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Hyb r id com posi tes based onpo lyethy lene and carbon f ib resPa rt 3: Im pac t resistant structuralcomposites through dam ageman agemen tA.A .J.M . PEIJS, R.W. VENDERBOSCHand P.J. LEMSTRA(Eindhoven University of Technology, The Netherlands)D a m a g e t o le r a n c e a n d i m p a c t r e si st a nc e h a v e b e c o m e k e y p a ra m e t e r s f o rc o m p o s i t e m a t e r i a l s in s tr u c t u r a l a p p l ic a t i o n s . I n t h i s p a p e r a t o u g h e n i n gc o n c e p t fo r s t r u c tu r a l c o m p o s i t e s b a s e d o n t h e h y b r i d i z a t i o n o f c a r b o n f i b r e sw i t h h i g h p e r f o r m a n c e p o l y e t h y l e n e (H P-P E) i b r e s i s p r e s e n t e d . I m p a c tb e h a v i o u r o f h y b r i d H P -P E /c arb on la m i n a t e s w a s s t u d i e d u s i n g a f a l l in gw e i g h t i m p a c t t es t . T h e e f fe c t o f th e a d d i t i o n o f H P-PE i b r e s a s w e l l a s th ee f fe c t o f t h e a d h e s i o n l e ve l o f th e s e f i b r e s o n t h e i m p a c t r e s i s ta n c e o f h y b r i dH P-PE/c a rbon s t r uc t u r es w as i nv e s t i ga t ed . H y b r i d i z a t i on r es u l t s in s t ruc t u r a lc o m p o s i t e s e x h i b i t i n g a s i g n i f i c a n t l y b e t t e r re s i st a n c e t o i m p a c t d a m a g et h a n a l l - c a r b o n l a m i n a t e s d u e t o a c h a n g e i n e n e r g y a b s o r p t i o n m o d e . A f t e rh y b r i d i z a t i o n m o r e e n e r g y i s s t o r e d i n t h e H P-PE c o m p o n e n t a n d c o n s e -q u e n t l y l es s e n e r g y i s a v a i l a b l e f o r d a m a g e i n t h e s t ru c t u r a l c a r b o n c o m p o -n e n t , r e s u l t i n g i n a r e d u c t i o n i n i m p a c t d a m a g e a n d i m p r o v e d p o s t - i m p a c tp r o p e r t i e s .K e y w o r d s : comp os i te ma te r i a l s ; hyb r i ds ; po l ye thy lene f ib res ; ca rbonf ib res ; epox y res ins ; imp act tes t ing ; laminates ; res idua l s t rength

T h e u s e o f c a r b o n / e p o x y c o m p o s i te s a s s t r u c tu r a lm a t e r i a l s d e v e l o p e d t o r e p l a c e m e t a l s h a s b e c o m e w e l le s t a bl i sh e d . C o m p o s i t e s b a s e d o n e p o x y m a t r i c e s h a v es e v e ra l a t t r a c t i v e f e a t u r e s . Th e s e r e s i n s y s t e m s sh o w ag o o d c o m p a t i b i l i t y w i t h c a rb o n f i b r e s a n d s a t i s fym a t r i x - d o m i n a t e d p r o p e r t ie s s u c h a s h o t / w e t p e r f o r -m a n c e . H o w e v e r , t h e a d v a n t a g e s o f t h e s e m a t e r ia l s a r es i g n i fi c a n t ly r e d u c e d b e c a u s e o f t h e i r s u s c e p t ib i l it y t od a m a g e b y l o w - v e lo c i ty im p a c t s. T o a c c o u n t f o r i m p a c td a m a g e , s t r u c t u r e s a r e o f t e n ' o v e r -d e s i g n e d ' re s u l t in gi n n o n -o p t i m u m s t ru c t u r e s i n t e rm s o f s tr e n g t h / s ti f f n e s sp e r u n i t o f w e i g h t . Th e s u s c e p t i b i li t y o f c o m p o s i t em a t e r i a ls t o f o r e i g n o b j e c t i m p a c t d a m a g e h a s b e e ns t u d i e d r a t h e r e x t e n s i v e l y1. U p o n i m p a c t l o a d i n g st r a n s v e r s e c r a c k i n g , d e l a m i n a t i o n s , f i b r e / m a t r i xd e b o n d i n g a n d f i b r e f r a c t u r e a r e t h e p o t e n t ia l f a i l u rem o d e s i n la m i n a t e d c o m p o s i t e s. I t h a s b e e n d e m o n -s t r a t e d t h a t d a m a g e d u e t o i m p a c t s u b s ta n t i a l ly r e d u c e st h e r e s i d u a l s t r e n g t h a f t e r i m p a c t o f a c o m p o s i t es t r u c t u r e , e v e n w h e n d a m a g e c a n n o t b e v i s ua l lyo b s e r v e d2-'4. Th e p r i n c i p a l m e c h a n i s m o f c o m p re s s i v es t r e n g t h r e d u c t i o n i s l o c a l b u c k l i n g o f t h e s u b - l a m i n a t e sf o r m e d in t h e d e l a m i n a t e d a r e a 5 . In t en s i l e load ings thes t r e n g t h r e d u c t i o n m e c h a n i s m i s d o m i n a t e d b y f i b re

f r a c t u r e . F o r t h e s e r e a s o n s , i m p a c t d a m a g e i s g e n e ra l l yr e c o g n i z e d as t h e m o s t s e v e r e t h r e a t t o c o m p o s i tes t ru c t u r e s6.S e v e r a l a p p r o a c h e s h a v e b e e n t a k e n t o i m p r o v e t h ep e n e t r a t i o n r e s i s ta n c e a n d d a m a g e t o l e r a n c e o f c a r b o n/e p o x y c o m p o s i te s . T h e m a i n d e v e l o p m e n t s i n t o u gh -e n e d c o m p o s i t e m a t e r i a l s i n c lu d e : t o u g h e n e d t h e r m o s e t m a t r i c e s - - m o d i f i c a t i o n ise s t a b l i s h e d t h ro u g h t h e a d d i t i o n o f r u b b e r o rt h e rm o p l a s t i c c o m p o u n d s T-9; t h e r m o p l a s t i c s - - a n e x a m p l e o f a h i gh p e r f o r m a n c e

t h e r m o p l a s t ic c o m p o s i t e s y s t em i s A P C - 2 b a s e d o n apolyether-etherk etone (PEEK)m a t r i x r e i n f o r c e dwi th c arbo n f ib res1; i n t e r l e a f i n g - - t h i s c o n c e p t o f c o m p o s i te t o u g h e n i n gw a s i n t r o d u c e d b y A m e r i c a n C y a n a m i d a n d i n vo l ve st h e i n c o rp o ra t i o n o f d i s c r e t e l a y e r s o f a h i g h s h e a rs t r a i n r e s i n a t t h e l a m i n a i n t e r f a c e s t h e r e b y g i v in gt h e c o m p o s i t e t h e a b i l it y t o u n d e rg o h i g h e r s h e a rd e fo rm a t i o n s w i t h o u t f o rm i n g d e l a m i n a t i o n s 1~.A d i f f e r e n t t o u g h e n i n g a p p ro a c h i s t o r e p l a c e b r i t t le

0010-4361/90/060522-09 (~) 1990 But terworth-H einem ann Ltd522 COMPOS ITES. VOLUME 21 . NUM BER 6. NO VEMBER 1990

