Composite materials often fail by delamination. As composite materials with tougher matrices are developed to give better delamination resistance, their delamination behavior needs to be fully characterized. In this paper the onset and growth of delamination in AS4/PEEK, a tough thermoplastic matrix composite, was characterized for Mode I and Mode II loadings, using the double-cantilever beam (DCB) and the end-notched flexure (ENF) test specimens, respectively. Delamination growth per fatigue cycle, da/dN, was related to strain energy release rate, G, by means of a power law. However, the exponents of these power laws were too large for them to be adequately used as a life prediction tool. A small error in the estimated applied loads could lead to large errors, at least one order of magnitude, in the delamination growth rates. Hence, strain energy release rate thresholds, Gth, below which no delamination would occur were also measured. Mode I and II threshold G values for no delamination growth were found by monitoring the number of cycles to delamination onset in the DCB and ENF specimens. The maximum applied G for which no delamination growth had occurred until at least 106 cycles was considered the threshold strain energy release rate. The Gth values for both Mode I and Mode II were much less than their corresponding fracture toughnesses. Results show that specimens that had been statically precracked in shear have similar Gth values for Mode I and Mode II for R-ratios of 0.1 and 0.5. An expression was developed which relates Gth and Gc to cyclic delamination growth rate. Comments are given on how testing effects (e.g., facial interference and damage ahead of the delamination front) may invalidate the experimental determination of the constants in the expression.

This paper presents a summary of the tests performed within a VAMAS (Versailles Project on Advanced Materials and Standards) round robin to examine the measurement of mode II interlaminar fracture toughness using four different test methods based on: End Notched Flexure (ENF), Stabilised End Notched Flexure (SENF), End Loaded Split (ELS), and four point End Notched Flexure (4ENF) carbon fibre reinforced epoxy specimens. Tests were performed by members of ESIS (European Structural Integrity Society), JIS (Japan Industrial Standards group) and ASTM (American Society for Testing and Materials).

Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam (DCB) specimen is used to measure fracture toughness, G Ic , and strain energy release rate, G Imax , for delamination onset and growth in laminated composites under mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy DCB specimens to evaluate the draft standard "Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional FiberReinforced Polymer Matrix Composites." Static results were used to generate a delamination resistance curve, G IR , for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic G Imax level equal to 90% of the fracture toughness, G Ic , to determine the delamination growth rate. Two different data reduction methods, a 2-point and a 7-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the nonnormalized results. Paris Law exponents were found to decrease by 5.4% to 46.2% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at 3 lower cyclic G Imax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material.

SUMMARY I;'S t a t i c and c y c l i c end-notched f l e x u r e (ENF) tests were conducted on three materials t o d e t e r m i n e t h e i r i n t e r l a m i n a r shear fracture toughness and f a t i g u e t h r e s h o l d s f o r d e l a m i n a t i o n i n terms o f l i m i t i n g v a l u e s of t h e mode I1 s t r a i n e n e r g y release r a t e , G I I , f o r delamination growth. Data were g e n e r a t e d f o r three d i f f e r e n t materials: a T300/BP907 g r a p h i t e epoxy, a n S2/SP250 g l a s s epoxy, and a n AS4/PEEK g r a p h i t e t h e r m o p l a s t i c . The i n f l u e n c e of p r e c r a c k i n g and data r e d u c t i o n schemes on t h e mode I1 toughness and f a t i g u e b e h a v i o r are d i s c u s s e d .F i n i t e element a n a l y s i s i n d i c a t e d t h a t t h e beam t h e o r y c a l c u l a t i o n f o r G w i t h t h e t r a n s v e r s e shear c o n t r i b u t i o n included was r e a s o n a b l y a c c u r a t e over t h e e n t i r e r a n g e of c r a c k l e n g t h s . However, compliance measurements f o r t h e three materials t e s t e d , and t h e v a r i a t i o n i n compliance w i t h crack l e n g t h , d i f f e r e d from t h e beam t h e o r y p r e d i c t i o n s . For m a t e r i a l s t h a t e x h i b i t e d l i n e a r l o a dd e f l e c t ion b e h a v i o r , measurements provided the most c o n s e r v a t i v e and accurate estimate of t h e i n t e r l a m i n a r shear f r a c t u r e toughness. C y c l i c l o a d i n g s i g n i f i c a n t l y reduced t h e c r i t i c a l CII f o r d e l a m i n a t i o n . A t h r e s h o l d v a l u e of the maximum cyclic GII below which no d e l a m i n a t i o n o c c u r r e d a f t e r one m i l l i o n c y c l e s was i d e n t i f i e d f o r each material t o q u a n t i f y t h e d e g r a d a t i o n i n i n t e r l a m i n a r shear f r a c t u r e t o u g h n e s s i n f a t i g u e . In a d d i t i o n , r e s i d u a l s t a t i c t o u g h n e s s t e s t s were conducted on g l a s s epoxy specimens t h a t had undergone one m i l l i o n cycles w i t h o u t d e l a m i n a t i o n .These residual s t a t i c tests, and t h e i n i t i a l s t a t i c tests on t h e tough AS4/PEEK g r a p h i t e t h e r m o p l a s t i c , e x h i b i t e d n o n l i n e a r l o a d -d e f l e c t i o n b e h a v i o r . For these cases, t h e l o a d a t d e v i a t i o n from n o n l i n e a r i t y was used t o d e t e r m i n e t h e i n t e r l a m i n a r shear f r a c t u r e toughness. A l i n e a r mixed-mode d e l a m i n a t i o n criterion used t o c h a r a c t e r i z e t h e s t a t i c toughness of s e v e r a l composite materials;however, a t o t a l C t h r e s h o l d c r i t e r i o n a p p e a r s t o be s u f f i c i e n t f o r c h a r a c t e r i z i n g the f a t i g u e d e l a m i n a t i o n d u r a b i l i t y o f composite materials w i t h a w i d e r a n g e of s t a t i c t o u g h n e s s e s . Flexural compliance c a l c u l a t e d from beam t h e...