Fatigue Crack Growth by FEM-DBEM Approach in a Steam Turbine Blade

Abstract

This study is concerned with the integrity assessment of cracked steam turbine rotor components, made of steel X20Cr13, that operate under cyclic loading. Damage accumulation and growth occurred on the leading edge of turbine blade starting from part-through surface flaws. Tensile tests are performed for assessment of the main mechanical and fracture steel properties: specimens are cut out from critical zones of turbine blade body after a given operating time. The subject for experimental studies is a steel bar of circular cross-section with straight-fronted edge crack. The optical microscope measurement and the COD method are used to monitor crack growth during the tests. An automatic crack propagation simulation on the blade is performed by a coupled approach which, starting from the results of a global finite element method (FEM) analysis, proceeds through the sub modelling of the cracked volume in a DBEM dual boundary element method (DBEM) environment and subsequent propagation analysis. displacement (COD) method [8][9][10][11].Residual fatigue life of turbine blades could be predicted with reasonable accuracy from knowledge of 3D stress-strain state, SIFs, fracture resistance material properties and in correspondence of different types of operation loadings.In operation, the analysed blades experiences two main types of damage: the first is the erosive and corrosive pitting, with an initial flaw size approximately equal to 0.5 mm; the second is a welding defect introduced during the protective coating application, on the leading edge of the blade, with an initial flaw size approximately equal to 1.5 mm. The former kind of defect is modelled in this work.Critical crack lengths discovered on in service blades were varying from 28 to 32 mm.The analysed blades belongs to the 27 th stage of a 200 MW power steam turbine with operation time t=92245 hours. Fatigue failures of blades in rotating turbine disk were detected in service. Their fracture was the result of fatigue crack initiation and propagation up to the formation of a critical crack. In all of these failures, crack propagation started from leading edge as a part-through surface flaw (Figure 1) produced by erosive and corrosive pitting. Experimental TestsTensile tests were performed for determination of the main mechanical properties of blade materials. Smooth and notched specimens were cut out from critical zones of turbine blade body with given operating time in order to allow for degradation of mechanical properties such as strength and ductility. The material analysed by experimental tests is a steel X20Cr13 bar of circular cross-section with The optical microscope measurement and the COD (crack opening displacement) method are used to monitor and calculate both crack depth and superficial crack length during the tests. The validity of effective crack length approximation, estimated by continuous measurements of COD data for each fatigue crack path under cyclic loading, is considered.Growth rate tests were performed with an harmonic test-cycl...