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c a r b o n f i b r e s b y m o r e d u c t i le f i b r e s su c h a s g l a ss a n da r a m i d . A r e l a t iv e l y n e w , t o u g h r e i n f o r c in g f i b r e ish i g h p e r f o r m a n c e p o l y e t h y l e n e ( H P- PE ), c u r r e n t l yp r o d u c e d b a se d o n so l u t i o n ( g e l )- sp i n n in g o f u l t ra - h i g hm o l e c u l a r w e i g h t p o l y e t h y l e n e ( U HM W - P E) , p o s se s s i n gu n i q u e m e c h a n i c a l p r o p e r t i e s i n t e r m s o f h i g h sp e c if i cs t reng th and s t i f fness 12 . M or eo ve r the se HP-PE f ib resp o s se s s a h i g h e l o n g a t i o n a t b r e a k l e a d i n g t o h i g hv a l u e s o f w o r k t o b r e a k c o m p a r e d w i th o t h e r r e in f o rc -i n g f i b r e s su c h a s c a r b o n , a r a m i d a n d g l a s s . D u e t ot h e se p r o p e r t i e s , H P -P E f i b r e s h a v e a h i g h p o t e n t i a l f o ru se i n c o m p o s i t e s t r u c t u r e s r e q u i r in g g o o d i m p a c tpro pe r t i e s r3 -16D u r i n g t h e m i d - 1 9 7 0 s v a r i o u s r e p o r t s c o n c e r n i n gi m p a c t i m p r o v e m e n t o f c a r b o n - r e in f o r c e d c o m p o s i t e sv i a h y b r id i z in g b y g l a s s a n d a r a m i d f i b r e s w e r ep r e se n t e d 17 -2 . M o s t p a p e r s w e r e d i r e c t e d t o w a r d se n h a n c e d e n e r g y a b so r p t i o n c a p a b i l i ti e s u t il iz i ngC h a r p y - t y p e t e s t s . A l t h o u g h w i d e l y u se d , t h e se t e s tsa r e n o t n e c e s sa r i l y a ll su i te d t o a n u n d e r s t a n d i n g o fi m p a c t b e h a v i o u r o f c o m p o s i t e l a m i n a t e s s i n c e t h e t e s tg e o m e t r y d o e s n o t r e p r e s e n t t h e e n d - u s e a p p l i ca t io n o ft h e c o m p o s i t e s t r u c t u r e 21 . I n s t r u m e n t e d d a r t o r f a ll in gw e i g h t i m p a c t t e s t s o n l a m i n a t e d p l a t e s h a v e t h ea d v a n t a g e o f a c lo se r a p p r o x i m a t i o n t o a c o m p o s i t ema te r i a l app l i ca t ion .R e c e n t l y s o m e s t u d ie s o n h y b r i d c o m p o s i t e s w e r ed i r e c t e d t o t h e s y s t e m c a r b o n /H P - P E22--25. It wa s sh ow nt h a t i m p a c t p e r f o r m a n c e a n d d u c t i l it y o f c a r b o n / e p o x yc o m p o s i t e s c a n b e i n c r e a se d b y h y b r i d i z in g w i t h H P - PEf i b r e s . P r e v i o u s s t u d i e s d e m o n s t r a t e d t h a t t h e a m o u n to f i m p r o v e m e n t w a s s tr o n g ly d e p e n d e n t o n t h ea d h e s i o n l e v e l o f t h e H P -P E f i b r e s24'25. Experimentsu s i n g i n s t r u m e n t e d d a r t i m p a c t t e s t s o n l a m i n a t e dp l a t e s a lso i n d i c a te d t h a t , a p a r t f r o m t h e a d h e s i o nleve l , t he p os i t i on of t he HP-PE p l i e s i n a l aye re dc a rb o n /H P - P E h y b r i d s t r u c t u r e i s a n i m p o r t a n t p a r a m e -t e r f o r i m p a c t r e s i s ta n c e 2 4. T h e h i g h e s t i m p a c t e n e r g yv a l u e s fo r p e n e t r a t i o n w e r e o b t a i n e d b y p o s i ti o n in g t h eH P -P E p l ie s a t t h e o p p o s i t e s i d e t o t h e i m p a c t e d su r f a c ei n t h e t e n s i le z o n e a n d a t a l o w l e v e l o f a d h e s io n .T h e o b j e c t i v e o f t h e p r e se n t s t u d y w a s t o i n v e s ti g a t et h e e f f e c t o f h y b r i d i z a t i o n w i t h H P -P E o n i m p a c td a m a g e i n th e s t r u c tu r a l c a r b o n c o m p o n e n t o f th eh y b r i d u n d e r l o w - v e l o c i t y n o n - p e n e t r a t i n g i m p a c tc o n d i t i o n s i n c l u d in g t h e e f f e c t o f su r f a c e t r e a t m e n t o ft h e H P - PE f i b r e s o n t h e i m p a c t t o u g h n e s s o f t h e h y b r i d s .

EXPERIMENTAL DETAILSMater ia lsH y b r i d l a m i n at e s w e r e m o u l d e d f r o m w o v e n f a b ri c s o fh i g h s t r e n g t h c a r b o n f i b r e ( G r a f i l X A - S / 3 K ) a n d H P -P Ef i b r e ( S p e c t r a 1 0 0 0, 6 5 0 d e n i e r ) i n a n e p o x y sy s t e m o fC i b a - G e i g y ( A r a l d i te L Y 5 5 6 / H Y 9 1 7 / D Y 0 7 0 ) b a s e do n b i sp h e n o l - A w i t h a n a n h y d r i d e c u r i n g a ~ e n t .C a r b o n a n d H P -P E f a b r i c s , b o t h o f 2 0 0 g m -* , w e r e p l a i nw e a v e f a b r ic s w i t h 5 0 % o f f i b r e in t h e w a r p a n d 5 0 % i nt h e w e f t d i r e c ti o n . O n e p l y o f H P -P E w a s a p p r o x i m a t e l yt w i c e a s t h i c k ( 0 . 43 m m ) a s o n e p l y o f c a r b o n f a b r i c( 0 . 2 0 m m ) .L a m i n a t e d s t r u c tu r e s w e r e p r e p a r e d b y s ta c k in g

