High Temperature Component Life Assessment

High Temperature Component Life Assessment

High Temperature Component Life Assessment G.A. Webster Professor of Engineering Materials Department of Mechanical Engineering Imperial College of Science, Technology and Medicine London, UK and R.A. Ainsworth Nuclear Electric pic Berkeley Technology Centre Springer-Science+Business Media, B.Y.

First edition 1994 1994 Springer Science+Business Media Dordrecht Originally published by Chapman & Hali in 1994 Typeset in 10/12 pt. Palatino by Thomson Press (India) Ud., New Delhi ISBN 978-90-481-4012-1 ISBN 978-94-017-1771-7 (ebook) DOI 10.1007/978-94-017-1771-7 Aparl from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries conceming reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Catalog Card Number 93-74446 r Printed on permanent acid-free text paper, manufactured in accordance with ANSIINISO Z39.48-1992 and ANSI/NISO Z39.48-1984 (Permanence of paper).

Dedication We are indebted to our wives, Sheila and Gina, without whose patience, encouragement and dired help this book might never have been completed. We, therefore, gratefully dedicate the book to them.

Contents Preface 1 1.1 Introduction Outline 1.2 Improvements in plant utilization and efficiencies 1.3 High temperature design procedures 1.4 Residual life assessment 1.5 Aims of the book 2 2.1 2.2 2.3 2.4 Processes of deformation and fracture at high temperatures Nature of creep Stress and temperature dependence of secondary creep Time dependence of creep Characterization of fracture 2.5 Time-temperature creep parameters 2.6 Creep under variable stress and temperature 2.7 Complex stress creep 2.8 Damage mechanics concepts 2.9 Fatigue effects 2.10 Summary 3 Stress analysis of uncracked bodies 3.1 Creep bending theory 3.2 Axisymmetric creep stress analysis 3.3 Energy methods 3.4 Reference stress concepts 3.5 Failure due to creep damage propagation 3.6 Summary xi 1 1 2 3 5 7 8 10 10 14 16 19 21 26 32 34 38 46 46 48 50 50 55 59 61 67 75 75 77 4 4.1 Stress analysis of cracked bodies Linear elastic fracture mechanics concepts 79 79

viii Contents 4.2 4.3 4.4 4.5 Small-scale yielding Elastic-plastic fracture mechanics concepts Creep fracture mechanics concepts Influence of stress redistribution 4.6 Summary Appendix A4 Some fracture mechanics solutions 83 85 100 110 121 121 124 129 5 Models for creep crack initiation and growth 132 5.1 General observations 132 5.2 Characterizations of creep crack growth 133 5.3 Mechanism of creep crack growth 136 5.4 Steady state creep crack growth models 136 5.5 Models of incubation period 148 5.6 Transient analysis of 'tails' 157 5.7 Crack propagation into damaged material 160 5.8 Residual life assessment 167 5.9 Relationship between fracture by crack growth and continuum damage 169 5.10 Summary 170 171 172 6 Creep-fatigue crack growth 175 6.1 Introduction 175 6.2 Types of loading cycle 176 6.3 Fatigue crack growth 177 6.4 Elevated temperature cyclic crack growth 201 6.5 Prediction of creep-fatigue crack growth 223 6.6 Modelling of displacement controlled cyclic crack growth 226 6.7 Summary 242 243 245 7 Experimental determinations of high temperature crack growth 248 7.1 Specimen geometry and testing arrangement 248 7.2 Methods of measuring crack extension 250 7.3 Measurements of deflection 254 7.4 Analysis of data to obtain C 255 7.5 Presentation of crack growth rate data 257 7.6 Presentation of crack incubation data 259 7.7 Validity criteria 260 7.8 Creep-fatigue crack growth data 261 7.9 Testing of service exposed material 263 7.10 Summary 265

Contents ix 8 Practical applications 8.1 Procedure 8.2 Material properties 8.3 Defect assessment 8.4 Worked examples 8.5 Summary Index 265 266 268 268 276 280 288 315 315 318 321

Preface There is a trend towards the progressive use of higher operating temperatures and stresses to achieve improved efficiencies in, for example, electric power generation equipment, gas turbines and chemical reactors. This trend is resulting in an increased need for more reliable lifetime prediction methods for components subjected to creep and fatigue loading. Traditionally, the design of equipment for operation at elevated temperatures has been based on the assumption that it is' defect free. However, frequently engineering components have to undergo periodic mandatory inspections to assess their suitability for further use. If cracks are detected, some procedure is required for determining whether the cracks are acceptable, or whether they constitute a risk to safety and must be repaired or the plant taken out of service. The increased sensitivity of crack monitoring equipment is causing smaller and smaller cracks to be detected. In addition, hypothetical defects often have to be assumed present where inspection is not possible. Consequently defect assessment calculations have to be made more frequently than in the past. It is important that these calculations are realistic because serious economic penalties could be incurred if the plant is taken out of service unnecessarily or a disastrous failure takes place. Recently, rapid advances have been made in characterizing high temperature crack growth behaviour. The main concern of this book is with explaining these developments. Initially some industrial applications are considered to set the scene. The fundamentals of creep deformation and fracture are described and used as a basis for developing relevant stress analysis principles for uncracked bodies. Fracture mechanics and limit analysis methods are then introduced for dealing with cracks. Models for the initiation and propagation of cracks under static and cyclic loading are discussed and procedures developed for identifying the mechanisms controlling failure. Recommendations are included for obtaining reliable high temperature crack growth data experimentally. Finally some practical applications are considered to indicate how lifetime predictions can be made. Throughout the book, complicated mathematics has been minimized and emphasis placed on identifying fundamental principles and showing how these can be applied, in conjunction with simplified procedures, to obtain solutions to real problems without the use of extensive numerical analysis. The treatment is intended to be suitable for practising engineers, metallurgists, materials scientists and research workers who are involved in the design and operation or maintenance of equipment which operates at high temperatures.

xii Preface Most of the research and developments on which the book is based were carried out at Imperial College and within the former Central Electricity Generating Board. The authors acknowledge the considerable contributions of a succession of research students and their colleagues. They hope that readers will find the book stimulating and useful. August 1993 G.A. Webster R.A. Ainsworth