Study of Fatigue Behavior for Spot Welded Tensile Shear Specimens of Advanced High Strength Steels Gert Weber,  Stephan Brauser, Holger Gaul, and Michael Rethmeier 1. Introduction In the automotive body-in-white manufacturing, modern steel sheet components are mainly joined by the resistance spot welding procedure. For safe design of spot welded body shell components the knowledge of the fatigue behavior is still of main interest. Therefore, great efforts are made to quantify the fatigue behavior of spot welded joints for modern advanced high strength steels (AHSS). [1,2] However, the rough conditions encountered in spot weld- ing fabrication of automobiles, i.e., sheet thickness toler- ances and positional uncertainties [3,4] in combination with the complex microstructure and the greater springback of modern steel types like AHSS may lead to production- related disturbances such as gaps between sheet metal components or weld imperfections in the form of cracks in the spot welded joints. [5,6] The inﬂuence of these factors on the fatigue behavior of spot welded joints made of AHSS has not been sufﬁciently taken into account. FE-simulation as well as experimental investigation of spot welded structures with gaps of up to 0.2 mm have shown that the amount of stress near the notch root of the nugget increased, and this effect caused slightly lower fatigue strength. [7,8] Despite the reported observations there seems to be a lack of systematic studies especially for gap sizes between 1 and 4 mm which gener- ally can happen in body shell mass production. A similar situation exists for the inﬂuence of cracks on the fatigue behavior. Up until now, the inﬂuence of weld imperfections, i.e., cracks, has mainly been investigated using specimens with simulated imperfections, e.g., holes drilled into the spot welded joint. [9] The main reason for such simulation is the high effort necessary to prepare fatigue test specimens containing reproducible imperfec- tions, especially in the form of cracks. Despite this fact, it is greatly desirable to investigate the fatigue behavior of spot welded components with spot weld cracks provoked by the welding procedure itself, because the inﬂuences of the different types of notches of an artiﬁcial imperfection and of a weld imperfection can hardly be estimated, which limits the possibilities of investigating the inﬂuence of weld imperfections on the fatigue strength by artiﬁcially created imperfections. As a contribution to a manufacture-oriented analysis of the fatigue behavior of spot welded joints, tensile shear specimens were produced with gaps between the sheet metal parts as well as reproducible surface cracks in the weld area. The fatigue behavior of the test specimens was analyzed in terms of cycles to failure and the specimen stiffness was determined by plotting history curves during Wo ¨hler testing. The failure criterion was a loss of 30% of the specimen stiffness relative to the initial stiffness at the beginning of the test. The results were compared to those of equally produced imperfection-free test specimens, tested in the same manner. 2. Requirements and Test Conditions For this investigation two different low alloyed TRIP steels HCT690T were selected. Table 1 shows an extraction and the mechanical properties of the tested steels. The TRIP steel with 1 mm thickness was used for analyzing the inﬂu- ence of gaps while the 1.5 mm TRIP steel was applied so see the inﬂuence of surface cracks. To examine the inﬂuence of gaps on the fatigue strength of spot welded joints, tensile shear specimens were pro- duced containing a gap of 3 mm typically occurring in automobile mass production, see Figure 1. The spot welded gapped and gap free (reference speci- mens) tensile shear samples consisted of two sheets, 105 mm long and 45 mm wide, which were overlapped by 35 mm and single spot welded in the center of the overlapped region. The cracked and crack free (reference specimens) tensile shear samples consisted of two sheets, 135 mm long and 45 mm wide, which were overlapped by 16 mm. It is well established that the weld size has an impact on the strength and fatigue performance of spot welded joints. In simple terms, a smaller nugget diameter can lead to decrease in fatigue strength. [10] Consequently, to avoid an inﬂuence of the weld size on the fatigue behavior, a con- stant nugget diameter of 5 mm for the gapped samples and of 6.8 mm for the cracked samples was chosen. The appro- priate reference specimens, i.e., without imperfections, had the same nugget diameters. It should be noted that [  ] G. Weber, S. Brauser, H. Gaul, M. Rethmeier Federal Institute for Materials Research and Testing, Berlin, Germany Email: gert.weber@bam.de DOI: 10.1002/srin.201100286 www.steel-research.de 988 steel research int. 83 (2012) No. 10 ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim FULL PAPER