MECHANISMS OF ENVIRONMENTALLY-INFLUENCED
FATIGUE CRACK GROWTH IN
LOWER STRENGTH STEELS
by
SUBRAMANIAN SURESH
B.
Tech., Indian Institute of Technology, Madras
(1977)
M.S., Iowa State University
(1979)
SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQ UIREMENTS FOR THE
DEGREE OF
DOCTOR OF SCIENCE
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May 1981
0
Massachusetts Institute of Technology,
1981
re- redacntedr
Signature of Autho r
e)rtment of Mechanical, Engineering
May 7,9 1981
Ce rti
fi ed by
/
/Signature reaactea
Robert 0. Ritchie
I
C
so
/
s UPe
Accepted by
Signature redacted
Chairman,
Archives
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
OCT 29 1981
U1BRARES
Department Committee
i
-
-
MECHANISMS OF ENVIRONMENTALLY-INFLUENCED FATIGUE CRACK GROWTH
IN LOWER STRENGTH STEELS
by
S. SURESH
Submitted to the Department of Mechanical Engineering on
May 7, 1981 in partial fulfillment of the requirements for
the Degree of Doctor of Science in Mechanical Engineering.
ABSTRACT
been
traditionally
have
steels
strength
Lower
of
presence
the
in
brittlement
to
em
considered to be immune
steels,
strength
lower
on
work
In this thesis
hydrogen gas.
it is clearly demonstrated that contrary to conventional
where
wi sdom, there are two regions of fatigue crack growth
crack
in
accelerations
cant
causes signifi
hydrogen gas
influences
Environmental
propagation rates compared to air.
these two regimes are sh own to be governed by entirely
in
different mechanisms.
In particular, in the near -threshold regi on of fatiguo
than 10
smal 1 e r
rates
(growth
propagation
crack
mm/cycle) , it is seen that with refere nce to a moist air
are
growth
rates
crack
near-threshold
environment,
dry helium, are
ga s and in
accelerated in dry hydrogen
marginally decelerated in di stilled water and remain the
are
results
surp rising
Such
same in wet hydrogen.
"oxide-induced
termed
pproach
a
new
a
of
terms
in
interpreted
several
crack closure" which is seen to be con si stent with
in
Evidence
.
al
observations
hitherto unexplained experiment
using
ind
iirectly
obtained
support of this model has been
analyses of fracture surface
spectroscopy and ESCA
Auger
ferri tic-ba i ni tic,
in ferritic-p earl itic,,
oxide deposits
steels.
Direct
fully
bainitic, and fully martensitic
is provided with
evidence for oxide-induced crack closure
of variable
Effects
aid of ultrasonic techni ques.
the
amplitude lo ads, in the form of block underload cycles below
are explained based on this mechanism and their
threshold,
to the
impl ications are noted with pa rti cul ar reference
Environmental influences
measurement of fatigue threshold s.
strength steels are
in the near- threshold region of lower
The consequences
compared wi th those in an alumi num alloy.
of this work are examined in the light of the effects near
o f not only various environments, but also such
threshold
ratio,
load
as
factors
and microstructural
mechanical
and
grain
si
ze.
trength
s
frequency,
Th esi s Su pervi sor:
Title:
Robert 0. Ritchie
Associ ate Professor
Department of Mechanical Engi ni eei ng
ii
-
-
ACKNOWLEDGEMENTS
It is my pleasure to than k
for
Professor
R.O.
Ritchie
me to enjoy the challenges of research.
motivating
His dedication and achieveme nts have constantly
of
source
a
been
I sincerely thank him for his
inspiration.
encouragement, stimulating ideas, and close friendship.
I
am very grateful
for
R.M . Latani sion
to Professors
D. M.
Parks
and
their e nthusiastic p articipation in
this work and many valuable comments and suggestions.
Thanks are due to Mr.
Palmer
J.
Lewis
and
Dr.
I.G.
of Lockheed Palo Alto Research Laboratories, Mr.
Martin, Mr.
M.I.T.,
Dr.
Lindley
of
England,
R.E.
A.
A.
Rudolph,
and
Mr.
M.Kurkela,
Joshi of Perkin-Elmer,
Electricity
Central
and Dr.
Generating
of
T.C.
Board,
for their help during the course of this work.
I am also very thank ful
Mr.D.DeLonga
for
to
Professor
their
assistance
J.H.Williams
with
and
ultrasonic
measurements.
The information, facilities
by
Lockheed
Missile
and
and materials
provided
Company,
Rockwell
Space
International Science Center, Westinghouse
RID
Center,
NASA-AMES Research Center, and Surface Sciences Division
of Perkin-Elmer are gratefully acknowledged.
iii
-
-
The support and friendship of my office-mates, both
past and present, are sincerely appreciated.
Finally, I wish to thank my parents and
who,
my
sister
despite 12000 miles of separation, have constantly
kept my spirits high through their weekly letters.
-
- iv
TABLE
OF
CONTENTS
Page No.
i
ABSTRACT.......................
ACKNOWLEDGEMENTS...............
ii
TABLE OF CONTENTS..............
iv
LIST OF FIGURES................
LIST OF TABLES.................
viii
NOMENCLATURE...................
ix
I.
INTRODUCTION...............
1.1
CONCEPTUAL BACKGROUND.
1.2
SCOPE OF PRESENT WORK.
...........
2.
N..
7
MATERIALS AND EXPERIMENTAL PROCEDURES....
2.1
MATERIALS............. ............
2.2
EXPERIMENTAL PROCEDURE ....
.........
9
9
19
3.
TWO REGIMES OF HYDROGEN-IND
24
4.
AN EXPLANATION FOR ENVIRONMENTAL EFFECTS NEAR
THRESHOLD ...............................................
51
5.
EVIDENCE SUPPORTING OXIDE-INDUCED CRACK CLOSURE..........
70
6.
INFLUENCE OF UNDERLOADS NEAR THRESHOLD ..................
79
7.
THRESHOLD BEHAVIOR IN DIFFERENT ALLOY SYSTEMS............
91
8.
CONCLUDING REMARKS ......................................
102
9.
CONCLUSIONS AND SUSUGGESTIONS FOR FURTHER RESEARCH.......
109
REFERENCES...................................................
115
APPENDIX.....................................................
121
LIST OF FIGURES
Schematic variation of fatigue crack growth rate with the
stress intensity range in steels, showing primary crack
growth mechanisms.
Fig. 2.1
Optical micrograph of SA542 Class 3, 174 mm thick base
plate at a)surface, b)quarter thickness ( T), and
c)mid-thickness ( T) locations (2% nital etch).
Fig. 2.2
Transmission electron micrograph of SA542-3 steel showing
globular Cr 2 3 C 6 and needle-shaped Mo 2 C particles.
Fig. 2.3
Micrograph showing the structure of BA2021 alloy; white
globular particles are CuAl
2
Fig. 2.4
Hydrogen permeation cell.
Fig. 3.1
Influence of frequency on fatigue crack growth rate in
SA542-3 steel tested at R = 0.05 in moist air and dry
hydrogen.
Fig. 3.2
Influence of load ratio R on fatigue crack growth rate in
SA542-3 steel tested at 50 Hz in moist air and dry hydrogen.
Fig. 3.3
Schematic showing regimes of hydrogen-assisted crack growth
commonly observed in lower strength steels.
Fig. 3.4
Fractography of fatigue crack growth in 24 Cr-lMo steel at
50 Hz in moist air and dry hydrogen gas (R = 0.3).
Fig. 3.5
Schematic illustration of the possible steps involved in the
embrittlement of steels by gaseous hydrogen atmospheres.
a is the maximum principal stress as a function of distance
xyyahead of the crack tip.
Fig. 3.6
Basic types of corrosion fatigue crack growth behavior
defined in ref. 26. a) true-corrosion fatigue, b) stresscorrosion fatigue, and c) mixed behavior.
Fig. 3.7
Effect of load ratio on the near-threshold fatigue behavior
of SA542-3 steel.
Fig. 3.8
Fatigue crack propagation in SA542-3 steel tested at R = 0.05
and 0.75 (50 Hz) in moist air and dry hydrogen.
Fig. 3.9
Fatigue crack propagation in bainitic SA542-3 steel tested at
R = 0.05 in environments of moist air, water, dry and wet
hydrogen and helium.
Fig. 3.10
Fractographs of near-threshold fracture surfaces of SA542-3
steel from tests in air, water, dry hydrogen and dry helium
environments.
.
Fig. 1.1
-
- vi
Fig. 4.1
Bands of corrosion deposits visible on near-threshold fatigue
fracture surfaces tested in moist air at a) R = 0.05 and
b) R = 0.75.
Fig. 4.2
ESCA analysis of crack tip corrosion debris in SA542-3 steel
showing the presence of predominately Fe 0
'
2 3
Fig. 4.3
Measurements of oxide thickness as a function of crack length
and crack growth rate for SA542-3 steel tested in moist air
and hydrogen gas at R = 0.05 and 0.75.
Fig. 4.4
Idealizations of oxide wedge inside near-threshold crack.
Fig. 4.5
Correlation between oxide thickness and cyclic CTOD.
Fig. 4.6
Variation of threshold stress intensity parameters with load
ratio.
Fig. 4.7
Nomenclature for stress intensity parameters at threshold and
their variation with load ratio R (after Schmidt and Paris).
Fig. 5.1
Schematic showing the location of transducers on the compact
tension specimen for ultrasonic testing.
Fig. 5.2
Variation of receiver signal with stress intensity.
Fig. 5.3
Schematic showing receiver signal output for uncracked,
saw-cut and fatigue-cracked specimens.
Fig. 6.1
Nomenclature of different parameters used to describe the
underload interactions near threshold.
Fig. 6.2
Crack length versus the number of cycles showing the effects
of underloads.
Fig. 6.3
Variation of cyclic CTOD, oxide thickness d, and crack growth
rate with stress intensity range and a scheme for finding the
critical underload stress intensity range.
Fig. 7.1
Variation of crack growth rate with stress intensity range in
SA516 steel tested in moist air and dry hydrogen environments
Fig. 7.2
Variation of crack growth rate with stress intensity range in
X-60 pipeline steel in moist air and dry hydrogen environments.
Fig. 7.3
Fatigue crack propagation behavior of BA2021 alloy in moist
air, dry hydrogen, distilled water and hydrazine environments.
Fiq. 7.4
Variation of excess oxide thickness with crack length in BA2021
allov.
-
- vii
Fig. 8.1
Schematic showinq the differerent modes of crack closure in
steels.
Fig. 8.2
Variation of threshold stress intensity range with yield
strength in several classes of steels.
Fig. 8.3
Fatigue crack growth behavior of Cr-Mo-V steel in moist air
and vacuum at 550'C (ref. 12).
Fig. Al
Schematic showing a typical deDth comoosition orofile of
Fe3, 01, and Cl peaks.
Fig. A2
Schematic showing oxide formation on crack faces and the
excess oxide thickness.
-
- viii
LIST OF TABLES
Table No.
2.1
TITLE
Page
Chemical composition, heat treatment,
ambi ent temperature mechanical properti es of SA542-3 steel.
10
2.2
Plate Chemistries of SA516, X-60
steels and 2021 aluminum alloy.
15
2.3
Ambient temperature mechanical properties of SA516, X-60, and BA2021
alloys.
17
3.1
Conditions for onset of hydrogenassisted crack growth in SA542-3.
29
4.1
Threshold data for 24 Cr-lMo steels.
63
6.1
Results of variable amplitude tests
involving effects of underloads on
near-threshold growth rates.
84
Threshold data for SA516 and X-60
steels.
95
7.1
-
- ix
NOMENCLATURE
a
crack length
a*
crack length over which retardation occurs
CH
local hydrogen concentration at point of
maximum dilatation ahead of crack tip
Co
equilibrium hydrogen concentration in
unstressed lattice
d
excess oxide thickness measured on fracture
surface
da/dN
fatigue crack growth rate per cycle
(da/dN)B
baseline fatigue crack growth rate at
AKB
fatigue crack growth rate following
initial
underload
(da/ dN
D
diffusion coefficient for hydrogen
permeation
E
elastic modulus
h
initial
h
final asperity height
K
linear elastic stress intensity factor
(Mode I)
Kn
plane strain fracture toughness
Kmax
maximum stress intensity during fatigue cycle
0
asperity height
minimum stress intensity during fatigue cycle
transition maximum stress intensity
K
Iscc
threshold stress intensity for environmental
attack
half the distance between location of peak oxide
layer and crack tip
x
-
-
N
number of cycles
N*
number of cycles over which retardation occurs
n'
work hardening exponent (cyclic)
R
load ratio (K
t
time
T
absolute temperature
)
RO
/K .
max min
universal gas constant
partial molar volume of hydrogen in iron
(2 cm 3 /mole)
x
hydrogen permeation distance ahead of the
crack tip
ACTOD
cyclic crack tip opening displacement
AK
alternating stress intensity range (Kmax
AKB
baseline alternating stress intensity range
AKff
effective stress intensity range
AK
threshold stress intensity range for. crack
growth (measured)
AKth
actual threshold stress intensity range
AKU
alternating stress intensity range at underload
AKUC
critical value of underload stress intensity
range
Kmin
)
-
AU F1reduction in cohesive strength due to hydogen
F_
ftrue
fracture strain
GO
hydrostatic tension
TVr
monotonic yield strength
AK
transition stress intensity range in mid-growth
regime
poisson's ratio
INTRODUCTION
1
1.1 CONCEPTUAL BACKGROUND
design
Modern approaches to
against
fatigue
failure
involve the application of linear elastic fracture mechanics
and take int
consideration th at all engin eering
are
The useful
flawed.
lif
e span of such components, then,
is estimated from the number 0 f cycles or time to
the
largest
assessed
undetected
ba ed
Because
the
of
approach in
attempted
on
to a crit
cr ac k
increasing
propagate
ical size which is
mechanics
fractu re
esign, a large
t
components
considerations.
of this "defect tolerant"
use
number
of
i nvestigators
characterize t he fa tigue cra ck growth behavior
of many engi eering material s in terms of such variables
mean
stress
forth.
as
frequency, mic rostr uctu r e, environment, and so
Al though a major portion
generated
have
of
thes e
data
has
been
for the mid-growth reg ime o f cr ack growth (growth
rates typi cally in excess of 10- 5 mm/cy cl e) ,
da ta
there has been a
rapidly
growing
extremely
low growt h rates (less than 1o-6 rm/cycle or of the
one
intensity rang e
below
la tti ce
fa tigue
spac ing per c ycl e
pertaining
to
where the stress
approaches a so called threshold value
,
of
for
)
order
need
which cracks remain dormant or grow at experimentally
undetectable rates (1).
-2-
The
of
use
in
essential
data
threshold
been
to
found
be
applications as large scale nuclear and
such
coal conversion pressure vessels,
turbine
shafts,
turbine
alternator rotors, and acoustic fatigue of welds in
blades,
gas
has
circuitry
nuclear
in
systems
reactor
high
where
frequency, low amplitude loads may be present (1).
It is now well known
from
available
the
crack growth rates are sensitive to several mechanical
that
and microstructural
variables, such as mean stress
/Kax ),
ratio (characterized by R= K,
monotonic
frequency,
forth.
with
and
cyclic
yield
A recent review of the effects
be found in Ref.
the
1.
propagation
load
of
size,
cyclic
strength,
and so
these
variables
The various mechanisms associated
of
fatigue
cracks
are
shown
1.1 for the near-threshold regime as
in Fig.
schematically
or
prior stress history,
grain size, grain boundary composition, crack
can
information
well as for higher growth rates.
The effect of
behavior
of
environment
materials,
some controversy.
information
in
influences near threshold.
has
resulted
in
near-threshold
fatigue
however, has remained a subject of
First
available
on
of
the
all,
there
very
little
literature on environmental
Even this limited extent of data
contradicting
inconsistent descriptions.
is
interpretations
and
10~2
10~4
PRIMARY MECHANISMS
REGIME B
I
NON - CONTINI. UM
MECHANISM S
CONTINUUM MECHANISM
(striation growth)
large influence of:
Ilittle influence of:
i. microstruc ture
ii. mean stres s
i. microstructure
ii. thickness
iii. environmen
11
KC
FINAL
IFAILURE
N
X
Imm/min
d
dN
B
z
2
-C
C
STATIC MODE"
MECHANISMS
I -6
large influence of:
"-I
lat tice spacin g
Iiii. certain combinations
of environment,meon
per cycle
stress
8frequency
A1
10-8
1mm/hour
(cleavage, intergranular
8 fibrous
large influence of:
i. microstructure
~
0
)
N
I m
Wc
C(A K)'
REGIME
E
E
0
-
-
REGIME A
ii. mean stress
1mm/day
iii. thickness
Ilittle influence
liv. environment
aT
U
of:
1mm/week
THRESHOLD A Ko
log A K
Fig. 1.1
Schematic variation of fatigue crack growth rate with the
stress intensity range in steels, showing primary crack
growth mechanisms.
-4-
Fatigue tests on low carbon steel
pressure
vess el
crack propagat ion
decrease
threshold AK 0
in
hydrogen gas c ompared
threshold
near
rates
was
fatigue
proc esses
involving
ev en
no
embri ttl
explained
though
air.
