tips on fatigue
TRANSCRIPT
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N A V W E P S00-25-559
58 *
TIPS
O N
FATIG U E
r atBU*nO&
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N A V A L
W E A P O N S
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TIPS
O N
F A T I G U E
by
Clarence
R.
mith
Structures
Design
Specialist
Fatigue
Laboratory-
General
Dynamics/Convair
Prepared
for
the
Bureauof
NavalW eapons
D E P A R T M E N T
O F
TH EN A V Y
1963
For
sale
by
theSuperintendent
of
Documents
U.S.
Government
Printing
Office
Washington
25 ,
D.C. --
Price0
ents
Reproduced
From
Best
AvailableCopy
2 0 0 1 1 1 3 0
2 7
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TIPS
O N
FA T IGU E
PREFACE
/Sooner
orlater,
metal
structures
underrepeated
load
wearout. The
prob-
lemis
to
becertainthat
it
islater,rather
thansooner./For
thisreason,
theNavy
has
always
encouraged
research
and
develop-
ment
to
find
ways
of
making
structures
last
longer.
It
has
beenfound,however,
hat
itis
not
enough
for
scientistsand
research
en-
gineers
to
knowthesecrets
of
fatigue.
If
designers,hopmen,
and
inspectors
do
not
recognize
the
signsof
fatigue,
then
the
purpose
of
research
and
development
has
notbeen
realized.
To
get
thismessageacross,plain
simple
languageandforthrightpictures,
are
used,
unhampered
by
superfluous
tech-
nicaljargon,theory,
nd
detaileddatadis-
plays.
This
approach
is
on eway
to
ensure
thatthe
findings
ofresearchbecome
the
usableknowledge
of
themanintheshop
and
theman
on
the
drawing
board.
The
premise
is
that
research
an ddevelopment
ardworth
every
cent
theycostifand
only
ifwemake
full
use
of
thenewideas
they
produce.
Lack
of
communicationbetween
those
who
knowand
those
who
need
toknowis
often
theprimecauseof
structural
failure.
If
the
knowledgegained
through
fatigue
researchoverthe
last
100
yearswere
ap-
plied,
many
fatigue
problems
would
never
occur
(or
recur).
Thousandsofdocumentsonclassic
and
appliedresearch
of
metaland
struc-
tural
fatigue
literally
bury
facts
by
their
weight
and
profundity.
These
documents
shouldbelefttotheexperts.
O n
theother
hand,
themanatthe
drawing
board
is
a
practicalman. H e
needspractical
answers
to
suchfunda-
mental
questionsas:
Is
he
continually
makingerrorsthat
will
result
in
fatigue
problems?
The
factthatnew
airplanes
arestillfailing
infatigue
indicates
this
to
be
true.
Examination
of
such
failures
in-
dicates
thatmanydesigners
are
not
even
aware
thatsharpnotchesare
fatigue-
prone.
Correctionsforthison e
fault
could
savemillions
of
dollars
per
yearandpos-
sibly
a
fewlives.
HencejJJiis
book
proposestobenoth-
ingmore
nor
less
than
a
simple
guideon
ho wto :
1 .
Recognizepotential
fatigue
prob-
lems.
2.
Rectify
existing
problems.
3.voidgettingintosituations
that
maycause
problems.
?-U'
1
in
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TIPS
O N
F A T I G U E
fpr:
Principlesmentionedhereinhaveto
do
with
1 )
relationshipsof
one
structural
member
toanother;
and
2)
paths
of
load
carriedwithinthendividualmembers.
Thissnotto
mply
hat
themoreabstruse
principles
of
solid
state
physics
would
not
also
be
helpful,
butthese
are
far
beyond
the
scope
of
this
work.]
Before
fatiguewas
considered,airplanes
were
designedto
withstand
a
given
static
oad. Thismaybe
in
terms
of
thenumberofG's
the
airplane
maybe
expected
to
encounter
duringa
maneuver,orin
some
casesthe
largest
gust
thatmay
be
encountered
oncen
a
lifetime).
In
any
event,
here
was
some
design
number.
Knowing
thestrengthofthe
ma-
terial,
t
waspossibleto
figure
out
how
muchmaterialwas
requiredto
carrythe
load.
A
perfectdesignwasonewhereinthe
structure
would
carry
100
percent
ofthe
designload
and
fail
at101percent. Not
only
that,
no
component
wouldberelatively
stronger
than
the
next.
Just
like
the
"won-
derful
one-hossshay
that
was
built
n
such
a
ogicalwaythat
t
ran
for
ahundred
years
toaday."
Inhispoem,"TheDea-
con's
Masterpiece,"
Oliver
Wendell
Holmes
(father
of
Supreme
CourtJustice)
chroni-
cleshatnopartcould
fail
firstbecause
each
was
constructed
ofthevery
bestma-
terialfor
theunctionto
be
served.
Un-
doubtedly,
design
also
had
somethingtodo
witht.
.
n
_w
)Whileairplanesare/stillIdesignedto
carry
acertain
static
oad,
atigueposes
theadditional
problem
ofestimatinghow
long
the
airplane
will
last.
Ideally,
t
shouldlastas
ong
as
he
designer
ntended
itshould.)
Indesigningforstatic
strength,
he
designer
was
given
asetof
rules
governing
thestressevels
to
whichhis
materials
could
be
worked. This
gave
some
uniform-
ity
n
design.
Thenominal
stress
evels
mayhave
differed
withlocationorpurpose.
Forexample,hecompressionallowable
would
depend
on
stringer
andbulkhead
spacing,while
tension
allowablesmight
de-
pendon
thetype
of
fastener.
Infatigue,designingto
auniformnom-
inal
stresswouldnotensureauniformfa-
tiguestrength. Auniformdesignforfatigue
would
involve
he
product
of
the
nominal
stress
times
the
stress
concentration.
Not
knowing
the
stress
concentrations,
his
would
be
animpossibility.Acknowledg-
ment
s
duetoall
thosewhose
encourage-
ment
and
assistance
have
made
this
book
possible.
While
theirs
s
the
glory
for
any
meritnthe
work,
blameforanyfault
herein
belongsto
theauthor
alone.
Assistance
came
frommany
sources,
all
remembered
and
deeply
appreciated,
thoughspaceimitsmentionto
M
.
Rosenfeld,
Naval
Air
Engineering
Center,
R.L.CreelandC.P.Baum,NavyBureau
ofWeapons
for
reviewoftheentirebook.
The
authorwishestoespecially
ac-
knowledgehecartoonsofW .Goldsmith
and
T.
Adams;
heeditorial
assistance
of
R.J.
Prichard;
heorganizationalhelp
of
Ralph
DeSola
in
the
earlystagesof
the
work;
and
theechnicalassistanceof
G.
G .
Green.
For
the
data
and
photographs
thatgive
thisnformal
work
a
concrete
set
of
ex-
amples,
especially
n
Chapter
5,
he
author
wants
to
thankthe
followingaircraftman-
ufacturers,operatorsandorganizations:
>
Aeronautical
Research
Laboratories,
Melbourne,
Australia
IV
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T I PSO NF A T I G U E
Aeronaves
de
Mexico,
S.A.
Aerospace
Development
Center,
Wright-PattersonAir
Force
Base
Aircraft
PlatingCo.
AmericanAirlines
OverhaulBase
American
Air
motive
Co.
Beech
AircraftCorp.
BellHelicopterCo.
Boeing
Airplane
Co.
BraniffAirlines
BristolAircraft,
Winnepeg,
Ontario
BritishEmbassy,Washington,D.C.
Canadair,Ltd.
CessnaAircraftCo.
Aeronauticsand
Missile
Division
Chance-Vought
Corporation
ADivision
of
Ling-Temco-Vought,
Inc.
Chapman
Laboratories,
nc.
ContinentalAirlines
DeltaAirlines
DouglasAircraftCo.,
nc.
EasternAirlines
FairchildEngine
&Airplane
Corp.
General
Dynamics
Corp.
GeneralElectricCo.
Grumman
Aircraft
EngineeringCo.
