Suivi électrochimique de l’endommagement de matériaux métalliques en fatigue

Electrochemical method of corrosion fatigue damage monitoring in metallic materials

Ecole des Mines, Centre SMS, URA CNRS 1884, Saint-Etienne

Abstract

The damage of metals and alloys under cyclic straining occurs through the progressive development of surface roughness and short crack nucleation and growth before the beginning of fatal crack propagation in the bulk. During the major part of the fatigue lifetime (> 80% N_F), a development of short cracks preceeding the fatal crack propagation takes place. Thus, fatigue lifetime is mainly determined by the resistance against short crack propagation.

In aqueous solutions, the reduction of fatigue lifetime results often from complex corrosion- deformation interactions which can lead to the activation of new damage mechanisms which involve generally local electrochemical reactions.

An experimental method based on the monitoring of current or potential transients during cyclic straining is proposed. Data related to the elementary damage mechanisms and to the damage development during the test can be collected from the shapes and amplitudes of current (potential) peaks within individual cycles and from the evolution of corresponding peak values versus number of cycles.

Examples of applications illustrate the possibilities of the method in the field of corrosion fatigue studies of passivating alloys. First, the damage localization in a duplex stainless steel and relationship between strain rate and damage evolution in a single phase c.c. stainless steel in a chloride solution are discussed. Then, the application of the method to identify the iodine induced corrosion fatigue mechanisms in Zircaloy-4 is presented. In all cases, the stage of short crack nucleation as well as transitions between different damage mechanisms can be precisely determined.