FAILURE ANALYSIS OF A FIBERGLASS-REINFORCED PLASTIC PRESSURE VESSEL

S. J. Glass, E. K. Beauchamp, M. Carr, T. R. Guess, S. L. Monroe, R. J. Moore, A. Slavin, and N. R. Sorenson, Sandia Nat ional Laborator ies , Albuquerque, NM 87185.

ABSTRACT

A f i b e r g l a s s - r e i n f o r c e d p l a s t i c (FRP) p re s su re v e s s e l con ta in ing s u l f u r i c acid f a i l e d c a t a s t r o p h i c a l l y i n s e r v i c e . P re l imina ry i n v e s t i g a t i o n s r u l e d out f a i l u r e due t o sabotage and chemical o r mechanical ove rp res su re . Subsequent examinat ion of t h e f iber f r a c t u r e su r faces and measurements of mi r ro r rad i i i n d i c a t e d t h a t fiber f a i l u r e had occurred a t stresses s i g n i f i c a n t l y b e l o w t h e f ibers ' e x p e c t e d s t r e n g t h . F u r t h e r examinat ion by scann ing e l e c t r o n microscopy and energy d i spe r s ive spectroscopy i n d i c a t e d t h a t t h e g l a s s fibers had been exposed t o s u l f u r i c acid, a reagent t h a t corrodes t h i s type of glass and degrades i ts s t r eng th . F i n i t e element a n a l y s i s i n d i c a t e d stresses i n an exposed reg ion of t h e vessel t h a t exceeded t h e s t r eng ths of t h e FRP during normal v e s s e l operation. Numerous c racks were detected i n t h i s region us ing a vicinal optical illumination technique. We concluded that vessel f a i l u r e was caused by p rogres s ive deg rada t ion and r u p t u r e of f ibers s t a r t i n g a t t h e outer surface of t h e FRP and extending inwards and l a t e r a l l y , u n t i l a crack of c r i t i c a l s i z e was produced.

BACKGROUND

A s u l f u r i c acid v e s s e l , p r e s s u r i z e d a t t h e normal o p e r a t i n g p r e s s u r e of 0 -31 MPa ( 4 5 p s i ) , b u r s t du r ing s e r v i c e . T h e vessel was one of two i d e n t i c a l 200 l i t e r , t e f l o n - l i n e d , f i b e r g l a s s - r e i n f o r c e d p l a s t i c (FRP) v e s s e l s used as pa r t of an a c i d d i s t r i b u t i o n system. The FRP vesse l contained s e v e r a l l a r g e cracks from which t h e b u r s t t e f l o n l i n e r bulged a s shown i n Fig. 1A. The even t a l s o s e v e r e l y damaged t h e containment c a b i n e t and showered t h e room w i t h debris a s shown i n Fig. 18. Although t h e room w a s unoccupied a t t h e t i m e of t h e f a i l u r e , a thorough i n v e s t i g a t i o n of t h e event was performed i n t h e i n t e r e s t of equ ipmen t r e l i a b i l i t y and p e r s o n n e l s a f e t y . F o r e n s i c

S. Jill G l a s s June 3, 1995

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

reconstruction of the event, fractographic analysis of the failed vessel, computer modeling of stresses in the vessel walls, and mechanical testing of sections of the failed vessel provided information about the cause of failure.

PRELIMINARY INVESTIGATION

During the preliminary investigation dozens of failure event scenarios were considered, all based on observations or recorded data. At each stage of the investigation, care was taken not to destroy or obscure evidence needed 'by a competing scenario. The various failure scenarios considered were grouped as follows: 0 Sabotage 0 Excessive internal pressure

Failure at elevated pressure due to chemical reaction.

Failure at elevated pressure due to mechanical over-

(Sulfuric acid reacts violently with many compounds, including water)

pressurization 0

0 Failure at normal operating conditions (pressure, temperature, environment)

There was no evidence to suggest that a person had the means or ability to cause this type of failure by sabotage. No evidence was found to uniquely support overpressure by chemical and/or mechanical means. Forensic evidence indicated that the pressure in the failed vessel was not unusually high when it failed. Trajectory analysis of debris, including a level sensor found fifteen feet away, indicated that the failure sequence was consistent with the FEU? cracking, followed by the teflon bladder bulging out of the cracked FRP and bursting. A thorough analysis of the gas system also eliminated the possibility of mechanical over-pressurization. It was concluded that the failure occurred under normal operating conditions.

