Fatigue Strength and Fracture Mechanisms in the Very‐High‐Cycle‐Fatigue Regime of Automotive Steels

• Published in: Steel research international Vol. 91; no. 8
• Main Authors: Sadek, Mohamed, Bergström, Jens, Hallbäck, Nils, Burman, Christer, Elvira, Roberto, Escauriaza, Borja
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.08.2020
• Subjects: automotive steels; Carburizing; Chromium manganese steels; Crack initiation; Crack propagation; Crack sensitivity; Emission analysis; Failure mechanisms; Fatigue failure; Fatigue life; Fatigue strength; Fatigue tests; Ferritic stainless steels; fish eyes; Fracture mechanics; Inclusions; initiation mechanisms; Martensitic stainless steels; Materials Engineering; Materials Science; Materialteknik; Materialvetenskap; Metal fatigue; Microalloying; Notch sensitivity; Quenching and tempering; Steel; stress life-curve; Structural steels; Surface cracks; Surface defects; Tensile strength; Ultrasonic testing; very high cycle fatigue
• ISSN: 1611-3683; 1869-344X; 1869-344X
• DOI: 10.1002/srin.202000060

Very‐high‐cycle‐fatigue (VHCF) strength properties are of interest to several technical applications assessed globally at different laboratories with long‐life fatigue testing capabilities. Also, VHCF failure mechanisms are a scientific topic with remaining open research questions. Herein, three automotive bar grade steels are studied with respect to VHCF strength and initiation mechanisms. A microalloyed ferritic–pearlitic steel (38MnSiV5, 870 MPa tensile strength), a quenched and tempered martensitic steel (50CrV4, 1410 MPa tensile strength), and a carburizing steel (16MnCr5, 1180 MPa core structure tensile strength) are studied to reveal characteristics regarding initiation and VHCF failure mechanisms. A 20 kHz ultrasonic fatigue testing instrument is used to obtain fatigue lives up to and above 109 load cycles in uniaxial loading. Hour‐glass specimens, smooth or notched, are tested at R = −1 and R = 0.1. Fatigue strength and stress life (SN)‐diagram data are achieved, and crack initiation and growth mechanisms are studied using primarily field‐emission gun–scanning electron microscopy (FEG–SEM). Fatigue strengths are explained by a modified life‐dependent Murakami‐expression, the Haigh diagram, and notch sensitivity. Interior and surface crack initiations by surface defects, triple points, and inclusions are found. The fine granular area (FGA) to fish‐eye crack growth transition conditions are explored and schematic descriptions are given. The mechanisms for fatigue failure in the very‐high‐cycle‐fatigue (VHCF) regime commonly involve unique features revealed in fracture surfaces, such as fish eyes and fine granular areas, controlling total fatigue life. Fracture mechanisms and fatigue strengths have been determined for VHCF in automotive steels. Estimates of crack tip plastic zone sizes are related to microstructure and crack growth region transitions.