Corrosion fatigue damage mechanisms of a new high strength steel for aeronautical applicationsThursday (28.05.2020) 14:50 - 15:10
To meet the highest performance and reduce the emissions, a new martensitic age-hardening steel has been developed for aircraft engines structural parts. Its very high mechanical resistance guarantees the reliability of structures over long services life. However, airplanes may operate in environments likely to cause pitting corrosion and impact their fatigue performance. Indeed, pure tensile cyclic loading tests conducted in 5 g/L NaCl solution have shown a significant reduction of fatigue strength compared to dry air. Similar results are found for high strength steel, mainly attributed to corrosion defects as preferred sites for crack initiation, crack growth assisted by electrochemical dissolution or hydrogen embrittlement [1–3]. Concerning the material under study, detailed postmortem sample observations allow to draw the following conclusions. First, localized corrosion is enhanced by cyclic loading. The higher the maximum applied stress, the more pits on surface at failure. Then, fatigue cracks are initiated on critical size pits. The early stage of crack growth is strongly reliant on corrosion. Both material dissolution and applied stresses contribute to propagation by a competitive process. As the crack is growing, the impact of dissolution is decreasing. Finally, growth rate becomes mainly controlled by applied stresses. However, the role of hydrogen introduced during corrosion processes is still questioned. Considering the above, the total corrosion fatigue lifetime can be divided into three contributions: the number of cycles to initiate one or several cracks from corrosion pits with a critical size, the number of cycles for “pit-to-crack” transition, and the number of cycles for crack growth until failure. Based on existing modelling approach for each contribution , a comprehensive model is proposed. A reasonable agreement is found between empirical and predicted lifetimes.
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