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The impact res is tance ofc o m p o s i t e m a t e r i a l s -
- - a r e v i e wW.J. CANTWELL andJ. MO RTO N(* Ecole Polytech
nique Fdd#rale de Lausanne, Sw itzerlandl Virginia
PolytechnicInstitute an d State University, USA)I n th i s paper
the im pac t response o f con t inuo us f i b re - re in fo rced co
mp os i tesis r e v ie w e d . A n a t t e m p t i s m a d e t o d
r a w t o g e t h e r m u c h o f t h e w o r k p u b -l ished in
the l i t e ra tu re and to iden t i f y the funda me nta l p a
ramete rs de te rm in -i n g th e i m p a c t resistance of con t
inu ous f ib re - re in fo rced com pos i te mate r ia l s.The e f
fec t o f va ry ing the p roper t ies o f the f ib re , m at r i x
and in te rphase a reexam ined as we l l as the ro le o f ta rge t
ge om et ry and load ing ra te on thedyn am ic response o f these
mate r ia ls .Key words: com pos i te m ater i a ls ; imp ac t t es
t i ng ; impac t res i stance ; res idua ls t reng th ; f a i lu re
m ode ; f i b re p roper t i es ; m at r i x p roper t i es ; i n
te rphase ;l amina te s tack ing sequence; geom et r y ; l oad ing
ra te
F i b r e - r e in f o r c e d c o m p o s i t e m a t e r ia l
s s u c h a s c a r b o n ,g l a ss a n d K e v l a r ~ f i b r e -
r e i n f o r c e d p l a s t ic s a r e f i n d i n gi n c r e as
i n g u s e in a w i d e r a n g e o f b o t h l o w a n d h i g ht
e c h n o l o g y e n g i n e e r i n g a p p l i ca t i o n s . C
o m p o s i t e s o f f e ra n u m b e r o f d is t in c t a d v a
n t a g e s o v e r m o r e c o n v e n -t i o n a l e n g i n e e
r i n g m a t e r i al s s u c h a s a l u m i n i u m a n ds t e e
l . T h e s e i n c l u d e h i g h e r s p e c i fi c s t r e n g
t h s a n ds t i ff n e s s e s , s u p e r i o r c o r r o s i o n
r e s i s t a n c e a s w e l l a si m p r o v e d f a t i g u e p
r o p e r t i e s . C o u p l e d w i t h th e s ei m p r o v e m e
n t s i n g e n e r a l o v e r a l l p e r f o r m a n c e i s t h
ef a c t t h a t t h e c o s t o f m a n u f a c t u r i n g c o m
p o n e n t s f r o mf i b r e - r e i n f o r c e d p l a s t i c
is o f t e n l e s s t h a n w i t h m o r ec o n v e n t i o n a l
m e t a l s . T h i s is t r u e n o t o n l y a t t h e l o w -c o
s t e n d o f t h e m a r k e t o c c u p i e d b y s h e e t m o u
l d i n gc o m p o u n d s ( S M C) b u t a l so i n t h e a e r o
s p a c e s e c t o rw h e r e c o m p l e x l o a d -b e a r in g
s h a p e s c a n b e p r o d u c e d i na l i m i t e d n u m b e
r o f s te p s , s a v in g t i m e i n b o t h j o i n i n ga n d
a s s e m b l y .C o m p o s i t e m a t e r i a ls d o , h o w e v
e r , s u f fe r s o m e s e r io u sl i m i ta t i o n s. P e r h
a p s t h e m o s t s i g n i fi c a n t a m o n g s t t h e s ei s
t h e i r r e s p o n s e t o l o c a l iz e d i m p a c t l o a d
i n g s u c h a st h a t i m p a r t e d b y a d r o p p e d t o o
l o r r u n w a y d e b r i s . I nr e c e n t y e ar s m a n y r e
s e a rc h p r o g r a m m e s h a v e b e e nu n d e r t a k e n i
n a n a t t e m p t t o b e t t e r u n d e r s t a n d t h ei m p
a c t r e s p o n s e o f t h e s e m a t e r i a l s 1 -9 . T h e
m a j o r i t y o ft h is w o r k h a s b e e n u n d e r t a k e n
o n c o n t i n u o u s fi b re ,h i g h p e r f o r m a n c e c o
m p o s i t e s s in c e t h e s e m a t e r i a l s a ref i n d i
n g i n c r e a s in g u s e i n t h e d e s i g n o f a l a r g e
n u m b e r o fc i v il a n d m i l i t a r y a i r c r a ft , i .
e . , i n c i r c u m s t a n c e s w h e r et h e c o n s e q u e
n c e s o f i m p a c t a r e l i k e ly to b e m o s ts e r i o u
s .T h e m a n n e r i n w h i ch c o m p o s i t e m a t e ri a ls
r e s p o n d t oi m p a c t l o a d i n g a n d d i s s i p a te t
h e i n c i d e n t k i n e t ic e n e r g yo f t h e p r o j e c t
i l e i s v e ry d i f f e r e n t t o t h a t o f m e t a l s. F o
r
l o w a n d i n t e r m e d i a t e i n c i d e n t e n e r g i
e s , m e t a l s a b s o r be n e r g y t h r o u g h e l as t ic
a n d p l a st ic d e f o r m a t i o n 1.A l t h o u g h t h e la
t t e r m a y c a u s e s o m e p e r m a n e n t s tr u c -t u r a
l d e f o r m a t i o n , i ts c o n s e q u e n c e s o n t h e l
o a d -c a r ry i n g c a p a b i li t y o f t h e c o m p o n e n
t a r e u s u a l l ys m a l l 1 1. A t h i g h i n c i d e n t i m
p a c t e n e r g i e s t a r g e t p e r f o -r a t io n m a y o c
c u r a n d t h e p a s s a g e o f t h e i m p a c t o r w i llg e
n e r a l l y r e s u l t i n p e t a l l i n g , c r a c k i n g a
n d s p a l l i n g 12.A l t h o u g h s u c h d a m a g e w i ll d
e g r a d e t h e l o a d - b e a r i n ga b i l i ty o f t h e s t
r u c t u r e , i ts e f f e c ts c a n g e n e r a l l y b ep r e
d i c t e d u s i n g f r a c t u r e m e c h a n i c s p r i n c i
p l e s 2 . I nc o m p o s i t e s , h o w e v e r , t h e a b i li
ty t o u n d e r g o p l a s ti cd e f o r m a t i o n i s e x t re
m e l y l i m i t e d w i th t h e r e s u lt t h a te n e r g y i
s f r e q u e n t l y a b s o r b e d i n c r e a t i n g l a r g e
a r e a s o ff r a c tu r e w i t h e n s u i n g r e d u c t io n
s i n b o t h s t r e n g t h a n ds t if f n e ss 1 .1 3. F u r t
h e r m o r e , t h e p r e d i c t i o n o f t h e p o s t -i m p
a c t l o a d - b e a r i n g c a p a b il i ty o f a d a m a g e d
c o m p o s i t es t r u c t u r e i s m o r e d i f fi c u l t t h
a n f o r m e t a l s s i n c e t h ed a m a g e z o n e i s g e n
e r a ll y c o m p l e x i n n a t u r e a n dc o n s e q u e n t l
y v e r y d i f fi c u l t to c h a r a c t e r i z e 12. T h ep r
o b l e m i s f u r t h e r c o m p l i c a t e d b y t h e l a ck
o f e x i s ti n gs t a n d a r d s o r e s t a b l is h e d t e s
t in g t e c h n i q u e s f o r i m p a c to f c o m p o s i t e m
a t e r ia l s . M u c h o f t h e w o r k p u b l i s h e d i nt h
e l i te r a t u r e h a s b e e n c o n d u c t e d o n p u r p o
s e - b u i l tm a c h i n e s u si n g c o n v e n i e n t s p e c
i m e n g e o m e t r i e s . A s ar e s u lt , d i r e c t c o m p
a r i s o n s b e t w e e n d i f f e re n t m a t e r i a ls y s t
e m s i s o f t e n v e r y d i f f i c u lt a n d i m m e d i a t
e c o n c l u -s i on s a re s o m e t i m e s h a r d t o d r a w
.T h e o b j e c t i v e o f t h i s a r t ic l e i s t o d r a w t
o g e t h e r t h ep e r t i n e n t f i n d in g s o f m a n y o f
t h e a r ti c le s p u b l i s h e d i nt h e f ie l d o f i m p a
c t o f f ib r e - re i n f o r c e d c o m p o s i t e s a n dt h
e r e b y p r e s e n t a g l o b a l v i e w o f t h e s t a t e -
o f - th e - a r t .I n i t ia l l y , t h e p a p e r w i l l r e
v i e w b r i e f l y t h e t e c h n i q u e sm o s t f re q u e n
t l y e m p l o y e d f o r a s s es s in g t h e i m p a c t
0010-4361/91/050347-16 ~) 1991 Butterw orth-Heine ma nn LtdCOMPO
SITES. VOLUM E 22. NUMB ER 5. SEPTEMBER 1991 3 4 7
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8/6/2019 Cantwell 91
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r e s p o n s e o f c o m p o s i t e m a t e r ia l s . F o l l
o w i n g t h i s, t h ee f fe c t o f v a ry i n g f u n d a m e n
t a l p a r a m e t e r s s u c h a s f ib r ea n d m a t r ix p r
o p e r t i e s a n d s t a c k i n g s e q u e n c e o n t h ed y
n a m i c re s p o n s e o f t h e s e m a t e r i a l s w i ll b e
c o n s i d e r e d .S i n c e a s u b s t a n ti a l a m o u n t o
f t h e w o r k p u b l i s h e d i n t h el i t e r a t u r e r e
l a te s t o t e s ts o n c a r b o n f i b r e c o m p o s i t e s
,t h e m a i n f o c u s o f t h i s r e p o r t w i l l b e o n t
h i s m a t e r i a l .H o w e v e r , w o r k u n d e r t a k e n
o n o t h e r m a t e r ia l s w il l b eu s e d i n o r d e r t o
s u p p l e m e n t a n d s u b s t a n t i a t e i d e a s a n dt
h e o r i e s .IMPAC T TEST TECHNIOUES FOR COMPO SITEMATERIALSI d e
a l ly , t h e i m p a c t t e s t fi x t u re s h o u l d b e d e
s i g n e d t os i m u l a t e t h e l o a d i n g c o n d i t i o
n s t o w h i c h a c o m p o s i t ec o m p o n e n t is s u b j e
c t i n o p e r a t i o n a l s e rv i c e a n d t h e nr e p r o d
u c e t h e f a i lu r e m o d e s a n d m e c h a n i s m s li k e
ly too c c u r . I n s im p l e t e rm s , t h e i m p a c t p r o
b l e m c a n b ed i v i d e d i n t o t w o s e p a r a t e c o n
d i t i o n s: l o w v e l o c i t yi m p a c t b y a l a rg e m a
s s ( d r o p p e d t o o l ) a n d h i g h v e l o c i t yi m p a
c t b y a s m a l l m a s s ( r u n w a y d e b r i s , s m a l l a
r m s f i r e ,e t c . ) . T h e f o r m e r i s g e n e r a l l y
s i m u l a t e d u s i n g a f a l li n gw e i g h t o r a s w i n
g i n g p e n d u l u m a n d t h e l a t t e r u s i n g ag a s g
u n o r s o m e o t h e r b a l li st ic l a u n c h e r . H o w e
v e r , a ss t a t e d p r e v i o u s l y , d u e t o a l a c k o
f e x p e r i m e n t a l s t a n -d a r d s a w i d e v a r i e t
y o f t e s t in g t e c h n i q u e s i s p r e s e n t l yb e i n
g e m p l o y e d i n o r d e r t o a s se s s th e d y n a m i cr
e s p o n s e o f r e i n f o r c e d p l a st ic s 1 - 4 , 1 4 - 1
7 , m a k i n g d i re c tc o m p a r i s o n d i f f i c u l t .I
n t h is s e c ti o n t h e m o r e c o m m o n l y u s e d t e c h
n i q u e s w i l lb e p r e s e n t e d a n d d i s c u s s e d as
w e l l a s t h e p r o b l e m sa s s o c i a t e d w i t h t h e
e n s u i n g d a t a a n a l y s i s.Low velocity impa ctM a c h i
n e s c u r r e n t l y u s e d f o r s i m u l a t in g t h e l o
w v e l o c i tyi m p a c t r e s p o n s e o f c o m p o s i t e m
a t e ri a l s i n c lu d e t h eC h a r p y a n d I z o d p e n d
u l u m s , t h e f a ll in g w e i g h t fi x t u re ss u c h a s
t h e G a r d n e r a n d d r o p d a r t t e s t s as w e l l a sh
y d r a u l i c m a c h i n e s d e s i g n e d t o p e r f o r m b
o t h i n - p l a n ea n d o u t - o f - p l a n e t e s t i n g a
t v e l o c i ti e s u p t o 1 0 m s 1.
1. Charpy pen dulumM a n y o f t h e e a r l y im p a c t s t u
d ie s o n c o m p o s i t e m a t e r i-a ls w e r e u n d e r t a
k e n u s in g t h e C h a r p y t e s t m e t h o do r i g i n a l
ly d e v e l o p e d f o r t e s t i n g m e t a l s 11 '1 s'1 9. T
h er e a s o n f o r t h i s ch o i c e w a s t h e f a c t t h a t
t h e C h a r p yp e n d u l u m i s b o t h s i m p l e t o u s e
a n d c a n b e i n s t ru -m e n t e d , a n d t h e r e f o r e ,
i n p r in c i p l e , c a n y ie l d i n f o r m a -t i o n o n t
h e p r o c e s s e s o f e n e r g y a b s o r p t i o n a n d d i
ss ip a -t i o n i n c o m p o s i t e s . T h e t e s t s p e c i
m e n i s g e n e r al l y at h i c k b e a m , s o m e t i m e s i
n c o r p o r a t i n g a n o t c h a t i ts m i dp o i n t a s s h
o w n i n Fi g . l ( a ) . T h e s p e c i m e n i s s u p p o r t
e di n a h o r iz o n t a l p l a n e a n d i m p a c t e d b y t h
e s w i n g i n gp e n d u l u m d i re c tl y o p p o s i te t h e
n o t c h . T h e e n e r g yd i s s ip a t e d d u r i n g i m p a
c t i s u s u a ll y r e c o r d e d b y a d i a lo n t h e t e s t
a p p a r a t u s . F u r t h e r i n f o r m a t i o n c a n b eo
b t a i n e d b y i n s t r u m e n t i n g t h e i m p a c t o r w
i t h a st r ai ng a u g e , t h e r e b y e n a b l i n g t h e d
e t e r m i n a t i o n o f t h ev a r i a ti o n o f t h e i m p a
c t f o rc e w i t h t im e . T h e e n e r g ya b s o r b e d d u
r i n g i m p a c t c a n a ls o b e d e t e r m i n e d b yi n t e
g r a t i n g t h e f o r c e / t i m e s i g n a l .
S p e c i m e nN o t c h I / - - ~ p a c t o r
W e i g
I n e r t i a a r
S t ra in a u ge - I [
S p e c i m e n
~ o rS p e c im e n [ _ _ / IN o t c h S tr a in a u g e s
' P " t rb e
F i g . 1 ( a ) C h a r p y i m p a c t t e s t ; (b ) Iz o d i
m p a c t t e s t ; ( c ) t e n s i l eH o p k i n s o n - b a r t
e s t; ( d ) g a s g u nT h e t e s t s e t - u p s u ff e rs a n u
m b e r o f d i s a d v a n t a g e s s u c ha s t h e f a c t t h
a t t h e l o a d / t i m e c u r v e s o f t e n c o n t a i n h i
g hf r e q u e n c y h a r m o n i c o s c i l l at io n s r e s u
l ti n g f r o m t h en a t u r a l r e s p o n s e o f t h e i m p
a c t o r , e t c. T h e s e e f fe c t s c a ng e n e r a l ly b e
f i l t e re d o n c e t h e h a r m o n i c f r e q u e n c i e s
o ft h e v a ri o u s c o m p o n e n t s h a v e b e e n d e t e r
m i n e d 2 . A ss t a t e d p r e v i o u s l y , t h e t e s t s
p e c i m e n i s a s h o r t , t h i c kb e a m a n d i s n o t t
h e r e f o r e t y p ic a l o f e n g i n e e r i n gc o m p o n e
n t s . F u r t h e r , t h e t e s t is d e s t r u c t iv e , i n
d u c i n gf a il u r e m o d e s t h a t a r e n o t n e c e s s
ar i ly o b s e r v e d u n d e rl o w v e l o c i t y im p a c t l
o a d i n g o n o p e r a t i o n a l s t ru c t u r e s..21B a d e
r a n d E l li s s h o w e d t h a t t h e C h a r p y e n e r g y
o fc a r b o n f i b r e - r e i n f o r c e d p l a s t i c ( CF R
P) v a r i e d w i t hs p e c i m e n g e o m e t r y a n d d o u b
t , t h e r e f o r e , t h e a p p li c a -b i l it y o f t h e t
e c h n i q u e . T h e C h a r p y t e s t i s o n l y s u i t a b
l ef o r r a n k i n g t h e i m p a c t p e r f o r m a n c e o f
c o n t i n u o u s f ib r e
348 COM POSITES. SEPTEMBER 1991
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c om p os i t e s a nd a s a f ir s t s t e p i n de t e r m in
ing thed y n a m i c t o u g h n e s s o f t h e s e m a t e r ia l
s .2. Izod testTh e I z o d im pa c t t e s t i s show n sc he m a
t i c a l ly i n F ig .l ( b ) . T h e t e s t s e t -u p a n d p r
o c e d u r e a r e s i m i la r t o t h o s eo u t l i n e d a b o
v e . I n t h e I z o d t e s t s p e c i m e n is c l a m p e d i
nthe ve r t i c a l p l a ne a s a c a n t i l e ve r be a m a n d
im pa c te d bya s w i n gi n g p e n d u l u m a t t h e u n s u p
p o r t e d e n d . T h e t e s ts u f fe r s s i m i la r p r o b
l e m s t o t h o s e r e p o r t e d a b o v e a n da ga in is be
s t su i t e d a s a t oo l f o r r a nk ing the im pa c tr e s i s
t a nc e o f c om pos i t e m a te r i a l s .3 . D ro p-we igh t
imp ac t t es tsH e r e a w e i g h t is a ll o w e d t o f a l l f
r o m a pre-determinedhe i gh t t o s t r i ke t he t es t spec i
men o r p l a t e sup po r t ed i nt he ho r i zon t a l p lane . I
n g ene r a l , t he imp ac t even t doesno t cause com p l e t e
de s t r uc t i on o f t he t es t spec imen h u tr e b o u n d s ,
e n a b l in g a r e s id u a l e n e r g y t o b e d e t e r m i n
e di f necessary. T he i nc i de n t ve l oc i ty o f the i mpa c t
o r canb e d e t e r m i n e d f r o m t h e equat ions o f m o t i
o n o r b yus ing op t i c a l s e nso r s l oc a t e d ju s t a
bov e the t a r ge t .F r e q u e n t l y , t h e i m p a c t o r
is i n s t r u m e n t e d , e n a b l i n g t h ef o r c e / ti m
e c h a ra c t e ri s t ic s to b e d e t e r m i n e d , a n d m a
ya l so c o n t a i n a d i s p l a c e m e n t t r a n s d u c e r
i n o r d e r t op e r m i t t h e d e t e r m i n a t i o n o f e
n e r g y d i s s ip a t io n d u r i n gt h e i m p a c t e v e n
t . O n e o f t h e a d v a n t a g e s o f t hi s t e stw i th r e
spe c t t o t he C h a r py a n d I z od t e s t s is t ha t a w
ide rr a n g e o f ( e s t g e o m e t r i e s c a n b e t e s t e
d , t h e r e b ye n a b l i n g m o r e c o m p l e x c o m p o n
e n t s t o b e t e s t e d .A l t h o u g h t e s t i n g is g e n
e r a l ly u n d e r t a k e n u s i n g ahe m i - sph e r i c a l
im pa c to r , i t is pos s ib l e t o u se o the rim pa c to r sha
pe s suc h a s b lun t c y l inde r s o r sha r p p o in t s .V a r
i a t io n s o n t h e d r o p - w e i g h t t h e m e i n c l u d
e th eG a r d n e r t e s t w h e r e a h e m i - s p h e r ic a l
im p a c t o r s t r i k es asm a l l d i a m e te r c i r c u l a
r p l a t e a nd th e d r ive n da r t t e s tw h e r e a he m i -
sphe r i c a l p r ob e i s d r ive n in to t hes p e c i m e n a t
a p r e - d e t e r m i n e d r a t e 22.4 . Hyd rau l i c t es t
ma ch inesI n r e c e n t y e a r s a n u m b e r o f w o r k e r s
h a v e u s e d h y d r a u -l ic t es t mach i nes f o r assessing
t he de f o r ma t i on andfa i l u re c harac ter i s t ics o f
mater i a l s a t h igh ra tes o fst rain 23'24. H er e, test
geometr ies such as tens i l e do g-bone spec i mens o r dou b l e
can t i l eve r beam (DCB) y p espec i mens can be t es ted ove r a
w i de r ange o f s t r a i nr at es T he s t r a i n h i s t o r y
o f t he spec i men can bemeasu r ed us i ng bon ded s t r a i n
gauges o r an op t i ca lt r ansduce r . I f a s tr a i n gauge o r
any o t he r d i sp l acem en tme asu r i ng dev ice i s bonded t o
t he sp ec i men , t he s t r a i nr a t e sens i t iv i ty o f t
he adhes i ve shou l d be cons i de r ed .T he advan t age o f t h
i s techn i que i s t ha t t he t es t spec i-mens pe r m i t t he
eva l ua t i on o f bas i c ma t e r i a l p r ope r t i essuch as
t ens i le s t r eng t h , m odu l us and i n t e r l am i na rf r
ac t u r e t oughness w i t hou t t he c on t ac t e f f ec ts
associ-a t ed w i t h f a l l ing w e i gh t i mpac t . E x t r em
e ca r e has t o bet aken i n o r de r t o ensu r e t ha t t he
mass o f t he l oad ce l land g r i pp i ng sys tem a r e as low as
poss i b l e s ince i ne r t i a le f fec t s resu l t i ng f r om
t hese com pone n t s may concea lt he t r ue ma t e r i a l r
esponse24. C a u t i on shou l d a l so bee x e r c i s ed w h e n
a p p l y in g f r a c t u r e m e c h a n i c s p r in c i p le
s
to ge o m e t r i e s suc h a s t he D CB s inc e t he spe c im
e n m a yn o t b e d e f o r m i n g i n t h e s a m e m o d e a s
a s i m il a rs t a t i c a ll y - loa de d spe c im e n24.I n te
rmed ia te and h igh ra te imp ac t1. Hop k inson-bar t echn iqueT
h e H o p k i n s o n - b a r t e c h n i q u e i s s i m i la r t
o t h e p r e v i o u st e s t p r o c e d u r e i n t h a t i t p
e r m i t s t h e d e t e r m i n a t i o n o fthe va r i a t i on
o f ba s i c m a te r i a l p r op e r t i e s a s a f unc t iono f
s t r ai n r a t e. S e v e r a l d i ff e r e n t t y p e s o f H
o p k i n s o n - b a ra r e c u r r e n t l y e m p l o y e d ; t
h e s e i n c l u d e t h e p u n c h -l o a d e d H o p k i n s o
n - b a r , t h e c o m p r e s s i o n b a r , t h et e n s il e b
a r a n d t h e H o p k i n s o n - b a r s h e a r t e s tz~-27. T
h es e t -u p a n d e x p e r i m e n t a l p r o c e d u r e s a s
s o c i at e d w i t hthe se t e s t s ha s be e n d i s c us se d
in de t a i l e l s e w he r e 25.H e r e , a b r i e f de sc r ip
t ion o f t he t e ns i l e t e s t w il l beg i v en . T h e t e s
t s e t -u p f o r u n d e r t a k i n g d y n a m i c t e n si l
et e s t s i s show n in F ig . l ( c ) ~ . H e r e , spe c im e ns
w a i s t e dt h r o u g h t h e t h i ck n e s s a r e b o n d e d
i n t o s lo t s in t h eine r t i a ba r a nd inpu t ba r a s show
n in t he d i a g r a m . T h i sc on f igu r a t ion is t he n p l
a c e d w i th in t he w e ighba r . S t r a inr a t e s a pp r oa
c h in g 1000 s 1 c a n the n b e a c h ie ve d u s ingga s - d r
ive n p r o j e c t il e s t o a c c e l e r a t e t he w e ig hba
r a nd ,in t u r n , l oa d the i npu t ba r 25. The ine r t i a o
f t he i n e r t i ab a r t h e n e n a b l e s t h e te s t c o u
p o n t o b e l o a d e d d y n a m i -c a l ly . S t ra in ga uge
s bond e d to t he i npu t a nd ine r t i aba r s e na b le t h e i
nc ide n t a nd r e f l e c t e d s tr e s s w a ve s t ob e a n a
l y s e d a n d p e r m i t t h e d e t e r m i n a t i o n o f a d
y n a m i cs t r e s s/ s t ra in c u r ve f o r t he m a te r i a
l . The H opk inson - ba rt e ns i l e t e s t i s de s t r uc t
ive .C a r e h a s t o b e t a k e n i n o r d e r t o e n s u r e
t h a t t h e i n t e r -f a c e b e t w e e n t h e s p e c i m e
n a n d l o a d i n g b a r s i s g o o d ,o the r w i se a she a r
f a i l u r e w i th in t he g r ipp ing se c t ion isl i ke ly t o
oc c u r . F u r the r , i n o r de r t o m in im iz e s t r e ssc
onc e n t r a t i ons a s soc i a t e d w i th t he g r ipp ing a r
e a ,r e l at i v el y l o n g s p e c i m e n s ( a p p r o x i m
a t e l y 6 0 m m ) a r e
28r e q u i r e d w h e n t e s ti n g c o m p o s i t e m a t e
r i a ls .