By design, the FEU? vessels are expected to be able to withstand 200 psi pressure. That, of course, assumes that the vessel retains its integrity. Thus it was assumed that the vessel integrity must somehow have been compromised and subsequent investigation focused on the fracture surfaces, the mechanical properties of the FRP material, and the stress state in the vessel under normal operating pressure.

FRACTOGRAF'HIC ANALYSES

Following an in situ examination of the ruptured FRP vessel, a three by twenty inch section (Fig. lA, section in dashed lines) shown in Fig. 2, containing the primary fractures, was cut from the vessel for detailed examination. The axial location of these primary fractures is near the region on the vessel that was supported/constrained by the metal cradle and rubber gasket (Fig. 1 A ) . The section of composite material containing the primary cracks is constructed from, nominally, seven layers (each -0.05 cm

S. Jill Glass 2 June 3, 1995

t h i c k ) of f i b e r - r e i n f o r c e d p las t ic with a l t e r n a t i n g + and -32O oriented fibers, termed cross pl ies . The cross p l i e s a r e surrounded by a c i r cumfe ren t i a l (circ) wrap (-0.23 cm thick) of f i b e r - r e i n f o r c e d p las t ic around t h e c y l i n d r i c a l p o r t i o n of t h e ves se l . The c r o s s pl ies support t h e a x i a l stresses i n t h e v e s s e l and t h e circ wraps support t h e hoop stresses.

Under normal condi t ions , ove r s t r e s sed FRP f a i l s by a combination of epoxy ma t r ix c racking and g l a s s f i b e r pul l -out . Where f i b e r s are p a r a l l e l o r o r i e n t e d a t some sma l l angle t o t h e p r i n c i p a l stress d i r e c t i o n , t h e f r a c t u r e s u r f a c e w i l l be very rough wi th ex tens ive f i b e r pu l l -out . I f t h e g l a s s f i b e r s a r e s u b s t a n t i a l l y weakened, l i t t l e o r no f iber pul l -out occurs and t h e f r a c t u r e s u r f a c e w i l l be p l a n a r because t h e f a i l u r e mode is c o n t r o l l e d by t h e mat r ix s t rength . Where f i b e r s are o r i en ted perpendicular t o o r almost perpendicular t o t h e p r i n c i p a l stress d i r e c t i o n , as t h e y are i n t h e circ wrap, t h e f r a c t u r e s u r f a c e w i l l a l s o be p l a n a r with no pro t ruding fibers.

Macroscopic examination of t h e primary f r a c t u r e su r face revea led two areas (Fig. 2, A r e a s A and B) of p l ana r c i r cumfe ren t i a l ( o r hoop) f r a c t u r e , normal t o t h e l o n g i t u d i n a l axis of t h e ves se l , extending almost completely through t h e circ wrap and c r o s s plies as shown i n t h e schematic i n Fig. 3A. If t h e v e s s e l i s exposed t o s u f f i c i e n t l y high a x i a l t e n s i l e stresses, t h i s type of p l a n a r hoop crack is expected i n t h e circ wrap (a r e s u l t of i n t e r f a c i a l f a i l u r e between t h e f i b e r s and t h e epoxy ma t r ix ) , bu t no t i n t h e c r o s s plies. F rac tu re i n t h e c ros s plies should e x h i b i t ex t ens ive f iber pul l -out producing a f r a c t u r e s u r f a c e t h a t i s n o t p l a n a r (Fig. 3B). The p l a n a r f r a c t u r e s u r f a c e of t h e c r o s s ply reg ion was t h e first evidence t h a t t h e v e s s e l had been compromised i n some way t h a t weakened t h e f i b e r s .

Areas A and B i n Fig. 2 w e r e examined us ing o p t i c a l microscopy and scanning e l e c t r o n microscopy (SEMI. SEM examination of a replica, t a k e n from f r a c t u r e A, p rovided ev idence t h a t t h e f r a c t u r e propagated from t h e e x t e r i o r toward t h e i n t e r i o r of t h e ves se l . Ind iv idua l f i b e r s , i n a l l of t h e c r o s s p l i e s , e x h i b i t e d v e l o c i t y hackle (Fig. 41, t h a t i n d i c a t e d t h e d i r e c t i o n of c rack propagat ion was from t h e e x t e r i o r t o t h e i n t e r i o r . The f r a c t u r e m i r r o r r a d i i a l s o provided an e s t i m a t e of t h e f i b e r f a i l u r e stress .' Calcula ted f a i l u r e stresses of t h e s e f i b e r s ranged from 620 t o 1207 MPa (90 t o 175 k s i ) , s u b s t a n t i a l l y lower than t h e nominal 3100 MPa (450 k s i ) s t r e n g t h f o r E-glass f i b e r s used i n t h e ves se l cons t ruc t ion .

