Effects of Rotating-Bending and Torsional Fatigue Loads on Gas Tungsten Arc (GTA) Welded AISI 1018 Low Carbon Steel Joints

Most welded structures are subjected to multiaxial fatigue load and majority of the fatigue failure initiated from weld joints. Therefore, it is important to evaluate multiaxial fatigue behavior of commonly used welded materials. In the current investigations, the influence of rotating bending fatigue load along with torsional pulsed load was appraised for most commonly used AISI 1018 low carbon steel. A rotating-bending-torsional fatigue testing unit was designed and manufactured for biaxial fatigue test of welded and un-welded specimens. For welded specimens, Gas Tungsten Arc welding (also known as Tungsten-Inert-Gas (TIG) welding) was carried out on 19.05 mm diameter round bar of AISI 1018 steel using ER70-S2 filler metal. For rational comparison, only defects-free specimens were carefully chosen and tested. After welding, uniaxial tensile test was conducted to understand the fatigue loading criteria during rotating-bending fatigue test. Due to TIG welding, tensile strength was decreased considerably about 18% as compared to base metal. Rotating-bending (RB) and rotating-bending-torsional (RBT) fatigue tests were conducted to obtain a systematic understanding of biaxial fatigue behavior. RB fatigue life of welded specimens reduced compared to base metal as a result of complex thermal cycle during welding process and microstructural changes. Under combined loading conditions (RBT), base metal specimens did not exhibit significant difference on the fatigue behavior. However, for the welded specimens, the fatigue strength was reduced by about 12.8%. Moreover microstructural characterization and fracture surface analysis were performed to understand the fracture behavior of the tested specimens.