5-7),
conventional
corrosion
threshold (1,3 4).
nuclear
press u re
of
corrosion
em brittlement
The results of
are
fatigu e
Paris
and
with
the
inconsistent
(vi z.,
mechanisms
metal
corrosion as well as hydrogen
path
suggested
embrittlement)
in fluence
p recise mechanism f or hydrogen
however,
active
or
dissolution
This
hydrogen
ement was suggested (4) .
co-workers
c onsiderable
and
in te rms of conventiona 1
hydrogen
arguments,
normalized
the presence of d ehumidified
in
humi d
tu
and
sho w significant acce leration in
(3)
steel
(2)
for
environmental
ef fects
near
For ex ampl e , data on A533 B 1ow strength
v essel
s tee 1
(5)
tested
in
air
and
distilled wate r show no va ri ati on in threshold be havior with
change in envi ronmen t for tests at a frequency
tes ts
Threshold
0f
160
Hz.
on a low strength steel did not reveal an y
appreciable ch ange in behavior among
envi ronment s
of
air
distilled wate r, and gaseous hydrogen of u nspeci f ied purity
Results
threshold
air and
although
h igh
in
str ess
dry
and
strength
intensity
wet
argon
D6ac
steel
showed
that
th e
range values were u nchanged in
atmospheres
at
100-374
Hz,
near- threshold growth rates were mar g inal ly 1ower
in dry argon compared to air (8).
There is al s o
the
added
-5-
complication of a seemingly more severe environment actually
reducing the near-threhol d growth rates compared to air.
and Seth
higher
(9) repored reduced near-threshold growth rates and
AK 0 values in steam than in air at 1000 C.
On the other hand,
several
studies
have
found
vacuum increases the thresholdK0 compared to air.
data on high purity heats of tempered E24 steel
Cr
Tu
stainless
martensitic
have clearly demonstrated
steels (En
that
the
that
Threshold
(10) and 13%
56 and FV 520b)
near-threshold
(11),
growth
rates are decreased and threshold AK 0 values are increased in
vacuo compared to
re-welding
has
behavior (1).
air.
The
possibility
of
crack
flank
been suggested as a partial reason for this
The
results
of
Skelton
and
however, show that in zero-tension loading it
Haigh
(12),
is possible to
propagate cracks in vacuum at much lower values
of AK
than
the threshold values in air.
It
is
evident
from
the
above
that
conventional
corrosion fatigue mechanisms do not explain the wide variety
of near-th reshold environmental influences on
in
the
literature.
Also
arguments
embrittlem ent are inconsistent with the
that
low
based
steels
on
traditional
cited
hydrogen
notion
strength steels are relatively immune to hydrogen
attack at low stress values.
Moreover, the purity levels of
the vacuum and inert gases used in different studies are too
-6-
different to allow any reasonable comparisons.
has
been
speculated
for
Although
a number of years th at corrosion
deposits on crack faces might play some role in
the
it
rates of crack growth,
determining
no quantitative anal ysis has yet
been carried out to substantiate the exact
natu re
of
this
clear
that
influence.
In
with
light of the above discussions,
the
limited
extent
of
it
is
information avai lable in the
literature,
it
mechanistic
interpretations for environmental
threshold
stress
proposed
thus
is
not
possible
intensity
far,
is
to
levels
capable
make
of
i nfluences at
that
and
viable
any
no
providi ng
model,
possible
explanations for the experimental observations.
The purpose of this work is
data
base
for
the
effect
of
to
establish
environments
a
reliable
in
the near
threshold fatigue crack growth regime of low strength steels
and
to
develop
a mechanism to explain the influences near
threshold of not only various environments,
mechanical
and
microstructural
size, load ratio, and so forth.
but
also
such
factors as strength, grain
-7-
1.2 SCOPE OF PRESENT STUDY
In the present study,
it is clearly
demonstrated
that
the
signifi cant influence o f environments in the mid-growth
and
in
the
propagation
near-threshol d
are
(chapter 3).
closure"
h as
termed
approac h
been
devel oped
experi mental
ne ar-thre s hol d
ratios,
reduced
in
compared
to
moi st
dry
which
hydrogen
in
an d
0f
enhan ces
cl osure
pressu re
vessel
hel ium
is
role
of
consistent
at
are
as
(such
low
load
significantly
water
or
air)
such as h ydrogen or helium)
corros io n
excess
crack
be
t hat
rates
growt h
envi ronments
oxide-induce d crack
fatigue
i s seen
It
to
the
crack
in a 1ow strength pressure
env ironments
due to the f ormati on
faces
shown
obs ervations
(cha p ter 4).
ex pl ain
c osure.
to
deposits
crack
The significance of
th reshold
near
-
vessel steel .
crack
"ox ide-induced
to
environment near threshold a nd is
with
fatigue
of
by entirely di fferent mechanisms
governed
A new
regions
corrosion
t es ted in air, water,
steels
noted.
In
particular,
a
characteriza tion of crack flank oxide de po0 sits is made using
X-ray photoe lectron
Evidence
spectroscopy
su pporti ng
the
concepts
closure is p rovi ded in chapter 5.
observations
are
highlighted
and
of
spectroscopy.
0 xide-induced crack
The consequences of these
with p articular reference to
the measurement of near-threshold data
for
Au ger
and
an
explanation
the effect of underloads on fatigue crack growth at low
-8-
stress intensities is given (chapter 6).
environmentally-affected
threshold
strength pressure vessel steel
different
classes
behavior
7).
Further,
surface
mechanical,
patterns
additional
intensities
low
in
2021
aluminum
alloy
contributions to crack
resulting
from
varying
are discussed in light of the
morphology
microstructural,
commonly
the
low strength steels, viz., SA516 and
of
closure at low stress
fracture
of
are c ontrasted with those
X-60 pipeline steels, as well as in a
(chapter
The mechanisms for
and
observed
envi ronmental
at
near-threshold
behavior
levels
(chapter8).
It is hoped that
cl earer
insight
the
into
corrosio n fatigue crack
which
w ill
material s
form
with
results
the
of
mechanics
growth
at
this
and
work
provide
mechanisms
ne ar -threshold
of
levels
the basis for alloy design procedures for
greater
i nfl uenc ed fracture.
resistance
to
environmentally
-9-
2
MATERIALS AND EXPERIMENTAL PROCEDURES
2.1 MATERIALS
low
The
strength
investigation
is
a
steel
in
used
the
pressure vessel
&4Cr-1Mo
present
steel, namely
ASTM A524 Class 3 (he reafter referred to as SA5 42-3)
which
is a ca ndidate materi al for coal conve rsi on app I i cati ons.
thick
section of thi s steel
heat
(Lukens
370 7)
no.
mm from Westinghouse Research and Developme nt
chemi ca 1
compositio n
tr eatment,
heat
Center .
the
and
tempera ture mechanica 1 proper ties of this stee 1
Tab le
2.1.
thickne ss,
as
used
than
shown
mi crosc opy s tudies
molybde num
micros truc ture
The
bai ni ti c wit n I ess
(13).
was
with a size of approximately 140 mm x 300 m m x 175
obtai ne d
in
A
carbide
3%lo
in
polygonal
Fig.
showed
of
the
2.1.
as - rec e i ved
steel,
SA516
condition.
listed
a re
SA5 42- 3 wa s fully
1errite
presence
of
at
cent er
a nd
chromium
particles (Fig.
For c omparison purposes, three additional
plain C -Si
ambi ent
Transmission electr on
(Cr, C. and Mo 2C)
f or th e environmental
The
2. 2)
alloys we re
fatigue crack growth studies.
Grade
70,
was
Microstructures
studied
of
in
A
t he
this pipeli ne
steel w ere f ound to be 30% pearlite and 70% ferrite, with an
average
grain
si ze of about 35pm-.
The X-60 pipeline steel
-10-
TABLE 2.1
Chemical compositi-m, heat treatment, and ambie;t tm-perature
mechanical properties of SA542-3.
i)
Base Plate Chem.;try
C
Mn
0.12
0.45
Si
N__i
0.21
0.11
Cr
Mo
2.28
1.05
Cu
0.12
p
S
0.014 0.015
Heat
Lukens
Heat
#3707
ii)
As-received Heat Treatment
a)
Austenitize
5 1/2 hr
at
954*C
b)
Temper
8
at
663 0 C
c)
Stress Relieve 15 hr
at
593 0 C
22 hr
at
649 0 C
18 hr
at
663 0 C
hr
water quench
-11-
TABLE 2.1
iii)
(continued)
Ambient Temperature as-received Mechanical Properties (mean values)
Yield
strength
(MPa)
496 7
U.T.S.
Elongation
Reduction in
Charp* K
Impact
Area
Energy
(2Pa)
(2)
(%)
(J)
610 17
25
77
197 18
Computed from ,J..
-neasurements;
For dehumidifie. hydrogen at 138 kPa pressure
For 45 mm gau0e length
K
Iscc
(1Par)
295
>85
-12-
Fig. 2.1
Optical micrograph of SA542 Class 3, 174 mn thick base
plate at a)surface, b)quarter thickness (4 T), and
c)mid-thickness ( T) locations (2% nital etch).
-13-
Fig. 2.2
Transmission electron micrograph of SA542-3 steel showing
globular Cr 2 3 C 6 and needle-shaped Mo 2C particles.
-14-
was obtained from
Science
International
Rockwell
Center.
Metal 1ography of the samples revealed a fine, equiaxed grain
structure (approximat ely 10pm
ferrite.
The
al ong
the
of
direction
the
b anded
in the longitudinal direct ion .
structure, i .e.
In
appl icat ions,
addition, an aluminum alloy u sed in aerospace
namely,
"ba nded"
with
crack growth spec imens were m achi ned so that
the crack propagated
ferrite
diameter)
in
British Alumi num 2021 wi th a c op per content of about
6.4%, was investigate d
material
in
produc ed
was
prese nt
the
ingo t
as
show n
microstructure,
in
F ig.
2.3,
The
plate
th e British Aluminum
by
A1 can
Company and rolled to plate b y
work.
Pl a te
Company.
The
is characterized by a
distribution of globu lar part ic e s of CuAl 2 phase, which are
This mate rial also contains
located on grain boundarie S.
0.15% Cd
which
precipitates.
acts
for 10h.
in
a
a
he at
agent
for
the 0'
treatment
at
529'C
for
5h.,
gly col sol u tion, and dup lex aging at 163*C
plus 171 0 C for 12h.
temperature
nucl eati ng
Prior heat tre atment of the 19 mm thick plate
consisted of solution
quenching
as
The com p 0 si tions, and ambient
mechanic 1 properties of SA516 , X-60 and BA 2021
alloys are given in Tables 2.2 and 2.3, respectively.
-15-
U.
0
-
10
0
fftk~2
Micrograph showing the structure of BA2021 alloy; white
globular particles are CuAl 2
.
Fig. 2.3
,A-m
16-
TABLE 2.2
Plate Chemistries
Material
C
0.23
ISA 516
Mn
Si
1.2
0.25
Grade 70
Composition of X-60 steel plate, weight percent
0.147
Va
Mn =
1.4
Cu
=
0.049
p
=
0.008
Nb
=
0.047
S
=
0.012
AZ
=
0.029
Si =
0.255
Co
=
0.014
Cr =
0.008
Mg
Mo =
0.240
Ca
c
=
0.004
0.003
=
0.010
P
S
0.006
0.019
-17-
TABLE 2.2 (continued)
Chemical analysis of 2021 aluminum alloy, weight percent
Si
=
0.1
Sn
=
0.06
Fe
=
0.17
Zn
=
0.03
Cu
6.4
Ti
=
0.05
Mn
0.32
Zr
=
0.18
0.005
Cd
=
0.15
Mg
<
V
=
0.09
-18-
TABLE 2.3
Ambient Temperature Mechanical Properties.
Yield
Material
Strength
Ultimate
Strenguh
Elongation
K
(MPa)
(MPa)
(%)
SA 516
327
496
28
233
X-60
386
552-
418
489
8.5
r20
*
2021 AZ
25 mm gauge length
**
Computed from Jc measurements
(MPa m)
-19-
2.2 EXPERIMENTAL PROCEDURES
Fat igue crack propagation
12 .7
with
mm
cri ter ion
the
50k N
under
pla st ic zone sizes did not
c Yc lic
that
contr ol .
in
temperatu re
mo ist
Tes ts
performed
ai r
pct
(30
H ydrogen
gases.
clamped
onto
sieves
sm all
ring
o-
and
were
sealed
system
involving
cold tr a ps, and heat bakeable lines (14).
techni ques
achie ved
using
DC
electrical
cap a ble of detec ting changes in crack
length of the ord er of 0.0 1 mm.
measured
atmospheres
puri f i cati on
Crack growth moni toring wa s
potential
hydrogen
th e test p i ece, where gas purity was
obtained using an extensiv e
molecular
humidity),
relative
hel i um
and
ambient
at
co nducted
wer e
maintained at 138 kPa pres s ure in a
were
Testing was
a nd high p urity d ehumi dified
distilled water,
chamber
based
In stron el ectro-s e r vo-hydra ul ic machines operating
lo ad
helium
performed
compact specimens machined in the T-L
exceed 1/ 15 of tes t piece t hi ckness.
on
were
Plane strain conditions were maintained
orientation.
on
thick
experiments
N ear-t hreshold growth rates
oad-she ddi ng
under
(decreasing
stress
intensity) condit ions, wit h the th resho 1Ld L'defined in terms
of
a
mm/c ycle.
g rowth of rate 10-
maximum
For decreasing
load conditions, at each 1 oad level the crack was propagated
a
distance
at
least
fo ur
times
th e size of the maximum
plastic zone size formed a t the previou s load level.
rates
above
10-0 mm/cycl e
were
measured
Growth
at constant load
-20-
amplitude.
Full
experimental
details
have
been
described
elsewhere (1,3,4).
Characterization
fracture
surfaces
was
performed
controlled PH I Model 550,
ESCA
on
deposits
usi ng
ESCA/scanni ng
fatigue
a fully computerAuger
spectometer.
on fr actu r e surfaces a nd at 25 A depth
studies
ion sputter-e tchi ng)
of
corrosion
of
SA542-3
(after
were conducted on the fracture surfaces
SA516 and a luminum 2021 s amples.
By comparing
the resul ti ng ESCA s urvey spectra with standard spectra, the
compositi on
of the surfa ce oxide was deduced.
the exten t of ox idat ion,
obtained
depth
correspon ding
rates
tantalum
oxide
standard.
The
on
io n
argon
sputtering
yields
from Veeco Brochure V60 and Physical Electronics
0 and Fe Auger
monitore d
were
for iron oxide an d aluminum oxide were
estimated u si nq the data
(14).
profiles
compos ition
as a funct ion o f Auger sputte ring time, calibrated
using a k nown thi ckn ess o f
(atom/ion )
To determine
and
their
peaks,
as
amplitudes
well
From this informat ion,
thicknes s
fracture
surface
Auger sp ectrosc opy measurements
detail in the Appendix.
C
peaks,
were
measured as a function of
sputter time.
of
as
an
estimate
of
the
oxide debris was obtained.
are
discussed
in
greater
-21-
Experiments to measure the rate of
SA542-3
in
procedures of Kurk el a and
develop ed
technique
electrochemical
the
utilized
by
L atan ision
(15 ),
in
2.4
Fig.
permeation
De vana than and S tachurski
cell
the
on
based
schematic diagram of the hyd roge n permeati on
shown
transport
hydrogen
(16).
is
used
The mat erial unde r investigation is
made a bi-electrod e i n the f orm of a membr ane (about 1.5
thickness)
entry)
of
and anode
c1amped
an d
(
in
be tween
both solutions
experiment
by
(hydrogen
An el ectr olyte solu tion of O.1N H2 S0
4
is used in the cat hode half-cell
m inimize
To
the cathode
mm
hyd rogen e xit) hal f-cell s exposing 1.27 cm
the surface ar ea.
half-cell.
A
ar e
the
de-aerated
and 0.1N NaOH in the
anode
effects of oxygen reduction,
prior
bu bbling nitroge n gas.
the
during
and
to
Hy drogen charging is
controlled by a ga lvanostatic ci rcuit whic h insures that the
cathodic
charging
resistance
and
current
not
the
is
cell
a
functi on of the external
resistanc e.
is
Hydrogen
adsorbed into the membrane and d iffuses th rough to the other
The anodic side of the sp ecimen is
side.
at
maintained
a
constant potential versus a satu rated calo mel electrode by a
potentiostat so th at the concentration of hydrogen
surface
side,
it
is zero.
0n
that
Thus, when hydrogen emerges at t he anodi c
is immedi ately oxidized and the
current
thereby is a direc t meas ure of the permeation rate.
sure that the oxidation of hydrogen is
the
sole
g enerated
To make
reaction,
C ATlHODE SIDE
-Electto-
E
me
It
Polenliostal
qSi
clptW IR A
Ga
A mmele
Gas
-Gas
input
output
Gas input
N2
N2
*recorder
Colomel
electrode
N)
Deta
0 - in pPb
i
rei
P.1
e lectrode
%.ucim
Iu
coutr
n terl
electro2de
n probeio
5on Idion
soIN
drainn
~2
To i he
grips of
ANODE SIDE
on Instron
Externalt
e auae
machine
Power
supply
Fig. 2.4 HYDROGEN PERMEATION CELL
K
-23-
the
anodic
side
is
plated with palladium to suppress the
oxidation of iron (this has
behavior).