2. Gas gun im pa ct tes t ingI m p a c t t e s t i ng a t ba l l
is t ic r a t e s o f s t r a in c a n b e a c h ie v e dus ing a h
igh p r e s su r e ga s gun suc h a s t ha t show n in F ig .~9l (
d ) . Typ ic a l ly , a gas suc h a s n i t r oge n i s f e d t o
ac h a m b e r l o c a t e d a t o n e e n d o f t h e b a r r el .
H e r e t h e g a si s r e s t r a ine d by a p l a s ti c d i a ph
r a gm . W he n the ga s ha sr e a c h e d a p r e - d e t e r m i
n e d v a l u e t h e d i a p h r a g m i s b u r st( by e l e c t
r ic a l he a t ing o r a m e c h a n ic a l punc tu r ingde v ic e
) , a c c e l e r a t i ng a p r o j e c t i l e dow n the ba r r e
l t os t r ik e a s p e c i m e n o r c o m p o n e n t s u p p o r
t e d v e r ti c a ll y .T h e v e l o ci t y o f t h e i m p a c t
o r c a n b e d e t e r m i n e d j u s tp r io r t o im pa c t u s
ing op ti c a l s e nso r s3 or by us ing as im p le b r e a k - w
i r e t e c hn iq ue 29. Ge ner a l ly , the tes t i sno t c om ple
t e ly de s t r uc t ive bu t f r e q ue n t ly r e su l ts i nl a
r ge - sc a l e da m a ge a nd /o r t a r ge t pe r f o r a t i on
. U n t i lr e c e n t l y t h e t e c h n i q u e s u f f e r e d
th e d i s a d v a n t ag e t h a tv e r y l it tl e i n f o r m a
t i o n c o u l d b e o b t a i n e d f r o m t h e t e s ti ts e
lf . H o w e v e r , i n s t r u m e n t e d g a s g u n s h a v e
n o w b e e nde ve lo pe d , e na b l ing f o r c e /d i sp l a c e
m e n t h i s to r ie s t o bem e a s u r e d a n d t h e i m p a c
t e v e n t t o b e a n a l y s e d i n m o r ede ta il31,32 . Ga s
gu ns can be used to te s t la rg e s t ruc tu resa nd a r e t he r
e f o r e u se f u l f o r a s se ss ing the h igh ve loc i tyim pa
c t r e spons e o f c om p os i t e m a te r i a l s.
C O M P O S I T E S . S E P T E M B E R 1 99 1 3 4 9
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8/6/2019 Cantwell 91
4/16
ConclusionsA t p re s en t , n o accep t ab l e s t an d a rd t
e s t in g p ro ce d u resa re av a i lab l e fo r i m p ac t t e s
t i n g o f co m p o s i t e m a t e r i a ls .C o n s e q u en t l
y , a w i d e v a r i e t y o f te s t i n g p ro ced u res ,s p e
c im e n g e o m e t r i e s a n d d a t a r e d u c t i o n t e c
h n iq u e s a r ep r e s e n tl y b e i n g e m p l o y e d . P e
n d u l u m t e c h n i q u e s s u c has th e C h a rp y an d I zo
d t e s t s o f t en r eq u i r e s p ec i m e ng e o m e t r i e s
t h a t a r e n o t r e p r e s e n t a ti v e o f c o m p o n e n
td i m en s i o n s an d s o a r e e s s en t ia l l y s u i t ab
le o n l y fo rr a n k i n g t h e i m p a c t r e s p o n s e o f
c o m p o s i t e s . D r o p -w e i g h t r ig s an d g as g u n s
o f f e r m o re r ep re s en t a t i v eap p ro a ch es fo r a s s
e s si n g t h e i m p ac t r e s p o n s e o f t h es em a t e r i
a l s . G rea t e r u s e o f i n s t ru m en t ed i m p ac t o r s
h asl e d t o a d e e p e r u n d e r s t a n d i n g o f th e p r
o c e s s e s o fen e rg y a b s o rp t i o n an d d i s s i p a t
io n i n t h es e m a t e r i a l s .M o re r ecen t l y , h y d
rau l i c m ach i n es h av e b een u s ed t oex am i n e t h e r e
s p o n s e o f co m p o s i t e m a t e r i a ls a t h i g hr a te
s o f s tr a in . A l t h o u g h s o m e w h a t m o r e e x p e n
s iv et h a n o t h e r , m o r e c o n v e n t i o n a l t e c h n
i q u e s, s u c hm ach i n es a r e i d ea ll y s u i t ed fo r an
a l y s in g t h e r a t ed ep e n d e n cy o f b asi c m a t e r i
a l p ro p e r t i e s .INFLUENCE OF CONSTITUENT PROPERTIES ONTHE
IMPACT RESPONSE OF COMPOSITEMATERIALSIn s i rq p le t e rm s , a f
i b r e - r e i n fo rced co m p o s i t e isco m p o s ed o f t h
r ee co n s t i t u en t s: t h e f i b r e s, t h e m a t r i xan
d a f i n e i n t e rp h as e r eg i o n ( s o m e t i m es r e f e
r r ed t o a st h e i n t e r f ace ) r e s p o n s i b l e fo r a
s su r i n g t h e b o n db e t w e e n t h e m a t ri x a n d f i
b re . T h e m a n n e r i n w h i c h th em a t e r ia l d e f o r
m s a n d f r a c t u r e s d e p e n d s u p o n b o t h t h ech
em i ca l an d m ech an i ca l p ro p e r t i e s o f th es e t h r
eeco n s t i t u en t s . D e t a i l ed an a l y s es u s in g b o
t h o p t i ca l an ds can n i n g e l ec t ro n m i c ro s co p es
h av e i d en t i f i ed an u m b er o f f a il u r e m ech an i s
m s in f i b r e - r e in fo rcedco m p o s i t e m a t e r ia l
s29 '3336 . Th ese include delam ina-t i o n , i n t r a l am i n a
r m a t r i x c r ack i n g , l o n g i t u d i n a l m a t r i xs
p l it t in g , f i b r e /m a t r i x d eb o n d i n g , f i b r e
p u l l -o u t an df i b re f r ac t u re . T h e r e l a ti v e en
e rg y ab s o rb i n g cap ab i l it yo f t h e s e f r a ct u r e
m o d e s d e p e n d s u p o n t h e b a si c p r o p e r -t ie s
o f t h e co n s t i t u en t s a s w e ll a s t h e l o ad i n g m
o d e .T y p i ca l v al u e s m e a s u r e d f o r a n u m b e r
o f c o n t i n u o u s
f i b re co m p o s i t e s a r e g i v en i n T ab l e 1 . G en
e ra l l ys p eak i n g , f a i lu r e m o d es t h a t i n v o l v
e f r ac t u re o f t h em a t r i x o r i n t e rp h as e r eg i o
n r e s u l t i n l o w f r ac t u reen e rg i e s w h ereas f a i
lu r e s i n v o l v i n g f i b re f r ac t u re r e s u l tin s
ign i f i can tly gre ater ene rgy d i ss ipat ion .Th e fo l low
ing sect ions wil l inves t igate the ro le o f them a t e r ia l c
o n s t i t u e n ts i n d e p e n d e n t l y a n d e x a m i n e
t h ei n f l u en ce o f v a ry i n g th e i r p ro p e r t i e s o
n t h e o v e ra l l
m p ac t r e s i s tan ce o f t h e c o m p o s i t e .FibreT h
e ro l e o f t h e f i b r e s i n a co m p o s i t e s tru c t u
re i sex t r em e l y im p o r t an t s i n ce t h ey a r e r e s p
o n s i b l e fo rb ea r i n g a s i g n i fi can t p e rcen t ag e
o f t h e ap p l i ed l o ad . A tp re s en t , m an y t y p es o f
f ib r e a r e av a i l ab le . I n ae ro n au -t i ca l app l ica
t ion s these includ e carb on , g lass , andKevlaP~ f ib res . Wi
th in each of these ca teg or ies f ib resex h i b i ti n g a w i d
e r an g e o f m ech an i ca l p ro p e r t i e s a r eavai lab le
. U nfo r tun ate ly , i t is o f te n d i f f icu l t to sepa ra
tet h e e f f ec t s o f m ech an i ca l p ro p e r t i e s ( s u
ch a s s t r en g t hand s t if fn ess ) f ro m t h o s e a r i s i
n g f ro m g eo m et r i ca lf ac t o r s (s u ch a s f ib r e s h
ap e an d d i am e t e r ) an d i n t e rf a -c i al p ro p e r t i
e s ( s u ch a s t h e s t r en g t h o f t h e ch em i ca lbond b
e t w e en f i b re an d m a t r i x ).E a r l y s t u d i e s3 6,
i n w h i ch t h e r e l a ti v e p e r fo rm a n ce o f an u m b e
r o f c o n t i n u o u s f ib r e c o m p o s i t e s w a s e x a
m i n e d ,s u g g es t ed t h a t t h e C l i a rp y i m p ac t r
e s i s t an ce o f S -gl as sa n d K e v la r~ f i b r e - r e in
fo rced co m p o s i t e s w as o v e r f i v et im e s g r e a t e
r t h a n t h a t o f a M o d m o r I I ca rb o n f i b re -r e i n
fo r c e d c o m p o s i te . B e a u m o n t e t a l . i s ex am i
n ed t h eC h arp y l o ad / t i m e t r aces o f a n u m b er o f
m a t e r i a l s an ds h o w e d t h a t t h e c u r v e c o r r e
s p o n d i n g t o a n H M Scarb o n co m p o s i t e w as ex t r
em e l y b r i t tl e , f a i l in g ca t a -s t ro p h ica l l y a
t m ax i m u m l o ad . T h e E -g l a ss an d K ev l a r~4 9 co m
p o s i t e s f a i led i n a m o r e p ro g res s i v e m a n n e
ri n d i ca ti v e o f en e rg y d i s s i p a ti o n t h ro u g h
d e l am i n a t i o n ,s p l it t in g an d o t h e r f a i lu r e
p ro ces s es. T h e au t h o r sq u an t i f i ed t h e C h a rp y
l o ad / t im e t r aces b y d e f i n i n g aduct i l i ty index (
D I ) , t h is b e i n g t h e r a t i o o f t h e en e rg i e sas
s o c i a ted w i t h th e c r ack p ro p a g a t i o n p h as e (
t h e a r eaa f t e r m ax i m u m l o ad ) an d t h e i n i t i a
ti o n p h as e ( t h e a r ea
T a b l e 1 . T y p i c a l v a l u e s o f t h e e n e r g y a
b s o r b i n g c a p a b i l i ty o f v a r i o u s c o n t i n u
o u s f ib r e c o m p o s i t e s f o rd i f f e r e n t fa i lu r
e m o d e sF a i lu r e m o d e M a t e r i a l T y p i c a l f
racture energy
(kJ m 2)Re f e re n c e
Sp l i t t i n gD e l a m i n a t i o nTransversef ibre f ractu
reF ibre p ul l -out
Debond ing
Typ e II CF/epoxy 0.1-1 11AS4/PEEK 3.8 60T300 /epoxy 0.1 102I M
6 / P E E K 2.2 62T re a t e d CF/epoxy 20 72Untreated CF/epoxy 60
72AS4/PEEK 128 60CFIpolyester 26 104CF/b ismale imide 8 0 0 1 0
5CF/epoxy 6 106
350 COMPO SITES. SEPTEMBER 1991
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8/6/2019 Cantwell 91
5/16
up to m a x im um loa d ) . T he r e su l t i ng duc t i l i ty
i nd i c e sf o r t h e K e v l a r- 4 9 , E - gl a ss a n d H M S
c a r b o n / e p o x yc om p os i t e s w e r e 23 , 0 . 4 a nd 0
. 0 r e spe c t ive ly , ind i c a t ingc l e a rl y t h e s u p e
r i o r e n e r g y a b s o r b i n g c a p a b i l i ty o f t h eK
e v la r f i b r e .H a n c o x 37 a nd B a de r a nd E l l is 21 c
om pa r e d th e l owv e l o c it y i m p a c t r e s p o n s e o f
c o m p o s i t e s c o n t a i n in g t y p eI a nd I I c a r bo n
f ib r e s . I n bo th c a se s i t w a s show n tha tthe m a te r
i a l s c on ta in ing type I I f i b r e s ( h ighe r f a il u r
es t r e ng th ) o f f e r e d supe r io r im pa c t r e s i s t a
nc e . S im i la rr e su l t s ha ve be e n obse r ve d f o l low
ing low ve loc i tyim pa c t t e s t s on E - a nd S - g la s s f
ib r e c om pos i t e s 15.I n a n a t te m p t to b e t t e r u n
d e r s t a n d t h e f u n d a m e n t a lp a r a m e t e r s c o
n t r o l l in g t h e p r o c e s se s o f e n e r g y a b s o r p
-t i on a nd d i s s ipa t ion in c om pos i t e m a te r i a l s ,
C ha m is e tal . 3s u n d e r t o o k I z o d i m p a c t te s t s
o n a w i d e r a n g e o fs y s te m s . T h e y c o n c l u d e d
t h a t f l e x u r e a n d i n t e r l a m i n a rs h e a r d e f
o r m a t i o n s a r e d o m i n a n t e n e r g y - a b s o r b i
n gm e c h a n i s m s i n c o m p o s i t e s a n d t h a t t h e
a r e a u n d e r t h em a te r i a l ' s l i ne a r s t r e ss / s
t ra in d i a g r a m r e p r e se n t s au s e f u l a p p r o a c
h f o r p r e d i c t i n g t h e i m p a c t r e s i s ta n c e o
fa c om po s i t e . Es se n t i a l l y , c om pos i t e s w i th
l a r ge a r e a su n d e r t h e s t re s s /s t ra i n c u r v e
a r e m o r e e f f e c t i v e e n e r g ya bso r be r s . F ig .
