Hole-making and cutting effects on fatigue performance of ultra high strength steels

Ultra high strength steels provide opportunities for reduced-weight designs since they have higher tensile and fatigue strengths than the conventional mild steel grades. For typical applications such as telescopic cranes, tippers, chassis, and agricultural machinery, lower weight leads to lower fuel consumption and better road homologation. Before the start of their service life, steel components must be machined to their final shape as they are supplied in standardized shapes and sizes. Mechanical and thermal cutting strategies invoke various physical phenomena within the cut material affecting the in-use properties. The objective of this study is to quantify the effect of hole-making processes and edge-preparation on fatigue performance of ultra high strength steels. A series of fatigue experiments were conducted on steel grades with yield strengths from 700 to 960 MPa using axially loaded coupon samples with a hole manufactured by punching and laser cutting. Fatigue performance is compared to the detail categories provided by design standards. Additional tests under four-point bending loading were conducted to focus on flat edges machined by laser cutting and milling. Especially at high-cycle and infinite-fatigue lives, tested grades were superior to the typical design curves when the holes were punched with low clearance or laser cut.