13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- [7]. The purpose of this research work is to extend this approach for partial slip fretting fatigue situations. This analysis also addresses the influence of the size domain over which the hydrostatic stress gradient is computed thus to establish the stability of this approach regarding the spatial contact stress resolution. pressure cyclic tangential loading + ( MPa) Σ ± heterogeneous stress distribution (severe stress gradients) ( MPa) Σ ± Fretting Loading Fatigue Loading quasi‐homogeneous stress gradient gradient of the hydrostatic stress Fatigue (smooth specimen) 0 MPa/µm Fatigue (notch, KT=2.35) 500 MPa/µm Fatigue‐Fatigue R= 40 mm Fretting 5000 MPa/mm h,max ∇σ X10 !! Fatigue Ø = 5mm Figure 1. Illustration of the stress condition characterizing the fretting fatigue loading. 2. Materials and experimental procedure 2.1. Materials The studied material is a tempered a 35 Ni Cr Mo 16 low alloyed steel displaying a tempered Martensitic structure. The original austenite grain size is about Ø =20 µm. The mechanical and fatigue properties of this steel, are summarized in the following table 1. Table 1. Mechanical and fatigue properties of the studied 35 Ni Cr Mo 16 low alloyed steel. E(MPa) ν σy0.2% (MPa) σu (MPa) σd (MPa) τd (MPa) ΔKth (MPa√m) 205000 0.3 950 1130 575 386 3.2 Ε: Young's modulus; ν: Young's modulus, σy0.2% : Yield stress (0.2%); σu: ultimate stress; σd : traction – compression fatigue limit (Rσ= σmin/σmax=-1 for 107 cycles); τ d : shear fatigue limite (Rτ=-1 for 10 7 cycles); ΔKth : long crack threshold (R=-1). Chromium 52100 steel was chosen for the cylindrical pads in order to maintain elastically similar conditions whilst simultaneously ensuring that cracks arose only in plane and fatigue 35NiCrMo16 specimens. Both plane and cylindrical pad surfaces were polished to achieved a small Ra=0.05 µm surface roughness.
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