T ab le 1 . P late th icknesses and densi t iesLay-up Average Averagethickness density(mm ) (gc m -'~)[Cs] 1.19 1.57[Cs /PE 1 ] 1 .57 1 .4 5[Ce/PE2] 1.92 1.37[Ce/PE3] 2.25 1.30p r e - i m p r e g n a t e d p l i es o f c a r b o n a n d H P - PE f i b r e w o v e nf a b r i c to g e t h e r i n a ( 0 , 9 0 ) l a y -u p i n a m o u l d t o e n su r ee v e n c o m p a c t i o n a n d c u r i n g f o r 4 h u n d e r c o m b i n e dv a c u u m a n d p r e s su r e c o n d i t i o n s i n a h o t p r e s s a t 8 0 C .A f t e r c u r i n g , s a m p l e s w e r e p o s t - c u r e d f o r 1 2 h a t110C.T h e r e f e r e n c e m a t e r i a l w a s a si x -p l y c a r b o n / e p o x yl a m i n a t e [ C 6] . A v a r i e t y o f h y b r i d c o n f i g u r a t i o n s w a sp r e p a r e d b a se d o n t h i s b a s ic s i x -p l y c a r b o n l a m i n a t e t ow h i c h , o n t h e b o t t o m s i d e , o n e , t w o o r t h r e e p l ie s o fHP-PE fabr i c s were add ed . The se l am ina tes a re des ig-na ted as [C6/PE1] , [C6/PE2] and [C6/PE3] resp ect iv ely.H P - P E f a b r ic s w e r e s t a c k e d a t th e n o n - i m p a c t e dsu r f a c e in t h e t e n s i o n z o n e t o o b t a i n a n o p t i m u m e f f e c ti n im p a c t i m p r o v e m e n t . T o i n v e s t i g a t e t h e e f f e c t o fi m p r o v e d a d h e s i o n o f t h e H P -P E f i b r e s o n t h e i m p a c tb e h a v i o u r o f su c h h y b r i d s , c o m p o s i t e s i n c o r p o r a t i n gu n t r e a t e d a n d t r e a t e d p o l y e t h y l e n e fi b r es w e r e m a n u -f a c tu r e d . I m p r o v e d a d h e s io n w a s o b t a i n e d b y im m e r s -ing HP-PE f ibres in ch rom ic acid for 15 minE6'27; thisp r o v e d t o b e a n e a sy a n d e f f e c t i v e w a y t o i n c r e a se t h ei n t e rf a c i a l b o n d s t r e n g t h w i t h e p o x y m a t r i ce s w i t h o u treduc ing s ign i f i can t ly the t ens i l e s t rength o f t he f ib re .T h e t o t a l v o l u m e f r a c t io n o f f ib r e i n a ll l am i n a t e s w a sa p p r o x i m a t e l y 6 0 % . P l a t e t h i c k n e s se s o f t h e v a r i o u sl a m i n a t e s a r e p r e se n t e d i n T a b l e 1 . P l a n e s a m p l e s o fd i m e n s i o n s 1 0 0 x 1 00 m m w e r e c u t f r o m t h e se la m i -n a t e s u s i n g a d i a m o n d c u t t in g w h e e l .

Imp act testingI m p a c t t e st s w e r e c o n d u c t e d w i t h a n i n s t r u m e n t e df a ll in g w e i g h t i m p a c t t e s t e r . I m p a c t e n e r g i e s w e r eg e n e r a t e d b y d r o p p i n g a h e m i sp h e r i c a l i m p a c t o r( d i a m e t e r o f 1 2 m m ) , l o a d e d w i t h d i f f e r e n t w e ig h t s a td i f f e r e n t h e i g h ts . T y p i c a l i n c id e n t e n e r g i e s w e r e 2 . 5 , 5 ,1 0 a n d 1 5 J . T h e l a m i n a t e s w e r e c l a m p e d b e t w e e n t w op l a t e s w i t h a sq u a r e o p e n i n g o f 4 0 m m . B o t h f o r c e v s .t i m e a n d i m p a c t v e l o c i t y j u s t b e f o r e i m p a c t w e r er e c o r d e d a n d s t o r e d i n a d i g it a l s to r a g e o sc i l l o sc o p e .P o s t p r o c e s s i n g o f d a t a r e su l t s in t h e c o m p l e t e e n e r g yh i s t o r y d u r i n g i m p a c t a n d t h e e n e r g y a b so r b e d b y t h ec o m p o s i t e . A t l e a s t f o u r sp e c i m e n s o f e a c h l a m i n a t ec o n f i g u r a t i o n w e r e i m p a c t e d a t a f i x e d e n e r g y l e v e l .A f t e r i m p a c t t h e sp e c i m e n s w e r e e x a m i n e d f o r v i si b led a m a g e a n d i n d e n t a ti o n d e p t h . I n o r d e r t o d e t er m i n et h e i m p a c t d a m a g e o f t h e c a r b o n c o m p o n e n t i n th e- h y b ri d , t h e o u t e r l a y e r s o f H P -P E w e r e r e m o v e d f r o mt h e b o t t o m su r f a c e o f t h e la m i n a t e . U l t r a so n i c C - sc a nw a s u s e d t o d e t e r m i n e t h e e x t e n t o f i m p a c t - in d u c e dd e l a m i n a t i o n s i n t h is c a r b o n c o m p o n e n t . O p t i c a lm i c r o sc o p y o n p o l i sh e d l o n g i t u d i n a l c r o s s - s e c ti o n s o f

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t e s t e d s a m p l e s w a s u s e d t o r e v e a l th e t y p e o f i m p a c td a m a g e i n th e c a r b o n l a m i n a t e s a n d t o d i s c r im i n a t eb e t w e e n m a t r i x c r a c k i n g , f ib r e b r e a k a g e a n d d e l a m i n a -.t i ons .R e s i d u a l f l e x u ra l s t r e n g t h o f t h e s t r u c tu r a l c a r b o n p a r to f t h e h y b r i d w a s m e a s u r e d b y l o a d i n g s t a t ic a l ly t of a i l u r e o n a Z w i c k t e s t i n g m a c h i n e u s i n g a t h r e e - p o i n tb e n d i n g f i x tu r e (L/d = 4 0 ) a t a r a t e o f 6 m m m i n - z.S p e c i m e n s w i t h a w i d t h o f 35 m m ( s li g h tl y l a rg e r t h a nt h e w i d t h o f th e l a r g e s t d e l a m i n a t i o n ) a n d a l e n g t h o f1 00 m m w e r e c u t f r o m t h e i m p a c t e d l a m i n a t e s .RESULTS AND DISCUSSIONLam inates wi th untrea ted HP-PE f ibres1. Im pac t energ iesP o s t p r o c e ss i n g o f t h e m e a s u r e d i m p a c t p a r a m e t e r s -- - f o r c e , t i m e a n d i m p a c t v e l o c i t y - - r e s u l t s i n th ec o m p l e t e e n e r g y h i s t o ry a n d t h e e n e r g y a b s o r b e dd u r in g i m p a c t b y t h e c o m p o s i t e la m i n a t e. U p o n i m p a c tt h e t o t a l i m p a c t e n e r g y c a n b e d i v i d e d i n to t w o p a r t s :1 ) t h e e l a s ti c a ll y s t o r e d e n e r g y i n t h e c o m p o s i t e p l a t ew h i c h is r e l e a se d a f t e r m a x i m u m d e f l e c t i o n b yr e b o u n c i n g o f t h e l a m i n a t e. T h i s r e b o u n c i n g e n e r g yi s su c c e s s i v e l y t r a n s f e r r e d b a c k t o t h e i m p a c t o r ; a n d2 ) t h e e n e r g y a b s o r b e d b y t h e c o m p o s i t e la m i n a t ew h i c h i s a v a i l a b le f o r d a m a g e a n d c o n se q u e n t l yc o n t r o l s t h e e x t e n t o f d a m a g e a n d r e s i d u a l s t r e n g t ha f t e r imp ac t 2a.I n F i g . i t h e e l a s ti c e n e r g y ( - - i m p a c t e n e r g y m i n u sa b s o r b e d e n e r g y ) i s p l o t t e d a g a i n s t t h e i m p a c t e n e r g yf o r th e a l l - c a r b o n l a m i n a t e a n d t h r e e h y b r i d s w i th ,r e sp e c t i v e l y , o n e , t w o a n d t h r e e p l i es o f H P - PE f a b r i ca d d e d t o t h e n o n - i m p a c t e d s u r f a c e o f t h is c a r b o nl a m i n a t e . F o r t h e [C 6 ] l a m i n a t e a n a p p a r e n t m a x i m u mi n e la s t ic e n e r g y s t o r a g e a p p e a r s a t a n i m p a c t l e v e l o fa p p r o x i m a t e l y 4 J . A t h i g h e r i m p a c t e n e r g i e s e l a s t ice n e r g y s t o r a g e d e c r ea s e s . C o n s e q u e n t l y m o r e e n e r g y isa b s o r b e d i n t h e l a m i n a t e a n d i s a v a i l a b l e f o r d a m a g e .T h e a d d i t i o n o f H P- PE r e su l ts i n a c h a n g e o f t h e e n e r g ya b s o r p t i o n m o d e . A f t e r h y b r id i z a ti o n , m o r e e n e r g y i ss t o r e d e l a s ti c a ll y i n t h e H P -P E f i b r e s a n d c o n se q u e n t l yt h e r e i s a n i n c r e a se i n e l a s ti c e n e r g y t r a n s f e r r e d b a c k t ot h e i m p a c t o r . W i t h i n t h e c o v e r e d i m p a c t e n e r g y r e g io na ll h y b r i d s sh o w e d a l i n e a r i n c r e a se i n e l a s t ic e n e r g yw i t h i n c r ea s i n g i m p a c t e n e r g y v a l u e s .

