Fatigue Life Prediction of Cruciform Joints Failing at the Weld Toe

Transcription

1 WELDING RESEARCH SUPPLEMENT TO THE WELDING JOURNAL, AUGUST 1992 Sponsored by the Americn Welding Society nd the Welding Reserch Council Ftigue Life Prediction of Cruciform Joints Filing t the Weld Toe A two-stge model ppers to offer the most ccurcy in predicting ftigue life BY M. SKORUPA ABSTRACT. The objective of this study is to select theoreticl model to predict the ftigue life of weld. The experimentl prt consists of ftigue tests within the life rnge of from 1 5 to 2 X 1 6 cycles for cruciform welded joints in two structurl steels under constnt mplitude xil loding with constnt positive stress rtio: the ftigue crck inititing t the weld toe. The ftigue lives were then clculted ccording to the two-stge pproch in which both the ftigue crck initition phse nd the crck propgtion phse were considered, nd ccording to the one-stge pproch which ssumes tht the totl ftigue life is spent in crck propgtion. The locl strin method ws pplied to clculte the ftigue crck initition life, while the ftigue crck propgtion life ws estimted using frcture mechnics concepts. The importnt feture of these nlyses ws tht the mteril properties needed were estimted vi hrdness mesurements. The ftigue lives obtined by the two-stge model fll within the.95 confidence limits of the experimentl dt nd re conservtive in the sense tht the verge rtio of the observed-to-clculted life is The one-stge model results in life estimtes n order of mgnitude lower thn those derived from the two-stge model. The study suggests tht the two-stge model rther thn the one-stge model enbles ccurte ftigue life predictions even for welds contining undercuts. Introduction Anlyticl procedures used for predicting ftigue behvior of notched structurl members cn be brodly clssified s the one-stge models ssuming tht N f = N p (l) or N f = N (1b) M. SKORUPA is with the University of Mining nd Metllurgy, Krkow, Polnd. nd the two-stge models where N f = N, + N p (2) The pproches ccording to Equtions 1, 1b nd 2 will be further termed s the P, I nd l-p models, respectively. In Equtions 1 nd 2, Nf is the totl ftigue of life, N p is the crck propgtion life nd N, is the crck initition life. For smll metl components, N, is defined s the number of cycles required to develop dominnt crck of so-clled initil size (i), usully of the order of tenths of mm. The mening of N p in Equtions 1 nd 2 is not the sme. N p in Eqution 2 is the number of cycles spent in crck growth from, to finl frcture while N p in Eqution 1 ccounts lso for the erliest phse of crck growth from flw or microstructure defect. Usully, low-cycle-ftigue theory is dopted to estimte Nj, nd frcture mechnics is employed to clculte N p. Disgreement previls on which of the models defined by Equtions 1 nd 2 best reflects ftigue behvior of welds. A widespred opinion tht defects, usully crcklike in shpe, re unvoidble even in high-qulity welds involves the predominnt use of the P model (Refs. 1-3). However, successful estimtes of weld ftigue life hve lso been mde utilizing the two-stge pproch (Ref. 4). Ftigue nlysis of welds requires tht strength effects of the welding process be KEY WORDS Modeling Ftigue Life Cruciform Joints Structurl Steels Weld Toe Crcking Crck Initition Crck Propgtion Locl Strin Frcture Mechnics Hrdness Mesure ccounted for. The most importnt of these re: geometricl vribility, chnge of mteril properties in the weld neighborhood nd residul stresses. The objective of this pper ws to select the theoreticl model most dequte for predicting the totl ftigue life of weld. The study focused on cruciform welded joint in two structurl steels under constnt mplitude xil loding. The dimensions of the weldment were chosen such tht filure