Effects of the geometry of welded joints on fatigue design

Abstract

The effect of the welding heat input on the geometry of the butt welded joints and the effect of the geometry on the fatigue resistance were investigated in the study. The ultra-high strength structural steel with the yield strength of 700 MPa was welded with two different welding heat inputs. The geometries of the cross sections of the welded joints were imported to FE-software for the calculation of the fatigue notch factors. The calculated fatigue notch factors were compared with the factors from literature, and S-N-curves were determined with the aid of the fatigue notch factors. In addition, the effect of the element size on the notch factor and the effect of initial crack size on the fatigue life with the aid of fracture mechanics were investigated. It was observed that the geometry of the welded joint does not explain the changes of the fatigue resistance of the welded joint. At least the half of the decrease of the fatigue resistance compared with base material is due to other than geometry, in example microstructural changes caused by welding. If the S-N-curve is calculated only with a fatigue notch factor, the fatigue resistances are over-optimistic in comparison with literature and design guidelines. Utilizing the finite element method in design, the adequate dense element mesh must be used that the geometry of the structure and welded joint corresponds to the reality.