2. D ama ge character izat ionF o l l o w i n g t h e i m p a c t t e s t s t h e l a m i n a t e s w e r ee x a m i n e d f o r th e a m o u n t a n d e x t e n t o f d am a g e . I t c anb e e x p e c t e d t h a t h i g h e r l e v el s o f a b s o r b e d e n e r g yr e s ul ts i n m o r e d a m a g e .F i g . 2 sh o w s t h e v i su a l o b se r v a t i o n s m a d e o n t h et e n s i o n s i d e o f t h e c a r b o n l a m i n a t e a f t e r r e m o v a l o f t h eHP-PE p l i e s . Fo r the bas i c [C6] l amin a te a s t rongi n c r e a se i s s e e n i n v i su a l t e n s i le f a c e d a m a g e w i t hi n c r e a s in g i m p a c t e n e r g i e s , r e su l t i n g i n f u l l p e n e t r a t i o no f t h e l a m i n a t e a t a n i m p a c t e n e r g y o f 1 5 J . A f t e rh y b r i d i z a t i o n a s t r ik i n g r e d u c t i o n i n v i su a l d a m a g e i so b se r v e d i n t h e l a m i n a t e , m a i n l y a s a r e su lt o f th ec h a n g e i n e n e rg y a b s o r p t i o n m o d e .

T h e e x t e n t o f in t e rn a l d a m a g e i n t h e c a r b o n c o m p o -n e n t r e su l t i n g f r o m t h e i m p a c t s w a s e x a m i n e d u s i n gu l t r a so n i c C - sc a n im a g i n g . T h e i n t e r n a l d e l a m i n a t e da r e a s f o r b o t h t h e n o n - h y b r i d i z e d a n d h y b r i d i z e dc a r b o n l a m i n a t e s a r e c o m p a r e d i n F i g . 3 . T h e o v e r a l ll e v e l o f d a m a g e i n a ll t h e c a r b o n l a m i n a t e s i n c r e a se sp r o p o r t i o n a l l y w i t h i n c re a s i n g i m p a c t e n e r g y .H o w e v e r , h y b r i d i z e d l a m i n a t e s sh o w e d a sm a l l e r le v e lo f i m p a c t d a m a g e a t a g iv e n i n c id e n t e n e r g y a n d l e s sse n s i ti v i ty t o h i g h e r i m p a c t e n e r g i e s a s r e f l e c te d b y t h edec r ease in s lope of t he s t ra igh t l i nes w i th inc reas ingn u m b e r o f H P -P E p l ie s .I n F i g . I i t w a s sh o w n t h a t o n e o f t h e p r i n c i p al t o u g h e n -i n g m e c h a n i sm s f o r H P - P E / ca r b on h y b r i d l a m i n a t e s i st h e i m p r o v e d a b i l i t y o f e l as t ic e n e r g y s t o r a g e . C o n se -q u e n t l y , l e ss e n e r g y i s a b so r b e d i n th e h y b r i d l a m i n a t et o b e c o n v e r t e d i n t o d a m a g e . A n a d d i t i o n a l e f f e c tw h i c h r e su l t s i n a r e d u c t i o n i n e n e r g y a v a i l a b le f o rd a m a g e i n th e s t r u c tu r a l c a r b o n c o m p o n e n t is e n e r g ya b so r p t i o n b y d a m a g e i n t h e H P - PE l a m i n a t e a n d a t t h eca rbon/HP-PE in t e r face .B y p l o tt i n g t h e d e l a m i n a t e d a r e a i n t h e c a r b o n c o m p o -n e n t v s . t h e e n e r g y a b so r b e d b y t h e h y b r i d l a m i n a t ea n d a s su m i n g t h a t t h e r e i s a d i r e c t c o r r e l a t io n b e t w e e nt h is d e l a m i n a t e d a r e a a n d t h e a b s o r b e d e n e r g y i n t h ebas i c [C6] l amina te* , i t is poss ib l e to d i sc r imina teb e t w e e n e n e r g y a b so r p t i o n i n t h e c a r b o n a n d H P -P Ec o m p o n e n t s ( F ig . 4 ) . A d d i t i o n o f H P -P E p l ie s c a u se s ash i f t o f t he s t ra igh t l i nes which sugg es t s t ha t , a se x p e c t e d , e n e r g y i s a l so a b so r b e d i n t h e H P - PE c o m p o -n e n t . A t a g iv e n d e l a m i n a t e d a r e a i n t h e c a r b o nc o m p o n e n t a n i n c r e a s e in a b s o r b e d e n e r g y a f te rh y b r i d i z a t io n i s o b se r v e d . T h i s i n c r e a se in a b so r b e de n e r g y c a n b e r e l a te d t o t h e e n e r g y a b s o r b e d b yd a m a g e i n t h e H P- PE c o m p o n e n t a n d a t th e i n t e r f a c ebe tw een th e ca rb on and HP-PE p l i e s . S ince the l a t t e r i si n d e p e n d e n t o f t h e n u m b e r H P -P E p l ie s ( t h e r e i s o n l yo n e d e l a m i n a t i o n p o s s i b l e b e t w e e n c a r b o n a n d H P -P Ec o m p o n e n t s ) , a d d i t i o n a l e f fe c t s i n a b so r b e d e n e r g y f o r

