V - Limite d’application de l’approche locale de la rupture fragile étudiée sur des aciers chargés en hydrogène

Limit of the local approach application of the brittle fracture on hydrogen charged steels

Abstract

The local approach of the brittle fracture by cleavage developped by BEREMIN relies the macroscopic mechanical properties to local criteria. It allows to predict the probability of failure of the structure by performing detailed calculation of the stress and deformation fields in the different element volumes within this structure. It also takes into account the distribution of the defects initiating the fracture in a specific critical zone. The local approach allows then the determination of a statistical criterion to be applied on cleavage fracture.

The cumulative distribution function P_R, over a small volume V₀ ahead of a crack tip or defect can be expressed as: P_R = 1 - exp[- (σw / σu)^m] where σw WEIBULL stress and σu mean cleavage stress defined as the stress/volume leading to PR = 0.63 and m is an empirically determined parameter presenting the degree of scatter in measured strength values.

The paper deals with the application of this approach on three steels in absence and in presence of hydrogen : railway steel FM80, with pearlitic structure, 35CD4 steel employed in tool’s joints in a tempered martensitic state and a bainitic A508.3 used in nuclear power plants.

The goal of this work is to show that in the case of an hydrogenated steel, the local approach is improved if the defects promoted by high stress triaxiality and local critical hydrogen concentration do not exceed the element volume V₀ in which the material is considered to be statistically homogeneous. The results show that in the two first steel the local approach is improved even in presence of hydrogen. In the hydrogenated bainitic steel (A508.3), the application of this method is not possible due to development in the material of fish eyes which the size is very large with respect to V₀.