2 p r e se n t s t he e xp e r im e n ta l da t ao b t a i n e d b
y C h a m i s a n d S i n c l ai r39 p lo t ted as a func t io no f
t he e n e r gy a bso r b ing c a pa b i l i t y o f t he f i b r e
a sd e t e r m i n e d b y t h e e n e r g y u n d e r t h e s t at
i c te n s i les t r e s s /s t r a in c u r ve . A n e x a m in a
t ion o f t he da t a sugge s t sa p o s s ib l e r e l a ti o n s
h i p b e t w e e n t h e s e t w o p a r a m e t e r s ,w i th m a
te r i a l s c on ta in ing f i b r e s w i th a g r e a t e r s t
r a ine n e r g y a b s o r b i n g c a p a c i ty o f f e r in g i
m p r o v e d l z o de ne r g i e s . I t a ppe a r s , t h e r e f
o r e , t ha t th i s t e c hn iq uef o r m s a u s e f u l gu ide
f o r a s se ss ing a nd e va lua t ing theim pa c t r e s i s t a
nc e o f c om pos i t e m a te r i a l s . Th i s t e c h -n ique a
l so e xp la in s t he r e su l t s o f H a n c ox 37 a nd B a de
ra nd E l l is 2~, r e po r t e d a bove , w h o f oun d tha t c o
m po s i t e sb a s e d o n t y p e I I c a r b o n f ib r e s o u
t p e r f o r m e d t y p e Ic a r b o n f i b r e c o m p o s i t
e s. T h e f o r m e r h a s a n e l a s ti ce n e r g y a b s o r
b i n g c a p a b i li t y s o m e f o u r t i m e s g r e a t e
rtha n th e l a t t e r.A l t h o u g h t h e a b o v e a p p r o a
c h i s v a l u ab l e f o r e v a l u a ti n gthe im pa c t r e s
is t a nc e o f a c om pos i t e , a c om p le t ea na ly s i s
shou ld t a ke in to c ons id e r a t ion e ne r g y d is s ipa -t
i on in f a i l u r e p r oc e s se s suc h a s f i b r e /m a t r
ix de bon d ing
4 0 ,
IE
Q ..E
3530 ~-25 ~-E2O1 5 - -10
0 I t { ,0.0 0.2 , I , , , I , i { , , , 1 ,0.4 0.6 0.8
1.0Normalized strain energy
F ig . 2 Va r ia t i on o f C h a r p y i m p a c t e n e r g y
w i t h n o r m a l i z e ds t r a in e n e r g y absorb ing capac
i ty o f fibres (da ta taken f r o mR ef 39)
I I
1. 2
a n d f i b r e p u l l -o u t . B e a u m o n t4 g ives express
io ns forw o r k a s s o c ia t e d w i t h m i c r o - m e c h a n
i c a l f r a c t u rep r oc e s se s suc h as de bond ing a nd f
ib r e pu l l -ou t . Th ew o t ' k fo r de bond ing i s g ive n a
s :Wd = ad2o2fld24Ef (1)
w h e r e d = f i b r e d i a m e te r , o f = f a i l u r e s t
r e ng th o f t hef ibre , ld = l e n g t h o f t h e d e b o n d e
d z o n e a n d E f = f i b rem od u lus . Th e w or k to pu l l -
ou t is g ive n a s:W p = ~ d ~ F g 2 4 (2 )
w he re lc = c r i t ica l t r ans fe r length ( - - o td /2x)
and x =c ons t a n t f r i c t i ona l she a r s t r e s s .B e a u
m o n t c o n c l u d e d t h a t p o s t - d e b o n d f i b re s
l id i ng i sthe p r im a r y e ne r g y a bso r b ing m e c ha n i
sm in g la s s f i b r ec om p os i t e s , w he r e a s f i b r e
pu l l- ou t i s r e spons ib l e f o rm u c h o f t h e t o u g h
n e s s in a c a r b o n f i b r e c o m p o s i t e . A ne x a m i
n a t i o n o f E q u a t i o n ( 2) i n d i c a te s a s t r o n g
d e p e n -d e n c y o f w o r k t o p u l l -o u t o n f i b re d
i a m e t e r . I n t h e o r y ,inc r e a s ing the d i a m e te r
o f t he r e in f o r c ing f i b r e shou ldr e su l t in a c om
pos i t e w i th a n im p r ove d r e s i s t a nc e topu l l - ou
t a n d pe r ha ps im p r ove d toughn e ss 1L34 . M or r i sa n d
S m i t h41 r e po r t e d r e su l t s tha t a ppe a r t o suppo r
t af i b re d i a m e t e r d e p e n d e n c e i n c o n t i n u o
u s f i b r e-r e in f o r c e d c om pos i t e s . Th e i r c onc
lu s ions a r e ba se d ,h o w e v e r , o n d i f f e re n c e s b
e t w e e n g l as s a n d b o r o n f i b rec om pos i t e s w i
th d i f f e r e n t f ib r e vo lum e f r a c t ions a nda r e no
t , t he r e f o r e , c onc lu s ive .I n r e c e n t y e a r s ,
f i b re m a n u f a c t u r e r s h a v e b e e n i m p r o v -ing
the s t r a in t o f a i l u r e o f c a r bon f ib r e s by r e
duc ingthe i r d i a m e te r . Typ ic a l ly , t he f ir s t ge ne
r a t ion o f c a r bonf ib r e s suc h a s T300 a nd A S 4 ha d d
i a m e te r s o f 7 -- 8 ~tm .M o r e r e c e n t fi b r es s u c
h a s IM 6 h a v e d i a m e t e r s o fa pp r ox im a te ly 5 t im
. B y im pr ov ing the s t r a in t o f a i l u r eo f t he f i b r
e s i n t h is w a y the m a nu f a c tu r e r s ha ve a l soi m p
r o v e d t h e s t ra i n e n e r g y a b s o r b in g a b il it y
o f c o m -p o s i te s a n d t h e r e b y i m p r o v e d t h e i
r i m p a c t r e s is t a n ce .D a v i e s e t a l . 42 show e d
tha t t h e i n t e r l a m ina r f r a c tu r et o u g h n es s o
f I M r / p o l y e t h e r e t h e r k e t o n e (PEEK)( supe r io
r s t r a in e ne r gy a bso r b ing c a pa b i l i ty ) i s supe r
io rto tha t AS4/PEEK. Curso n e t a l . 4 dr e w s im i l a r c
onc lu -#on s f o l low ing low ve loc i ty im pa c t t e s t s on
the sem a te r i a l s .U n d e r c o n d i t io n s o f l o w v e
l o c i t y im p a c t l o a d i n g , i ta p p e a r s , t h e r e
f o r e , t h a t t h e i m p r o v e d t o u g h n e s s g a in e
dby inc r e a s ing the f i b r e d i a m e te r i s o f f s e t by
the r e duc -t ion in f ibre fa i lure s t ra in , r esu l t ing in
a loss of s t ra ine ne r g y a bso r b ing c a pa b i l it y . I t
m a y be , ho w e ve r , t ha ti n c r ea s i n g t h e f i b re d
i a m e t e r m i g h t l e a d t o i m p r o v e -m e n t s i n t
h e h i g h v e l o c it y im p a c t r e s p o n s e o f c o m p o
s -i t e m a te r ia l s . U n de r suc h loa d ing c ond i t i ons
, t he t a r ge tr e sponse i s m o r e l oc a l i z e d w i th e
ne r g y be ing d i s s ipa t e do v e r a n a r e a i m m e d i a
t e t o t h e p o i n t o f i m p a c t . C o n s e -que n t ly , t
he ove r a l l e ne r gy a bso r b ing a b i l it y o f t hes t r
uc tu r e i s le s s im p or t a n t a nd loc a l m e c h a n i sm
s suc h a sf i b re p u l l -o u t a n d f r a c t u r e b e c o m
e d o m i n a n t .A s w e l l a s i n f lue nc ing the im pa c t r
e s i s t a nc e o f a c om -pos i t e , t he p r ope r t i e s o f
t he f i b r e a l so ha ve a s ign i f ic a n te f f e c t upo n
th e r e s idua l l oa d - be a r ing c a pa b i l i t y o f t hem
a te r i a l . C om p os i t e sy s t e m s tha t o f f e r e xc e
l l e n t im pa c tr e s i s t a nc e do no t ne c e s sa r i l y e
xh ib i t supe r io r pos t -i m p a c t r e s id u a l p r o p e
rt i e s . F o r e x a m p l e , p o l y e t h y l e n e
C O M P O S I T E S . S E P T E M B E R 1 99 1 3 51
-
8/6/2019 Cantwell 91
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f i b r e c o m p o s i t e s a r e c a p a b l e o f a b s o r
b i n g l a r g ea m o u n t s o f e n e r g y b e f o r e f a i l
u r e b u t s t il l o f f e r r e la -t i ve l y p o o r c o m p r
e s s i v e p r o p e r t ie s d u e t o t h e l o ws t i ff n e s
s o f t h e f i b r e s t h e m s e l v e s . I n m o s t c i r c u
m s t a n c e sa c o m p r o m i s e is r e q u i r e d . T h e f o
ll o w i n g s e c t io nc o n s i d e r s b r i e fl y t h e e f f
e c t o f fi b r e p r o p e r t i e s o n t h ep o s t - im p a c
t s t r e n g t h o f c o m p o s i t e m a t e r i a ls .1. Ef fec
t o f f ib re p roper t ies on pos t - impa ctres idua l s t reng
th
I n r e c e n t y e a r s a c o n c e r t e d e f f o r t h a s
b e e n m a d e b y t h em a n u f a c t u r e r s o f h ig h p e r
f o r m a n c e f i b re s t o i m p r o v eb o t h t h e s h o r
t- a n d l o n g - t e r m m e c h a n i c a l p r o p e r ti e s o
fa d v a n c e d c o m p o s i t e s . M u c h o f th i s i m p r o
v e m e n t h a sb e e n a c h i e v e d b y i n c r e a s i n g t
h e s t r a i n t o f a i l u r e o f t h er e i n f o r c i n g fi
b r e s . I n t h e p r e v i o u s s e c t i o n , t h e e n e r g
ya b s o r b i n g c a p a c i t y o f t h e f i b r e s w a s i d
e n t i f i e d a s a ni m p o r t a n t p a r a m e t e r i n d e
t e r m i n i n g t h e l e v e l o fd a m a g e i n c u r r e d i
n a c o m p o s i t e l a m i n a t e . O f t e n ,m a t e r i a l
s t h a t s a t i s fy t h i s c o n d i t i o n a l s o o f f e r
e x c e l l e n tr e s i d u a l p r o p e r t i e s . C a n t w e
l l e t a l . 43 s h o w e d t h a t t h i sw a s t h e c a s e f o
r l o w a n d h i g h s t ra i n c a r b o n f i b r ec o m p o s i
t e s . A n A S 4 c a r b o n f i br e c o m p o s i t e w i t h as
u p e r i o r s t ra i n e n e r g y a b s o r b i n g c a p a c i
t y t h a n t h a t o fa n X A S c a r b o n f i b r e c o m p o s
i t e o ff e r e d s u p e r i o rr e s i d u a l p r o p e r t i e
s , F i g . 3 . T h i s i s n o t a l w a y s t h e c a s e ,h o w
e v e r . F o r e x a m p l e , i f t h e s t i f f n e ss o f t h
e f i b r e i sv e r y l o w a n d i t s s tr a i n t o f a i l u r
e h i g h , a c o m p o s i t ec o n t a i n i n g t h e s e f i b
re s w i l l b e c a p a b l e o f a b s o r b i n gl a rg e a m o
u n t s o f e n e r g y b u t w i l l e x h i b it p o o r r e s id
u a lc o m p r e s s i v e p r o p e rt i e s . I n o r d e r t o o
v e r c o m e t h i s,h y b r i d c o m p o s i t e s a r e f r e q
u e n t l y u s e d , c o m b i n i n g t h ee n e r g y a b s o r
b i n g c a p a b i l i t y o f l o w m o d u l u s f i b r e s w i
