High Temperature Fatigue of Welded Joints—Experimental Investigation and Local Analysis of Butt Welded Flat and Cruciform Specimens

Austenitic stainless steel of type X6CrNiNb18-10 exhibits advantageous mechanical and chemical properties and is a common material for numerous applications in the nuclear power plant and chemical industries. Besides the mechanical strain induced by high pressure, the fatigue life in welded pipelines is affected by additional thermomechanical strains due to thermal loading. The welding process mainly determines the geometry and metallurgical constitution of the welded joint. Therefore, the butt welds additionally influence the strain gradient along the component and reduce its lifetime. While the base and weld material are similar, they show different softening and hardening behavior, especially at ambient temperature. Cyclic hardening occurs in the base material, whereas cyclic softening can be observed in the weld material. The hardness distribution along the welded joint reveals no clear differentiation of the base material, the heat affected zone, and the weld material. The attributes of the individual materials cannot be transferred to the welded joint automatically. Thus, the analysis of the interaction between the materials along the welded joint is a main topic of this research. To this end, digital image correlation (DIC) is used for different kinds of specimens and load conditions. The position along the testing area at which fatigue failure occurs depends on the specimen type and the load condition but not on the temperature. Further, isothermal and anisothermal fatigue tests on welded cruciform specimens are presented. The common practice of the effective strain is discussed for the analyzed conditions.