P l a n a r f r a c t u r e r e g i o n s w e r e bounded by f r a c t u r e r e g i o n s e x h i b i t i n g ex tens ive f iber pul l -out , which w e r e l i k e l y c r e a t e d dur ing t h e f i n a l moment of ves se l rup ture a s t h e regions of p lanar f r a c t u r e l i n k e d t o g e t h e r . Fibers i n t h e s e l i n k i n g r e g i o n s e x h i b i t e d t h e mir ror r a d i i c h a r a c t e r i s t i c of f a i l u r e stresses of 3100 MPa. T h e s e o b s e r v a t i o n s sugges ted t h a t t h e v e s s e l was

S. Jil l Glass 3 June 3, 1995

f a b r i c a t e d from E-glass f i b e r s of t h e correct s t r e n g t h l e v e l , bu t t h a t t hey had been degraded and weakened i n some regions.

A p o r t i o n of f r a c t u r e B was examined using energy d i s p e r s i v e spec t roscopy (EDS) t o o b t a i n chemical i n fo rma t ion on f i b e r composition and on t h e presence of co r ros ive agents . Because t h e f r a c t u r e s u r f a c e had been f looded with s u l f u r i c acid d u r i n g t h e v e s s e l r u p t u r e and wi th aqueous s o l u t i o n s d u r i n g cl’ean-up, unequivocal chemical data could not be obta ined from t h i s area. Areas were sought f o r examination where c racks would not have been exposed t o p a r t i c u l a t e debris, ac id , o r aqueous s o l u t i o n s . Chemical ana lyses were a l s o performed on t h e f r a c t u r e s u r f a c e s of f l e x u r e test specimens (described elsewhere i n t h i s paper) t h a t were obta ined from chemically uncompromised regions of t h e ves se l . One specimen was selected f o r t e s t i n g because it w a s l o c a t e d i n t h e c r a d l e reg ion and conta ined a hoop crack i n t h e circ wraps. This p r e - e x i s t i n g c rack did not ex tend completely through t h e specimen. The specimen w a s loaded i n f l e x u r e such t h a t t h e e x t e r i o r su r face conta in ing t h e crack was placed i n tens ion . This caused t h e p re -ex i s t ing crack t o propagate t o completion through t h e innermost c ros s p l i e s .

Figure 5 shows t h e fracture su r faces of a f l e x u r e specimen which did n o t c o n t a i n a p r e - e x i s t i n g c r a c k (A) and t h e specimen described above t h a t did (B). The d i r e c t i o n of crack propagat ion i s from t h e bottom of t h e f i g u r e proceeding towards t h e t o p . F i g u r e SA shows no p l a n a r f r a c t u r e i n t h e c r o s s p l y r eg ions (Area 2 ) . F i g u r e 5B shows t h e two types of macroscopic f r a c t u r e f e a t u r e s i n t h e c ros s plies, i.e., t h e pre-ex is t ing crack e x h i b i t s p l a n a r f r a c t u r e ( F i g . SB, Area 11, whereas t h e f r a c t u r e s u r f a c e produced d u r i n g t e s t i n g e x h i b i t s t h e expected f iber p u l l - o u t ( F i g . SB, Area 2 ) .

SEM examination of t h e f r a c t u r e s u r f a c e of t h e f l e x u r e b a r with t h e p r e - e x i s t i n g c rack was performed. The p r e - e x i s t i n g c rack contained p l ana r f r a c t u r e both i n t h e circ wrap and i n some of t h e c r o s s pl ies immediately adjacent t o t h e circ wrap. The remainder of t h e c r o s s p l i e s , which f a i l e d during f l e x u r e t e s t i n g , exh ib i t ed f i b e r pu l l -out ( F i g . 6A) and t h e f i b e r s i n t h i s region f a i l e d a t h igh stresses. EDS v e r i f i e d t h e presence of Ca and A 1 as major c o n s t i t u e n t s o f t h e E-g lass f ibers ( F i g . 6B). T h i s EDS information can be con t r a s t ed w i t h t h e EDS spectrum from m a t e r i a l from t h e p l a n a r reg ion of f r a c t u r e ( F i g . 7A) . F i b e r s i n t h i s reg ion fa i led a t low stresses (-689 MPa)) . EDS of t h e s e f i b e r s ( F i g . 7B) i n d i c a t e d Ca and A 1 deple t ion .