Hiller
AircraftCorp.
Kaman
Aircraft
Corp.
Lockheed
Aircraft
Co.
Martin
Co.
Metal
Improvement
Co.
Mexicana
de
Aviacion
McDonnell
Aircraft
Corp.
NationalAeronauticalEstablishment,
Ottawa,Ontario
NationalAirlines
National
Luchtvaartlaboratorium,
Amsterdam,
TheNetherlands
Naval
Air
EngineeringCenter
A S L )
North
American
Aviation,
nc.
Northeast
Airlines
Northrop
Aircraft,nc.
PanAmericanAirlines
Republic
Aviation
Corp.
Standard
Pressed
SteelCo.
Transcanada
Airlines
Trans-WorldAirlines
United
Airlines
WesternAirlines
/Thisbook
represents
an
initial
attempt
to
make
all
evels
ofpersonnelaware
of
thefatigue
problem
thatexists
n
aircraft
structures./
It
is
anticipated
that
revision 4.
willbe
required
in
the
future;
consequently
users
comments
are
solicited
so
that
a
meaningful
revision
maybe
accomplished.
Similarly,
he
photographs
represent
the
best
illustrations
presently
availableto
depicttheproblems
discussed. These
photographs
were
not
specificallytaken
for
thispurpose;
hence
they
are
notallas
clear
andunclutteredas
would
bedesired
Photographs
that
llustrate
more
clearly
theproblemsdiscussedhereinor
any
other
fatigue
problemsoccurring
in
aircraft
structures
are
desired.
Users
comments
and
newphotographs
suitablefor
illustra-
tionshould
be
forwardedto:
Director
S-3)
Aeronautical
StructuresLaboratory
Naval
Air
EngineeringCenter
Philadelphia,
Pa.,
19112
San
Diego,
California
30
Oct
1963
C.
R.
SMITH
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TIPS
O N
FATIGUE
C O N T E N T S
PREFACE
Chapter
1
heFatigueProblem
Chapter
2asic
Principles
Chapter
3
oints ndJoining
Chapter
4
evelopingan
IntuitionforFatigue
Chapter
5ast
Experience
Chapter
6aking
the
Mostofa
Bad
Situation
Chapter7heckList
A P P E N D I C E S
A.
atigueTest
Data
B.
tress
Concentrations
C.
uggestedReading
Vll
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ESTABLISHED
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TIPS
O NF A T I G U E
TH E
F A T I G U EP R O B L E M
1.1
A
C E N T U R YOFS T U D Y
N D
F A T I G U E
STILL
F A I L SS T R U C T U R E S
T hefatigue
problem
relatingto
metals
and
structures
has
been
investigated
experi-
mentally
formorethan
a
century.
In
8 4 9 ,
Jonesand
Galton
investigated
cast
ironbars
inbending. They
found
thatfailureoccur-
red
inless
than
100,000cycles
if
loaded
to
more
than
one-third
of
ultimate
bending
strength. Similarwork
on
wrought
iron
built-upgirdersbyFairborn
(1860-1861)
showedsimilar
results.
Wohler's
workfor
thePrussianStateRailwaysgoes
back
to
the
1850's
whenhemadeanextensive
series
of
tests
of
various
grades
of
iron
and
steel
subjected
to
repeated
direct
tensile
and
compressive
oads,
orepeated
bending
loads,andto
repeated
torsional
loads.
Yet
wecontinuetoreadaboutandhear
about
railroad
wrecks,
automobile
smashups,
airliner
crashes,
andother
catastrophes
directlyattributabletofatiguen
metallic
structures.
another
report
inmuchsmaller
type:
"Jet
BouncesnAir:
10
Hurt."
How
many
such
bouncescanastructure
sustain
before
t
fails
andbecomeshefact
behind
an
even
bigger
headline?
W hyan
even
bigger
headline?
The
first
planewas
acharteredplane
n
an
unscheduled
flight.
Itwent
down
nearth e
coastofWestAfrica,
and
whatever
happens
in
remote
placesneverseemsasrealor
critical
as
what
happenscloser
o
home.
T he
second
planecarriedmore
passengers
and
wason
aregularlyscheduled
flight.
Itsroute
involved
the
ives
andemotions
of
hundreds
of
thousands
of
people
locally.
W h e n
fatiguefailure
overcomes
the
second
plane,astmay
in
the
course
of
time,t
is
safeo
predictthat
theheadlinewillbe
bigger,
he
casualty
figures
morestartling,
and
the
impacton
the
raveling
public
even
greater.
1.3
T O D A Y
F A T I G U ES
A
B IG G E R
P R O B L E M
T H A N
E V E R
1.2 F A T I G U E
CA N
BE
BIG,
BA D
N E W S
"Airliner
Crashes
with
10
Aboard:
"
read
the
black
headlines
on
5
March
1962.
O n
the
same
front
page,
and
at
the
foot
ofthe
columndescribing
the
loss
ofthe
airliner,
her
crew,
and
allher
passengers,was
Airplanesnthe
past
were
not
subjected
to
loads
experienced
by
present
dayhigh
speed
aircraft.
Also,
they
were
builtof
materials
whosetensile
strengths
were
soowhat
in
ordertosatisfystatic
strength
require-
ments,
stresses
forserviceloadingwould
automatically
fallwithinranges
that
would
provideanadequate
fatigue
ife.
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TIPS
O N
F A T I G U E
The
fatigueproblemhas
risen
at
an
alarming
rate
withpresent
dayairplanes
having
highspeed
andperformance. To
makematters
worse,he
materials
used
in
presentday
airplanes
are
stronger,
yet
haveno
betterand
insomecases,poorer)
fatigue
properties
han
those
used
formerly,
The
esult
has
eenanaccumulationof
service
failures,
omewith
fatalities
involved.
Such
situationshave
nvolved
the
designer
n
unfamiliarareas.
Besides,
more
accuratemethods
of
stress
analysis
enable
us
to
design
structureswith
greater
efficiency
and
precision.
1.4 THEPANICFACTOR
Designers
are
usually
shocked
byservice
failures;
hence
heir
subsequent
designs
andmodifications
f
ailedpartsften
includeahighpanicfactor. Thisanic
factorislikely
tobefar
out
of
proportion
o
the
design
mprovement
needed.hepanic
factor
s
born
of
sudden
fright. Some-
times
t
is
compoundedwith
ignorance,
and
certainly
its
use
s
contrary
to
all
the
principles
of
gooddesign.
Unanticipated
fatigue
failurescause
designers
o
become
appalled
at
the
amount
of
information
that
seems
neces-
sarytoestimate
he
service
ife
ofany
partorstructure.
The
factthattheir
knowledge
of
the
ordinary
mechanical
properties
of
materials ultimateand
yield
strengths,
longation,
modulus
of
elasticity
has
failedthem,
eads
hem
to
feel
fully
justified
inusing
the
high
panicfactor. The
natural
tendency
so
"beef-up"
hestructurehat
failed,
ven
though
his
change
may
notbehe
solution
and
the
weight
penalty
extreme,
hisome
cases,removalofmaterialmight
solve
he
problem,
whereasa
"beef-up"
may
create
a
newproblem
just
outside
of
the"beefed-up"area.
1.5
HE
VICIOUSCIRCLE
Inaircraft
design,
verypound
of
structure
addedrequiresadditionalweightin
he
form
of
added
power
andfuelnecessary
to
carry
the
addedstructure.
Thisquicklybecomes
aviciouscirclebecausemore
support
structureshenneeded
tosustainhe
added
engineand
fuel
required
tocarry
the
"beefed-up"redesignedstructure.
1.6
REAK
THE
BIG
ONE
NTOLITTLE
ONES
Asong
asfatigue
s
treated
asone
enor-
mous
problem,
t
neverseemsoget
solved.
However,
when
fatigue
s
consid-
eredasanumber
ofsmall
problems,
he
solution
of
eachproblembecomesappar-
ent.
An
initialapproach
to
any
problem
s
tolist
the
factorsnvolved,
uch
as:
1.
hat
are
the
oads?
