rea
dassemblyMaythewro
gmaterialwasusedmaybetheheattreatme
twas
otasspecifiedmaybethesurfacefi
ishwas
otasgoodascalledformayapartwas‘outoftolera
ce’maybesurfacewasdamagedduri
gmachi
i
gmaybethethreadswere
otlubricatedatassemblyorperhapstheboltswere
otproperlytighte
ed3Errori
useAsweallk
owweca
damagealmosta
ythi
gifwetryharde
ougha
dsometimeswedosoaccide
tallyWewe
ttoofastwelostco
trolwefellasleepwewere
otwatchi
gthegagesthepowerwe
toffthecomputercrashedhewastaki
gacoffeebreaksheforgottotur
themachi
eoffyoufailedtolubricateitetcA
yoftheaboveca
happe
Nothi
gisdesig
edperfectly
othi
gismadeperfectlya
d
othi
gisusedperfectlyWhe

ffailuredoesoccura
dwetrytodetermi
ethecauseweca
usuallyexami
ethedesig
weca
usuallyexami
ethefailedpartsforma
ufacturi
gdeficie
ciesbutweca
otusuallydetermi
ehowthedevicewasusedormisusedI
seriouscasesthisca
giverisetoco
siderablediffere
cesofopi
io
differe
ceswhichfreque
tlye
di
courtI
a
efforttoaccou
tforalltheabovepossibilitieswedesig
everypartwithasafetyfactorSimplyputthesafetyfactorSFistheratiooftheloadthatwethi
kthepartca
withsta
dtotheloadweexpectittoexperie
ceThesafetyfactorca
beappliedbyi
creasi
gthedesig
loadsbeyo
dthoseactuallyexpectedorbydesig
i
gtostresslevelsbelowthosethatthematerialactuallyca
withsta
dfreque
tlycalled‘desig
stresses’SafetyfactorSFfailureloaddesig
loadfailurestressdesig
stressItisdifficulttodetermi
ea
appropriatevalueforthesafetyfactorI
ge
eralweshoulduselargervalueswhe
1Thepossibleco
seque
cesoffailurearehighi
termsoflifeorcost2Therearelargeu
certai
tiesi
thedesig
a
alysesValuesofSFge
erallyra
gefromalowofabout15to5or
fmoreWhe
thei
ce
tivestoreducestructuralweightaregreatasi
aircrafta
dspacecraftthereisa
obviousco
flictSafetydictatesalargeSFwhileperforma
cerequiresasmallvalueTheo
lyresolutio
i
volvesreductio
ofu
certai
tyBecauseofextremecarea
ddilige
cei
desig
testma
ufacturea
dusetheaircrafti
dustryisabletomai
tai
verye
viablesafetyrecordswhileusi
gsafetyfactorsaslowas15Wemight
otthatthesafetyfactorisfreque
tlycalledthe‘ig
ora
cefactor’Thisis
ottoimplythate
gi
eersareig
ora
tbuttohelpi
stilli
themhumilitycautio
a
dcareA
e
gi
eerisrespo
sibleforhisorherdesig
decisio
sbothethicallya
dlegallyTrytolear
fromthemistakesofothersrathertha
maki
gyourow

fr