Study On Torsional Fretting Fatigue Behavior And Damge Mechanism Of Two Metal Materials

Failure phenomena of fretting fatigue widely exists in modern industrial fields,such as aerospace,machinery,transportation,nuclear power and so on.Fretting damage becomes the main reason for the failure and fracture of many key components.A large number of engineering failure analysis proved that the fatigue life of structure is greatly reduced coused by the fretting damage.The early failure of the structure leads to catastrophic accidents and caused casualties and property losses.According to the load of the component,the fretting fatigue can be divided into three modes: the tension-compression(or tension-tension)fretting fatigue;the bending fretting fatigue and the torsional fretting fatigue.However,the fretting fatigue in the components of actual working conditions is often the mutual coupling of the three.At present,the researcher mostly focused on the tesion-compression fretting fatigue and the bending fretting fatigue in the field of fretting fatigue,and the reports for the torsional fretting fatigue were less.Therefore,the systematic study of torsional fretting fatigue behavior is not only to recognize and reverse the damage mechanism of torsional fretting fatigue,but also to improve fretting tribology.The theory has important scientific significance and can be used to anti-failure of torsional fretting fatigue damage in the engineering application.In this study,a pneumatic device is designed based on multi-axis fatigue testing machine which can provied normal loading.The tests adopted with the point contact configurations of cylinder/cylindrical vertical cross.Under different torques,nomal contact loads and cycle numbers,the torsional fretting fatigue tests of 316 L austenitic stainless steel and LZ50 steel has been carried out.The S~N curves of plain and fretting fatigue for two materials were set up accordingly.Base on this,micro-analysis metohods of optical microscope(OM),scanning electron microscope(SEM),electron energy disperse spectroscopy(EDX),X-ray photoelectron spectrometer(XPS),electron probe microscope(EPMA),transmission electron microscope(TEM)and white light interference surface profile-meter were used to analyze the fatigue fracture surface,fretting damage zone,cross-section and microstructure of the torsonal fretting fatigue.The torsional fretting fatigue behavior and damage mechanism of two typical metal materials are systematically studied.The main conclusions are as follows:(1)Due to the fretting actions,the torsional fretting fatigue life was much shorter than the plain fatigue which might decrease to 20%~80%,or even lower.The amplitude of torque has a significant effect on the torsional fretting fatigue life.With increasing torque load,the fretting fatigue life of materials showed decreased firstly,and then increased,finally decreased again.Corresponding to the fretting behavior of torsional fretting fatigue,the S-N curve can be divided into three areas: the partial slip zone(PSR),the mixed zone(MFR)and the slip zone(SR).In the partial slip zone,the fretting damage is slight and fatigue life is long.In the maxed zone,the fretting crack is easy to initiation and propagation,so the fatigue life is the shortest.In the slip zone,the wear rate of the material is higher than the nucleation rate of micro-crack,so the fretting fatigue life can be prolonged.When the amplitude of torque is higher enough,the fretting damage action of the materials were diminished,the fatigue damage of materials should play a leading role.Thus,the fretting fatigue life of materials decreased with the increase of torque again.(2)The main fracture types of torsional fretting fatigue were 45° slopes normal stress fracture.The fretting fatigue cracks tended to occur in fretting damage zone,which extend along the sample surface with 45° to the axis of the sample,and the final fracture was 45° slopes.Under the torsioanl load,the unit of fatigue specimen obtains the maximum normal stress on the inclined section of an angle of 45°.For the material,when the shear strength is higher than the tensile strength,the failure is caused by the outer surface anlong the axis of sample into spiral 45° and fatigue fracture is tensile rupture to 45° slopes.(3)In the mixed zone,the wear mechanisms of the torsional fretting damage zone were abrasive wear,oxidative wear and delamination.In the initial stage of torsional fretting fatigue,the damage of fretting zone is slight,some oxide debris and ploughing grooves were found in the wear zone.The damage zone formed a ring state due to elasic deformation regulation.With the increase of the cycles,the damage of fretting zone was aggravated.Some spalling and wear debris accumulation phenomenon were found in the wear zone.The wear mechanisms of fretting damage zone were abrasive wear,oxidatve wear and delamination.No surface crack and fatigue crack initiated in this stage.With the further increase of the cycles,the damage of fretting zone was further aggravated.On the side of the damage zone near the loading end,micor-cracks can be found on the surface and profile of the specimen.The wear degree of the damage zone further aggravated.In the final stage of torsional fretting fatigue,a visible fatigue crack can be found in the damage zone.The wear degree of the damage zone continues to aggravated,and the wear was more serious.At this stage,the wear mechanisms were still mainly by abrasive wear,oxidation wear and delamination.(4)Under the same normal load and with the increase of torque,the friction coefficient of the final stable value was increased.In the torsional fretting fatigue tests,the friction coefficient vriation trends were consistent at different torque.The greater of torque amplitude,the faster of friction coefficient reaches a maximum value and enters to a relatively smooth phase.Meanwhile,the friction coefficient has the same trend with the torsion angle.Under the different normal loads,the friction coefficient curve changes.By the increasing of the normal load,the relatively stabel of the friction coefficient was small,and the friction coefficient curve was more smooth.(5)The microstructure evolution in damage zone of torsional fretting fatigue was closely related to the microstructure of the materials.The TEM analysis indicated that for the austenitic stainless steel with FCC structure,the evolution of dislocation configuration changes from the twinning deformation mechanism into dislocation cell deformation mechanism with the increase of load.But for the carbon steel with BCC structure,the evolution of dislocation configuration was mainly the tangles,accumulation and climb of dislocation.Finally,the evolution of dislocation configuration mechanism was the formation and deformation of dislocation cell.