Strojniški vestnik - Journal o f Mechanical Engineering 54(2008) 11,807-816 UDC 62-112.81:624.014
Paper received: 20.12.2007 Paper accepted: 25.09.2008
Safe Operation of Welded Structure with
Cracks at Elevated Temperature
M eri B u rz ić ’ - R a d ic a P ro k ić -C v e tk o v ić 2 - B ilja n a G ru jič 3 - Iv an a A ta n a s o v s k a 4 - Ž iv o s la v A d a m o v ič 5
1 In stitu te "G O Š A " , B e lg ra d e , S erb ia
2 F a c u lty o f M e c h a n ic a l E n g in e e rin g , B e lg ra d e , S erb ia 3 In n o v a tio n C e n tre , B e lg ra d e , S erb ia 4 In stitu te "K irilo S av ič ", B e lg ra d e , S erb ia 5 F a c u lty o f T e c h n ic a l E n g in e e rin g , Z re n ja n in , S e rb ia
The fatigue crack growth rate param eters and conditions fo r abrupt fracture o f thick jo in t in steel at room and operating temperature were analysed. Fatigue cracks generated fro m sharp weld defects are initial cracks that grow through either the w eld jo in t region or the base metal in accordance with Paris law. Service life o f a welded structure depends on position and orientation o f the existing sharp weld defect. Different pre-cracked specimens were used in this experimental investigation.
They were cut from the base and w eld metals and heat-affected zone. In comparison with the base metal, w eld jo in t region show ed higher crack growth rate at operating and room temperatures. Fatigue crack growth rate was higher at operating temperature irrespective o f the position. Reliability o f structure with initial longitudinal cracks positioned in the heat-affected zone was lower than with initial transversal cracks located in the weld metal.
© 20 0 8 Jo u rn al o f M e c h a n ic a l E n g in e e rin g . A ll rig h ts rese rv e d .
Keywords: welded structures, fatigue cracks, fracture toughness, proof test
0 IN T R O D U C T IO N
W e ld d e fe c ts a rise in m a n u fa c tu re o f w eld ed stru c tu re s. D u rin g n o rm a l o p e ra tio n o f such stru c tu re s, sh a rp d e fe c ts in w e ld jo in t reg io n act as sites fro m w h ic h fa tig u e c ra c k s fo rm a n d th en g ro w . C ra c k s o f c ritic a l siz e je o p a r d iz e stru ctu ra l in te g rity . D u e to th e c o m p le x n a tu re o f fu sio n w e ld in g an d h u m a n fa c to r, th e size, lo catio n , ty p e , a n d o rie n ta tio n o f d e fe c ts c a n n o t b e p re d ic te d . O n ly a th o ro u g h p o st- m a n u fa c tu re in sp e c tio n o f w e ld jo in ts p ro v id e s n e c e s sa ry in fo rm a tio n a b o u t th e a c tu a l w e ld d e fe c t p re s e n c e in w e ld e d stru ctu re s.
T h e m o s t je o p a r d iz in g ty p e o f sh a rp w e ld d efe cts are tw o -d im e n s io n a l d e fe c ts o rie n te d n o rm al to th e d ire c tio n o f flu c tu a tin g stress. T h e ir d irec t e ffe c ts o n th e s tre n g th o f w e ld jo in ts are tre a te d as th e e ffe c ts o f c ra c k s (c ra c k -lik e d efects). F ra c tu re m e c h a n ic s is u s e d to a s se ss th e se v erity o f m a c ro s c o p ic tw o -d im e n s io n a l d efe cts in re la tio n to d e s ig n lo a d in g . T h e re are so m e co m p lex fe a tu re s th a t m a k e th a e a p p lic a tio n o f fra ctu re m e c h a n ic s d iffic u lt su c h as re s id u a l stresses, in te ra c tio n o f c ra c k s w ith w e ld g eo m etry , m a te ria l p ro p e rtie s v a ria tio n s d u e to
d is s im ila r w e ld a n d b a s e m e ta ls a n d e s p e c ia lly m ic ro s tru c tu ra l h e te ro g e n e itie s o f w e ld jo in t re g io n re s u ltin g fro m th e w e ld in g th e rm a l cy c le s.
T e n s ile w e ld in g re sid u a l stre s se s a p p e a ra n c e a n d lo c al m a te ria l e m b rittle m e n t are fu n d a m e n ta l fo r v a rio u s c ra c k in g p h e n o m e n a o f
w e ld s [1], In te ra c tio n o f stre s se s d u rin g stru c tu re o p e ra tio n a n d e x is tin g cra ck s o fte n le ad s to u n e x p e c te d w e ld jo in t fra ctu re. A c tu a lly , cra ck s o rig in a tin g fro m sh a rp w e ld d e fe c ts g ro w d u rin g se rv ic e life o f w e ld e d stru ctu re s. T h e y h a v e to re m a in s m a lle r th a n th e c ritic a l c ra c k size, o th e rw is e , th e y w ill c a u se w e ld jo in t fra c tu re in b rittle o r q u a s i-b rittle m a n n er. D is in te g ra tio n o f a w e ld e d stru c tu re is v e ry lik e ly i f h ig h ly lo a d ed w e ld jo in t c o lla p s e s. A s u p e rio r re s is ta n c e o f w e ld jo in t a g a in s t th e fa tig u e c ra c k g ro w th is o f g re a t im p o rta n c e fo r lo n g e r s e rv ic e liv e s o f w e ld e d stru c tu re s.