2.
hat
are
he
stresses?
3.
hat
are
the stressconcentrations
?
4.
ow much
is
the
material
good
for?
5.
ouldweuseabettermaterial?
6.
s
hat
shape
necessary?
7.hataboutcareless
shop
practices?
8.hat
is
the
matter
with
nspection?
9.
hy
didn't
the
engineer
say
so
f
that
iswhat
he
wanted?
10. What"birdbrain"calledoutthis
heat
treat?
This
list
ouldgo
n
ndn.
he
pointis,hatin
lookingverven
this
short
list,
he
designer,henspector,
and
theshop
mancan
each
find
at
east
oneand
probably
moretemshat
he
can
personally
do
something
about.
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TIPSO N
F A T I G U E
1.7
IFE V E R Y O N E
D O E S
H IS
O W N
JO B
WELL...
Accordingly,
if
everyone
ook
pains
o
correct
faults
n
areaswhere
he
has
influence, lessening
of
fatiguefailures
wouldsurelyresult.
Take
he
caseoffeathered
edges.
Just
because
designers
di dnot
takethetrouble
tocallfor
corner
radii, nd
inspectors
di d
notrejectpartshavingsharporners,
repairswere
necessary
onseveral
recent-
ly
built
airplanes.
Wh i l e
the
repairs
n
themselves
maynothave
been
costly,
he
interestat
6
percentonan
idleairplane
costing
$5,000,000willamounttoover
$800 .00
perday. A dd
to
that
the
rental
value
offacilitiesforrepairand
wages
of
an
idle
crew,
and
thedailycostis
appalling.
A
fleet
of
30 0
military
airplanes
was
recentlymodified
to
bring
them
up
tode-
sired
fatigue
life.
It
cost
3200manhours
perairplaneo
the
tune
of
more
than
$11,000,000for
thejob. Downtime
amountedto
about
3
months
perairplane.
1 . 8
LL
FOR
W NT
OF
FATIGUE
RESISTANCE
THE
B TTLECOULD
BE
LOST
In
the
case
of
amilitaryairplane,
he
cost
can
befailuretocompleteamission,which
in
a
criticalsituationwouldbe
mpossible
to
measuren
terms
of
dollars
andcents.
WM
Figure1.1.
"Beach
Marks"dentify
Progressive
Fatigue
Failure
See
Section2.8)
L-3
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TIPS
O NF A T I G U E
1.9
IFYOUC A NGETTHEM
WHEN
atigue
design
superfluous
and
THEY'RELITTLE, FEW
ostly.
Here
s
where
he
atigue
PROBLEMS
WILL
GET
BIG
xperts
should
be consulted. In
other
cases,
the
ptional fatigue
The
mportance
of
considering
fatigue
esign
is
so
simple
that
little
or
in
design
cannotbe
overemphasized.
o
cost
s involved.
These are
Inmanycases,
it
may
seemhat
ainlyhe items discussed
n
theffort
necessaryfor
an
adequatehis book.
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TIPS
O N
F A T I G U E
BA S I CPRINCIPLES
2.1
LET'S
KEEP
IT
SIMPLE
This
chapter
willpresentonly
those
principles
thatpracticingengineers,
nspectors,
and
shop
personnel
can
apply.
In
doing
so,
some
ofthe
morebasicfundamentalsof
crystallinestructure,uchas
slip
planes,
dislocations,
nd
others
willbeomitted.
These
are
beyond
the
scope
of
this
work.
While
theprinciples
discussed
are
adequate
forthe
purpose
intended,
hereader is
directed
to
AppendixC,
Suggested
Further
Reading,
"
for
moredetailedtechnical
information.
Knowing
the
behavior
of
metals
under
load
is
vital
to
theunderstandingoffatigue.
Every
day,ommonoccurrences
showhow
metals
actwhenloaded.
Athoughtful
con-
sideration
of
theseexamples
s
probably
theeasiestway
to
summarizesomeof
the
morebasic
principlesof
metalfatigue.
Have
you
ever
noticed
that
you
canbreak
awire
quicker
by
increasing
the
bend
angle?
2.2
T'S
NOT
NLY
WHAT
OU
DO,
IT'S
HOW
OUDOITAND
HOW
MANYTIMES
2.2.1UNIDIRECTIONAL
VERSUS
REVERSE
BENDINGRepeated
bend-
ingsafamiliar
example
offatigue.
5
CYCLES
90-DEGREE
REVERSE
BENDING
A
galvanized
14-gage
wirehat
breaksn
our
or
ive
ycles
f 90-
degree
everseending
.
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T I P S
O N
F A T I G U E
+
90
.
will
last
or
5r
6
bends
of
from o90degrees.
15 CYCLES
0
o90-DEGREE
BENDING
Kbent
only
from 5o90
degrees,
t
may
last
from
60
to
70
cycles.
7 0CYCLES
45-to
90-DEGREE
BENDING
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TIPS
O N
F A T I G U E
83,000POUNDS
Similarly,
heife
of
a
structuremay
dependmoreonho wtisoadedthanon
he
total
number
of
timest
is
oaded,r
onthe
maximum
amount
of
theloads
themselves.
This
s
bestshownbyloa
xg
an
nch-
squarebar
of
aluminum
alky
hat
breaks
at83,000
pounds
when
loaded
once.
ONE-INCH-SQUARE
BARBREAKS
AT
83,000
POUNDS
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T I P S
O N
F A T I G U E
60,000OUNDStension)
AAA
--)-e y le
60,0 0 0POUNDS
APPLIED
25,
000
TIMES
WILLBREAKTHEBAR
60,000
P O U N D S (tension)
~H*1cycle
60,000
P O U N D S
(compression)
FatigueFailure
Identifiedby"Beach
Mark"
Appearance
See
Section
2.8
However,floads
of
from to
60,000
pounds
wereapplied,t
would
last
about
25,000cycles.
Loadingsfrom
60,000
poundsension
o
60,000
pounds
compression
(commonly
called
"plusominus60,000pounds")
would
fail
he
bar
in
about
4,000cycles.
REVERSING THE
6 0 000-POUND
LOADS
BREAKS
THE
BAR
I N
4
0 0 0 CYCLES
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TIPSO NFATIGUE
C/3
Q
I D
o
0.
o
1,000
L0.000
00,000
CYCLES
TO
FAILURE
1,000,000
Figure2.1
LoadVersus
Fatigue
Life
for
One-Inch-Square
Bar
of
7075-T6
Aluminum
Alloy-
Similarrelationshipsbetweenre-
peated
tension
and
reversed
loadsare
plotted
n
Figure
2.1.
Load
is
shown
on
the
vertical
axis
ordinate)
and
he
number
ofcycles
onthehorizontal
axis
abscissa).
S o
hat
the
ifetime
scale
could
be
condensed
tofitononepage,heabscissahasbeen
compressed
in
whatis
commonly
knownas
a
ogarithmic
scale.
Notehateachmajor
division
represents
0
times
he
valueof
thepreviousdivision. Schematic
diagrams
of
load
cycles
are
shown
to
facilitate
read-
ing.
2.3
CONVERTING
LOADTOSTRESS
Thench-squarebar
was
used
in
the
pre-
cedingexamplefor
woreasons.
First,
squarench
is
astandardunit
of
measure.
Second,
when
fatigue
effects
are
understood
inermsofasquare-inch
cross-section,
t
iseasyto
compare
heoad-carrying
ability
of-our
known
example
with
the
load-carrying
abilities
of
structures
havingother
dimen-
sions.
In
other
words,
oad
carrying
ability
is
hen
expressed
in
erms
of
pounds
per
squarenchofcross-sectional
area.
Commonly
expressed
in
termsof
load
dividedbycross-sectional
area,
heshort-
handor
algebraic
descriptionforstress
s
where
S
=
S =stressn
pounds
per
square
inch
P
=oadin
pounds
A
=ross-sectionalarea
n
squarenches
730-755
0-642
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TIPS
O N
F A T I G U E
2.4 FATIGUE
SHORTHAND
Sincefatigue
ife
s
not
only
dependent
on
the
amount
of
stress,
utalso
on
how
he
stress
s
applied,
a
system
has
been
de-
vised
identifying
theype
ofloading,hus
R =
'mm
J
max
This
ssimply
the
ratioofthe
minimum
stress
dividedbythemaximumtress.
