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SHEAR BEHAVIOUR OF UNIDIRECTIONAL GFRP AND CFRP AT HIGH STRAIN RATES
P. Kumar, B. Rai
To cite this version:
P. Kumar, B. Rai. SHEAR BEHAVIOUR OF UNIDIRECTIONAL GFRP AND CFRP AT HIGH STRAIN RATES. Journal de Physique Colloques, 1988, 49 (C3), pp.C3-97-C3-103.
�10.1051/jphyscol:1988314�. �jpa-00227737�
JOURNAL DE PHYSIQUE
Colloque C3, Suppl6ment au n " 9 , Tome 49, septembre 1988
SHEAR BEHAVIOUR OF UNIDIRECTIONAL GFRP AND CFRP AT HIGH STRAIN RATES
P. KUMAR and B. R A I
Department o f Mechanical E n g i n e e r i n g , I n d i a n I n s t i t u t e of Technology, Kanpur, Kanpur 208016, I n d i a
Resume - Le comportement dynamique en contrainte deformation par cisaillement plan d'un unidirectionnel GFRP (composite verre-resine) et CFRP (composite carbone resine) a etb etudie A. grande vitesse de dkformation d'environ 300/s A l'aide d'une machine en torsion de Kolsky. La courbe contrainte-deformation en cisaillement h grande vitesse de deformation montre un ecoulement du materiau ?I contrainte de cisaillement constante pour des deformations excedant respectivement 3 A. 5% et 5 h 7% pour le GFRP et le CFRP.
La contrainte d'ecoulement de chacun des matbriaux n'est que tres faiblement superieure A celle des echantillons testCs en quasi-statique.
A b s t r a c t
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The dynamic i n - p l a n e s h e a r s t r e s s - s t r a i n b e h a v i o u r of u n i d i r e c t i o n a l GFRP ( g l a s s f i b r e r e i n f o r c e d p l a s t i c ) and CFRP ( c 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 ) h a s been s t u d i e d a t h i g h s t r a i n r a t e s of a b o u t 300/s u s i n g t h e !:olsky t o r s i o n machine.The s h e a r s t r e s s - s t r a i n c u r v e s a t h i g h s t r a i n r a t e s e x h i b i t f l o w o f m a t e r i a l a t c o n s t a n t s h e a r s t r e s s e s f o r s t r a i n s excee- d i n g 3-5% and 5 7 % f o r GFRP and CFRP r e s p e c t i v e l y . The f l o w s t r e s s f o r each m a t e r i a l i s o n l y . m a g i n a l l y h i g h e r o v e r t h e s t r e n g t h of q u a s i s t a t i c a l l y l o a d e d specimen.
1
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INTRODUCTIONF i b r e c o m p o s i t e m a t e r i a l s a r e known t o be weak u n d e r s h e a r l o a d s . Study of t h e i n - p l a n e s h e a r s t r e s s - s t r a i n b e h a v i o u r a t q u a s i s t a t i c d e f o r m a t i o n h a s b e e n s t u d i e d w e l l and s e v e r a l d i f f e r e n t t e s t i n g methods have been d e v e l o p e d /1,2/. However, i n v e s t i g a t i o n on t h e s h e a r b e h a v i o u r a t h i g h s t r a i n r a t e s i s s c a n t y . Vlerner and Dharan/3/
employed a l < o l s k y p r e s s u r e b a r machine t o a p p l y i n t e r l a m i n a r a n d t r a n s v e r s e s h e a r a t h i g h s t r a i n r a t e s r a n g i n g from 6000 t o 18000/s.
They f o u n d t h a t s t r a i n r a t e e f f e c t i n i n t e r l a m i n a r s h e a r i s s m a l l b u t i s s i g n i f i c a n t i n t r a n s v e r s e s h e a r . However, pure s h e a r l o a d was n o t a p p l i e d d i r e c t l y on s p e c i m e n s ,
I n t h i s i n v e s t i g a t i o n we r e p o r t t h e dynamic s h e a r b e h a v i o u r o f u n i d i r e c t i o n a l g l a s s f i b r e r e i n f o r c e d p l a s t i c and c 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 d e t e r m i n e d by a t o r s i o n machine b a s e d on t h e Kolsky p r i n c i p l e ( s p l i t Hopkinson b a r ) . The r e s u l t s a r e compared w i t h t h o s e o f t h e epoxy o b t a i n e d under dynamic c o n d i t i o n s .
