13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- values as a function of the Hertzian contact radius for different contact pressures. A quasi constant evolution is observed which allows us to consider a constant friction value (i.e. µ=µPS=µt=0.8). 0 0.2 0.4 0.6 0.8 1 1.2 0 0.5 1 1.5 pmax = 600 MPa pmax= 800 MPa pmax = 1000 MPa Hertzian contact radius, a (mm) COF at the sliding transition, µt meanvalue: µt=0.8 * CN Q 0 5 10 15 20 25 30 35 40 45 200 220 240 260 280 300 tangential force amplitude, Q* (N/mm) projected crack length, (mean value) bp(µm) experimental value extrapolated bp (µm) cross section (a) (b) Figure 3. a) Evolution of the coefficient of friction (COF) at the sliding transition as a function of the contact radius and the maximum Hertzian pressure for the studied interfaces - b) Illustration of the destructive methodology used to identify the crack nucleation condition for plain fretting and fretting fatigue conditions. Table 2. Compilation of the studied test conditions (PF : plain fretting, FF: fretting fatigue) R (mm) W (mm) P (N/mm) σmax (MPa) Rσ pmax (MPa) a (mm) µt * CN Q (N/mm) a c k = e (mm) Y VM σ σ PF_1 20 3 353 0 0 800 0.28 0.8 186 0.58 - 0.97 PF_2 20 3 552 0 0 1000 0.35 0.8 218 0.71 - 1.05 PF_3 40 5 398 0 0 600 0.42 0.85 271 0.38 - 0.83 PF_4 40 5 707 0 0 800 0.56 0.88 287 0.70 - 0.85 PF_5 40 5 1100 0 0 1000 0.70 0.75 310 0.80 - 0.89 PF_6 80 8 795 0 0 600 0.84 0.79 305 0.72 - 0.62 PF_7 80 8 1414 0 0 800 1.12 0.78 399 0.80 - 0.71 PF_8 80 8 2209 0 0 1000 1.40 0.74 470 0.85 - 0.77 FF_1 80 8 795 100 0.1 600 0.84 - 273 0.75 0.020.69 FF_2 80 8 795 200 0.1 600 0.84 - 261 0.76 0.040.78 FF_3 80 8 795 400 0.1 600 0.84 - 137 0.88 0.080.87 FF_4 40 8 398 100 0.1 600 0.42 - 210 0.58 0.010.83 FF_5 40 8 398 200 0.1 600 0.42 - 192 0.62 0.020.90 FF_6 40 8 398 400 0.1 600 0.42 - 100 0.83 0.040.93 FF_7 80 8 795 200 1 600 0.84 - 300 0.72 - 0.80 FF_8 80 8 795 400 1 600 0.84 - 280 0.74 - 0.97 3.2. Crack nucleation analysis The cracking investigation consists in identifying the partial fretting loading (i.e. Q*<µP) inducing
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