occurred t the weld toe. Ftigue lives clculted ccording to the two-stge nd one-stge models for defectless weld geometery nd in the presence of undercut were compred with the observed results. Experimentl Tests Specimens Cruciform welded specimens for ftigue testing were fbricted from structurl steel pltes. The steels were St3S (crbon mild steel) nd 18C2A (low-lloy steel). The chemicl compositions nd mechnicl properties of these steels re given in Tble 1. The welding procedure ws mnul metl rc in the horizontl position. Bsed on Ref. 1, the rtios of the weld leg length I to the plte thickness t were chosen such to promote filure t the weld toe under xil ftigue loding. The St3S pltes were welded in longer pieces nd then sw-cut into nrrow specimens. Ech 18G2A specimen ws welded seprtely. The specimen side surfces were milled to discrd weld strt nd stop res, then ground. The specimen geometry is shown in Fig. 1 nd the dimensions re given in Tble 2. Vrible geometry prmeters (Fig. 2), knowledge of which is vitl to nlyticl ftigue life prediction, nmely, the weld contct ngle fi, the notch root rdius t the weld toe r, the depth of possible undercut t the weld toe d nd mislignment (eccentricity e nd ngulr distortion ), WELDING RESEARCH SUPPLEMENT I 269-s

2 Tble 1 Chemicl Composition nd Mechnicl Properties of the Steels Used for Welded Specimens Steel C Mn Chemicl Composition ("o) Si P S Cr Cu Su (MP) Mechnicl Properties Sy (.2%, MP) RA " m St3S 18C2A Fig. 7 Test specimen geometry. were mesured. The mesurements of the fi nd r vlues were mde in side surfces of the specimens using n opticl microscope t mgnifiction of 1X. While only smll vritions in the <3 vlues were observed due to very smooth weld profiles (Fig. 3A), the mesured r vlues showed lrge mount of sctter (Fig. 3B). Undercuts were mcroscopiclly exmined t mgnifiction of bout 7X in 19 longitudinl sections of the St3S specimens nd in 35 sections of the 18G2A specimens; four weld toe regions being observed in ech section. Preprtion of tht inspection involved wet polishing with successively finer grdes of emery pper. Undercut depths were mesured perpendiculrly to the intended direction of loding from horizontl projection of the bse plte line Fig. 2. No undercuts except severl very shllow (of depth be- Tble 2--Dimensions of Welded Specimens low.5 mm) depressions t the weld toe were detected in the St3S specimens. However, in the 18G2A specimen sections 82 undercuts were found for the totl number of 14 weld toe regions inspected. The mesured depths d did not exceed.1 mm (Fig. 3C), the notch root rdius t the bottom of undercut r' being lwys lrger thn d. The wy of fbriction of the 18C2A specimens chieved very good joint lignment. Mislignments, nmely eccentricity up to 2% of the plte thickness nd ngulr distortions up to 3 deg (Fig. 2) were only found in prt of the St3S specimens. As rule, the vlues of e nd mesured in both sides of given specimen were not the sme. The mximum vlues of e nd for the St3S specimens re listed in Tble 3. Ftigue Tests The specimens were ftigue tested until filure by breking into two prts in n electrohydrulic mchine under constnt stress mplitude t frequency of =15 Hz. The stress rtio R ws.1 for the St3S specimens nd.2 for the 18G2A specimens. All the 1/t = specimens filed t the weld toe, while in prt of the 1/t = 1 specimens weld filures were observed in ccordnce with other reported results (Ref. 1). Tbles 3 nd 4 give the ftigue test results for the specimens filing t the weld toe. A sttisticl nlysis of the ftigue dt for the Series 1/t = St3S specimens nd 18G2A specimens ws crried out ccording to Ref. 5 ssuming the following liner reltionship between the ftigue life N t - nd the nominl stress rnge AS: lognf = A + B logas (3) The smll