Using
thisnotation,
he
curve
forrepeated
tension
loading
inFigure2.1wouldbe
identifiedasR=0,ecausehe
minimum
loadwas
zero
and
zero
divided
by
anything
isstill
zero.
2.5
TENSIONANDCOMPRESSION
LOADING
Accordingto
convention,
ension
stresses
arealways
dentified
asplus(+
)
and
compression
stresses
areminus
-). In
Figure2.1hecurvefor
reversed
loading
would
have
astress
ratio
of
J
mm
=
-
'max
sinceS
opposite
signs
mn
is
equal
o
max
,
except
forhe
2.6
COMPRESSION
S-NCURVES
FOR
SMOOTH
SPECIMENS
Topresent
fatigue
datainbriefform,
curvesof
stress
versusthenumberofcy-
cles
to
failure,
called
S-N
curves)
are
used. Since
the
dimensionofthe
bars
n
Figure
2.1
wasone-inch
square,pplying
S=P/A,
he
curvesshown
are
alsoS-N
Curves.
Frequently,
whole
family
of
curves
s
given
in
order
to
show
lives
for
other
ratios
ofstress
R). Figure2.2
shows
a
familyof
curves. Appendix
C,
"Suggested
FurtherReading,"
contains
references
to
S-Ndatafor
other
materials.
Curves
for
typical
airplane
structures
are
given
n
Appendix
A.
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T I P S
O N
F A T I G U E
80,0001-.
70,000
a
0,000
C O
f
0,000
I
g
0,000
w
0,000
R
20,000
10,000
1,000
10 000 100 000 1 000 000
CYCLESTO
FAILURE
Figure
2.2
A
Whole
Family
of
Curves,ShowingLives
for
VariousRatios
of
Stress,
R.
S
P/A
2.7
STRESS
CONCENTRATIONS
Inthe
case
of
airplane
structures,
he
fatiguebehavior
will
be
substantially
differ-
ent
fromhat
observed
n
thesquare-inch
bar,
because
free
flow
of
stress
s
nter-
rupted
byobstacles
such
as
holes,notches,
bumps,
and
changes
of
section.
Piling
up
ofstressat
obstacles
such
asheses
commonly
called
aconcentration
of
stress,
andtheobstacleshemselves
are
knownas
stress
raisers.
2.7.1
NFILLED
HOLE
- Inthe
case
fhe
ne-inchar,
asmallhole
throughts
enter
would
ause
he
actual
tress
the
dge
f
he
ole
to
ebout
hree
imes
hat
away
rom
the
ole.
S P/A
Figure2.3.
Bar
withCentrally
DrilledHole
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TIPS
O N
F A T I G U E
Figure.4 PhotoelasticModels,howing
Stress
atEdgeof
Hole
and
at
V
Notch
The
tress
distributions
around
twodifferentconcentrationsarehown
in
Figure
.4.
The
eft
picturehows
thetressdistribution
at
he
edgeof
anunfilled
hole
and
he
ightpicture
shows
what
happens
arounda
V-notch.
Here,an
experimental
tressanalysis
technique,
alledphotoelasticity
,
s
usedovisuallydemonstrateocations
ofhighly
stressed
areas.
This
ech-
nique
mploys
polarized
ight
and
clear
plasticmodelsnwhich
stressed
areasbecomeopaque.
Thissanes-
pecially
valuable
ool
for
demonstrat-
ing
relativemerits
of
design.
Amount
ofstresssdirectly
related
o
he
number
of
opaque
ines
and
concentra-
tions
proportionalo
he
inepacing.
Asnjudging
he
teepnessof
the
er-
rain
bythecontourines
of
aopo-
graphicalmap,photoelastic
patterns
tell
he
teepness
of
stress.
2.7.2 WATCH
U T
FOR
OPEN
HOLES- Most
structures
have
holes.
Open
holes
aresually
worse
hanrivet-filledholes. Stress
at
an
open
hole
s
hree
imes
hat
away
from
he
hole.
Forhiseason,
open
holes
houldbe
avoided
nregions
of
highstress. Where locationholes
are
an
absolute
necessity, plug
hem
withrivets
if
possible. Neverplug
holes
with
weLd,
as
hisreates
high
residual
locked p)
ensile
tresses.
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TIPS
O NF A T I G U E
VinylPlastic
i
Polaroids
=ZZ
S S
Figure2.5.
Photoelastic
Model,howingDistributionof
Load
inFasteners
ofa
ClevisJoint
2.7.3
RIVETSANDB O L T S
-Riveted
or
bolted
jointsalwaysconstituteproblems.
Onereasonis
hatthe
oadintroducedby
the
rivet
or
bolt
increases
he
stress
at
pointsofconcentration.
The
second
reason
shatitis
virtually
impossible
o
distribute
the
loadevenlybetweenrivetsorbolts,irre-
spective
ofworkmanship.
This
s
because
the
secondand
successive
rows
of
rivets
cannot
carry
their
share
of
load
without
some
stretchinthe
splicing
material
betweenhe
firsttwo
rows. Infact,
he
stretchinthe
splicing
materialshould
be
greater
han
that
of
thematerial
being
spliced
at
this
point.
Note
in
Figure
2.5
thatthe
fastener
nearestthe
oad
hashe
highest
stress.
Asolutionto
this
problem
wouldbeo
make
hisfastener
incapable
of
carrying
so
muchof
the
load. Unfortunate-
ly,educingthesizeofthefasteneris
not
always
a
solution.
Infact
it
s
an
nvitation
to
rouble
with
the
fasteners
hemselves.
Perhaps
awiserchoicewouldbeoremove
some
of
the
splice
materialso
that
it
would
notbeable
ooverloadthe
first
fastener
or
fasteners.
The
hinner
splice
material
stretches,husallowingsome
ofthe
oadto
be
carried
by
the
second
ow
of
fasteners.
This
s
llustrated
in
Figures2.6
and
2.7,
where
edge
views
of
photoelasticmodels
are
shown. The
modelin
Figure
2.6
s
very
similar
totheclevis
jointshownn
Figure
2.5.
Being
cutfromonepieceof
material,he
model
inFigure2.6clearly
showshat
a
good
load
distribution
cannot
beattainedbyprovidingabetterfit.
Figure
2.6.
Photoelastic
ModelofClevis
Joint,
howing
Edge
Viewof
Load
DistributionBetweenFasteners
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TIPSO N
F A T I G U E
Figure
2.9. ScarfedLap
Joint
A
properadiussespecially
criticaln
machined
parts
where
nly
alightchangeould
determine
whetherhe
part
were
atisfactory
ornot.
Most
mportant,
owever,
isomake
ertainhatanadditional
notchsot
created yhemachined
radii
ailingo
meet
he
lat
surface
smoothlyashownnhe
photoelastic
modeln
Figure
2.11.
Further
examplesf
theffectsf
radii
on
fatiguearegiveninChapter5.
2.7.7
F R E T T I N G T h e
erosion
oftw o
surfaces
rubbing
against
each
Other
is
known
as"fretting." Thenotcheffect
of
the
pittedsurfaces
tends
to
exaggerate
the
effect
ofothernotchessothatthecombina-
tion
is
a
superimposed
stress
concentra-
tion.
Frettingiseasilyrecognizedby
Figure
.0
PhotoelasticModeLsShowing
Effect
of
Fillet
Radii
onStress
Figure
2.
1
PhotoelasticViewof
Machined
RadiiNotMeeting
Stresss
ndicated
by
Number
ofDark
Lines
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TIPSO N
F A T I G U E
powderthat
siftsout
from
between
the
surfaces.
Since
fretting
s
caused
by
rub-
bing,
anything
that
will
reduce
the
amount
of
rubbingwillreduce
fretting. Equalizing
theamountofstretchbetweenmaterialbe-
ing
spliced
andsplicematerial,as
llus-
trated
n
thephotoelasticmodels
of
scarfed
joints,
willhelp.
