Fos Presentation Final

8/13/2019 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*



+,-% .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



+,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



/%.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



+!.= 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



->>+-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



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



/%.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



/%.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



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



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



3=reater FS

>ess FS



=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



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



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



=raphical representation ofdetermination of FS



/>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



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



//>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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