Evaluation of fatigue crack propagation in spot welded joints by stiffness measurements H. Gaul ⇑ , G. Weber, M. Rethmeier BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany article info Article history: Received 3 September 2010 Received in revised form 6 December 2010 Accepted 9 December 2010 Available online 16 December 2010 Keywords: Resistance spot welding Frequency Stiffness Advanced high strength steels Crack growth abstract Resistance spot welded joints made of two advanced high strength steels (AHSS) and one high strength steel (HSS) were examined. In the high cycle fatigue regime tests were realised for one loading condition, and the load and the displacement were measured. From the load and displacement range, the stiffness was calculated and in situ normalised to its value after 1000 cycles. It was thus possible to use the normalised stiffness evaluated during the experiment as failure criterion for the fatigue test of resistance spot welded joints. In order to evaluate this failure criterion, fatigue cracks were produced in the three different steel grades mentioned above according to user-specified levels of the normalised stiffness. It was found that the fatigue crack area increases linearly and the through-sheet-thickness crack length exponentially with decreasing stiffness. The comparison of the fatigue crack propagation during the fatigue test of the three different tested base metals showed very similar values. The comparability of fatigue tests performed on spot welded joints might thus be improved by the in situ evaluated stiffness as a failure criterion. This can be also used in further work to verify results of computer simulations of the stress intensity factor at spot welds with the help of experimental results. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In the car body-in-white production, steel sheet components are mainly joined by resistance spot welding. Although this tech- nology is wide spread and has been used for a long time, the fati- gue resistance of spot welded joints is still in the focus of research and development. Especially with the introduction of new materi- als like advanced high strength steels (AHSS), new issues concern- ing the fatigue behaviour of the body shell structures do arise [1,2]. It is shown there that the fatigue life does not increase with increasing base metal strength, which limits the sheet thickness reduction. When single spot welded joints break, the body shell stiffness, which is an important factor of the vehicle comfort, can suffer [3]. For a fast and cost-efficient prototyping process it is therefore desired to calculate and, more and more, to simulate the fatigue behaviour of a new body shell design. Especially the knowledge of stress intensity factors and fracture mechanics of the spot welded structures are in the focus of the investigations [4–6]. Moreover, a unique failure criterion is desired to improve the comparability of fatigue experiments made with different materials, joining methods, component designs, etc. Such criteria are well known for base metal investigations and described in [7], but for resistance spot welded joints no unique failure criterion exists [8]. A crack size determination in through-sheet-thickness direction in combination with the potential drop method is reported in [9]. The investigated mild steel showed an exponential correlation of the crack size with the measured voltage drop across the spot welded joint during the fatigue test. Presumably due to the high effort that is involved in the potential drop technique, no further development of this method regarding the evolution of the fatigue crack is known. A promising criterion, i.e. the stiff- ness of the measurement setup as a function of cycle numbers, has only been analysed randomly by now [8,10]. In the latter pub- lication, the crack length as a function of the stiffness is reported for riveted joints. The results are promising, and simulations of the fatigue behav- iour of body shell structures can be improved by experiments revealing the crack propagation in spot welded joints in the high cycle fatigue regime. Numerical results of the approach of Paris are reported in [11,12]. By using the two crack propagation direc- tions reported there, i.e. the sheet width and thickness direction, an experimental evaluation of stress intensity factors is made pos- sible by the method presented in this contribution. Fatigue crack growth models as reported in [13,14] can thus be improved based on experimental findings. 2. Correlation between the fatigue crack propagation and the change in specimen stiffness Fatigue experiments are usually controlled by two magnitudes measured during the experiment, i.e. the force or the displacement. 0142-1123/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2010.12.005 ⇑ Corresponding author. Tel.: +49 30 8104 3123; fax: +49 30 8102 1557. E-mail address: holger.gaul@bam.de (H. Gaul). International Journal of Fatigue 33 (2011) 740–745 Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue