13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 2.2. Test conditions As illustrated in Figure 3, two different test apparatuses were involved to quantify respectively the fretting and the fatigue influences in cracking processes. Plain Fretting ‐ Friction law (µ=0.8) ‐crack nucleation analysis under plain fretting stressing ‐crack nucleation and crack propagation analysis under Fretting Fatigue loadings. Fretting Fatigue (double actuator system) P Q R Steel : 35NCD16 [1&2] Counter body : 52100 steel Studied Contact (2D Cylinder/Plane) fretting loading (RQ* = ‐1) σ P Q R Ball bearing fatigue loading (Rσ= ‐1 to 1) fretting loading (RQ* = ‐1) Fretting Fatigue σu = 1130 MPa σ y0.2% = 950 MPa σd(‐1) = 575 MPa τd(‐1) = 386 MPa ΔKth= 3.2 MPa√m Contact under partial slip condition P (N/mm) : linear normal force Q (N/mm) : linear tangential force δ(µm) : displacement σ(MPa) : fatigue stressing 35NCD16 In phase loading Figure 2. Illustration the experimental strategy based a combined plain fretting and fretting fatigue analysis involving similar contact configurations. Plain Fretting tests were applied by imposing a nominally static normal force P, followed by a purely alternating cyclic displacement amplitude (δ∗), so that an alternating cyclic tangential load Q was generated on the contact surface. During a test, P, Q and δ are recorded, from which the δ - Q fretting loop can be plotted. The studied plane specimen is not subjected to any fatigue stressing. The fretting fatigue experiments were performed using a dual actuator device inspired by Fellows et al. [8]. This test system allows the separate application and control of fretting and fatigue loadings. Like for the plain fretting, the system is instrumented to measure the contact loading (P, Q*, δ*) but also the fatigue stressing (σ, Rσ=σmin/σmax). In order to analysis both contact pressure, fatigue stress and stress gradient effects, various cylinder radius from R= 20 to 80 mm, Hertzian contact pressures from pmax=600 to 1000 MPa and fatigue stress conditions from σmax= 0 and 400 MPa considering two stress ratio Rσ=0.1 and 1.0 were investigated. The details of the studied conditions are compiled in table 2. Note that the lateral width of the cylinder pads (W) was chosen to satisfy plain strain conditions. 3. Experimental results 3.1. Friction analysis Because the partial contact stress field depends on the coefficient of friction, it is important to establish this value. H. Proudhon et al. show in [10] that the friction coefficient measured at the transition between partial and gross slip conditions (µt) may be used to provide a representative value of the friction under partial slip conditions (i.e. µPS=µt). Figure 3b compares the obtained µt
RkJQdWJsaXNoZXIy MjM0NDE=