Surface residual stresses induced by torsional plastic pre-setting of solid spring bar Vinko Močilnik, Nenad Gubeljak n , Jožef Predan University of Maribor, Faculty of Mechanical Engineering, Smetanova ul.17, 2000 Maribor, Slovenia article info Article history: Received 31 July 2014 Received in revised form 29 November 2014 Accepted 5 January 2015 Available online 12 January 2015 Keywords: Residual stresses Torsion Pre-setting Spring bar abstract Residual stresses could be induced by the plastic torsion loading of a solid round bar. This article deals with the residual stresses generated at the surface during the process of plastic pre-setting. Residual stresses were measured on the surface of a specimen by x-ray diffraction for different angles of subsequent plastic pre-setting. In addition, the residual stresses were calculated using analytical and numerical modelling by finite element methods. The analytical approach was based on the torsional characteristic, τ–γ, of the material and tension test results. It has been found that the direction of cold rolling on the surface has a significant influence on residual stresses, as it is reflected in the initial stress state. A good agreement between analytical modelling, finite element analysis, and experimental residual stress measurement was obtained. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Residual stresses in mechanical components are a result of technological processes. Residual stresses usually arise due to additional deformation during cold surface deformation such as cold drawing, stamping, shoot peening, cold rolling, and/or pre- setting. Residual stresses play a significant role in stress magnitude and the failure of a component. One example of residual stresses preventing failure is the cold rolling of the surface of mechanical component in order to induce surface compressive stresses that improve the fatigue life of the component. Torsion specimens were surface cold-rolled and plastic pre-strained in the torsion direc- tion. Surface rolling increases the compressive stress in the surface layers of the specimen, whilst the pre-setting acts in-depth. Both tensile and compressive stresses are in equilibrium within the material. During the usage of the mechanical part residual stresses are cumulative with applied ones. If the latter are time changing, and if the mechanical component is loaded with fatigue, then residual stresses can significantly affect crack initiation. Maximum applied stresses are mostly on the surface of mechanical parts (bending, torsion, or a combination), and the geometric changes and rough surface contribute an additional stress concentration. When the tensile residual stresses cumulate with the tensile applied stress on the surface of the mechanical component, it leads to a very unfavourable situation. Compressive residual stress on the surface is much more desirable and is subtracted from the applied tensile stress during use of the mechanical part. With the use of appropriate manufacturing processes (for example cold surface rolling, or pre-setting into the plastic range) it is possible to add compressive residual stress on the surface of the mechan- ical component. The authors in [1] have investigated the influence of residual stresses due to pre-setting on the lifetime of the torsion specimens from spring steel. It was proposed that with an increased pre-setting angle, the lifetime falls at the same amplitude of the applied stress ratio (minimum stress/maximum stress) R ¼ 0 torsional fatigue. The analytical procedure for determining residual stresses based on the experimentally determined τ–γ characteristics was discussed. In Ref. [2] authors investigated the influence of residual compres- sive stress on the lifetime of a torsion alternately-loaded hollow specimen made of spring steel. Residual compressive stress was simulated by a constant axial compressive force, and the specimen was loaded by alternating torsional fatigue at the same time. It has been established that the lifetime of the specimen with the increas- ing of the compressed axial pre-loading at alternating torsional fatigue, increases as well up to a certain point. The authors in [3] studied crack shapes and their growth rate for S45 steel specimens under various combinations of torsion and constant axial force loads. They state that the crack propagation angle is about 451 for various loading amplitudes, that the static tension axial force together with cyclic torsion causes the accelerated growth of a crack and lowers the service life, and that the compression axial force, together with torsion, considerably increases the service life Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijmecsci International Journal of Mechanical Sciences http://dx.doi.org/10.1016/j.ijmecsci.2015.01.004 0020-7403/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.:+386 2 220 7661; fax: +386 2 220 7994. E-mail addresses: vinko.mocilnik@siol.net (V. Močilnik), nenad.gubeljak@um.si (N. Gubeljak), jozef.predan@um.is (J. Predan). International Journal of Mechanical Sciences 92 (2015) 269–278