On the effect of Lüders bands on the bending of steel tubes

Abstract In several practical applications hot-finished steel pipe that exhibits Luders bands is bent to strains of 2–3%. Luders banding is a material instability that leads to inhomogeneous plastic deformation in the range of 1–4%. This work investigates the influence of Luders banding on the inelastic response and stability of tubes under rotation controlled pure bending. Part I presents the results of an experimental study involving tubes of several diameter-to-thickness ratios in the range of 33.2–14.7 and Luders strains of 1.8–2.7%. In all cases the initial elastic regime terminates at a local moment maximum and the local nucleation of narrow angled Luders bands of higher strain on the tension and compression sides of the tube. As the rotation continues the bands multiply and spread axially causing the affected zone to bend to a higher curvature while the rest of the tube is still at the curvature corresponding to the initial moment maximum. With further rotation of the ends the higher curvature zone(s) gradually spreads while the moment remains essentially unchanged. For relatively low D / t tubes and/or short Luders strains, the whole tube eventually is deformed to the higher curvature entering the usual hardening regime. Subsequently it continues to deform uniformly until the usual limit moment instability is reached. For high D / t tubes and/or materials with longer Luders strains, the propagation of the larger curvature is interrupted by collapse when a critical length is Luders deformed leaving behind part of the structure essentially undeformed. The higher the D / t and/or the longer the Luders strain is, the shorter the critical length. Part II presents a numerical modeling framework for simulating this behavior.

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