Adhesive
bondingsee
Section3.9)
alsohelps.
Lubricants
can
be
helpful
n
special
cases.
Consult
the
spe-
cialist
on
this.
2.8
IDENTIFYING
FATIGUE
FAILURE
Thoseofuswhohavehadthedubious
pleasure
oflookingatthe
brokenendofa
drive
shaft
or
arear
automobile
axle,
cannever
forget
what
it
looked
like. W e
may
also
rememberhe
mechanic
saying
thattheshaft
was
oldandcrystallized,
and
that
waswhy
tfailed.
Thisexplanation,owever,
s
not
necessarily
accurate.
All
metal
s
crystalline.
However, because
fatigue
cracks
propagatethroughthe
crystals
nstead
ofaroundthemas
nthe
caseoftheone-timeoading,rstatic
failure
in
a
ductilematerial)
outlines
of
the
crystals
come
nto
clear
view.
Fatigue
cracks
propagate
at
various
rates,ependingonthematerial
and
loading.
The
stressconcentrationat
the
end
of
a
crack,
eing
extremely
high,
causesthe
material
tofatiguelocally
so
thatthe
crack
continuesuntil
enough
fresh
material
notyetfatigued)
s
engaged
to
resistloading
for
another
interval.
This
gives
rise
to
the
"beach"
markappearance
offatiguefailedpartsasshown
in
Figure
1.1.
Sometimes
called
"tide"marks,
they
are
usefulforlocatingorigins
of
fatigue
failure.
Thebeachmarks
eft
by
high
loading
are
usually
spaced
farther
apart
than
those
caused
by
low
oading.
In
eithercase,
heorigin
is
usuallyata
pointof
stress
concentration
or
nucleus,
andthe
beach
markspropagatencircular
patterns
with
thenucleus
as
he
center.
Usually,
hemarks
near
the
origin
are
obliteratedbyrubbingoffractured
sur-
facesagainst
each
other.
ThusnFigure
1.1,he
origins
wereprobablyatlower
cornersofthe
hole;
however,each
marksdo
notappearuntilsomedistance
away. After
fatigue
crackinghadpro-
gressed
to
the
last
beachmark
at
the
right,herewasnsufficientremaining
areatocarrytheload.
Static
failure
finally
resulted
as
ndicated
bythe
rough
surface
at
theright.
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TIPS
O NF A T I G U E
3
J O I N T S
A N D
J O I N I N G
3.1O O
M U C H
S T R E S S
INTH E
W R O N GP L A C E S
There
was
no
fatigue
problem
in
aircraft
during
the
era
when
they
were
made
of
wood.
Itwasonlywiththe
advent
of
all-
metalairplanes,
nd
inparticular
of
high-strength
metal
airplanes,
hatfatigue
became
a
problem.
W h y ? T oo
much
stress
in
the
wrong
places
Thereason
w e
haveoo
much
stress
in
hewrongplaces
s
hat
w e
have
hrown
awaythe
simple
approach
used
ingluing
wood,andinsteadw eresort
totheobvious
boiler
plateconstruction.
N o w ,
boiler
plateconstructionis
fine
when
used
on
boilers,ut
w e
don't
haveo
build
air-
planes
ike
hat.
W ooden
airplanes
were
built
with
cabinetmaker
techniques,
ndthecabinet-
maker
tried
tojoinhisstructureinsucha
manner
that
the
joint
was
notapparentto
theeye-nortohestress. W h e nthe
same
echnique
was
usedon
wooden
air-
planes,hestress
flowed
fromon epiece
to
another
as
f
they
were
one.
Maybe
it
was
uck,but
the
resultwas
acontinuity
of
stress
flow.
3.2H Y
H A V E
J O I N T S ?
Joining,
o
begin
with,
s
technique
used
only
when
thestructurecannot
bebuiltin
on e
piece. Ideally,
heload
is
evenly
distributed
throughout
the
structure
to
afford
acontinuity
ofstressflow. Accord-
ingly,
hemorenearly
the
joiningresembles
a
singlepiecenmisrespect,hebetter
the
joint.
The
cabinetmaker 'slong-scarf
joint
very
nearlysatisfied
thiscondition.
3.3
T Y P E SO F
J O I N T S
The
mostobvious
wayto
jointw osheets
of
material
togetherisoaptheedges
of
on epiece
over
the
other
and
fasten
them
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T I P SO NF A T I G U E
with
some
device. Historiansellus
hat
man's
firstattemptofthissortwaspro-
bablyfasteningtwopieces
of
animalskin
togetherwith
afishbone. The
result
would
be
known
today
as
a
ap
joint.
4^
LAP
JOINT
DOUBLESHEARBUTTOINT
Anotherype
of
butt
jointholds
the
two
pieces
of
materialwith
two
splice
plates. This
s
calleda
doubleshearbutt
joint. Being
symmetrical,
t
has
afatigue
strength
superior
to
hat
of
either
the
ap
joint
or
singleshear
buttjoint. Thedouble
shearbutt
joint
ispreferable
wherever
cost
andaerodynamics
permit.
A
neaterway
isobutt
the
wo
sheetsedge-to-edgeandfasteneachtoa
thirdsheet(spliceplate)underneath.
This
gives
a
smooth
surfaceon
one
side
that
isnot
only
pleasing,utalso
s
aero-
dynamically
superior
ifthesheet
happens
to
be
he
outsideskin
of
an
airplane.
The
buttjoint
witha
singlesplice
plate
ordou-
bler
sknown
asa
single
shearbuttjoint.
* = f * = *
SINGLE
SHEAR
BUTT
JOINT
LUG
A
fourth
type
ofjoint
is
he
simple
lug.
It
usually
consists
ofa
clevis
anda
singlefitting
that
is
pinnedbetween
the
clevisby
a
singleboltor
other
fastener.
Thistype
s
generally
used
for
moving
partswhere
bushings
orbearings
are
used
for
esseningfriction.
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TIPSO N
F A T I G U E
Manyvariations
of
th eabove
jointscould
bementioned;
however,
heir
problemsandsolutionsaresimilar.
Several
exceptions,
such
as
hooks
and
piano
hinges,
needaspecialist'sattention.
3.4H A T ' S
TH E
M A T T E R
W I T H
J O I N T S ?
C L A M P S ?-N otso
bad
ifbending
is
away
from
rivets
3.4.1
LAP
J O I N T S
While
thelap
jointisthesimplest
of
all
joints,tsmain
problem
ismat,
when
thetw o
sheetsof
materialarejoined,heytendto
align
themselves
witheach
other. This
causes
the
sheettobe
bent
at
the
first
fastener,
which
is
alreadysuffering
from
too
much
load
see
Figure2.8). Thisoffset
inalign-
ment
is
commonly
called
eccentricity.
The
logical
solutionwouldbeto
letthesheet
bend,
as
ongas
t
didn't
bendright
where
the
oad
was
greatest.
Ideally,
oucould
clampthesheetto
make
t
bend
at
some
other
point;
his
would
separatethebending
stress
from
the
load-carrying
shear
stress.
A
trick
suchasthis
s
frequently
called
"confus-
in g
the
stress"
or
"stress
confuser."
S ee
Chapter
6
or
other
stressconfusers.
LAP
JOINT
-heet
bendsight
where
t
hurtsmost
T H F S SO N F U S E R
STRESS
CONFUSER
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TIPS
O N
F A T I G U E
While
usinga
C-clamp
wouldbe
highly
mpractical
nanairplane,he
same
results
can
beobtained
by
driving
extra
rivets
hrough
the
edges
of
doublers.
In
fatiguetests,
an
ordinaryap
jointwithtwocountersunkrivetsasnA
same
problem
as
he
lapjoint.
One
ofits
advantages
s
thatthedoublercanbe
made
thicker
than
the
material
being
spliced.
