Analytical and experimental studies on the buckling of laminated thin-walled structures

The b u c k l i n g and p o s t b u c k l i n g b e h a v i o r o f some t h i n w a l l e d s t r u c t u r e s made o f l a m i n a t e d compos i t e m a t e r i a l s a r e s t u d i e d by f i n i t e e lement ana l yses and by expe r imen ta l i n v e s t i g a t i o n s . The p r e d i c t e d b u c k l i n g l o a d s and modes ag ree reasonab l y w e l l w i t h exper iments . The f a i l u r e l o a d s however canno t be e s t i m a t e d from t h e p r e s e n t p o s t b u c k l i n g s o l u t i o n s wh i ch a r e based on t h e c l a s s i c a l l am ina t i o n t h e o r y . 1. I n t r o d u c t i o n C u r r e n t a i r c r a f t d e s i g n p r a c t i c e a l l o w s some s t r u c t u r a l components t o b u c k l e p r i o r t o f a i l u r e under c e r t a i n l o a d i n g c o n d i t i o n s . These components a r e expected t o a c h i e v e t h e i r u l t i m a t e l o a d s i n t h e p o s t b u c k l i n g range. Thus, t h e b u c k l i n g and p o s t b u c k l i n g b e h a v i o r o f compos i te s t r u c t u r e s must be w e l l unders tood. Many s t u d i e s have been done f o r s t i f f e n e d and u n s t i f f e n e d compos i t e pane l s under compressive l o a d i n g (e.g., Refs. 1-5). However, t h e r e i s s t i l l a l a c k of unde rs tand ing o f t h e c h a r a c t e r i s t i c s under compress ion f o r t h i n w a l l e d s t r u c t u r e s t h a t a r e , i n gene ra l , o f a n i s o t r o o i c l am ina tes w i t h unsymmetr ic l ayups . The p r e s e n t paper concerns a combined f i n i t e element and expe r imen ta l s t u d y o f t h e b u c k l i n g and p o s t b u c k l i n g b e h a v i o r o f some t h i n w a l l e d s t r u c t u r e s made o f l a m i n a t e d compos i te m a t e r i a l s . The s t r u c t u r e s under c o n s i d e r a t i o n a r e squaresec t i oned channe ls , tubes, and s t i f f e n e d pane l s which c o n s i s t of non -cop lana r and a n i s o t r o o i c pane ls o f e i t h e r symmetr ic o r unsymmetr ic l ayups . The most conven ien t scheme t o accoun t f o r noncop lana r p l a t e e lements , i s t h e use o f semiLoof elements6 f o r wh i ch t h e normal r o t a t i o n s a l o n g t h e e lement edges a r e d e f i n e d by t h e va lues a t nodes wh ich a r e n o t l o c a t e d a t c o r n e r s o f t h e element. The a n a l y t i c a l s t u d y i s based on a 32-DOF r e c t a n g u l a r semiLoof e lement f o r combined i n p l a n e and o u t o f p l a n e a c t i o n s , c o n s t r u c t e d by t h e assumed s t r e s s h y b r i d f i n i t e e lement m e t h ~ d ~ , ~ . ~ . The expe r imen ta l s t u d y c o n s i s t s o f compression t e s t s o f s e v e r a l k i n d s o f specimens made o f l a m i n a t e d compos i tes w i t h t h e purpose o f examin ing t h e b u c k l i n g and p o s t b u c k l i n g b e h a v i o r o f t h i n w a l l s t r u c t u r e s w i t h a n i s o t r o p i c l am ina ted pane l s . Another o b j e c t i v e i s t o v e r i f y -*Research A s s i s t a n t , now a t Department o f T h e o r e t i c a l & A p p l i e d Mechanics, U n i v e r s i t y o f I l l i n o i s a t ~ rbana -champa i gn **Pro fessor ***Associate P r o f e s s o r Copyright O American Institute of Aeronautics and Astronautics, Inc., 1987. All rights reserved. t h e co r respond inq f i n i t e e lement s o l u t i o n s . I n t h i s paper t h e r e s u l t s o f t h r e e k i n d s o f specimens a r e p resen ted . More d e t a i l e d r e s u l t s and a n a l y s i s can be found i n t h e r e p o r t by ~ a n g l o .