t hs t i ff e r f i b re s c a p a b l e o f r e s i s t i n g c o
m p r e s s i v e l o a d s* * .A s s t a t e d p r e v i o u s l y
, m a n y o f t h e l a t e s t g e n e r a t i o n o fc o m p o s
i t e s a r e b a s e d o n f i b r es w i t h s m a l l e r d i a
m e t e r s .S i n c e t h e c o m p r e s s i v e s tr e n g t h o
f a c o m p o s i t e d e p e n d su p o n t h e s t a b i l it y o
f t h e f i b r e s , i t w o u l d b e e x p e c t e dt h a t s m
a l l e r d i a m e t e r f i b r e s w o u l d r e s u l t in a m
a t e r i a lw i t h p o o r e r c o m p r e s s i v e p r o p e r
t i e s. T h i s i n d e e da p p e a r s t o b e t h e c a s e 45
. H o w e v e r , r e d u c i n g t h e f i b r ed i a m e t e r i
n c r e a s e s t h e e n e r g y a b s o r b i n g c a p a b i l i
t y o ft h e c o m p o s i t e , r e s u l t i n g in l o w e r l e
v e ls o f d a m a g e f o r a
1 2 0 0~ - 1 0 0 0
e-?~ 6 0 0
~, 400n .-
200
~ . ~ ~ " ' - o . . . . . o . . T e n s io n
C o m p r e s s i o n / I ' I "I I I I0 2 4 6 8
Im pac t ene rgy ( J )F i g. 3 V a r i a t i o n o f t e n s i l
e a n d c o m p r e s s i v e r e s i d u a l s t r e n g t hw i t
h i m p a c t e n e r g y f o r l o w a n d h i g h s t r a i n f
ib r e s ( R e f 4 3 ): e ,h i g h s t r a i n f ib r e s ; o , l o
w s t r a i n f ib r e s
10
g i v e n i n c i d e n t e n e r g y . T h e r e d u c t i o n
s in p l a i n c o m p r e s -s iv e p r o p e r ti e s o f t h e c
o m p o s i t e a p p e a r t o b e o f f se t b yt h e r e d u c t
i o n i n d a m a g e a r e a .2, ConclusionsThe prop er t ies o f
the f ib res in a con t inuous f ib recompos ite have a s ign if i
can t e f fec t on the im pac tres is tance and subsequent load-bea
r ing capaci ty o fcomponents m ade f rom such mater ia ls. Fo r
lowve loc i ty impac t load ing , the ab i l i ty o f the f ib res
to s to reenergy e las t ica ll y appears to be the fun dam enta
lparam eter in de te rm in ing impac t res is tance . Kev la rf
ibres, w hich h ave large areas un de r their s tress/stra incurves
, o f fe r exce l len t impac t resistance . The ro le o fthe f ib
re d iam eter is no t com ple te ly c lear . A s imp lepu l l -ou t
m ode l sugges ts ha t compos i tes w i th la rgerd i a m e t e r f
i b re s s h o u l d b e i n h e r e n t l y t o u g h e r.H o w e
v e r , c u r r e n t t r e n d s a r e t o w a r d s s m a l l e r
d i a m e t e rf i b r es o f f e r i n g h i g h e r s t r ai n s
t o f a i l u re . A n y r e d u c t i o ni n t o u g h n e s s i s
t h e r e b y h i d d e n b y t h e i n c r e a se d e n e r g ya b
s o r b i n g c a p a c i t y o f t h e f i b r e s .Mat r i xT h e
p o l y m e r i c m a t r i x in a f i b r e - r e i n f o r c e d
c o m p o s i t es e r v e s t o p r o t e c t , a l i g n a n d s
t a b i l i z e t h e f i b r e s a s w e l l a sa s s u r e s t r
e ss t r a n s f e r f r o m o n e f i b r e t o a n o t h e r . I
ng e n e r a l , b o t h t h e s t i ff n e s s a n d s t r e n g t
h o f th e m a t r i xa r e c o n s i d e r a b l y b e l o w t h o
s e o f t h e r e i n f o r c i n g f i b re .T h e l a t t e r i s
t h e r e f o r e r e s p o n s i b l e f o r c a r ry i n g m o s
t o ft h e a p p l i e d l o a d in a c o m p o s i t e c o m p o n
e n t . T h e r o l e o ft h e m a t r i x i s n e v e r t h e l e
s s c r i ti c a l. F o r e x a m p l e ,d a m a g e t o t h e m a
t r ix s u c h a s i m p a c t - i n d u c e d d e la m i n a -t i
o n c a n r e d u c e t h e l o a d - b e a r i n g c a p a b i f i
ty o f t h ec o m p o s i t e b y u p t o 5 0 % 4 6. A s a r e s u
l t o f t h i s r e l a ti v e l yp o o r b e h a v i o u r , m u c
h w o r k ha s b e e n u n d e r t a k e n i nr e c e n t y e a r s
in a n a t t e m p t t o i d e n t if y t h e f u n d a m e n t a
lm a t r i x p r o p e r t i e s t h a t i n f l u e n c e t h e i
m p a c t r e s i s t a n c eo f c o m p o s i t e m a t e r i a l
s . S i n c e t h e f ir s t g e n e r a t i o n o fm a t r i x s y
s t e m s f o r a d v a n c e d c o m p o s i t e s la c k e d t o
u g h -n e s s , a n u m b e r o f te c h n i q u e s h a v e b e e
n d e v e l o p e d t oi m p r o~ , e t h e t o u g h n e s s o f
th e s e m a t e r i a l s. T h e s ei n c l u d e : t h e u s e o
f p l a s ti c i z i n g m o d i f i e r s 1 1 ; t h e a d d i t i
o n o f r u b b e r p a r t i c le s su c h a s c a r b o x y
l-47--49.t e r m i n a t e d b u t a d i e n e - a c r y l o n i t
r i l e (C T B N) ; t h e a d d i t i o n o f t h e r m o p l a s t
i c p a r t ic l e s s u c h a s
polyethersulphon e (PES)a n d p o l y e t h e r i m i d
e(pE0~52; a r e d u c t i o n i n t h e c r o s s - li n k i n g d
e n s i t y o f t h e r m o s e t ssuc h as e po xy r es ins53 '54
; t h e u s e o f t h e r m o p l a s t i c m a t r i c e s s u c h
as PE E K ; a n d t h e i n c l u s i o n o f th i n , t o u g h l
a y e r s a t p lyin t er f ac es 5 ,5s-57.E a r l y w o r k b y B
r a d s h a w e t a l . 11 s h o w e d t h a t a d d i n g ap l a s
t i d z e r t o E p i k o t e 8 2 8 e p o x y re s i n in c r e a s
e d t h eM o d e I f ra c t u r e t o u g h n e s s b y o v e r tw
o o r d e r s o fm a g n i t u d e . W h e n u s e d a s a m a t r
ix s y s t e m in a c a r b o nf i b r e c o m p o s i t e , i n c
r e a s e s i n t o u g h n e s s d i d r e s u l t;h o w e v e r ,
i n t h is c a s e t h e I z o d i m p a c t e n e r g y w a s
3 5 2 C O M P O S I T E S . S E P T E M B E R 1 99 1
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improved by only 25%. This disappointing transfer oftoughness
was explained by the fact that the Izod testinduces crack propagat
ion across fibres rather thanbetween them. Improvements in the
interlaminartoughness and resistance to splitting are
hidden,therefore, by the higher energy fracture energy associ-ated
with transverse fibre failure.Husman e t a l . 17 conducted high
velocity impact tests ona limited number of Modmor II carbon fibre
compos-ites. Their data suggested that the impact resistance
ofthese materials did not depend upon the properties ofthe
polymeric matrix.In a more detailed analysis by Williams and R h o
d e s 49 ,the impact resistance of 24 modified and unmodifiedcarbon
fibre/epoxy composites was examined. Theirexperimental analysis
showed that both the level ofdamage incurred as well as the
residual compressiveproperties of the laminates varied enormously.
It wasfound that the brittle laminates tended to fail byextensive
delamination whereas the tougher systemsfailed in transverse shear
near the impact location. Theauthors concluded that the tensile
performance of theneat matrix has a significant influence on the
impactbehaviour of a composite structure. For improvedimpact
resistance, the strength of the matrix shouldexceed 69 MPa and its
strain to failure should begreater than 4%. Finally, in order to
ensure adequatecompressive strength, the shear modulus should
begreater than 3.1 GPa.Hirschbuehler 7 examined a large number of
plain andinterleaved composite systems under a variety ofdifferent
loading conditions in an attempt to relatematrix properties and
post-impact compressivestrength. It was found that the residual
compressiveproperties of the composites increased with the
flexuralstrain to failure of the pure resin, Fig. 4.Hunston47
analysed data from three sources in anattempt to identify a link
between matrix propertiesand composite fracture toughness. He
identified adefinite correlation between the resin Mode I
fracturetoughness and composite interlaminar fracture energyas
measured by the DCB specimen, Fig. 5. With brittle
a . 3 5 0 I,'- 3002 5 0
2 15 0~ l I$~ : 5 0
0 ,0 . 0
0 0 . . "0
, I , , , I I ~ , I. ,. , , I , , , I , L ,2 .0 4 .0 6 .0 8 .0
10 .0 12 .0Ne a t r e s in f l e xu r a l s t r a i n to fa i l u r
e ( %)
F i g. 4 V a r i a t i o n o f re s i d u a l c o m p r e s s i
v e s t r e n g t h o f i m p a c t -d a m a g e d c o m p o s i t
e s w i t h n e a t re s i n f l e x u r a l fa i l u r e s tr a i
n( R e f 5 7 )
polymers, the resin toughness is fully transferred to
thecomposite, whereas with tougher polymers the resintoughness is
only partly transferred to the composite.In the latter it is
proposed that the presence of thefibres restricts the crack-tip
plastic zone size, therebyreducing the positive effect of the
tougher matrices.Masters3s'~ extended this approach by
conductingMode I, Mode II and compression after impact (CAI)tests
on a number of epoxy and bismaleimide-basedcarbon fibre composites.
He showed that no corre lationexisted between the Mode I inter
laminar toughness andthe C A I properties, Fig. 6, whereas a very
good agree-ment was found between the Mode II resistance
andresidual compressive strength, Fig. 7. Similar observa-5 8 tions
have been made by other workers . It is clearthat the matrix in a
flexurally loaded composite will besubjected to a large Mode II
component and that theforward shear properties of the matrix will
be impor-tant in determining the level of damage incurred. It
issomewhat less clear why the residual compressiveproperties of the
composite should be Mode II con-trolled since the failure process
is undoubtedly complexcontaining a significant Mode I component.
