Issue 35

C. Gandiolle et alii, Frattura ed Integrità Strutturale, 35 (2016) 232-241; DOI: 10.3221/IGF-ESIS.35.27 235 A continuous current I=5A is applied through the fatigue test sample. The contact potential is measured between two platinum wires, of 0.1mm of diameter, welded on the sample surface. The potential is measured on each side of the sample in order to take into account the possible non homogeneity of the crack propagation. The potential measured depends on the sample resistance. When a crack nucleates and propagates, the sample cross section diminishes and thus the electrical resistance increases. It is then possible to establish a calibration curve linking the crack length to the measured potential. E XPERIMENTAL RESULTS Crack propagation calibration curve he calibration curve of the studied contact was established following an empirical method like Meriaux et al [10]. Considering a constant fretting fatigue solicitation of: R=4.6mm, P, σ F,moy /σ y,flat =0.78, R σ =0.85, Q*/P=0.30 ; tests were interrupted after various number of fretting fatigue cycles. Crack lengths were measured by destructive method [11] and plotted as a function of electrical potential (Fig. 3). Potential values of each test were normalized by their respective initial value V 0 . Figure 3 : Identification of the calibration curve (R=4.6mm, P, σ F,moy /σ y,flat =0.78, R F =0.85, Q*/P=0.30). Fig. 3 plots the calibration curve. Its evolution is complex and was described by 2 successive polynomial functions: For 1.01 < V/V 0 < 1.14 : b p = 643212.(V/V 0 ) 3 - 2040300.(V/V 0 ) 2 + 2157715.(V/V 0 ) - 760573 (5) For 1.14 < V/V 0 : b p = 5487.(V/V 0 ) 3 - 30610.(V/V 0 ) 2 + 57230.(V/V 0 ) - 33000 (6) We have no clear hypothesis to explain the plateau evolution observed for a crack length of b=200µm. Meriaux et al. observed a similar behavior on Ti-6AL-4V [10]. A first hypothesis suggests that it is induced by the fretting solicitation: the plateau corresponds to the change of crack propagation mode from a mixed mode I and II (fretting fatigue) to pure mode I. A second hypothesis states that the plateau could be connected to the presence of debris in the crack, in particular in the zone dominated by the mode II, which could perturb the electric conduction. Close to the breaking points, there was a high dispersion of crack lengths which results in a second plateau. From this curve, it can also be deduced that when the potential stabilizes then a crack arrest condition is reached. This point was studied thoroughly in a previous work [12]. A crack propagation criterion was established from the slope of the potential as a function of fretting fatigue cycles K e .  K e <10 -9 cycles -1 : crack arrest;  K e >10 -9 cycles -1 : crack propagation. T