The des ign of t h e acid v e s s e l i nco rpora t e s a t o p cover, provide suppor t f o r p ipes and va lves . During r o u t i n e va lve maintenance s u l f u r i c acid was spi l led on to t h e t op of t h e vessel and subsequent ly r i n s e d o f f with water. The f i t of t h e t o p cover on t h e vessel provided a m e c h a n i s m t o t r a p acid and/or acid r i n s e water i n t h e v i c i n i t y of t h e circ wrap. Given t h i s opportuni ty f o r

S. Jil l Glass 4 June 3, 1995

exposure of t h e f i b e r s t o s u l f u r i c acid, it is q u i t e probable t h a t Ca and A 1 w e r e l eached from t h e f i b e r s by t h e a ~ i d . ~ , ~ A c i d d i s s o l u t i o n o f Ca and A 1 s i g n i f i c a n t l y reduces t h e t e n s i l e s t r e n g t h of E-glass fibers, c o n s i s t e n t with o u r f r a c t o g r a p h i c measurements. Fur ther a n a l y s i s of depos i t s on t h e f r a c t u r e su r face i n d i c a t e d t h e presence of Ca(S0412, a l i k e l y product of prolonged exposure of t h e material t o s u l f u r i c ac id .

The fa i led vessel was examined f o r c racks i n reg ions o t h e r t han t h e pr imary f r a c t u r e s u r f a c e us ing a s i m p l e op t ica l i n s p e c t i o n technique . A fiber o p t i c l i g h t sou rce is p l a c e d against t h e s u r f a c e of t h e t r a n s l u c e n t FRP vessel. I n t h i s manner, l i g h t i s i n t e r n a l l y scattered. With t h i s v i c i n a l i l l u m i n a t i o n , c r a c k s o r i e n t e d normal t o t h e s u r f a c e scattered t h e l i g h t and w e r e observable as shadows i n t h e ha lo around t h e fiber o p t i c l i g h t source as shown i n Fig. 8. Magnif icat ion of 10-20X is h e l p f u l , b u t no t necessary f o r d e t e c t i n g t h e cracks. Using t h i s technique cracks w e r e l oca t ed i n regions of the vesse l away from t h e primary c rack and i n t h e companion un fa i l ed vesse l .

C i r c u m f e r e n t i a l c r a c k s w e r e found i n t h e circ wraps of b o t h ves?e ls . The c racks w e r e always loca ted i n t h e same a x i a l region; wi th in f ive c m of t h e upper "knuckle" t r a n s i t i o n from t h e dome t o p t o t h e c y l i n d r i c a l wa l l of t h e ves se l (see F i g . 1). T h i s reg ion i s very close t o , o r may inc lude t h e t o p of t h e gasketed upper c r a d l e support band. The c racks va r i ed i n l eng th from a f r a c t i o n of a c m t o more than 25 c m i n length. There were i n d i c a t i o n s t h a t some c r a c k s had j o i n e d with neighbors t o form longe r c racks . Severa l c racks in t e rcep ted t h e edge of a c u t s e c t i o n and appeared t o p e n e t r a t e t h e circ wraps. None of these cracks pene t r a t ed t h e e n t i r e w a l l , s o as t o be v i s i b l e on t h e i n t e r i o r sur face . No hoop cracks w e r e found on t h e i n t e r i o r of t h e v e s s e l o r a t any o t h e r l o c a t i o n on t h e e x t e r i o r sur face .

F INITE ELEMENT ANALYSIS

A f i n i t e element a n a l y s i s of t h e pressure vessel w a s performed t o estimate t h e stress s ta te p r i o r to f a i l u r e . T h e stress es t imates , a long w i t h expe r imen ta l ly determined m a t e r i a l s t r e n g t h s , w e r e e s s e n t i a l f a c t o r s i n t h e de te rmina t ion of t h e v e s s e l f a i l u r e scenar io . The p res su re v e s s e l was modeled as axisymmetric thereby neg lec t ing any smal l asymmetries i n t h e area of p e n e t r a t i o n s i n t h e t o p of t h e v e s s e l . 4308 four-noded q u a d r i l a t e r a l e lements , each w i t h f o u r i n t e g r a t i o n p o i n t s , w e r e used i n t h e model. To a c c u r a t e l y r e s o l v e t h e stresses through t h e t h i c k n e s s of t h e ves se l and i n t h e f a i l e d region, t h e t e f l o n l a y e r was modeled with two elements through t h e th i ckness , t h e c r o s s pl ies wi th t h r e e elements through t h e thickness , and t h e circ wrap w a s modeled with f o u r e lements through t h e th i ckness . F igures 9 and 1 0 show t h e model geometry and m e s h . I n modeling t h e o r tho t rop ic p r o p e r t i e s of t h e FRP, appropr i a t e p r o p e r t i e s w e r e ass igned t o both t h e ( c ros s