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t he [C6/PE2] and [C6/PE3] l amina tes can be a t t r i bu tedt o a n i n c r e a se i n d a m a g e e n e r g y i n t h e H P -P E l a m i n a t e .S i n c e t h e r e w a s n o e v i d e n c e o f H P- PE f i b r e f r a c t u r ew i t h i n t h e i m p a c t e n e r g y r a n g e t e s t e d , e n e r g y a b so r p -t i o n p r o c e s se s sh o u l d i n v o l v e d e l a m i n a t i o n , d e b o n d i n ga n d m a t r i x c r a c k in g .T o i l lu s t r a te t h is w e w i l l d is c u s s t h e e n e r g y a b so r p t i o nf o r [ C 6 ] a n d [ C JP E 1 ] ' l a m i n a t e s a t a n i m p a c t e n e r g y o fa p p r o x i m a t e l y 1 0 J . F r o m F i g s 1 a n d 3 i t c a n b e sh o w ntha t i n the c ase of t he [C6] l amin a te th i s y i e lds a va lueo f 0 . 6 J f o r e l a s ti c e n e r g y s t o r a g e a n d 8 . 8 J f o r e n e r g ya b so r b e d i n t h e l a m i n a t e , r e su l t in g i n a c a r b o n d a m a g ea r e a o f 4 .5 c m 2 . W i t h t h e a d d i t i o n o f o n e p l y o f HP-PEf a b r i c e la s t ic e n e r g y s t o r a g e i n c r e a se s u p t o 2 . 9 J ,l e a v in g 6 . 6 J f o r e n e r g y a b so r p t i o n i n t h e h y b r i dl a m i n a t e a n d c a u s i n g a d e l a m i n a t e d a r e a i n t h e c a r b o nc o m p o n e n t o f 2 . 8 c m2 . F r o m F i g . 4 it c an b e s h o w n t h a tt h e a m o u n t o f a b s o r b e d e n e r g y n e c e s s a r y to c r e a t e t h is

d a m a g e , a s su m i n g a d i r e c t c o r r e l a t io n b e t w e e n d e la m i -n a t e d a r e a a n d a b so r b e d e n e r g y , i s 4 . 3 J in a [ C 6]l a m i n a t e . C o n se q u e n t l y 2 . 3 J ( = 6 . 6 - 4 . 3 J ) isa b so r b e d i n t h e H P - PE c o m p o n e n t a n d a t t h e c a r b o n /HP-PE i n t e r f ac e . I n c o n c l u s i o n t h e t o t a l e f f e c t o f h y b r i d -i z a t io n o n t h e e n e r g y a b so r p t i o n m o d e i n a [C 6 /P E 1]l a m i n a t e a t a n i m p a c t e n e r g y o f a p p r o x i m a t e l y 1 0 J i s2 . 3 J ( = 2 . 9 - 0 . 6 J ) o f a d d i t i o n a l e l a s ti c e n e r g y s t o r a g ea n d 2 . 3 J a b so r b e d e n e r g y b y d a m a g e i n t h e H P- PE p l yand a t t he ca rbon/HP-PE in t e r face . Thi s l eaves 4 .9 Ja b so r b e d b y t h e c a r b o n l a m i n a t e a n d a v a i l a b l e f o rd a m a g e i n t h e s t r u c t u r a l p a r t o f t h e h y b r i d .M i c r o g r a p h s o f c r o s s -s e c t io n s o f h y b r i d i z e d c a r b o nl a m i n a t e s su b j e c t e d t o a n i m p a c t e n e r g y o f 1 0 J a r esh o w n in F i g . 5 ( a ) - ( d ) . F o r c o m p a r i so n a n o n -h y b r i d i z e d [ C 6 ] l a m i n a t e su b j e c t e d t o a n i m p a c t o f 7. 5 Ji s sh o w n b e c a u se d a m a g e i n t h e 1 0 J i m p a c t e d sp e c i-m e n s w a s t o o s e v e r e , p r e v e n t i n g a c l e a r o v e r a ll p i c tu r eo f t h e d a m a g e . A l t h o u g h t h e [ C 6 ] l a m i n a t e sh o w n w a si m p a c t e d a t a l o w e r e n e r g y l e v e l t h a n t h e h y b r i d i z e dl a m i n a t e s , t h e s t r o n g r e d u c t i o n i n e x t e n t o f d a m a g e i sd e a r l y v i su a l i ze d . A t a n i m p a c t o f 7 .5 J t h e r e i s g r o w t ho f l a rg e f i b r e c r a c k s u n d e r t h e p o i n t o f im p a c t a s ar e su l t o f t h e f l e x u r a l r e sp o n se o f th e l a m i n a t e . F a i l u r ei s i n i t ia t e d a t t h e b o t t o m s i d e o f th e l a m i n a t e b y t e n s il es t r e s se s e x c e e d i n g t h e f a i l u r e s t r e s s o f t h e c o m p o s i t ea n d r e su l ti n g i n t h e e j e c t i o n o f a sh e a r p l u g . A f t e rh y b r i d i z a t i o n t h e a m o u n t o f d e l a m i n a t io n , f i b r e a n dm a t r i x c r a c k i n g i s g re a t l y r e d u c e d so t h a t , f o r t h ec a r b o n c o m p o n e n t o f t h e [ C6 /P E 3] l a m i n a t e , d a m a g e i sm o r e o r l e s s t h e c o m b i n a t io n o f o n e s t r o n g ly r e d u c e dd e l a m i n a t i o n a n d o n e s i n g l e t r a n sv e r se c r a c k .I m p a c t d a m a g e s i g n if i ca n t ly r e d u c e d t h e r e s i d u a lf i e x u r a l s t r e n g t h o f b o t h n o n - h y b r i d i z e d a n d h y b r i d i z e dca rb on l am ina tes (F ig . 6 ) . In a ll ca ses the curves exhib i tb a s i c a ll y t h e s a m e sh a p e . H o w e v e r , t h e r e d u c t i o n i ns t r e n g t h i s m o r e p r o n o u n c e d i n t h e n o n - h y b r i d i z e dc a se . T h e f l e x u r al s t r e n g t h o f t h e [ C 6 ] l a m i n a t e s a f t e r ,f o r e x a m p l e , a n i m p a c t o f 1 0 J w a s r e d u c e d t o 3 0 % o ft h e u n d a m a g e d v a l u e c o m p a r e d w i t h a p p r o x im a t e l y4 0 % , 5 0 % a n d 6 0 % o f th e u n d a m a g e d v a l u e s f o rh y b r i d s w i t h o n e , t w o a n d t h r e e p l i es o f H P -P E r e sp e c -t i v el y . T h e r e a so n f o r t h is i m p r o v e d r e s i d u a l s t r e n g t h o ft h e c a r b o n c o m p o n e n t , b y a s m u c h a s 1 0 0% f o r th e[ C6 /P E 3] l a m i n a t e , i s u n d o u b t e d l y t h e r e d u c t i o n i nd e l a m i n a t i o n a n d f i b r e f r a c t u r e. P a r t ic u l a r l y t h e l a t t e rw i ll h a v e a s t r o n g e f f e c t o n r e s i d u a l s t r e n g t h a s t e s t e dh e r e i n a t h r e e - p o i n t b e n d i n g m o d e .