2
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EXPERIhENTAL DETAILS 2.1 Kolsky T o r s i o n iAachineI n t h e K o l s k y t o r s i o n n a c h i n e / 4 / ( s p l i t Hopkinson b a r ) a s h o r t C y l i n d r i c a l specimen i s bonded between two l o n g - e l a s t i c - r o u n d b a r s known a s i n c i d e n t and t r a n s m i t t e d b a r s . I n o u r s t u d y b o t h t h e b a r s were made f r o m aluminium a l l o y . The i n c i d e n t a n d t r a n s m i t t e d b a r s a r e 25 mm i n d i a m e t e r and 2170 mm and 2120 m l o n g r e s p e c t i v e l y .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988314
C3-98 JOURNAL DE PHYSIQUE
A mechanism f o r g e n e r a t i n g a f a s t r i s e t i m e t o r s i o n a l p u l s e i s made a t t h e nonspecimen e n d of t h e i n c i d e n t b a r . A s t a t i c t o r q u e i s s t o r e d i n a p o r t i o n of t n e i n c i d e n t b a r w i t h t h e h e l p o f a l a r g e p u l l e y a t t a c h e d a t t h e end and a clamp a t a b o u t 600 rnm f r o m t h e p u l l e y . I n o r d e r t o p r o d u c e a t o r s i o n a l p u l s e , t h e clamp i s opened q u i c k l y . The m a g n i t u d e o f t h e p u l s e d e p e n d s upon t h e m a g n i t u d e o f t h e s t o r e d t o r q u e a n d t h e l e n g t h o f t h e p u l s e i s t w i c e t h e t i m e t a k e n by a s h e a r wave t o t r a v e r s e between t h e c l a m p and t h e p u l l e y .
When t h e i n c i d e n t p u l s e r e a c h e s t h e specimen, p a r t of
i t
i s r e f l e c t e d back i n t o t h e i n c i d e n t b a r and t h e r e s t i s p a s s e d t o t h e t r a n s m i t t e d b a r , The i n c i d e n t and r e f l e c t e d p u l s e s a r e r e c o r d e d t h r o u g h a p a i r o f r o s e t t e s t r a i n g u a g e s bonded t o t h e i n c i d e n t b a r . S i m i l a r l y t h e t r a n s m i t t e d p u l s e i s m o n i t o r e d by a p a i r of r o s e t t e s t r a i n g u a g e s bonded t o t h e t r a n s m i t t e d b a r . These t h r e e p u l s e s a r e m o n i t o r e d t h r o u g h e l e c t r i c b r i d g e s on a d u a l beam N i c o l e t O s c i l l o s c o p e No, 2090. F i q . 1 shows a l l t h e t h r e e r e c o r d e d p u l s e s o f a t y p i c a l expe-F i g . l
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I n c i d e n t , r e f l e c t e d and t r a n s m i t t e d p u l s e s (GFRP Exp.NO. 86-53 ).
r i m e n t . It may be n o t e d t h a t t h e r e f l e c t e d p u l s e r e p r e s e n t s t h e s t r a i n r a t e and t h e t r a n s m i t t e d p u l s e p r o v i d e s t h e s t r e s s i n t h e specimen. Our e x p e r i m e n t a l s e t - u p i s s i m i l a r t o one r e p o r t e d by H a r l e y e t a1/5/.
2.2 M a t e r i a l F a b r i c a t i o n and Specimen P r e p a r a t i o n
A t y p i c a l specimen ( F i g . 2 ) i s a t h i n t u b e of a b o u t 1 2 mm mean d i a m e t e r , 0.75 mm t h i c k n e s s and a b o u t 3 mrn l e n g t h h e l d between two f l a n g e s of 22 mm d i a m e t e r . The f l a n g e s and t h e specimen a r e made a s a n i n t e g r a l u n i t f r o m one p i e c e . The f l a t f a c e s of t h e f l a n g e s a r e bonded t o t h e i n c i d e n t and t r a n s m i t t e d b a r s . The specimen were machined f r o m 22 mm t h i c k p l a t ~ s of u n i d i r e c t i o n a l f i b r e c o m p o s i t e s k e e p i n g f i b r e s p a r a l l e l t o t h e a x i s o f t h e specimen. The 22 mrn t h i c k p l a t e s were c a s t i n t h e l a b o r a t o r by a f i l a m e n t w i n d i n g o v e r a f l a t m a n d r e l u s i n g E - g l a s s - f i b r e YFGP, I n d i a ) and epoxy r e s i n
( k r a l d i t e CY230 of C I B A , I n d i a ) f o r GFRP. S i m i l a r l y CFRP p l a t e s were p r e p a r e d from c a r b o n f i b r e s ( G r a f i l E/A-S, C o u r t a u l d s Ltd., E n g l a n d ) and t h e epoxy. To s u p p r e s s v o i d s b o t h p l a t e s o f a m a n d r e l a r e p r e s s e d f o r a day between two s t i f f s t e e l p l a t e s i m m e d i a t e l y
a f t e r t h e v i n d i n g o p e r a t i o n . The f i b r e volume f r a c t i o n of G F R D a n d CFRP a r e 44% a n d 3 0 A r e s p e c t i v e l y .