S. Jil l Glass 5 June 3, 1 9 9 5

plies) and (circ wrap) . The t e f l o n l i n e r w a s modeled as an i s o t r o p i c m a t e r i a l . A l l m a t e r i a l s were assumed t o be l i n e a r l y elastic. The c r a d l e w a s modeled by r a d i a l l y cons t ra in ing the model i n t h e region of t h e cradle gasket . The app l i ed loading w a s a n e t i n t e r n a l p re s su re of 0.31 MPa. The a n a l y s i s was run on a CRAY YMP us ing t h e commercial f i n i t e element a n a l y s i s code ABAQUS ver s ion 4 . 9 . 4

The a n a l y s i s shows a region of high a x i a l o r l ong i tud ina l t e n s i l e stress ( -49 MPa (7100 ps i ) ) j u s t above t h e c r a d l e gaske t i n t h e reg ion where c r a c k s w e r e observed. This h igh stress reg ion is shown i n Fig. 11. Such stresses would t e n d t o cause c racking i n t h e circ w r a p i f i t s s t r e n g t h is a t or below t h i s l e v e l .

MECHANICAL TESTING

F l e x u r a l s t r e n g t h tests ( th ree -po in t bend) were conducted t o ob ta in t h e s t r e n g t h of uncracked composite ma te r i a l i n t h e region where e l e v a t e d t e n s i l e stresses w e r e predicted ( j u s t above t h e m e t a l cradle). F lexura l s t r e n g t h specimens were a l s o m a d e from cracked m a t e r i a l (cracks l o c a t e d by v i c i n a l i n spec t ion technique) i n t h i s region t o compare t h e f racture surfaces. Specimens were 5.1 mm t h i c k by 22.9 mm wide by 8 3 . 8 mm long. Specimens were c u t from t h e v e s s e l and loaded such t h a t t h e circ wrap l a y e r s from t h e e x t e r i o r o f t h e vessel w e r e i n t e n s i o n , w i t h t h e glass r e in fo rcemen t fibers o r i e n t e d p e r p e n d i c u l a r (normal) t o t h e app l i ed stress. The span w a s 6 9 . 9 xtun and specimens were loaded a t 0.13 mm/min.

The i n i t i a l f a i l u r e mode w a s ma t r ix c racking i n t h e c i rc wrap. Under cont inued loading, t h e crack continued t o propagate through t h e mat r ix of t h e c i rc wrap u n t i l it reached t h e circ wrap/cross p l y i n t e r f a c e . Here t h e c r a c k e i t h e r changed d i r e c t i o n and produced de laminat ion of t h e circ wrap/cross p l y i n t e r f a c e , o r cont inued a long a t o r t u o u s pa th through t h e c r o s s plies, wi th ex tens ive f i b e r pu l l -out . The average t e n s i l e stress t o i n i t i a t e mat r ix c racking i n uncracked specimens was 55 k 9 MPa (8000 4 1325 p s i ) . This value f a l l s wi th in t h e repor ted range of s t r e n g t h s f o r t h i s t y p e of m a t e r i a l (28-62 MPa). Th i s measured range of s t r e n g t h s is a l s o wi th in t h e predicted stress l e v e l of 4 9 MPa (7100 p s i ) . Thus t h e t e n s i l e stresses developed under normal ope ra t ing cond i t ions ( 0 . 3 1 MPa i n t e r n a l pressure), i n t h e region of t h e c i rc wraps a d j a c e n t t o t h e cradle, a r e l i k e l y t o have produced mat r ix cracking. Such cracks would then provide access t o c o r r o s i v e acid t h a t a t t a c k e d t h e glass f i b e r s , degrading t h e i r s t r e n g t h . During each p r e s s u r i z a t i o n cyc le weakened g l a s s f i b e r s f a i l e d , producing a slowly growing crack, which provided f u r t h e r access t o acid, and f a i l u r e of a d d i t i o n a l f i b e r s . When t h e crack reached c r i t i c a l p ropor t ions , it propagated c a t a s t r o p h i c a l l y and produced vessel f a i l u r e .