Master'sresults are impressive nevertheless.Materials that satisfy
the above condition and therefore
3IEo
2
E 1 J - ~ Th e r mo s e t- - / o + o E xp er im e n ta l 0 O 0'
- O J 0 T o u g h e n e d~ t h e r m o s e t' ~ + T h e r m o p l a
s t i cEO 0 I 1 I L I Lo 0 1 2 3 4 5 6
Re s in f r a c tu r e e n e r g y ( kJ m - 2 )F i g . 5 V a r i
a t i o n o f co m p o s i te M o d e I i n t e r l a m i n a r f r
a c t u ree n e r g y w i t h r e s in f r a c t u r e e n e r g y
( R e f 4 7 )
5 0 0 ~ -t 450E 7
4 0 0 ~ -~ 3 5 0==_= 300
2 5 0
'~ 15 0- 100" oo 5 0
0 0
A S 4 / 9 0 7 A S 4 / 1 8 0 8 - f i ( m EC 6 0 0 0 / 1 8 2 7 - f
i l m A M 6 / 1 8 0 8 - f i l m A
A S 4 / 1 8 0 8 - f i l r n A [ M 6 / 1 8 0 8
A S 4 / 1 8 0 8 AS4/1808-fitm C C 6 0 O 0 / 1 8 2 7 A $ 4 / 3 5
0 2 - f i { m A
A S 4 / 3 5 0 2
I 1 I I I1 0 0 2 0 0 3 0 0 4 0 0 5 0 0Re s id u a l co mp r e ss
i o n s t re n g th a f te r imp a c t ( MP a )
F i g. 6 V a r i a t io n o f re s i d u a l c o m p r e s s i o
n s t r e n g t h a f t e r i m p a c tw i t h M o d e I s t r a i
n e n e r g y r e l e a s e r a t e (R e f 5 6 )
C O M P O S I T E S . S E P T E M B E R 1 99 1 3 5 3
-
8/6/2019 Cantwell 91
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4 0 0 0E 3 5 0 0~ 3ooo_~ 2500
2000
c 1500
1000
00
A S 4 / 1 8 0 8 : f i l m EI M 6 / 1 8 0 8 - f i l m A e A S 4 /
1 8 0 8 - f i i m AA S 4 / 3 5 0 2 . f i l m A
A S 4 / 1 8 0 8 - f i l m C C 6 0 0 0 / 1 8 2 7 4 i l m EI M 6 /
3 1 0 0 - f i l m E
I M 6 ( 3 1 0 0 - 2 1 - f i l m E A S 4 / 9 0 7
A 5 4 / 1 8 0 8 . . . . . . . A S 4 / 1 8 0 8l M o / I ~ u ~ C 6
0 0 0 / 1 8 2 7A S 4 / 3 5 0 2 I M 6 / 3 1 0 0 e I M 6 / 3 1 0 0 -
2 1
t I i I I1 0 0 2 0 0 3 0 0 4 0 0 5 0 0R e s i n co m p re ss i o
n s t re n g t h a f t e r i m p a c t (M P a )
F i g. 7 V a r i a t i o n o f r e s i d u a l c o m p r e s s i
o n s t r e n g t h a f t e r i m p a c tw i t h M o d e I I s t r
a i n e n e r g y r e l e a s e r a t e ( R e f 5 6 )offer supe
rior imp act propert ies inc lude therm oplast ic-based compo si
tes and interleaved laminates. In recentyears considerable interest
has been generated byca rb o n f i b r e - r e in fo rced PEEK (A P
C2 ) , a s emi -crys ta l line the rm opla s t i c com pos i t e '~
'59-62. In ter l ami -n a r f r ac t u r e t e s t i n g an d i mp
ac t l o ad i n g h av e s h o w n t h a tt h i s ma t e r i a l o
f f e r s ex ce l l en t st a ti c an d d y n am i c t o u g h -n
es s an d i s cap ab l e o f ab s o rb i n g a co n s i d e rab l e
am o u n to f en e rg y w h i l s t i n cu r r i n g o n l y s ma l
l amo u n t s o fdamage4 ,63 ,64 . Scan n ing e lec t ron m icrogr
aphs o f thef r ac t u r e s u r f aces in d i ca t e ex t en s i v
e d r aw i n g an d p l a s t i cf lo w 6 5. A n o t h e r ad v an
t ag e o f t h i s ma t e r i a l i s t h a t i t st h e rmo p l a
s t i c ma t r i x a l lo w s r ap i d r ep a i r u s i n g fu s io
nt ech n i q u es s u ch a s t h e h o t p r e s s t ech n i q u
e66. H er e,i m p a c t d a m a g e c a n b e r e d u c e d o r r e
m o v e d b y s im p l yh e a t i n g t h e c o m p o n e n t t o a
t e m p e r a t u r e a b o v e t h em e l t i n g p o i n t o f t
h e m a t r i x , r e f o r m i n g a n d c o o li n g . T h eh i g
h v e l o c i t y imp ac t r e s p o n s e o f ca rb o n f ib re
/PE E K h asrece ived very l i tt l e a t t en t ion . In i t i a l
tes t ing has sug-g es t ed t h a t i ts h i g h v e l o c i ty imp
ac t r e s p o n s e i s p e rh ap sr e l a ti v e ly p o o r . D
an - J u m b o e t a l . 5 9 s h o w e d t h a t b e y o n da ce r
t a i n v e l o c it y t h r e s h o l d , A PC 2 ex p e r i en ced
as u d d en d ro p i n f l ex u ra l s t r en g t h . S i mi l a r
o b s e rv a t i o n sh a v e b e e n o b s e r v e d b y M o r t o
n a n d G o d w i n63 fol lowin gbal l is t i c imp act t es t s on
th i s mater i a l . Th i s wi l l bed i s cu s sed i n mo r e d e
t a i l i n t h e s ec t i o n o n r a t e e f f ec ts .T h e s e o
b s e rv a t i o n s su g g es t , t h e r e fo re , t h a t ca r e
s h o u l db e ex e rc i s ed w h en a t t emp t i n g t o r e l a
t e s t a ti c p ro p e r t i e ss u ch a s i n t e r l ami n a r t
o u g h n es s an d s t r en g t h t o ch a rac -t e r ize t ly n
ami c p ro p e r t i e s s u ch a s i m p ac t r e s i s t an ce.Po
l y m er i n t e r lea f i n g i n v o lv es th e u s e o f h i g h
t o u g h n es sf i lms o r l ayers a t p ly in ter faces in re la
t ive ly b r i t t l ema t e r i a l s . T h e i n c l u s io n o f
s u ch l ay e r s i n c reas e s t h el ami n a t e ' s i n t e r l
ami n a r f r ac t u r e t o u g h n es s56 as w el l asr ed u c i
n g t h e l ev e l o f d am ag e i n cu r r ed fo r a g i v eni n c
i d en t en e rg y67. T h e l o ad -b ea r i n g p ro p e r t i e s
o fd am ag e d i n t ed ea fed co m p o s i t e s a r e s i g n if
i can t lys u p e r i o r t o t h o s e o f co n v en t i o n a l
ep o x y co mp o s i t e s67.In ter l a yer t ec hno logy is s ti l
l i n i ts in fancy ; how eve r ,ea r l y r e s u lt s a r e v e ry
f av o u rab l e an d t h e t ech n i q u eo f f e r s en o rm o u
s p o t en t i a l.1. E f fec t o f m at r ix p roper t ies on pos
t - impa ctres idua l s t reng thImpact-damag ed com posi tes are
prob ably m ost sensi -
t iv e t o c o mp res s i v e l o ad i n g s in ce i mp a c t -g
en e ra t edd e l ami n a t i o n s t en d t o r ed u c e t h e s t
ab i li ty o f t h el o ad -b ea t i n g p l i e s r e s u lt i n g
in p r em a t u re f a i lu r et h ro u g h b u ck l i n g . I n t
h e p r ev i o u s s ec t i o n i t w as s h o w nt h a t co m p o
s i t e s w i t h h i g h M o d e I I i n t e r l am i n a rf r ac
t u r e t o u g h n es s e s o f f e r s u p e r i o r CA I p ro p
e r t i e s s5,56.I t ap p ea r s t h a t t h e m a j o r i t y i f
n o t a l l o f th i s i mp ro v e -m en t i n CA I b eh av i o u r
r e s u l ts f r o m t h e l o w er l eve l s o fd a m a g e i n c
u r r ed d u r i n g i m p a c t .T h e r e s id u a l t e n si le
p r o p e r t i e s o f t o u g h e n e d c o m p o s -i te s d o n
o t ap p ea r t o b e s i g ni f ican t ly b e t t e r t h an t h o
s eo f s t an d a rd ep o x y s y s t ems , F i g . 8 . T h i s r e
s u lt s f r o m t h efac t t h a t co mp o s i t e s w i t h t o u
g h e r ma t r i ce s t en d t o b em o re n o t ch - s en s it i v
e d u e t o r e d u ce d s p l it t in g an dd e l am i n a t i o n
a ro u n d s t re s s co n cen t r a t i o n s s u ch a sn o t c h
e s o r d a m a g e6s.T o u g h en i n g co mp o s i t e s u s i n
g e l a s to me r i c p a r ti c l e sr ed u ces t h e l ev e l o f
d e l am i n a t i o n an d t h e r e fo reen h an ces r e s i d u
a l co mp res s i v e p ro p e r t i e s , F i g . 9 .H o w e v e r
, t h e p r e s e n c e o f s u c h i n c lu s io n s o f t e n r e
d u c e st h e g l a ss t ran s i t io n t em p era t u r e o f t h
e m a t r i x ma t e r i a lw h i ch i n t u rn r ed u ces t h e h
o t -w e t p ro p e r t i e s o f t h eco m p o s i t e . I n ma n
y s i t u a ti o n s a co m p ro m i s e i s t h e r e -fo r e n
eces s a ry .
1 4 0 0
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8/6/2019 Cantwell 91
9/16
2. ConclusionsI t i s c l e a r t ha t m a t r ix p r ope r t i
e s p l a y a s ign i f i c an t r o l ei n d e t e r m i n i n g t
h e i m p a c t r e s i s ta n c e a n d s u b s e q u e n tloa d -
be a t ing c a pa b i l i t y o f a f i b r e - r e in f o r c e d
p l a s ti c . A tp r e se n t , a s ign i fi c a n t e f f o r t
is be ing m a d e to im pr ovet h e p o s t - im p a c t c o m p r
e s s i v e p r o p e r t i e s o f c o m p o s i t e s .I t a ppe
a r s t ha t m a te r i a l s w i th h igh M o de I I i n t e r l a
m i -n a r f r a c t u r e t o u g h n e s s e s o f f e r s u p e
r i o r c o m p r e s s i o ns t r e ng ths a f t e r im pa c t . C
a r e shou ld be e xe r c i s e d ,h o w e v e r , w h e n c o r r
e l a t i n g d a t a o b t a i n e d u n d e r s t a ti cloa d ing
c ond i t i ons w i th r e su l t s f r om dy na m ic te s t s .In
te rphaseT h e s t r e n g t h o f t h e b o n d b e t w e e n t h
e m a t r i x r e si n a n dthe f i b r e r e in f o r c e m e n t
is a c on t r o l l i ng f a c to r in de t e r -m i n i ng t h e m
e c h a n i ca l p e r fo r m a n c e o f m o s t p o l y m e rc om
p os i t e s . I n ge ne r a l , t he su r f a c e o f t he f i b r
e s ist r e a t e d b y a n o x i d a ti v e p ro c e s s i n o r d
e r t o i m p r o v e t h el e ve l o f a dhe s ion be tw e e n m a
t r ix a n d f ib r e . I n i ti a l ly ,t h i s i n te r r a c i a
l z on e w a s c on s ide r e d a s be ing a tw o-d im e ns iona l
su r f a c e w i th e f f e c t ive ly z e r o t h i c kne s s .H o
w e v e r , m o r e r e c e n t s tu d i e s h a v e s h o w n t h
a t t h i sr e g ion i s in f a c t t h r e e - d im e n s iona l ,
ha v ing i ts ow ndis t inc t pro per t i es 69.S t u d i es h a v
e s h o w n t h a t v a r y i n g t h e l e v el o f s u r f a c et
r e a t m e n t a p p l i ed t o a c a r b o n f i b r e c o m p o
s i t e c a nc h a n g e t h e m o d e o f f ai l u re a s w e ll a
s m a n y f u n d a m e n -t a l m e c ha n ic a l p r ope r t i e
s H '69-7z. C o m po s i t e s w i th l owle ve ls o f f i b r e su
r f a c e t r e a tm e n t f a il a t r e l a t i ve ly l ows t r e
sse s w he n lo a de d t r a nsve r se ly t o t he f i b r e s , l
e a v ingsm oo th f i b r e s on the f r a c tu r e su r f a c e .
I nc r e a s ing thel e ve l o f t r e a tm e n t a p p l i e d to
t h e f i b r e s inc r e a se s t het r a nsve r se f a i l u r e
s t r e s s a nd f a i l u r e oc c u r s w i th in t hem a t r ix
, i . e . , t he i n t e r ph a se r e g ion i s no lon ge r t hew
e a k e s t l i n k i n t h e c o m p o s i t e .R o g e r s e t a
l . 71 s h o w e d t h a t i m p r o v i n g t h e f i b r e /m a t
r i xb o n d s t r e n g t h i n a c a r b o n f i b r e -r e i n f
o r c e d e p o x yr e su l t e d in a f ou r f o ld i nc r e a se
i n t he i nc ide n t im pa c te n e r g y r e q u i r e d t o i n
i t ia t e d a m a g e . A t h i g h e r e n e r g ie st h e l o a
d - b e a ri n g p r o p e r t i e s o f c o m p o s i t e s w i t
h s u r f a c e -t r e a t e d f i b r e s d r ops d r a m a t i c
a l ly un t i l t he pe r f o r a t i onl im i t is r e a c h e
d72. B les s a nd H a r tm a n 73 show e d tha t t hep e r f o r a
t i o n t h r e s h o l d e n e r g y i n a s u r f a c e - t r e a
t e dc om pos i t e i s s ign i f i c an t ly l ow e r t ha n tha t
o f a s im i l a ru n t r e a t e d l a m i n a t e .T h i s b e h
a v i o u r h a s b e e n e x p l a i n e d b y D o r e y ~'2, wh
os h o w e d t h a t t h e t r a n s v e r s e f r a c tu r e e n e
r g y o f a c o m -p o s i te , a f u n d a m e n t a l p a r a m e
t e r f o r d e t e r m i n i n gr e s is t a n c e t o p e n e t r
a t i o n a n d p e r f o r a t i o n , d e p e n d ss t ro n g l y
u p o n t h e f i b r e / m a t ri x b o n d s t r e n g th . C a r
b o nf ib r e - r e in f o r c e d e pox ie s w i th un t r e a t e
d f i b r e s o f f e rt r a nsve r se f r a c tu r e e ne r g i e
s a s h igh as 60 k J m - 2 ( R e f72 ) . T r a nsve r se f a i l u
r e in c om pos i t e s w i th h igh l e ve l s o ff ib r e su r f
a c e t r e a tm e n t a bso r bs c ons id e r a b ly le s se ne r
g y , w i th q uo t e d ~2 t r a nsve r se f r a c tu r e e ne r g
i e sbe ing a s low a s 20 k J m - 2. A t e n e r g i e s l e ve ls
a bove tha tr e q u i r e d t o a c h i e v e p e r f o r a t i o n
, d a m a g e i n a s u r f a c e -t r e a t e d c o m p o s i t e
t e n d s t o b e l o c a l iz e d a r o u n d t h epo in t o f im
pa c t , o f t e n t a k ing the f o r m o f a c l e a n ho le 71.T
h e p o s t - p e r f o ra t i o n r e s i d u a l p r o p e r t ie
s o f t r e a t e dc o m p o s i t e s a r e g e n e r a l l y s u
p e r i o r t o t h o s e o f u n t r e a t e dc om pos i t e s a s
show n in F ig . 10.
