Characterization of fatigue crack growth in ultra-thin bainitic steel under mixed-mode loading conditions for railway applications
Abstract
This research focuses on carbide-free bainitic steel, specifically designed for railway infrastruc-ture with an emphasis on high speed and heavy loaded freight tracks. The bainitic steel developed in this study exhibits significantly higher tensile strength (min. 1250 MPa), yield strength (min. 700 MPa), and enhanced wear resistance and rolling contact fatigue (RCF) properties compared to con-ventional pearlitic rail steels. Considering the complex and mixed-mode stress conditions on the rails running surface, a crucial challenge is managing fatigue crack propagation processes. This study aims to provide an understanding of fatigue crack growth rates (FCGR) under combined load-ing conditions, specifically tension-shear (mode I+II) and tension-torsion (mode I+III). In this investigation, both numerical modeling (finite element method) and experimental ap-proaches were utilized to evaluate the FCGR under mixed-mode loading conditions, relevant to real service conditions of railway tracks. Fracture surfaces were analyzed using scanning electron mi-croscopy (SEM) to examine fracture mechanisms and microstructural influences. The microstruc-ture of the analyzed bainitic steel consists of lath like bainitic ferrite, retained austenite (both filmy and blocky), and martensite/austenite (M/A) constituents, with limited carbide precipitation. It was found that fatigue lifetime increased with the loading angle in mixed-mode conditions. The domi-nant cracking mode observed was transgranular, characterized by secondary fracture facets aligned with bainitic ferrite laths. The fraction of intergranular fracture decreased with an increasing loading angle. Additionally, secondary cracks indicated preferential cracking directions correlated with the crystallographic structure of bainite. The results obtained are fundamental for accurate fatigue life prediction of ultra-fine carbide-free bainitic rails, significantly contributing to enhancing rail reliability through advanced material design and precise fatigue modeling.