S. Jil l Glass 6 June 3, 1995

As mentioned i n t h e s e c t i o n desc r ib ing t h e f l e x u r e specimens used f o r EDS a n a l y s i s of glass f i b e r s , when specimens from cracked and uncracked regions of t h e v e s s e l i n t h e v i c i n i t y of t h e circ wrap w e r e examined, two types of f r a c t u r e s u r f a c e s w e r e observed. I n t h e p rev ious ly uncracked specimen, t h e f r a c t u r e s u r f a c e of t h e c r o s s p l y reg ion e x h i b i t e d t h e expected non-planar f r a c t u r e with e x t e n s i v e f iber debonding and p u l l - o u t (Fig. 3B). I n t h e specimens e x h i b i t i n g pre-cracks i n t h e circ wrap, t h e f r a c t u r e surface of t h e c r o s s p l y region exh ib i t ed p l ana r f r a c t u r e with no f i b e r pu l l -out ad jacent t o t h e circ wrap, bu t non-planar f r a c t u r e with extensive f i b e r pu l l -out i n t h e remainder of t h e cross p l y (see F i g . 3 A ) . The arrangement (sequence) of t h e s e two types of f r a c t u r e s u r f a c e s w a s s i m i l a r t o t h a t observed on t h e primary c racks of t h e f a i l e d vessel. The presence of p l a n a r f r a c t u r e i n t h e cross p l y m a t e r i a l c l e a r l y demonst ra tes t h e e x i s t e n c e of reg ions of ex tens ive crack growth i n t h e cross pl ies under normal o p e r a t i n g p res su res . Thus o t h e r c racks existed t h a t could have even tua l ly caused f a i l u r e ; t h e a c t u a l f a i l u r e w a s no t a f luke o r t h e r e s u l t of a unique f l a w , b u t r a t h e r was s imply t h e first cracked region t o reach c r i t i c a l dimensions.

The proposed f a i l u r e s c e n a r i o and s u p p o r t i n g ev idence a r e summarized as follows:

There w e r e two primary c i r c u m f e r e n t i a l through-cracks j u s t above t h e steel c rad le . The primary c racks w e r e o r i e n t e d p a r a l l e l t o t h e c i rc wrap d i r e c t i o n and t h e f i b e r s i n t h e circ w r a p . The f r a c t u r e su r face of t h e primary crack was p lanar even i n t h e c r o s s p l i e s , with no evidence of f i b e r debonding o r pu l l -out . The o r i e n t a t i o n of m i r r o r , m i s t , and hack le f e a t u r e s on g l a s s f i b e r f r a c t u r e su r faces from t h e p lanar region of t h e f a i l e d c r o s s pl ies i n d i c a t e d t h a t c racks i n i t i a t e d a t t h e o u t e r su r f ace of t h e v e s s e l and propagated inward. Fractographic evidence revea led t h a t t h e load-bearing f i b e r s on p l a n a r f r a c t u r e s u r f a c e s i n t h e cross p l y reg ion f a i l e d a t 1/4 t o 1 /2 of t h e i r nominal s t r e n g t h , suggesting t h e p o s s i b i l i t y t h a t e n v i r o n m e n t a l a t t a c k and s t r e n g t h degradat ion had occurred. Maintenance was performed on t h e va lves on t h e t o p of t h e ves se l , which r e s u l t e d i n s u l f u r i c a c i d be ing spi l led and washe9 down onto t h e region of t h e circ wraps. Microanalysis da t a showed t h a t t h e Ca and A 1 conten ts of t h e weak E-glass fibers i n t h e p l a n a r crack growth a r e a w e r e s i g n i f i c a n t l y reduced from t h a t of f u l l s t r e n g t h f ibers e l sewhere . The d e p l e t i o n l e v e l s and r e s u l t i n g s t r e n g t h degradat ion were s i m i l a r t o those repor ted i n t h e l i t e r a t u r e f o r fibers of similar E-glass compositions a f te r c o n t r o l l e d exposure t o s u l f u r i c ac id . Frac tographic evidence i n d i c a t e d t h a t fibers which f a i l e d d u r i n g t h e f i n a l ove r load even t (away from t h e i n i t i a l