Fig.- 2-
.;
>FK:J s p e c i m e n . 3-
RESULTS AND DISCUSSION3.1 G l a s s F i b r e R e i n f o r c e d P l a s t i c (GFRP)
S i x e x p e r i m e n t s w e r e c o n d u c t e d on u n i a x i a l g l a s s f i b r e r e i n f o r c e d epoxy t o d e t e r m i n e t r a n s v e r s e s h e a r p r o p e r t i e s . The s p e c i m e n w e r e machined t o h a v e t h e i r a x e s p a r a l l e l t o g l a s s f i b r e s t o o b t a i n in- p l a n e s h e a r s t r e s s - s t r a i n p r o p e r t i e s . A l l t h e s i x e x p e r i m e n t s a r e l i s t e d i n T a b l e 1. F i g . 3 shows t h e r e s u l t i n g dynamic s t r e s s - s t r a i n c u r v e f o r t h e e x p e r i m e n t s , I t i s c l e a r from t h e f i g u r e t h a t v a r i a - t i o n i n s t r a i n r a t e ( f r a n 2 7 0 / s t o 4 2 0 / s ) d o e s n o t p r o v i d e a s y s t e m a t i c v a r i a t i o n i n s t r e s s - s t r a i n r e s u l t s . T h e r e f o r e t h e s e e x p e r i m e n t s may b e c o n s i d e r e d t o d e f o r m a t a n a v , - r a g e s t r a i n r a t e of 3W/s
.
TABLE 1
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D e t a i l s o f dynamic e x p e r i m e n t s o n GFRPS1. S p e c i m e n S p e c imen s i z e s S t r a i n
NO. No. L e n g t h T h i c k n e s s I.iain D i a R a t e
(
1
(mm) ( m m > (s'l)1 86-49 3.1 0.65 12.7 3 0 0
2 86-51 3.0 0.49 12.6 300
3 86-53 3.1 0.52 12.5 280
4 86-56 3.1 0.53 12.4 2 7 0
5 86-57 2.8 0 . 3 2 1 2 . 1 4 2 0
6 86-59 3.0 0.45 12.2 2 9 0
The dynamic s t r e s s - s t r a i n c u r v e r a r e compared w i t h q u a s i s t a t i c r e s u l t s o b t a i n e d on a n I n s t r o n m a c h i n e . T h r e e s p e c i m e n a r e t e s t e d a t q u a s i - s t a t i c r a t e s t h r o u g h a s t a n d a r d t o r q u e f i x t u r e . d l 1 t h e s p e c i m e n f a i l e d a t s t r a i n s of a b o u t 0.1. On t h e c o n t r a r y dynarnic s p e c i r n e n s d i d
JOURNAL DE PHYSIQUE
1 I I 1
0 0.02 O.W 0.06 0.08 0.10
Sheor Strain f
F i g . 3
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Dynamic and q u a s i s t a t i c s h e a r s t r e s s - s $ r a i n c u r v e s o f u n i a x i a l GFRP a t v a r i o u s s t r a i n r a t e s y.n o t f a i l a t t h i s s t r a i n . They c o u l d n o t be l o a d e d t o h i g h e r s t r a i n s d u e t o t h e l i m i t a t i o n s of o u r Kolsky t o r s i o n machine. A n o t h e r d i f f e - r e n c e i s i n t h e n a t u r e o f s t r e s s - s t r a i n c u r v e s . Compared t o q u a s i s - t a t i c t e s t s dynamic s t r e s s - s t r a i n c u r v e s e x h i b i t h i g h e r s t i f f n e s s u p t o t h e s t r a i n of 0.03 t o 0.05. A t h i g h e r s t r a i n s dynamic c u r v e s show f l o w of m a t e r i a l a t c o n s t a n t s t r e s s e s which i s q u i t e d i f f e r e n t f r o m t h e q u a s i s t a t i c b e h a v i o u where s t r e s s i n c r e a s e s w i t h s t r a i n g r a d u a l l y and c o n t i n u o u s l y t i l l t h e specimen f a i l s . The a v e r a g e f l o w s t r e s s of dynamic c u r v e i s a b o u t 10/ h i g h e r o v e r t h e q u a s i s t a t i c s t r e n g t h .