1 0 0 0~ Surface-treated f i b res(3 . 8 0 0 0 U n t r e a t e d
f i b r e s
" - - - P e n e t r a t io n6 0 0 . . - - O - -- = E I / J' \ l
/
/0 2 4 6 8 10 12 14
I n c i d e n t e n e r g y ( J )F i g . 1 0 R e s i d u a l f l
e x u r a l s t r e n g t h v s . impact energy for b a l l i s t i
-c a l l y impacted s u r f a c e - t r e a t e d a n d u n t r e a
t e d c a r b o n f i b r ecomposites ( R e f 7 2 )M c G a r r y e
t a l . TM a t t e m p t e d t o e n h a n c e t h e i m p a c tr e
s i s t a nc e o f T300 /M Y - 720 c a r bon f ib r e / e poxy byc
oa t ing the su r f a c e s o f t he f i b r e s w i th a t h in l
a ye r o fC TB N r ubbe r . G a r d ne r im pa c t t e s t s show e
d tha tt r e a t ing the f i b r e s in t h i s w a y im pr ove d
the t h r e sh o lde ne r g y f o r f i r st da m a ge s ign i f ic
a n t ly . F u r the r , a boveth i s t h r e sho ld t he l e ve l
o f da m a ge f o r a g ive n im pa c te ne r gy w a s l e s s i n
t he m od i f i e d c om pos i t e .1. Ef fec t o f in te rphase
proper t ies on pos t - impac tres idua l s t reng thD o r e y75 ha
s show n tha t i nc r e a s ing the s t r e ng th o f t hef ib r e
/m a t r ix bon d inc r e a se s t he i n t e r l a m ina r she a
rs t r e ng th ( ]LS S) o f t he c om pos i t e un t i l a p l a t
e a u i sr e a c h e d be yon d w h ic h po in t no s ign i f i c
an t i nc r e a se ispos s ib l e , F ig . 11 . O v e r t h i s r a
nge o f t r e a tm e n t l e ve l sthe no tc h e d t e ns i l e s t
r e ng th f a l ls d r a m a t i c a l ly . I nc r e a s -ing the I
LSS in t h i s w a y supp r e s se s t he f o r m a t ion o fd e l
a m i n a t e d z o n e s i n t h e r e g i o n o f s t re s s c o
n c e n t r a -t i ons , r e nde r ing the m a te r i a l m or e no
tc h - se ns i t i ve .C o n s e q u e n t l y , e v e n t h o u g
h s u r f a c e t r e a t m e n t o f t h ef ib r es r e duc e s t
he l e ve l o f da m a ge f o r a g ive n e ne r gy ,the i n c r e
a se d no tc h - se ns i t i v i ty o f t he l a m in a t e r e su
l t s inpoo r e r r e s idua l t e n s i le p r ope r t i e s , F
ig . 1 2 7 5 . C o n -ve r se ly , t r e a t i ng the f i b r e s
im pr ove s t he po s t - im pa c tc o m p r e s s iv e p r o p e r
t i e s a s s h o w n i n t h e l o w e r p a r t o fF ig . 12 . C
le a r ly , t he l e ve l o f su r f a c e t r e a tm e n t a pp l
i e dto t he f i b r e s i n a m u l t i d i r e c t iona l c om po
s i t e w il l de p e ndu p o n t h e o p e r a t i o n a l c o n d
i t io n s t h e c o m p o n e n t w il le n c o u n t e r . I n g
e n e r a l , a c o m p r o m i s e i s s o u g h t i n w h i c
hthe f i b r e s a r e g ive n in t e r m e d ia t e l e vel s o f
t r e a tm e n t .2. ConclusionsT h e l e v el o f t r e a t m e n
t a p p l i e d t o t h e s u r f ac e o f t h ef ib r e s i n a c
om p os i t e m a te r i a l ha s a s ign i fi c a n t e f f e c
tupo n bo th i ts im pa c t r e s i s t anc e a s w e l l a s i ts
r e s idua lloa d - c a r r y ing ca pa b i l it y . I n ge n e r a
l , im pa c t on c o m po s -i t e s w i th l ow l eve l s o f f i
b r e su r f a c e t r e a tm e n t g e ne r a t e sl a r ge a r e
a s o f sp l i tt i ng a nd de l a m ina t ion w i th s e ve r ee f
f ec t s o n t h e c o m p r e s s i v e p r o p e r ti e s o f t h
e m a t e r ia l .Loc a l i z e d im pa c t l oa d ing on h igh ly
t r e a t e d f i b r ec om p os i t e s r e su l ts i n a sm a l l
e r , m o r e l oc a l i z e d da m a gez o n e , a l o w e r p e r
f o r a t io n t h r e s h o l d a n d i m p r o v e d
COMPOSITES. SEPTEMBER 1991 355
-
8/6/2019 Cantwell 91
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' [1 .0
0 .5
T / T 1 0 0
O n / O n l O 0
0 I I I I0 1 0 0 2 0 0 7 0 0Su r f a ce t r e a t m e n t
(%)
Fig . 11 Var ia tion o f in ter lamina r shear s t rength and
notchedst rength o f carbon f ib re composi tes as a funct ion o f
f ib resurface t reatm ent (Ref 75)Treatment level 0%o . . . . .
5%
1.00~
AtOnt3v.c 0.6
-~ 0.4" 0
0.20.0
Fig. 12composi tes vs . impact energ y (Ref 75)c o m p r e s s i
v e p r o p e r t ie s . H o w e v e r , t h e i n c r e a s e d n
o t c hse ns i ti v i ty a s soc i a t e d w i th f i b r e su r f
a c e t r e a tm e n tr e su l t s i n a r e duc t ion in t he p os
t - im pa c t t e n s i l e s t r e ng tho f t h e m a t e r i a l.
T h e l e v e l o f t r e a t m e n t a p p l i e d t o t h es u r
f a ce o f t h e f i b r e s w i ll d e p e n d , t h e r e f o r e
, u p o n t h ede s i r e d a pp l i c a tion .
~ _ Tension " - - - - 100%
~,+\ \ . . . . . . . . . . . . o\~ , \ ".. . . . . . . ...' ~ ,
, . ~ .
X ~ O ~ ~ ' I I ~~+ ~ ~ . . . . . .. . . . . "~_ =t - - _ -- . .
. . +
/Compression
I I I I0 2 4 6 8 10Impact ene rgy (J)
Residual s t rength o f t reated and un t reated carbon f ib
re
Fibre stacking sequenceC o m p o s i t e m a t e r i a ls o f f
e r a u n i q u e a d v a n t a g e i n t h a tp r ope r t i e s
suc h a s s t r e ng th a nd s t i ff ne s s c a n be t a i l o r e
dto m e e t spe c if i c de s ign r e qu i r e m e n t s t h r oug
h a c a r e f u ls e l e c t ion o f t he f i b r e s t a c k ing
se que nc e . C ons ide r a b lew o r k h a s s h o w n t h a t t h
e i m p a c t r e s is t a n ce o f c o m p o s i t em a te r i a l
s a l so de p e nds u pon the spe c i f ic o r d e r i n w h ic
hthe p l i e s a r e s t a c ke d6 '~ '76-s . For ex am ple , unid
i rec -t i ona l c om po s i t e s ha v ing a l l t he i r f i b r
e s a l igne d in on ed i r e c t ion f a i l by sp l it t ing a t
ve r y l ow e ne r g i e s a n d a r et h e r e f o r e h i g h l y
u n s u i t a b le f o r a p p l i c at i o n s w h e r ei m p a c
t l o a d in g m i g h t o c c u r11. Fol low ing im pac t te s
tson a s e r i e s o f ( 0 , + / - 4 5 ) l a m ina t e s , D or e y
1~ as we ll a sM o r t o n a n d G o d w i n 63 show e d tha t c om
p os i t e s ha v ing+ / - 4 5 su r f a c e p l ie s o f f e r e d
a sup e r io r im pa c t r e s is -t a nc e a nd im pr o ve d r e s
idua l s t re ng ths . I t w a s sug -g e s t e ds l t ha t + / - 4
5 p l i e s i nc r e a se d th e f l ex ib i li t y o f t h ec om
pos i t e , t h e r e b y im pr ov ing i ts a b i li t y t o a bso
r be ne r gy e l a st i c al l y . F u r the r , p l a c ing suc h
p l i es on thes u r f a c e o f a c o m p o s i t e s e r v e s t
o p r o t e c t t h e l o a d -b e a r i n g 0 p l i e s a ga in s
t da m a g e indu c e d by the im p ing ,ing p r o j e c t i l es l
. T h e s e i d e a s w e r e s u p p o r t e d b y S t e -va nov
ic e t a l . s , w h o c o n d u c t e d i n s tr u m e n t e d C h
a r p yte s t s on a s e r i e s o f m u l t i d i r e c t iona l
T300 c a r bo n f ib r ec o m p o s i t e s. T h e y s h o w e d t
h a t ( + / - 4 5 c o m p o s i t e sw e r e c a p a b l e o f a b
s o r b i n g c o n s i d e ra b l y m o r e e n e r g ytha n
(0,90) , (0,+/--45 ) and (00,900 ,+/-45 ) lami-na t e s .A s o m e
w h a t d i ff e r e n t a p p r o a c h w a s a d o p t e d b y N
o l e ta n d S a n d u s k y i n t h e d e s i g n o f a c o m p o
s i t e l e a d in g e d g ef o r th e A 1 0 T h u n d e r b o l
t8z. Th e y p r opo se d th a t t hel e a d ing e dge be ve r y s t
if f i n o r d e r t o ' sp li t o f f ' t heim p ing ing b i r d w
i th l i tt l e o r no lo s s in s t r e ng th o rs t if f ne ss .
The y sugge s t e d tha t i n o r de r t o a c h ie ve th i sr e q
u i r e m e n t , a h i g h p e r c e n t a g e o f c a r b o n f i
b r es s h o u l dbe o r i e n t e d in t he spa nw ise d i r e c t
ion o f t he a i r c r a f t ' sl e a d i n g e d g e .Th e a u tho
r s s3 show e d tha t da m a ge in i t i a t i on in a s e r ie so
f ( + / - 4 5 l a m i n a t e s s u b j e c te d t o l o w v e l o
c i t y i m p a c td e p e n d e d u p o n t h e t h i c k n e s s
a n d t h e r e f o r e t h e s t if fn e s so f t he c om p os i t
e . I n i t i a l f a i l u r e in t h in , f l e x ib l e t a r ge
t soc c u r s i n t he l ow e r m o s t p ly as a r e su l t o f t
he t e ns i l ec o m p o n e n t o f t h e f l e x u r a l s t re s
s f ie l d . D a m a g e i nth i c ke r , s t i f f e r t a r ge t
s i n i t ia t e s a t t he t o p su r f a c e due to
1. 6' 1 , 41,21.0o.830. 6E0.40,20.0
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Fig . 13 Low veloci ty impact energy to in it ia te dam age vs
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t he c on ta c t s t r e s s f ie ld . Th i s i s show n in F ig
. 13 . Th i sc u r ve c l a r i fi e s D or e y ' s s l c l a im
tha t i nc r e a s ing thef l e xu r a l s ti f fne s s o f a t a r
ge t , f o r e x a m p le , by p l a c ingf i b re s o n t h e s u
r f a c e o f a l a m i n a t e , c a n e n h a n c e i t sim pa c
t r e s i s t a nc e . Th i s i s t r ue f o r t he r a nge o f s t
if f-ne s se s w he r e i n i ti a l fa i l u r e oc c u r s a t t
he t o p su r f a c e o ft h e c o m p o n e n t . I n m o r e f l
e x ib l e t a r g e t s, h o w e v e r ,r e duc ing th e f l e xu
r a l s t i ff ne s s m a y p r e c ip i t a t e f a i l u r e a ta
l o w e r i n c id e n t e n e r g y .A d e t a i l e d s t u d y b
y H o n g a n d L i u6 ide n t i f i e d f unda -m e n t a l a s p
e c t s in t h e d e v e l o p m e n t o f d a m a g e i n g la ssf
ib r e - r e in f o r c e d p l a s ti c ( G F R P) sub je c t e d
to h ighve loc i ty im pa c t l oa d ing . Th e y show e d tha t i
nc r e a s ingthe a ng le q i n a ( ~ , q , 0~ ) l a m in a t e r e
su l t e d i n g r e a t e rd e l a m i n a t i o n - t y p e d a m
a g e f o r a g i v e n i n c i d e n t e n e r g y ,F ig . 14 . I
nc r e a s ing q i n t h i s w a y a l so ha d the e f f e c t o fr
e d u c i n g t h e f i rs t d a m a g e t h r e s h o l d e n e r
g y . T h ea u tho r s a l s0 show e d tha t f o r a g ive n e ne r
gy , i nc r e a s ingthe t h i c kne s s o f t he G F R P t a r ge
t r e su l t e d i n a n inc r e a sei n d e l a m i n a t e d a r
e a . T h i s i n c r e as e in d a m a g e a r e a m a yr e s u lt
f r o m t h e r e d u c t i o n i n t h e t a r g e t ' s e n e r g
y a b s o rb -ing c a pa b i l i t y a s p r opose d by D or e yS L
L iu s4 e x t e nde dt h is w o r k b y d e v e l o p i n g a s i m
p l e m o d e l f o r p r e d i c ti n gt h e l ik e l y d e la m i
n a t i o n s i te s i n a n u m b e r o f d i f f e r e n tc om p
os i t e s . I t w a s sugge s t e d tha t de l a m ina t ion inm
ul t i - a ng le c om po s i t e s is m or e l i ke ly t o oc c u r
a ti n t e r f a c e s w he r e t he m i sm a tc h in be nd ing s t
i f f ne ss isg r e a t e s t , f o r e xa m ple , be tw e e n + /-
-45 p l i es . L ius h o w e d e x p e r i m e n t a l l y t h a t
t h e l e v el o f d e l a m i n a t i o n i na g l a s s / e poxy
c om pos i t e i nc r e a se d a s a ng le q i n a ( 0 , q)l a m in
a t e i nc r e a se d , i . e . , a s t he be nd ing s t i ff ne s
sm ism a tc h inc r e a se d Th i s e v ide nc e sugge s ts t ha t
i fd e l a m i n a t i o n n e e d s t o b e s u p p r e s s e d ,
l a m i n a t e s w i ths u d d e n l a rg e c h a n g e s i n f i
b r e d i r e c ti o n s h o u l d b ea v o i d e d .O t h e r t e
c h n i q u e s t o r e d u c e i m p a c t - i n d u c e d d e la
m i n a -t i o n i n c l u d e t h e u s e o f w o v e n f a b ri c
s46"85,hyb r id i z a t ion 16'a6-88 ( for exa mp le , ca rb on f
ibres w i thK e v la r f i b r e s ) a nd th r e e - d im e ns ion
a l s t i tc h ings9'9. T h ef i rs t o f t he se t e c hn ique s i
nvo lve s r e p l a c ing the u n id i r e c -t i ona l + / - 45 p
l i es i n a m u l t i d i r e c t iona l c om pos i t e by a+ / -
4 5 w o v e n f a b ri c . T h e t h r e e - d i m e n s i o n a l
n a t u r e o ft h e f a b r i c h e l p s s u p p r es s t h e f o
r m a t i o n o f d e l a m i n a t e dz one s a t t h i s c r i t
i c a l in t e r f a c e .
