Influence of Niobium on the Beginning of the Plastic Flow of Material during Cold Deformation
Investigations were conducted on low-carbon steel and the steel with same chemical composition with addition of microalloying element niobium. While tensile testing was carried out, the thermographic measurement was tacking place simultaneously. A specific behavior of niobium microalloyed steel was noticed. Test results have shown that, in the elastic deformation region, thermoelastic effect occurs, which is more pronounced in niobium microalloyed steel. Start of plastic flow in steel which is not microalloyed with niobium begins later in comparison to the microalloyed steel, and it is conducted so that, at the point of maximum stress, deformation zone is formed within which stresses grow. In steel microalloyed with niobium after proportionality limit, comes the occurrence of the localized increase in temperature and the occurrence of Lüders band, which propagate along the sample forming a deformation zone.
Lüders bands at the beginning of the plastic flow of materials
Intensive research into inhomogeneous deformations at the beginning of the plastic flow on different metal materials has been carried out during the last few years. Luders bands are detected using methods of static tensile testing, thermography and visioplasticity with digital image correlation. This paper is a review of insights into the mechanism of formation and parameters influencing the occurrence and propagation of Luders bands. Current knowledge indicates that there is not a full explanation of the cause of the Luders bands formation. There are also certain differences in the way some parameters influence the occurrence of Luders bands, primarily in different materials. Therefore, modern methods are used to explain the formation of Luders bands.
On the lüders band front in mild steel—II. Elastic-plastic analysis of the front by the finite element method
Abstract The stress distribution at the Luders front is analyzed in the plane stress approximation by means of the finite element method. The stability of a planar interface and that of a ledge geometry, as suggested by previous experimental work, are examined. Results indicate that the associated stress distribution makes the former geometry intrinsically unstable while for the latter it ensured its stability during the propagation of the front. In addition to the theoretical support given to Luders front propagation by the ledge mechanism, the calculations give a stress concentration factor of 1.5 ± 0.2 for the elastic material at the tip of the advancing ledges. The stress distribution behind the front becomes homogeneous at a distance of the order of the grain diameter from the interface indicating thus that a significant fraction of the final Luders deformation should be accountable for by creep. Combining the calculated stress distribution with an experimentally determined strain profile the stress-strain history of the material behind the front is reconstructed. The inhomogeneous deformation is finally described in terms of homogeneous deformation processes.