number of Series l/t = 1 St3S specimens filing t the weld toe prohibited performing sttisticl nlysis of the corresponding ftigue dt. The ftigue test results together with the relevnt regression lines nd.95 confidence limits re shown in Figs. 4 nd 5. The higher width of the sctter bnd for the St3S welds (Fig. 4) compred to tht for the 18G2A welds (Fig. 5) my predominntly result from the occurrence of mislignments in the former specimens. The reson ftigue resistnce of the St3S specimens is significntly higher thn tht of the 18G2A specimens my be ttributed to Mteril St3S 18G2A Rtio l/t 1 t Dimensions (mm) B L I Fig. 2 Vrible geometry prmeters nd crck pth for test specimens. Tble 3 Ftigue Test Results nd Mislignments for St3S Specimens Mteril Rtio l/t Nominl Stress Rnge AS (MP) Totl Ftigue Life Nf (cycles) St3S Unbroken Mislignments Angulr Eccentricity Distortion s I AUGUST 1992

3 15i iio LO _ o fi Weld Contct Angle /3, ^ Weld Toe Rdius r. mm 81 g6 o o fi2 " Depth ot Undercut d^m F/g. J Mesured vritions of geometry prmeters for test specimens: A Weld contct ngle fi, B Root rdius t the weld toe r; C Depth of undercut d. Tble 4 Ftigue Test Results for 18G2A Specimens Nominl Stress Rtio Rnge Mteril l/t AS (MP) 18C2A B Totl Ftigue Life N fl (cycles) Unbroken Ftigue Dt Points Regression Line Q95 Confidence Limits N f Observed Results 1 5 1, 6u 2*1 u 6 N, N f, cycles Fig. 4 Observed nd predicted with two-stge pproch ftigue lives for St3S l/t = 7.5 welded specimens. more fvorble weld toe geometry (lck of undercuts) in the former. The present ftigue dt fll within the reported sctter bnds obtined with similr specimens (Ref. 1). Microscopic exmintion of the crcked specimens reveled tht ftigue crcks initited nd propgted within the hetffected zone (HAZ). Ftigue crck pths were pproximtely stright lines sloped to the direction trnsverse to the min plte t n ngle <p ~ 15 deg for l/t = nd rj> «8 deg for l/t = 1 (Fig. 2). An observtion of the frcture surfces indicted tht the specimens filed when the crck depth reched on n verge bout.35l Ftigue Life Clcultions Geometricl Vribility Bsed on the geometry mesurements (Fig. 3), the vritions in the ngle fi re ignored nd n verge vlue of 8 = 45 deg is ssumed. In order to cope with the vrible nture of other geometry prmeters t the weld toe, it is ssumed, fter Lwrence, ef l. (Ref. 4), tht ftigue crcking strts t the loction where the ftigue notch fctor clculted through Peterson's eqution dopts its mximum vlue. Peterson's eqution reds K t -1 Kf 1 + (4) 1 +oc/t where K t is the elstic stress concentrtion fctor, r is the notch root rdius nd is mteril constnt given by 2.32 X 1 4 S U -1 ' 8 (mm), S being the ultimte strength of mteril in MP. If the nlyticl K t vs. r reltionship is known, Eqution 4 cn be differentited with respect to r to determine the criticl notch root rdius r c for which Kf obtins mximum. For the defectless weld toe geometry, the following K t vs. r reltionship ws obtined from finite element nlysis (Ref. 6): K, = 1 + Cf(r/tp (5) where the constnts Ci nd C2 depend on the rtio of 1/t. The Kf function given by Eqution 4 combined with Eqution 5 psses through mximum for C 2 -E 1 r c = - ^ - - (6) In the presence of undercut (Fig. 2) the theoreticl stress concentrtion fctor t the weld toe ws pproximted by (Ref. 7) K tu = K,(1+2V(d7FT] (7) where K t is given through Eqution 5. Using Equtions 4 nd 7, the ftigue Tble 5 Empiricl Reltionships between Mechnicl Properties nd Hrdness DPH of Steel Property Ultimte tensile strength, S u (MP) Yield strength.2%, S y (MP) Ftigue strength coefficient, o-j'(mp) Ftigue strength exponent, b Cyclic yield strength, o- y'(mp) Trnsition ftigue life, 2N tr (reversls) Function of DPH 3.45 DPH 2.68 DPH DPH -I glog( /DPH) 2.1 DPH 5.7 X 1 s exp (-.17 DPH) Source Ref. 8 Ref. 9 Ref. 9 Ref. 9 Ref. 8 Ref. 9 WELDING RESEARCH SUPPLEMENT I 271-s

