The Effect of Specimen Thickness on the Lüders Phenomena in AISI 1524 Steel Alloy: Experimental Observations Using DIC

Background At certain temperatures and strain rates, low carbon steels, as well as some aluminum and magnesium-based alloys, exhibit plastic flow instability at the onset of their respective yield points, known as the Lüders phenomena. Such phenomenon is recognized by a distinct yield point and subsequent plateau on the stress-strain curve, and takes the form of a band when full-field tensile strain contours are observed experimentally. Objective This paper aims to investigate the specimen thickness dependence of the Lüders effect in AISI 1524 hot-rolled steel alloy. Sixteen samples were cut from hot-rolled plates and uniaxial testing in conjunction with digital image correlation were performed to gain insight and quantify the Lüders band’s spatial characteristics during the extent of the plateau. More specifically, 1 mm, 2 mm, 3 mm, and 4 mm thick flat specimens of mild steel were tested under identical conditions, most notably strain rate, in order to isolate and understand the effect of specimen thickness. Results Results revealed that both the Lüders band width and velocity depend on the specimen thickness. An increase in specimen thickness resulted in an increase in Lüders band width, and an increase in Lüders band velocity on 1524 steel alloy. Moreover, the angle at which the Lüders band appeared on the front surface of the specimen with respect to the different thicknesses revealed the 3D nature of the band’s formation. Conclusions The obtained results and observations suggested a 3D nature of the band’s formation in AISI 1524; whereby nucleation takes place at the core of the material before propagating in the thickness direction towards the structure’s surface. Moreover, it was concluded that the Lüders band special features, width and velocity is dependent on a structure’s thickness.