Full-Scale Fatigue Testing of Components and Structures

335 Pages·1988·14.227 MB·English

by  K. J. Marsh (Eds.)

Full-scale Fatigue Testing of
Components and Structures
Edited by
K. J. Marsh, ME, DPhii
Controller, Structures, Design and Materials Department, National Engineering
Laboratory, East Kilbride, Glasgow, UK
Butterworths
London Boston Singapore Sydney Toronto Wellington
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First published 1988
© Butterworth & Co. (Publishers) Ltd, 1988
British Library Cataloguing in Publication Data
Full-scale Fatigue Testing of Components and Structures
1. Materials. Fatigue. Measurement
I. Marsh, K.J.
620.1Ί23Ό287
ISBN 0-408-02244-2
Library of Congress Cataloging in Publication Data
Full-scale Fatigue Testing of Components and Structures
Includes bibliographies and index.
1. Materials—Fatigue—Testing. 2. Structural stability
I. Marsh, K. J. (Kenneth James), 1935-
TA418.38.F85 1988 620.1Ί23 88-10566
ISBN 0-408-02244-2
Photoset by Butterworths Litho Preparation Department
Printed in Great Britain at the University Press, Cambridge
Preface
Much has been written over the last hundred years on the subject of metal fatigue,
from many viewpoints: metallurgical or engineering; analytical or experimental;
research, design or development. In more recent years there has been a growing
realization in many industries, initially in the aircraft industry, but increasingly in
other fields, that the design and development process often requires a combination
of approaches involving both materials properties data and the testing of full-scale
components or structures. Relatively little has been published on the latter
approach; this book attempts to fill that gap. After examining why full-scale fatigue
testing is necessary or desirable, the subsequent chapters give examples of such
testing, and of how it fits into the design and development process, in a wide range
of industries. It is hoped that the reader will compare and contrast the different
approaches, thus gaining a broad overall picture relevant to his or her own field.
This book is published with the permission of the Director of the National
Engineering Laboratory of the Department of Trade and Industry, UK. I
acknowledge gratefully the assistance, in a variety of ways, of many colleagues at
NEL, and particularly that of Dr Joe Fairbairn, Manager of Structural Testing and
Analysis Division, who was associated with the early definition of the book. Many
thanks are also due to the authors of individual chapters, both from NEL and from
other establishments, many well known to me through the Fatigue Group of the
Engineering Integrity Society. Without their cooperation the book could not have
been written. Finally, thanks to my Secretary, Mrs Anne Black, for meticulously
typing much of the NEL material.
K. J. Marsh
East Kilbride
January 1988
v
Contributors
G. Asquith Chief Mechanical Technologist, Rolls-Royce PLC, Derby
J. V. L. Barker Manager, Bridge Test Section, Royal Armament Research
and Development Establishment, Christchurch
A. M. Clayton Structural Integrity Centre, Northern Research Laboratories,
UK Atomic Energy Authority
H. Crawford Components, Fatigue and Standards Division, National
Engineering Laboratory, East Kilbride
K. Denton Formerly Structural Testing and Analysis Division, National
Engineering Laboratory, East Kilbride
J. L. Duncan Structural Testing and Analysis Division, National
Engineering Laboratory, East Kilbride
J. H. Edwards Managing Director, Testwell Ltd, Daventry
Dr P. R. Edwards Managing Director, PP Data Ltd, Formerly Materials and
Structures Department, Royal Aircraft Establishment,
Farnborough
D. R. Everitt Head of Driveline Technology Section, GKN Technology Ltd,
Wolverhampton
Dr J. Fairbairn Manager, Structural Testing and Analysis Division, National
Engineering Laboratory, East Kilbride
R. L. C. Greaves Head of Structural/Mechanical Test Facilities, Westland
Helicopters Ltd, Yeovil
Dr S. J. Hill Head of Advanced Automotive Systems Group, GKN
Technology Ltd, Wolverhampton
R. Holmes Components, Fatigue and Standards Division, National
Engineering Laboratory, East Kilbride
Dr K. J. Marsh Controller, Structures, Design and Materials Department,
National Engineering Laboratory, East Kilbride
R. McLester Formerly Head of Fracture Mechanics, British Railways
Research Division, Railway Technical Centre, Derby
VII
viii Contributors
Dr A. C. Pickard Chief of Materials and Mechanical Engineering, Rolls-Royce
PLC, Derby
Dr G. Sumner Structural Integrity Centre, Northern Research Laboratories,
UK Atomic Energy Authority
Dr G. P. Tilly Head of Structures Group, Transport and Road Research
Laboratory, Crowthorne
D.M. Waters Structural Testing and Analysis Division, National
Engineering Laboratory, East Kilbride
Dr D. Webber Bridge Test Section, Royal Armament Research and
Development Establishment, Christchurch
Acknowledgement
Chapter 9, The fatigue of military bridges, by
D. Webber and J. V. L. Barker, is reproduced with
the permission of the Controller of Her Majesty's
Stationery Office. © Crown copyright 1988.