T h e re s is ta n c e o f m e ta llic m a te ria ls to fa tig u e c ra c k g ro w th a n d b rittle fra c tu re d ep e n d s o n th e m ic ro stru c tu re . P ro p e rtie s o f th e b a s e m e ta l (B M ) m e e t re q u ire m e n ts in sta n d a rd s. T h e w e a k e s t lin k s are a lw a y s w e ld jo in ts c o n s is tin g o f h e a t-a ffe c te d z o n e (H A Z ) an d w e ld m e tal (W M ). M ic ro s tru c tu re o f b o th in a s -w e ld e d c o n d itio n
Corr. Author's Address: Institute "GOŠA", Belgrade, Serbia, [email protected]
re su lts fro m ch e m ic a l c o m p o sitio n a n d w e ld in g
th e rm al cycles.
M ic ro s tru c tu re a n d p ro p e rtie s o f w e ld jo in t re g io n can b e a n a ly se d in d e ta ils b y u sin g sa m p le s o f m a teria ls c u t fro m actu al w e ld jo in ts [2], A lte rn a tiv e is an an a ly sis p e rfo rm e d w ith sa m p le s o f m a teria ls w ith sim u la te d m ic ro stru ctu re. In th is ca se , th e rm al c o n d itio n s d u rin g w e ld in g h a v e to b e s im u la te d e ith e r on sa m p le s o f B M in o rd e r to p re p a re p a rtic u la r H A Z a re a s [3] o r o n sa m p le s o f sin g le -ru n W M in o rd e r to p re p a re p a rtic u la r m u lti-ru n W M are as [4], U s in g c o m b in a tio n o f b o th , i.e. sam ples from w eld jo in t and sam ples w ith sim ulated m icrostructure is an effective approach, too [5],
T h e ex p e rim e n ta l re su lts o f fra ctu re to u g h n e ss m e a su re m e n t a n d fa tig u e -c ra c k g ro w th rate m e a su re m e n t are sh o w n a n d d isc u sse d in th is p a p e r fro m th e le a k -b e fo re -b re a k c o n c e p t p o in t o f v iew . F ra c tu re p ro p e rtie s o f w e ld jo in t re g io n a n d n o n -a ffe c te d b a s e m e ta l w e re d e te rm in e d u sin g th e sp e c im e n s m a c h in e d fro m th e sa m p le s o f m a teria l cu t fro m a rea l w e ld e d plate.
1 E X P E R IM E N T A L S
1.1. Materials and Specimens
T h e steel u se d in th is study is A -3 8 7 G r. 11 C lass 1 ste e l d e s ig n e d fo r o p e ra tio n at e le v a te d te m p e ra tu re s. Its ch e m ic al co m p o sitio n an d b asic m ech a n ica l p ro p ertie s a t ro o m te m p eratu re are
sh o w n in T ab le 1 [6]. A 96 m m th ic k d o u b le -U
sh a p ed w e ld e d te st c o u p o n w a s a v a ila b le fo r the re se a rc h . R o o t p asses w ere d e p o site d by m etal m a n u a l arc w e ld in g (M M A ) w ith c o a te d ele c tro d e L IN C O L N Sl 19G (A W S : E 8 0 1 8 -B 2 ), th e re st o f th e w e ld -g ro o v e w a s fille d a t b o th sid e s by su b m e rg e d arc w e ld in g (S A W ) w ith w ire L IN C O L N L N S 150 a n d flu x L IN C O L N P 230. C h e m ic al c o m p o sitio n s o f b o th co n su m ab les are sh o w n in T ab le 2. B a sic m e ch a n ica l p ro p ertie s for all-w e ld m e tals a t ro o m te m p e ra tu re are sh o w n in T ab le 3 [6]. T h e W M is s tro n g e r th a n B M (o v e rm a tc h in g ).
W e ld in g te ch n o lo g y sp e cific atio n w as p rep a re d a c co rd in g to sta n d ard E N 2 8 8-3 [7], O p e ra tin g te m p eratu re is n o t sp e cifie d in this standard. It is n o t req u ired to test th e b e h a v io u r o f th e w eld jo in ts at th e o p era tin g tem p eratu re.
