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Mat Chapter 30
  WELDS - STATIC AND FATIGUE STRENGTH - I  WELDS – STATIC AND FATIGUE STRENGTH – I 1.0INTRODUCTION The need for connections and the importance of connection desins in stee! str ct resha#e $een co#ered in an ear!ier chapter% It has $een pointed o t that stee! sections are!inear e!ements prod ced in certain con#enient !enths d e to constraints onman fact rin and transportation% Therefore connections are necessar& to pro#idecontin it&' (here re) ired' as (e!! as to create three-dimensiona! stee! str ct res% *ne of the most efficient and possi$!& direct (a&s of pro#idin connections in stee! str ct res is $& (a& of welding  %We!din is the process of +oinin t(o pieces of meta! $& creatin a stron meta!! rica! $ond $et(een them $& heatin or press re or $oth% It is distin ished from other forms of mechanica! connections' s ch as ri#etin or $o!tin' (hich are formed $& friction or mechanica! inter!oc,in% It is one of the o!dest and re!ia$!e methods of +ointin%We!din (as ) ite an art pre#a!ent in ancient Greece to ma,e $race!ets% It (as pro$a$!& aforin process' (here meta!s (ere heated and hammered toether% odern (e!din has $een in e.istence since Wor!d War I% It (as main!& sed for repairin damaed ships%After /0/0' the se of (e!din as a constr ction and fa$rication method $ean tode#e!op% Since then man& impro#ements and de#e!opments ha#e ta,en p!ace% Toda& thereare o#er 12 different (e!din processes' (hich can $e sed to +oin #ario s meta!s andtheir a!!o&s% 2.0ADVANTAGES OF WELDING We!din offers man& ad#antaes o#er $o!tin and ri#etin% Some of the ad#antaes are!isted in the fo!!o(in% ã We!din ena$!es direct transfer of stress $et(een mem$ers% Hence' the (eiht of the +oint is minim m% 3esides efficienc&' desin detai!s are #er& simp!e% Less fa$ricationcost compared to other methods d e to hand!in of fe(er parts and e!imination of operations !i,e dri!!in' p nchin etc% The most stri,in ad#antae of (e!dedstr ct res is in the area of econom&% We!ded str ct res a!!o( the e!imination of a!are percentae of the  sset and sp!ice p!ates necessar& for ri#eted or $o!tedstr ct res% Time is sa#ed in detai!in' fa$rication and fie!d erection% In some $ridetr sses it ma& $e possi$!e to sa#e p to /14 of the stee! (eiht $& resortin to(e!din% We!din a!so re) ires considera$!& !ess !a$o r for tin the (or,% ã We!din offers air tiht and (ater tiht +oinin of p!ates and hence idea! for oi!storae tan,s' ships etc%5 Cop&riht reser#ed Version II30 - 1 30  WELDS - STATIC AND FATIGUE STRENGTH - I ã We!ded str ct res s a!!& ha#e a neat appearance as aainst the c! ttered s rface of  $o!ted or ri#eted connections% Fi% / sho(s a comparison of ri#eted p!ate irder and a(e!ded p!ate irder% F rther' (e!ded connections offer the desiner more freedom for inno#ation in his desin concept% It ena$!es him to se an& cross section and the $estconfi ration to transmit forces from one mem$er to another% ã The rane of app!ication of (e!din is #er& (ide% For e.amp!e' connection of a stee! pipe co! mn to other mem$ers can $e made #er& easi!& $& (e!din (hereas it is#irt a!!& impossi$!e $& $o!tin or ri#etin% We!din is practica$!e e#en for comp!icated shapes of +oints% ã There is no need for ho!es in mem$ers connected $& (e!din e.cept possi$!& for erection p rposes% This has direct inf! ence in the case of tension mem$ers as the pro$!em of determinin the minim m net section is e!iminated% This a!so res !ts in amem$er (ith a sma!!er cross section% ã We!ded str ct res are more riid compared to str ct res (ith ri#eted and $o!tedconnections% The riidit& of (e!ded str ct res is d e to the direct connection of mem$ers $& (e!din% In $o!ted or ri#eted str ct res' the connection is esta$!ishedthro h an!es or p!ates' (hich def!ect nder !oads' ma,in the str ct re f!e.i$!e% ã It is easier to ma,e desin chanes and to correct mista,es d rin erection' if (e!dinis sed% It is a!so a simp!e proced re to strenthen the e.istin str ct res (ith(e!din% Version II30 - 2 (a) Bolted girder section(a) Welded girder section Fig.1 Comparison of appearance of riveted and welded plate girders  WELDS - STATIC AND FATIGUE STRENGTH - I ã A tr !& contin o s str ct re is formed $& the process of f sin the mem$ers toether%This i#es the appearance of a one-piece constr ction% Genera!!