The fretting fatigue life gradually decreases with increasing contact stress under SP and DP. The study by Liu et al.24showed multiaxial fretting fatigue life of 35CrMoA steel had a trend of decreasing first, then increasing and finally decreasing with increasing contact stress. With the same contact stress range, the same trend of fretting fatigue life was achieved in this paper. Compared with plain fatigue, the contact stress was added between the fretting pad and the specimen during the fretting fatigue, which caused the fretting wear on the surface of the specimen. The larger the contact stress, the more severe the surface damage of the specimen, and the stress concentration was more obvious in this situation, which accelerated the initiation and propagation of microcracks and reduced the fretting fatigue life.23,31 As the contact stress of 35CrMoA steel increased, the equivalent strain also increased, indicating that the sample had undergone severe plastic deformation during the fretting fatigue process. At this situation, serious fretting wear occurred, so the greater the contact stress, the more serious the surface damage of the sample, the more obvious the stress concentration phenomenon, which led to a faster crack initiation rate, and greater plasticity also accelerated crack propagation; In addition, at the beginning of fretting fatigue, there was adhesive wear between the specimen and the fretting pad. When the adhesive part was torn due to fretting, fine metal debris was generated.32 This process gone on and the debris continued to increase, which caused the change of fretting wear from adhesive wear to abrasive wear. After the fretting surface was wiped by the abrasive wear, the surface microcracks may be erased, thereby reducing the source of cracks and improving the fretting fatigue life.33,34 The greater the contact stress, the lower the relative displacement between the specimen and the fretting pad, which reduced the wiping effect of abrasive wear on microcracks, resulting in a low fretting fatigue life. Combining the above two reasons, the fretting fatigue life of 35CrMoA steel under SP and DP showed a downward trend with increasing contact stress. Under the same contact stress and equivalent stress, the fretting fatigue life of DP was slightly lower than the fretting fatigue life of SP. The fretting fatigue damage of the 35CrMoA steel was greater under DP.
Analysis of fretting region