Growing demands on gas turbines, meaning lower consumption and higher efficiency, is accompanied by higher temperatures inside the turbine. Better lifetime prediction concepts for thermomechanical fatigue (TMF) loading of gas turbine components are needed to ensure safe operation. The influence of the TMF loading history—either in‐phase (IP) or out‐of‐phase (OP)—on crack initiation and short crack growth is studied for the cast nickel‐based super alloy IN100, which is commonly used as blade material. Therefore, smooth specimens are loaded at two different mechanical strain levels with, respectively, two phase relationships, using a temperature cycle T = 300–950°C and T = 300–850°C. The replica technique is used to detect cracks in the order of tens of micrometers. The differences in crack initiation and short crack growth are studied for the different loading conditions and the development of the crack length evolving from multiple short cracks is investigated.

The aim of the presented work is to develop improved concepts for the prediction of fatigue lifetime under variable amplitude loading including plastic deformation and thermomechanical fatigue of welded joints made of the austenitic stainless steel 1.4550. Local strain amplitudes are located in the regime of low cycle fatigue, which is dominated by short crack growth. This can best be described by nonlinear fracture mechanics. To develop a fracture mechanics based lifetime model, different experiment types are carried out to estimate the influence of loading history, thermal cycling, and mean stresses. For the evaluation, different estimation concepts based on Palmgren‐Miner's classic linear damage accumulation rule are used: solely calculated by strain ranges, damage parameter established by Smith, Watson and Topper, and a damage parameter based on the cyclic effective J‐integral. The differences in the concepts are highlighted and used for further considerations on how to improve the lifetime prediction models.

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’.

Combustion chamber tiles of aircraft engines are subject to locally high thermomechanical fatigue (TMF) loading during flight operations. The TMF loading in these components results in a cyclic bending load over the wall thickness. Additional stress concentrations occur at geometric features such as fillets or effusion cooling holes. Thus complex loading conditions arise that cannot be reliably assessed using conventional material database and computational design methods. In the context of this article, a new test method is presented that consists of a bending test rig for applying component-related loading conditions to corresponding specimens, here called subelements, that represent significant geometric, lifetime-limiting features of the component. In various test series on additively manufactured subelements, geometric, manufacturing, thermal, mechanical and time-dependent parameters are investigated concerning their influence on fatigue crack growth and lifetime. The results from these tests are compiled in a database, which is used to validate an advanced computational assessment approach for crack growth and lifetime accounting for stress gradients in the depth direction due to the bending and along the surface due to possible geometric features. The method can be applied to component geometries such as combustion chamber tiles using a mechanical finite element analysis (FEA) followed by postprocessing steps to increase reliability of lifetime predictions.

Abstract. The change in operation of power plants due to the increasing use of renewable energies causes additional stresses to the components by a high number of smaller load cycles. This fact results in a demand for validated new approaches to estimate fatigue life especially for welded joints that are the weak parts within the piping. The components are operated in the LCF regime where short fatigue crack growth determines the life. Therefore, a non-linear fracture mechanics based approach is appropriate. For the development and validation of the model, an experimental campaign was performed including fatigue tests of smooth specimens with various microstructures of X6CrNiNb18-10 (AISI 347) as well as specimens containing mechanical and microstructural notches. Experiments are performed with all types of specimens with an increasing complexity from constant to variable amplitude loading, the latter also applied as pseudorandom sequence derived from measurements in power plants. The developed non-linear fracture mechanics based 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 evident when comparing calculated with experimentally determined fatigue lives. Remaining differences serve for further considerations of how to improve the life prediction for variable amplitudes.