The
permeation
no
effect
current
on
is
el ectrometer and recorded on chart paper.
were carried out at room temperature.
the
permeation
measured
All
by
an
experiments
-24-
TWO REGIMES OF HYDROGEN-INDUCED CRACKING
3
Lower
strength
steels,
yield
with
typically below 1 000 MPa, hav e traditional
immune to em brittlement i
to be relatively
hydrogen gas, a notion which has persisted
K Iscc
stress
threshold
inte nsities
cracking under su stained load ing.
strength
values
been considered
the presence
of
ue to their high
ydrogen-assi sted
for
However
from the initial
results obtained in this work on low stren, h steels, it has
become evident th at such stee ls may be susce ptible to marked
hydrogen-assisted cracking un der cy clic
Ioa ding
at
stress
The ambi ent temperature
KIs Xc
in
fatigue cr ack pro pagation beh av i or of SA542- 3 is shown
well
below
.
intensities
3. 1
Figs.
pressure
for en vi r onments of
dehumi d ified
(0.5
frequencie s
Fig.
3.2
and
gaseo us
to 50 Hz)
or
hydrogen
and load rati
3.1 shows t he effect of fr equency at
3 .2,. the effe ct of load ra
0.05, and Fig.
of 50 Hz.
fatigue
It
is apparent tha t
crack
propagation
signi fican t incre ases in
moist ambi ent air
cra ck
oist air and low
a
range
(0.05 to 0.75).
load
ratio
are
two
where
dry
hydrogen
rat es
regions
examined in the f 0 llowing sections.
in
these
of
causes
compared
as shown schematically in Fig.
characteri stics o f hydrogen effects
of
o at a frequency
there
growth
of
to
3.3.
The
regimes
are
5
6
7
8
9
K)
20
20
0
30n
An
I
I
2 /4 Cr - I Mo STEEL
';n
AC~
-
-2
4
)
AK ( k sif
3
SA 542 Class 3
R = 0.05 Ambient Temperature
50 Hz air
V 2 HzJ
(30% RH)
A
50 Hz
S
Ol
I0
0
VV
Environment
Frequency
*
E
-cc
2
5 Hz
dry
2 Hz
hydrogen (138 k Pa)
1-5
0 VIr
0.5 Hz
1-4
A
L4
ai
-6
0)
U
c
N
o di*
z
.0
aa
-8
-
010
ci
,'-
-3
0
le
a0
0
00 0
I lattice
0
spacing
per cycle
0
0
air
10
0
-9
10
Threshold AK 0
0
10
RI
81
I
3
4
Fig. 3.1
4
5
I
I
6
7
I
I
I
8 9 10
20
30
ALTERNATING STRESS INTENSITY,AK (MPaVm)
40
50
Influence of frequency on fatigue crack growth rate in
SA542-3 steel tested at R = 0.05 in moist air and dry
hydrogen.
60
0
A K ( k si 4F)
2
3
4
5
I I
N
L)
7
8
9 10
20
I
I
I I I
I
30
40
|
|
50
I
10-4
21/4 Cr- I Mo STEEL
3
z E0-4
}
0.7 5
0
V
H 2 (R=0.05)
SA 542 Class 3
Frequency = 50 Hz, Ambient Temperature
R
0.05
moist
K
-*
x
vx
H (R= 0.3)
air
V A
00.*
A0
-5
10
of
0.05
0.3
x
x
H 2 (R=0.5)
0.5
0.75
.
0.05 (0.5 Hz)
2
VA
00
".0.
H
H2
000
jf
*0
10
1
-6
>
RO
0
10-4
F-6
0
7
0
z
'0
0 5
6*
*0
H2
H 10
I
10
of
0
I lattice
spacing
per cycle
0
Air
0
a
: a 0
o -61
cr
-7
0 1
.
0
9
-8
10
13
Air
H2
R=0.75
a
R=0.05
1-9
I
2
Threshold AKO
14
3
I~
4
5
6
7
8
9 10
20
~I
I
30
40
ALTERNATING STRESS INTENSITY, AK (MPaVi)
Fig. 3.2
Influence of load ratio R on fatigue crack growth rate in
SA542-3 steel tested at 50 Hz in moist air and dry hydrogen.
-
E
6
-
1
-2
50
60
0
I
10-2
decreasing v
increasing
R
AIR (independent
of v and R)
-
E
E
-o
4-
-6
10
increasing
R
0-8
T
HYDROGEN
GAS
Kmax
m
AIR
*AKo(H 2 )j AKO(air)
log AK
Fig. 3.3
Schematic showing regimes of hydrogen-assisted crack growth
commonly observed in lower strength steels.
I
I
-28-
REGIME
excess
of
causes
enhancements
to
compared
air.
growth
in
the
r ates
times)
20
ma x (roughly
at lo wer AK levels with
provide d t he frequency is
3.2),
o n R).
below a critical value (dependent
and load ratio have absolutely
propagation
20
value,' termed
present case), i.e.
increasing load ratio (Fig.
crack
to
up
These accelera tions s eem to occur at an
approximately constant Kmax
MPa/-m
in
the pre sence o f g aseous hydrogen
10-5 mm/cycle),
in
rates
propagation
(crack
In the mid-growth regime
(
MID-GROWTH
no
How ever, frequency
0n
effect
the
for tests condu cted in mo ist
3.1 summarizes the Knax
fatigue
air.
Table
values for different freque ncies and
load ratios obtained for tests in h ydrogen.
The sudden
hydrogen
is
transition
observed
to
to
a
result
in
rate
growth
higher
from
cha nge
a
in
a
predomin ately transg ranul ar to a predomi nately i nte rgranular
fracture
in Fig.
is
at stress i ntens ity values of around K
3.4.
F
nax
as shown
The c haract eristic fractu re in ai r,.
however,
pred omi na tely
t ransgr anular with sp oradic i nte rgranuiar
both below a nd abo ve KT
This effect of hydrogen
max
also app ears to be reversi ble in the sen se that rem oving the
facets,
hydrogen envi ronment durin g the test,
val ues
above
KT
max
resul ts
in
at
stres s
intensity
up to 20 times de crease in
growth rates, whereas re-introducing hydrogen
leads
to
an
-29-
TABLE 3.1
Conditions for Onset of Hydrcgen-Assisted Crack Growth in SA542-3
Frequency
K
K
(Hz)
(MPa/M)
0.05
50
no effect
0.01;
5
no effect
0.05
2
21.8
22.9
0.05
0.5
21.2
22.3
0.30
50
15.3
21.9
0.30
5
14.4
20.6
0.50
50
11.8
23.6
-30-
AIR
HYDROGEN
AK=
14 MPa.iFi~
AK=
20MPa./m-
AK=
25 MPa /i-
Fig. 3.4
Fractography of fatigue crack growth in 24 Cr-lMo steel at
50 Hz in moist air and dry hydrogen gas (R = 0.3).
-31-
acceleration in propagation rates by a similar magnitude.
Although many mechanisms have been
for the hydrogen embri ttl
years
proposed
ement of steels, such as the
the
Pf ei l-Troi ano-O ri ani de cohes ion models (18) and
p res sure
(19
model
Zappfe
is unlikely that one s ingle theory
it
can provide a c ompl ete under standing of the prob 1em.
precise
of
mechanisms
the
over
(Since
embrittl ement are in most
hyd rogen
cases unknown, particul arly in low strength stee ls, w e imply
here
by
mechanisms
which
hydrogen e nters the lattice and
fracture).
promotes some degradati on in resistance to
Fo r
high strength steels, h oweve r, decohesi on theories appea r to
provide at least a part ial d escription of observed behav ior,
monoto nic
for
both
cy cli c
and
in
such as H 2 0 and H 2 S.
at mosph eres,
"hydrogen-producing"
loading
Such
models propose that hyd rogen is ad sorbe d f rom the gas phase,
or evolved by
reactive
electroc hemi c al
surface
at
the
re a ction
freshly
on
tip, thereby entering the
crack
und er the
lattice and being trans porte d,
(20) ,
driving
force
of
the stress gradient, to the region of maximum dilation ahead
of the crack tip where
place
presumably
(Fig.
3.5).
point
of
by
some
form
embrittlement
(Unlike b ehavi or ahe ad of a blunt
maximum
takes
a red u cti on in cohesion at interfaces
notch,
the
dil atati on ahea d of a sharp crack may be
very close to the crack tip, i .e.
crack
of
tip opening displacement.
of the order of twice the
Accordingly, in this latter
0y
yy
elastic /plastic
stress distribution
crack tip region
00
gaseous H2
2
embrittlement
~
F1
x
o
eg ion
4
Gas Phase Transport
2 . Physisorption
3 . Chemisorption (dissociation)
4. Solution in Metal Lattice (hydrogen entry)
5. Hydrogen Transport in Lattice
I
Fig. 3.5. Schematic illustration of possible transport processes involved in the embrittlement
of ferrous alloys by gaseous hydrogen environments.
IIC
-33-
critical
case, the
process
may
involved
event
rather
in
at
occur
the
some
embrittlement
microstructu rally
significant characteristic distance ahead of the crack tip).
reduction
in cohesive strength (a
can be assumed to
)
The
be proportional to the local concentration
of
hydrogen
at
this point, where
CH
C0
is the
unstressed
=
Co - exp(% /ROT)
equilibrium
iron
(3.1)
concentration
lattice,
the
v
hydrogen in iron, a0 the hydrostatic
constant,
of
partial
in
the
molar volume of
tension,
T the absolute temperature
and
hydrogen
R.
(2).
the
gas
Such models
are consistent with an increasing susceptibility to hydrogen
embrittlement
with increasing material
hydrogen enrichment factors
(CH
/Co
)
strength (21),
are
an
since
exponential
function of the hydrostatic tension.
The mechanistic aspects of hydrogen-induced cracking in
lower strength steels, on the other hand, are still unclear.
Classical decohesion models
they
predict
only
(18,21)
marginal
pose
a
problem
since
enrichment factors due to the
small magnitude of the hydrostatic tension in lower strength
materials.
For
external
environments, classical
models (19), where hydrogen is assumed to accumulate
internal
are
also
voids
in
pressure
within
molecular form and exert gas pressures,
inappropriate
since
internal
pressures
cannot
-34-
proposed
tha t
hyd rogen
a nner
unspecified
Beachem has
hydrogen.
of
pressure
external
the
exceed
in
deformation
assists
some
and thereby promotes fracture, whi ch is
after
consistent wi t h observations of reduced flow stresses
(22
hydrogen
ato m s
)
charging
.
disl ocations,
and
t0
importance
the
embri ttlement
undefined at t his time (23 ).
(24,25)
hav e
precis e interaction between
the
However,
of
clearly
iron alloys, remains
of
studies
Recent me tallographic
may
hydrog en
that
indicated
al though
the
enhance
nucleation an d growth of m icrovoids at internal surfaces
strength
low
it
bu t
steels,
is
no t
known whether such
fatigue,
"micro-decohesiv e" mechanisms are impor tant during
where a complete mechanistic ba sis for crack growth
the absence of environmental ef fects, is sti I1
strength
a
bainitic pressure vessel
load
marked
ratio/freq uency
with
coincident
a
growth
crack
fatigue
Hydrogen-assisted
in
even in
,
lack ing.
low
the
in
steel is characterized by
depen dence,
often
is
fractur e mode change from predo mi nantly
transgranul ar to intergranul ar cracking, and occurs above
critical Km.
Many
autho rs
cracking
in
which is smal 1
value, KTmax
have
observ ed
higher
streng th
ratio/frequ ency dependence,
The
(Fig.
phenom enon,
3.6b)
(26),
referred
has
when Kmax
compared to KIscc'
hydrogen- assisted
similar
steels,
a
with identi cal load
exc eeds Kscc
(26-29).
to as "stress corrosion fatigu e"
been
interpreted
in
terms
of
a
I
I
(c
)
)
(b)
(a
Og gressiveI
agg ressive
a ggressive
z
0
"0
inert
inert
inert
U
E'I
0
EI
log AK
Fig. 3.6 Basic types of corrosion fatigue crack growth behavior: (a) true corrosion fatigue;
(b) stress corrosion fatigue; (c) s.c.f. and t.c.f. combined. (after McEvily and Wei,1971).
I
-36-
superposition
(27)
or process competition (28)
load hydrogen-induced cracking and pure
of sustained
fatigue
mechanical
Reaction kinetic studies (20,30) indicate that
components.
such environmentally-induced cracking can be attributed to a
mechanism of hydrogen embrittlement,
Specifically, it
reactions at the crack tip.
in
gaseous
hydrogen
rate-limited by
atmospheres
this
appears
crack
growth
that
rate-limiting step
involves chemisorption of hydrogen atoms, whereas
environments,
surface
in
water
is limited by the oxidation of
freshly exposed iron surface at the crack tip (20,30).
Similar kinetic studies (30)
in a lower strength
steel
(SA542-2) have shown that hydrogen-assisted cracking in this
regime
(i.e.
chemisorption
rate-i imited
by
KT
aiso
)
is
max
for crack growth i n ga seous hydrog en, and the
above
acceleration i n growth rates has been attr ibu ted to hydrogen
embrittlement
mechanisms
(altho ugh
the
these mechanis ms for lower streng th s teel s
In
of
view
for
fracture
study,
it
the
are
uncertain).
presence of a pred omi na ntly in tergranular
hydrogen- assisted
is
preci s e nature of
not
c ra ck ing
unreasona bl e
to
in
t he
presume
present
that
the
embrittlement mechanism involves hydrogen- induced decohesion
at
grain
bou ndari es,
strength steel s (18).
similar
to
that
observed
in high
-37-
data
In terms of macroscopic growth rate behavior, the
in
3.1 and 3.2 indicate that an effect analogous to
Figs.
stress corrosion fatigue takes place in the mid-growth rates
6
10-
(above
mm/cycle) in lower strength steels, with the very
notable exception that it
well
occurs at stress intensities
T
KIscc
below
i.e.
at
where no such hydrogen
Kmax < KIscc
embrittlement would normally result under
sustained
purely
This suggests some synergistic interaction
loading.
than additive or mutually-competitive interactions)
and
environment
growth.
thus
continuously
providing
that the role
is felt
a
freshly
basic
some
through
However, rather than occuring
of cyclic loading is simply to maintain
tip,
between
fatigue contributions to crack
mechanical
in embrittlement mechanism, it
change
(rather
sharpened
crack
exposed
metal
surface there for hydrogen to adsorb.
One is now faced with a terminology problem
of
nature
the
characteristic
since
hydrogen-assisted growth observed, with its
fracture
mode
ratio
frequency/load
and
dependence, conforms to stress corrosion fatigue (c.f.
3.1 with Fig.
effect
clearly
in
lower
occurs "below
KIscc ".
3.6b),
yet
strength
been termed "true corrosion fatigue"
differ
from
steels
the
below
KIscc
has
(26,31,32) , as shown in
3.6a, yet the characteristics of
clearly
Fig.
Conventionally, such
environmentally-assisted fatigue behavior
Fig.
the
the
present
those depicted in this figure.
effect
It is
-38-
apparent that the commonly
termi nol ogy (26,31 ,32)
"above
used
steel s,
strength
high
KIscc
for corrosio n fatigue, whil st perhaps
provi ding si mple a nd u seful ci assi fication
insensiti ve
11
below
and
lower str engt h materials.
s train-rate
for
be misl eading for
can
This fol lows from the
fact
that
in such d uctile alloys, the st ress intensities nec essary for
the onset of such hydrogen-ind uced cracking
loading
for
(
embrittle ment)
K Iscc9
i.e.
this .
First,
ax
as
fatigue b y Ford (33) and
strength
high
threshold s
be
can
be
steels
should
o ut
for aqueous corrosion
Daws on
a nd
Pelloux
such
(3 4),
tha t
K Iscc
env ironmental
dy nami c
Second,
K Iscc
since
stressed under fixed-load
(i.e.cyclic)
measurements
or
ri si ng-load
critical event for determining the KIscc
p rot ective
In
expected
straining
are
condit ions,
often
fact,
environment
oxide
by
under
the
or fra cture
of,
scale previ ously formed at the crack
pre-crack i ng
test
and
threshold may often
simpl y in volve hydrogen permea ti on through,
tip.
be
on test pieces prev iousl y pre-cracked in air,
performed
the
lower
not be tak en as material const ants for a
under
reduced
condition s.
provided
point ed
particul ar alloy/environmental syst em, and can
to
and cyclic
sustained
Two r easons
).
hydrogen
are more str ain-r ate sensitive c ompared to
steels
str ength
und er
different
are
(from
and
in
the
employing
conditions on side-grooved specimens, Shaw
actual
hydrogen
fixed di splacement
(35) has recently
-39-
signifi cantly
that
demonstrated
approaching Kx
mat
lower
KIscc
lo w er
values, can be measured in
values,
strength
steels includin g 2 k Cr-Mo al 1 oy s.
Thus,
an
it is reasonable to regard the value of
in
intensity
stress
threshold
effective
which
threshold in
KIscc
steels
(26),
yet
max
as
fatigue for
embrittlement
gaseous hydrogen-induced cracking by hydrogen
mechanisms,
KT
is nominally equal to the sustained load
is
high
rate-insensitive
strain
in the
less than KIscC
considerably
present lower strength steels.
strength
hydrogen
of
role
(The
in
influencing fatigue crack growth at near-threshold levels is
to
considered
be
largely
of
independent
conventional
hydrogen embrittlement mechanisms as discussed later in this
chapter).
