fos presentation final
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Factor of Safety of EngineeringMaterials
Prepared by:Sanghamitra Das (MEB11061
Bedanta !ashyap (MEB1106"
S#ra$ %i&ari (MEB1106'
ripen !alita (MEB1106)Madh#r$ya D#tta (MEB1106*
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+,-% .S F-/% F S-FE%233 .t is a term describing the str#ct#ral
capacity of a system beyond the e5pected
loads or act#al loads .t is a meas#re ofho& strong the system is than it #s#allyneeds to be for an intended load
.t ens#res s#fficient reser7e strength incase of an accident
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+,2 FS33-ll the factors in7ol7ed in design cannot be
estimated acc#rately Some of these factors are:
1 8ncertainty in the magnit#de of e5ternal forceacting on the component
" 9ariations of yield strength or #ltimate tensilestrength of materials
' 9ariations in the dimensions of the componentd#e to imperfect &ormanship
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/%.8ES; %he FS ens#res against these #ncertainties
and #nno&n conditions
.f the FS is large< the probability of fail#re isred#ced
,o&e7er< the man#fact#ring cost of thecomponent increases
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+!.= FM8>-: %he factor of safety is defined as:
FoS= Failure Stress/Allowable Stress
or
FoS= Failure Load/Working Load
Example .f a component needs to &ithstand a load
of 1*0 < and a FS of ' is selected< then it is
designed &ith strength to s#pport )*0
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->>+-B>E S%ESS %he allo&able stress is the stress 7al#e< &hich is #sed indesign to determine the dimensions of the component .tis considered as a stress< &hich the designer e5pects &illnot be e5ceeded #nder normal operating conditions
For d#ctile materials< the allo&able stress ? is gi7en bythe relationship:
? @S ytA FS
For brittle materials< the relationship is: ? @S #tA FS
&here S yt and S #tare the yield strength and #ltimatestrength of the material respecti7ely
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F-/%S +,./, FS DEPEDSEffect of failure:
%he FS is high for applications &here fail#re may res#lt in ma$orloss
Type of load:
static load lo& FS
impact load high FS
Degree of accuracy in force analysis:
- lo& FS is selected &hen the system is s#b$ected to forces &hichare precisely determined n the contrary< a high FS is taen &henthe magnit#de or direction of force applied is #ncertain and#npredictable
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/%.8ES;aterial of co!ponent:
brittle materials- high FS ( based on #ltimate tensilestrength
Ductile materialslo& FS ( based on yield strength
"eliability of co!ponent:
%he FS increases &ith increasing reliability s#ch as inapplications lie contin#o#s process eC#ipment< po&erstations etc
#ost of co!ponent:
-s the factor of safety increases< dimensions of thecomponent< material reC#irement and cost increase %he FSis lo& for cheap machine parts
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/%.8ES;Testing of !ac$ine ele!ent:
.f a machine component can be tested #nder act#alconditions of operation< a lo& FS is #s#ally selected or elsea high FS is chosen if there is de7iation bet&een the test
conditions and act#al ser7ice conditions
Ser%ice conditions:
.f a machine component operates in ad7erseatmospheric conditions s#ch as high temperat#re or
corrosi7e en7ironment the SF is ept high
&uality of !anufacture:
- lo& FS is selected &hen the C#ality of man#fact#re ishigh /on7ersely< a greater 7al#e of FS is reC#ired to
compensate for poor man#fact#ring C#ality
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DES.= SPE/.F./-%. %he factor of safety is often specified in a design code or
standard< s#ch as:
-merican .nstit#te of Steel /onstr#ction (-.S/ steelb#ildings bridges
-merican Society of Mechanical Engineers (-SME press#re 7essels < boilers< shafts
-merican /oncrete .nstit#te (-/.
ational Forest Prod#cts -ssociation (FP- &ood
str#ct#res
-l#mini#m -ssociation (-- al#mini#m b#ildings bridges
Society of a#tomoti7e engineer(S-Ea#tomobile<
aerospace
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DE%EM.-%. F FS:Factors to be considered in determining FOS:
is of accidental o7erloading of str#ct#re abo7e act#aldesign tolerances
%ype of load (static or dynamic
ate of load applications (intermittent or repeated
>oad sie
Possibility of str#ct#ral fatig#e fail#re
9ariability in C#ality of &ormanship
9ariation in material properties
8ncertainty
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3=reater FS
>ess FS
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=8.DE>.ES F SE>E/%. F FS F
D.FFEE% M-%E.->S:CAST IRON S!"ECT TO STATIC #OADIN$:
%he indicator of fail#re is #ltimate tensile orcompressi7e strength
Fail#re is e5pected to occ#r &hen the tensile orcompressi7e stress reaches the #ltimate tensile orcompressi7e strength at any location
Material is not as homogeneo#s as steel and oftencarries resid#al stresses from the casting process
- 7al#e of ' to * is recommended for factor of safety
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D8/%.>E M-%E.->S S8BGE/% % S%-%./ >-DS%EE>
%he indicator of fail#re is yield strength
Fail#re is e5pected to occ#r &hen the
tensile or compressi7e stress reaches thetensile or compressi7e yield strength atany location
- 7al#e of 1* to " is recommended forfactor of safety to be applied on the yieldstrength in tension or compression
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D8/%.>E M-%E.->S S8BGE/% % 9-.-B>E >-DS%EE> %he indicator of fail#re is end#rance or fatig#e strength
Fatig#e fail#re depends on both the magnit#de of stress and the n#mber ofloading cycles
Fatig#e strength also 7aries &ith material properties as &ell as other factors
that are #niC#e to the part s#ch as: 2ield strength
Stress concentration
S#rface finish
Sie
%emperat#re of part
Fatig#e fail#re is s#dden and total< in a similar manner to the static fail#re ofbrittle materials
- 7al#e of 1' to 1* is recommended for factor of safety to be applied on thefatig#e strength
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=raphical representation ofdetermination of FS
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/>8M F-.>8E B2 B8/!>.=
Members loaded in compression are liely to fail byb#cling
B#cling is ca#sed by elastic instability &hich res#lts inlarge deflection of the compression member
B#cling load depends on/ompressi7e load
2ield strength of the material
Mod#l#s of elasticity of the material
End conditions for the col#mn< s#ch as fi5edAfi5ed<pi7otedApi7oted< freeAfi5ed< pi7otedAfi5ed
- factor of safety of ' to 6 is recommended to be appliedon the b#cling load
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P-%S S8BGE/% % /%-/% S%ESSES
Parts s#ch as cams< gear teet%< bearings< rail &%eel on rail<are s#b$ect to contact stresses bet&een the s#rfaces &hichres#lt in local fail#re of the s#rface by pitting
%his is local fail#re ca#sed by contact stress reachingendurance strengt% of the s#rface material at the point ofcontact
S#ch local fail#re does not render the entire machine elemento#t of operation Pitting is therefore a form of &ear since themachine element contin#es to operate
- factor of safety of 1H to "* is recommended to be applied onthe s#rface end#rance strength
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//>8S..t is common practice to sie the machineelements< so that the ma5im#m design stress isbelo& the 8%S (8ltimate %ensile Stress or yield stress
by an appropriate factor the Factor of Safety< basedon 8%S(8ltimate %ensile Stress or 2ield Strength %he factor of safety also no&n as Safety Factor< is#sed to pro7ide a design margin o7er the theoreticaldesign capacity to allo& for #ncertainty in the design
process
%here are other factors s#ch as Margin of Safetyand eser7e Factor &hich are also considered in thedesign process
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