4 5
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C30 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0
I m p a c t e n e r g y ( J )F ig . 1 4 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 i m p a c t e d( 0 s *, 0
5 ", 0 5 ") G F R P l a m i n a t e s ( R e f 6 )
T h e i m p a c t r e s i s ta n c e o f c a r b o n f i b r e c
o m p o s i t e s c a nb e e n h a n c e d c o n s i d e r a b l y
b y i n c o r p o r a ti n g p l ie s o fl o w e r m o d u l u s f
i b r e s16,s6-s8. In or de r to assure com pat-ib i li t y , t he
m a t r ix r e s in i s u sua lly t he s a m e in t he tw o o rm o
r e c o n s t i t u e n t m a t e r ia l s . H a n c o x a n d W e
l lsa7s h o w e d t h a t t h e I z o d i m p a c t e n e r g y o f
a n H T - S c a r b o nf i b re c o m p o s i t e c o u ld b e i n
c r e a s e d b y 5 0 0 % t h r o u g hhybr id i z a t ion w i th E
- g l a s s f ib r e s . A s w e l l a s r e duc ingt h e b a s i c
p r ic e o f t h e c o m p o s i t e , t h e a d d i t i o n o f t
h eg las s f i b r e s w a s f oun d to c ha n ge the m ode o f f r
a c tu r ef r o m a c l e a n b r e a k t o a d e l a m i n a t i o
n - t y p e n o n -c a t a s t r oph ic f a i lu r e S im i l a r c
onc lu s ions w e r e d r a w n byH e l f in s t i ne 16 f o l low
ing C ha r p y im pa c t t e s t s on K e v la r -T 3 0 0 c a r b o
n f i b r e h y b r id s . H o w e v e r , t h e a b s o l u t em a
g n i tude o f t he i nc r e a se s w a s l e s s im pr e s s ive t
ha ntha t r e p o r t e d by the p r e v ious w or ke r s f o r G F
RP .D o r e y e t a l . ~ a s se s se d the h igh ve loc i ty im pa
c tr e s p o n s e o f a n u m b e r c a r b o n - K e v l a r h y
b r i d l a m i n a te s .T h e y s h o w e d t h a t t h e a d d i
t i o n o f t h e l o w e r m o d u l u sK e v l a r f ib r e s i n
c r e a s e d t h e t h r e s h o l d e n e r g y f o r t h eo n s
e t o f d a m a g e b y u p t o f o u r t i m e s .A t p r e se n t
m a ny w o r ke r s a r e a s se s s ing the f e a s ib i l i ty o
fus ing w e a v ing a nd b r a id ing t e c hn ique s i n o r de r
t oi m p r o v e t h e d a m a g e t o l e r a n c e o f f i b r e
- r e in f o r c e dp o l y m e r c o m p o s i t ess9-93. Su 93 c
o n d u c t e d M o d e Ide l a m ina t ion t e st s on bo th s t i
t c he d a nd non - s t i t c he dA S 4 c a r bon f ib re / J1 ( a
s e m i - c r ys t a l l i ne t he r m op la s t i c ) .H i s r e
su l t s show e d tha t s t i tc h ing w i th K e v la r f i b r er
e su l t e d i n a 100% inc r e a se i n i n t e r l a m ina r f r
a c tu r et o u g h n e s s . I n s t r u m e n t e d d r o p - w e
i g h t i m p a c t t es t s o n an u m b e r o f 2 - a n d 3 - D c
o m p o s i t es i n d i c a t ed t h a t t h el a t t e r o f f e
r e d a supe r io r im pa c t r e s i s ta nc e , t he p r e se n c
eo f t h e t h i r d d i m e n s i o n r e i n f o r c e m e n t s
e rv e d t o i n h ib i tt h e p r o p a g a t i o n o f d e l a m
i n a t e d z o n e s . F a b r i c a t in g 3 - Ds t r u c tu r e
s i s c le a r ly m o r e e x p e n s i v e a n d t i m e c o n s u
m -ing tha n c ons t r uc t ing w i th c onve n t iona l 2 - D p r
e p r e gs .I n o r d e r t o r e d u c e t h e s e c o s ts , it h
a s b e e n p r o p o s e dtha t a s e l e c t ive p r o c e d u r
e be a do p te d , t ha t i s , 3 - Dr e in f o r c e m e n t be u
se d a t c r i ti c a l p ly i n t e r f a c e s o r a tc o m p o n
e n t edges 93.1. Ef fect o f f ibre stacking sequen ce onpost - im
pact res idua / s treng thI n t h i s s e c t ion the r o l e t ha
t t he f i b r e s t a c k ing se que nc ep la ys in de t e r m in
ing the r e s idua l p r ope r t i e s o f im pa c t -da m a ge d c
om pos i t e s is di s c usse d A s ou t l i ne d a bove ,m u c h o
f t h e w o r k p u b l i s h e d i n t h e l i t e r a t u r e c o
n c e r n st h e r e s i d u a l c o m p r e s s iv e p r o p e r t
i e s o f d a m a g e dc om p os i t e s s inc e th i s is c ons
ide r e d to be t h e m os tc r i ti c a l f o r m o f l oa d ing c
ond i t i on C e r t a in c on f l ic t s m a ye x is t , h o w e v
e r , w h e n c o n s i d e r i n g t h e o p t i m u m f i b
restackin.8 s e que n c e f o r r e s idua l c om p r e s s ive s t
re ng th .D o r e y M s u g g e s t ed t h a t f o r i m p r o v e
d i m p a c t r e s is t a n c ethe + /- -45 f i b r e s shou ld be
l oc a t e d on the o u te r m os ts u r f ac e o f t h e c o m p o
s i t e . T h i s m a y n o t b e a n i d e a ls t a c k ing se qu
e nc e f o r s t a b i li t y in c om pr e s s ion H e r e ,s t i f
fe r l a m ina t e s , f o r e xa m p le , t hose w i th su r f a c
e 0f ib r e s , a r e b e t t e r su i t e d t o i n - p l a ne c
om pr e s s ive 63l o a d in g . N e v e r t h e l e s s , M o r t
o n a n d G o d w i n h a v es h o w n t h a t a n A P C 2 ( ~ , +
/ - 45 )2 s l a m i n a t e o ff e r sin f e r io r p r ope r t i e
s t o t hose o f a ( + / - 45 , ~ , + / - - 45 , 0 ) spla te , F ig
. 15 .
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0 1 2 3 4 5 6I m p a c t e n e r g y ( J }F i g. 1 6 E f f e c t
o f r e p l a c i n g th e + / - 4 5 p l ie s i n a 1 6 - p l y
(0,+/-45) C F R P c o m p o s i t e w i t h a w o v e n f a b ri c
( R e f 4 6 )
I n t he p r e v ious s e c t ion i t w a s s t a t e d tha t t
he u se o fw ov en +/--45" fabr ics in (0" , + / --45 ) lam ina tes
se rve s tor e d u c e t h e o v e r a ll l e v e l o f d e l a m i
n a t i o n u n d e r i m p a c tl o a d in g . T h e s u b s e q u
e n t r e s i d u a l s t r e n g t h s o f t h em i x e d - w o v
e n c o m p o s i t e s w e r e s u p e r i o r t o t h o s e o f t
h es t a n d a r d m a t e r i al m a n u f a c t u r e d f r o m u
n i d i r ec t i o n a lp l ies , F ig . 16 .S i m i la r i m p r o
v e m e n t s i n r e s id u a l s t r e n g t h h a v e b e e nn o
t e d f o ll o w i n g i m p a c t o n s t i tc h e d c a r b o n f
i b re c o m -92p o s i te s . C o m p r e s s i o n a f t e r i m
p a c t t e s ts o n a n u m b e r92o f A S 4 /3501-- 6 l a m ina t
e s show e d tha t s t i t c he d l a m i -na t e s o f f e r e d r
e s idua l s t r e ng ths up to 100% g r e a t e rt h a n t h e i r
u n s t i t c h e d c o u n t e r p a rt s . O n e o f t h e d i s
a d -v a n t a g e s o f th i s p ro c e s s is t h a t t h e u n d
a m a g e d c o m -p r e ss i v e s tr e n g t h o f t h e m a t e
r i a l is r e d u c e d b y u p t o20% 92.2 . Conc lus ionsThe im
pa c t r e s i s t a nc e o f a m u l t i d i r e c t iona l l a m
ina t e i ss t r o n g ly d e p e n d e n t u p o n t h e s p e c
if ic o r ie n t a t i o n o f t h ep l i es . U n id i r e c t
iona l l a m ina t e s shou ld be a vo id e d s inc e
the y sp l it a nd f a il a t l ow e ne r g i e s . The m i sm a
tc h inbe nd in g s t if f ne ss be tw e e n tw o p l i e s a ppe a
r s t o ha ve as ign i fi c a n t e f f e c t upo n th e l e ve l o
f da m a ge inc u r r e d a tt h a t i n t e rf a c e . D a m a g e
a p p e a r s t o b e g r e a t e s t w h e r ep l y o r i e n t a
t io n c h a n g e s o f 9 0 oc c u r . Th i s sugge s t s t ha tf
o r c o n t a i n m e n t o f d a m a g e l a m i n a t e s w i t h
a b r u p tc ha nge s i n f i b r e d i r e c t ion shou ld be a vo
ide d . O the rw a y s t o s u p p r e ss d a m a g e i n c l u d e
t h e u s e o f w o v e nf a b r ic s , t he u se o f hyb r id c om
po s i t e s o r s t i t c h ing a tde s i r e d loc a t ions .G e
o me t r yG e o m e t r y is a f u n d a m e n t a l parameter n
determin ingt h e i m p a c t r e s p o n s e o f a c o m p o s i
te c o m p o -n e n t a , r t A 5,6 3,7 5,8 3. L o w v e l o c i t
y i m p a c t t e s t s o n C F R Ph a v e s h o w n t h a t t h e
m o d e o f f a i l u r e i n a s i m p l e b e a mm a y v a r y
depend ingu p o n i t s s p a n - t o - d e p t h r a t i o . S h o
r tt h i c k spe c im e ns t e nd to f a i l i n a n i n t e r l a
m ina r m od ew h e r e a s a s l ong th in b e a m s f a i l e d i
n a fl e xu r e 11.B
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