3.2 Dynamic B e h a v i o u r of Epoxy
To compare t h e dynamic c u r v e s of u n i a x i a l FRP w i t h dynamic s h e a r s t r e s s - s t r a i n c u r v e s of epoxy, t e s t s w e r e c o n d u c t e d on t h e Kolsky t o r s i o n machine f o r epoxy specimen. The d e t a i l s of t h e t h r e e e x p e r i - ments a r e l i s t e d i n T a b l e 2. S t r e s s - s t r a i n c u r v e s o f t h e s e e x p e r i - ments a r e shown i n Fig.4. Two o f t h e t h r e e specimen f a i l e d a t a b o u t 10% s h e a r s t r a i n and t h e t h i r d specimen c o u l d n o t be s t r a i n e d t o f a i l u r e . U n l i k e GFRP m a t e r i a l s s t r e s s i n epoxy s p e c i m e n s i n c r e a s e s w i t h s t r a i n o v e r t h e e n t i r e r a n g e of s t r a i n . I n o t h e r words, epoxy
specimen do n o t e x h i b i t f l o w of m a t e r i a l a t c o n s t a n t s t r e s s e s a t h i g h s t r a i n r a t e s . F u r t h e r m o r e , e f f e c t of g l a s s f i b p e r e i n f o r c e m e n t i s q u i t e c l e a r f r o m t h e f i g u r e . The f l o w s t r e s s of dynamic s t r e s s - s t r a i n c u r v e s of u n i a x i a l GFRP i s a b o u t 40./! more o v e r t h e dynamic s t r e n g t h o f epoxy.
TMLE 2
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D e t a i l s o f dynamic e x p e r i m e n t s on Epoxys1. Specimen Spec h e n s i z e s S t r a i n
No. No. L e n g t h T h i c k n e s s Mean Dia R a t e
(mm) ( m ) (mm) ( s - l )
Shear Stmln f 70
a-
SO -
:.
40-X
i B
30-C ul
Fig. 4
-
Comparison o f dynamic s h e a r s t r e s s - s t r a i n c u r v e s of epoxy w i t h dynamic i n - p l a n e s h e a r s t r e s s - s t r a i n b e h a v i o u r o f GFRP.20-
3.3 Carbon F i b r e R e i n f o r c e d P l a s t i c (CFRP) ,, ,
,/;y
,/,',/
-
Unioxial GFRP',',, ---- EPOXY
S i x e x p e r i m e n t s whose d e t a i l s a r e l i s t e d i n T a b l e 3 were c o n d u c t e d on u n i a x i a l c a r b o n r e i n f o r c e d epoxy. Specimen a r e machined t o have t h e i r a x e s p a r a l l e l t o t h e f i b r e s . Fig. 5 shows t h e e x p e r i m e n t a l l y o b t a i n e d dynamic s t r e s s - s t r a i n c u r v e s f o r a l l t h e s i x specimen a l o n g w i t h q u a s i s t a t i c c u r v e s . L i k e GFRP c a s e s t h e CFRP specimen a l s o e x h i b i t f l o w of s t r e s s a t c o n s t a n t s t r e s s e s which b e g i n s a t t h e s t r a i n s r a n g i n g between 0.5 and 0.7. The CFRP specimen d i d n o t f a i l a t t h e maximum c a p a c i t y o f o u r t o r s i o n machine. I n c a s e o f two specimen deformed a t s t r a i n r a t e s of 1 5 0 / s and 335/s t h e bond between t h e specimen and t h e 1:olsky b a r s f a i l e d d u r i n g t h e e x p e r i - ments and t h e r e f o r e s t r e s s - s t r a i n s c u r v e s were n o t o b t a i n e d f o r h i g h s t r a i n s . Cornparision o f dynamic CFRP s t r e s s - s t r a i n c u r v e s w i t h q u a s i s t a t i c r e s u l t s , shows t h a t t h e f l o w s t r e s s a t a v e r a g e dyamic S t r a i n r a t e of 2 8 0 / s i s a b o u t 28/ h i g h e r o v e r q u a s i s t a t i c s t r e n g t h .