1 Introduction
K. J. Marsh
1.1 Historical background
Metal fatigue has been studied for many years and particularly in recent decades
there has been a deluge of published work. The earliest publications appeared well
over 100 years ago and some of these very early works are discussed by Norman
Frost in a typically incisive and amusing review[l]. As he pointed out, Wöhler's
classic experiments were published in Britain in 1871 [2]. These set the foundation
for the fatigue testing of specimens and the determination of S/N curves, and
involved the development of several ingenious fatigue testing machines. Although
Wöhler's name is now often associated with rotating-bending machines, he in fact
devised machines for repeated tension, repeated torsion and repeated three-point
bending in addition to the rotating cantilever machine. The principle of most of the
machines was similar and involved setting the maximum desired load by the tension
in a spring acting at one end of a complicated lever system, the specimen being at
the other end. The load cycle was achieved by relaxing this maximum load by
means of a load applied through a reciprocating connecting rod at some suitable
point in the lever system. This connecting rod could be driven by any appropriate
power source. Using these machines Wöhler carried out many investigations on the
fatigue strength of specimens of various materials, particularly looking at the effect
of smooth fillet radii instead of sharp corners.
However, as early as 1850, there had been many important discussions on fatigue
(although that term was apparently not introduced until somewhat later) in the
Institution of Mechanical Engineers[3], particularly relating to failures in
wrought-iron railway axles, and one Archibald Slate spoke of a machine he had
made for subjecting square bars to reversed loading 'equivalent to 90 years of
railway service'.
In parallel with this early work on specimen testing, there was much interest in
components and structures and repeated loading tests were carried out around this
period on chains and beams[l]. One particularly well designed set of experiments
was carried out by Sir William Fairbairn, involving a series of repeated bending
tests on wrought-iron beams; the results were published in 1864[4]. Here a rivetted
wrought-iron girder, 6.7 m long and 0.4 m deep, fabricated from plate and
angle-irons, was subjected to a repeated central load of several tons, by a
mechanism driven by a water wheel. Figure 1.1 shows the arrangement, B being a
lever system at right angles to the girder A. The apparatus was designed to lower
the load quickly on to the beam, on each application, producing a considerable
1
2 Introduction
Figure 1.1 Repeated bending tests on awrought-iron beam, reported by Sir William Fairbairn in 1864
amount of vibration. The series of tests, which lasted from March 1860 to January
1862, and involved over 3 million repetitions of loading at one load level,
demonstrated quite clearly that whereas the girder had a virtually indefinite life at
one-quarter of the static failure load, at a load of one-third of the static failure load
failure could occur after a few hundred thousand repetitions.
Fairbairn had been requested to carry out this very extensive investigation by the
Board of Trade who established the regulations applying to the safety of bridges
and girders subjected to 'vibration and impact from a rolling load' at that time. The
intention was to verify, under realistic conditions, the design criterion for bridge
girders derived from tests on simple bar specimens of wrought iron, which
restricted the maximum design stress level to 5 tons per square inch. This criterion
was applied for many decades to the design of innumerable iron and subsequently
mild-steel components and structures.
