Fatigue Crack Propagation in Metals and Alloys: Microstructural Aspects and Modelling Concepts

312 Pages·2007·3.8 MB·English

by  Prof. Dr.?Ing. Ulrich Krupp(auth.)

Ulrich Krupp
Fatigue Crack Propagation
in Metals and Alloys
Fatigue Crack Propagation in Metals and Alloys: Microstructural Aspects
and Modelling Concepts.Ulrich Krupp
Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31537-6
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Ulrich Krupp
Fatigue Crack Propagation
in Metals and Alloys
Microstructural Aspects and
Modelling Concepts
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ISBN: 978-3-527-31537-6
To my children
Lukas and Lorenz

VII
Foreword
Although it was understood very early that fatigue was a matter of creating and
propagating a crack to the point of ultimate failure of a specimen or structural
component, indeed long before “crystallization” theories of fatigue eventually died
from natural causes, no significant measurements of crack propagation kinetics
existed before the early 1960’s. Theories of crack growth existed before then, of
course, highly speculative and based on unrealistic models. Only after appropriate
fracture mechanics parameters were developed to allow the driving forces for
crack growth to be expressed for a wide variety of component geometries did pre-
dictive capability greatly improve for fatigue design purposes. The traditional
approach of designing against fatigue, known as the “total-life” approach, worked
adequately on the whole before the arrival of linear elastic fracture mechanics,
mainly because it was backed by extensive (and expensive) experimentation. It also
has undergone improvements, for examples in counting cycles during variable
load cycling and in strain (rather than stress)-based predictions, and continues in
wide use today, even in certain aerospace applications where usually fracture
mechanics considerations now dominate. The total-life approach can be valid in
situations where inspections for cracks are difficult or are economically prohibi-
tive. The success of fracture mechanics in applications to aircraft probably arises
from the necessity to use high stresses in aircraft and where relatively infrequent
very high stress cycles promote early crack initiation, even in the absence of signif-
icant manufacturing defects, on which some fracture mechanicians rely to justify
the approach.
Like most books on fatigue, the aim of Dr. Krupp’s book is to help to make bet-
ter fatigue life predictions. Although the title of his book implies an emphasis on
fracture mechanics, and does focus on fracture mechanics (and almost exclusive-
ly on metals), it contains an elaborate chapter on crack initiation, and it also deals
with the underlying physics of fatigue mechanisms. It is a well-balanced book in
other respects as well, in the referencing between early and recent publications,
between fundamental aspects such as dislocation considerations and modeling,
and particularly in the balance between mechanics and materials considerations.
It covers the range from elementary dislocation behavior to recent advances in
modeling. It is thus suited to a wide range of students from different backgrounds
and developmental stages; veterans of fatigue studies will find it interesting. In
Fatigue Crack Propagation in Metals and Alloys: Microstructural Aspects
and Modelling Concepts.Ulrich Krupp
Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31537-6
VIII Foreword
that it contains detailed treatments of microstructural effects on short crack
growth behavior, the book goes far to bridge gaps of understanding between mate-
rial scientists and mechanical engineers. Thus, Dr Krupp’s worthy book repre-
sents another important step in the evolutionary development of books dealing
with fatigue.
It is a pleasure to acknowledge a highly productive collaboration with Dr. Krupp
during his sabbatical visit to the University of Pennsylvania. His Penn colleagues
gained much from his industry and insights, of which his book is an excellent
example.
November 2006 Campbell Laird
Stockbridge, Michigan
IX
Preface
As long as machines are moving we have to care about fatigue and fatigue-crack
propagation. What we don’t know about fatigue damage mechanisms is hidden in
safety factors. However, in terms of light-weight design and economic engineering
design safety factors need to be chosen as small as possible, i.e., it has to be accept-
ed that technical materials exhibit microstructural changes and crack propagation
during service life. This book outlines shortly the present state in fatigue-life pre-
diction paying particular attention to fracture mechanics concepts. Today we know
that interactions between a growing fatigue crack with local features of the
material’s microstructure are most relevant for understanding and modeling fa-
tigue damage in the fatigue-limit range under high-cycle fatigue (HCF) and very-
high-cycle fatigue (VHCF) conditions. It is hoped that this book gives students in
advanced courses in materials engineering and researchers in industry and acade-
mia a picture of the sum of implications of short-crack propagation in fatigue life,
helping them to interpret failure cases and to identify strategies for mechanism-
based service-life prediction and the design of fatigue-damage-resistant material
concepts.
Beside a literature review this book contains the results of research work the au-
thor carried out at the University of Siegen, Germany, and the University of Penn-
sylvania, Philadelphia, USA. The complete work was encouraged and strongly sup-
ported by Prof. Dr. Hans-Jürgen Christ, holder of the chair of materials science
and testing at the University of Siegen. Working with such an outstanding scien-
tist and teacher was most enjoyable and I am greatly indebted not only for his great
effort in supporting my scientific career but also for his friendship. I was lucky al-
so to have the chance to closely collaborate with the Institute of Mechanics, Prof.
Dr. Claus-Peter Fritzen. This collaboration, which is greatly acknowledged, al-
lowed me to combine the experimental work with complex modeling concepts.
A very strong impact on the research presented in this book was the chance to
work together and to discuss the results with Prof. Dr. Charles J. McMahon, Jr. and
Prof. Dr. Campbell Laird at the University of Pennsylvania. Thanks to their hospi-
tality and friendship my family and me spent a great and unforgettable time in
Philadelphia and for that I wish to express my gratitude.
Of course, this book was only possible by the huge amount of work carried out
by my former colleagues and PhD students at the University of Siegen, Dr. Wil-
Fatigue Crack Propagation in Metals and Alloys: Microstructural Aspects
and Modelling Concepts.Ulrich Krupp
Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31537-6
X Preface
helm Floer, Dr. Alexander Schick, Dr. Olaf Düber, Dr. Boris Künkler, Helge
Knobbe, Philipp Köster, and Arne Ohrndorf and the great support by my col-
leagues at the University of Pennsylvania, Dr. William M. Kane and Dr. Alex
Radin. Beside their invaluable scientific contribution to this book it was and is their
friendship, which made work really enjoyable and for which I am very thankful.
My special thanks goes to Prof. Dr. Hans-Jürgen Christ, Prof. Dr. Campbell
Laird, Prof. Dr. Charles J. McMahon, Jr., and Dr. Boris Künkler for critically read-
ing parts of the manuscript and to Wiley’s editing and publishing team for their pa-
tience and support during the completion of this book.
However, without financial support successful research is hardly possible. I was
lucky that I received several grants from the German research foundation, Deuts-
che Forschungsgemeinschaft, and a personal fellowship from the Alexander von
Humboldt foundation allowing me to live and work in the United States. This sup-
port is gratefully acknowledged.
Finally I am deeply grateful to my family, especially my grandmother Johanna
Schlaadt, my parents Hiltrud and Peter and my two children Lukas and Lorenz.
Without their help and their love I never would have come so far and I wish to ded-
icate this book to them.