Similar
environmental
embrittlement mechanisms.
onset of the
above
a
in
or
threshold as
titanium alloys.
environmentally-assited
constant
place
in
Dawson and Pelloux
refer to their environmental
fatigue
systems,
alloy
where
contribution to cracking involves active
path corrosion in addition to,
corrosion
for
fatigue crack growth for the mid-growth regime in
aqueous environments for a number of
the
proposed
maximum
stress
of,
hydrogen
(34), in fact,
AKscc
for aqueous
However, since the
crack
growth
intensity
occurs
(K
max
)
corrosion
been
have
(33,34)
arguments
-40-
(independent of R) for the present case of gaseous
envi ronements
(Table
it is preferable to define the
3.1),
envi ronmental threshold in terms of
than
a
parti cular
alternating
perhaps worth noting here that
who
a
value,
ra ther
intensity.
I t is
Kmax
stress
certain
(2,36- 39),
authors
have exam ined fatigue crack propagation behavior ov er a
limited range of growth rates, have
this
hydrogen
environm ental
threshold
fatigue crack growth.
interpr eted
mistakenly
as
threshold fo r no
the AK0
in
As shown by the data
and 3.2, this is clearly erroneous, since AK
Figs.
3 .1
thresholds are
generally an order of magnitude smaller.
Finally, the question arises as to the reason
existence
of
environmental
an
for
the
th res hold stress intensity
) for a hydrogen embrittlement con tribution to fatigue
(KT
max
From the present da ta, the value of KT
crack growth.
max
and
and 3.2)
3.1
appears dependent upon frequency (Fig S.
hydrogen
pressure
(discussed
lat er
independent of load ratio (Table
are
consistent
the
critical
embrittlement
stress
is
intensity
Such
hydrogen
Thus,
abo ve
and
observations
which
predomi nant with a c ritical
tip.
concentrati on
hydrostatic stress and the
lattice).
3 .1).
7),
with modelling stu dies (18,21) which equate
hydrogen ahead of the crack
critical
Chapter
in
by
Fro m
(I
CH)
hydrogen
increasing
Eq.
hydrogen
solubility of
(3.1),
this
is dependent upon the
concentration
in
the
the hydrogen pressure, an
-41-
expected
earlier onset of hydrogen-assisted growth is to be
(this
is shown to be true in Chapter7), whereas the similar
least
consistent with the kinetic asp ects rf
q ualitatively
there b e ing sufficie n t time for t he
considered
several
processes
in
terms
of
t ce
This critic al
st ep
the
step
competition
betwe en
principally i nvol v ing t he
3.5)
(I Fig.
in
critical
hydroge n embrittleme nt process t0 occur.
can be
at
3.1) is
effect observed with decreasing frequency (Fig.
tr anspo rt
rate of hydrogen ads 0 r ption at t he crack tip, its
into th e lattice, an d the rate of formation of fresh surfa ce
as the crack grows.
dismissed
qui ckly
diffusion
hydrogen
permeation
0f these,
transport
rate- 1 imiting
a
as
hydrogen
d ata.
Us i ng
step
simpl y
sampl es
of
2
cm/sec.
fr om
(1 7), o ur
a
yie 1 ded
SA542-3
of rough ly
diffusion coeffici ent for hydroge n permeation (D)
5x10 - 7
be
electro-c hemi c al
the
techni que of D evanath an and Stachurski
measurements on un str es sed
can
Us ing the simple relation for the dis tance x
for hydrogen diffusion into the lattice in time t, namely,
(3.2)
implies that hydrogen will permeate through the
distances
roughly
4
and
40pm
during
frequencies of 50 and 0.5 Hz, respectively.
several
times
one
lattice
cycle,
Since
this
by
for
is
larger than the crack advance per cycle, and
furthermore actual diffusion rates may be in excess of
this
-42-
due
to
dislocation
aided
transport
in stres sed
(23,40)
samples, hydrogen transport can clearly not be considered as
rate-limiting
noted above,
co-workers
more
detailed
(20,30)
kinetic
equate
chemisorption of hydrogen
hydrogen
the
crack
t hreshold
for the analogous effect of an environmental
fatigue
in
aqueous
T
frequency dependence on K max
the
interaction
between
environments
or
hydrogen
for
the
where
,
is assumed to be a function
the
of
rate of oxide r upture at the
crack tip, the rate of passivation and
dissolution
to
gase ou s
for
tip
and
step
rate-limit ing
the
at
Wei
by
studies
as
Related arguments have been advan ced
atmospheres.
corrosion
However,
for the frequency range studied.
rate
the
metal
the bared surface
at
production
of
(33,34,41).
NEAR-THRESHOLD
In the
where
REGIME
near-threshold
growth
increasing
threshold
the
regime
smaller
rates
are
load
ratio
of
crack
than
results
in
propagation,
10-6
a
mm/cycle,
decrease
in
AK0 up to a certain load ratio value, beyond which
it is load ratio-independent,
as
shown
in
Fig.
3.7
for
moist
air environment over a range of R values from 0.05 to
0.75.
The most
growth
is
seen
pronounced
in
the
effect
of
hydrogen
on
crack
near- threshold regime, where the
presence of dry gaseous hydrogen enhances the fatigue
crack
AK (ksi'-In)
2
I
1022
-
7 8
I
I
910
I
I
20
30
30
20
150n
40
40
I
100
0
0
0
0
o
R
v
-
6
I
SA 542 Class 3
Frequency = 50 Hz
Moist air 23*C, 30% RH
-
-o
I
5
I
2 1/4 Cr- IMo STEEL
0
z
4
I
--
E
E
3
00
0.05
0.30
0.40
0.50
-5
00
00
0
0
0.75
0P
o 00
0
z
0
0v
N
I
0
d-
c
0
-6
0
A
A
A
_0
0
40
I lattice
V
z
0
00
2
0
0
spacing
per cycle
V
0
510
0
a
a:
R=075o
o
R=O.4
vR=0.3
'R=0.05
Th reshold AKO
-8
10 1
A
AR=0.5
4I
2
3
i
4
i |
4 |
|
I I
5
6
7
8
_
9
I
.
10
20
I-9
30
ALTERNATING STRESS INTENSITY, A K(MPavri)
Fig. 3.7
600
.
U_
0
2
.
-7
10
z
0
AA
106
-7
00
Effect of load ratio on the near-threshold fatigue behavior
of SA542-3 steel.
40
50
LA.)
-44-
by
rates
propagation
orders of magnitude and
two
to
up
30%
decreases the threshold str ess intensity range by up to
moist
to
compared
influence
signifi cant
This
air.
hydrogen is o bserved only at low load rat ios, and
there
of
is
and hydrogen at high R valu es
the
helium
(
little differ ence between near threshold growth rates in air
3.8).
Fig.
Although in
dry
g rowth rate s are lower compared to moist
crack
air in the mi d-growth regi o n of
crac k
it
propagation,
is
seen in the n ear-thre shol d regi me tha t the presence of a dry
inert gas act ually ac cel era tes growth rates by over an order
of
magnitude
compar ed
to
mo ist ai r, the threshol d
moist air bei ng 24% h igher comp ared t o helium.
a
hand,
water
dis till ed
env i ronme nt
A Ko value
humid air,
w hilst
fractionally
w et
above
re sults
hydrogen
On t he other
leads to a marg inal
deceleration in near thresh old crack growth
threshold
AK 0 in
rates,
a
that mea sured in
a
t hreshold
3.9).
At these
in
behavior simi lar to t hat of moist a ir (Fig.
low stress in tensi tie s, the fractur e mode
with
is
predo mi nantly
transgranul ar with ev idence of some i ntergranular se paration
3.10).
(Fig.
are
not
maximum
The pr oportion of in te rgranular facet s, which
see n at R=0 .75,
of
typic ally
increases wi th increasing
30-40%
at AK
decreasing to zero above AK=20-25 MP
Hz, where abr upt
observed
in
accelerations
in
z
15
MPa /i,
AK to a
before
(at a frequen cy of 50
g rowth
rates
are
not
hydrogen or water) in air, water, and hydrogen
AK (ksi friT)
3
4
5
6
7
8
9 10
I
I
I
I
I I
20
30
40
50
I
-
n2
2
1
21/4 Cr-I Mo STEEL
SA 542 Class 3
Frequency = 50Hz, Ambient Temperature
R
1 3
104
S
U
0.
.E
.*
0 S0
10
air (30% RH)
5
0
-
z
10-4
0
-)
~0
0.05) dry hydrogen
0.75i
(138 k Pa)
00
1-)
-6
10
7
0
0-5
0
L.. 10
03100
0
l0
C
o
C.D
I)
7
0
I3
vu
S
S
0
H2
-7
6
Air
0
0
0
8
N
spacing
per cycle
R=0.05
1?0
H2
R=0.75
I lattice
*
M
a
0 I
0
0
0
0
0
10- 8
Air
_
a
1- 9
Threshold AK 0
2
3
4
1 1
.
0
I
5
6
7
{{ |
8
9 10
I
I
I
I
20
30
40
50
ALTERNATING STRESS INTENSITY, AK (MPa/m)
Fig. 3.8
Fatigue crack propagation in SA542-3 steel tested at R = 0.05
and 0.75 (50 Hz) in moist air and dry hydrogen.
60
A K ( ksi /In)
3
-2
10
5
II
4
6
I
7 8
T I
20
1
9 10
I1
30
40
50
_ I
21/4 Cr- I Mo STEEL
E -3
E 10
-4
10
SA 542 Class 3
Frequency = 50 Hz, R =0.05
Ambient Te mperature
z
--
air (3(
-5
10
% RH)
--- dry hy irogen (138 kPa)
-74
0
a
distille d water
0
dry heli urn (138 kPa)
o
wet hyd rogen (138kPa)
10
6
00
0
S00
~0
6
-----
E
0_
0
-0
/
-
0
0
to-8
3
4
5
0
4
6
-8
10
-- Air
0
dry H 2
I lath ce
spaci ng
per cyycle
H2 0
10- 9
wet
H2
Threshold AKO
1 U l
I I
7 8 9 10
20
I
30
I
40
.
0
-7-.-.
-* (9
10
5
z
He
0
0
1
41--
000
50
60
ALTERNATING STRESS INTENSITY, AK (MPa/im)
Fig. 3.9
Fatigue crack propagation in bainitic SA542-3 steel tested at
R = 0.05 in environments of moist air, water, dry and wet
hydrogen and helium.
-47-
Fig. 3.10
AIR
HYDROGEN
HELIUM
WATER
Fractographs of near-threshold fracture surfaces of SA542-3
steel from tests in air, water, dry hydrogen and dry helium
envi ronments.
-48-
environments.
Little evidence of such facets
was
seen
in
helium tests.
The significant influences of hydrogen
near
threshold
have also been interpreted previously as possible effects of
(1,4).
Although
conventional corrosion fatigue mechanisms (i.e.
in the case
embrittlement
hydrogen
mechanisms
of hydrogen environments, hydrogen embrittlement mechanisms)
have
been
suggested
the
earlier,
mechanisms had not been explained.
has
exact
nature
of such
work
In the present
it
become obvious that such models are not consistent with
the near-threshold data observed and with the existing
data
in the literature.
First, although there is
a
significant
of
influence
at low load ratios near threshold, there is little
hydrogen
effect of hydrogen at high load ratios.
as
Arguments
well
as
models based on hydrogen embrittlement do not explain as
to
why
hydrogen
near -
deteriorates
fatigue
threshold
resistance only at low load ratios.
Second, the accelerations
in
ultra-low
crack
growth
rates in the presence of hydrogen are not accompanied by any
apparent change
mid-growth
in
regime.
fracture
At
low
mode,
load
as
ratios,
growth rates are similar in hydrogen and
proportion
of
was
seen
in
the
near-threshold
helium
where
the
intergranular fracture is different, whereas
-49-
in
the proportion of intergranular fracture is similar
rates are different (Fig.
growth
the
where
hydrogen
and
air
3.10).
Third,
strength
have
(42,43)
recent studies
shown
that
high
which are known to suffer marked h ydrogen
s teel s,
embrittleme nt even under static loading co nditions, s how
acceleratio n
in
near-t hreshold
presence of hydrogen.
growth
is
This behavior
conventiona 1 corrosion fatigue arguments.
not
to the
due
rates
explai ned
by
Most impor tant of
AK 0
the pr esent study h as shown that the threshold
all,
no
in
dry helium for the low strength steel unde r investiga tion to
AK in humid air an d similar t o
be below th e threshold
hydrog en.
significant
compared
to
moist
air
environment
dry
hydrogen
causes
growth
rates
near-threshol d
in
increases
behavior si milar to that
water
whereas
Moreov er,
wet
hydrogen
leads to th reshol d
,
in
that
in
moist
air.
Also,
di stilled
results in a marginal reduction in ultra
low growth rates compared to air.
The above observations clearly
conventional
question
role
of
corrosion fatigue mechanisms, such as hydrogen
embrittlement and active path corrosion (metal
on
the
dissolution),
environmentally-affected near-threshold crack growth.
A
new approach termed "oxide-induced crack closure", which has
been developed in this thesis work,
is seen to be consistent
-50-
with the observed effects of
environments
and is presented in the next chapter.
near
threshold,
-51-
4.
AN EXPLANATION FOR ENVIRONMENTAL EFFECTS NEAR THRESHOLD
It was demonstrated in the previous chapter that
conventional
corrosion
embrittlement
explanations
fatigue concepts involving hydrogen
mechanisms
for
might
near
offer
environmental
rates, such arguments are
seen
threshold.
viable
effects
inconsistent
at
the
following
mechanistic
higher growth
with
the
behavior
A new model, based on crack closure
due to the oxide debris on fracture surfaces,
in
while
paragraphs
and
is
the
presented
influences
environmental and mechanical factors in the low growth
regime
are
described
of
rate
using the concepts of this mechanism
(44-46).
During fatigue crack growth,
the
crack
tip,
surroundi ng elastic material
cl osure
of
the
the
tress
propagati on
,
effective
value,
due to the restr aint of
and
on this residua l
of
intensity
AK
eff
(=K
intensity
phenomenon
range
loading
AK (=K
max -K c2
cycle
approaches
the
a
respons ible
range,
from a nominal value
the
s tretch
The result o f this is
some
,
).
and
the
nominal
-K
re ducti on
for
crack
for
to
mi n
However,,
max
load rati 0 is raised, the crack remai ns open
porti on
plastically
crack faces may occ ur at p ositive I oads of
the loadi ng cycle (47).
in
is
.
at
strained
material
as
a
an
the
1 arger
effective stress
val ue.
This
by which crack tip stress intensity range values
-52-
are
reduced
fracture
our
due
residual
to
surfaces
occurs
contact
of
is known as crack closure, or in
"plasticity-induced
terminology,
and
stretches
crack
closu re"
(44,46-48).
Al though it has long been k nown that the role of
closure
the near-threshold regime of crack growth could
in
be quit e
significant
no
(1,49),
has been carried out .
phenome non
detailed
study
of
the
It has become apparent in
this wo rk, however, that by cons idering the closure
arising
crack
from not only the resid ual
effects
plastic stretches in the
wake of the crack, but also the enlarged excess oxide debris
on
the
crack flanks, a mechani sm for environmental
effects
near th reshold could be derived.
Fracture surfaces obtained after threshold tests
show
bands
of
corrosion
products
examination of such surfaces reveal
thickest
in
specimens
analysis
Macroscopic
that the oxide layer
relatively
at high load ratios (Fig.
information obtained from X-ray
(ESCA)
them.
4.1).
insignificant
Comparison of
photoelectron
spectroscopy
with standard spectra for iron oxides show
that deposits present on the surfaces are predominantly
(Fig.
4.2).
is
tested in moist environments at low
load ratios and is found to be of
proportions
on
often
Actual
oxide
function of crack length (and
thickness
thus
a
measurements,
function
of
Fe 2 03
as a
growth
-53-
AK 0 = 7.7 MPa
notch
M
AKO= 3.1 MPoj
s
-
notch
b
R= 005
Fig. 4.1
R=075
Bands of corrosion deposits visible on near-threshold fatigue
fracture surfaces tested in moist air at a) R = 0.05 and
b) R = 0.75.
-54-
11
7
1
CAuger
6
OAuger
5
Wj
1
|
--
Fe2p1 / 2
Fe2P 3/ 2
4
Z 3
T eAuger
Ols
Cis
Fe3s
NI\s
0
-1000
1
-800
-600
BINDING
Fe3p
2p
1
1
-400
-200
ENERGY, EV
2P3/2
1
0
Fe 203
2 P1/2
-
6
-
51
L
2k
-740
-724
BINDING
-708
-716
ENERGY, EV
-700
SA542-3 steel
ESCA analysis of crack tip corrosion debris in
showing the presence of predominately Fe 2 03
.
Fig. 4.2
-732
-55-
rate)
a re shown in Fig.
4.3, estimated using Ar+sputtering
analysis in the Auger spectrometer.
(Details of
the
oxide
thicknes s measurements using Auger spectroscopy can be found
in Appen di x).
It is apparent that the oxide thicknesses are inversely
related to the crack growth rate, an d are at a maxi mum close
to AKo
of
.