It can be seen, therefore, that the earliest fatigue work included full-scale fatigue
testing. However, in the first half of this century fatigue research was dominated by
specimen testing, presumably because it was simpler, considerably cheaper, and
pioneering investigators such as Wöhler had shown the way. For example, two
important books on fatigue appeared in the mid-1920s, one by Gough[5] in 1924 in
Britain, and a very similar book in many ways in 1927, by the American authors
Moore and Kommers[6]. Both were concerned largely with descriptions of fatigue
testing machines and experimental methods for determining the fatigue limit of
simple specimens, as well as much discussion on the current theories of fatigue
failure.
By 1946, in the International Symposium on Fatigue held in Melbourne,
although specimen testing still dominated, we find Pugsley[7] stating that 'far too
little information existed on the application under controlled conditions of repeated
loads on structures (as distinct from material test bars)'. There was much discussion
Historical background 3
at this time about failures in service, in various industries, but little evidence of
full-scale testing, although Jackson and Grover[8], in a paper reviewing fatigue
failure in structures in fairly general terms, do mention 'the construction of large
equipment to test full-size structural units to failure under repeated stress'. They
quote full-scale tests on railroad wheels and axles, heavy springs, structural steel
joints, aircraft wing beams and landing gear and other large structures, giving a few
references, but also state that no complete analysis of dynamic stress distribution
and corresponding fatigue tests of a large structure appears to have been published.
Perhaps the next major milestone was the comprehensive International
Conference on Fatigue organized by the Institution of Mechanical Engineers in
1956. As well as the more basic studies, there was again much discussion on failures
in service, indeed three of the nine sessions were on 'the Engineering and Industrial
Significance of Fatigue'. However there was an increasing emphasis on full-scale
fatigue testing of components and structures, particularly in the transportation
industries - railways, automotive and aircraft. Tests on relatively small full-scale
components were clearly becoming much more common, particularly related to the
automotive industry. Love[9] described numerous simple constant-amplitude
fatigue tests on crankshafts, mainly single-throw, and gear teeth, while Duckworth
and Walter[10] presented extensive constant-amplitude results on plain bearings,
using two special-purpose test rigs. Coates and Pope[ll] developed a special
resonant fatigue machine for testing coil springs of some 60 mm diameter and
150mm overall free length, presenting numerous constant-amplitude results.
The testing of rather larger components reported included test rigs for complete
railway axles[12], a resonance fatigue machine for lengths of rail[13], and
alternating plane bending tests on welded thin-gauge box-section beams simulating
car chassis members, using a resonant vibration rig[14]. However, the only tests
described on complete structures were in two papers from the area of aircraft
fatigue studies. Payne[15] described a lengthy series of constant-amplitude tests on
complete Mustang wings, and Walker[16], in a useful review article, mentions
complete airframe tests, following the Comet crash investigations in 1954, including
repeated pressurization of the cabin plus gust loading on the wings. It is interesting
to note that gust loadings of wings were then apparently still simulated by
constant-amplitude 'equivalent gusts', although the amplitude of these loads was
known to vary considerably.
The question of the necessity for variable-amplitude fatigue testing was just
beginning to be discussed seriously at this time, although mainly in the context of
small specimen testing, to evaluate the validity of the Palmgren-Miner linear
cumulative damage rule. In this 1956 Conference, fatigue machines to subject small
materials testpieces to random loading, based on a random noise generator and
vibrator[17], or to what we would now call randomized multi-level programme
loading[18] were described. However, Gassner[19] had been propounding the need
for variable-amplitude fatigue testing for many years and had developed eight-level
block-programme testing extensively at his LBF Laboratory in Darmstadt,
proposing stress probability distributions appropriate to vehicles and aircraft. He
showed good agreement between the results of laboratory programme tests on a
simple vehicle component and lives of components in service, but inaccurate and
unsafe predictions from the Palmgren-Miner rule.
