Fig.2 Diagrams of fretting pad clamping device
  1. RESULTS AND DISCUSSION
  2. Strain response behavior
During the fretting fatigue process, due to the effect of alternating stress, the phenomenon of ”cyclic hardening” or ”cyclic softening” frequently occurred. The phenomenon of cyclic hardening or cyclic softening depended on the original state of the material, crystal structure, strain amplitude (stress amplitude), and the working environment of the material; At the micro level, it was related to the dislocation structure, dislocation density, and stacking fault energy in the material.27-29 G.Z.kANG et al.30found that 40Cr3MoV bainitic steel showed cyclic softening in uniaxial cyclic loading after certain cycles and no stable state existed if strain amplitude was higher than 0.9%. 35CrMoA steel also experienced cyclic softening during multiaxial composite fretting fatigue. Fig. 3 shows the equivalent strain response behavior of 35CrMoA steel under SP and DP with the equivalent stress of 400MPa. It can be clearly seen that in the first 300 cycles of the square path, the equivalent strain (εe) gradually increases with the same equivalent stress, different degrees of cyclic softening occur. When it reaches 300 cycles, the equivalent strain basically stabilizes in each condition, and the cyclic softening phenomenon disappears meanwhile. After the fretting fatigue cycle reaches 7000 cycles, the equivalent strain is slightly reduced, and the material is strengthened internally, and a lesser degree of cyclic hardening occurs; Under the DP, the material shows a continuous cyclic softening trend. As the number of fretting fatigue cycles increases, its cyclic softening rate gradually decreases, and cyclic hardening does not occur in the later stage of fretting fatigue, and as the contact stress increases the slower the cyclic softening rate decreases.