Fatigue performance and the effect of manufacturing quality on the uncoated and hot-dip galvanized ultra-high-strength steel laser cut edges

Abstract

Fatigue performance is a key issue in weight critical applications. In this study, the effect of local surface characteristics influenced by laser cutting, hot-dip galvanizing (HDG) and stress concentration factors, is considered and a multiparametric model for numerical evaluation is introduced. The Laser cutting is one of the most important processing methods for ultra-high-strength steels (UHSSs). The development of laser cutting has led to the utilization of nitrogen as an assistant gas with high-power laser systems. A fiber laser with nitrogen used to blow molten material from kerf was found to induce initial sub-surface defects to cut edges. HDG is an important surface finish for steel providing corrosion protection and been previously used with mild and high-strength steels.

Experimental fatigue tests were conducted to characterize the effect of steel grade, manufacturing quality, and surface finish on fatigue performance. HDG was found suitable for a UHSS S960 UHSS grade. Based on the fatigue test results, revised design guidelines for components with cut edges and hot-dip galvanized surfaces were proposed. FAT 150 MPa m = 3 could be used for laser-cut edges in as-cut condition and extended up to FAT 315 MPa m = 5 for high-quality cut edges. The definition of cut edge quality in this work was connected to cut edge features like initial crack-like defects instead of surface quality.

The use of stress concentration factors in fatigue strength assessment was clarified, and notched components were fatigue tested. In addition to design recommendations, a multiparametric fatigue strength assessment model based on modelling local defects. The defects were characterized with scanning electron microscopy from fracture surfaces. In the model, surface quality is characterized as an equivalent crack and theory of critical distances (TCD) is used in result analysis. Local stresses can be obtained utilizing finite element analysis where measured or estimated crack length is used as an input value. The introduced local cyclic behavior based 4R method application was found valid for fatigue strength assessment for uncoated and HDG specimens with a single master curve.