0.2
In moist air the maximum t hickness is of the order
pm
hydrogen.
at
R=0.05,
compared
0.liam in
than
less
to
(Even after intensive pur if ic ation of gas streams
some residual traces of moisture rem ai n in the system which,
aided
by
formation
a
fretting
in
mechanism,
some
in
re su lt
dehumidified environm en ts ).
At R=0.75
thicknesses in either environment ar e an order of
smaller.
It
is
also
found
th e
that
oxide
,
oxide
ma gni tude
oxide
background
deposits obtained by exposing a fres hly bared surface of the
same material to the same laboratory ai r environment for the
same length of time as the duration of a threshold te st
roughly
50A to 150A in
thickness
the
substantial
oxide
formation
similar to the
whi c h is
amount of oxide formation at higher grow th
rates.
fatigue
process.
This
Clearly
at n ear-threshol d stress
intensities at low load ratios resul ts f rom some
the
are
asp ect
of
proce ss h as been descr ibed as
"fretting oxidation", where plastici ty-i nduced crack closure
(47)
and
Mode
II
displacements
(50)
fracturing and compacting of the oxi de
lead
scale
to ab rasi on,
at
lo w
load
E
3
2 1/4 Cr-I Mo STEEL
0.2 -SA
542 Class 3
Ambient Temperature (23*0C)
U)
Frequency
iu
z
air (R=0.05)
50 Hz
0
0
0
air (R =0.75)
10-5
E 10-6
E
z 10~7
-
Threshold-
0 10-
0
0
1
2
3
4
5
6
7
8
CRACK LENGTH (mm)
Fig. 4.3
9
10
11
Measurements of oxide thickness as a function of crack length
and crack growth rate for SA542-3 steel tested in moist air
and hydrogen gas at R = 0.05 and 0.75.
-57-
ratios, to yield local heating, new zones of exposed surface
and hence more
corrosion
rates
oxidation
(46,51).
The
lack
of
visible
deposits at high load ratios and at higher growth
(Fig.
4.1)
is
certainly
consistent
with
this
promoting
crack
mechanism.
MODEL TO COMPUTE CLOSURE K DUE TO OXIDE DEPOSITS
The signifi cance of oxide
closure,
i.e.
the
where
cyclic
crack
layers
in
tip displacements are small,
at near-th reshold levels, can be estimated by means of
following
simple
crack, shown in Fig.
model.
Consider first
an idealized
4.4a, wher e the excess oxide
layer
taken to be a ri gid wedge of co nstant thickness d,
along the crack length a distan ce 2Z behind the
Assuming
only
is
extending
crack
tip.
a mechanical cl 0 sure phenomenon arising from
the presence of the excess oxid e debris on the
crack
and ignoring pla sticity and hys teresis effects,
calculations
based on elastic
integral
(53)
superposit ion
equations
(52)
using
Barenblatt
faces
singular
or the Westergaard s tress function
yiel d a closure stress intensity value at the crack tip
(54)
dE
Kcz
where d is
x=O
the
(4.1)
42)
maximum
excess
oxide
thickness,
2z
the
-58-
y
i===, I
dl
so X
-
e- 24
(a)
C 00/2
Kfra/o-y E
-0.50
OXIDE PROFILE
d /2/
-0.02
-0.25
r
(Kmax/my )2
I-- 2t --a4
(b)
Fig. 4.4
Idealizations of oxide wedge inside near-threshold crack.
-59-
behind the tip corresponding to the thickest oxide
location
formation and E/1- -2 the effective Young's modulus in plane
strain.
2 z c in be obtain ed from scanning
of
estimate
An
el ectro n mi croscopy of fati gu a
liquid ni tr ogen after cycl ing at threshold
tip for SA542-3 steel
roo t
square
numerical
a
pm.
It
dependenc e
in air (R=0.0 5)
of
thic knes s
oxide
2
in
of
1
1A
Eqn.
MPaVin.
0.1 pm, KcZ
For tests at R=0 .05
a
), the exact
W ith 2 = 2 pm,
and
4.3), an
d = 0.2 pm (Fi g.
found to be
estimate of the closure stres s intensity K
4.5
the
sho uld be noted t hat b ecause of
v alue becomes less critical).
maximum
From
was found to be
.
between 1/2 to 5
AKO levels.
the 1ocati o n of the oxid e peak behind
such microg raphs,
crack
in
open
broken
specimens,
h ydroge n, where d =
in dry
is estimat ed to be about 2.3 MPa /m.
Thus
the
predicted d ifference in cl osu re stress int ensi ti es for moist
to
air and dry hydrogen near- thr eshold condit ions, due
the
presence of oxide debris, is of order 2 MP a/m, a pproximately
equal
the
AK, values for
to th e difference in ob served thresh old
two
e
nature of t
plasticityenvironment
crack
face
influence o
levels,
ironmen ts
(Fig.
3.1).
Thus despite the crude
model and the a ssumption tha t
duced
it
closure
are
the
ide ntical
levels
in
is clear tha t corrosion d eposits formed
of
both
on
at the thicknes ses measured have a significant
closure stress
intensities
at
near-threshold
where crack tip displacements are small1.
-60-
of the situation
A more realistic idealization (55)
in
shown
where
Fig
in
SA542-3 steel
on
superimposed
mo ist
the
mea sured oxi d e profile, for
the
ai r
(Fi g.
4.3),
is
for a solid of
crack opening profile
as determi ne d from
y ielding finite element sol uti o n,
Shih's
for monotonic
.
loading using J2
maximum
R =0 .05
at
strain hardeni ng exponent n'=0 .1),
small-scale
is
deformation theory (56).
thickness
oxide
By comparing the
wi th the normalized crack opening
COD/(K
)
(
2
/Ea
, as a function of normalized
max
y
/cr )2 ), it is again
t ip (r/(K
distance from the crack
max
y
C
ar ise from the
can
apparent that significant cra k closure
displacement
presence of su b-micron corros ion deposits.
Ho wever, because
of uncertantie s associated wi t h the location o f the
oxide thicknes s behind the cr a ck tip at
nature of cont act between the
oxide
profile
from
the
crack
AK
faces,
with the exact
and
by
Purushothaman
and
al so
was derived in
of the in terference between mating fracture surfaces.
In th eir surface roughness model
the
have
Tien (57) , where an
estim ate of the closure stress intensity Kck
terms
the
are not feasible at present.
Approximate calculations for closure levels
performed
with
ox ide peak to the c rack tip, more
rigorous calculations of Kcz
been
maximum
for fatigue crack
cl osure,
influence of fracture surface undulation is considered,
such that
h =
h0exp (cf)
.(
(4.2)
-61-
where h0 and h are length dimensions describing
and
final
fracture
surface
roughness,
the
and
initial
the
true
fracture ductility representative of the stress state
of the crack tip.
a(h-h0 ),
the
ahead
By equating the change in asperity height
where a is an unknown constant less than unity,
crack
tip opening displacement, estimates of Kcz
were
derived.
The model does not, incidentally, incorporate
role
crack
of
crack
(50),
growth
influence the extent of interference
which
will
(58).
markedly
However, despite
this and uncertainties in the empirical CTOD expression
the
and
constant a, this simple model does yield reasonable
(57).
estimates of KCP
Thus, at the
associated
with
small
crack
tip
near-threshold
opening
fatigue
displacements
crack
growth, it
appears that crack opening or closing loads can be
both
the
tip Mode II displacements characteristic of
near-threshold
in
to
by
the
presence
surface
roughness.
assume
more
of
foreign
Whereas
importance
debris and by fracture
oxide-induced
with
enhanced
regard
to
closure
will
environemental
effects, surface roughness-induced crack closure may well be
significant
known
to
in
have
coarser-grained
superior
microstructures, which are
near-threshold
crack
resistance and rougher fracture surfaces (59-61).
growth
-62-
Comparisons of the maximum excess
computed cyclic crack
different
2 4 Cr-lMo
of
cl as s es
derived f ro in Sh ih's
pressure
calcul ations
parameter s are o f similar
Despite
4.5).
sputterin g measu rements and
one
suggests
in
physically
when
environme nts,
The
at
on
this
t his
varia t ion
differen t
AK0
in
cease
will
load
wet
for
model,
thresho 1d f or te sts in
on e
vac U o
the
for
rationale
appealing
AK
and
to
eff
dry
ratios, and with varying
yield str ength a re consiste n t with this mechanism.
based
Au ger
become s wedged-closed, such that
it
tends to zero.
F i g.
and
computations,
ACTOD
existence of a t hreshold, i n that the crack
propagate
) the two
the
in
uncertainties
inherent
0
4.1
(Table
magnitude
(5 6),
strain
that at the thresho ld stress intensity (AK
reveal
stee is,
vessel
plane
for
for
ACTOD)
displacements
opening
with
4.3),
(Fig
measurements
Auger
from
threshold,
at
thickness
oxide
not
would
except
to
expect
perhaps
Further,
at
find a
smaller
CTODs w here , for example, r ughness-induced closure may play
a role.
masked
Wh ile i nterpretati 0 ns
of
such
a
result
may
be
by rewel ding effect '5 , the vacuum data of Cooke et al
(10) and Skelton and Haigh (12)
on medium strength steels do
not in fact show a well-defined threshold.
-63-
Table 4.1.
Threshold Data
Environment
Steel
for 2tCr--1M o Steels
R
cyclic
___________
j(MPa
) (ijm) (jim) (iym)
SA542-3
moist air
0.05
7.7
0.18
0.31
0.20
a-500.Pa)
moist air
0.75
3.2
0.03
0.78
0.01
dry H2
0.05
5.2
0.08
0.14
0.09
dry H2
0.75
3.3
0.03
0.83
0.01
dry He
0.05
6.2
0.11
0.20
0.10
water
0.05
7.8
0.18
0.32
0.25
SA542-2
moist air
0.05
7.1
0.11
0.17
0.12
ayz769MPa)
moist air
0.75
2.8
0.02
0.41
-
__
max
dry H2
0.05
4.6
0.04
0.07
0.04
dry H2
0.75
2.8
0.02
0.38
-
__
Max.
dry He
0.05
4.9
0.05
0.08
0.04
dry He
0.75
2.7
0.02
0.35
-
___
Oxide
Thic kre ,s
CTOD
0
SA542-2/
T690
moist air
0.05
8.6
0.19
0.34
0.16
a -Y575MPa)
dry H 2
0.05
6.9
0.12
0.22
0.11
dry He
0.05
7.1
0.13
0.23
0.11
Plane strain values defi ned as 0.50 (AK
0.50 (Kmaxo /aYE)
- maximum
where
0
/2'E) - cyclic, and
y
,
AK 0 and Kmax,o are the
cyclic and maximum threshold stress intensities, a'
and a
y
are the cyclic and monotonic yield strengths, and E is the
elastic modulus.
-64-
21/4 Cr -I Mo Steels
0.21
. SA542-3
e SA542-2
A SA542-2/T690
A
6
0
A
cm,
0.\
4-.
0
0~
0'
0
0.1
0.2
maximum excess oxide thickness,d (AL m)
Fig. 4.5
Correlation between oxide thickness and cyclic CTOD at 6KO
-65-
From the above results and
that
in
dry
environments
is
it
discussions,
such as dehu midified hydrogen or
helium (where the availabil i ty of moistu re is less
to
an d
environments)
humid
high
load
smaller),
the
at
closure due to plasticity
is
excess
r estricted.
oxide
debris
is
closure due to corrosion de b ris is
stress
intensity
range
resul ting threshold
atmospheres.
In
v a lue
values
other
lar ger
air,
not
to
thickness
of
such cases, the
the
and
effective
hence
to
the
humid
w 0 rds, near-th reshold crack growth
hydrogen
compared
because of h ydrogen em brittlement per se, but
moist
air.
measurements,
influences
formation
compared
lower
due to a smaller amount of closure in
opposed
compared
ratios (where
smal ler,
is
are
In
rates are accelerated in th e presence of
to
clear
threshold
gaseous
hydrogen
as
This is co nsistent with the oxide
t he
observ ation
that
hydrogen
only at low load ratios and
beha vior
the fact that there is no c hange in c haracteristic
fracture
mode.
The influences
threshold
(44).
values
of
also
load
ratio
follow
and
from
strength
on
the
such closure concepts
The variation of the measured threshold AK0
values in
both
moist
4.6.
Low load ratio threshold values are apparently greater
compared
residual
to
air
and hydrogen environments is shown in Fig.
those
plastic
at
higher
stretches
load
and
ratios because of the
the
resulting
-66-
10
8
SA 542 Class 3
Frequency = 50 Hz
e mojit &ar(23*C,30%RH)
8_
K6
o dry hycregen (138kPa)
CL Y
w<
00
--
4-
04
u
WO
z
0.2
0
2
0.6
0.4
0.8
1.0
C,,
-12
C,)
W
E
C;o 8 -P
4
8
00
-
0
0.2
0
0.4
0.6
0.8
1.0
LOAD RATIO, R = Kmin/ Krr.
.
Fig. 4.6
Variation of threshold stress intensity parameters with load
ratio.
-67-
plasticity-induced
this closure is
closure
enhanced
further
crack
oxi de-induced
by
However, above a
envi ronments.
dry
to
compared
In moist environments,
closure.
crack
these
critical R value (about 0. 58 for SA542- 3 steel), both
c1osure
modes
The nomencl ature
load rat io or environment.
intensity
stress
ratio ar
4 .7
positive
open
crack
a
intensity
a fatigue crack is a constant.
Kc9,
necessary to
,is
threshold
At low
load
ratios
higher
than
those
plasicity-induced
for
higher
closure
in
load
dry
(Kmin
are apparently
where the meaured threshold values AK0
),
stress
), necessary to propagate
AKeff(=Kmax -Kcz
range,
Kc
a
that
assumed
is
It
effecti ve
the
that
and
(49).
intensity,
stress
threshold
for
and thei r variation with load
paramet ers
shown in Fig.
minimum
and the re is no influence of
minim ized
are
ratios
of
because
environments
and
both
platicity-induced closure and oxide-induced closure in moist
is seen that the maximum stress intensity
it
value corresponding to threshold
= Kcz
KOmax
+AK
.
is
no
a
constant,
K mi
is
greater
than
becomes equal to K
to
same.
at which the minimum stress intensity
R
value of
corresponding
Kc
closure and hence the measured and the actual
threshold alternating stress intensity values are the
The
i.e.
However, at high load ratios, where
the minimum stress intensity
there
AK 0 is
,
environments,
, i.e.
the
R
, is the critical load
transition
from
a
ratio
constant Koe max
-68-
K
AKth
(A)
Kmax
-
4
4
Ki
Kmin
4
T IM E
Ln Ko
Kma
I
Kmin=
cI
(B)
LKth
K
'I,
'
IRcl
1.0
R
Kmax
KC
Komax
(C)
AKth
I-R
Kmax
IRcl
R
1.0
Fig 4,7 : Nomenclature for stress intensity parameters
at threshold and their variation with load ratio R
(after
Schmidt and Paris)
behavior to a constant
important
AK
,
-69-
R-independent behavi or.
to note from Fig.
4.6 that R
The
fact
that
is
corre sponding to
moist air and hydrogen enviro nments is about
respectively.
It
0.6
and
0 .3,
this critical load ratio is
higher in moi s t ai r than in hydr ogen is indicative of higher
closure loads due to enhanced ox ide debr is formatio n.
Finally, the reduction in
increasing
material
strength
hydrogen in ultra-high
consistent
with
threshold
and
values
with
and the la ck of an effect of
strength
steels
(1)
may
also
be
this model, since plasti city-induced crack
closure would be expected to be reduced in
materials
AK0
further,
fretting
higher
strength
damage from oxide debris
would be somewhat less severe on the harde r fracture surface
of a high strength material.
Results of a number of studies show evidence supporting
the
oxide-induced
crack
closure
model.
Such data, along
with measurement of closure using ul trasonic techniques
discussed in the next chapter.
are
-70-
5.
EVIDENCE
SUPPORTING OXIDE-INDUCED
concepts
Evidence in support of the
of
oxide-induced
an be readily fo und in the literature.
closure
crack
CRACK CLOSURE
The
idea of corrosion deposits form in g in cr ack s and influencing
subsequent
growth
example, threshold results for
enti rely
not
is
behavi or
ve ssel
pressure
have been found to show so mewhat high er
steel
water than in air
near-threshold
(6,46).
Tu a nd
Seth
new.
(9)
and
For
rotor
AK0 values in
slower
report
rat es i n se veral rotor steels tested
growth
at 100 0 C in the seemingly
more
ag gressive
environment
of
a f act they attribute to a number of
steam compared to air;
factors including crack tip cor rosi on
deposits.
In
fact,
observation of corrosion de bri s has traditionally provided a
means to detect and monitor the gro wth
under
show
clearly
incipient
conditions in m any aircraft components
service
Studies by Swanson and Marc us
(51)
of
that
the
63)
as well as Benoit
amount
of
dif fusi on
oxide
in to
far
greater
than
that
abs ence of crack growt h.
