Abstract
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.
Highlights
With the large variety of different steel alloys and steel condi- spent during initiation
Due to the of properties is important for the selection of steel alloy development and use of ultrasonic fatigue testing instruments, the VHCF regime is reached in a reasonably short time
4) Both interior and surface failure initiations occurred depending on the presence of initiation defects, local strength (in the carburized case), and stress distribution
Summary
As in VHCF, the major part of the fatigue life is. With the large variety of different steel alloys and steel condi- spent during initiation. The initiation processes become more tions being used in automotive manufacturing, the mapping critical and have been studied in more detail.[9–17]. Due to the of properties is important for the selection of steel alloy development and use of ultrasonic fatigue testing instruments, the VHCF regime is reached in a reasonably short time (with 20 kHz load frequency, 109 cycles are reached within 14 h). Escauriaza Department of Development of New Products SIDENOR IþD S.A., Barrio Ugarte s/n, Basauri E-48970, Spain
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