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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Two propagation domains are observed for all the curves presented in Fig. 7. First, a micro propagation phase, clearly visible for each test, extends over depth intervals ranging from a few hundred microns to about 1 mm. It is characterized by a strong dependence on the initial imperfection depth. Thus, in this micro propagation domain, the fatigue crack propagation rates associated with the deepest imperfections are higher than those related to imperfections with smaller depth. Moreover, there is a decrease in the propagation rate for the deepest notches and overall significant fluctuations. From a depth threshold value, generally slightly less than 1 mm and depending on the initial imperfection depth, the crack propagation rate curves are similar for a given level of strain amplitude, whatever the initial imperfection depth is. A micro-propagation stage with a growth rate different from that observed for long cracks has already been observed in 304L by several authors [Poulain (2015)], [Ould Amer (2014)]. In the context of this study, a significant notch effect, associated with a large strain concentration, can be put forward to explain the observed differences. The lifetimes considered correspond to the number of cycles necessary to achieve a 5% drop with respect to the stabilized peak stress. They are noted N 5_with_flaw for the specimens with imperfection and N 5_ref for the reference samples (without initial imperfection). Fig. 8 shows the evolution of N 5_with_flaw /N 5_ref versus the imperfection depth for the different studied strain levels. It can be observed that whatever the strain amplitude considered, the fatigue resistance significantly drops even for the smallest depths of imperfections (close to 100 μm). Furthermore, the deeper the initial imperfection is, the greater the decrease in the lifetime is. Thus, for the largest depths of imperfections (around 300 μm), the relative life can decrease in a factor of 10 in the most detrimental cases (Table 1). 3.2. Reduced fatigue life in the presence of surface imperfections

Fig. 8. Evolution of the relative fatigue life as a function of the surface imperfection depth

Figure 8 also illustrates an influence of the strain level. Indeed, the surface imperfection presence seems more harmful for the lowest strain amplitudes. For similar depths, the relative lifetime reduction is approximately twice larger for Δε t /2 = 0.2% than for Δε t /2 = 0.6%. This effect of the deformation level is potentially related to the decrease in the time (in cycle number) spent during the initiation stage caused by the presence of surface irregularities.