C. Gandiolle et alii, Frattura ed Integrità Strutturale, 35 (2016) 232-241; DOI: 10.3221/IGF-ESIS.35.27 234 For R K >0; ΔK eff = K 1max -4 with R K = K Imin /K Imax (2) For -1<R K <0; ΔK eff = K 1max -(4.R K +4) (3) For R K <-1; ΔK eff = K 1max (4) The overall C and m parameters of the Paris law expressed as a function of ΔK eff are given in Tab. 1. R σ =σ min /σ max C m 0.1 8.32x10 -9 2.88 0.5 1.22x10 -8 2.88 overall 1.27x10 -9 3.27 Table 1 : Crack propagation laws from british standard BS7910-05 Fretting fatigue test The fretting fatigue test device is shown in Fig. 2a. It consists in three hydraulic actuators to control independently the normal force P, the tangential force amplitude Q* and the fatigue stress σ. The fretting actuator imposes a purely alternating sinusoidal cyclic displacement δ(t) on the plane, generating an alternating cyclic tangential load Q(t) on the contact surface. During the test, displacement δ, normal force P and tangential force Q were recorded, enabling the Q-δ fretting loop to be plotted and thus the fretting regime to be identified. As cracking, and more specifically crack nucleation, was to be investigated, the displacements were kept small enough to maintain partial slip conditions. Sinusoidal cyclic tangential force and fatigue forces were applied in phase. That is the maximum fretting load Q* max was applied at the same time as the maximum fatigue stress σ max . All tests were performed at constant normal force P. The frequency was fixed at 12 Hertz, high enough to investigate a long test condition and low enough to guarantee test control stability. The fretting stress ratio was kept constant at R Q* =Q* min /Q* max =-Q*/+Q*= -1. (a) (b) (c) Figure 2 : (a) sketch of the fretting fatigue test device, (b) Picture of the fretting fatigue contact with the measurement wires, (c) sketch of the implementation of the PDT method. A monitoring system using the potential drop technique method was implemented on the fretting fatigue sample. The potential drop technique (PDT) allows direct information on the crack propagation kinetic during test. It was introduced in 1957 by Barnette et Troiono to observe crack propagation in fatigue tests [8], and first applied on fretting fatigue test by Kondo et al. [9]. Since then, the technique has been widely used. Fig. 2b&c show a picture and a sketch of the application of PDT on a fretting fatigue test.

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