The integrity of structures exhibiting flaws in nuclear power plants, and in particular for class 1 components (pipes, vessels, internals), has to be assessed to meet safety criteria. Technical rules along with simplified assessment methods are available in various standards: ASME, RSE-M, RCC-MR, R6 etc…. They rely on conservative hypotheses and parameters (loading, material properties, failure diagram). They are developed using analytical or numerical approaches and are validated with adequate demonstrative experimental testing. In particular, RSE-M and RCC-MR codes include a large set of compendia for the calculation of the fracture parameter J, depending on component and defect geometries, loading and material characteristics. However, it is essential first to consider the type of failure that the structure is likely to undergo and then to apply the proper assessment methods with the criteria required by the safety rules. For structures made of a material with a high strain-hardening behavior combined with a large enough value of ductile tearing resistance, it is not so easy to identify properly the failure mode between ductile failure or plastic collapse. This paper deals with the fracture mechanical test on a specimen made on Inconel 600 up to the ultimate failure of the sample. Due to the high mechanical properties of this material, the determination of ductile fracture initiation and propagation is quite difficult. Nonetheless the tests were carried out on a Center Cracked specimen under Tension (CCT). A numerical analysis of this experiment is provided using Finite Element (FE) calculations. In this case, the small-scale-yielding hypothesis is not verified and a complex load path is observed. The classical fracture mechanics parameter J-integral is not relevant anymore. The use of an energy fracture parameter in thermo-plasticity, called GTP, seems to be a more adequate parameter to describe correctly the effect of the crack growth. Different models are evaluated and a comparison with more classical parameters in a Non-Linear Elastic (NLE) behavior is done.

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