Acta Mech 223, 829–839 (2012) DOI 10.1007/s00707-011-0603-y Samrand Rash Ahmadi · Soran Hassanifard · Masoud Mohammad Pour Fatigue life prediction of friction stir spot welds based on cyclic strain range with hardness distribution and finite element analysis Received: 6 November 2011 / Revised: 11 December 2011 / Published online: 14 January 2012 © Springer-Verlag 2012 Abstract The main aim of the present study is to investigate the fatigue behavior of single friction stir spot welds (FSSW) using strain-based modified Morrow’s damage equation. The correlation between microhard- ness, cyclic material constants, and mechanical strength of different zones around the FSSW are assumed to be proportional to the base material hardness. Experimental fatigue tests of friction stir spot welded specimens have been carried out using a constant amplitude load control servo-hydraulic fatigue testing machine. ANSYS finite element code has been used to simulate a single tensile shear friction stir spot welded joint, and non-linear elastic-plastic finite element analysis has been employed to obtain the values of local equivalent stress and strain near the notch roots of the joints. The results based on the numerical predictions have been compared with the experimental fatigue test data. It has been shown that the strain-based approach does a very good job for estimating the fatigue life of friction stir spot welded joints. 1 Introduction Lightweight materials such as aluminum and magnesium alloys are increasingly used in the automotive indus- try instead of the classical iron-based alloys in these days to reduce the fuel consumption and therefore gas emissions. Resistance spot welding (RSW) is widely used for joining steel sheets; however, this method of joining for aluminum sheets lead to poor mechanical performance, as reported by Gean et al. [1] and Thornton et al. [2]. Recently, the solid-phase joining technique for thin sheets of the lightweight alloys innovated that named friction stir welding (FSW). This process developed by TWI in 1991 as reported by Thomas et al. [3] has received considerable attention, because it offers various advantages such as good retention of baseline mechanical properties, little distortion, low residual stresses, and few weld defects. Newly German factory GKSS [4] has applied FSW to spot welding, which is called friction stir spot welding (FSSW) or friction spot joining (FSJ). Since FSSW has the same advantages as FSW, this technique is expected for extensive applications for joining of body parts in automotive industry. Mazda Motor Corporation was the first automo- tive FSSW application, with the rear door of their 2003 RX-8 [5]. The FSSW technology includes a process similar to friction stir welding (FSW) except that, instead of moving the tool along the weld seam, the tool only penetrates into the sheets. The resulting weld has an inherited characteristic hole in the middle of the joint that was left by the probe. Generally, the parameters that induce FSSW are tool geometry, rotational speed, holding time, and downward force. S. Rash Ahmadi (B ) · M. Mohammad Pour Mechanical Engineering Department, Faculty of Engineering, Urmia University, Urmia, Iran E-mail: s.rashahmadi@urmia.ac.ir Tel.: +98-441-2972954 S. Hassanifard Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran