Strain distribution during air bending of ultra-high strength steels

Abstract

Air bending is a widely used method for forming ultra-high strength steels (UHSS). However, the limited formability of UHSS poses some challenges for the bending process in the form of strain localisation, surface defects, punch detachment (multi-breakage) and pseudo-polygonal “nut-like” shape of the bend. In this study, the bendability of three UHSS grades (700, 900 and 1100 MPa) is investigated with 3-point bending tests, utilising Digital Image Correlation (DIC) for measuring the strain distributions on the outer curvature. The differences in the extent of multi-breakage and the bend shapes are also studied, and these observations are correlated with the findings from the bending force and strain measurements. The differences between the investigated UHSS grades are significant. The 900 MPa grade produces more localised strain distributions and pronounced multi-breakage compared to the other grades, along with a more polygonal “nut-like” geometry. The reasons and effects of the multi-breakage phenomenon, as well as the causes for the observed differences in the behaviour of the materials are discussed in this paper. The presented results and the measurement data provide more information about the behaviour of the investigated materials in bending, and can be used for improving bending simulation, numerical models, and workshop instructions.

Total strain on the outer surface of steel sheets in air bending

“…The distribution of strain and curvature on the outside bend surface can vary significantly depending on the work-hardening properties of the material. When bending materials with poor global formability (i.e., low work-hardening and uniform elongation), the inside radius of the bent sheet may decrease far below the punch radius, leading to high local strains and bend shapes that may be difficult to predict [1]. Various terms have been used referring to this phenomenon in previous studies: multi-breakage, gap formation, punch-sheet separation, punch detachment, punch-sheet-liftoff, loss of contact with the punch/sheet etc.…”

“…However, research on the root of the phenomenon, i.e., the causes and effects of the strain distribution development in air bending, has been relatively scarce. Strain distributions in air bending have been studied by several authors [1,[9][10][11][12][13][14], but not for a large selection of materials and punch radii, that would allow thorough analysis of the effects of certain tool parameters and material properties. Research in this area is therefore necessary.…”

Effect of surface roughness on strain distribution during bending

“…Bendability is generally measured as the minimum ratio of the punch radius to the strip thickness that the steel can tolerate without the appearance of surface defects during bending to an angle of 90º in three-point bending [1]. However, in this study, the bendability of UHSS grade is investigated with 3-point bending tests, utilising Digital Image Correlation (DIC) for measuring the strain distributions on the outer curvature [2][3][4][5] and not to angle of 90°. Earlier studies [6,7] have shown that strain localization is the precursor to failure in the air bending of ultrahigh-strength strip steels and shear bands have been observed to initiate at depths in the range of 1-6 % of the total strip thickness from the surface at angles of ~45° to the surface, i.e.…”