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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Table 1. Examples of N 5 lifetimes with and without imperfection and associated number of cycles at initiation N i Δε t /2 Imperfection depth N 5_ref N 5_with_flaw N i 0,2% 320 116 100 12 000 1 000 0,6% 280 4 840 [3] 970 110

In fact, the analysis of the experimental measurements of the electrical potential, considering an initiation threshold corresponding to a 100 µm crack (as previously defined), demonstrates that the fraction of lifetime spent in the initiation stage is greatly reduced in the presence of surface imperfection. Table 1 illustrates this phenomenon with initiation cycle numbers (N i ) representing approximately 10% of the lifetime in the two considered cases. In support of that, without imperfection, the part of the lifetime associated with initiation and micro propagation represents a significant part of the total lifetime [Ould Amer (2014)], increasing when the strain level decreases [Maiya (1975)]. As a result, the surface imperfections, by drastically reducing the initiation time, induce a more severe reduction in the total lifetime at the lowest strain levels. The experimental campaign carried out on 304L austenitic stainless steel has demonstrated a significant influence of the presence of surface imperfections on the low-cycle fatigue resistance. The main parameter, governing the lifetime reduction, is the value of the depth of the anomalies. A minor influence of the strain amplitude has also been established. The crack propagation history from imperfections is reconstructed by analyzing the fracture surfaces of the specimens that were ink-marked. These results also allowed to calibrate the electrical potential drop technique. The growth rates of cracks emanating from the surface imperfections could be determined thanks to the use of the experimental measurements resulting from the monitoring of the electrical potential. Two distinct propagation domains have thus been highlighted. In order to obtain a fatigue crack propagation law taking into account the various test parameters, one of the main perspectives will be to analyse the crack growth data collected here by means of an adequate driving force parameter in a generalized plasticity context. 4. Conclusions

Acknowledgements

The authors thank the Region of Nouvelle Aquitaine and the Framatome company for co-financing this thesis work.

References

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