22 Anisotropic Measurements on Single-Ply Lamina
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Composites WAYNE J. MIKOLS and JAMES C. SEFERIS D e p a r t m e n t of C h e m i c a l E n g i n e e r i n g , U n i v e r s i t y of W a s h i n g t o n , Seattle, WA 98195
E n v i r o n m e n t a l and c h e m i c a l effects significantly influence the physical properties o f composite materials (1,2). Environmental effects cause property alterations due to various h y g r o thermal histories and states of the c o m p o s i t e ( 3 ) . C h e m i c a l effects manifest themselves through structural considerations. Such things a s resin/catalyst stoichiometry, matrix impurities, fiber surface impurities, and o t h e r p r o c e s s related phenomena c a u s e c h e m i c a l alterations in t h e c o m p o s i t e w h i c h affect t h e macroscopic and/or microscopic structural n a t u r e of the s y s t e m (4,5). Many investigations h a v e f o c u s e d upon t h e influence of either c h e m i c a l or environmental effects upon various c o m p o s i t e system properties. I t s h o u l d be a p p a r e n t t h a t t h e s e two effects are strongly coupled. Physical properties of c o m p o s i t e s a r e of c o u r s e d e p e n d e n t upon t h e ultimate physio-chemical structure of the system. C o n s e q u e n t l y , this basic structure is a n overriding consideration w h i c h is responsible for t h e n a t u r e and e x t e n t to w h i c h e n v i r o n m e n t a l effects c a n influence s y s t e m properties. By examining basic anisotropic viscoelastic properties of a composite s y s t e m , this s t u d y r e l a t e s m o i s t u r e a b s o r p t i o n t o p r o p e r t y and s t r u c t u r a l c o n s i d e r a t i o n s a f f e c t i n g t h e a b s o r p t i o n p r o c e s s . The p a r t i c u l a r g r a p h i t e - e p o x y c o m p o s i t e s y s t e m employed i n t h i s s t u d y was made w i t h T-300 c a r b o n f i b e r s ( U n i o n C a r b i d e ) a n d 5208 epoxy ( N a r m c o ) . U n i d i r e c t i o n a l c o n t i n u o u s s i n g l e - p l y T-300/5208 l a m i n a t e s w e r e u s e d . Sample m a t e r i a l s w e r e e x p o s e d t o e i t h e r d e s i c c a t e d o r soaked environments p r i o r t o t e s t i n g . I n a composite system, p r o p e r t i e s can be viewed on a macroscopic or m i c r o s c o p i c s c a l e . M a c r o s c o p i c a l l y , composites e x h i b i t a n i s o t r o p i c b e h a v i o r due t o t h e n a t u r e i n w h i c h c a r b o n f i b e r s a r e o r i e n t e d i n the m a t r i x m a t e r i a l . Dynamic m e c h a n i c a l e x p e r i m e n t s w e r e p e r f o r m e d o n b o t h s o a k e d and d e s i c c a t e d s a m p l e s . These measurements o n t h e two s e t s o f s a m p l e s w e r e u s e d t o s t u d y t h e m a c r o s c o p i c b e h a v i o r o f t h e c o m p o s i t e s a t 0°, 15°, 30°, and 45° d i r e c t i o n s w i t h r e s p e c t t o the f i b e r d i r e c t i o n . Similar experi0-8412-0567-l/80/47-132-293$05.00/0 © 1980 American Chemical Society May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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294
RESINS FOR
AEROSPACE
merits w e r e c a r r i e d o u t u s i n g s o n i c m o d u l u s measurements. Micro s c o p i c a l l y , the composite m a t r i x m a t e r i a l e x h i b i t s v i s c o e l a s t i c b e h a v i o r t h a t c a n s t r i c t l y be a s s i g n e d t o t h e c r o s s l i n k s t r u c t u r e of the r e s i n . G r a p h i t e r e i n f o r c e m e n t o n l y s e r v e s t o damp o u t v i s c o e l a s t i c t r a n s i t i o n s of the r e s u l t a n t composite. By c o m b i n i n g dynamic m e c h a n i c a l d a t a w i t h m o i s t u r e d i f f u s i o n s t u d i e s , a f u n d a m e n t a l u n d e r s t a n d i n g o f t h e n a t u r e and mechanisms i n v o l v e d i n t h e m o i s t u r e a b s o r p t i o n (and p r o p e r t y d e g r a d a t i o n ) p r o c e s s e s c a n be i n f e r r e d . Experimental The c o m p o s i t e s t u d i e d i n t h i s w o r k was made w i t h a commer c i a l epoxy (Narmco 5208) whose p r i m