S. Jil l Glass 7 June 3, 1995

e

e

e

p l a n a r cracks) and dur ing subsequent l abora to ry tests failed a t f u l l s t r eng th . F i n i t e element c a l c u l a t i o n s were performed f o r t h e vessel p r e s s u r i z e d t o 0.31 MPa under two cond i t ions : wi th and without a stiff cradle. The radial c o n s t r a i n t provided by t h e cradle produced a bending moment which resulted i n a t e n s i l e stress of approximate ly 48 MPa i n t h e a x i a l d i r e c t i o n of t h e vesse l , on t h e o u t e r su r f ace , j u s t above t h e cradle. This stress was t r a n s v e r s e t o t h e glass f i b e r s i n t h e circ wrap. Without t h e c o n s t r a i n t of t h e c rad le , a stress of t h i s magnitude does not develop. Tens i l e tests were conducted on undamaged m a t e r i a l c u t from a reg ion of t h e f a i l e d v e s s e l away from t h e primary c racks . That m a t e r i a l had an average s t r e n g t h of 55 f 9 MPa, confirming t h a t c racks could form i n t h e c i rc wrap a t t h e t e n s i l e stress l e v e l s produced by t h e r a d i a l c o n s t r a i n t of t h e stiff steel c rad le . Cracking of t h e ma t r ix m a t e r i a l of t h e c i rc wrap a t t h e predicted stress l e v e l s provided a c c e s s of acid t o t h e fibers i n t h e c r o s s plies. Subsequent c racking of t h e c r o s s plies proceeded non-catastrophical ly as acid-weakened fibers fa i led s e q u e n t i a l l y a t t h e c rack t i p d u r i n g subsequent

e A sample taken from an a rea which contained c i r cumfe ren t i a l c racks , bu t away from t h e primary f r a c t u r e , was broken i n a th ree-poin t bend test. The f r a c t u r e su r face revealed a crack t h a t p e n e t r a t e d about h a l f way through t h e load-bearing c r o s s plies. The p l ana r cha rac t e r of t h i s pa r t i a l c rack was s i m i l a r t o t h a t observed on t h e two f l a t areas of t h e primary cracks. One o r more c racks growing through t h e c ros s plies above t h e cradle reached t h e c r i t i ca l s i z e f o r c a t a s t r o p h i c c rack propagation, r e s u l t i n g i n ves se l f a i l u r e a t normal ope ra t ing pressure .

- p r e s s u r i z a t i o n cyc les .

SUMMARY

A f a i l u r e i n v e s t i g a t i o n was conducted on a ruptured s u l f u r i c a c i d v e s s e l . F o r e n s i c r e c o n s t r u c t i o n of t h e even t , f r a c t o g r a p h i c a n a l y s i s of t h e f a i l e d ves se l , computer modeling of stresses i n t h e v e s s e l w a l l s , and mechanical t e s t i n g of s e c t i o n s of t h e failed v e s s e l provided information about t h e probable cause of f a i l u r e . It w a s concluded t h a t t h e f a i l u r e occurred a t normal o p e r a t i n g cond i t ions and was t h e r e s u l t of a l o c a l i z e d high t e n s i l e stress caus ing c r a c k i n g of t h e matr ix , l e a d i n g t o acid-exposure and weakening of load-bearing g l a s s f ibers i n t h e FRP. Sequen t i a l f a i l u r e o f weakened f ibe r s led t o c r a c k growth d u r i n g p r e s s u r i z a t i o n c y c l e s u n t i l one c r a c k reached t h e c r i t i c a l d imens ions f o r c a t a s t r o p h i c f a i l u r e . Ceramic f r a c t o g r a p h i c t e c h n i q u e s and v i c i n a l i l l u m i n a t i o n w e r e key e l e m e n t s i n r econs t ruc t ing t h e f a i l u r e scenario.

S. Jil l Glass 8 June 3, 1995

REFERENCES

1. J. J. Mecholsky, "F rac tu re Sur face Analys is of Optical F ibers , " Engineered Mater ia l s Handbook, Vol. 4, Ceramics and Glasses, ASM 663-668 (1991) .

2. B. D a s , B. D. Tucker, and J. C. Watson, " A c i d Corrosion Analys is of F i b r e Glass," J. Mater. Sc i . 26 1241 -6606-12 (1991).

3. B. D. Caddock, K. E. Evans, and I. G Masters, "Diffusion Behaviour of t h e Core-Sheath S t r u c t u r e i n E-Glass F i b r e s Exposed t o Aqueous HC1," J. Mater. Sc i . 24 [ l l ] 4100-4105 (1989).

