Experimental Study Of Several Metal Materials On Bending Fretting Fatigue

Failure phenomena of bending fretting fatigue broadly occur in the modern industrial fields, such as railway wheel-axles, electrical motor shaft and pinion shaft of train traction motors, aircraft engines, overhead conductors, and so on. So far, the bending fretting fatigue, which may lead to the failure of key parts or even cause disastrous accidents, has been rarely studied, and the understanding of its failure mechanism is still not systematical and thorough. Thus, a systematical study on the running behavior and damage characteristics of bending fretting fatigue not only has a great scientific significance to reveal the fretting fatigue damage mechanism and to improve the fretting theory, but also offers some engineering guides for practical applications of anti-failure of bending fretting fatigue.On a self-designed high-precision bending fretting fatigue test rig with the contact configurations of point and line, the bending fretting fatigue tests of316L austenitic stainless steel,7075aluminum alloy, LZ50steel and17CrNiMo6steel has been carried out under different bending loads, normal contact loads and cycle numbers. The S-N curves of conventional and fretting fatigue for non-identical materials were set up accordingly. Base on this, micro-analysis methods of optical microscope(OM), scanning electron microscopy(SEM), electron energy disperse spectroscopy(EDS), transmission electron microscope(TEM) and surface profile-meter were used to analyze the fracture surface, fretting damage zone, cross-section and microstructure of the bending fretting fatigue. The damage mechanisms of bending fretting fatigue for the four materials have been investigated systematically. The main obtained conclusions are summarized as follows:(1) The bending fretting fatigue life was much shorter than that of plain fatigue due to the fretting actions, which might decrease to30%, or even lower. The life of bending fretting fatigue was significantly influenced by the alternating stresses. The higher stress level was, the lower fatigue life occurred. The S-N curves of bending fretting fatigue appeared shapes of C type. The nose-shaped part of the C-curve was always corresponding to the mixed fretting regime, where the cracks were easy to initiate and propagate, and the life was the lowest. The partial slip regime located beneath the C-curve, where the fretting damage was slight and the life was longer. Above the C-curve, the slip regime presented higher wear rate, where the surface crack nucleuses were wiped off, resulting in lengthened the life.(2) The wear mechanisms of the fretting damage zone were abrasive wear, oxidative wear and delamination, and the evolution of fretting damage can be summarized as follows: a) In the initial stage, the damage zone exhibited the annular morphology, and some ploughing grooves, few oxidative debris particles and no crack were found in the wear zone, when the wear mechanisms were slight abrasive and oxidative wear. b) With the increase of the cycles, the damage was aggravated and the delamination also occurred. Also, no surface crack and fatigue crack initiated in this stage. However, the wear mechanisms of fretting damage zone were changed to abrasive wear, oxidative wear and delamination. c) The damage was further aggravated with the further increase of the cycles. The surface incline cracks and cross-section micro-cracks have been formed. The wear mechanisms has been never changed during this stage, d) Finally, the subsurface fatigue crack propagated along a direction with a small angle to the contact surface and linked to the surface incline crack at the another side. The wear mechanisms of this stage were abrasive wear, oxidative wear and delamination accompanied with cracking.(3) Different from the plain bending fatigue, the bending fretting fatigue cracks initiated at the subsurface usually accompanied with some impurity particles or second phases. The main influencing factors of the bending fretting fatigue process included:material properties, cyclic loads, contact stresses, contact geometries, loading velocity (frequency), and so on.(4) The fretting fatigue crack propagation can be divided into3stages:Stage Ⅰ—the direction of crack propagation was at a certain angle (～60°) to the contact surface which controlled by the contact stresses, and the main crack linked to the surface incline cracks which propagated alone; Stage Ⅱ—the crack propagation was both dominated by the contact stress and plain bending fatigue stress, and the propagation angle switched towards the direction vertical to the contact surface; Stage Ⅲ—the crack propagation was controlled only by the plain bending fatigue stress, and the propagation direction was perpendicular to the contact surface, its propagation behavior matched the plain fatigue.(5) The microstructure and sub-structure of fretting fatigue damage zone was close related to the original microstructure of the materials. The TEM analysis indicated that there were two types of microstructure and sub-structure evolution laws. For the steels with BCC structure and aluminum alloys with FCC structure and high-level stacking fault energy, the nucleation of fretting fatigue cracks accorded the mechanism of dislocation cell deformation. However, for the austenitic stainless steel with low-level stacking fault energy, its nucleation of fretting fatigue cracks deferred to the twinning mechanism.