4 to occur in the St3S welds before cutting up the pltes, they were relieved by the slicing opertion (Ref. 1). Therefore, residul stresses were ignored in the present nlysis. Residul stresses when present cn be llowed for in the crck initition nlysis s sttic prestress (Refs. 12 nd 13). Crck Initition Anlysis o Ftigue Dt Points Regression Line.95 Confidence Limits N f Observed Results 1-1C fc 2*1 6 Nf>Nj,Np, cycles Fig. 5 Observed nd predicted with two-stge pproch ftigue lives for I8G2A welded specimens. notch fctor for weld with undercut ws obtined s K t (1 4- v w ) ) - 1 From Eqution 8, the Kf mx condition for weld with undercut exists if V( 2 (K, f d)- 1) 2 4-4K t 2 2 K, V^" J Bsed on the inspection of the weld toe regions described erlier, the St3S specimens were ssumed to be defectless while for the 18G2A specimens the presence of undercuts ws llowed for. According to the Kf mx concept, Equtions 6 nd 9 define the vlues of the notch root rdius dopted in the crck initition nd crck propgtion nlyses for the St3S nd 18G2A specimens, respectively. As the ftigue notch fctor ccording to Eqution 8 is monotonic function of r nd d, the minimum mesured r vlue equl to.5 mm nd the mximum mesured d vlue equl to.1 mm were ssumed for the 18G2A specimens. Estimting HAZ Mteril Properties through Hrdness Mesurements (9) The properties of the HAZ mteril tht re involved in the ftigue process of the test specimens my be different from those of the bse metl. As HAZ mteril is difficult nd expensive to test, ll the required mteril prmeters were either estimted from hrdness mesurements t the weld toe using empiricl reltionships given in Tble 5 or ssumed. The ftigue ductility exponent c =.6 ws ssumed. The reltionships of S y nd <7 y ' with hrdness depend on the type of steel. As discussed in Ref. 7, those ssumed in Tble 5 correlte best with reported test results for vrious HAZ mterils in steels (Refs. 4, 9). Other mteril properties of the HAZ were clculted through the following reltionships tht re known in the low-cycle-ftigue theory: cyclic strin hrdening exponent n' = b/c (1) cyclic strength coefficient K' =o- y '/(.2) n ' (11) ftigue ductility coefficient s the men of = (^'/K') 1 (12) nd tf' =,'(2N tr ) b -VE (13) Tble 6 lists the mteril properties clculted from the reltionships given in Tble 5 nd from Equtions 1 to 13. The vlues of r (, r c ' nd K tmx in Tble 6 result from Equtions 4 to 9. Initil nd Finl Crck Size Where the l-p model (Eqution 2) is imployed, the initil crck size ; must be defined in order to bridge between the initition nd propgtion nlyses. A common prctice is to ssume the i vlue rbitrrily (usully.25 mm), lthough nonrbitrry definitions of j were lso proposed (Refs. 1, 11). For the considered geometry nd mteril it ws shown in Ref. 7 tht the vlue of.25 mm lies between those clculted ccording to Refs. 1 nd 11. Therefore, : =.25 mm ws dopted in this study. Bsed on the exmintion of frcture surfces, the finl crck size f = 3.5 mm ws ssumed. Residul Stresses The wy of fbriction of both St3S nd 18G2A specimens precluded occurrence of high residul stresses. With the nrrow 18G2A specimens, mking weld produced more or less even temperture cross the pltes nd consequently could only crete low residul stresses. Although reltively high residul stresses were likely The crck initition lives (Nj) were estimted with the locl strin pproch (Ref. 14). The mthemticl procedure to clculte the locl stress nd strin mplitudes (<r nd e, respectively) nd the locl men stress (<r ) is illustrted in Fig. 6 where R is the stress rtio, S nd e denote the nominl stress nd strin, respectively, nd the subscripts "1" nd "" to stress or strin correspond to