Abstract
Ultra-high-strength steels (with tensile strength higher than 980 MPa) are widely used in automobile manufacturing owing to their lightweight that contributes to fuel efficiency. The fatigue strength of ultra-high-strength steels with a notch tends to decrease, which is known as the effect of notch sensitivity. In this study, 4-point bending fatigue tests were performed to examine the fatigue strength and notch sensitivity of four steels; namely 590 MPa class steel, 980 MPa class martensitic steel, 980 MPa class bainitic steel, and 980 MPa class precipitation hardening steel plates with three different stress concentration factors. The results indicate that the fatigue strength and notch sensitivity of 980 MPa class steel specimens were higher than those of 590 MPa class steel specimens. The notch sensitivities of tested plate specimens were lower than those reported for cylindrical specimens of bainitic ultra-high-strength steels. Fatigue crack observation revealed that the cracks initiated in 590 MPa class steel, 980 MPa class bainitic, and martensitic steel propagated southward from the lowest bottom of notch. Although similar initial crack propagation pattern was detected in precipitation hardening steel, the crack changed direction when it reached the central part of the specimen.
Highlights
R ecently, automobile fuel efficiency has been receiving worldwide attention to reduce automotive carbon emissions and to surpress global warming
Despite the high fatigue strength of Ultra-high-tensile-strength steel (UHTS) compared to conventional steel, its applications in vehicle parts are limited owing to the significant decrease in UHTS fatigue strength when it is subjected to any stress concentration
The fatigue cracks initiated in 590 MPa class steel, 980 MPa class bainitic steel, and martensitic steel specimens propagated 130
Summary
R ecently, automobile fuel efficiency has been receiving worldwide attention to reduce automotive carbon emissions and to surpress global warming. Reducing the vehicle weight is one of the most efficient methods to improve fuel efficiency This can be achieved by adopting lightweight steel with higher strength. Ultra-high-tensile-strength steel (UHTS) exhibits lightweight properties with relatively high tensile strength, higher than 980 MPa, compared to conventional mild steel with tensile strength of approximately 300 MPa. Multiple methods can be used to enhance the tensile strength of steels, such as heat treatment and microstructural control. The fatigue strength of UHTS used in fillet welded joint has shown a behavior similar to that of conventional steel because of the high stress concentration factor between the welded metal and substrate. The reduction in fatigue strength caused by stress concentration is known as notch sensitivity This fatigue property is inherent to each material. Notch sensitivity is quantified by the fatigue notch factor Kf, defined as
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
