INTRODUCTION
Fretting refers to a very small relative displacement (in the order of microns, usually 5-50 microns) between two surfaces in contact with each other under a certain contact stress.1-3 Fretting fatigue is an accelerated failure process under the combined action of fretting wear and cyclic stress.4,5 In many practical engineering applications, such as the aerospace industry,6 the machinery industry,7the transportation industry8 etc, there are inevitable vibrations and small relative movement between different contact parts of components,4 the interaction causes cracks to easily occur at the contact points, which causes fatigue fracture of the material. The fretting fatigue process is essentially a combination of fretting wear and fatigue caused by the presence of contact stress and cyclic stress. Fretting plays a nonnegligible role in the initiation and propagation of cracks, leading to material failure in advance,9 therefore, fretting has a great impact on fatigue life. The current researches on fretting fatigue are mainly focused on the effect of different influencing factors on different materials, and it is generally believed that the factors that have a greater effect on fretting fatigue are contact stress, displacement amplitude, cyclic stress, coefficient of friction, material microstructure, temperature and humidity,10-12 such as C. Giummarra6 and Sachin R. Shinde,13 et al. studied the effects of different microstructures of different aluminum alloys on fretting fatigue life, Yang14 and Jin15 et al. reported that the effect of contact stress on the fretting fatigue process of different materials. In addition, many studies had also used the finite element method to simulate the stress and strain changes of the fretting process, analyzed the fretting fatigue mechanism on the basis of experiments, and had proposed some fretting fatigue life prediction models based on plain fatigue life models, wear damage, and crack initiation and propagation processes. 14,16,17 In the process of analyzing fretting fatigue, the fretting maps had a great effect on analyzing the fretting mechanism and fretting fatigue behavior prediction of materials. Z.R. Zhou18 et al. made a good summary of the types and development process of fretting map, and Sachin Shinde19 et al. also made a supplement to the development of fretting map.
35CrMoA steel is a medium carbon alloy structural steel that has both good hardenability and good toughness. It’s also featured in high static strength, high fatigue limit and excellent stress-rupture properties at high temperature. Therefore, it was widely used for important structural parts such as large cross-section gears, heavy-duty transmission shafts, and turbine generator main shafts.20 In recent years, due to the requirements of high-speed railways, 35CrMoA steel had been used to manufacture the axles of high-power electric locomotives. In actual operation, due to the close cooperation between this component and its tight fittings, composite fretting usually occurred at the contact parts. Therefore, the performance research of 35CrMoA steel must be carried out in accordance with the actual conditions. Currently, some studies had been done on the fretting fatigue of steels. For example, GH Farrahi21 et al. studied the fretting fatigue behavior of 316L stainless steel under the combination of bending and tensile load, and analyzed the effect of grain size in fretting fatigue process; Teng22 et al. investigated the high-cycle fretting fatigue behavior of GCr15 and performed 2D and 3D analysis of the fretting area morphology; Kozo Nakazawa23 et al. reported the effect of contact stress on austenitic stainless steel, and concluded that the effect of relatively low contact stress on the fretting fatigue life can be neglected, and at high contact stress, plain fatigue experiments performed before fretting fatigue test can increase fretting fatigue life. For 35CrMoA steel, there were limited studies referring its fretting fatigue performance. Liu24 et al. used experimental and finite element analysis methods to study the multiaxial fretting fatigue of 35CrMoA steel, and obtained that with the increase of contact stress, the fretting fatigue life tended to decrease first, then rose and decreased again at the next stage, and stress concentration occurs at the edge of the contact area; Tian25 and Lv26 et al. also investigated the multiaxial fretting fatigue of 35CrMoA steel, but levels of variables were quite limited in that study, which didn’t perform an integrated research on the fretting fatigue behavior of 35CrMoA steel. Moreover, the comparison of the strain response behavior under different paths was limited, the explanation of the fretting zone wear mechanism was incomplete, and the analysis of crack initiation and propagation was lacking.
In this paper, 35CrMoA steels were both used as the specimens and the fretting pads, and the contact form of the cylindrical surface was used to explore the multiaxial fretting fatigue behavior and mechanism of 35CrMoA steel under the square path (SP) and diamond path (DP). The research analyzed the strain response under different loading paths during fretting fatigue, compared the morphology of the fretting area under different contact stresses and different loading paths. The abrasive debris in slip region and stick region of the SP was examined to analyze the composition and describe the process of abrasive debris generation. Moreover, the fretting fatigue fracture morphology was observed to analyze fracture features of crack source zone (CSZ), crack propagation zone (CPZ) and instantaneous fracture zone (IFZ). The cracks’ initiation and propagation were also concluded from the fracture morphology.
EXPERIMENTAL PROCEDURES
The samples and fretting pads materials used in this experiment were 35CrMoA steel. The heat treatment processes used for fretting fatigue specimens and fretting pads were quenching and tempering. The specific treatment processes were: solution treated at 850℃ for 25min in a box type resistance furnace, quenched in oil, and followed immediately by aging at 550℃ for 1 hour, finally cooling in oil. The chemical composition and main mechanical properties of the 35CrMoA steel are shown in Tab.1 and Tab.2, respectively.
Table1 Chemical composition of 35CrMoA steel (wt %)