Fatigue life enhancement of aluminium joints through mechanical and thermal prestressing Ba ˚rd Wathne Tveiten a, * , Arne Fjeldstad b , Gunnar Ha ¨rkega ˚rd b , Ole Runar Myhr c , Børge Bjørneklett c a SINTEF Materials and Chemistry, N-7465 Trondheim, Norway b Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway c Hydro Aluminium Structures, N-2830 Raufoss, Norway Received 17 January 2005; received in revised form 26 January 2006; accepted 30 January 2006 Available online 22 March 2006 Abstract This paper presents some simple and flexible methods to enhance the fatigue life of welded aluminium components. Besides enhancing the fatigue life, the proposed methods can easily be implemented into manufacturing processes. The key element of the methods is to change residual stresses from tension to compression at locations vulnerable to fatigue. This is accomplished by mechanical prestressing using elastic pre-deformation or by thermal prestressing using induction heating. The specimens tested are welded aluminium rectangular hollow section T-joints. Prior to fatigue testing, welding FE-simulations were carried out to verify the magnitude and pattern of the resid- ual stress fields (through process modeling). Fatigue testing was later carried out on four different batches. One batch was produced using elastically pre-deformed chords, two batches were treated by means of thermal prestressing (induction heating), and one batch was ‘‘as welded’’ representing a ‘‘reference case’’. Based on statistical evaluation of S–N data, the introduction of superimposed compressive stress fields results in a significantly improved fatigue life. Among the different batches, induction heating turned out to be the most promising method with a fatigue strength improvement factor of 1.5 on stress, compared to ‘‘as welded’’ components. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Fatigue testing; Induction heating; Residual stress; Weld geometry; Welding simulation 1. Introduction It is well established that manipulations of residual stress fields will influence the fatigue life either by retarding (superimposed compressive stress field) or by accelerating (superimposed tensile stress field) fatigue crack growth. Residual stresses are defined as those stresses existing in a structure or a part of a structure in the absence of exter- nally applied loads. Residual stresses can be categorised as short-range or long-range stresses. Short-range stresses exist in the weld metal and the heat affected zone (HAZ) of welded components and are self-equilibrating over the cross section of the local member. They are caused by inho- mogeneous thermal expansion and contraction of the material in the weld and HAZ region. It is generally assumed that short-range stresses in welded built-up mem- bers may reach a level not far from yield stress both parallel and transverse to the weld. Long-range stresses are uniform throughout structural members, but not self-balanced within local members. They are generally small compared to the yield stress and exhibit small stress gradients. In small-scale welded specimens typical of S–N testing, only short-range residual stresses are present. Various methods to enhance the fatigue life by introduc- ing favourable residual stresses have been suggested in the literature, e.g. peening, overload, and thermal methods [1]. However, common to these methods are that they are gen- erally time-consuming, costly, and labour-intensive. Thus, they are not feasible in high-volume productions, e.g. 0142-1123/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2006.01.006 * Corresponding author. Tel.: +47 73593890; fax: +47 73592931. E-mail address: Bard.W.Tveiten@sintef.no (B.W. Tveiten). International Journal of Fatigue 28 (2006) 1667–1676 International Journalof Fatigue www.elsevier.com/locate/ijfatigue