Fatigue Life of Welded Joints of AISI 347 Stainless Steel Under Thermomechanical and Variable Amplitude Loading

The change in operation of conventional power plants — due to the increasing use of renewable energies — from a stationary to a more flexible operation, causes additional stresses to the components by a high amount of smaller load cycles. This fact results in a demand for validated new concepts to estimate fatigue life especially for welded joints which are the weak parts within the piping. Resulting from the measured stains during operation in the LCF regime, a non-linear fracture mechanics based concept was chosen. For the development and validation of the model, different experiment types are carried out using various types of specimens. To consider the influence of different microstructures within a welded component, specimens made of X6CrNiNb18-10 (AISI 347) with the microstructure found in the base material on the one side, and as found in the HAZ on the other side are used. To take the influence of a mechanical and microstructural notch into account, notched specimens of X6CrNiNb18-10 (AISI 347), and welded specimens made of X6CrNiNb18-10 (AISI 347, base material) and X5CrNiNb19-9 (weld material) are used. Experiments are performed with all types of specimens with an increasing complexity from constant amplitude loading to operational loading. The developed nonlinear fracture mechanics based lifetime model uses the effective cyclic J-Integral normalized to the crack length to replace crack growth calculation by a linear damage accumulation. To consider the loading history an algorithm for the calculation of crack opening and crack closure is used. The advantages of this approach are shown by a comparison with damage calculations based on the damage parameter by Smith, Watson and Topper and based solely on the strain ranges. The differences in the concepts will be highlighted and used for further considerations of how to advance the lifetime prediction model for variable amplitudes. The presented work gives an overview of the preliminary results of the current work on the AiF research project 18842 N ‘Extended damage concepts for thermomechanical loading under variable amplitudes and plastic deformation’.