Thisreducesthe
effects
of
bending,
ut
it
creates
an
additional
problem:
he
rivet
nearest
the
doubler'
s
edge
now
carries
most
of
the
load
just
as
n
the
caseofthe
clevis
joint
shown
in
Figure
2.5.
ZBZB2ZBB02.
A=55,000Cycles
>>->>^>.>.>.-*VVVN--V^
1WMM
s s
B= 248,
0 0 0
Cycles
SameA s
A "
ExceptExtra
Rivets
Are
Driven
Through
Edges
O f
Splice
Plates.
To
visualize
this,
stakehe
end
of
awiderubberband
to
aboardwith
three
thumb
tacksandpull.
Note
he
amount
of
deformationrequiredat
the
No.
fastener
before
heNo.
starts
o
carry
he
oad.
This
would
indicate
that
some
provision
must
be
madeforthespliceplatetostretch
failedafter55,000cycles
of
repeated
load-
ing.
Bydrivingextra
rivets
throughdou-
bler
edgesas
n
B,
he
atigue
ife
was
raisedto
248,000cycles.
The
secondjoint
lasted
longer
because
he
edge-driven
rivet
couldtake
noload
otherhanthat
caused
by
sheetbending,
hus
passing
the
shear
oad
ontohenextrivet,which
was
husre-
lieved
of
the
bending
load.
ThumbTacks
Rubber
Wood
3.4.2
SINGLE
SHEAR
BUTTJOINTS
The
single
shearbutt
joint
is
reallytwo
lapjoints
acing
each
other,o
ithas
the
EXPERIMENT--
howing
oad
divisionbetweenfasteners
i:
n
HEAVY
SPLICEPLATE-
Reduces
bending,
but
makes
ivet
carryoo
much
load
ifthe
No.
and
No.
fasteners
are
o
carry
heirfair
share
ofthe
load.
Somedegreeof
deformation
can
be
achieved
by
hinning
thedoubler
materialbetweenthe
firsttworowsof
fasteners
so
the
secondrowcan
carry
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TIPS
O NF A T I G U E
Very
thin
at
fastener
No.
1
Useexra
fasteners
if
necessary
to
carrystatic load
Bending
stress
is
relieved
at firstfastener
and
alsosome
of the
load
TAPERED
SPLICE
PLATE
some
of
the
oad.
Since
doubler
material
muststretch
in
order
to
dothis,
he
thicknessat
the
first
fastener
should
be
less
than
half
that
ofthe
material
being
spliced.
Thevalue
ofone-half
isarbi-
trary;
hepointbeing
that,
with
this
thickness,heN o.
fastener
won't
over-
load
the
spliced
material.
Highloads
would
cause
doubler
material
at
the
N o.
fastener
to
yield
inbearing--which
is
good,fthe
remaining
fastenerscan
carry
thedesignload.
Thus,
or
static
strength
it
might
beagoodpolicynotto
relyn
the
first
row
of
rivets
but,
nstead
to
provideextra
fasteners
forthe
job.
Theoretically,
he
doublershould
taperto
almost
nothing
so
that
theN o.
fastener
carries
n
infinitesimal
partof
the
load
something
on
the
order
of
the
cabinetmaker'sscarfed
wood
joint.
Gen-
erally,he
extramachining
isimpractical,
sometimes,
however,
heweight
saving
does
make
it
worthwhile.
Almostasgood
asthe
thick,
apered
spliceplateis
the
thin
auxiliary
doubler
next
tothe
material
being
spliced. The
auxiliary
doubler
should
belong
enoughto
engage
an
extra
row
of
rivets
outsidethemain
splice
area.
Here
again,here
isacompromise
between
the
practical
and
theoretical
optimum
thickness
ofauxiliary
doublers.
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T I P S
O NF A T I G U E
AUXILIARY
DOUBLER
TWO
AUXILIARY
DOUBLERS
For
splicesnaluminumalloy,
an
aluminum
alloy
auxiliary
doubler
about
one
thirdas
hick
as
he
splicedmaterial
is
about
right. Wherewo
auxiliary
doublers
are
used,
he
one
nearest
the
butt
shouldbeaboutonefifthas
hick
as
thematerial
spliced.
doubleshear
type
alsohashe
problem
of
load
distribution
between
fasteners.
(See
photoelastic
modelsn
Figures2.5and
2.7). Scarfing,
rproviding
auxiliary
doublersas
for
single
shearjoints,
will
improve
fatigueife.
Auxiliary
thin
doublerswhen
properly
usedwillincrease
heifetime
morehan
ten
imes.
3.4.3
DOUBLE
SHEAR
BUTTJOINTS
Doubleshear
butt
joints
are
superior
to
those
of
the
single
sheartype.
This
s
becausethesymmetry
of
thedouble
shear
typeeliminates
the
bendingeffects
found
inthe
single
shear
However,he
3.4.4
LUGS
The
lug
isa
simple
form
of
thedoubleshearjoint. Sincethejoint
has
but
one
fastener,
heproblem
ofload
distribution
between
fasteners
does
not
arise. Whetherornotthefastenerusu-
ally
abolt)its
tightly
hasmadeasub-
stantial
differenceintest
results.
A
oose
bolt
tends
tobendmore
andwill
some-
times
fail
in
the
middleof
the
tongue,
r
male
fitting.
Italsointroduces
an
ex-
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TIPSO N
F A T I G U E
W I D ET HINU G
-
asyon
bolt
but
bad
on
fatigue
tremelyhigh
bending
stressonthe
corners
ofthe
clevis,
or
femalefitting.
Thick
lugswithcloselyfittedboltsor
bushings
willhavetwicethefatigue
life
of
lugs
with
sloppy
fits. A
goodnterference
it
will
mprove
he
life
manyimes.
See
Chapter ormore
n
his.
Becausefhebendingeffect,
its
a
good
dea
not
o
tinton
bolt
size.
Indications
re
hat
t
would
be
helpful
ohavehebolt
even
wices
strong
ashe
ug.
This
eeps
rom
overloadingheornersfheug.
NARROWTHICKLUG--
etter
thanwide,
thinlug,
but
bends
bolt
too
much;
Also,
om won't have
o
worryabout
thebolt.
Shape
of
the
lug'scrosssection
is
very
important. Awide
thin
lug,
while
relieving
bolt
bending,
causes
the
stress
at
theedgeofthe
holetobemany
times
the
averagestress
away
from
the
hole.
(See
concentrationfactorsin
Appendix
B ).
O n
he
other
hand,
a
narrow,
thicklug
requires
a
ong
bolt,
which
bends
and
overloads
he
lugedges.
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TIPS
O N
F A T I G U E
Testshavehownthatthelug
with
cross
sectionswithratiosofA /Bbetween
1
and
3would
bebest
where
theboltshear-
ingstrengthisequaltothestrengthofthe
lug.
3
m
U
U
1,000,000
100,000
10,000
i
Calculated
if
Bolt
Didn't
Bend
Testfor
lughaving
static
strength
equal
to
that
of
bolt
Stronger
bolts
permit
using
larger
A /B
forbetterfatigue
performance
A /B
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TIPS
O N
F A T I G U E
3.5
F A S T E N E R
P A C I N G
Much
has
beenwritten
aboutspacing
fastenersna
joint,
mostof
itfroma
static
strength
point
of
view.
Accordingly,
handbooks
recommend
fastener
sizes
and
spacing
that
leave
as
much
of
the
area
as
possible
in
thesplicedmaterial.
For
static
strength,
he
net
cross
sectional
area
is
usually
noless
than75percent
of
theareaawayfrom
the
splice.
(Net
cross
section
is
the
area
remaining
after
removal
of
material
for
holes.) This
would
give
a
fastener
spacing
commonly
called
pitch)
ofaboutfour
times
the
fas-
tener
diameter.
To
further
enhance
the
static
strength,he
first
fastener
nearest
theloadissometimes
reduced
insize.
ThisRow
of
Fasteners
has5.7-Diameter
Spacing
o
Frequently,
astenersare
stag-
gered. Joints
of
this
type
have
never
proven
tobeanybetterthan
the
tandem
pattern. Staggering,however,sde-
sirableforfuel
sealing
orother
uses
where
joints
should
not
leak.