)
(64
reveal
that
in
et
al
and that
the mat eri al in the
pre senc e of moist envi ronments duri ng fatigue
is
(62).
phe nomenon of enlarged oxide
formation is a result of th e fa ti gu e process itself
the
flaws
c rack
growth
i n th e same enviro nment in the
Resul ts 0 f
h um id
Mahulikar
and
environments
di spl acements on the faces of fatigue specimens
Marcus
resi dual
are
higher
compared to those in vacuum due to the build up of oxides on
-71-
fracture faces.
explain
in
Fatigue data by Nordmark
and
Fricke
(65)
crack arrest during fatigue of 7475 alumi num alloys
sump
water
using
arguments
Skelton and Haigh (12)
deposits.
of
crack
tip
corrosion
interpret their results of
lowered AKO values in vacuo and at R=-1 in Cr-Mo-V steels at
550 C
terms
in
of reduced crack tip opening dis placements
from shea r lips and oxide-filling
become
of
the
crack.
has
clear that several hitherto unexplained ex perimental
observati ons cited in the literature (and briefly
in
It
chapt er
described
be understood ba sed on the c oncepts of
1) can
crack closure induced by corrosion deposits.
The
on
results
environmental
effects
near-thres hold region of crack prop agation
previous
chapt er)
oxide-i ndu ced
clearly
crack
closure.
supp ort
in
the
(presented in the
the
concepts
of
The fact that with reference
to moist a ir, near-threshold growth rates are accelerated in
dry
hydro gen
and
dry helium and marginally decelerated in
distilled water while
hydrogen
the
point
corrosion
showing
the
same
behavior
in
wet
may b e somewhat surprisin g (and unexplainable from
of
view
fat igue
of
arguments
mechanisms).
based
However,
on
conventional
these
results
cl earl y demonstrate that the si gni f icant role of environment
near
th reshold
de pends
princ i pal ly on the availability of
moi stu re and not on the embritt 1i ng tendency of any specific
envi ronmental
species.
This
behavior is obvious from the
-72-
result that while dry
crack
hydrogen
accelerates
growth rates by up to 20 times, wet hydrogen leads to
a threshold behavior similar to moist air.
on
near-threshold
SA387,
Class 2, Grade 22 steel
Recent data (66)
specimens taken from the
heat-affected as well as the
weld
moisture
is responsible for thick oxide
(and
formation.
not
In
oxygen)
this
near-threshold
pressure
growth
similar to those
in
rates
dry
air
zones
have
vessel
shown
that
steel,
the
in dry oxygen are found to be
and
dry
hydrogen
are
and
significantly higher than those in moist air.
Actual
procedures
measurements
Williams
of
of
and
closure
crack
DeLonga
techniques developed by Frandsen et al
usin g
(67)
(based
(68))
are
the
on the
de scri bed
below.
MEASUREMENT OF CLOSURE USING ULTRASONIC TECHNIQUES
Near-threshold crack
using
cl osure
ultrasonic waves(68).
phenomenon
was
studied
Compact tension specimens were
prepared by conducting thresh old tests in moist air and
hydrogen environments
length,
frequency
i.e.
of
about
2
MHz,
and by pre-cracking to the same crack
26
mm
were
specimen and were received at
schematically
in
Fig.
dry
5.1,
Ultrasonic
waves,
with
a
transmitted at one edge of the
the
opposite
end
as
shown
while the specimen was mounted
-73-
ULTRASONIC
GENE RA TOR
TRANSMITTER
CT
LOAD
SPECIMEN
RECEIVER
OCLLOCOPE
Fig. 5.1
Schematic showing the location of transducers on the compact
tension specimen for ultrasonic testing.
-74-
specimen
was
subjected
intensity
values
(slightly
above
to
the
K
ze ro
values
max
thresholdAK0 of 7.7 MP a/in at R=0.05).
signal
load.
The
corresponding to stress
loads
from
ra nging
the
of
on the testing machine for application
to
8.5 [A Pa1/m
about
corresponding
to
the
The amplitude
0f
the
to the specimen was kept a constant and
transmitted
the transmitted outpu t from the receiver was mon itored
an
oscilloscope.
T h ere
no variation in the receiver
was
signal up to a certai n load level beyond whi ch t here
was
a
This is due to the fact
the sig nal amplitude.
in
decrease
with
that as the crack sta rts to open, a
of
arno unt
lesser
the
signal is transmitted to the receiver leadin g to a reduction
in th e signal
s i gna 1
After a gradual
de crea se, reciever
amplitude agai n reached a constant v al ue .
droppe d
then
were
ampl itud e.
repea tability.
to
There
the
zero
level
to
The loads
check
for
was no noticeable difference between
the receiver signals for increasing and decreasing loads.
The variation of
against
the
stress
conducted on specimens
values
receiver
output
signal
intensity
factor
for
fatigue
cracked
to
is
plotted
closure
threshold
in both air and hydrogen atmospheres (Fig.5.2).
seen that there is no steep decrease in the receiver
implying
so
that the crack does not open all
rather
observation
gradually.
of
Bachmann
This
is
and Munz
in
AK0
It is
output
at once, but does
agreement
(69)
tests
with
the
that crack surfaces
-75-
MOIST AIR (R=O.05)
IR
-5
-
a:
DRY HYDROGEN (R=O.05)
-
w
MOIST
1n
-
'-I
W-
AIR(R=0.5)
3
w
u
w
- 15
X
Z
201
-
o, Ai ,- INCREASING K
O,AO - DECREASING K
SIGNAL FREQUENCY = 2 MHz
251
0
2
4
6
8
,
STRESS INTENSITY
Fig. 5.2
10
12
KI (MPaJ/n)
Variation of receiver signal with stress intensity.
-76-
can remain in contact 1local ly,
be
considered
open.
fully
for the c rack c losure
obtained, it
while globally the crack
Although no precise estimati on
stress
intensities,
of the c losure K values
threshold
coul d
KcZ
be
is observed that the stress intensi ty levels at
which the recei ver output signal
order
can
in
air
and
value decreases is
of
the
(roughly equal to K levels at
hydro.gen).
The
most
importa nt
observati on of these tests is that crack opening occurs at a
higher sti ress i ntensity value for
com pared
aiir
moist
the
specimen
tested
in
to that tested in hydrogen, confirmi ng
the occurrence of increased crack closure near threshold due
to oxide deposi
To
check
the
validity
of
the
closure
addi ti onal experiments were conducted.
signal
wa s
the
same
(Fig.
5.3),
with a saw cu t equal
and
output
wi th
and
this
hydrogen.
the
constant
Al so,
a
in length to the crack length
of the fat igue-cr acked specimens showed
receiver
in
as that obtained with no load on the
specimens thresho 1d- tes ted in air
specimen
variation
whi 1e the applied loads were increased from
zero to th reshol d 1evel s
output
two
When a specimen with
no fatigue crack was te sted there was no
receiver
tests,
increasing
no
change
1oads.
in
the
However,
the
amplitude of the output signal for the case of the
specimen was much smaller than the initial
saw
cut
output amplitudes
of the uncracked and the precracked specimens.
-77-
UNCRACKED
SPECIMEN
_j
AIR (R =0.05)
H 2 (R=0.05)
AIR (R =0.5)
SPECIMEN
0
Fig. 5.3
WITH SAW-CUT
LOAD
Schematic showing receiver signal output for uncracked,
saw-cut and fatigue-cracked specimens.
-78-
The tests conducted
that
using
ultrasonic
waves
indicate
the closure loads for threshold tests in moist air are
higher
than
uncertainties
those
involved
of the fact that
locally
in
the
dry
hydrogen.
of
Because
the
in using this technique and because
crack
surfaces
may
be
in
contact
(similar to the growing region of Hertzian contact)
while macroscopically the crack may remain closed, a
closure load cannot be meaningfully estimated.
single
-79-
INFLUENCE OF FATIGUE UNDERLOADS NEAR THRESHOLD
6
Transients in growth rate behavior
or
overloads
attributed
stresses
blunting
cycli c
(70,71) ,
hardening (72) ,crack
strain
(73), and plasticity-induced closure
such
phenomena
these
studies
where mechanisms such as
Since
(47).
sheets
thin
plane stress conditions,
essentially
under
(70,71,74-76),
with
performed
been
have
tip
to airframe components many of
common
are
compressive
residual
as
mechanisms
such
single
spectrum loading sequences have been
various
to
by
induced
plasticity-induced
closure
crack
can be predominant (47).
In this work, however, the role of
loading
under
fully
Specific ally, the
ed
so-cal
is considered
(AKO)
certai n
loading
standard
load
near-thr eshold
example,
of
spectra,
shedding
blo ck
intensi ty
stress
(1).
amplitude
strain con ditions is examined.
plane
influence
threshold
variable
in
procedures
the
occ ur
within
laborato ry
during
(1 ,77)
for
measuring
growth rates at low s tre ss intensi ties.
since measurement
of
such
the
for no cr ack growth
Such underl oad s may
and
below
cycling
ne ar -thresh old
For
crack
growth a t conventional frequencies ca n b e so time consuming,
it
or
is often necessary to interrupt the test,
"parking"
where
overnight
presumably
no
by say stopping
at aAK level below the thresholdAK0
damage
can
occur.
Where
such
-80-
interruptions
have
been
transients
crack
growth
behavior
in
reported
behavior
for nominally
,
result
often
after the interruption is initially
from behavior before
unexplained
(78),
and
very different
identica 1
conditions.
In the present work, the influence of periodic cycling at AK
levels below the threshold AK0
fatigue
is examined on near-threshold
crack propagation in SA542-3.
rate behavior is
rationalized
in
The tr ansient growth
terms
of
oxide-induced
crack closure.
The
examined
of
influence
under
variable
constant AK
cycling
sequences illustrated in Fig.
near-threshold
levels
,
amplitude
loading
was
using
load
(R=0.05)
After load shedding
6.1.
cracks were grown for 1 -1.5
whereupo n a
(AK
mm at a
s ub-threshold
AK
underload
(15
level
(
)
constant baseline AK 1evel
to
AK U)
was a pplied for 2.7 x 106
AKB baselin e
original
level.
serie s
A
performed for baseline levels of AKB
underloads
(below AK0
length (a)
the
retur ned
hrs at 50 Hz) , bef ore condit ions were
4 -
versus numb er of cycl es (N)
sequence,
the
6.1), namelv (da/dN)B
(da/dN)s
of AKU
= 8.3
the initial
of
-
number
the
were
10 MPa/iKn,
with
7.5 MPa/I.
From crack
dat a moni tored during
following p arameters were defined (Fig.
the baseli ne crack growt h rate at AKB,
crack grow th rate at AKB
of cycles at LKB
before growth rates
to
tests
following the
underload period, and a and N*representing the crack
and
cycles
returned
length
following the underload period
to
their
original
baseline
-81-
H-
constant
Cr)
z
R=0.05
LJ
0L
CT) Ile
Vl)
AKB
AKU
AKS
LUJ
Hn
+-27x
106cycles+
(a)
AKB
-
E
-
N
Cr
LU
-
d--
dN) B
---+
z
AKU
E
da
N8
dN /S
dN JB
N
NUMBER OF CYCLES
(b)
Fig. 6.1
Nomenclature of different parameters used to describe the
underload interactions near threshold.
-82-
value
(da/dN)B'
of
Effects
near-threshol d
sub-threshold
crack
underloads
growth
between 8.3 and 10 MPaiii (R
)
on
at baseline AK B level s
rates
a re
=0. 05 )
< AK
(AKU
shown
in
Fig.6.2.
Data pertinent to this figure i n term s of paramete rs defined
in Fig.
growth
6.1 are listed in Tabl e
was
cycles,
ever
affected.
Little
AKU
=
4 MPa/iii
(Fi g.
AKB = 10 MPaVi
at AKU = 6.5 MPaii
had
growth at AKU = 9 MPa/i
underloads
AKU
=
gr owth
crack
underload of
a
smal 1
4.2b).
(Fig.
be 1ow
and
10
underload periods, initial
slower
MPai
cases
apparen t after an
rates
growth
influence
at
Fig.
on
crack
Appl i cation of cycl ic
the
resul ted
however,
(
8.3
underload
Simi 1 arly, underl 0ad cycling
decelerations in subsequent bas eli ne
between
was
crack
cert ain
in
base line
6.2a).
only
was
e ffect
on
fractionally
7.5 MPa/i,
no
Although
du ri ng t he 2.7 x 106
detected
subsequenct
severely
6.1 .
threshold
in
growth
at
significa nt
6.2c-f) .
AK B
at
rates
Following t he
grow th rates were up to
5
tim es
than pre-underload base line rates, w ith retardatio ns
experienced over crack growth i ncrements a*
(equivalent
up
0.3
to
mm
to several times the maximum pl astic zone siz es
at AKB ).
Thus periods of underload cycles beneath the
,
AK 0
threshold
where presumably no crack growth damage can occur, can
give rise to transient retardations in near-threshold
crack
-83-
A K = 10 -- a- 4
2 2.3 -
-+10 MPa
/
22.4
3 x 1O~ 6 mm/cycle
22.2
22. 1 -
-
3 x 10-6mm/cycle
22.0
21.9 0
4
AK=9- 6.5-
22.8 -
0
8
I
282
278
I
286
9 MPaGV'
8x l~7 mm/cycle
22.7
10- 6 mrn/cycle
22.6
23.2
282
12
I
294
288
-
E
E
6
0
AK
8.3 -- 7.5 -+e-8.3 MPa
y
42xxlO- mm/cycle
23.0
0
3
S24.0 -AK=9-7.5 --- 9MPa
303
20 4
30
20
10
M-
10 7 mm/cycle
).5x
24.06
253. B
24.
6
12
906. x
28
6.
5
Fig. 6.2
10
280
mm /cycle
i-
x
-/mm/cycle
285
290
Number of Cy cle s (N x 104)
Crack length versus the number of cycles showing the effects
of underloads.
TABLE 6.1
Test 'No.
a
B~~ U
10
10+ 4
96.5
8.3
d
9 + 7.5 - 9
e
9.5
7.5
7.5
8.3
+
9.5
10+7.5+i0
(mm/cycle)
d(
x-6
1 x 10
+
+8.5
+2
(mm)
N,
(no. of cycles)
0.01
1.5 x 10
x 10
0.008
1.5 x 104
x 10~7
0.1
8.0 x 10 5
0.14
6.5 x 105
x 10 7
1
2.5 x 10 6 +6.5x
3 x 10- 6
a
3 x 10J
4 x 10~7 +1.4
1 x 10- 6
_____I____________
f
dKB
. 3
9
c
S
on Near-Threshold Growth
1.2 x1
6
0.3
5.0 x 105
-
of Variable Amplitude Tests involving Effects of Underloads (AKU
)
Results
I
CO
4:11
.2
0_________I
x1
0.24
10
15
--------------
-85-
growth
rates over distances comparable wi th several maximum
plastic
zone
dependin g
sizes,
both
on
the
the
magnitude
baseline
of
and
the
retardation
under - load
stress
i ntensi ty ranges applied.
It has
been
shown
that,
certai n
in
circumstances,
below the threshold stress intensity AKO can retard
cycling
subsequent near-threshol d crack growth rates,
somewhat
ana logous
to
found
to
posses s
failure. Howev er, whereas
strain
phenomenon
well know n effect of "coaxing"
the
where smooth specimens cycled below the
often
a
improved
coaxing
fatigue
re sistance
has
been
limit
to
are
fatigue
attributed
to
aging effects (79), a mechanism for the influence of
sub-threshold underloads is
presented
below
in
terms
of
oxide-induced crack clos ure arguments.
It
was
shown
thicknesses
from
the
chapter
that
oxide
propagat e
equals
displacement (ACTOD).
AKB
threshold
is
approached
oxide thickness measurements it was inferred
thickness
baseline
previous
levels as t he
that cracks would not
oxide
the
increase with decreasing growth rates and hence
with decreasing AK
and
in
and
the
when
the
pulsating
crack
Thus for particular
underload
AKU,
it
maximum
excess
tip opening
combinations
follows that if
excess oxide thickness d formed at baseline AKBis less
the
ACTOD
at
underload
AKU
then
the
application
of
the
than
of
sub-threshold underload cycles, while not generating fatigue
-86-
per se, can result in further oxide formation due to
damage
Thus
fretting oxidation mechanisms.
level
AKB
baseline
following
underload cycling, one
the
retardation
initial
might expect in this instance some
the
to
returning
on
in
growth rates due to the increa sed crack closure loads
crack
from the extra oxide deposits formed at AKU
larger than the underload ACTOD,
underload
the
on
the excess oxide thickn ess d at AK B is already
hand,
other
If,
.
the
of
then little effect
cycling would be expected since during the latter
period the crack would be effectively wedged-closed
thereby
limiting further oxidation.
This c once pt is illustrated in
variation
cr ack
of
thi ckness
oxi de
measured
pul sating
growth
crack
(d)
and
tip cyclic CTOD
a
the
where
6.3,
experimentally
(da/dN),
rates
Considering first the situation o f
MPaVi,
Fig.
underload
computed
is
values
basel i ne
plotted.
