Cumulative Damage of Welded Joints

451 Pages·2006·3.839 MB·English

by  T.R. Gurney (Auth.)

Cumulative damage of welded joints
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Cumulative
damage of welded
joints
Tim Gurney
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
W
OODHEAD PUBLISHING LIMITED
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Preface
So far as welded joints are concerned, research on the problem of fatigue did
not really start until the time of the Second World War. It was not, after all,
until then that welding began to become the fabrication method of choice for
many structures; up to that time the use of rivets had been almost universal.
Indeed, if welding had not been invented, many structures, and even whole
industries, such as nuclear power generation, simply could not exist.
However, as welding started to become more widely used, fatigue soon
made itself apparent and it was not long before fatigue failures in service
began to become quite numerous. That was not particularly surprising because,
in the early days of welding, structures were necessarily being designed, so
far as fatigue was concerned, to very inadequate rules, but it took a few years
for them to suffer enough loading cycles for the problem to become apparent.
By the mid-1960s one could almost say that the problem had reached epidemic
proportions and at that time it was estimated that, in Britain alone, the loss
resulting from fatigue failures of welded structures was running into tens of
millions of pounds each year.
In retrospect, and speaking in general terms, it is fairly easy to define the
general phases in which the research was carried out. In the beginning, when
everyone was effectively starting from scratch, the main objective was to
define, by testing, the basic constant amplitude fatigue strengths of the various
types of joint that are commonly used, and there are many of them. This
involved producing constant amplitude S-N curves for each type of joint,
primarily under pulsating tension loading but also, to a rather limited extent,
under other stress ratios. The results of that work, carried out in many countries,
were subsequently used to define the basic design S-N curves that now exist
in various design Standards around the world. In order to set the background
to the remainder of this book, the results of that work are summarised in
Chapter 2 and Appendix A.
Rather surprisingly, again in retrospect, the work on defining constant
amplitude S-N curves tended to start by looking at transverse butt welds. At
the time they were considered to be the most important joints; obviously
they are important, but it was not then realised that fillet welded joints would
x Preface
give much lower fatigue strengths and that fillet welded joints were going to
exist in most structures. However, the work on butt welds also highlighted
another potential problem, namely the influence of weld defects.
Thus, following the work on ‘good quality’ butt welds, research followed
two main routes. The first involved looking at the effects of defects in butt
welds (e.g. porosity, slag inclusions and lack of penetration) and ultimately
led to the fatigue part of various defect acceptance Standards, such as the
British PD6493. The second was concerned with obtaining basic fatigue data
for a wide range of different types of fillet welded joints. It was at this stage
that it was realised that residual stresses, an inevitable consequence of welding,
could be an important variable in the overall fatigue problem; a brief summary
of their significance is included in Chapter 3. It was also discovered that the
fatigue strength of fillet welded joints could be very low indeed and attempts
were made to see if any improvement could be obtained by such simple
methods as using high tensile steel. Unfortunately, however, that produced
no improvement whatever.
It was at about this time that fracture mechanics was developed and shown
to be applicable to the fatigue problem. More or less concurrently metallurgical
investigation showed that fatigue cracks in welded joints starting at the weld
toe were almost invariably associated with small crack-like intrusions. Thus
for the first time it became possible to analyse, admittedly in relatively
simple terms at that time, the growth of fatigue cracks in welded joints,
particularly as crack propagation data for various parent materials was also
then becoming available. As a result of that work it quickly became obvious
why increasing the strength of the parent material did not lead to an increase
in the fatigue strength of welded joints.
At this stage the work on fillet welded joints also divided into two parts.
The first was devoted to investigating other methods (such as spot heating,
peening, toe grinding and TIG dressing) of improving the fatigue strength of
such joints, but by then there was a reasonably sound knowledge of what the
problems really were and consequently the work could be approached with
specific aims in view, such as the elimination of toe defects, the introduction
of compressive residual stresses, etc. The second branch of the work was
more concerned with the exploitation of fracture mechanics principles to
examine, theoretically, the influence of various aspects of joint geometry and
then to confirm the findings by testing. It was as a result of this work that
such features as thickness effect first became apparent, many of which are
now incorporated in current design Standards.
All the work summarised above was carried out under constant amplitude
loading. In the beginning that was because it was the only type of test that
could be performed, but more recently it was primarily because constant
amplitude testing machines were fairly readily available and, in any case, it
was desirable to obtain strength data for Standards which were consistent
with what already existed.
Preface xi
The fact remains, however, that many people realised that constant amplitude
tests did not adequately represent service conditions which, in most instances,
involved variable amplitude loading. In the relatively early days an attempt
was made to ‘bridge the gap’ by means of tests under block programme
loading, that being the only form of variable amplitude loading that was
possible with the testing machines of the time. Unfortunately, however, as
discussed in this book, that could give very wrong answers. It was not until
the arrival of servo-controlled machines that it became possible to carry out
realistic tests.
In general, variable amplitude testing has involved two types of approach.
The first has involved tests on various types of joints under loading spectra
which were intended to simulate the loading that was expected to occur in
service, mainly with a view to assessing whether or not the experimental
lives obtained were better or worse than those predicted by Miner’s rule.
Effectively, therefore, they were aimed at justifying the use of that rule for
design purposes. The second approach has been to try to define what particular
features of a loading spectrum are likely to lead to safe or unsafe predictions.
The results obtained in both approaches are summarised in this book, although
it is concentrated rather more on the second approach, since much of that
work has probably been seen only by a relatively restricted audience. While
it cannot be said that we yet know all the answers to the problem, we do at
least know some of the situations where the current design rules ‘do not
work’. It is to be hoped that, by bringing all the facts together in one place,
it may help others to tidy up some of the loose ends. No apology is made for
attempting to set out the experimental data results in considerable detail.
In the preparation of this book I am particularly indebted to the Welding
Institute, by whom I used to be employed, for their permission to make use
of material which has previously appeared in the Institute’s research reports.
I am also most grateful to the many other investigators and organisations
who have so willingly and generously permitted me to make use of their
results. So far as possible the source of all such information is referenced at
the point of use.
Nomenclature
Note: In general, the following list does not include variables which occur in
only one specific chapter; they are defined at the point of use.
a Crack length
B Plate thickness
C, m Material constants in crack propagation equation and in
equation of S-N curve
E Young’s modulus
E No. of cycles per block in which stress equals or exceeds
i
Pσ
i
I Irregularity factor
k Constant defining spectrum shape of Weibull distribution
(for Rayleigh k = 0.5, for Laplace k = 1.0)
K Stress intensity factor
K Stress concentration factor
t
L Attachment length (toe to toe)
M Correction factor for K allowing for crack being situated at
K
position of stress concentration
M Correction factor for K allowing for crack tip plasticity
p
M Correction factor for K allowing for crack mouth being at a
s
free surface
M Correction factor for K allowing for plate thickness
t
N Cycles to failure
N No. of blocks to failure
B
N Constant amplitude life under top limit stress in the spectrum
c
N Number of exceedences (cycles)
E
N Block length (cycles)
L
N Predicted life (cycles)
pred
p Proportional stress (= Stress/Maximum stress)
i
( )
∆s
P or P(S) Probability of load exceedence
σ
R Stress ratio (S /S )
min max
xiv Nomenclature
R Ratio of experimental life/life predicted by the Area rule
G
S Stress
S Minimum stress in cycle
min
S Maximum stress in cycle
max
S Mean stress
m
S Stress range
r
S Fatigue strength of joint using basic S-N curve
B
T Thickness
t Thickness corresponding to basic S-N curve
B
T′ Apparent thickness
v No. of cycles per block of stress range p
i i
y Correction factor for K (function of geometry) =
M M M M
k s t p
φ
o
( )
α Non-dimensional crack length = a
b
∆S
β Clipping ratio = max
σ
∆K range of stress intensity factor
∆K Threshold value of ∆K for crack propagation to occur
th
σ rms of process standard deviation of log N
σ Yield stress
y
φ Complete elliptic integral
o
Σ n
Miner summation
N
1
Introduction
1.1 Background
Fatigue can be defined as a mechanism of failure which involves the formation
and growth of a crack or cracks under the action of repeated stresses. Ultimately,
of course, the crack may propagate to such an extent that total fracture of the
member may occur. In other words, fatigue failure can only occur in a
structure in which the stresses vary with time in a repeated manner. Note,
however, that this does not imply that the various stresses all have to be of
the same magnitude; it is much more usual for them to have a wide range of
magnitudes, but both types of loading can lead to fatigue failure.
Fatigue has, in fact, been recognised as a problem, for more than a century,
dating at the very least from the classic fatigue tests on railway axles carried
out by Wohler, details of which were published in 1871 (Wohler, 1871). So
far as welded joints are concerned, however, the problem really dates only
from the Second World War, or even slightly after it, since it was not until
then that welding became a major production process, and after that it took
a little time for structures to amass sufficient load cycles for fatigue to rear
its head. More recently, of course, welding has become the fabrication method
of choice for almost all types of structure, so that scope for running into
fatigue problems in welded structures is immense.
Potentially there are many different sources of cyclic loading (often referred
to as fatigue loading, for obvious reasons), any or all of which may be
relevant in particular situations, and the following list includes some typical
types of structure in which they may occur. The list is not by any means
exhaustive but is merely intended to show examples. It includes, for example:
1. Fluctuating live loads
Bridges, crane gantry girders, diesel engine frames, locomotive underframes
and bogies, lorry chassis frames and axles, ships, cranes, earth moving
equipment, farm machinery, rock crushers and presses.
2. Acceleration forces in, or due to, moving structures
1