Department of Mechanical Engineering

Optimisation of Low Cycle Fatigue Life in Jet Engine Frame

Albert Ericsson Finn Rundstrm

Master Thesis in Solid Mechanics Linkping University

December 2006

LITH-IKP-EX--06239--SE

ii

Albert Ericsson and Finn Rundstrm Optimisation of Low Cycle Fatigue Life in Jet Engine Frame LITH-IKP-EX--062398--SE Trollhttan, Sweden, December 2006

iii

Abstract The aim of this is master thesis where the aim is to analyse the factors that limits the fatigue life of the GEnx TRF engine and, if possible, ways to prevent such problems to should be presented. Thermal and structural analyses have been carried out using the finite element program Ansys. Models from previous studies at Volvo Aero Corporation have been used. There have been three main studies, all of them concerning LCF life: Firstly, about 80 finite element models with local changes of the lug section have been performed; the thickness in the lug section has been varied, sometimes together with different types of flanges. This study increased the life to 280% of the original model. Secondly, global changes have been studied; in this case six different strut angles have been analysed. It was found that the current strut angle is the optimal one. Finally, a sensitivity study of the thermal boundary conditions has been carried out in order to investigate the importance of the uncertainty of the initial thermal assumptions. A good correlation between the life and thermal boundary conditions was found around the mount lugs whilst it is hard to draw any conclusions in the hub. Sammanfattning Syftet med detta examensarbete r att underska vilka faktorer som styr livslngdsbegrnsningarna i bakre turbinstativet (TRF) i GEnx -motorn, mestadels med inriktning p omrdet kring motorfstena, och om mjligt hindra dessa problem att uppst. Arbetet har huvudsakligen utfrts med hjlp av FE -programmet Ansys och utgtt frn modeller tidigare konstruerade p Volvo Aero Corporation. Tre strre studier betrffande lgcyklisk utmattning har utfrts: Frst och frmst har ca 80 finita element -modeller byggts med lokala ndringar av omrdet runt motorfstena, dr godstjockleken har varierats, ibland tillsammans med olika sorters flnsar. Denna studie resulterade i en kning av livslngden till 280 % av den ursprungliga. Utver detta har globala ndringar studerats; i detta fall i form av sex modeller med olika vinklar p ledskenorna. Slutsatsen r att nuvarande vinkel r den mest optimala. Slutligen har ven en knslighetsanalys av de termiska randvillkoren utfrts fr att underska inverkan av oskerheten hos de ursprungliga termiska antaganden som gjorts. Ett tydligt samband mellan de termiska randvillkoren och livslngden pvisades runt motorfstet, medan det var svrt att dra ngra slutsatser betrffande navet.

iv

v

Preface The GEnx jet engine is a light-weight engine that has recently been developed by Volvo Aero Corporation. In this work, the expected life of the Turbine Rear Frame will be investigated. Since the engine mount relies on this component, the demands on expected life are high and it is also a cost issue. In order to improve the expected life of parts around the engine mount lugs, the authors were asked to perform a master thesis and analyse if there is anything more that can be done to increase the life without doing too large changes of the structure. Previous studies have been made by our supervisor at Volvo Aero, Mr Hans Rydholm mostly regarded thermal stresses. This master thesis has mainly been carried out using computer models and other data from the work by Mr Rydholm, but in one part of the study Lars-Ola Normark has contributed. The work of this master thesis has been carried out during autumn 2006 at Volvo Aero Corporation, Trollhttan, Sweden. The authors are students from KTH (Kungliga Tekniska Hgskolan), Stockholm, and LiTH (Linkpings Tekniska Hgskola), Linkping, and the supervisors at the universities have been Professor Jan-Gunnar Persson and Professor Tore Dahlberg, respectively. Detailed values as well as material data and Ansys scripts will not be presented in this report due to Volvos secrecy. The authors would like to thank the supervisors at the universities and Volvo Aero as well as everybody else who has helped us with models, scripts and good advices during the work. Trollhttan, 2006-12-19 Finn Rundstrm, KTH, and Albert Ericsson, LiTH

vi

vii

Contents 1 Nomenclature.........................................................................................................1

1.1 Acronyms.......................................................................................................1 1.2 Explanations...................................................................................................1

2 Introduction............................................................................................................2 2.1 Company presentation, Volvo Aero Corporation ..........................................2

2.1.1 Volvo Group ..........................................................................................2 2.1.2 Volvo Aero History................................................................................2 2.1.3 Volvo Aero today...................................................................................2 2.1.4 Business areas ........................................................................................3

2.2 Principles of Turbo Fan Jet Engines ..............................................................4 2.3 The GEnx engine ...........................................................................................5 2.4 The GEnx Turbine Rear Frame (TRF)...........................................................6 2.5 Assumptions and limitations..........................................................................6

3 Theory ....................................................................................................................8 3.1 Cumfat 5.7......................................................................................................8

3.1.1 Introduction of Cumfat ..........................................................................8 3.1.2 Hypotheses for evaluation of multiaxial stress field..............................8 3.1.3 Maximum principal strain hypothesis..................................................10 3.1.4 Linear elastic to nonlinear elastic-plastic correction ...........................10 3.1.5 Cycle counting .....................................................................................12 3.1.6 Mean stress correction .........................................................................13 3.1.7 Fatigue life ...........................................................................................13

3.2 Concept generation ......................................................................................15 3.2.1 Basic methods ......................................................................................15 3.2.2 Morphological method.........................................................................15 3.2.3 QFD......................................................................................................16

3.3 Concept Evaluation......................................................................................17 3.4 Example of Concepts ...................................................................................18

4 Approach to a thermal-stress and LCF analysis ..................................................20 4.1 Creating the geometry and databases...........................................................20 4.2 Generation of thermal Boundary Conditions (BC) input tables ..................21 4.3 Running the analysis in Ansys.....................................................................23

5 Modelling.............................................................................................................25 5.1 The FE models .............................................................................................25 5.2 The 1B model...............................................................................................27 5.3 The 2B model...............................................................................................28 5.4 Element type shell181..................................................................................30 5.5 Element type Shell57 ...................................................................................31

6 Conceptual design................................................................................................32 6.1.1 Axial flange..........................................................................................34 6.1.2 Tangential flange .................................................................................34 6.1.3 Irregular flange.....................................................................................34 6.1.4 Move and modify material...................................................................34 6.1.5 Heat shield ...........................................................................................34 6.1.6 Thermal barrier coating (TBC) ...........................................................35 6.1.7 Preheating ............................................................................................35 6.1.8 Cold air flow ........................................................................................35

6.2 Go/no-go screening......................................................................................35 7 Sensitivity study.........................................................................................