Tips On Fatigue

8/11/2019 Tips On Fatigue

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N A V W E P S00-25-559

58 *

TIPS

O N

FATIG U E

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2/123

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


3/123

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


4/123

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


5/123

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


6/123

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


7/123

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8/123

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.

1-1


9/123

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.

1-2


10/123

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


11/123

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.

1-4


12/123

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

.

2-1


13/123

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

2-2


14/123

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

2-3


15/123

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

2-4


16/123

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

2-5


17/123

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.

2-6


18/123

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

2-7


19/123

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.

2-8


20/123

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

2-9


21/123


22/123

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

2-11


23/123

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.

2-12


24/123

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

3-1


25/123

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.

3-2


26/123

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

3-3


27/123

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

3-4


28/123

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.

3-5


29/123

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-

3-6


30/123

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.

3-7


31/123

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

3-8


32/123

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

3-9


33/123

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

5-31


75/123


76/123


f

:

,?:J'f?Pii

FORWARD

I

1

5-33


77/123

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.

5-34


78/123

TIPS

O NF A T I G U E

J**'

-;f.

*

V

$>

5-35


79/123

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.

5-36


80/123

TIPSO NF A T I G U E

7 3 0 -7 5 5

0-64--G

5-37


81/123

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.

5-38


82/123


5-39


83/123

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.

5-40


84/123

TIPS

O N

F A T I G U E

5-41


85/123

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.

5-42


86/123

TIPS

O N

F A T I G U E

5-43


87/123


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.

5-44


88/123

TIPSO N

F A T I G U E

5-45


89/123

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.

5-46


90/123


l

%

^'M4a*i'

5-47


91/123

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).

5-4*


92/123

TIPS

O NF A T I G U E

5-49


93/123


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.

5-50


94/123

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.

6-1


95/123

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

6-2


96/123


97/123

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).

6-4


98/123

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

6-5


99/123

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