TheseRows
Have
4-Diameter
Spacing
T A N D E M
P A T T E R N
Good
StaticStrength
Joint
S T A G G E R E D
P A T T E R N
O Kfor
leak
prevention
730-7550-643
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T I P SO NF A T I G U E
Designpractices
used
for
optimum
tatic
strengths seldom
apply
o
fatigue. Again,inastener
spacing,
the
est
practice
or
tatic
s
trength
fall
sarhortfgood
fatigue
e
sign.
As
as
been
shown,
the
stress
at
the
dgef
he
irst
fastenerholeshe
mostc
rit
icalfor
fatigue.
Accordingly, the
best
fastener
pattern
would
e thatwhich
would
essen
he
tresshere.
Inerms
f
he
verage
tress
away
romhe
irst
owffasteners,
the
attern
an
be
ikened
o
eries
oflugs. Thiswouldndicate
hat
theptimum
pacing
or
fasteners
normaloheirection
f
oading
wouldebout
2.5
iameters.
In
terms
ftatic
trength,
this
would
amounto ointwhosetrengthwas
60
percentf
he
tructural
trength
7.0
U
^yH
,
'. w
-
* ~
v
.
'viv''c
^ -I
*
_**'^^^1
'.*
a-*?'-
*m
> . . -
^
I
..
*
'i
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T I PSO NF A T I G U E
f
:
,?:J'f?Pii
FORWARD
I
1
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TIPS
O N
F A T I G U E
5.5.3 RADIUS
AT
CHANGE
OF
SECTIONThere
always
has
o
be
a
radiusof
some
sort
at
any
change
of
section. Likewise,herehaso
be
a
radius
at
achange
of
direction. However,
you
don't
have
to
make
one
radius
righton
top
of
the
other. The
radius
for
change
n
section
should
havebeen
made
at
another
location.
Ifthis
werempossible,oth
radii
should
have
beenenlarged
to
permit
amoregentletransition.
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TIPS
O NF A T I G U E
J**'
-;f.
*
V
$>
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TIP NF A T I G U E
5.
.
ROUGH
SURFACEFINISH
A
finishsuch
as
thatillustratedherealso
constitutes
asuperimposition
of
stress.
Where
astressraiseralready
exists,
s
in
this
case,
tis
oolish
o
et
a
rough
surfaceike
this
get
by,
specially
with
toolmarksnormalo
the
directionof
loading.
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TIPSO NF A T I G U E
7 3 0 -7 5 5
0-64--G
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TIPS
O NF A T I G U E
5.
. ROSS GRAIN--heparts
shown
failedorwo
reasons,
rough
surfaces,
and
he
material's grain
structuresnormal
o
heirection
of
Loading.
Whilet
might
havebeen
possible
o
avert
failurebymachining
asmooth
surface,
the
wrongdirection
of
grainmakes
uch
a
olution
highLy
speculative.
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TIPSO NF A T I G U E
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TIPS
O NF A T I G U E
5.6
UXILIARY
ATTACHMENTS
The
part
shownwasfrom
a
anding
gearassembly.
Failure
occurredthrough
arivet
hole
used
forattaching
a
schafing
shield. Similarthingshappen
to
frames
having
attachment
screws
o
support
hy-
draulicines,upholstery,
or
whathave
you?
It
would
have
been
better
to
tiehe
part
onwith
rope
also
ry
adhesive
bond-
ing)han
totake
chanceson
fatiguing
as
shown.
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TIPS
O N
F A T I G U E
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TIPS
O N
F A T I G U E
5.7
H A R DLATING
Here
s
atypicalexample
of
wherea
part
waschromeplated
to
make
t
morewear
resistant. It
wasn't
more
atigue
resistant.
Thecracksn
the
plating
act
as
stress
raisershat
eventually
fail
the
part
the
plating
s
supposed
to
protect.
Shotpeeningprioro
plating
s
a
common
inhibitoroffatigue
cracking
n
chrome
plated
parts.
It
s
unwise
o
chrome
plate
paints
for
dimensional
buildup
orwearresistance
withoutthe
help
of
the
specialist.
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TIPS
O N
F A T I G U E
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TIPSO NF A T I G U E
5.
SHOPBLUNDERS
Whilemost
of
the
examples
previously
showncanbeblamed
directly
on
design,
a
number
could
equally
well
have
been
causedby
shop
blunders.
Thus,wehave
theholethatwas
not
tapped
all
theway
through
andthe
rough
surface
hatwas
not
smoothed.
Theollowingexamplescanbe
blamed
almostentirely
on
shop
practices.
5.8.1
NO
EDGE
DISTANCE The
engineering
drawing
maynot
have
speci-
fiedtheexactlocationsofholesfornut
plates;
however,tandardshoppractices
should
besuchthatthiswouldnever
happen. Asshown,
here
was
nsufficient
roomfor
nut
plates
to
be
spaced
in
sucha
manner
that
holes
wouldfall
between
nut
plates.
Also,
otethat
the
edge
surface
finishwasnothing
to
bragabout.
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TIPSO N
F A T I G U E
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TIPS
O NF A T I G U E
5.8.2
MISMATCH-- Evenxcellent
machinists
ftenmachine
urved
surfacehatdoesn'tmeettstraight
counterpart,
leaving
what
amounts
o
auperimposedtress
aiser.
While
it
s
otoad
where
hewourfaces
are
onvex,
the
oncavenes
sually
result
n
ailure
s
ndicated
nhe
illustration.
Many
rawingoom
manuals
specify
iemaximumllowablemis-
match. Ito
appens
not
n
hese
cases)
hatatigue
ailures
ave
resulted
wherehemismatch
was
withinolerance. Care
houlde
exercisednermittingmismatches
in
ritical
areas-evenwithin
specifiedolerances. Whilet
would
e
irtuallympossible
o
definehe.amountf
mismatchhat
can
e
ermitted
n
very
ase, a
rule
fhumb
so
sextreme
carewith
oncave
urfaces.
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TIPSO NF A T I G U E
l
%
^'M4a*i'
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TIPS
O NF A T I G U E
5.8.3
EXCESSIVECLAMPING-Thebolt
onhispartwastightenedwithouthaving
the
proper
spacer
bushing.
Fatiguefailure
finally
setn,
as
might
be
expected.
Make
sure
youhave
he
right
engthbushing
and
the
right
engthbolt
andTHINK
TWICE
BEFORE
TIGHTENING
see
4.4).
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TIPS
O NF A T I G U E
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T I PSO NF A T I G U E
i.9
O R T G T N A L
FITTINGS
A .
R O N G
G R A I N
DIRECTION
B.
POTFACES
C.
NADEQUATE
FILLET
R A D I U S
D.
HARP
CORNERS
E.
EATHERED
E D G E S
F.
O U G HSURFACE
G.
TS=240-250
ksl
MURPHY'S
L A W
Murphy'sLawstateshat
f
twere
possi-
ble
obotchupa
job,
omeonewillsurely
findawayodot. DON'T
UNDERESTI-
MATEGROUP
EFFORT
Asshownnthe
upper
eft
photograph,hecombinedefforts
of
engineering
and
shop
very
nearly
suc-
ceededndoingeverythingwrong.
Botchesn
this
one
partnclude
sharpedges,badspotfaces,mall
fillet
radii,
ough
surface,
cross
grain,
and
others.
In
addition,here
were
signsof
hydrogen
embrittlementdue
o
cyanide-
bath
cadmium
plating.
Hydrogen
embrit-
tlements
a
termused
for
owductility
causedbyabsorptionoftoomuchhydrogen
during
processing.
These
nice
big
words
REDESIGNED
FITTING
H.
NLARGED
FILLETR A D I U S
I.O U N D E D
CORNERS
J.
M O O T H
SURFACE
I N I S H
K .
