PSI - Issue 46

L. Frank et al. / Procedia Structural Integrity 46 (2023) 3–9 L. Frank and S. Weihe / Structural Integrity Procedia 00 (2019) 000–000

4

2

expansion efficiency. The demanded flexibility of the turbomachinery to successfully master the energy transition affects the loading situation of almost all fossil power plant components, resulting in increased wear or a decreased lifetime. The end stage blades of steam turbines are subjected to high centrifugal forces which results in low-cyclic stress during startup and shutdown. In addition, bending stresses of high frequency induced by blade vibration during the operation, e.g. high-cyclic stress, are superimposed. The highest stresses or stress gradients due to those operational forces and bending stresses appear in the blade root. Depending on the superposition of centrifugal and bending loads, different damage mechanisms occur at different spots in the blade root (Frank et al. (2018), EPRI (2008)). On the one hand, the high centrifugal force causes fatigue stress, especially in the notch areas in the blade root, and on the other hand, the high-frequency alternating bending stresses at high compressive forces lead to wear on the bearing flanks. This leads to wear of the surfaces with the formation of pits and small cracks, which accelerate the fatigue damage (fretting fatigue). Cracking starts primarily at the edge of the contact, since an increase in stresses occurs here. The utilization level of the used materials for the turbine blades of the end stage of the steam turbines’ low-pressure section is already very high in relation to the mechanical properties. In order to ensure safe and steady operation in the future, improved materials and optimized design variants with appropriate evaluation concepts for the design need to be used. To investigate the fatigue behavior of end stage blades in detail, it is essential to carry out component tests with original-sized end stage blades since the same geometries, dimensions, and manufacturing processes are present. Against this background, a test concept for examining original-size end stage blades of steam turbines under near practical fatigue loads was developed in the COOREFLEX 4.1.2 project (Frank et al. (2018)) and the test rig was set up at the Materials Testing Institute of the University of Stuttgart. The presentation of the test rig, the procedure of the component tests and the results obtained are the content of chapter 2. For the lifetime prediction in the area of the connection between the blade and the turbine shaft, suitable numerical lifetime prediction concepts must be reviewed for applicability and developed further. Against this background, numerical investigations by means of elastic-plastic finite element analysis were carried out to estimate the local multiaxial stress condition acting in the blade root. The local stresses and strains were post-processed using a critical plane approach and advanced fatigue damage parameters. The estimated lifetimes were compared with the experimental results of the component tests on original-sized end-stage blades from chapter 2. The methods used and the results obtained are presented and discussed in chapter 3. In the COOREFLEX 4.1.2 project (Frank et al. (2018)) a test concept was developed for examining original-size end stage blades of steam turbines under near-practical fatigue loads. The main challenge of setting up suitable test rigs is the appropriate application of huge centrifugal forces in combination with bending loads on full-scale components to simulate real operating conditions. The realization of a centrifugal test rig on a laboratory level is very difficult and costly and due to the existing airfoil section, there is no possibility for a load introduction by tension forces on the airfoil without breaking off the airfoil. Under these circumstances, a new test concept was developed in which the load introduction of the centrifugal force is applied inversely by means of a compressive force. The developed concept is shown in Fig. 1 (left). The blade root with a fir tree profile is mounted in the groove part. The load application of the centrifugal force by means of a compressive force is applied via a stamp and pins inside the blade root. The load application of the bending force can take place at the airfoil. The advantage is that both force vectors are decoupled and can be varied without any interaction. Free oscillation of the blade is also possible, which enables excitation with the first resonance frequency of the blade. By excitation of the blade eigenmodes at their resonance frequency, it can be ensured that similar dynamic stress distributions as in real operation are present in the test rig. Due to the cut-outs for the stamp and pins, the cross-sectional area of the blade root is reduced and the stress distribution in the blade root is changed. The verification of the transferability as well as the design of the load transfer components required detailed FE analyses. Frank et al. (2018) demonstrated the validity of the concept by means of 2. Experimental investigations 2.1. Test rig and test set-up