Laminates with treated HP-PE fibresF i g . 7 sh o w s t h e e l a s t ic e n e r g y v s . i m p a c t e n e r g y f o r t h eh y b r i d s i n c o r p o r a t i n g c h r o m i c - a c i d t r e a t e d H P -P E f i b r e sw i t h a n i m p r o v e d l e v e l o f a d h e s i o n . I n a c c o r d a n c e w i t ht h e d a t a sh o w n f o r u n t r e a t e d H P -P E t h e r e i s a l so a ni n c r e a se i n e la s t i c e n e r g y s t o r a g e w i t h t h e a d d i t i o n o ft r e a t e d H P -P E p l i es . H o w e v e r , i n c o n t r a s t t o u n t r e a t e dHP-PE, a max imu m in e l a s t ic ene rg y s to rag e ab i l i t yo c c u r s w i th i n t h e i m p a c t e n e r g y r a n g e t e s t e d , r e v e a l in gt h e e m b r i t t l e m e n t o f H P -P E c o m p o s i t e s w i t h i n c r e a s in gl e v e ls o f a d h e s i o n . T h i s m o r e b r i t tl e f a i lu r e b e h a v i o u rresu l t s i n the f r ac ture of HP-PE f ib res a t h igh e r im pac te n e r g i e s a n d c o n se q u e n t l y a l o ss o f i m p a c t p r o t e c t i o nb y t h e se f i b r e s ( F ig . 8 ) . T h i s i m p l i e s t h a t a b o v e t h e

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I I I

5 1 0 1 5 2 0 2 5I m p a c t e n e r g y ( J)

Fig. 7 Elastic energy vs. impact energy for hybrid laminatesi n c o r p o r a t i n g t re a te d HP-PE: - ~ - , [ C6] ; I , [ C s / P E 1] t r e a t e d ;& , [C JPE2] treated; and I , [C6/PE2] treatedres idual fl exural st rength o f t he carbon co mp onen t i sshow n in F ig . 9 . In t h i s typ ica l case t he e f f ec t i s show nf or t he [ C 6/ PEI ] and [ C 6/PE3] laminat es . A s ex pect e d ,res idual s t rengt h decrea ses w i t h increas ing impactenergy . Ho w e ver , i t i s dem onst rat ed t hat t he hybr id -ized ca rbon laminat es incorporat ing t reat ed HP-PE havea subs t ant ia l h igher reduct ion in s t rengt h at h igherimpact l ev e l s t han t he laminat es w i t h unt reat ed HP-PEf ibres . A s t he or ig in of t h i s mo re prono unced reduct ionin res idual f l exural st rength can not be exp la ined by t he

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~.- :i,:Fig. 8 Ten sile face damag e in a [Ca/PE~] laminate incorporat ingtreated HP-PEshowing fibre fracture in the HP-PE ply after animpact of 15 J

1 0 0

8 0Cm 6 0

-~ 40D"0~ 2 0cc

o,u ,

\ + ~ 0 " "1 " - ,. ,. D

0 I I 10 5 1 0 1 5 2 0

I m p a c t e n e r g y ( J )Fig. 9 Residual f lexural strength of carbon compone nt vs .impact energy-- - effect of HP-PE ibre surface treatment: + , [Ca] ;O , [ C a /P E 1 ] ; O , [ C a /P E a l t r e a t e d ; D, Ca/PEal; and I , [Ca/PEaltreatedef f ect o f f ibr e t r eat men t on t he de lam inat ed ar ea in t hecar bon com pon ent , o t her s t r engt h r educing fact or smust be t he or ig in of t hese ad d i t ional reduct ions instrength.By p lot t ing t he res idual s trengt h of a l l t he impact edcarbon lam inat es , hybr id ized as w e l l as non- hybr id ized ,agains t de lam inat ed area it can be dem onst rat ed t hat ,f or a g iven de la minat ed area , t he res idual s t rength oft he carbon laminat e hybr id ized w i t h unt reat ed HP-PEf ibres i s super ior t o t hat of lam inat es incorporat ingtreated f ibres (Fig. 10) . This indicates that in the caseo f t r e a te d f i b re s t h e d a m a g e i n th e c a r b o n c o m p o n e n ti s mo re loca l ized arou nd t he impact cen t re and moreharmf u l t o t he f l exural load-carrying capac i t y of t hecom pos i t e t han in t he case of laminat es incorporat ingunt reat ed HP-PE f ibres . T h is m ore in t ens i fi ed dam age int h e c a r b o n c o m p o n e n t c a n b e t h e r e s u l t o f m o r ede lam inat ion , d ebon d ing , m at r ix crack ing and f ibrefracture of which, as already stated, f ibre fracture wil lhave a predomina nt ro le in f lexural s t rength reduct ion .Th is in t ens if i ed loca l ized dama ge canno t be revealedus ing C - scan because t h i s techn iqu e i s on ly capab le o f

1 0 0

v 8 0C

6 0

X

_e 40,.-

~ 2 0cr

00 00:oe 0 o

o Oo oo o% o

0 I I I I I0 1 2 3 4 5

D e l a m i n a t e d a r e a ( c m ~)Fig. 10 Residual f lexural strength of carbon component vs .delaminated area of carbon comp onent of a ll impacted lam i-nates: O, untrea ted HP-PE; O, treated HP-PE

5

4-ga 3-ot-o~z 2( -

1

00 2 5

I I

/ II II i i

I II i

/ / = 1 " / r ~ / o / ~ , /

5 1 0 1 5 2 0I m p a c t e n e r g y ( d)

Fig. 11 Indentat ion depth v s. impact energy~ effect of HP-PE ibresurface treatment: + , [Ce ] ; O , [Ca /PE1 ] ; 0 , [Ca/PE1 ] treated;D, [Ca /PEa l , an d I , [Ca /PEa ] treated

giv ing an in t egrat ed p ic ture of t he overal l damage . F ig .11 compares t he indent at ion dept h of t he carboncom pon ent o f hybr id laminat es incorporating t reatedand untr eated HP-PE f ibres with increasing impa ctenergies . I t can be observe d t hat at h igher impactenergies t he indent at ion dep t h i s muc h great er forhybrid lam inates reinforc ed w ith treated HP-PE f ibrest han f or t he hybr ids w i t h unt reat ed f ibres as a resu l t o ff ibre fracture in the HP-PE plies . A s in denta t ion in thecase o f a bri t tl e mat er ia l such as carbon / ep oxy a lw aysinvolve s f ibre f ract ure , i t support s t he ear l ier as sum p-t ion tha t hybrids with tr eated I -n'-PE f ibres hav e a m orein t ens i f i ed loca l ized dama ge area in t he carbonlaminat e w i t h mo re severe f ibre f rac ture around t heimpact cen t re .A / I -c a r b o n la m i n a t e sBe s ides a change in energy absorpt ion mo de t here i s anadd i t ional e f f ec t o f increased lam inat e s t i f fnes s on t he

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8

d"E 6

~ 4CEa 2

[]/[ ]/

0 I J I0 5 1 0 1 5 2 0

I m p a c t e n e r g y ( J )Fig. 12 D e l a m i n a t e d a r e a v s . i m p a c t e n e r g y f o r a l l -c a r b o nl a m i n a t e s : +, [C8]; O, [Ca]; A, [Clo ] ; and Fi , [C12]

8

2 5

E

CEd3

6

4[]o J z ~

2

t o "/0

0 5I I

1 0 1 5 2 0A b s o r b e d e n e r g y ( J )

