PSI - Issue 48

Bernadett Spisák et al. / Procedia Structural Integrity 48 (2023) 326–333 Spisák et al / Structural Integrity Procedia 00 (2023) 000 – 000

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environments, the actual mechanical behavior of the components must be assessed with confidence, preferably without excessive conservatism. Therefore, the fracture toughness of reactor pressure vessel (RPV) beltline materials is monitored throughout the lifetime of the nuclear power plant. RPVs are constructed from ferritic steels, which break with a ductile mechanism at elevated temperatures, but undergo brittle fracture at low temperatures. However, when exposed to neutron radiation, the transition from ductile to brittle behavior is shifted to a higher temperature. Mini compact tensile (CT) specimens are increasingly being used to directly measure the fracture toughness of reactor pressure vessels (RPVs) in the transition region using the Master Curve methodology (Lucon et al. (2006), Sokolov (2018)), with this the ductile to brittle range could be determined (DBTR). There are several research related to this topic. Sokolov (2022) used mini-CT specimen to determine the fracture toughness of irradiated, highly embrittled weld, while Sirkiä (2017) wrote her thesis about the applicability of this type of specimen for the determination of DBTR. Sánchez et al. (2023) also made a review about the fracture characterization of ferritic steels with mini-CT specimen. These show that the topic is still highly researched to this day. The aim of the research project was to develop a simulation technique which could be used for the determination of the fracture toughness from mini-CT specimens. For this the following steps were carried out.  fabrication of 1T CT, 0.16T CT and small sized, flat notched tensile (NT) specimens,  carrying out measurements,  usage of local approach method (Gurson-Tvergaard-Needleman (GTN) damage parameters (Gurson (1977), Tvergaard and Needleman (1984))) ○ determination of damage parameters with the help of artificial neural network (ANN) from the flat NT specimens, ○ validation of the determined damage parameter with normal and mini-CT specimens  implementation of GTN parameters into the virtual crack closure technique (VCCT),

○ validation of the modified model with normal CT specimens ○ determination of fracture toughness from mini-CT simulation,

From these steps the complexity of the research work can be noted, therefore in the following sections these are going to be introduced in more detail. The introduced finite element simulations were carried out in MSC.Marc software, where in case of the VCCT simulations the model was combined with a subroutine.

Nomenclature a

crack length [mm]

a 0

initial crack length [mm]

F f 0 f c f f f n G J Q S n q 1 q 2  n  a  E

force [N]

initial void volume fraction (GTN parameter) critical void volume fraction (GTN parameter) failure void volume fraction (GTN parameter) volume fraction for void nucleation (GTN parameter)

energy realise rate

fracture toughness [kJ/m 2 ]

standard deviation (GTN parameter) yield surface multiplier (GTN parameter) yield surface multiplier (GTN parameter) mean strain for nucleation (GTN parameter)

crack propagation [mm]

energy released from the crack when the crack length (a) is extended by  a [J]