W e ld jo in t w ith sh a rp in itial w e ld d efe cts in th e W M a n d H A Z a re a s w a s e v a lu a te d in re s p e c t o f th e stru c tu re s a fe o p e ra tio n . T he sp e c im e n s f o r th e fra c tu re to u g h n e s s a s se ss m e n t a n d fa tig u e c ra c k g ro w th ra te m e a s u re m e n t w ere m a c h in e d fro m th e w e ld e d te st c o u p o n as sh o w n in F ig u re 1.
T h re e -p o in t b e n d s p e c im e n s (T P B ) w ere u s e d fo r fra c tu re to u g h n e ss te s tin g at ro o m te m p e ra tu re . T h e ir sh a p e an d d im e n sio n s are s h o w n in F ig u re 2. D u e to sp e c ific d e s ig n o f th e h ig h -te m p e ra tu re c h a m b e r, th e s p e c im e n s lo o k ed lik e c o m p a c t te n s ile sp e c im e n s (C T ) u s e d for fra c tu re to u g h n e s s te s tin g a t o p era tin g te m p e ra tu re . T h e ir sh a p e a n d d im e n sio n s are also s h o w n in F ig u re 2.
T a b le L Chemical composition and basic mechanical properties o f the steel
c
Si M n P S C r M o Y ie ldstress
T e n sile
stren g th E lo n g a tio n
Im p ac t en e rg y
M a ss. °/ M P a %
j
0.15 0 .2 9 0.5 4 0 .0 2 2 0.011 0.93 0 .4 7 325 4 95 35
165
T ab le 2. Chemical composition o f fille r metals
F ille r m e ta l C Si M n P S C r M o
M a ss %
L IN C O L N Sl 19G 0 .08 0 .0 4 5 0.35 0 .0 2 5 0.0 2 5 1.10 0 .5 0 L IN C O L N L N S 150 0.11 0.18 0 .37 0 .0 2 0 0 .0 2 0 1.04 0 .47
T ab le 3. Mechanical properties o f all-weld metals
F ille r m e tal Y ie ld stress T e n sile stren g th E lo n g a tio n Im p a c t e n e rg y
M P a % J
L IN C O L N Sl 19G 505 6 40 23 > 9 5
B e n d in g s p e c im e n s (B ) w e re u se d fo r fa tig u e -c ra c k g ro w th -ra te m e a su re m e n t. T h e ir d im e n sio n s w e re 1 0 x 1 0 x 5 5 m m . T h e ty p e o f all sp e cim en s, th e ir p o sitio n , a n d o rie n ta tio n in th e te st c o u p o n in re la tio n to th e w e ld ax is is c le a rly in d ic a te d in F ig u re 1. T h e n o tc h e s a n d cra ck s w ere lo c a te d in th e B M , W M an d H A Z.
F ig. 1. View o f the welded test coupon with specimens sampling
T h e v a lu e s o f J -in te g ra l w e re c a lc u la te d from re g is te re d d e p e n d e n c e o f fo rc e v e rs u s lo ad - p o in t d is p la c e m e n t ju s t w h e n u n lo a d in g se q u en c e started . T h e d ia g ra m s J - A a w e re p lo tte d u sin g th o se v a lu e s a n d c o rre s p o n d in g c ra c k in c re m en ts. A t first, a re g re s s io n lin e w a s d ra w n in th e lin e a r se ctio n o f e a c h J - A a d ia g ra m . Jic-v alu e , c ritic a l J-in te g ra l, w a s o b ta in e d w ith in te rs e c tin g J - Aa d ia g ra m a n d a p a ra lle l lin e to re g re s s io n line. T h is line in te rse c ts x -a x is at Aa = 0 .15 m m . T h e
d ia g ra m s J - A a a t ro o m a n d o p e ra tin g te m p e ra tu re v a lid f o r th e B M a re s h o w n in F ig s. 3b and 4 b , th e d ia g ra m s v a lid fo r th e W M in F igs. 5b an d 6b, w h e re a s th e d ia g ra m s v a lid fo r th e F1AZ in F ig s. 7b a n d 8b. A v e ra g e v a lu e s o f
d e te rm in e d J lc-v a lu e s e x tra c te d fro m th o se d ia g ra m s are lis te d in T a b le 4 [10].
2 R E S U L T S
2.1. Fracture toughness
F ra c tu re to u g h n e s s w a s e x p e rim e n ta lly
d e te rm in e d a c c o rd in g to th e sta n d a rd s [8] a n d [9]. S in g le -s p e c im e n m e th o d w a s u sed . T h re e s p e c im e n s w e re n o tc h e d a n d p re c ra c k e d in th e B M , th re e in th e W M an d th re e in th e H A Z . T h e s p e c im e n s w e re lo a d e d a n d s u c c e s s iv e ly p a rtly u n lo a d ed at ro o m te m p e ra tu re an d at 5 4 0 °C . T h e re g is te re d slo p e b e tw e e n fo rc e, F, a n d cra ck m o u th o p e n in g d isp la c e m e n t, 8 (C M O D ), in th e c o u rs e o f s p e c im e n u n lo a d in g e n a b le d d e te rm in a tio n o f th e c ra c k siz e , a, an d e v e ry c ra c k in c re m e n t, A a, re sp e c tiv e ly . T h e e x a m p le s o f d ia g ra m s F - 8 p lo tte d a t ro o m a n d o p e ra tin g te m p e ra tu re v a lid fo r th e B M a re s h o w n in F igs. 3 a a n d 4 a, d ia g ra m s v a lid fo r th e W M in F ig s. 5a a n d 6a, w h e re a s d ia g ra m s v a lid fo r th e H A Z in F ig s. 7 a a n d 8a.