Fig . 13 D e l a m i n a t e d a r e a v s . a b s o r b e d e n e r g y f o r a l l -c a r b o nl a m i n a t e s : +, [Ce]; , [Ca]; A, [Clo ] ; and r - i , [C~2]

e x t e n t o f d a m a g e i n th e c o m p o s i t e s w i th t h e a d d i ti o n o fHP-PE p l i e s . Af t e r hy br id i za t ion the in c rease in s t i f fnesso f t h e l a m i n a t e r e su l ts i n sm a l l e r d e f l e c t i o n s a n dc o n s e q u e n t l y in l o w e r f l e x u r al s t r e s se s. T h e se s t r e s se sc a u se f i b r e a n d m a t r i x f r a c t u r e i n t h e l o w e r o u t e r p l y .T h i s c r a c k d e f l e c ts w h e n e n c o u n t e r i n g t h e n e x t p l y ,w h e r e i t f o r m s a p l a n e o f d e l a m i n a t i o n . C a n t w e l l a n dM o r t o n sh o w e d t h a t l a m i n a t e s t i f f n e s s i s a d o m i n a n tp a r a m e t e r i n i m p a c t r e sp o n se , t h e r e b y c o n t r o l l i n g t h em o d e o f f r a c tu r e 29. I m p a c t d a m a g e i n t h i n f l e x ib l el a m i n a t e s i n i ti a t es d u e t o f l e x u r a l a c t io n o f t h el a m i n a t e , w h e r e a s i n t h i c k e r s ti f f e r l a m i n a t e s d a m a g e isi n i t ia t e d b y c o n t a c t s tr e s se s c o n c e n t r a t e d a r o u n d t h ei m p a c t c e n t r e .I n o r d e r t o e n s u r e t h a t t h e o b s e r v e d i m p r o v e d t o u g h -n e s s w a s n o t s i m p l y d u e t o t h e e f f e c t o f i n c r e a se dt h i c k n e s s o f th e h y b r i d l a m i n a t e s , a l l - c a r b o n l a m i n a t e sw e r e f a b r i c a t e d c o n t a i n i n g 8 , 1 0 a n d 1 2 p l i e s o f c a r b o nf a b r i c , c o r r e sp o n d i n g t o t h e t h i c k n e s s o f t h e h y b r i d sw i t h o n e , t w o a n d t h r e e p l i es , r e sp e c t i v e l y , o f HP -P E

f a b r i c a d d e d . T h e e f f e c t o f t h e a d d i t i o n o f c a r b o n p l i e st o t h e b a s i c [C 6 ] l a m i n a t e o n t h e d e l a m i n a t e d a r e a i nt h e se c o m p o s i t e l a m i n a t e s i s g iv e n i n F ig . 1 2 . T h e s l o p eo f t h e se s t r a i g h t l in e s c a n b e c o n s i d e r e d a s a s t a n d a r df o r im p a c t t o u g h n e s s . C o n se q u e n t l y i t m a y b e c o n -c l u d e d t h a t t h e su sc e p t i b il i ty to i m p a c t d a m a g e o fc a r b o n / e p o x y c o m p o s i t e s is n o t a l t e r e d w i t h i n c r ea s i n gt h i c k n e s s. T h i s i n c o n t r a s t t o t h e h y b r i d l a m i n a t e sw h i c h sh o w e d a d e c r e a s i n g s l o p e w i t h i n c r e as i n gam ou nt o f HP-PE, re su l t i ng in a fa r m ore e f fec t ivereduc t ion in impac t suscep t ib i l i t y . Thi s supe r ior i tyb e c o m e s e v e n m o r e p r o n o u n c e d i f t h e w e i g h t re d u c t io ni s t ake n in to accoun t . The h ybr id [C6/PEa], fo re x a m p l e , i s m o r e t h a n 2 0 % l i g h te r t h a n t h e [ C m ]l a m i n a t e a t a p p r o x i m a t e l y t h e s a m e t h i c k n e s s.B y p l o t ti n g t h e d e l a m i n a t e d a r e a o f t h e a l l -c a r b o nl a m i n a t e s a g a i n s t t h e a b so r b e d e n e r g y i t c a n b e sh o w ntha t a l l the d a t a fa l l on the s am e s t ra igh t l i ne (F ig . 13) .T h i s su p p o r t s o u r e a r l i e r a s su m p t i o n , m a d e f o r d is c ri m -i n a ti n g b e t w e e n e n e r g y a b so r p t i o n i n c a r b o n a n d H P -P Ec o m p o n e n t s , t h a t t h e d e l a m i n a t e d a r e a i n t h e c a r b o np a r t o f th e h y b r i d c a n b e d i r e c t ly c o r r e l a t e d t o t h ea b so r b e d e n e r g y i n t h i s c o m p o n e n t .CONCL US lONSI t h a s b e e n sh o w n t h a t t h e a d d i t i o n o f H P -P E p l ie s t o t h en o n - i m p a c t s i d e o f a c a r b o n l a m i n a t e r e su l t s i n s tr u c -tu ra l hybr id composi t e s exhib i t i ng a s ign i f i can t ly be t t e rr e s i s t an c e t o i m p a c t d a m a g e b y a lt e r in g t h e e n e r g ya b so r p t i o n m o d e v i a h y b r i d i z a t io n . W i t h th e a d d i t i o nof HP-PE, mo re ene rg y i s s to red e l a s t i ca lly in the HP-PEf i b r e s a n d c o n se q u e n t l y l e s s e n e r g y i s a v a il a b l e f o rd a m a g e i n th e s t r u c tu r a l c a r b o n c o m p o n e n t o f th eh y b r i d . A n a d d i t i o n a l e f f e c t w h i c h l e a d s t o a r e d u c t i o ni n e n e rg y a b s o r b e d b y t h e c a r b o n c o m p o n e n t is e n e rg ya b so r p t i o n b y d a m a g e i n t h e H P -P E l a m i n a t e a n d b yde lam ina t ion a t t he ca rbon/HP-PE in t e r face . Th einc rease in dam age to l e ran ce i s r e f l ec t ed by a s ign if i-c a n t r e d u c t i o n i n d e l a m i n a t e d a r e a o f t h e c a r b o nl a m i n a t e a f t e r h y b r i d i z a t io n a s i n d i c a te d b y t h e C - sc a nd a t a . M i c r o sc o p i c i n v e s ti g a t io n s o f s e c t i o n e d sp e c i m e n sa l so i n d i c a t e d t h a t t h e n u m b e r o f d e l a m i n a t i o n s a n de x t e n t o f fi b r e / m a t r ix f r a c t u r e w a s a l so r e d u c e d i n t h eh y b r i d i z e d c a r b o n l a m i n a t e s . A s a r e su l t o f th i s a ni m p r o v e m e n t i n r e s id u a l f l e x u ra l s t r e n g t h a f t e r i m p a c ti s o b t a i n e d .H y b r i d s i n c o r p o r a t i n g su r f a c e - t r e a t e d H P -P E f i b r e s a l sos h o w e d i m p r o v e d d a m a g e t o l e ra n c e . H o w e v e r , a t h ig himpa c t l eve l s f ib re f rac ture occu rs in the HP-PE p l i e s duet o a m o r e b r i t tl e f a i l u r e b e h a v i o u r o f H P- PE c o m p o s i t ew i t h i n c re a s i n g l e v e l s o f a d h e s i o n . A f t e r f r a c t u r e o f t h eHP-PE f ib res the e l a s t i c ene rg y s to ra ge capab i l i t y in th i sc o m p o n e n t d i m i n i sh e s , r e su lt i n g in m o r e e x t e n s i v el o c a l iz e d c a r b o n d a m a g e i n t h e v i c in i ty o f t h e i m p a c tc e n t r e . C o n se q u e n t l y t h e r e s i d u a l s t re n g t h o f t h e seh y b r i d i z e d c a r b o n l a m i n a t e s i m p a c t e d a t h ig h e r e n e r g yl e v e ls is l o w e r t h a n t h a t o f h y b r i d i z e d c a r b o n l a m i n a t e si n c o r p o r a t i n g u n t r e a t e d f i b r e s , w h i c h d i d n o t sh o w a n yf ibre f rac ture in the HP-PE p l i e s w i th in the t e s t ed im pac te n e r g y r a n g e .I t was a l so foun d tha t hybr id i za t ion w i th HP-PE f ib resr e su l t e d i n c o m p o s i t e s w h i c h w e r e f a r m o r e su p e r i o r i n

