13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Load history effects on fracture toughness of RPV steel Jérémy Hure*, Benoît Tanguy CEA, DEN, Service d’Etudes des Matériaux Irradiés, 91191 Gif-sur-Yvette, France * Corresponding author: Jeremy.hure@cea.fr Abstract Fracture toughness is required for assessing structural integrity of reactor pressure vessels (RPV). However, it has been long recognized that load history effects can increase the apparent toughness, which is not considered in practical rules assessment. This could lead to over pessimistic assessment in case of thermomechanical transient. The so-called warm prestress (WPS) effect corresponds to the absence of crack propagation after prestressing if the load is held constant or decreased as the temperature is decreased. This study investigates the WPS effect on A508 Cl3 RPV steel, focusing on transient loadings in which the load is increased during cooling, with a twofold aim. Firstly to investigate experimentally to which extent it is possible to go beyond the WPS effect by progressively increasing the slope of the transient. Secondly, to assess numerically the ability of local approach to fracture (LAF) models to quantitatively predict the experimental results. The evolution of local quantities such as opening stress, cumulated plastic strain, triaxiality close to the crack tip are finally described in details. Keywords Warm prestress effect, Cleavage fracture, RPV steel, Local approach to fracture 1. Introduction The warm prestress (WPS) effect describes the fact that, when a crack is loaded at a temperature T1 to a stress intensity factor K1, no fracture will occur if the stress intensity factor decreases or is held constant between T1 and T2 < T1, even if the fracture toughness of the virgin material (i.e without prior loading) is exceeded [1, 2]. The WPS effect has been the object of several experimental studies on ferritic steels (see e.g. [3]) as security assessment of nuclear reactor pressure vessels (RPV) requires to consider the case of a loss of coolant accident (LOCA) in which the vessel is subjected to a mechanical loading associated with a variation of the temperature. In this particular case, the embrittlement due to neutron irradiation has to be considered and, in the perspective of prolongation of nuclear power plant lifetime, the WPS effect has to be well understood in order to justify security margins. In this paper, we study the WPS effect on French RPV 16MND5 (similar to A508 Cl3) steel, focusing on transient loadings in which the load is increased while cooling the specimen (Fig. 1). Our aim is first, from an engineering point of view, to quantify how much it is possible to go beyond the conservative principle of WPS effect, i.e. no propagation if the load is held constant or decreased during cooling. Secondly, we use these WPS cycles to compare the numerical predictions of local approach to fracture (LAF) models, the differences being discussed by looking in details at the evolution of the crack tip stress/strain fields.
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