T h e v a lu e s o f J -in te g ra l w e re c a lc u la te d fro m re g is te re d d e p e n d e n c e o f fo rc e v e rs u s lo a d - p o in t d is p la c e m e n t ju s t w h e n u n lo a d in g se q u e n c e sta rte d . T h e d ia g ra m s J - A a w e re p lo tte d u sin g th o se v a lu e s a n d c o rre s p o n d in g c ra c k in c re m en ts. A t first, a re g re s s io n lin e w a s d ra w n in th e lin e a r s e c tio n o f ea c h J - A a d ia g ra m . Jjc-v alu e , c ritica l J -in te g ra l, w a s o b ta in e d w ith in te rse c tin g J - Aa d ia g ra m a n d a p a ra lle l lin e to re g re s s io n line. T h is
lin e in te rse c ts x -a x is a t Aa = 0.1 5 m m . T h e d ia g ra m s J - Aa at ro o m an d o p e ra tin g te m p e ra tu re v a lid fo r th e B M are s h o w n in F igs. 3 b a n d 4 b , th e d ia g ra m s v a lid fo r th e W M in F ig s. 5b a n d 6 b , w h e re a s th e d ia g ra m s v a lid fo r the H A Z in F igs. 7b a n d 8b.
A v e ra g e v a lu e s o f d e te rm in e d J lc-v a lu e s e x tra c te d
fro m th o s e d ia g ra m s are lis te d in T a b le 4 [10]. In re s p e c t o f fra c tu re to u g h n e ss th e h ig h e s t
q u a lity a re a o f tre a te d w e ld jo in t is W M w h e re a s th e lo w e s t H A Z . T e s tin g te m p e ra tu re d o e s n o t c h a n g e th is fact.
T a b le 4. Fracture toughness, JIc, plane-strain fracture toughness, KIc, threshold stress-intensity range and parameters o f fatigue crack growth rate C and m
A re a H O o J Ic, k J /m 2 K lc, M P a m '/2 A K th, M P a m '/2 C , n m /c y c le m
B M 63.5 120.9 6.8 4 ■ 1 ( f 3 2.1
W M 20 81.3 133.7 6.8 8 - 10'4 2.8
H A Z 51 .7 106.6 6.7 2 ■ i cr4 3,5
B M 4 3 .4 88 .4 5.9 1 ■10'3 3.0
W M 540 57 .2 101.6 6.2 5 ■10'4 3.5
H A Z 38 ,9 83.7 6.1 3 • i cr4 3.9
a) b )
F ig . 3. Diagrams F - ö and J- A a fo r the B M a t room temperature
a) b )
F
,
k
N
F
,
k
N
a) b)
F ig . 5. Diagrams F - S and J - A a fo r the W M at room temperature
a b
F ig . 6. Diagrams F - S (a) and J - Aa (b) fo r the W M at operating temperature
a b
F ig. 8. Diagrams F - ö (a) and J - / l a (b) fo r the H A Z at operating temperature
2.2. Fatigue-crack growth-rate parameters
F a tig u e -c ra c k g ro w th -ra te p a ra m e te rs w ere e x p e rim e n ta lly d e te rm in e d a c c o rd in g to th e sta n d a rd A S T M E 6 47 [11], C h a rp y -siz e sp e c im e n s w e re n o tc h e d to a d e p th o f 2 m m in th e W M an d H A Z a re a s an d in th e B M . T h e y w e re b e n d -lo a d e d in m o m e n t co n tro l at ro o m te m p e ra tu re an d at 5 4 0 °C o n a h ig h -fre q u e n c y re so n a n t p u lsa to r. R e sista n t fo il-g a u g e s w ere a tta c h e d on th e sp e cim en s in o rd e r to re g iste r c ra c k siz e ch a n g e s d u rin g o sc illa tin g lo a d in g (see F ig u re 9). In th e c o u rse o f su c c e ssiv e sm a ll c ra c k in c re m en ts, th e s tre s s-in te n sity fa c to r ra n g e , AK, w a s k e p t c o n stan t. S im u lta n e o u s c o rre c tio n s o f lo a d in g m o m e n t w e re p e rfo rm e d d u rin g fatig u e c ra c k g ro w th . F a tig u e c ra c k g ro w th ra te , d a /d N , w a s c a lc u la te d as a q u o tie n t o f c ra c k in c re m en t,
Aa, a n d n u m b e r o f c y c le s, N .