4. ABAQUS Version 4.9, H i b b i t t , Karlsson, and Sorensen, Inc.

ACKNOWLEDGMENTS This work was performed a t Sandia National Labs, supported by t h e U.S. Department of Energy Under Contract #DE-ACO4-76DPOO789(??) and as p a r t of Cooperative Research and Development Agreement 01082 with SEMATECH. The au thors acknowledge SEMATECH, I n t e l Corporation, and Fluoroware, Inc. for p e d s s i o n t o publ i sh and p resen t t h e r e s u l t s of t h i s i nves t iga t ion . The au thors a l s o thank t h e o t h e r Sandia p a r t i c i p a n t s i n t h i s i nves t iga t ion inc luding Mark Davis, John Geiske, Richard Grant, Fred Greulich, Chuck Gwyn, W i l l Hareland, Paul Hlava, A l i c e Kilgo, Bonnie McKenzie, Joe Michael, Gordon Pike, Ken Schuler, and Mark Stavig.

KEYWORD LIST (3-8) Fractography, f i b e r g l a s s , epoxy, f a i l u r e ana lys i s , case study, acid a t t a c k , f i n i t e element ana lys i s , v i c i n a l i l lumina t ion

S. Jil l Glass 9 June 3, 1995

LIST Fig.

F i g .

F i g .

F i g .

F i g .

F i g .

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OF FIGURES 1A. The FRP vessel contained s e v e r a l large cracks from which t h e

burs t t e f l o n l i n e r bulged. The region ind ica t ed with dashed l i n e s was c u t from t h e v e s s e l fo r f r a c t u r e su r face examination (Fig. 2 ) . The FRP v e s s e l f a i l u r e a l s o severe ly damaged t h e cabine t and showered t h e room with deb r i s . (no t s u r e whether w e have a '

photo of t h i s ) , Following an i n s i t u examination of t h e ruptured vesse l , a

. t h r e e . b y twenty inch s e c t i o n ( sec t ion i n dashed l i n e s i n Fig. l), conta in ing t h e primary f r ac tu res , was c u t from t h e vessel f o r d e t a i l e d examination. Macroscopic examination of t h e primary f r a c t u r e su r faces revea led two a reas ( A r e a s A and B) of p l ana r c i rcumferent ia l ( o r hoop) f r a c t u r e , normal t o t h e long i tud ina l axis of t h e vesse l , extending almost completely through t h e circ wrap and cross plies. Schematic c r o s s s e c t i o n of t h e p l ana r f r a c t u r e su r face observed for t h e circ wrap and c ross plies of Area A. Schematic cross s e c t i o n of t h e expected f r a c t u r e su r face topography for t h e circ wrap and c ross plies.

d i r e c t i o n of crack propagation (note arrows) . The f r a c t u r e su r face of a f l e x u r e specimen which did not conta in a pre-ex is t ing crack. There is no p lanar f r a c t u r e i n t h e c ros s p l y regions ( A r e a 2 ) . T h e f r a c t u r e su r face of a f l e x u r e specimen which contained a pre-ex is t ing c rack i n t h e circ wrap. The d i r e c t i o n of crack propagat ion i s from t h e bottom of t h e f i g u r e t o t h e top. The photo shows t h e two t y p e s of macroscopic f r a c t u r e f e a t u r e s i n t h e c r o s s p l i e s , i .e., t h e p re -ex i s t ing c rack e x h i b i t s p l a n a r f r a c t u r e ( A r e a 11, whereas t h e f r a c t u r e su r face produced dur ing t e s t i n g e x h i b i t s t he expected fiber pul l -out ( A r e a 2) . SEM micrograph of a f r a c t u r e d f i b e r from a region of f r a c t u r e e x h i b i t i n g ex tens ive f i b e r pul l -out . EDS v e r i f i e d t h e presence of Ca and A 1 as major cons t i t uen t s of E-glass fibers i n t h e region of F i g . 6A. SEM micrograph of a f r a c t u r e d f i b e r from a region of f r a c t u r e e x h i b i t i n g p lanar f r a c t u r e (Fig. 5B, Area 1). EDS of f i b e r from planar f r a c t u r e region ind ica t ed dep le t ion of Ca and A 1 r e l a t i v e t o t h e EDS spectrum i n Fig. 6B.

su r face s c a t t e r t h e l i g h t and a r e observable a s shadows i n t h e halo around t h e f i b e r o p t i c l i g h t source (see arrows) .

9 F i n i t e element model geometry and mesh of t h e FRP v e s s e l and t e f l o n l i n e r .

1 0 D e t a i l of t h e f i n i t e element mesh of FRP vesse l and t e f l o n l i n e r .

11 F i n i t e element a n a l y s i s model showing stresses i n FRP v e s s e l wal l . Regions of high t e n s i l e stress (-49 MPa) w e r e found above t h e r a d i a l c o n s t r a i n t of t h e c rad le , and correspond t o t h e cracked regions of t h e ves se l .

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DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied. or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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