Fig.2 Diagrams of fretting pad clamping device
- RESULTS AND DISCUSSION
- 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.