PSI - Issue 12

Venanzio Giannella et al. / Procedia Structural Integrity 12 (2018) 404–415 V. Giannella Structural Integrity Procedia 00 (2018) 000 – 000

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Keywords: FEM-DBEM; fatigue crack-growth; turbine vane; load spectrum.

1. Introduction

Fatigue life prediction for turbofan engine disks, traditionally, has involved two distinct problems: a numerical stress analyses involving the critical regions and a crack modelling to perform the simulation of advancing damages, pre-existent in the component or in-service initiated. In order to predict the residual fatigue life of mechanical components successful applications of Fracture Mechanics (FM) methodologies must be achieved (e.g. in Maligno et al., 2015; Citarella et al., 2013, 2014b, 2018). An accurate characterization of crack paths and Crack Growth Rates (CGRs) require knowledge of flight cycle loading, accurate stress assessment, reliable material properties definition and precise Stress Intensity Factors (SIFs) evaluations. Experience has shown that advances in FM proceed through mutual interaction between numerical analyses and experimental observations (Citarella et al., 2005, 2014a, 2014c, 2015a; Calì et al., 2003). In this context, component design becomes very demanding due to high temperatures, complex mechanical loads, corrosive environment and long expected lifetimes. It is therefore of interest to accurately evaluate the impact of detected defects on these components (e.g. in Citarella et al., 2014d, 2016b) to avoid the catastrophic consequences of an in-service structural failure. Several fatigue failures of rotating compressor disks of civil aircraft engines were recorded during service. Such failures were the result of fatigue crack initiations and their propagation up to reaching a critical size. In all of the failures, the crack propagation started from part-through defects, initiated in between the compressor disk and the blade attachments (both made of a two-phase titanium alloy). These quarter-ellipse corner cracks developed in the slot fillets under the blades and near the disk outer surface. This paper provides a numerical investigation of a crack-growth process, following previously performed full-scale experimental tests of the same rotating disk, as available in literature (Shlyannikov et al., 2001). In particular, a peculiar variant of the submodelling methodology is implemented, based on a FEM model of the overall uncracked component, useful to calculate the global stress to subsequently use as input for the fracture assessment by a DBEM submodel (the crack is introduced at this stage). In order to reduce the computational efforts, the DBEM analyses are performed on a submodel including a restricted volume embedding the crack. FEM stresses, once converted into tractions, are applied on the crack face elements of the DBEM submodel, representing the only needed boundary conditions to perform the stress analysis and calculate SIFs, kink angles and CGRs. Wilson (1979) has briefly presented the theoretical background of such an approach in the past and, more recently, Giannella et al. (2017a, 2017b) have applied the approach to simulate fatigue crack-growth in real structures, also for thermal-stress problems with allowance for complex load spectra. The J-integral and the Minimum Strain Energy Density (MSED) criterion (Sih, 1974) have been used for calculating SIFs and kink angles respectively. The Paris’ law has been used to calculate CGRs. The crack path predicted by the FEM-DBEM approach has been compared with the path calculated by means of a FEM-FEM global-local approach and with an experimental path taken from literature. The adopted FEM based fracture simulation tool is FRANC3D (Wawrzynek et al., 2009). The adopted FEM and DBEM codes are ABAQUS (Dassault, 2011) and BEASY (BEASY, 2016) respectively.

Nomenclature C

Paris’ law coefficient da/dN Crack-Growth Rate (CGR) E Young’s modulus G Shear modulus J J -integral K Stress Intensity Factor (SIF)