F ig . 9. Specimen with firm ly cem ented crack size gauge with indicated bend-loading
T h e fa tig u e c ra c k g ro w th ra te ra p id ly d ec reased w h e n A K w h e n a p p ro a c h in g th e th re s h o ld stress-
in te n sity fa c to r ra n g e , A K th. O n th e o th e r side, w h e n A K c a m e u p to K c-v a lu e fa tig u e c ra c k grew faster.
E x p e rim e n ta l d ia g ra m s d a /d N - AK re p re s e n te d in d o u b le -lo g a rith m ic sc a le at ro o m te m p e ra tu re v a lid fo r th e B M , W M , a n d H A Z are s h o w n in F ig u re 10a a n d th o se at o p era tin g te m p e ra tu re in F ig u re 10b.
T h e lin e a r p o rtio n o f re la tio n s h ip b etw e en th e fa tig u e c ra c k g ro w th ra te a n d stre s s-in te n sity fa c to r ra n g e is k n o w n as P a ris la w [12]:
s = c '<i K >- < "
w h e re c o n s ta n ts C a n d m are m a te ria l-d e p e n d e n t. F a tig u e -c ra c k g ro w th -ra te p a ra m ete rs e x tra c te d fro m d ia g ra m s a re lis te d in T ab le 4
[13].
3 E V A L U A T IO N O F T E S T R E S U L T S
I f m e c h a n ic a l p ro p e rtie s a re av ailab le, p la n e -s tra in fra c tu re to u g h n e s s , K ic, is in d ire ctly d e te rm in e d fro m th e Jic-v a lu e s. T h e fo llo w in g e x p re s s io n is u sed :
(2)
w h e re E is Y o u n g ’s m o d u lu s a n d y P o is s o n ’s n u m b e r.
E s tim a te d v a lu e o f E at te m p e ra tu re o f 5 4 0 °C is 8 2 % o f th e sa m e v a lu e a t room
ro o m te m p e ra tu re a n d 5 4 0 °C fo r th e c a lc u la tio n o f Kic-'values g iv e n in T a b le 4.
V a ria tio n s o f K lc in th e w e ld jo in t re g io n in c o m p a riso n w ith B M are e x tre m e ly sig n ific a n t. C ritica l c ra c k size, a c, w h e n w e ld jo in t fra c tu re s at a fix ed stress lev el is s tro n g ly K lc d e p e n d e n t. By a p p ly in g fo rm u la
K lc = ö - Y ( a ) A/7 t- a c (3)
C ritic a l c ra c k siz e is c a lc u la te d as
( v \2 a , = -1
71 K k
a ■ Y (a) (4)
w h ere Y (a) is sh a p e fa c to r an d a stress n o rm a l to the c ra c k p la n e.
W h en fa tig u e c ra c k g ro w s in a c c o rd a n c e w ith P aris la w , stre s s -in te n s ity fa c to r ra n g e is g re a te r th an th e th re sh o ld s tre s s-in te n sity fa c to r ra n g e ,
AK*-I f th e fa tig u e c ra c k g ro w th rate p a ra m e te rs C an d m are a v a ila b le , th e n u m b e r o f c y c le s to fra ctu re, N , w ill b e d e te rm in e d u sin g E q u a tio n 1. T he fo llo w in g e x p re ssio n h a s to b e so lv ed :
af j m N
}--- 5 _ = C - „ 7 . ( a ) - . J d N (5)
Y ( a ) - a 2 0
w h e re a s an d a f are in itial a n d fin al c ra c k size, re sp e c tiv e ly .
S h a p e fa c to r o f an in fin ite 9 6 -m m th ic k w all w ith d iffe re n t ty p e s a n d siz es o f c ra c k s are
[15]:
> S m all h a lf-e llip tic su rfa c e c ra c k w ith len g th - to -d e p th ra tio 2 .4 2 (F ig u re 1 la ): Y = 0 .7 3 4 > H a lf-e llip tic su rfa c e cra ck w ith le n g th -to -d e p th
ratio 2 .83 p e n e tra tin g h a l f o f th e w a ll-th ic k n e s s (F ig u re l i b ) : Y = 0 .8 6
> H a lf-e llip tic su rfa c e c ra c k w ith le n g th -to -d e p th ra tio 4 .2 a p p ro a c h in g o p p o s ite w all su rfa c e (F ig u re 11c): Y = 1.234
> T h ro u g h -th ic k n e s s c ra c k (F ig u re 1 Id ): Y = l .
