PSI - Issue 19

P. Cussac et al. / Procedia Structural Integrity 19 (2019) 463–471 P. Cussac / Structural Integrity Procedia 00 (2019) 000 – 000

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distinction neither of the deformation level applied nor of the initial notch depth. It is then clear that the obtained results do not depend on any of these parameters.

Fig. 6. Experimental calibration curve based on ink markings and observation of fracture surfaces

Moreover, experimental markings have also permitted the determination of an initiation threshold by the potential drop technique. In this aim, several markings were made at different values of the variation of the electrical potential and one of them showed a crack propagation threshold of about 50 μm for a variation of this potential of 0.5 %. Other markings were subsequently carried out for a 1% change in the normalized difference of potential V/V 0 and were associated with depths ranging from 70 μm to 120 μm. These results have allowed to associate the relative evolution of 1% of the electrical potential to a crack propagation of about 100 microns from the bottom of imperfection. The use of both the experimental measurement of the potential as a function of the number of cycles (N) and the experimental calibration curve (fig 6) enables to deduce the crack depth evolution during the tests a=f(N). By deriving this equation, the crack propagation rates (da/dN) can be deduced. The corresponding results obtained for the tests carried out for the three strain amplitudes are presented in Fig. 7.

Fig. 7. Crack propagation rates as a function of crack depth for different strain levels (Δε t /2 = 0.2%, 0.3% and 0.6%) and different initial imperfection depths