the vlue t the end of the first loding reversl nd to the mplitude, respectively. Since the mteril properties involved in the monotonic stress-strin curve eqution cnnot be relted to hrdness, cyclic softening of HAZ mteril ws ignored. The Ni vlues were clculted through the eqution from Morrow (Ref. 15) * = (fff' - <r )(2Ni) b (14) in which only the mteril properties tht cn be directly relted to hrdness re utilized. To llow for the effect of cyclic relxtion of the locl men stress on Nj the constnt o vlue in Eqution 14 ws replced by (Ref. 16) <ro.2n = <T (2N-1) k (15) where ff,2n is the current vlue of men stress t ny reversl 2N nd k denotes the relxtion exponent. From Ref. 12, k ws expressed through the following empiricl reltionship vlid for metls: k = fp /(Ee tr ) (16) where e p is the plstic strin mplitude, t Xr : the trnsition strin (hlf strin mplitude corresponding to the trnsition ftigue life 2N, r ), nd E: the Young modulus in MP. The opertionl detils nd the progrmming flow digrms of the procedure used to compute Nj cn be found elsewhere (Ref. 7). Crck Propgtion Anlysis Since the mteril constnts ssocited with ftigue crck propgtion rte cnnot be estimted from hrdness, only the concepts where crck growth rte is described in terms of low-cycle-ftigue mteril properties were used in the crck propgtion nlysis, nmely the LEFM model from Mjumdr nd Morrow (Ref. 17) nd the EPFM pproch of Usmi (Ref. 18). Both bove mentioned concepts were shown in Ref. 7 to provide for welded joint in mild steel the N p vlues, which correlted well with those obtined using the Pris eqution. 272-s I AUGUST 1992

5 According to the model of Mjumdr nd Morrow (Ref. 17) the mteril hed of the crck tip within the reversed plstic zone is composed of the unixil "ftigue elements" of width of 2 P* where p* is "microstructure size." Assuming tht ftigue crck extension occurs due to the successive ftigue filure of ech element Mjumdr nd Morrow deduce the following expression for ftigue crck growth rte: d dn -[4(1 + n')e y] iw ~ c (17) Tble 6 Mechnicl Properties of St3S HAZ nd 18G2A HAZ Mteril Property Hrdness, DPH Ultimte strength, S u (MP) Yield strength.2%, S y (MP) Young modulus, E (MP) Cyclic yield strength, y'(mp) Ftigue strength coefficient, 7'(MP) Ftigue strength exponent, b Ftigue ductility coefficient, tf' Ftigue ductility exponent, c Cyclic hrdening exponent, n' Cyclic strength coefficient, K'(MP) Trnsition ftigue life, 2N tr (reversls) Peterson's mteril constnt, (mm) Criticl notch root rdius, r c [r c'] (mm) Mximum ftigue notch fctor, Kf mx () l/t = (b) l/t = 1 St3S HAZ < ).157< b > w 2.93< b > 18G2A HAZ [.74] 3.12 where AK is the stress intensity fctor rnge, e/ is the cyclic yield strin, nd the mening of the other symbols is explined in Tble 6. The vlidity of Eqution 17 is limited by the condition R p» 2p* (18) where R p is the reversed plstic zone size. The physicl interprettion of p* given by Mjumdr nd Morrow (Ref. 17) suggests tht it represents the men distnce between the mjor microstructure deformtion brriers. This distnce is thought to be closely relted to the length of nonpropgting surfce crck t the ftigue limit (Ref. 19). According to Usmi (Ref. 18), the dimeter (d pc) of such crck in metllic mteril is mteril constnt which cn be expressed s d pc = X 1-7 (S y/e)" 2 (mm) (19) The bove mentioned findings from Refs. 18 nd 19 re the rtionle for ssuming in the present nlysis tht p* equls d pc. Eqution 17 ws only utilized to clculte the crck propgtion lives for the St3S specimens. The pproch of Usmi (Ref. 18) ws pplied to derive the N p estimtes for the 18G2A specimens since Eqution 18 ws not stisfied for 18G2A HAZ mteril even t the highest stress level considered. From experimentl tests on vrious steels, Usmi postultes correltion