Development of an Experimental Approach to Study the Synergy and Damage Mechanisms of Corrosion Fatigue in Metallic Materials

Abstract

The study of the effect of the environment on the fatigue strength of metallic materials has been, and still is, the subject of a great deal of research. Although the phenomena of crack initiation and propagation in fatigue-corrosion appear to be well understood, they remain highly dependent on several parameters linked to the environment studied and the material. Numerous dimensioning approaches exist to consider the effect of corrosion on fatigue life. Some treat the two problems (fatigue and corrosion) separately, while others attempt to model the complex effects of cyclic loading/corrosion synergy. A multi-physics and multi-scale experimental approach was developed to better understand crack initiation and propagation mechanisms in fatigue/corrosion coupling situations. This approach is based on the multi-instrumentation of fatigue-corrosion tests, combining electrochemical and optical microscope measurements. More recently, lambs-wave monitoring has been introduced to track the evolution of damage and to propose a self-health monitoring based on physical evidence. Based on the results of in-situ electrochemical measurements and optical observations, the mechanisms and laws of fatigue/corrosion coupling have been identified: • Corrosion pitting initiation is controlled by an original experimental setup developed to generate a controlled size of a single corrosion pits in the middle of the fatigue specimen equipped with four piezoelectric sensors to track the corrosion fatigue damage, • The size of the corrosion pit is estimated from the measured electrochemical current using the faraday law, • The first phase of crack propagation (short crack regime) is highly dependent on corrosion. Material dissolution and applied stresses contribute to lowering the crack initiation threshold and increase the kinetics of short fatigue crack propagation, • In the Paris fatigue crack propagation regime, crack growth kinetics are higher than in air. This detailed understanding of damage mechanisms has enabled us to: • identify the parameters and laws governing fatigue-corrosion crack initiation and propagation, • determine the pit-to-crack transition step using in-situ lambs-wave monitoring and optical microscope observation, • and propose an analytical model for estimating fatigue-corrosion life. Microstructure/cyclic loading/corrosion interactions and the role of hydrogen, introduced during fatigue-corrosion processes, in crack initiation and propagation mechanisms remain to be studied.