0f
A KB
10
the oxide thi ckness gener ated at thi s le ve1 h as been
measured to be app rox imately 0.11 pm (Fig.
underload
of
4 MPa/m
AKU
ACTOD is ro ughly 0 .05
thickness,
such
is
the
crac k
enhanceme nt in oxide-induce d crack
the
exi sti ng
remains c los ed.
closure,
,
observations
(Fig.
an
oxide
With no
th eref 0 re,
underload of 4 MPa/im would b e expected to have 1 ittl
on growth ra tes at 10 MPav/im
When
applied, t h e c orre s pondi ng
pm, smaller than
tha t
6 .3)
an
e effect
in accordance with exper imental
6.2a and Table 6.1).
an underload of AKU= 7.5 MPa/m is applied at
Conversel y, when
AKB= 10 MP a/f,
-2
2 1/4 Cr-I Mo STEEL
0.7
Z42
-3
10
C lass 3
Freq uency = 50 Hz, Ambient Temperature
Air (30% RH), R=0.05
Calculated ACTOD
0.6
.-
0.5
*& -
E
da/dN
E 10-5
0.4
0
z
~0
0
-10.3
I-)
-6
10
-
lattice
S pacing
p er cycle
0.2
Measured d
-7
10
0.1
AKuc AKO
-08
2
4
6
8
AK 8
10
20
30
40
A K (M Pa v/r)
Fig. 6.3
I
0
.
_0
E
Variation of cyclic CTOD, oxide thickness d, and crack growth
rate with stress intensity range and a scheme for finding the
critical underload stress intensity range.
0
50
I
-r-0
('_
underload ACTOD is now roughly 0.17 pm, larger than the
the
existing (baseline) oxide thickness, such that some
oxide
growth
can occur.
With the resulting enhancement in
oxide-induced crack closure, the 7.5 mpa
be
expected
rates at AKB
(Fig
to
cause
some
It also follows
level,
e
effect.
bel ow which
underl oad
retardation in growth
from
discussion
this
that
whi ch
underloads
ca n
remains
crac k
ACTOD
is
clo sed,
corresponds
to
an
underloads
at
7.5
(Table
AK,= 9 MPav'm
AKU= 7.1 MPavm,
rates
underloa d
MPa/mi
ca n
S0 .11ptm),
level of AK U =6 MPa/m.
lead
underloads
at
6.1).
Similarly
fo r
where
critical
un derload
whereas
to
4 MPa/-m
crack
sub sequent
h ave
Iy
by
ro ugh ly
five
lev el
time s,
negl i g ible effects are ap parent fol lo wing underlo ads
6.5 MP av/-i
AKUC
by
(Fig.
6.2b and 6.2d).
Using Fig.
1ittl
grow th
under loads at 7.5 MP a/m are seen to
initial
the
less than the existing oxid e th ickness
this
growth
have
whe re
i.e.
For crack growth at AKB = 10 MP a/m (where d
effect
each
This corresponds to an underloa d AK U level
the
retard ati ons
at
a critical u nderload stress intensity
(d).
Hence
results
6.2.
range (AKUC ) would exi st below
1 i ttl
would
Similar arguments are applicable
to the other results in Fig.
r
initial
underload
/m
10 NPa VF consistent with experimental
=
6.2f and Table 6.1).
basel i ne
further
e
at
becomes
reduce
whereas
at
at
6.3, values of
can be esti mated graphically for any baseline AKB level,
simply fol 1 owing the direction of the arrows i n equating
-89-
oxide thickness (d)
to
AKB
.
However,
AKU
crack closure is important
ACTOD
oxide-induced
that
recognizing
at
at
only at low stress intensities where crack tip d isplacements
are
deposits
corrosion
and
of the same size scale, it
is
likely
to
apparent that sub-threshold underl oads are
(proportional
rapidly
i ncreases
k levels, the corresponding ACTOD
increasing
With
r ates.
growth
near - threshold
on
effect
an
have
only
so
to AK 2 ) that further oxid ation during
the underload would be negligible in comparison.
has been shown that near-threshold
Thus, in summary, it
growth
crack
fatigue
suffer
can
following underload cycling (at the same load
to
Such crack growth
threshold.
the
generated
oxide
enhanced
from
result
ret ardations
transient
rati o)
below
retardations are reasoned
crack
induced
closure
during the underload, provided that the underload
ACTOD is large enough to allow further
close numerical correspondence between
oxide
The
grow th.
\'CTOD and cr ack flank
oxide thickness data, however, could be considered
somewhat
in view of the uncertainty in the two e stimates.
fortuitous
But conceptually such data are totally
of
explanation
experimental
the
with
consistent
an
observations in terms of
oxide-induced crack closure.
In terms of recommended procedures for
thresh old
(77),
fatigue
such
crack
closure
growth
concepts
measuring
and threshold
could
have
AK
near
values
important
-90-
Current practices for threshold determination
consequences.
utilize
c rack
grow th
(load-shed ding)
monitoring
an d/or
under
decreasing
AK
increasing
conditions,
continuous ly involvi ng variable amplitude 1oading.
generally
minimize
taken
to
crack
gro wth
considerat ions.
be
too
rapid
retardation
determi nation
threshold
may
avoid
and
in
that
a
"parking" overnight at AK levels below
yield simi lar premat ure retardations.
plasticity
tests,
the
growth
rate s
determined
incre asi ng
AK.
In
this
si gni f icant tha t material s which
unde r
aqueou s
this
str ess
behavi or
contri buti on
regar d,
are
say
by
may
More over, where oxide
conditions may be somewhat different f rom
under
factor
threshold,
induced cl osure is s ignificant, one might e xpect
threshold
in K to
further
interrupting
is
premature
from
It is now apparent that
important
hence
resulting
thus
Care
descent
a
AK
v ery
that
decreasing
those
it
near
AK
determined
is
perhaps,
susceptible
to
corrosio n cracki ng are oft en likely to show
(77,80)
from
possibl y
refl ect ing
a
differing
oxide induced crack closure arising from
the presence of sub stanti al crack fl ank corrosi on deposits.
-91-
7.
THRESHOLD BEHAVIOR IN DIFFERENT ALLOY SYSTEMS
It
was
in
shown
oxide-induced
crack
pressure
of
that
the
grow
rates
eels.
vessel
th ree
in
aracteristics
classes
teels
lower
strength
alloy are di scussed in the li
intensity
r ange
It
two
other
as well as an aluminum
pipeline
SA516
for
presure
(0.14
gas at load rati os of 0.15 and 0.75.
of this steel obtained by Wachob and
pressure
in
t of crack clos ure phenomena.
environments of moist air and low
hydrogen
different
7.1 shows t he v ariation of crack growth rate with
Fig.
stress
(6.9
Nelson
stee
in
MPa)
dry
Fa ti gue data
in
(37)
high
MPa) h ydro gen are also plotted in Fig.
7.1.
is observed that si mi 1a r to the behavior observ ed in
case
of
SA542-3,
the
pipeline
steel
of
crack
growth,
namely,
the
unde rgoes
also
significant accel erati ons in propagation rates
regions
of
thi s c hapter, the
In
near-thresho 1d crack growth
of
concepts
readily explain environmental
closure
influences at ultra- low
types
4
Chapter
in
the
two
the mid-growth and the
threshold regimes.
In
obtained
the
in
Nelson (37)
similar.
studies
mid-growth
the
present
region,
study
the
as
crack
growth
data
well as by Wachob and
f or environment of moist air
are
found
to
be
The difference in the testing frequency in the two
does
not
appear
to
result
in
any
significant
2
t072 t4~
[
3
4
5
AK (ksil/in)
6
8
7
9 10
20
30
40
50
SA 516 Grode 70 Pipeline Steel
z
1-4
Ambient temperature
E
E 0oz
'
R
015
0.15
0.75
dry hydrogen (0.14MPo)
o
0.15
0 15
laboratory air
dry hydrogen (6.9MPa)
o
o
*
0
0.75
Environment
rnoist air (30% RH)
Reference
present study
Hz)
present study
f50
high pressure
H2 (0.15)
10
.111 6
00e
(50Hz)
Wociob
and
(0
Nelson
(0.1 -OHz)
0
A~
*
*
.0
-6
10
>'
low pressure
H 2 and air (015)
-
A
1,0
z
0;
-6
76,
90
83
0
0
0
spacing
0
H2 E air ---- ,)
(0.75)
0
_
H 2 (0.15)
0
t
2
3
per cycle
8-air (0.15)
-69
b
4
I lottice
'
-
0
-8
L
Threshold AK,
5
6
7
8
9
10
2
0
30
40
50
60
ALTERNATING STRESS INTENSITY, AK(MParmn)
Fig. 7.1
Variation of crack growth rate with stress intensity range in
SA516 steel tested in moist air and dry hydrogen environments
-93-
in fatigue be havior.
differences
(This
on the influence of low pr essure
critical
in the
frequency
mid-growth
inferred
and the information
hydrogen
i mply
the
for a ny effect of low pr essure hydrogen
regime
is
below
0.1 Hz) .
It
can
be
from these resul ts t hat increasi ng the pressure of
hydrogen from 0.14 MPa to
acceleration
in
6.9
MPa
g rowt h
crack
leads
to
value of 11 MPa/'m (co p a r ed to a KTmax of1
for
low
strength
This
SA542- 3).
the
of
influence
about 20 MPa/m
effe ct of
consistent with the previo usly presented
that
considerable
rates wit h a relatively low
K ax
3)
that
pressure is
arg ument
(Chapter
hydrogen in accelerating crack
velocities at growth rates above 10 -5
mm/cycle is due
to
a
hydrogen embr ittlement mechanism.
In
the
threshold
environmental
regime
influences
seen
for
in
crack
the
case
growth,
the
of SA516 are
similar to those in SA542-3 for both low and high load ratio
values.
Also,
comparison of the results obtained on SA516
in the present study and in Ref.37
crack
shows
no
variation
in
growth rate with a change in frequency from 0.1 to 50
Hz above a
information
growth
on
rate
crack
of
10- mm/cycle.
growth rates near-threshold for high
pressure hydrogen or frequencies lower than 50
available for SA516.
Unfortunately,
Hz
are
not
-94-
The fatigue crack
steel
in
7.2.
Fig.
other
pipeline
X-60
In this case also, behavior similar to
that
of lower strength steels is observed.
classes
7.1 shows the fracture surface oxide
SA516 and X-60 pipeline steels
both
of
behavior
and hydrogen environments is shown in
air
moist
growth
Table
values
thickness
in
for
It is noted that
(81).
at low R ratios the thickness of excess oxide on crack faces
is several
times greater at ultra-low growth rates than that
observed at higher growth rates or load ratios.
From the above results, it can
be
that
inferred
the
explanation for environmenta 1 effects based on oxi de-induced
closure is
several
consistent
classes
of
low
s trength steels, i.e.,
(ferritic-bainitic) , SA542-2
(fully
(banded
bainitic),
ferritic)
observ ations
experimental
with
SA516
(fully
martensitic)
(ferritic-pearlitic)
SA542-3
and
X-60
IN 2021 ALUMINUM ALLOY
To contrast the
behavior
near-threshold environmental
in
lower
strength
steels,
influences were investigated in
2021 aluminum alloy which is used in aerospace
(40%
SA387-2-22
steels.
THRESHOLD BEHAVIOR
(82).
in
applications
Fatigue crack growth rates were measured in moist air
RH), dry
distilled
low-pressure
water,
and
(0.14
hydrazine
HPa)
gaseous
(N2 H 4 ),
hydrogen,
over a range of
10-3
EIT
E
A K (k sijIW)
8 9 10
7
I
6
I
5
4
IXI
3
I
20
30
I
X-60 PIPELINE STEEL
Frequency: 50 Hz, Ambient Temperature
U
E
z
10-4
10-5
Environment
R
0
0.1 )moist
A
0.5
(30% R.H.)
0
0.1
dry hydrogen
aa
oir
A
( 138 k Pa)
0
0
0
10-6
A0
< I00-5
PAO
4,
911
U
z
0
0
ba b
0
0
0
C
6
z
e
*
1Q
O <0-6
0
I lattice
0
spacing
per cycle
0
RR0.5
Air
0
*-R
(.1,
10-i 7
R =0.1
=0.1
H2
10-8
Air
a
Threshold A KO
I
I
3
4
5
6
7
8
9
I
10
I
20
I
30
ALTERNATING STRESS INTENSITY, AK(MPaViF)
Fig. 7.2
Variation of crack growth rate with stress intensity range in
X-60 pipeline steel in moist air and dry hydrogen environments.
-96-
TABLE 7.1
Threshold data for SA516 and X-60 steels
Steel
Environment
R
CTOD
AK
cyclic
SA516
(a
-
327 MPa)
moist air
0.15
7.2
0.24
0.57
0.45
moist air
0.75
3.5
0.07
1.8
0.06
0.15
5.7
0.15
0.36
0.1
9
0.32
0.68
0.28
0.1
5.9
0.14
0.29
0.1
dry H 2
X-60
(a m 386 MPa)
pm
-
lPalm rr Jm
max
Max.
excess
oxide thickness
moist air
dry H 2
A
-97-
growth
frequecy
Hz and a load ratio of 0.1 (Fig.
9
exces s of 10-J mm/cycle,
acce 1erate
Na H4
the
7. 3).
a
At
water
distill ed
rates wh ile the
growth
crack
at
.
and
of
rate s in
growth
10~ 7 mm/cycle to 10-2 mm/cycle)
(from
rates
presence of hydr 0 gen
similar
g as
results
cra ck
grow th
rates
The increase in growth ra tes due
tho s e in a ir.
to
in
to IN
2 H4 seems to be the result of attack of NH 3(form ed due to
of
dissociation
on
effect of wa te r,
appears
with
Cu in the
its
wh ile the
mat enial,
mode,
fractu re
characteristic
be a resul t of hydrogen embrit tilement pr ocesses
to
(commonly ob serv ed in s everal
(83,84)).
It
is
classes
acceleration
mid-growth re gime at KT
max
KIscc (82,85).
aluminu m
alloys
that j ust lik e the be ha vior in
observed
lower streng th steels, 2021 al umi n um
significant
of
in
al 1 oy
also
growt h
c ra ck
un dergoes
rates
the
in
than
values considerably smaller
In the near-threshold region of crack growth,
however,
moist air, dry hydrogen and distilled water environments are
found
to
analysis
moist
air
result
of
the
has
predominantly
fracture
threshold
in
identical
fracture
surfaces
behavior
(82).
(Fig.
shown
values.
ESCA
surfaces obtained from tests in
identified
Al2 03
threshold
the
surface
film
to
be
Auger spectroscopy study of the
7.4)
by
reveals
lower
that
strength
unlike
the
steels,
the
excess oxide thickness on the crack faces of aluminum
alloy
)
AK(ksiin
3
2
10*'
4
5
6
7
I
I
I
I'~-
8 9 10
20
1 II'IIIIII
BA 2021 ALUMINUM ALLOY
R=0.1, FREQUENCY =9Hz, 25 0 C
-
0
o moist oir (-40 % RH)
* dehumidified hydrogen (115kPo)
0
a distilled water
0
0
a
o hy dro z ini
Cf'
0
a0
a
0f
0
1- 6
0
0
C-)
C-)
0
0
E 10
A
A
I
.
A
0
0
10
0 0
a.
-10
I
0
V.4D~
-.
~
'-3
pc. cy'cie
I
I
2I
Threshold AKO
I
I
I
3
4
5
6I
7I
8I
,
I
9I 10
I
I
I
t~
I
I
1
ALTERNATING STRESS INTENSITY FACTOR, AK(MPa/-)
Fig. 7.3
Fatigue crack propagation behavior of BA2021 alloy in moist
air, dry hydrogen, distilled water and hydrazine environments.
1
1
1
I
1
20 3
-7
I
I
I
I
I
(I)
U)
SA542 -3 (50Hz)
--A2021 (9Hz)
Ambient temperature
moist air (30-40%RH)
R = 0.05 - 0.1
z
-
E 0.2
w
-
-
I
I
I
I
I
I
SA542 -3
A-202 I
(H 2 0, 9Hz)
0.11
0
0
-
O-- -
r
-
I.
A4_2021
0
10-5
E 10-6
E
E 10-7
Threshold
0
'a
)
10~8
.
.. , ,. ./
Fiq. 7.4
2
.
3
.
4
5
6
7
8
I
9
I
10
I
11
CRACK LENGTH (mm)
Variation of excess oxide thickness with crack lenth in BA2021
allov, ana SA542-3 steel.
-100-
tested in moist environments is about the same at all growth
rate levels .
will
Altho ugh it
based
concl usi ons
on
this
be
to
Sre mature
make
any
extent of data, it
limite
is
all
clear that oxide-in duced crack closu re may not occur in
the
alloy
capa bl e
that
rea dily
aluminum
though
even
thickening of the o xide film at thre shol d
occur
readily fo rming
of
just
A pl ausibl e explanation for this behavior may
oxides.
be
are
that
systems
because
aluminum
the
oxide
forms
oxides,
AK values may not
fo rmed on the fracture
surfaces is so tena cious that it is di ff icult to activate
mech ani sm
fretting
at
hydrogen has been k nown to
crack
growth
in
controversy on the
investigators
higher
influence
of
a
alloys.
on
on
near-threshold
all oys.
lower
effects
Ho wever, there is some
w ater
of
Gaseous
stresses.
in sign ificant
aluminum
observed
have
many
While
thresh old
val ues
and
near-t hresh old growth rates in water than in air for
some aluminum all oy s,
as
have
aluminum
beha vi or
fatigue
thresh old
low
a
result
of
and have interpreted such
conve nti onal corrosion mec hani sms
have found no di ffE rence betwee n
threshold
water
a nd
moi st
in certa in cl asses o f aluminum all oys
is probably du e to the
corrosion
a
fact
that,
from
the
(83
behavior
0 thers
ai r
at
This
convent ional
mechanisms point of view, moist air itself may be
a severe environment.