between the normlized crck growth rte (d/dn)/ e, e being the effective crck length (hlf length of centrl crck in n infinite body) nd the effective locl strin rnge given by Ae eff = Ae/(1-R), R < O Ateff = Ae, R > O (2) where Ae nd R re the totl locl strin rnge nd the locl stress rtio, respectively, t distnce of from the notch root in the uncrcked body. Usmi's experimentl dt points cn be fitted in with the following reltionships: (d/dn)/ e = 12.3(Ae eff) 2 Ae eff < 9 X 1~ 4 (d/dn)/ e = 648(Ae eff) X lo" 4 < Ae eff < 5 X lo" 3 (d/dn)(/ e = 11(Ae eff) 2 Ae eff 2= 5 X 1~ 3 (21) From Ref. 18, Eqution 21 proved vlid for number of steels. In order to clculte the N p vlues Equtions 17 nd 21 were integrted numericlly in 24 steps employing Simpson's U) (I) 1_ ti, u (_ F / /V / J) / A \ / V ~ ' ( 12 3 Number of reversls, 2N 1 Fig. 6 Schemtic of notch stress nd strin clcultion. rule. The stress intensity fctor solution ws derived by finite element method, s described thoroughly in Ref. 6. Comprison of Predicted nd Observed Results Two-Stge Approch In Figs. 4 nd 5 the totl ftigue lives Nf provided vi the two-stge l-p model (Eqution 2) re compred with the f- 6- i,=k f S,e. WELDING RESEARCH SUPPLEMENT I 273-s

6 2*1 c ~l I I I ll E 1 1- Life to Form Crck of Size Oj, cycles Fig. 9 Comprison of estimtes on life to form crck of size, ccording to two-stge nd onestge model for sound weld nd for weld with undercut. N.5-,N DEFECT y \ UNDERCUT Xf tf 2»1 o N f, cycles. Fig. 8 Predicted percentge of totl life devoted to ftigue crck initition s function of totl life for sound weld nd for weld with undercut. V i/t=io. * 18G2A *1 6 Observed Ftigue Life N fl cycles Fig. 7 Comprison between observed nd predicted ftigue lives for ll test specimens. tigue test dt for the welded specimens. The discrepncies between the predicted AS vs. Nf digrms nd the regression lines re depicted by rtios of Nf vlues given for the two stress levels tht correspond to the highest nd the lowest stress rnge considered in the experimentl tests. From Figs. 4 nd 5, it is seen tht the predicted AS vs. Nf curves fll within the sctter bnds of the ftigue dt. Since the slopes oo < 5 \MDEL WELD\. TOE ^ \ NO DEFECT (St3S) UNDERCUT (18G2A) l-p P The comprison between the ctul nd clculted ftigue lives for ll the ftigue specimens is given in Fig. 7. Except of the theoreticl AS vs. Nf digrms in Figs. 4 nd 5 re lower thn these of the regression lines, the men ftigue lives (represented by the regression lines) re underestimted t higher stress levels nd slightly overestimted t lower stress levels. single dt point, the predictions gree with the experimentl results within fctor of 2. The predictions re conservtive in the sense tht the verge rtio of the observed-to-clculted life is Generlly lower discrepncies between the ctul nd estimted results observed in Fig. 7 for the 18G2A specimens compred to those for the St3S specimens re prently due to the higher sctter in the ftigue dt in the ltter cse. In Fig. 8 the predicted percentge of totl life spent in crck initition is plotted s function of totl life. For sound weld, the figure shows tht within the considered life rnge crck initition consumes previling frction of life while in the presence of undercut, crck initition life domintes only in the long life regime (Nf > 5 X 1 5 cycles). One-Stge Approch As discussed previously, the difference between life estimtes ccording to the two-stge nd the one-stge (P model) pproches rises from the different wys of predicting the number of cycles to develop crck of size ;. In Fig. 9 the evlutions of tht number of cycles for the considered cruciform weld by using the l-p model nd the P model re compred, Usmi's concept (Ref. 18, see Equtions 2 nd 21) being employed in the ltter cse. It cn