Practical Airframe Fatigueand Damage Tolerance

L. J. Bent

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TESTIMONIALS

Practical Airframe Fatigue and Damage Tolerance the course Fantastic course the perfect balance of problems to teaching. The problems at the end really brought it all together. Your thorough knowledge of the subject really showed. Thanks for passing it on. Fiona Scolley, Lead Stress Engineer, Assystem UK Ltd (November 2010)

An extremely good course very practical. Dave Carter, Stress Engineer, Assystem UK Ltd (November 2010)

The course covers the fundamentals with detailed illustrations and examples which help to create a solid base to understand the various aspects of crack growth and specific details more easily. I am specifically happy with the simple and easy to understand explanations to various questions raised during the course. Sachin Kumaz Rostogi, Stress Engineer, Goodrich IDC (April 2011)

Training instructor has good practical knowledge and experience. Vimal Kumar Jaiswal, Stress Engineer, Goodrich IDC (April 2011)

Personally the course has been very good to consolidate my understanding and experience. I have learnt hand checks and a feel for things which were missing in my bag. Many thanks for a good course. Yogish B. E., Aerospace Engineer, Atkins Aerospace Bangalore (April 2011)

Overall very good and very well presented. Scott Ferry, Consultant Engineer, Airframe Designs Ltd (May 2011)

Excellent hand-outs and worked examples. Andrew Somers, Stress Engineer, Atkins Aerospace Glasgow (May 2011) F&DT is a difficult subject to pitch right. Les has done this extremely well. Rob Smith, Lead Stress Engineer, Spirit AeroSystems (May 2011) Good knowledge of subject well communicated. Andrew Thomas, Lead Stress Engineer, Spirit AeroSystems (May 2011) Practical Airframe Fatigue and Damage Tolerance the book I have to say I'm very impressed. This should be standard reading for any young engineer like myself ... I'm pleased to see it is full of clear explanations and the calculations are used to provide supporting evidence and reference information. Your service has been excellent and I appreciate it very much Tyrel Beede, Irvine, California See also http://mechanicalhacks.wordpress.com/2011/03/01/bearing-loads-cosine-and-gencoz-distributions/

PRACTICAL AIRFRAME FATIGUE AND DAMAGE TOLERANCE

by

Les Bent

Ilustrations by Derek Paul

Published by Sigma K Ltd

Coylton, Scotland

http://www.sigmak.ltd.uk

Printed and bound by Antony Rowe Ltd, Eastbourne

ISBN 978-0-9565174-0-1

All rights reserved

Copyright Sigma K Ltd 2010

No part of the material protected by this copyright notice may be reproduced or utilised or transmitted in any form or by any means, electronic or mechanical, including photocopying , recording or by any information storage and retrieval system without prior written permission from the copyright owner.

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Chapter 1. FATIGUE AND DAMAGE TOLERANCE 1.1 Fatigue of aircraft structures 1 1.2 The rise and fall of safe life and fail safe 2 1.3 Damage tolerance 6 1.4 Damage tolerance certification regulations 7 1.5 Damage tolerant repairs 9 1.6 Widespread fatigue damage 10 Chapter 2. FATIGUE PROCESSES 2.1 Fatigue crack initiation 13 2.2 Effect of stress concentrations on fatigue crack initiation 19 2.3 Fatigue crack propagation 22 2.4 Fractography 22 Chapter 3. FATIGUE CALCULATIONS 3.1 Fatigue cycle definitions 25 3.2 SN curves and master diagrams 27 3.3 Power law factors 28 3.4 Form of SN data equations or tabular 30 3.5 Effect of tensile strength amongst alloy class 31 3.6 Test data 31 3.7 Effect of mean stress on fatigue life 32 3.8 Example of a simple fatigue calculation for a constant amplitude cycle 34 3.9 Stress spectra and counting 35 3.10 Miners rule 42 3.11 Scatter factors 44 3.12 Fatigue calculation programs 45 3.13 Strain life fatigue calculations 47 3.14 Some statistical aspects of fatigue 49 Chapter 4. FATIGUE OF JOINTS 4.1 Introduction 53 4.2 Stress concentration factors in joints 54 4.3 Stress concentrations in the bearing mode 57 4.4 Stress concentrations in the clamped and mixed modes 61 4.5 Secondary bending effects 65 4.6 Fretting 67 4.7 Failure locations under fatigue 68 4.8 Analysis of multiple row fastened joints 69 4.9 SN data for multiple row attachment joints 79 4.10 Lugs 85 4.11 Interference fit 91

