Torsten-Ulf Kern

The Role of Advanced Fracture Mechanics Evaluation Methods for Turbine Components

The demand for higher plant cycling operation and reduced life-cycle costs are the main drivers for the design and assessment of turbine components today. Heavy cyclic loading increases the potential of fully utilizing the fatigue capabilities of the material which might lead to crack initiation and subsequent crack propagation.Fracture mechanics methods and evaluation concepts are widely applied to assess the integrity of components with defects or crack-like findings. The realistic modelling of the failure mechanism plays a key role for the accurate prediction of crack sizes at failure state.A basic treatment of material toughness typically leads to conservative assessments for components with sufficient ductility. A standard approach to describe material behavior with high ductility is to use the start of stable crack extension as a dimensioning parameter for the analysis. By definition a critical condition for a component is reached when the crack driving force is equal to the characteristic material parameter. On the other hand, advanced analysis methods allow determination of the instability point (ductile tearing analysis).This paper will discuss two cases for practical analysis from steam turbine design showing clear advantages for service application by using advanced analysis methods.© 2016 ASME

Mechanical Behavior of Dissimilar Welds for Steam Turbine Rotors With High Application Temperature

Fossil fired steam power plants of the latest generation require the elevation of steam parameters pressure and temperature to increase efficiency as well as to reduce greenhouse gas emissions. In order to achieve these goals for high temperatures, nickel-base alloys could play an important role for steam turbine applications in the future. Due to technological and economical restrictions, their application in turbine rotors shall be restricted to the most heavily stressed regions. Dissimilar welds offer a known solution to combine nickel-base alloys with ferritic/martensitic steels in this case. Thermal mismatch and differences in high temperature performance of the applied base materials make it very difficult to evaluate the lifetime of such dissimilar welds. Depending on temperature and type of loading, different failure mechanisms can be observed. Further, the type of weld material plays a major role for the service behavior of the weld. Therefore, this paper describes standard creep and fatigue tests which were conducted to identify failure mechanisms and failure locations at the weld zone. To simulate the outcome of the creep tests, a modified Graham-Walles approach is used that also accounts for the different creep behavior of the heat affected zones (HAZs) compared to the base material. For the simulation of the fatigue tests, the model type Chaboche–Nouailhas–Ohno–Wang (CNOW) is used. The results contribute to better knowledge in designing dissimilar welds between nickel-base alloys and martensitic steels under high temperature loading.