4 D IS C U S S IO N W IT H C O N C L U S IO N
I f d u rin g c y c lic lo a d in g an e x is tin g cra ck , p re v io u s ly g e n e ra te d fro m a sh a rp w e ld d e fe c t, h a d g ro w n to th e siz e w h e n a n y w h e re a lo n g th e c ra c k c o n to u r s tre s s -in te n s ity fa c to r a tta in s p la n e stra in fra c tu re to u g h n e s s , K |C-v a lu e , an a b ru p t fra c tu re o f w e ld jo in t w o u ld o c c u r. T h is siz e is d e fin e d as th e c ritic a l c ra c k siz e. I f siz e o f fa tig u e c ra c k a tta in s th e c ritic a l c ra c k siz e , w e ld jo in t w ill fra c tu re . T h e re s u lt ca n b e a c a ta stro p h ic d is in te g ra tio n o f th e w h o le stru ctu re .
AK, M Pa m l/2
a)
AK, M Pa m ',!®r
2ci 2ac
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!
i
i
◄
——
---—►
:
2c,
:
i
•
ar t
'
1
1
* d
1
2c,
—- = 4.20
ai
Fig. 11. Four types o f fatigue cracks: sm all initial half-elliptic surface crack a-, (a), fin a l half-elliptic surface crack af (h), wall-penetrating crack when leakage starts a, (c), critical and non-critical
through-thickness cracks ac,C / (d)
A ssu m e th a t in itial in n e r su rfa c e c ra c k in th e w e ld jo in t a re a is a lre a d y a lo n g crack . F o r th e lin e a r-e la stic tre a tm e n t, th e d e p th o f th is cra ck , ao, h a s to b e m u c h g re a te r th a n th e p la s tic z o n e size, ry (F ig u re 11a).
a ; > 5 0 r y (6)
P la stic z o n e a h e a d o f th e c ra c k tip d e v e lo p s as th e re su lt o f in te ra c tio n b e tw e e n a c ra c k a n d stress. Its siz e d e p e n d s o n th e stress- in te n sity fac to r, K , m a te ria l y ie ld stress, R p, a n d stre s s-fie ld co n fig u ra tio n .
vRry
—— plane stress condition
. 271
— p lane strain condition
. 671:
(
7
)
O p e ra tin g stre s s o f th e stru c tu re d o e s n o t e x c e e d 100 M P a. In n er s u rfa c e c ra c k 5 m m -d e e p fu lfils th e c o n d itio n in E q u a tio n 6. C ra c k s o f th is siz e w e re fo u n d in th e stru c tu re d u rin g re g u la r in- se rv ic e in sp e c tio n s. T h e re fo re , th o s e c ra c k s g ro w in a c c o rd a n c e w ith P a ris law . P r o o f p re s s u re te st p e rfo rm e d a fte r in -s e rv ic e in sp e c tio n s le d to th e stress 10 0 % h ig h e r th a n m a x im u m o p e ra tin g stress.
It is g re a t a d v a n ta g e to o p e ra te a p re ssu re - p la n t d e s ig n e d b a s e d o n le a k -b e fo re -b re a k . F a tig u e -c ra c k p e n e tra tio n th ro u g h th e w a ll ca n b e m a n ife ste d e ith e r b y lo ss o f p re s s u re o r b y leak a g e. T h a n k s to th o s e o b v io u s in d ic a tio n s , an o p e ra to r is c a p a b le to p re v e n t fin a l fra c tu re o f th e p la n t b y s h u ttin g it d o w n . A b ru p t fra c tu re o f
p re s s u riz e d c o m p o n e n t c o u ld b e je o p a r d iz in g for h u m a n liv e s a n d en v iro n m e n t. B e sid e s, a p la n t d e s ig n e d in a c c o rd a n c e w ith le a k -b e fo re -b re a k c o n c e p t a llo w s p e rio d ic a l re p a irs o r re p la c e m e n t o f th e d a m a g e d c o m p o n e n t.
T h e p ro c e d u re is as fo llo w s:
1) C a lc u la tio n o f th e th ro u g h -th ic k n e s s c ra c k size w h ic h re su lts in fra c tu re , a c'hroush (su b s c rip t c d e n o te s c ritic a l siz e fo r fra ctu re).
2 ) C a lc u la tio n o f th e d e p th o f s u rfa c e c ra c k in an in fin ite th ic k w a ll w h ic h re su lts in fracture,
^surface 9 6 - m m t h i c k W a l l
3) C o m p a ris o n o f th e a csurface w ith th e w all th ic k n e s s t. I f a csurface < t, c ra c k w ill not p e n e tra te th e w a ll in a sta b le m a n n e r befo re fra ctu re.
4 ) I f acsmface > t, th e c ra c k w ill p e n e tra te th e w all. D e p th o f c ra c k a p p ro a c h in g th e o p p o site su rfa c e o f th e w a ll, a,, is t, w h e re a s its length 2cj = 4 .2 x a i (s u b s c rip t 1 d e n o te s th e siz e for le a k a g e to start).
5) T h e c ra c k p e n e tra tin g th e w a ll g ro w s fu rth e r a n d b e c o m e s a th ro u g h -th ic k n e s s c ra c k o f le n g th 2 a = 2 Clsurface.
