In this study fatigue tests of GMAW (Gas Metal Arc Welding) welded joints were conducted on both 1.6mm body sheet (DQSK-GA, DP590-GA, DP780-GI, and TRIP 780-GI) and 3.4mm frame materials (SAE1008 HR 240MPa, HSLA420 HR, DP600 HR, and uncoated Boron). Further, mixed thickness joints were tested which combined 3.4mm SAE1008 HR with each of the 1.6mm separately – with the exception of DQSK. A number of different joint configurations were tested including single and double lap-shear, start-stop lap shear, butt weld, and perch mount. Great care was taken in this study to ensure that the geometry of the welds was consistent, not only within a given material lay-up, but between all of the specimens of a given type – this effort was made in order to substantially reduce life scatter and provide a better understanding of the role base material plays in the fatigue life of GMAW joints. Fatigue testing was limited to tension-tension loading (R=Pmin/Pmax=0.1), with limited elevated mean stress testing (DP590 and DP600, all specimen types) conducted at R=0.3.
It was determined that, regardless of specimen configuration, base material strength played no role in the fatigue performance of these joints. Further, for all materials and specimen configurations (except perch mount-shear and start-stop specimen configurations) that there was no influence of mean stress on fatigue performance. The thickness of the base metal and the joint had the most significant influence on the fatigue performance of joints. The fatigue performance of mixed thickness joints was either the same or poorer than the equivalent single material 1.6mm joint. Lastly, attention to consistent weld geometry yielded fatigue life scatter which was significantly reduced over that found in earlier studies.
