatigue specimens was normal stress fracture.The difference of fracture types was caused by the different range of torsional fatigue stress in the two kinds of experiment,instead of the effect of fretting.The torsional fatigue cracks located the surface whicle the torsional fretting fatigue cracks located in the subsurface at the edge of wear scar between the load end and the central position.(3)Under the same normal load,the steady-state value of friction coefficient increased with the increase of torsional fatigue stress.In the fretting mixed regime,the displacement amplitude near the loading end is larger,the wear and oxidation of the loading end was far more serious than the central position and the fixed end,the laocal contact fatige is intense,it is easy to initiate the cracks and leads to the shorter fatigue life.In the partial slip regime,the wear and oxidation of the different fretting damage area were relatively uniform,the fretting wear was main damge mode and there is longer fatigue life.In the slip regime,similar with the fretting mixed regime,but the wear of the surface is relatively servere,surface spalling and wear eliminated microcracks formed on the surface,the fatigue life is higher than the fretting mixed regime.(4)The mechanisms of torsional fretting damage were fatigue wear,oxidation wear and abrasive wear.Torsional fretting fatigue cracks nucleated on both sides of the wear scar section,it was mainly controlled by the normal contact stress at the cracks initiation stage and the angle between the crack propagation direction and the contact surface was 30 degrees.Then the angle was 60 degrees under the control of normal contact stress and torsional fatigue stress in the transition period of cracks propagation.And then the cracks propagation was mainly affected by the torsional fatigue stress when the propagation depth has exceeded the Hertz contact depth,Finally,two main cracks of the section vertically intersected in the process of cracks further propagation,the main crack after intersection continued propagating perpendicularly to the contact surface until the test sample was fratured... |
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