Design of Welded Joints

Design of welded joints problems

1. Design of Riveted and
Welded joints
Syllabus:
Riveted&Weldedjoints:BoilerJoints&
LozengeJoint,Designofrivetedjointsunder
eccentricloadingandeccentricallyloaded
weldedjoints
Unit-III

2. Thejointsusedinmechanicalassembliesare
classifiedintotwogroups-permanentandseparable.
Permanentjointsarethosejointswhichcannotbe
disassembledwithoutdamagingtheassembled
parts.Rivetedandweldedjointsarepermanentjoints.
Separablejointsarethosejointswhichpermit
disassemblyandreassemblywithoutdamagingthe
assembledparts.Boltedjoints,cotterjointsand
splinedconnectionsaretheexamplesofseparable
joints.

3. Weldingcanbedefinedasaprocessof
joiningmetallicpartsbyheatingtoa
suitabletemperaturewithorwithoutthe
applicationofpressure.Weldingisan
economicalandefficientmethodfor
obtainingapermanentjointofmetallic
parts.

4. Advantages of Welded joints over
Riveted Joints
Weldedjointsofferthefollowingadvantagescomparedwith
rivetedjoints:
(i)Rivetedjointsrequireadditionalcoverplates,gusset
plates,straps,clipanglesandalargenumberofrivets,
whichincreasetheweight.Sincetherearenosuch
additionalparts,weldedassemblyresultsinlightweight
construction.Weldedsteelstructuresarelighterthanthe
correspondingironcastingsby50%andsteelcastingsby
30%.
(ii)Duetotheeliminationofthesecomponents,thecostof
weldedassemblyislowerthanthatofrivetedjoints.

5. (iii)Thedesignofweldedassembliescanbeeasilyand
economicallymodifiedtomeetthechangingproduct
requirements.Alterationsandadditionscanbeeasilymade
intheexistingstructurebywelding.
(iv)Weldedassembliesaretightandleakproofas
comparedwithrivetedassemblies.
(v)Theproductiontimeislessforweldedassemblies.
(vi)Whentwopartsarejoinedbytherivetingmethod,holes
aredrilledinthepartstoaccommodatetherivets.The
holesreducethecross-sectionalareaofthemembersand
resultinstressconcentration.Thereisnosuchproblemin
weldedconnections.

6. (vii)Aweldedstructurehassmoothandpleasant
appearance.Theprojectionofrivetheadadverselyaffects
theappearanceoftherivetedstructure.
(viii)Thestrengthofweldedjointishigh.Veryoften,the
strengthoftheweldismorethanthestrengthoftheplates
thatarejoinedtogether.
(ix)Machinecomponentsofcertainshape,suchascircular
steelpipes,finddifficultyinriveting.However,theycanbe
easilywelded.

7. Disadvantages of Welded
joints over Riveted Joints
Weldedjointshavethefollowingdisadvantages:
(i)Ascomparedwithcastironstructures,thecapacityof
weldedstructuretodampvibrationsispoor.
(ii)Weldingresultsinathermaldistortionoftheparts,
therebyinducingresidualstresses.Inmanycases,stress-
relievingheattreatmentisrequiredtorelieveresidual
stresses.Rivetedorcaststructuresdonotrequiresuch
stressrelievingtreatment.

8. (iii)Thequalityandthestrengthoftheweldedjointdepend
upontheskillofthewelder.Itisdifficulttocontrolthe
qualitywhenanumberofweldersareinvolved.
(iv)Theinspectionoftheweldedjointismorespecialised
andcostlycomparedwiththeinspectionofrivetedorcast
structures.

9. BUTT JOINTS
Weldedjointsaredividedintotwogroups-butt
jointsandfilletjoints.
Abuttjointcanbedefinedasajointbetweentwo
componentslyingapproximatelyinthesame
plane.Abuttjointconnectstheendsofthetwo
plates.ThetypesofbuttjointsareillustratedinFig.
Theselectionofthetypesofbuttjointsdepends
upontheplatethicknessandthereliability.Some
guidelinesareasfollows:

11. FILLET/LAP JOINTS
Afilletjoint,alsocalledalapjoint,isajoint
betweentwooverlappingplatesorcomponents.A
filletweldconsistsofanapproximatelytriangular
cross-sectionjoiningtwosurfacesatrightangles
toeachother.Therearetwotypesoffilletjoints-
transverseandparallel

12. Types of Welded Joints

13. Afilletweldiscalledtransverse,ifthedirectionoftheweld
isperpendiculartothedirectionoftheforceactingonthe
joint.ItisshowninFig.(a)and(b).

