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Fatigue Strength and Fracture Mechanisms in the Very‐High‐Cycle‐Fatigue Regime of Automotive Steels
Author(s) -
Sadek Mohamed,
Bergström Jens,
Hallbäck Nils,
Burman Christer,
Elvira Roberto,
Escauriaza Borja
Publication year - 2020
Publication title -
steel research international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.603
H-Index - 49
eISSN - 1869-344X
pISSN - 1611-3683
DOI - 10.1002/srin.202000060
Subject(s) - materials science , fatigue limit , ultimate tensile strength , carburizing , fracture (geology) , scanning electron microscope , composite material , metallurgy
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 10 9 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.