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4.12 Cold working 92 4.13 Burr marks and scratches 95 4.14 Fatigue of bolts 96 4.15 Fatigue of bonded joints 97 Chapter 5. CRACKS UNDER STRESS 5.1 Fracture mechanics 102 5.2 Historical development 103 5.3 The effect of a crack in a stress field 107 5.4 Crack tip stress intensity factor 111 5.5 Fracture toughness 111 5.6 Crack growth 114 5.7 Crack growth programs 117 5.8 Threshold, closure and short crack behaviour 123 5.9 Crack growth retardation 127 Chapter 6. STRESS INTENSITY FACTORS 6.1 Centre cracked panel 130 6.2 Edge crack 132 6.3 Surface flaws 134 6.4 Small cracks at holes 136 6.5 Through cracks at holes 146 6.6 Stress gradients 147 6.7 Thickness changes 153 6.8 Crack tip bulging in fuselages 154 6.9 Compounding methods 160 6.10 Cracks in stiffened panels 165 6.11 Development of crack shape 178 6.12 Finite element and boundary element methods 179 6.13 Multiple site damage 179 Chapter 7. RESIDUAL STRENGTH 7.1 Nett section yield 181 7.2 Critical fracture toughness 185 7.3 Plastic zone size 186 7.4 R-curves 189 7.5 Residual strength of stiffened panels 196 Chapter 8. LOAD AND SPECTRUM DERIVATION 8.1 Loading spectra 200 8.2 Cumulative frequency (exceedence) diagrams 202

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8.3 Flight profiles 208 8.4 Gust and manoeuvre spectra 210 8.5 Ground spectra 225 8.6 Fatigue calculation using a cumulative frequency diagram 229 8.7 Fatigue calculation using a defined time sequence 236 8.8 Undercarriage loading 238 8.9 Omission of load levels during testing 239 8.10 Standard loading spectra 240 Chapter 9. DAMAGE DEFINITION AND INSPECTION 9.1 Identification of Principal Structural Elements and Structurally Significant Items 241 9.2 Fatigue stresses and damage mapping 242 9.3 Static stress levels 244 9.4 Geometry and stress concentrations 245 9.5 In-service data 248 9.6 Test data 249 9.7 Material properties 250 9.8 Areas prone to accidental damage 250 9.9 Initial damage assumptions 250 9.10 Detectable damage pre- and post-MSG3 252 9.11 Non-destructive testing (NDT) 258 9.12 The non-destructive testing manual 260 9.13 Eddy current testing (ET) 260 9.14 Magnetic testing (MT)/Magnetic Particle Inspection (MPI) 261 9.15 Penetrant testing (PT)/Penetrant Flaw Detection (PFD) 262 9.16 Radiographic testing (RT) 262 9.17 Ultrasonic testing (UT) 263 9.18 The interface between the Stress Office and NDT 264 Chapter 10. CERTIFICATION CALCULATIONS 10.1 Overview of calculation process 265 10.2 Report layout 271 10.3 Assemble background data 271 10.4 Load path, expected damage progression and detectable damage 272 10.5 Limit load and test/in-service factors 272 10.6 Fatigue loads 272 10.7 Stress intensity factor calculation 273 10.8 Critical crack length 273 10.9 Threshold calculation 273 10.10 Repeat inspection period calculation 274 10.11 Single load path structure 277 10.12 Multiple load path structure CASE 1: SD item intact until PD item failure 278 10.13 Multiple load path structure CASE 2: PD item fails before SD item 280

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10.14 Multiple load path structure CASE 3: SD item fails before PD item 282 10.15 Reporting and summary sheets 284 10.16 General considerations 284 Chapter 11. DAMAGE TOLERANCE OF TYPICAL STRUCTURE 11.1 Lugs 287 11.2 Spanwise/chordwise wing joints 303 11.3 Spar joints 313 11.4 Fuselage lap/butt joints 317 11.5 Fittings 321 11.6 Stiffened panels 323 11.7 Wing/empennage cut outs and holes 332 11.8 Spars 335 11.9 Fuselage cut-outs and holes 341 11.10 Stringer run outs 345 11.11 Ribs 347 11.12 Fuselage pressure bulkheads 349 Chapter 12. DAMAGE TOLERANCE OF REPAIRS 12.1 Introduction 352 12.2 AC 120-93 and the three stage repair approval process 353 12.3 Considerations before metal is cut 355 12.4 Stop drilling 358 12.5 Use and abuse of power laws 361 12.6 Oversizing and bushing holes 365 12.7 Oversizing and spotfacing holes 372 12.8 Cold working of holes 374 12.9 Blend repairs to skin and flange elements 375 12.10 Repair doublers 380 12.11 Inspection requirements 387 12.12 Repair doubler proximity 390 12.13 Widespread fatigue damage and repair doublers 393 12.14 Availability of data 393 REFERENCES 401 SUBJECT INDEX 409

INDEX 409

Practical Airframe Fatigue and Damage Tolerance Index Note: Figures are indicated (in this index) by

italic page numbers, Tables by emboldened numbers, and footnotes by suffix n. Abbreviations used in index: PD = primary damage; SD = secondary damage; SIF = stress intensity factor; SSI = structurally significant item

1 g on the ground (1GOG) stress 210 2024-T3 material 35, 43, 80, 189 2024-T4 material 358 2024-T351 material 291 AC 120-93 repair approval process 3535 access rating [in detectable crack

calculations] 257 accidental damage, areas prone to 250, 290 adhesively bonded joints see bonded joints adhesives

high-temperature curing compared with cold bonding 65

and stress concentrations at knife edged fastener holes 645, 317

AFGROW material data 291 output file for example 298 SIF