O
SPOTFACES
L.TS=210-220
ksi
sound
authoritative
when
usedtoexplain
failures
for
which
no
realreason
other
thanpoordesignorworkmanship,
which
we
hateoadmit)s
apparent.
ecarburi-
zation
another
mouthfulhat
means
oss
of
carbon
due
opoor
processing)
was
also
apparent
toaminorextent.
Proper
processing
was
nsuffi-
cientto
bring
thepart
up
to
required
ife,
sot
was
necessary
to
perform
amajor
overhaul.
This
ncluded
providing
a
better
surfaceinish,
removingsharp
edges,
and
providing
more
generousfillet
radii.
The
reworked
part
sshownn
the
upper
right
photograph. Testsonsimilar
parts
re-
vealed
alifeofapproximately
four
times
thatsustainedbyoriginal
partswith
no
n-
crease
n
weight.
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TIPS
O N
F A T I G U E
MAKING
THE MOST
OF
A
BAD
ITUATION
6.
BAD
SITUATIONS
6.
. THECASE
OF
THE
FLAT
TIRE
-
Have
ou
ever
hadfLat tire
while
driving
along
the
countryside
and
discovered
that
yoursparewasalsoflat?
...
o
pumpor
patching
material
...
o
youfinally
decided
todrive
t
flat.
Thenhere
was
theokerwhopassedyou
and
yelled,"Don't
you
know
you
got
a
flat
tire?"
And
you
felt
likestickinga
bigsign
on
yourrearbumper
saying,
I
knowit's
flat o
what?"
Ifyouhavehad
such
an
experi-
ence,henyoucan
magine
what
it's
ike
o
have
a
service
failure
n
an
areawhere
there
justisn'tenoughroomor
replace-
ment
with
a
huskier
part.
Maybe
you
are
already
using
materialasstrongas
you
dare.
What
now?
Shop
isstill
turning
out
parts
like
hosethatbroke)bythe
barrel-
full,
nd
you're
faced
with
the
need
for
a
quickdecision.
Youhave
hreepossible
decisions: (1 )youcandonothingandhope
that
the
rest
of
the
parts
won'tbe
sobad;
(2)
you
can
trytofixtupand
hope
hat
theix
s
O K ;
or,
3)
you
can
fix
afew
samples
and
est
the
partso
see
f
the
fixsanygood.
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T I P S
O N
F A T I G U E
6.2
DECISIONS
Oar
decision
on
a
structural
fix
shouLd
be
basedon
he
facts
we've
earned
sofar. Thepurpose ofhishapter
isoarrange
hese
facts
n
sucha
manner
hat
our
decisionsan
be
easier. Whatare
hese
facts?
fatigue
ife.
In
Chapter
we
ound
whathappens
when
wevioLate
principles
ofgood
design
andfabrication.
As
in
vioLation
of
principles
f
good
health,
the
orrective
measures
may
be
low,
painfuL, and
bad
asting.
6.3
SIZINGP
THE
SITUATION
Some
ofhebasicprinciples
werepresentednChapter
Also
shownwere methods for reducing
stressat
filletsby
providing
amore
generous
adius.
The
ubject
of
joints
was
introduced
n
Chapter3. O f
partic-
ular importance
washe
act
hat
a
small change
n
basic design
could
resuLt
in a vast
improvement
in
As
farasatigue
soncerned,
remember thatstructure
will
neverfail
exceptata stress
concen-
tration.
Accordingly,
Let's
worry
about
stress
at
he
concentration
and
never
mind
aboutwhatappens
else-
where, at least
ot
or the
time
being.
This
implifies
ur
problem.
The
next
hing
sovisuaLize
what
can
bedoneo
his
particularstresso
make
he most
of
he
situation.
DECISIONS --ALWAYS DECISIONS
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TIPSO N
F A T I G U E
While
here
maybe
others these
cases
will
suffice
ostart. Supposea
part
were
oaded
sohat
stress
at
the
concen-
trationfluctuates
rom to
60,000psi
(R
0).
Such
a
part
could
be
expected
to
60,000
J__J
90,000Cycles,Stress
Range
0,000
psi, R 1
90,000
cyclescurvefor
R
-1)
and
maxi-
mumstress=30,000
psi,igure
2.2.
30,
000
Cycles,Stress
Range60,000
psi,
R
lastforabout30,000
cycles,
ccordingto
the
S-N
curve
for
R
0
shown
n
Figure2.2.
Wherehe
stress
range
s
definedas
he
difference
between
he
maximum
and
minimum
tress,
otalstressrange
would
be
60,000
psi.
The
ntroduction
of
a
compressive
stress
at
theconcentration
sounds
ikea
nice
rick
if
youcandot.
That
is,
he
compressivestressshould
be
apermanent
affair
ocked
up
so
it
can't
getaway
and
should
be
njustexactlytherightspot.
Such
stresses
arecommonly
known
as
residual
stresses. Residualstresses
can
either
ben
tension
or
compression.
Similarly,
f
the
stress
could
be
madeo
behave
ashoughit
werecycling
6.3.
MOVETHE
WHOLE
STRESSD O W N
If
it
were
possible
o
do
something
to
the
structure
ocally
so
that
the
stress
at
the
concentration
would
be
30,000
psi
n
com-
pression
-30,000psi)
when
the
part
was
unloaded,
heoriginal
loading
shouldcause
the
ocalizedstressofluctuatebetween
-30,000psi
and
+30,000psi 30,000
psi).
This
would
correspond
to
thesamestress
range
asbefore,uttheifenowwouldbe
200,000
Cycles,
Stress
Range60,000psi, 2
from
minus
40,000
psi
toplus
20,000psi,
a
lifetime
of
200,000
cycles
would
result
(curveforR
-2
and
maximum
stress
20,000
psi,
igure2.2).
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TIPS
O N
F A T I G U E
Whenaparthavingastress
raiser
soaded
in
tension
soaso
cause
hematerial
at
heconcentra-
tion
to
yield
locally,hepermanently
deformed
material
mustgo
nto
com-
pressionwhen
the
oads
emoved
and
the
partsprings
back.
Such
tems
as
hooks
end
themselvesothis
ype
of
correction.
Generally,
heamountof
overloads
critical
and
should
be
specified
bythe
pecialist.
Apractical
method
called
"shotpeening"s
used
tontroduce
residual
compressive
tress
for
a
longer
ife.
In
the
first
nstance,
a
compressive
ayer
at
he
notch,
amountingo0,000
psi,
willdo
he
trick.
Inthe
econd,
a
layer
of
40,000
psi
would
be
equired.
Both
are
easily
achieved,tbeingcommonpracticeo
introduce
esidualcompressive
stressesas
highaswohirds
of
the
material's
compressive
yield
strength.
Other
methods
ofintroducingprotec-
tive
compressive
tress
ayers
nclude
controlledmechanicalpeening,
vapor
blasting,urface
olling,
and
a
process
called
"coining."
Surfaceollings
especially
suitable
forcylindrical
ob-
jects
uch
as
bolts. Figure.1 shows
how
thread
rolling
was
used
to
m-
proveheatigueifeofbolts.
Fatigueife
ofa
part
canbe
improved
by
providing
a
better
finish.
This
sparticularly
true
when
the
original
partfailed
because
f
machine
mismatch.
(A
mismatchoccurs
where
thewo
machine
urfacesdonotmeet-
seeection5.8.Z).
200 ,0 00
180 , 000
(Bolts
having
hreads
rolledprioroheat
treatmenthaveabout
he
ame
fatigue
strengthasboltswith
machined
hreads)
10 , 000
100 , 000
1 , 000 , 000
10 , 000 , 000
Figure
.
Thread
Rolling
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T I P S
O N
F A T I G U E
Figure
Z.llshows
howstresses
canbe
verysevere
asa
esultof
this.
Simply
moothingoutheadius
would
be
aolutionouch
a
mismatch.
Other
asesmay
ot
e
o
imple.
We
mayinda
part
made
with
a sharp
notchohatbearingoulditlose
Let's
ake
he
ase
fa
ug.
Assume
hathemachiningdirection
were
normalohedirection
of
he
Load Aswehave
aLreadyseen,
this
would
constitute
a
uperimposed
stress
oncentratio