C3-102 JOURNAL DE PHYSIQUE
F u r t h e r , i t i s n o t e d t h a t when s t r a i n r a t e i s i n c r e a s e d from q u a s i - s t a t i c t o h i g h v a l u e s of a b o u t 300/s changes i n t h e b e h a v i o u r of GFRP and CFRP a r e s i m i l a r i n n a t u r e .
TABLE 3
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D e t a i l s of dynamic e x p e r i m e n t s on CFRP1 . Specimen S p e c m e n s l z e s S t r a i n
NO s Length T h i c k n e s s Mean Dia R a t e
(mm
1
( m ) (mm) ( s - l )'"I I
Shear Strain T
F i g . 5
-
uynamic and q u a s i s t a t i c s h e a r s t r e s s - s t r a i n b e h a v i o u r s of u n i a x i a l CFRP.Dynamic s h e a r b e h a v i o u r s of CFRP and epoxy a r e shown i n F i g . 6 . I t shows t h e e f f e c t o f c a r b o n f i b r e r e i n f o r c e m e n t o n t h e epoxy. The a v e r a g e f l o w s t r e s s of CFRP specimen i s a b o u t 30g h i g h e r o v e r t h e s t r e n g t h of epoxy.
-
Unioaial CFRP---
EPOXYI I I I I I
0 0.02 0.W 0.06 0.08 0.W Shear Slrain f
Fig. 6
-
Comparing dynaniic I n - p ~ i l ~ ~ e s h e a r b e h a v i o u r o f CFRP w i t h dynamic s h e a r b e h a v i o u r o f epoxy.U n i a x i a l GFRP and CFRP specimen a r e t e s t e d a t h i g h r a t e s o f a b o u t 300/s
.
The dynamic i n - p l a n e s h e a r s t r e s s - s t r a i n c u r v e s show f l o w of m a t e r i a l a t c o n s t a n t s h e a r s t r e s s e s f o r s t r a i n s e x c e e d i n g 3-5/and 5-7% f o r GFRP and CFRP r e s p e c t i v e l y . Under dynamic l o a d i n g t h e specimen d i d n o t f a i l u p t o 10/ s h e a r s t r a i n . The f l o w s h e a r s t r e s s of each m a t e r i a l i s o n l y m a r g i n a l l y h i g h e r o v e r t h e s t r e n g t h of q u a s i s t a t i c a l l y l o a d e d specimen.
ACXi\JOWLEDGEIvlENT
The r e s e a r c h work r e p o r t e d h e r e h a s been funded by t h e S t r u c t u r e P a n e l of A e r o n a u t i c s . < e s e a r c h and Development Board, Government of I n d i a . The a u t h o r s wish t o e x p r e s s t h e i r s i n c e r e t h a n k s t o Dr.i:.M.
Raju o f NAL, D a n g a l o r e , 0r.i-.S.Venkataraman of I.IIT, hiadras a n d k. 1C.Rajaiah of ADE, B a n g a l o r e f o r t h e i r h e l p and encouragement i n c a r r y i n g o u t t h i s work.
REFERENCES
/1/ Agarwal, B.D. and Broutman, L.J. A n a l y s i s and Performance o f F i b r e C o m p o s i t e s ,John Wiley and S o n s , New York, 1980.
/2/ Zweban, C . and P i p e s , R.B. i:lechanical B e h a v i o u r , Composites Design Guide,Univ. of Delacvare, !'ewark
,
Vol. 1, J u n e 1980./3/ Warner, S.M. and Dharan, C.K.H. J.Compos. I.:ater.
,
20 ( 1 9 8 6 ) 365./4/ Duffy
J.,
Campbell, J.u. and Hawley, R.H. J.App. Llech., 3 8 ( 1 9 7 1 j 83./5/ H a r l e y , K.ri., Duffy J. and Hawley, R.H. M e t a l Handbook, N i n t h Ed. Volume 8, ASil, Ohio, 1985.