a r y c o n s t i t u e n t i s t e t r a g l y c i d y l 4,4* d i a m i n o d i p h e n y l methane epoxy c u r e d w i t h 4,4' d i a m i n o d i p h e n y l s u l f o n e (TGDDM-DDS). C o n t i n u o u s T-300 c a r b o n f i b e r s i m p r e g n a t e d w i t h epoxy p r o v i d e d a u n i d i r e c t i o n a l p r e p r e g t a p e h a v i n g a 34.7% r e s i n c o n t e n t and a n a e r i a l d e n s i t y o f 95 g/m . S h e e t l a m i n a w e r e p r e p a r e d f r o m a 30.48 cm w i d e r o l e o f p r e p r e g t a p e by vacuum bag c a s t i n g . Sheets o f p r e p a r e d p r e p r e g were p l a c e d i n a 120°C o v e n . The o v e n t e m p e r a t u r e u n d e r w e n t a l i n e a r ramp t o 177°C o v e r 30 m i n u t e i n t e r v a l . The s h e e t s w e r e h e l d a t t h i s e l e v a t e d temperature f o r 2 hours. The o v e n was p e r m i t t e d to r e t u r n t o room t e m p e r a t u r e . C u r e d s a m p l e s w e r e removed and a l l o w e d t o s e t a t a m b i e n t l a b o r a t o r y c o n d i t i o n s f o r a t l e a s t one month (101.3 k P a , 22°C, 60% R e l a t i v e H u m i d i t y ) . S e v e r a l s e t s o f samples were c u t from t h e c u r e d l a m i n a s h e e t s w h i c h had a t h i c k n e s s o f 0.02 cm. P o r t i o n s o f the mate r i a l w e r e c u t h a v i n g n o m i n a l d i m e n s i o n s o f 7cm χ 14cm. These samples were s u b j e c t e d t o subsequent d e s i c c a t i o n over anhydrous c a l c i u m s u l f a t e and/or s o a k i n g i n d e i o n i z e d water. An a m b i e n t l a b o r a t o r y t e m p e r a t u r e o f 22°C was m a i n t a i n e d t h r o u g h o u t t h i s p o r t i o n of the experiment. T y p i c a l data from these t e s t s are p r e s e n t e d i n F i g u r e 1. S w e l l i n g measurements w e r e c o l l e c t e d a t d e s i c c a t e d , 60% r e l a t i v e h u m i d i t y , and s o a k e d s a m p l e e n v i r o n ments. A v e r a g e s w e l l d i m e n s i o n s f o r t h e s a m p l e o f F i g u r e 1 a r e i l l u s t r a t e d i n F i g u r e 2. L e n g t h and w i d t h s w e l l d i m e n s i o n s c o r r e s p o n d t o r e s p e c t i v e f i b e r and. t r a n s v e r s e f i b e r c o m p o s i t e d i r e c tions. These measurements w e r e o b t a i n e d u s i n g c a l i p e r s a c c u r a t e to +0.001 cm ( i . e . l e s s t h a n 0.01% o f s a m p l e d i m e n s i o n s ) . Lami na t h i c k n e s s measurements w e r e o b t a i n e d u s i n g a d i s h e d p l a t e m i c r o m e t e r a c c u r a t e t o +0.0002 cm ( i . e . a p p r o x i m a t e l y 1% o f sam ple dimensions). S e v e r a l s t r i p samples h a v i n g approximate dimensions o f 0.4cm χ (10-17cm) χ 0.02 cm w e r e a l s o c u t f r o m t h e c u r e d s h e e t m a t e r i a l at v a r i o u s d i r e c t i o n s to the f i b e r d i r e c t i o n . These a n g l e s w e r e 0, 15, 30, and 45 d e g r e e s t o t h e f i b e r d i r e c t i o n . I t was f o u n d t h a t s t r i p s c u t a t a n g l e s g r e a t e r t h a n 45 d e g r e e s c o u l d n o t s u s t a i n any l o a d f o r t e s t i n g . E x t r e m e c a r e was t a k e n t o i n s u r e t h a t no v i s i b l e m i c r o c r a c k s r e m a i n e d a l o n g sample edges 2
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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MiKOLS AND SEFERis
-1.0' 0
' 125
Singh-Ply
< 250
' 375
295
Laminates
• ' ' 500 625 750 ABSORPTION TIME. (HOURS)
> 875
' 1000
1
' 1125
1250
Figure I. Soaking and desiccation of Τ-300/5208 composite sheets; percentage gain or loss as a function of time with respect to initial equilibrium at 60% rela tive humidity
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Figure 2.
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.50 -75 1.00 1.25 PERCENT MOISTURE IN SAMPLE
1 1.50
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Percentage swell of linear sample dimensions as a function of moisture content
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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296
RESINS F O R A E R O S P A C E
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Figure 3.