The arguments about the validity of this attractively simple design rule, and its
applicability to a wide range of materials and fatigue situations, were to rage for
many years. (Although frequently quoted, few realized that the original papers[20,
4 Introduction
21] proposed the rule for very restricted situations.) Much effort was expended in
investigating alternative rules during the 1960s[22] until it became generally
accepted[23] that the practical approach was to use some form of Palmgren-Miner
rule at the design stage, backed up by full-scale testing of prototypes if optimization
was essential.
However, the main reason that arguments about how to predict life under
variable-amplitude conditions from a constant-amplitude S/N curve were largely
swept away was the advent of a new type of fatigue testing equipment - the
servo-controlled electrohydraulic loading actuator - which enabled virtually any
kind of stress history to be applied to the testpiece. This had a major effect on
full-scale fatigue testing.
1.2 Servo-hydraulic fatigue testing equipment
In the UK servo-hydraulic equipment became readily available in the middle to late
1960s; for example, the first servo-hydraulic system was installed at NEL in 1968. It
is not an exaggeration to state that such equipment revolutionized fatigue testing
with its flexibility. No longer was the test engineer limited to constant-amplitude
sinusoidal loading or crude mechanical block-programme loading. Any waveform
within the positive and negative load or deflection limits of the loading actuator
could be applied (given sufficient hydraulic power). Inputs could be provided from
function generators, random noise generators, magnetic tape or, subsequently,
from digitally generated signals. The way was therefore open to applying
service-recorded load histories or other derived randomly varying inputs rather
than crude simulations of these. The control loop could be closed in load,
deflection or strain control modes. Large load capacities could be readily provided,
given sufficient hydraulic power, and much greater deflections accommodated than
was possible in conventional mechanical machines. In fact, the brute force of
hydraulic power had been harnessed to the sophistication of modern electronic
control systems.
The principle of a servo-hydraulic system is straightforward, and is shown in
Figure 1.2. The load generated by a hydraulic loading actuator is measured by a
strain-gauged dynamometer or load cell in series with the specimen. The signal
from the load cell is amplified and compared in a differential amplifier with the
desired input signal. The output of the differential amplifier is transmitted to the
servo-valve which controls the flow of pressurized oil into either end of the loading
actuator. The system thus forms a closed-loop control circuit. The loop may also be
closed by the output from a displacement transducer or a strain gauge on the
specimen if desired. The hydraulic power is provided by the power pack (oil supply,
pump, filters, accumulators) which operates at constant pressure, usually about
20MPa.
There are some disadvantages, of course. Power requirements are massively
greater than those of resonant machines, particularly if high frequencies and large
load or stroke capacity are required. As early as 1965, the merits of resonant
compared to servo-hydraulic fatigue machines were discussed by Haas and
Kreiskorte[24]. However, the advantages of realistic service-simulation testing
greatly outweigh any disadvantages, particularly when applied to fatigue testing of
full-scale components and structures. Servo-hydraulic equipment may be in the
form of a single actuator in a conventional fatigue machine frame, which is often
Reasons for full-scale fatigue testing 5
Input
Amplifier signal
generator
/ / / / /, Z
Load cell
v?
Specimen Differential
amplifier
-Strain gauge
Loading
actuator
-4- Accumulator
Displacement
transducer
Figure 1.2 Diagram of a servo-hydraulic loading system
appropriate for testing small components, as discussed in Section 1.5. For more
complex simulations and tests on structures, it is often necessary to use a number of
loading actuators in some form of test rig.
1.3 Reasons for full-scale fatigue testing
We have seen that some of the earliest fatigue studies involved full-scale testing and
that, after a period of concentration on small materials testpiece data, the second
half of this century has seen an increasing trend towards full-scale fatigue testing of
components and structures again, aided by the ready availability of appropriate
testing equipment. It is instructive at this stage, therefore, to examine the
justification for such testing today, since it is certainly not an inexpensive
procedure. In fact, there are a number of important reasons which occur at
different stages of the engineering design and development process. These may be
considered in chronological order as:
1. Design data acquisition
2. Prototype development
3. In-service modifications