6 ) I f th e c ritic a l c ra c k siz e a cthrough » c csurface, th e re is e n o u g h tim e to n o tic e p re s s u re lo ss or leakage.
w o u ld c e rta in ly p e n e tra te th e w a ll b e fo re fin al fractu re.
W h e n c ra c k w o u ld ap p ro a c h th e o p p o site w all su rfa c e , its h a lf-le n g th o n th e in n e r su rfa c e w o u ld b e Ci = 2 0 2 m m . T h is is n o t m u c h less th an critical th ro u g h -th ic k n e s s c ra c k siz e u n d e r n o rm al o p e ra tin g c o n d itio n in th e B M (2 4 9 m m ) and es p e c ia lly in th e H A Z (223 m m ). W ith o u t an y d o u b t, w e ld jo in t p ro p e rtie s o f th is p la n t sa tisfy th e le a k -b e fo re -b re a k co n c e p t, b u t its o p e ra tin g safety fa c to r is a b it q u e s tio n a b le .
C a lc u la te d c ritica l siz e s o f th e th ro u g h th ic k n ess c ra c k in p ro o f-te s t c o n d itio n are listed in T a b le 5. T h e y are m u c h sm a lle r th a n th e h alf- len g th o f th e c ra c k p e n e tra tin g th e w all. T h e p o ssib ility th a t th e e x is tin g d ee p c ra c k c a n n o t be d e te c te d d u rin g in -se rv ic e in sp e c tio n a fte r w h ich p ro o f-te st s h o u ld b e p e rfo rm e d m a y n o t b e n e g lec ted . C ra c k siz e acthrough in p ro o f-te s t co n d itio n c o u ld b e c ru c ia l fo r fu rth e r safe o p era tio n o f th e p la n t.
F o r h ig h e r re lia b ility , it is b e n e fic ia l to lim it m a x im u m a llo w a b le c ra c k siz e rig o ro u s ly , for in sta n c e , le t th e c ra c k siz e b e o n ly o n e - h a lf o f the w a ll-th ic k n e s s . S tre s s -in te n s ity fa c to r w h ic h is the re s u lt o f th is c ra c k at p r o o f stre s s is K = 67
M P a. T h is lev el o f s tre s s-in te n sity is lo w e r th an th e p la n e -s tra in fra c tu re to u g h n e s s o f B M , W M , an d e v e n H A Z . I f d u rin g in -s e rv ic e in sp e c tio n s a ca refu l e ffo rt w e re m a d e to fin d all c ra c k s on in n e r su rfa c e , sa fe o p e ra tio n o f p la n t w o u ld be g u a ra n te e d .
W h ic h c ra c k s are m o re d a n g e ro u s in th e w e ld jo in t re g io n - lo n g itu d in a l c ra c k s in th e H A Z o r tra n sv e rsa l c ra c k s in th e W M ? T h e se c ra c k s are o fte n th e re s u lt o f h y d ro g e n a tta c k o n h a rd a re a s o f w e ld jo in ts .
In te rm s o f e x p e rim e n ta l d a ta lis te d in T a b le 4 , th e c ra c k s in the H A Z w ill g ro w fa ste r th a n th e c ra c k s in th e W M . In itial c ra c k s in th e H A Z c o u ld g ro w all th e tim e th ro u g h th e m a te ria l w ith th e w o rs t p ro p e rtie s. In itial c ra c k s in th e W M g ro w at first in th e W M , th e n g o a c ro ss H A Z , an d c o n tin u e o u ts id e th e w e ld jo in t in th e B M w h ic h p ro p e rtie s are su p e rio r. C o m p a ris o n o f th e n u m b e r o f c y c le s fo r a n in itial 5 m m -d e e p c ra c k to g ro w to 5 0 % o f th e fin a l c ra c k siz e (a = 2 4 m m ) at ro o m an d o p e ra tin g te m p e ra tu re s is s h o w n in T a b le 6. T h e b a s is is n e c e s s a ry n u m b e r o f c y c le s fo r c ra c k g ro w th in th e H A Z at o p e ra tin g te m p e ra tu re .
T ab le 5. Critical sizes o f part-thickness and through-thickness cracks in normal operating and p r o o f test conditions
A re a H o n
surface
a c Cl a cthrough “ cthrough
o p e ra tin g c o n d itio n p ro o f-te s t c o n d itio n
m m m m
B M
20
864 2 02 4 65 116
W M 1056 2 02 5 69 142
H A Z 671 2 0 2 3 62 90
B M
5 40
4 6 2 2 0 2 2 4 9 62
W M 6 10 2 0 2 3 29 82
H A Z 4 1 4 2 0 2 223 56
T ab le 6. Time n eededfor the crack growth to h a lf o f the fin a l crack size at operating temperature in relation to the time fo r growth in the weakest area o f the w eldjoint region, i.e. H AZ
A re a T, °C N /N o
o p e ra tin g c o n d itio n
B M 51.8
W M 20 8.8
H A Z 4.5
B M 3.8
W M 5 40 1.8
Q u o tie n ts N /N 0 listed in T a b le 6 are a d d itio n a l sa fe ty fa c to rs fo r th e sa fe s tru c tu re o p era tio n . T h e m o st d a n g e ro u s s itu a tio n is th e p re se n c e o f lo n g itu d in a l c ra c k s in th e H A Z , w h ic h g ro w th ro u g h th e H A Z u n d e r c y c lic lo a d in g at te m p e ra tu re o f 5 4 0 °C .
