A fracture mechanics approach to prediction of pitting under fretting fatigue conditions

Within the framework of the calculational model for investigation of fatigue fracture and estimation of durability of bodies contacting under fretting fatigue conditions, the pitting formation in these bodies has been simulated and investigated. It is assumed, that a pit is generated by a surface macrocrack, which previously appeared on the contact zone, and developed due to its cyclic filling and wedging by fretting products. It is also assumed, that the crack propagates by normal opening mechanism (Mode I fracture). On the basis of the stress intensity factors analysis it has been established, that the surface cracks located near the contact zone edges and oriented to an unloaded body surface are most favorable for formation of pitting. For these locations the crack growth paths have been constructed. It is established, that the pressure of fretting products on the crack faces is greater and that the crack inclination angle to body surface is less, the pitting of a body surface is faster (paths turn more sharp to the surface). The pitting process is also intensitied with increase of the friction factor between contacting bodies. Residual durability was evaluated by the time of pit formation in the near-surface layer of TC-5 titanium turbine alloy. The theoretical results on pitting formation correlate with experimental data received on TC-5 samples and also with known Waterhouse observations.