14. Afilletweldiscalledparallelorlongitudinal,ifthe
directionofweldisparalleltothedirectionoftheforce
actingonthejoint.ItisshowninFig.(c).

15. Therearetwotypesofcross-sectionsforfilletweld-
normalandconvex,asshowninFig.Thenormalweld
consistsofanisoscelestriangle-atrianglehavingtwo
equalsides.ItisshowninFig.(a).
AconvexweldisshowninFig.(b).Aconvexweldrequires
morefillermaterialandmorelabour.Thereismorestress
concentrationinaconvexweldcomparedtoatriangular
weld.
Therefore,normalweldispreferredoverconvexweld.

17. Strength of Transverse Fillet Welded
Joints
Thetransversefilletweldsaredesignedfortensile
strength.Letusconsiderasingleanddoubletransverse
filletweldsasshowninFig.(a)and(b)respectively.

20. Ifσ
tistheallowabletensilestressfortheweldmetal,then
thetensilestrengthofthejointforsinglefilletweld,
P=Throatarea×Allowabletensilestress=0.707s×l×σ
t
andtensilestrengthofthejointfordoublefilletweld,
P=2×0.707s×l×σt=1.414s×l×σ
t
Note:Sincetheweldisweakerthantheplateduetoslag
andblowholes,thereforetheweldisgivenareinforcement
whichmaybetakenas10%oftheplatethickness.

21. Strength of Parallel Fillet Welded Joints

22. Theparallelfilletweldedjointsaredesignedforshear
strength.Consideradoubleparallelfilletweldedjointas
showninFig.(a).
Weknowthattheminimumareaofweldorthethroatarea,
A=0.707s×l
Ifistheallowableshearstressfortheweldmetal,then
theshearstrengthofthejointforsingleparallelfilletweld,
P=Throat area ×Allowable shear stress
= 0.707 s ×l ×
andshearstrengthofthejointfordoubleparallelfilletweld,
P = 2 ×0.707 ×s ×l ×τ = 1.414 s ×l ×τ

23. Ifthereisacombinationofsingletransverseanddouble
parallelfilletweldsasshowninFig.(b),thenthestrengthof
thejointisgivenbythesumofstrengthsofsingle
transverseanddoubleparallelfilletwelds.
Mathematically,
P = 0.707s ×l
1×σ
t + 1.414 s ×l
2×τ
wherel
1isnormallythewidthoftheplate.
Inordertoallowforstartingandstoppingofthebead,12.5
mmshouldbeaddedtothelengthofeachweldobtained
bytheaboveexpression.
Forreinforcedfilletwelds,thethroatdimensionmaybe
takenas0.85t.

24. Twoplatesarejoinedtogtherbymeansoffillet
weldsasshowninfig.Thelegdimensionsofthe
weldsis10mmandpermissibleshearstressat
thethroatcross-sectionis75Mpa.Determine
thelengthofeachweld,if15mmweldlengthis
requiredforstartingandstoppingoftheweld
run.

27. Aplate75mmwideand12.5mmthickisjoined
withanotherplatebyasingletransversefillet
weldandadoubleparallelfilletweld.Tensile
stress=70N/mm
2
and shear stress=
56N/mm
2
.Findthelengthofeachparallelfillet
weld.

28. Aplate100mmwideand10mmthickistobeweldedto
anotherplatebymeansofdoubleparallelfillets.Theplates
aresubjectedtoastaticloadof80kN.Findthelengthof
weldifthepermissibleshearstressinthewelddoesnot
exceed55MPa.

29. Asteelplate,80mmwideand10mmthickisjoinedtoanother
steelplatebymeansofasingletransverseanddoubleparallel
filletweds,asshowninfig.Thestrengthoftheweldedjoint
shouldbeequaltothestrengthoftheplatestobejoined.The
permissibletensilestressandshearstressfortheweldmaterial
andtheplatesare100and70Mpa.Findthelengthofeach
parallelfilletweld.Assumethatthetensileforcepassesthrough
centreofgravityofthesewelds.