Session: DA-12-02 Fracture 2-Fracture Prediction and Evaluation

Paper Number: 124301

124301 - Defect Stability Evaluation in a Nozzle Through the Gfr Criterion Under Thermal Loading 

French regulation for Fracture Mechanics Assessments of nuclear power plant components imposes to demonstrate a significant margin between the maximum load imposed to the component and the load at crack instability. All types of loading (pressure, seism, thermal transients …) and loading conditions (normal to accidental) are concerned by this assessment without any distinction with their nature. Next to high thermal stresses, the fracture mechanics criteria used for crack propagation evaluation in ductile regime are very rough and known to be very conservative when applied to large structures. This is the case for the JR-Δa curve, which is determined on CT specimens, and which drastically overestimates the crack growth rate or rapidly limits it to 3 or 5 mm. Based on such approaches, it becomes difficult to reach the criteria for some components encountering low ductile toughness.

To overcome those difficulties, an alternative energetic approach of the JR-Δa criterion has been proposed: the G_fr criterion which allows evaluating accurately the ductile crack growth even for complex materials and for large amounts of propagation. The main assumption of this protocol is that the global behavior of the structure with a growing crack can be approximated by a succession of stationary crack behaviors of different sizes. It can be either a set of stationary cracks with idealised shapes or a node-to-node crack growth approach where each node behaves independently. The passage between two stationary states is based on the difference of plastic part Jpl of the J integral, and a material parameter called G_fr.

The first crack propagation evaluation scheme of the G_fr criterion [1] is devoted to purely mechanical (primary) loading where the plastic part J_pl of the integral J is simply the difference between total elastic-plastic part J, and elastic part J_el: J_pl = J - J_el. However, for thermal loading conditions as thermal shocks, it is generally observed that the total elastic-plastic J is lower than the elastic one. From the previous decomposition, it results that J_pl becomes negative, which is a physical nonsense. To overthrow this difficulty, an extension of the approach has been proposed in [2] in order to take into account the specificities of the thermal (secondary) loading. This extension is based on the elastic-plastic stresses determined on the crack free model, which allows to define an effective elastic part of J called J_el-tild.

This study reports on the crack growth stability of an inclined nozzle through the G_fr criterion considering thermal and mechanical loading conditions. Crack propagation is simulated through iterative remeshing and calculation phases depending on the chosen criterion on the increment of propagation. This paper highlights the capability of the approach to predict large 2D crack growth in a component.

[1] S. Chapuliot, S. Marie, An energetic approach for large ductile crack growth in components, Fatigue and Fracture Mechanics, ASTM STP 1406 (2000).

[2] S. Chapuliot, O. Ancelet, S. Marie, Assessment of the defect stability for large structures submitted to thermal shock loading through the Gfr criterion, International journal of pressure vessel and piping, 2020.

Presenting Author: Walid HAMOUCHE FRAMATOME

Presenting Author Biography: I have finished my PhD in structural mechanics in 2016.
After I started my career as mechanical engineer at Framatome.
Today I have the position of specialist in fast fracture.

Authors:

Walid HAMOUCHE FRAMATOME
Edith Marques Vieira Framatome
David Albrecht EDF
Stéphane Chapuliot EDF
Olivier Ancelet Framatome
Stéphane Marie Framatome