-101-
In this light, recent results
Vasudevan
(86)
reveal
the
obtained
by
inconsistent
behavior o f aluminum alloys.
W h ile
Bucci
near-threshold
their
studies
si gni fi can tly more oxide growth at threshold
were
found
explain
limited
s uch
results
extent
susceptibi lity
of
to
It
has
not
been
possible
of
publishe
on
information
environmenta 1
limited
formation
such alloys.
i nformation
at
to
in alumi n um alloys because only very
influences
at
growth rat es is currently availa b le, despite the
use
oxide
be the same at all growth rate
to
levels in a 7075-T7 alloy.
showed
AK0 levels than
at higher growth rates in a 7075 I- T6 alloy, the excess
thicknesse s
and
their
ultra-low
widespread
However, it can be said even with the
that
the
mechanism
of
thick
oxide
threshold in alum Ii num alloys may be different
from that in low strength steels and may depend not only
on
the mechan ical and metallurgical properties of the material,
but also on the properties of the oxide.
-102-
8. CONCLUDING REMARKS
The present study has
clearly
demonstrated
the
that
of corrosion products on crack faces significantly
presence
influences the fat igue behav ior of lower streng th steel s
small
int ensi ty ran ge values.
stress
At this po int,
it is
pertinent to consi der other forms of crack clos ure that
the
excess
fracture surface r oughne ss (57,60
(87)
cannot be ign ored.
that
there
is
near-threshold
a
stro ng
gr owth
Mode
rat es.
pr esent
component
II
closure
(
87)
de scri be d
in
increased
thres hold
for
the
si gni fi cant
rol e
nea r
threshold
di ffe rent envi ronments and load ratios
grain
strength (1,4,11,91).
of
ab ove (and schematically shown in
(88) en able u s t 0 envi sage possible
as
AK
locations
The se different mecha nisms
8.1)
factors
at
This implies that a rougher
Fig.
other
m orphology
and irregular
because of the d isc rete c ontact at several
along the crack le ngth
crack
on
Th e work of Davidson (50) has shown
fracture surface w ould r esu lt
values
debris
oxide
the possi bib ity of crack closure ari sing from
faces,
crack
are
Si nce fracture morph ology may
of importance near threshol d.
be of the same siz e scal e as
at
size
but
exp lanations
of
not
only
also
of
such
(1,4,61,87,89,90) and yield
No Closure
Oxide Induced Closure
Roughness- Induced Closure
Kmin
C
Kmax
(V
AKeff Km x- Kmin
Fig. 8.1
Schematic showin
steels.
Ak'
~V
ef- 'max
-v
'CI
eff
= K
max
-v K
the differerent modes of crack closure in
CI
-104-
The variati on of measured
threshold
steels
The
(1,48).
decreasing
less
from
fre tting
surfaces (60).
in
strength
plasticity-induced
limited
closure
(44-46), or perhaps smoother fracture
Certainly higher strength
retardatio n
less
increased
of
values may simply result from reduced crack
AK0
closure, arising
(48),
role
with
classes of
8.2 for several
strength is shown in Fig.
yield
values
AK 0
steels
show
far
in crack growth following single positive
overload cycles (92), a phenomenon often explained in
terms
of crack closure arguments.
The role of finer grain si ze in
AK
0
may
again
values
closure from
1es s
is
explanation
simply
fracture
be
similarly
a result of reduced crack
surface
roughness.
far
often
less
than
it
sizes
a grain diameter.
verification of t hese ideas is in
but
most
near
instances
AK 0
lacking,
is inter esting to note that, similar to the role of
of
1 owe r
strength
beneficial
a nd coarser microstructures in
reducing nea r-thr eshold grow th rates are generally far
evident,
an
Quantitative
moist enviro nment s seen in the present work, the
effects
Such
more appealing than those based on inherent
mi crostructu ral r easons where p lastic zone
are
decreasing
an d
often
non-existent
observation which strongly
crack closure
(46).
suggests
less
high load ratios, an
at
a
prominent
role
of
(ksi)
ay
100
50
0A
10
200
250
I
I
i
Threshold Data for Steels at R=0-0.05
ambient temperature air
N
AA
A
10
8
N0
A
00
V
aAA
2 1/4 Cr
11Mo
0
-
8-
150
N%'%I
VI
12
4
o-
m-
W
-
R
*
o
*
martensitic
bainitic
y
pearlitic (< 20% ferrite)
U
0
austenitic
duplex (ferrite + martensite)
A ferritic (<20% pearlite)
A SA 387 (bainite + < 40% ferrite)
0 SA 542 -3 (bainite)
o SA 542-2 (martensite)
1
1
2
0
0
Fig. 8.2
200
400
*
M 4
0
'.""" """ ."" """
'"..
0
I
I
1200
1000
800
600
YIELD S.TRENGTH, o-y (MPa)
2
1400
1600
Variation of threshold stress intensity range with yield
strength in several classes of steels.
1800
-106-
interpreted
Overload effects on crack growth have been
based
on
crack
phenomena (73).
to
attempted
closure
as
well
as
crack
tip blunting
rationalize
effect of oxide deposits on
the
near-threshold growth rates by means of crack
blunting
tip
Such an interpretation certainly seems plausible
arguments.
because the
values
have
(93,94)
Similarly some investigators
of
threshold
AK
from
predicted
the
increased
crack tip radius (calculated from the procedures
of Kreager and Paris (95)) due to oxide formation appear
be
reasonably
close
to
and
crack
one
is
AK 0 values.
the actually measured
While it is not known at this point
whether
to
both
blunting
closure occur at ultr-low growth rates and which
more
ultrasonic
predominant,
techniques
the
results
(Chapter
5)
obtained
clearly
using
indicate
a
prominent role of crack closure.
It can also be inferred that the results
this
work
may
also
shows
in
possibly explain the behavior seen in
vacuum and the existence of a
8.3
presented
threshold
in
steels.
the threshold behavior of a Cr-Mo-V
Fig.
steel tested
in air and vacuum at 550 0 C (from the studies of Skelton
Haigh
(12)).
It
is
observed
from
definite threshold exists for vacuum
Although
a
number
of
the
existence
a
workers (see ref.1)
threshold value for no crack growth in
values,
of
such
figure that no
this
at
a
and
high
R
ratio.
have reported a
steels
at
threshold
lower
could
R
be
II
-107-
x
7-0
LOW R
(0.2 5)
HIGH R
(0.7)
E
E
/0
10-6
0
,0
I LATTICE SPACING
dz
,a" 0
CYCLE~
VACUUM
(R=0.7)
-
1
10Hz
10-8
4
|
5
I
.
I
I
I
I,
,
I
,
I
10
,
I
I
I
15
I
20
AK (MPa/in~)
Fig. 8.3
Fatigue crack growth behav ior of Cr-Mo-V steel in moist air
and vacuum at 5500C (ref. 12).
-108-
to
attributed
rewelding
crack
plasticity-induced
(1),
closure, and surface roughness-induced closure.
hi gh
of a threshold at
that
shows
clearly
load
the
ratios
in
The absence
"good"
a
vacuum
of threshold values in
existence
steels in seemingly d ry environments may merely
be
due
to
combined effects of oxidation, fracture morphology, and
the
residual
stresses.
In summary, it is clear that concepts of crack
closure
are of paramount im portance to fatigue crack growth behavior
at near-threshold 1evels although
may
closure
mech ani sms
the
At
lo w
stress
closur e mechanisms originate fr om the fact that
crack
cyclic
such
very different from the pl asti cit y-induced
be
closure originally proposed by Elber (47).
intensities,
of
tip
opening
are
displacements
of
a
size
comparable with fra cture surface roughness a nd the thickness
of corrosion debris within the crack.
their
speculative
These ideas,
despite
nature, clearly suggest that si gni fi cant
influences in fatig ue crack propagation may arise s imply due
to
a mechanical co ntact of fractured faces, rather than any
inherent
metallurgical
rea sons.
profound
influence
the direction which should be taken
for the
design
of
on
alloys
with
This
role
increased
may
have
resistance
fatigue crack propagation at near-threshold growth rates.
a
to
-109-
CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH
9.
a
Based on
of
study
corrosion
temperature
ambient
fatigue crack growth in SA542-3 pressure vessel
low strength steel, X-60 pipeline steel
steel, SA516
2021
and
aluminum
alloy, the following conclusions can be made:
Lower
1.
strength
steels
significant
undergo
in fatigue crack propagation rates due to the
accelerations
presence of hydrogen gas in mid-growth regime (growth
in
excess
near-threshold
mm/cycle)
as
mm/cycle)
(growth
region
at
threshold for
loading.
10
of
stress
hydrogen-assisted
far
below
cracking
in
than
smaller
rates
intensities
as
well
rates
the
under
the
10-
6
KIscc
sustained
Mechanisms for the influence of hydrogen in these
two regimes are entirely different.
2.
abrupt
In the
mid-growth
regime
of
crack
propagation,
in crack growth rates which depend on
accelerations
frequency, load ratio, and hydrogen pressure, are associated
with
to
a
be
characteristic intergranular fracture mode and seem
specific
containg/producing
enhanced
crack
to
gaseous
environments.
growth
embrittlement" mechanisms.
rates
hydrogen
It
result
and
appears
from
hydrogen
that
such
"hydrogen
-110-
3.
1evel s,
At near-threshold
growth
below
10
-0
mm/cycle,
in dry gaseous hydrogen and hel i um are larger
rates
than those compared to moist air and w et hy droge n by
two
orders
approximately
of
magnitude ,
with
40%
higher
in
air.
near-threshold
rates
propagation
thres hol d
On
in
AK0
the
up
values
other
di still e d
to
hand,
water are
marginally lower compared to those in air.
4.
Contrary to
in
propagation
the
the
behavior
a ssociated
mid-growth
characteristic fracture mode
for
regime,
crack
with
there
env ironmental
is
no
inf1 uences
near threshold.
5.
The corrosion
tested
at
ambient
products
temperature
formed
that
crack
growth
observed
at
the
the
specimens
have been identified using
ESCA analysis to be predominantly Fe 2 03
found
in
in
steels.
It
oxide thicknesses are inversely related to
rates.
The
thickest
oxide
formation
at
high
load
that
ratios or on specimens of the same
material exposed to the same environment for same length
time.
is
near-threshold growth rates at low load ratios
in moist environments and is several times larger than
observed
is
of
-111-
6.
Environmental
explained
based
on
influences
intensity
but
to
due
are
growth
According to this model, low
rates are accelerated in hydrogen
per
embrittlement
compared to air, not because of hydrogen
se,
threshold
the concepts of a new mechanism termed
"oxide-induced crack closure".
stress
near
reduced amount of crack closure resulting
from less oxide debris formation on crack surfaces.
7.
with
Concepts of oxide-induced
observed
effects
fracture
morphology,
behavior
and
of
and
load
closure
are
ratio,
environment
consistent
strength
on
level,
near-threshold
are in accord with observations that
further
growth
rates
Despite the normal uncertainties involved in
oxide
inert environments accelerate
near-threshold
compared to distilled water.
8.
it can be inferred that the exces s
measurements,
thickness
oxide deb ri s on the crack flanks is roughly of the order
of
the cycli c crack tip open ing displacement at the threshold.
9.
Measurement
ultrasonic
of
techniques
closure
cl early
amount of closure in moist air
even
though
be made.
It
spontaneous
near
using
threshold
demonstrate
the
increased
to
dry
hydrogen,
compared
precise meas urement of closure loads could not
is also seen that crack opening may
phenomenon,
but
a rather gradual
not
be
a
process with
crack surfaces remaining i n contact locally, while
globally
-112-
the crack can be considered fully open.
10.
Block
intensities
underload
below
the
cycles
subsequently
magnitude of the
underload
resumed
effect
stress
at
growth rates when
baseline
being
'KB
critically
cycling
levels,
the
dependent
upon
(Au ) and baseline (AKB ) stress intensities.
This
behavior appears to be
oxide-induced
consistent
crack closure.
baseline
load
with
the
arguments
of
The lack of such crack growth
retardation effects for certain
and
alternating
threshold can result in significant
transient retardations in initial
is
at
combinations
of
underload
levels is found to occur when pulsating
crack tip displacements at the underload
are
smaller
than
the existing excess oxide thicknesses.
11.
debris,
In addition to mechanisms generated
sources
of
enhanced
crack
by
corrosion
closure from fracture
morphology and roughness may arise at near-threshold
because
their
size
scales
are
comparable
to
levels
crack tip
displacements.
12.
that
Near-threshold data in 2021 aluminum alloy
reveal
the concepts of oxide-induced crack closure may not be
valid in all the
forming
oxide
materials
films.
that
are
capable
of
readily
Thick oxide formation at low growth
rates may depend on how tenaciously the oxide is attached to
the fracture surfaces.
113-
SUGGESTIONS FOR FURTHER RESEARCH
1.
levels
Thick oxide formation at low R values
has
threshold
been
found
behavior
to
significantly
understood
and
further
AKeff
influence
of lower strength steels.
kinetics of such an oxidation process are
and
the
However, the
still
not
fully
needs to be conducted to
research
elucidate the mechanism of excess oxide debris formation.
2.
Fatigue results on aluminum alloys
concepts
show
that
the
of oxide-induced closure are not applicable to all
materials.
The mechanism for thick surface
film
formation
under threshold fatigue conditions and its dependence on the
properties of the oxide itself
3.
appears
It
environmental
possible
interactions
influenced by surface film
corrosion
mec hanisms
effects may be
alloys,
which
and
operative.
are
need to be studied in detail.
being
at
that
in
threshold may not be solely
effects
and
that
other chemical
For
material s,
some
example,
developed
for
conventional
and metallurgical
aluminum-lithium
extensive use in
aerospace appl ication, readily form hydrides on exposure
hydrogen.
It
would
to
be interesting to study the effect of
hydrides on th reshold fatigue behavior
and
to
investigate
the process of oxide formation and the resultant closure, if
any , i n these all1oys due to the combined effects.
-114-
4.
steps
Although information
regarding
the
rate-limiting
for hydrogen embrittlement mechanisms associated with
the mid-growth regime is becoming available, further studies
on
chemical
reaction
kinetics
and hydrogen diffusion are
required to substantiate the currently-known concepts.
5.
Arguments of crack closure near
threshold
arising
from fracture surface roughness and irregular morphology are
conceptually
currently
appealin g, yet
no quantitative information
Further
available.
research
is
is
needed
to
closure
as
substantiate these con cepts.
6.
wel 1
The implicati ons of the
as
di f ference
roughness-in duced
in
thre shold
oxide-induced
closure mechanisms are that the
A K0
values
for
different
env ironments (and stre ngth levels and grain sizes) is due to
a p ure mechanical clos ure phenomenon.
it
is
possible
con siderations
t hat
may
met allurgical factors.
phi losophies is needed
be
grain
more
A
In the light of this,
size
and
important
environmental
than
various
systematic approach to aid design
-115-
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- 121
APPENDIX
AUGER SPECTROSCOPY STUDY OF OXIDE DEPOSITS
Ch aracteri zati o n of
surfaces
550
was
Scanning
Auger
01
of
profiles
the
monitor
were
Fe3
Depth
composition
obtained
using Ar
The composition of C1 was also measured
sputter-techniques.
to
spectrometers.
and
as PHI Model
590 as well
done using PHI Model
fracture
on
deposits
oxide
the
A typical depth
pr esence of hydrocarbons.
composition profile is shown in Fig.
Because of
count peaks plotted against the sputtering time.
sputter
the difficulties ass ociated with the calibration of
rate
by
a
preparing
thickness
known
tantalum oxide standard was
for
used
of
Auger
the
with
Al,
oxide, a
iron
Since
calibration.
there is no currently-available standard procedure, the time
the
taken for the 01 composition profile to drop from
to
a value which is half of the peak minus the steady-state
level, was arbitrarily defined as the sputter
from
the
time.
oxide
were
Also,
rates calibrated with the tantalum oxide
sputter
rates
standard using this procedure, the corresponding
iron
peak
using
estimated
sputtering yields (atom/ion) from
the
Veeco
data
for
on argon ion
Brochure
V60
and
Physical Electronics
(15).
Using a beam voltage of 5kV, ion
and
sizes
of
electron
beam
respectively (for PHI Model
roughly
200
and
20pm,
550), and rastering rates of 180
Fe 3
CI
0I
w
a-
C
PC
Fe3, 01, and Cl peaks.
-
- 123
Hz (X-directi on) and 30 Hz (Y-direction),
obtained
for
several
sputter times were
fracture surfaces.
The sputter time
mul tiplied by the modifi ed sputter rate provides an estimate
of the extent of oxidati on.
Simple calculations
oxidizes
of FeO.
Thus
showed
to 2.13 cc of Fe 2 03,
Similarly 1 cc of Al
that
a
cubic
cm
of
Fe
2.07 cc of Fe 3 0 4 , and 1 .76
cc
results
in
2
cc
of
Al2 03
the oxide thickness measurements represent rough ly the
total excess
material
inside
the
c rack
(assuming
thicknesses on each crack face), as sh own in Fig.
A2.
equal
124
-
-
B
I
/
Oxi d e
I
Unoxidized Steel
Fig. A2
Oxidized Steel
Schematic showing oxide formation on crack faces and the
excess oxide thickness.