Results o f an experimental study o f t h e postbuckling response and f a i l u r e c h a r a c t e r i s t i c s o f 16-and 24-ply o u a s i -i s o t r o p i c and 24-ply o r t h o t r o p i c f l a t rectangular graohite-epoxy p l a t e s loaded i n compression a r e presented. The r a t i o o f f a i l u r e load t o b u c k l i n g load was higher f o r specimens w i t h lower i n i t i a l b u c k l i n g s t r a i n s than f o r specimens w i t h higher i n i t i a l b u c k l i n g s t r a i n s . Some specimens supported more than f i v e times t h e i r i n i t i a l buckling l o a d b e f o r e f a i l i n g . A n a l y t i c a l r e s u l t s obtained from a nonlinear general s h e l l analysis computer code c o r r e l a t e we1 1 w i t h t e s t r e s u l t s up t o f a i l u r e . The specimens f a i l e d along a nodal l i n e o f t h e b u c k l i n o mode i n an induced shear f a i l u r e mode. Some specimens e i t h e r had c i r c u l a r holes o r were subjected t o low-speed impact damage before t e s t i n g . The postbuckling s t r e n g t h o f specimens w i t h high i n i t i a l buckling s t r a i n s i s degraded more by holes and imoact damage than soecimens w i t h lower i n i t i a l b u c k l i n q s t r a i n s . I n t r o d u c t i o n Current metal a i r c r a f t design p r a c t i c e a1 lows some s t r u c t u r a l comoonents (e.g., fuselage and s t a b i l i z e r panels) t o buckle p r i o r t o f a i l u r e under c e r t a i n loading conditions. These components a r e desioned t o c a r r y t h e i r u l t i m a t e load w h i l e i n a postbuckled s t a t e . The use o f advanced-composite m a t e r i a l s i n s i m i l a r postbuckl i n g appl i c a t i o n s could lead t o e f f i c i e n t s t r u c t u r a l designs. Before comp o s i t e s t r u c t u r a l components can be designed f o r these a p p l i c a t i o n s , however, t h e i r postbuckling response and f a i l u r e c h a r a c t e r i s t i c s must be understood w e l l enough t o p r e d i c t t h e i r s t r u c t u r a l behavior. E a r l i e r published work on t h e postbuckl i n g behavior o f composite s t r u c t u r e s (e.g., Refs. 1-4) has focused on a n a l y t i c a l s o l u t i o n s o f c l a s s i c a l o r t h o t r o p i c p l a t e problems. Only a l i m i t e d amount o f data has been published comparing t e s t r e s u l t s w i t h a n a l y t i c a l p r e d i c t i o n s . The compressive s t r e n g t h o f b u c k l i n g -r e s i s t a n t graphiteepoxy laminates has been shown t o be s e r i o u s l y degraded by c i r c u l a r holes (Refs. 5-7) and low-speed impact damage (Refs. 5 , 8 and 9). On t h e o t h e r hand, p r e l iminary t e s t s on graphite-epoxy laminates (Ref. 7) i n d i c a t e t h a t c i r c u l a r holes may have l i t t l e o r no e f f e c t on b u c k l i n g response o r postb u c k l i n g strength. This paper presents r e s u l t s o f a t e s t program t o study the postbuckling response and f a i l u r e c h a r a c t e r i s t i c s o f s...

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In the last 20 years NASA has worked in collaboration with industry to develop enabling technologies needed to make aircraft safer and more affordable, extend their lifetime, improve their reliability, better understand their behavior, and reduce their weight. To support these efforts, research programs starting with ideas and culminating in full-scale structural testing were conducted at the NASA Langley Research Center. Each program contained development efforts that (a) started with selecting the material system and manufacturing approach; (b) moved on to experimentation and analysis of small samples to characterize the system and quantify behavior in the presence of defects like damage and imperfections; (c) progressed on to examining larger structures to examine buckling behavior, combined loadings, and built-up structures; and (d) finally moved to complicated subcomponents and full-scale components. Each step along the way was supported by detailed analysis, including tool development, to prove that the behavior of these structures was well-understood and predictable. This approach for developing technology became known as the "building-block" approach. In the Advanced Composites Technology Program and the High Speed Research Program the building-block approach was used to develop a true understanding of the response of the structures involved through experimentation and analysis. The philosophy that if the structural response couldn't be accurately predicted, it wasn't really understood, was critical to the progression of these programs. To this end, analytical techniques including closed-form and finite elements were employed and experimentation used to verify assumptions at each step along the way. This paper presents a discussion of the utilization of the building-block approach described previously in structural mechanics research and development programs at NASA Langley Research Center. Specific examples that illustrate the use of this approach are included from recent research and development programs for both subsonic and supersonic transports.

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