5 R E F E R E N C E S
[1] Z. B u rzić, S. S ed m a k , M . M a n jg o (2 0 0 1 ) T h e a p p lic a tio n o f fra c tu re m e c h a n ic s in w e ld e d jo in t p ro p e rtie s a s se ss m e n t, R IM 2 0 0 1 , B ih ać , pp. 4 5 1 -4 6 0 .
[2] N . G u b e lja k (1 9 9 9 ) F ra c tu re b e h a v io r o f sp e c im e n s w ith su rfa c e n o tc h tip in th e h ea t a ffe c te d z o n e (H A Z ) o f s tre n g th m is m a tc h e d w e ld e d jo in ts , In te rn a tio n a l jo u rn a l o f f r a c tu r e , 100 (2) pp. 155-167
[3] V . G lih a (2 0 0 5 ) T h e m ic ro stru c tu re a n d p ro p e rtie s o f m a te ria ls at th e fu sio n line, [12] M e ta llu rg y , 4 4 (1 ), pp. 13-18.
[4] D . R o jk o , V . G lih a (2 0 0 5 ) S im u la tio n s o f tra n s fo rm a tio n k in e tic s in a m u lti-p a s s w eld , M a te ria ls a n d m a n u fa c tu rin g p ro c e s s e s , 2 0 [13] (5 ), pp. 8 3 3 -8 4 9 .
[5] I. R a k , V . G lih a , M . K o ? a k (1 9 9 7 ) W e ld a b ility an d to u g h n e ss a s s e s s m e n t o f T i- m ic ro a llo y e d o ffsh o re steel. M e ta llu rg ic a l a n d m a te ria ls tra n sa c tio n s A , 2 8 , pp. 199- 2 06.
[6] M . B u rz ić , Z. B u rz ić , J. K u ra i (2 0 0 6 ) T h e p re d ic tio n o f re s id u a l life o f re a c to rs in R N P , C e rtL a b P a n č e v o .
[7] E N 2 8 8 -3 , (1 9 9 5 ) A p p ro v a l o f w e ld in g p ro c e d u re s fo r m e ta llic m a te ria ls -P a rt 3:
W e ld in g p ro c e d u re te sts fo r a rc w e ld in g o f ste e ls, Q u a lity a s s u ra n c e in w eld in g , B e lg ra d e .
[8] A S T M E l 152-91 (1 9 9 5 ) S ta n d a rd T est M e th o d fo r D e te rm in in g J - R C u rv e , A n n u al B o o k o f A S T M S ta n d a rd s, V o i. 0 4 .0 1 . pp. 724.
[9] B S 74 4 8 (1 9 9 1 ) F ra c tu re m e c h a n ic s to u g h n e s s te sts. P a rt 1. M e th o d for d e te rm in a tio n o f K ic c ritic a l C T O D an d c ritic a l J v a lu e s o f m e ta llic m a te ria ls , B S I. [10] M . B u rz ić (2 0 0 7 ) P h D T h e sis, U n iv e rs ity o f
N o v i S ad, D e p a rtm e n t o f T e c h n ic a l F acu lty , S erb ia.
[11] A S T M E 6 4 7 (1 9 9 5 ) S ta n d a rd T e s t M e th o d fo r C o n s ta n t-L o a d -A m p litu d e F a tig u e C rack G ro w th R a te s A b o v e 10'8 m /c y c le , A n n u al B o o k o f A S T M S ta n d a rd s V o i. 03. 0 1 , pp. 7 14.
P .C . P a ris, a n d F. E rd o g a n (1 9 8 2 ) A C ritical A n a ly s is o f C ra c k P ro p a g a tio n L aw s, T rans. A S M E , Jo u rn a l B a s ic E n g ., V o i. 85, N o. 4, p p . 528.
M . B u rz ić , Z. B u rz ić , J. K u ra i, D ž. G ačo (2 0 0 7 ) F a tig u e B e h a v io u r o f A llo y e d Steel fo r H ig h T e m p e ra tu re , F irs t S e rb ia n (2 6 th Y U ) C o n g re s s o n T h e o re tic a l a n d A p p lie d M e c h a n ic s , K o p a o n ik , S erb ia, p p . 1085-
1090.
[14] D IN 4 1 3 3 (1 9 9 1 ) S c h o rn s te in e a u s Stahl, B e rlin .
