Case study Fracture analysis of a low pressure steam turbine blade Ernst Plesiutschnig *, Patrick Fritzl, Norbert Enzinger, Christof Sommitsch Graz University of Technology, Institute of Materials Science and Welding, Kopernikusgasse 24, 8010 Graz, Austria 1. Introduction Pitting corrosion at turbine blades in steam power plants is a frequent starting point of growing fatigue cracks [1–3]. In a recently conducted failure analysis [4], pitting corrosion has been detected at the first load bearing flank of a low pressure steam turbine blade during a major overhaul. Metallographic investigations as well as qualitative finite element analysis (FEA) of the cracks were carried out to determine the root cause of the failure. Questions about the crack propagating loads (important for operation), the excitation of natural frequencies (important within cyclic operating conditions) and the generation of a viable crack size (damage tolerant design) are important for operators and designers of steam power stations. For them it is important to determine inspection intervals; to calculate the operation time to the present (or a detectable) crack size. Inspection, maintenance and replacement cause downtimes are costly and therefore studied [5] to optimise intervals. Linear elastic fracture mechanics (LEFM) and FEA are typically used for damage tolerance or fail-safe design concepts (e.g. the evaluation of the residual life-time with the Paris law or the leak-before-break design) as illustrated by [6, p. 378]. Here LEFM and FEA are used to evaluate stresses in order to suggest the crack initiation, propagation and finally the operating conditions. Results from failure analysis with literature data are used as an input for the fracture mechanics analysis. Knowing three of the parameters in Fig. 1a (e.g. Y, a, DK) the fourth (Ds) can be calculated. FEA and determined input parameters (see Fig. 1b) are used as a proof and as an extension to the analysis. The cracks are illustrated in Fig. 2 to show the investigated turbine blade in top Fig. 2a and side Fig. 2b view. The red planes at the first load bearing flank mark the positions of the fatigue cracks. Crack 1 is the oldest fatigue crack and is therefore used for analysis. Case Studies in Engineering Failure Analysis 5–6 (2016) 39–50 A R T I C L E I N F O Article history: Received 15 December 2015 Received in revised form 11 January 2016 Accepted 8 February 2016 Available online 15 February 2016 Keywords: Turbine blade Pitting corrosion FEM 1.4021 (AISI 420) A B S T R A C T Cracks were analysed at the root of the third blade row of low-pressure steam turbine blades of different natural frequencies. The root cause of the fatigue crack initiation was pitting corrosion of the forged ferritic/martensitic X20Cr13 material. Metallographic investigations, finite element analysis and fracture mechanics analysis combined with experimental data from the literature are used to evaluate crack propagating stresses to discuss the operating conditions. The calculations show that corrosion pits at the root of the turbine blade increase the local stresses above yield strength. Excitation of natural frequencies by changing the rotor speed is not responsible for the crack propagation. The centrifugal load and superimposed bending load caused by unsteady steam forces are responsible for the crack propagation. ß 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). * Corresponding author. Tel.: +43 3168731657. E-mail addresses: ernst.plesiutschnig@tugraz.at (E. Plesiutschnig), norbert.enzinger@tugraz.at (N. Enzinger). Contents lists available at ScienceDirect Case Studies in Engineering Failure Analysis jo ur n al ho m ep ag e: ww w.els evier .c om /lo cat e/c s efa http://dx.doi.org/10.1016/j.csefa.2016.02.001 2213-2902/ß 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).