be seen in Fig. 9 tht the P model gives more conservtive predictions on the life to form crck of size j thn the l-p model. The discrepncies between tht life estimte from both pproches increse with decresing stress level to rech the vlue of 9.1 for the sound weld nd 45 for the weld with undercut. Considering tht the l-p model hs been previously shown to yield slightly conservtive Nj estimtes (Figs. 4, 5 nd 7), from Fig. 9, it is cler tht for lives greter thn 1 5 cycles the P model cnnot entirely ccount for the totl ftigue life even for weld contining undercut. Finl Remrks Although bsed on the mteril properties estimted in rough wy, the ftigue lives predicted using the two-stge 274-s I AUGUST 1992

7 pproch re in good greement with the experimentl results. Utilizing in ftigue nlyses of welds the HAZ mteril properties estimted vi hrdness mesurements t the weld toe ws first proposed by Lwrence, ef l. (Ref. 4). The subsequent nlyses, however, were confined to the predictions of the long life ftigue strength of welds with the use of the onestge I model (Ref. 2). This study including the crck propgtion period estimted ccording to the concepts in which the ftigue crck growth rte is expressed in terms of the low-cycle-ftigue mteril properties, enbled life predictions within the rnge of from 1 5 to 2 X 1 5 cycles. Conclusions The two-stge pproch, including both the ftigue crck initition nd propgtion phses, enbles one to estimte totl ftigue lives in the life regime of 1 5 to 2 X 1 6 cycles, which gree within fctor of 2 with experimentl dt for cruciform welded specimens filing t the weld toe. The study supports the pplicbility of het-ffected zone mteril properties estimted vi hrdness mesurements towrds ftigue life predictions of welds. The one-stge pproch, which neglects the ftigue crck initition phse, cnnot entirely ccount for the totl ftigue life, even for welds contining undercuts. References 1. Gurney, T. R Ftigue of Welded Structures. Cmbridge, U.K. 2. Ymd, K., nd Hirt, M.A Ftigue crck propgtion from fillet weld toe. /. of the Structurl Division. ASCE 18, No. ST7, pp Smith, I.F.C., nd Smith, R.A Ftigue crck growth in fillet welded joint. Eng. Frct. Mech. 18: Lwrence, F.V., Ho, N.)., nd Mzumdr, P.K Predicting the ftigue resistnce of welds. Annul Review of Mterils Science 8: Sttisticl nlysis of liner or linerized stress-life nd strintife ftigue dt ASTM Stndrd E Skorup, M Brm, H nd Prij, J Applicbility of pproximte K solutions towrds crcks t weld toes. Eng. Frct. Mech. 26: Skorup, M Predicting the ftigue life of welds. Scientific Bulletins of the Stnislw Stszic Acdemy of Mining nd Metllurgy. No Mechnics, Bull. 18 (in Polish). 8. Morrow, J Internl Friction, Dmping nd Cyclic Plsticity. ASTM STP 378, pp McMhon, ).C, nd Lwrence, F.V Predicting ftigue properties through hrdness mesurements. FCP Report No. 15, University of Illinois t Urbn-Chmpign. 1. Chen, W A model for joining crck initition nd propgtion nlyses. Ph.D. Thesis, University of Illinois t Urbn-Chmpign. 11. Cmeron, A.D., nd Smith, R.A Ftigue life prediction for notched members. Int. J. of Pressure Vessels nd Piping 1: Burk, J.D The effect of residul stresses on weld ftigue life. Ph.D. Thesis, University of Illinois t Urbn-Chmpign. 13. Skorup, M Ftigue crck initition life prediction for welded joints by low cycle ftigue pproch. Ft. Frct. Eng. Mter. Struct. 13: Dowling, N.E., Brose, W.R., nd Wilson, W.K Ftigue Under Complex Loding Anlysis nd Experiments. Advnces in Engineering. SAE 6: Morrow, J SAE Ftigue Design Hndbook, pp Jhnsle, H.R. nd Topper, T.H Cyclic Stress-Strin Behvior. Anlysis, Experimenttion, nd Filure Prediction. ASTM STP 519, pp Mjumdr, S nd Morrow, Frcture Toughness nd Slow Stble Crcking. ASTM STP 559, pp Usmi, S Smll Ftigue Crcks. The Metllurgicl Society Inc., pp Miller, KJ The behvior of short ftigue crcks nd their initition. Ft. Frct. Eng. Mter. Struct. 