& (e!ded +oints are asstron or stroner than the $ase meta!' there$& p!acin no restriction on the +oints%D e to this contin it& ad#antae' a #er& !are n m$er of stee! frames ha#e $eenconstr cted a!! o#er the (or!d% ã Stress concentration effect is considera$!& !ess in a (e!ded connection% Some of the!esser important ad#antaes of the (e!din processes are6 re!ati#e si!ence of the process of (e!din and fe(er safet& preca tions%Some of the disad#antaes of (e!din are6 ã We!din process re) ires hih!& s,i!!ed manpo(er  ã E.perienced manpo(er is needed for inspection of (e!ded connections% A!so' non-destr cti#e e#a! ation ma& ha#e to $e carried o t to detect defects in (e!ds ã We!ded +oints are hih!& prone to crac,in nder fati e !oadin ã Cost!& e) ipment is essentia! to ma,e (e!ded connections ã 7roper (e!din can not $e done in the fie!d en#ironment ã Lare resid a! stresses and distortion are de#e!oped in (e!ded connectionsIn the ear!ier da&s' com$ination of $o!tin' ri#etin and (e!din (as not practiced%Str ct res (ere comp!ete!& (e!ded' $o!ted or ri#eted% 7resent!& $oth are sed in astr ct re e.cept that $oth connection techni) es are not sed in one and the same +oint%The present trend is to se (e!din for (or,shop connections or sp!ices' and hihstrenth $o!ts for fie!d +oints% 3.0 FUNDAENTALS OF WELDING A (e!ded +oint is o$tained (hen t(o c!ean s rfaces are $ro ht into contact (ith eachother and either press re or heat' or $oth are app!ied to o$tain a $ond% The tendenc& of atoms to $ond is the f ndamenta! $asis of (e!din% The inter-diff sion $et(een themateria!s that are +oined is the nder!&in princip!e in a!! (e!din processes% Thediff sion ma& ta,e p!ace in the !i) id' so!id or mi.ed state% In (e!din the meta!!icmateria!s are +oined $& the formation of meta!!ic $onds%When t(o c!ean and f!at s rfaces are +oined' the $ondin ta,es p!ace $et(een s rfaceatoms and a perfect connection is formed 8Fi%9:% Version II30 - 3 Fig.2 Two ideal surfaces brought together to form a weld  (a) Before weld (b) After weld  Metal A Metal B  WELDS - STATIC AND FATIGUE STRENGTH - I The efficienc& o$tained for the +oint is /224 and its strenth (o !d $e as m ch as that of the $ase meta!s% In practice ho(e#er' it is #er& diffic !t to achie#e a perfect +oint; for' rea!s rfaces are ne#er smooth% When irre !ar s rfaces are +oined $& (e!din' contact isesta$!ished on!& at a fe( points in the s rface' (here atomic $ondin occ rs% Thereforethe strenth attained (i!! $e on!& a fraction of the f !! strenth% A!so' the irre !ar s rfacema& not $e #er& c!ean' $ein contaminated (ith adsor$ed moist re' o.ide fi!m' rease!a&er etc% as sho(n in Fi <8 a :% In the (e!din of s ch s rfaces' the contaminants ha#e to $e remo#ed for the $ondin of the s rface atoms to ta,e p!ace% This can $e accomp!ished $& app!&in either heat or  press re% In practica! (e!din' $oth heat and press re are app!ied to et a ood +oint%When heat is app!ied' the adsor$ed !a&ers are dri#en off' o.ide fi!ms are $ro,en and the&ie!d strenth of the $ase meta!s are !o(ered; (ith the app!ication of press re' p!asticdeformation ta,es p!ace and $rins into contact more atoms for (e!din% Heat app!icationres !ts in the me!tin of the meta!!ic mem$ers' ena$!in atoms to come into contact $&f! id f!o( and form $onds% Sometimes' a fi!!er materia!' of the same t&pe as the $asemateria! or compati$!e (ith it' is added to achie#e the $ond =Fi <8 b ::>% When press re isapp!ied to form a +oint it $rea,s the o$str ctin !a&ers and sharp edes to $rin the +oinin s rfaces toether for ma,in the $ond% Fi% <8 c : sho(s a (e!d formed $& theapp!ication of heat and press re% As pointed o t ear!ier' an& (e!din process needs some form of ener&' often heat' toconnect the t(o materia!s% The re!ati#e amo nt of heat and press re re) ired to +oin t(omateria!s ma& #ar& considera$!& $et(een t(o e.treme cases in (hich either heat or  press re a!one is app!ied% When heat a!one is app!ied to ma,e the +oint' press re is sedmere!& to ,eep the +oinin mem$ers toether% E.amp!es of s ch a process are GasT nsten Arc We!din 8GTAW:' Shie!ded eta! Arc We!din 8SAW:' S $mered Arc Version II30 - !  Metal A Metal B Adsorbed layer Oxide layer  Fig.3 (a) Surface contaminants  Metal A Metal B Fig.3(c) ear perfect weld   Metal A Metal B Fig. 3(b) !ddition of filler material 
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