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impact resistance than thicker and heavier laminatesconstructed entirely of carbon/epo xy.Finally , i t can be conc luded that the pro posed hybridtough ening conc ept results in comp osite laminates withno penalty from the standpoint of structural applicabil-i ty and with superior dam age toler ance and pene trationresistance c omp ared w ith that of non-hybridizedcarbon/epoxy laminates.Addi t iona l be nef i ts o f th is toughening con cept are thepossibil it ies of selective toughe ning, i .e . , application o fHP-PE plies at critical locatio ns in a c om pos ite structurewith respect to impact dama ge, and impact m odifica-tion of existing comp osite structures. A t the same tim eit doe s not exclud e the possibi l ity of combining hybrid-izat ion wi th o ther concep ts for compos i te tougheningsuch as toughen ed m atrices and interleafing whichshould result in comp osites with even mo re superiorimpact resistance than those p resented in this paper.A C K N O W L E D G E M E N T SThe authors would l ike to thank the C ompos i te M ateri-a l s Group o f the K U Leuven (Belg ium) for use o f the irimpact facilities, and in particular Luc Peete rs for hisassistance in performing the falling weight impactexperiments . They a lso acknow ledge the Ministry o fEc ono mic A ffairs for their support under grantI O P . P C B P 2 .2 .REFERENCES1 Greszezuk, L.G. 'Foreign object impact damage to composites'AST M STP 568 (American Society for Testing and Materials,1975)2 Starnes, J.H., Rhodes, M.D. and Williams, J.G. 'Effect ofimpact damage and holes on the compression strength of agraphite/epoxy laminate' Nondestructive Evaluation and Flaw

Criticality for Composite Materials, AS TM STP 696 (AmericanSociety for Testing and Materials, 1979) pp 145-1713 Labor, J.D. 'Impact damage effects on the strength of advancedcomposites' ibid. pp 172-1844 Starnes, J.i t. and Williams, J.G. 'Failure characteristics ofgraphite/epoxy structural components loaded in compression'NASA Technical Memorandum 84552 (Washington DC, Sep-tember 1982)5 Wang, A.S.D. 'Fracture mechanics of sublaminate cracks incomposite materials' Composite Technol Rev 6 (1984) pp 45-626 Whitehead' R'S" et a/" 'Damage tlerance f cmposite multisparwing structure' in Proc Seventh Conf on Fibrous Composites inStructural Design, A FW AL- TR-85-3094 (June 1985)7 Chan, L.C. , Gillham, J.K. , Kinloch, A.J. and Shaw, S.J.'Rubber modified epoxies I, cure transition and morphology'Advances in Chemistry No 209 (American Chem Soc, Washing-ton DC, 1984)8 Hunston, D.L. et al. 'Matrix resin effects in composite delamina-tion: mode I fracture aspects' Toughened Composites, AS TMSTP 937 edited by N.J. Johnston (American Society for Testingand Materials, 1987) pp 74-949 Hunston, D.L. 'Composite interlaminar racture: effect of matrixfracture energy' Composite Technol Rev 6 No 4 (1984) pp176-18010 Leach, D.C., Curtis, D.C. and Tamblin, D.R. 'Delaminat ion

behaviour of aromatic polymer composite APC-2' ASTM Sympon Toughened Composites, Houston, TX, 198511 Evans, R.E. and Masters, J.E. 'New generation of compositesfor structural applications: materials and mechanics' ToughenedComposites op. cit. pp 413-43612 Lemstra, P.J ., Kirschbaum, R. , Ohta, T. and Yasuda, H. inWard, I.M. (Ed) Developments in Oriented Polymers--2(Elsevier Applied Science Publ, 1987) pp 39--7713 Harpell, G.A., Kavesh, S., Parley, I. and Prevorsek, D.C. US Pat4 403 012 (Allied Corp, 1983)14 Adams, D.F., Zinunerman,R.S. and Chang, H.W. 'Properties ofpolymer matrix composites incorporating Allied A-900 polyeth-ylene fiber' SAMPEJ 21 No 6 (1985) pp 44--4815 Chang, H.W., Lin, L.C. and Bhatnagar, A. 'Properties andapplications of composites made of polyethylene fibers' Proc31st Int SAMP E Symp (1986) pp 859--86616 Zhnmerman, R.S. and Adams, D.F. 'Impact performance ofvarious fiber reinforced composites as a function of temperature'Proc 32nd Int SAM PE Symp (1987) pp 1461-147117 Beaumont, P.W.R., Riewaid, P.G. and Zweben, C. 'Methods forimproving the impact resistance of composite materials' ASTMSymp of Foreign Object Impact Behaviour (Sept 1973)18 Harris , B. and BunseH, A.R. 'Impact properties of glass fibre/carbon fibre hybrid composites' Composites 6 No 5 (1975)pp 197-20119 Mallick, P.K. and Broutman, L.J. 'Static and impact propertiesof laminated hybrid composites' J Testing and Evaluation 5(1977) pp 190-200

20 Perry, J.L. and Adams, D.F. 'Charpy impact experiments ongraphite/epoxy hybrid composites' Composites 6 No 4 (1975)pp 166-17221 Ireland, D.R. 'Ins trumented mpact testing for evaluatingend-use performance' Physical Testing of Plastics-Correlationwith End-Use Performance, AST M STP 736 (American Societyfor Testing and Materials, 1981) pp 45-5822 Adams, D.F. and Zimmerman, R.S. 'Static and impact perfor-mance of polyethylene fiber/graphite fiber hybrid composites'Proc 31st lnt S AMP E Symp (1986) pp 1456-146823 Poursartip, A., Riahi, G., T eghtsoonian, E. and Chinatambi, N.'Intralaminarpolyethylene fibre-carbon fibre hybrid laminates'Proc 19th Int SAMPE Tech Conf(1987) pp 503-51424 Peijs, A.A.J.M. and Lemstra, P.J. 'Hybrid composites based onpolyethylene and carbon fibres; compressive and impact behav-iour' Proc 3rd Rolduc Polymer Meeting (Elsevier Appl Sci Publ,1989) pp 218-22725 Peijs, A.A .J.M ., Catsman, P. , G ovaert, L.E. and Lemstra, P.J.'Hybrid composites based on polyethylene and carbon fibres.Part 2: influence of composition and adhesion level of polyethyl-ene fibres on mechanical properties' Composites 21 No 6(November 1990) pp 513-52126 Ladizesky, N.H. and Ward, I.M. J Mater Sci 18 (1983) pp533-54427 Mercx, F.P.M., Benzina, A., Langeveld, A.D. and Lemstra, P.J.to be published28 Verpoest, ., Marii~n,J., Devos, J. and Wevers, M. 'Absorbedenergy a test independent parameter for damage and residualstrength after impact?' Proc TE Q C 87, Guildford, UK (1987)pp 69-7729 Cantwell, W.J. and Morton, J. 'Detection of impact damage inCFRP laminates' Composite Structures (1985) pp 241-258

A U T H O R SThe authors are w i th the Departm ent o f Po lymerTechn ology , Eindhoven Universi ty o f Technology , POB ox 513 , 5600 MB Eindhoven, The Netherlands .Enquiries should be addressed to A .A .J .M . Peijs .

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