1: nd Yung, J.Y., nd Lwrence, F.V Anlyticl nd grphicl ids for the ftigue design of weldments. Ft. Frct. Eng. Mter. Struct 8: Testin, R.A., Yung, j.-y., Lwrence, F.V., nd Rice, R.C Predicting the ftigue resistnce of steel weldments. Welding Journl 66: 93-s to 98-s. Appendix e 3f i Crck size Equivlent crck size Finl crck size Initil crck size Peterson's mteril constnt Ftigue strength exponent c d dpc e E k Kl Kf (Kfmx) Ki (K tu) k' I n' N N f Ni N P Ntr r(r c ) r' (r c ') R R P S Su Sy P e P % «f «/ p* Co Cf' ffy <P Subscripts 1 A Abbrevitions EPFM HAZ LEFM Ftigue ductility exponent Depth of undercut Dimeter of nonpropgting surfce crck Nominl strin Modulus of elsticity Relextion exponent Stress intensity fctor Ftigue notch fctor (mximum vlue) Stress concentrtion fctor (t the bottom of undercut) Cyclic strength coefficient Weld leg length Cyclic hrdening exponent Number of cycles Totl ftigue life Crck initition life Crck propgtion life Trnsition ftigue life Weld toe rdius (criticl vlue) Root rdius t the bottom of undercut (criticl vlue) Stress rtio Reversed plstic zone size Nominl stress Ultimte tensile strength Yield strength.2% Angulr distortion Weld contct ngle Notch strin Cyclic plstic notch strin Trnsition strin Ftigue ductility coefficient Cyclic yield strin Microstructure size from Mjumdr nd Morrow eqution Notch stress Notch men stress Ftigue strength coefficient Cyclic yield strength Slope of crck pth Amplitude The first reversl Rnge Elstic-plstic frcture mechnics Het-ffected zone Liner-elstic frcture mechnics WELDING RESEARCH SUPPLEMENT I 275-s

8 Nitrogen in Arc Welding A Review WRC Bulletin 369 December 1991 By IIW Commission II In 1983, Commission II of the Interntionl Institute of Welding (IIW) initited n effort to review nd exmine the role of nitrogen in steel weld metls. The objective ws to compile in one source, for future reference, the vilble informtion on how nitrogen enters weld metls produced by vrious rc welding processes, wht forms it tkes in these welds, nd how it ffects weld metl properties. This bulletin contins 13 reports nd severl hundred references relted to Nitrogen in Weld Metls tht hs been prepred s review to show the importnce nitrogen hs in determining weld metl properties. Publiction of this report ws sponsored by the Welding Reserch Council, Inc. The price of WRC Bulletin 369 is $85. per copy, plus $5. for U.S. nd $1. for overses, postge nd hndling. Orders should be sent with pyment to the Welding Reserch Council, Room 131, 345 E. 47th St., New York, NY 117. Reserch on Modern High-Strength Low-Alloy Steel Welding WRC Bulletin 373 June 1992 (1) Influences of Steel Composition nd Welding Procedure on the HAZ Toughness of Thick-Section Structurl Steels By P.L. Hrrison nd P. H. M. Hrt (2) Het-Affected Zone Properties of Thick-Section Microlloyed Steels A Perspective By F. Heisterkmp, K. Hulk nd A. D. Btte (3) Experience in Fbricting New Types of Offshore Plte nd Linepipe By P. Tuvnes nd I. Hrneshug (4) Influence of Locl Brittle Zone on HAZ Toughness of TMCP Steels By S. Aihr nd K. Okmoto The four ppers contined in this Bulletin were presented t the Conference on "Metllurgy, Welding nd Qulifiction of Microlloyed (HSLA) Steel Weldments," held t Houston, Tex., November 6-8, 199. The Americn Welding Society holds the copyrights nd is the source of these ppers. Publiction of this document ws sponsored by the Welding Reserch Council, Inc. The price of WRC Bulletin 373 is $4. per copy, plus $5. for U.S. nd $1. for overses, postge nd hndling. Orders should be sent with pyment to the Welding Reserch Council, Room 131, 345 E. 47th St., New York, NY s I AUGUST 1992