PSI - Issue 13

Ghassen Ben Salem et al. / Procedia Structural Integrity 13 (2018) 619–624 Ben Salem Ghassen / 00 (2018) 000–000

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Figure 7. Three parameters weibull law identification

tip located on the ferritic side near the FL. SEM examination of fracture surfaces showed an intergranular fracture initiation for CT specimens with mixed fatigue precrack, resulting in lower toughness values. The intergranular fracture mechanism initiates at the MA interface after a ductile deviation of the crack to the FL. The analysis of toughness values classified by the proportion of austenite in the fatigue precrack indicated that the intergranular fracture mechanism is not volume dependent and seems to be correlated to the notion of threshold stress. A 3 parameters Weibull law was therefore used to model the previous brittle mechanism. Tensile tests on axisym metric specimen, which were machined to initiate intergranular fracture in the MA interface and tested at − 170 ◦ C, were used to calculate a threshold stress ( σ th ), below which brittle fracture cannot occur. In a first step, the most suitable NT specimen configuration for threshold stress calculation was chosen by coupling SEM examination with stress profiles numerical analysis. A threshold stress of 1620 MPa was then calculated. The other Weibull law parameters (m and σ u ) are temperature dependent and will be identified with CT fracture toughness tests in the brittle-to-ductile transition. Afterwards, the probabilistic model applied on CT specimen tests must be extended to a structure case. Finally, the e ff ect of thermal ageing on the DMW fracture resistance will be studied by performing the same tests on a DMW mock-up treated at 400 ◦ C for 10000 hours [18].

Acknowledgements

This study was sponsored by Framatome with the participation of the MSSMat and CEA / LISN laboratories. The authors wish to express their gratitude to all the partners who contributed in this work, especially partners from Framatome GmbH for the experimental program on the DMW mock-up, Eva He´ripre´ (MSSMat) for her help in the SEM analysis and Olivier Ancelet and Gregory Perez (CEA / LISN) in the experimental development.

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