Interlaminar Shear Fracture Toughness and Fatigue Thresholds for Composite Materials

Abstract

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...

Interlaminar shear fracture of laminated composites

“…Use of tough matrix systems was shown to greatly improve composites' damage tolerance [1], but unfortunately the accompanying reduction in Gnc/G~c for such materials necessitates the additional consideration of shearing modes in any fracture analysis. This is manifested under cyclic loading, where threshold Gnc values for tough resins may be as little as the mode I toughness of brittle epoxy resins [2,3].…”

Refinements to the Mixed-Mode Bending Test for Delamination Toughness

“…The extensional modulus, E 11, in the longitudinal direction of the test specimen would control deformation of a homogenous material. However, it has been shown that the measured stiffness of this type of composite bending specimen is significantly less than that predicted by a simple bending model using a modulus value measured in tension [8]. In other words, the effective modulus for these specimens in bending is normally less than the value of modulus measured in tension.…”

“…First, the moduli of these types of bending specimens vary significantly from specimen to specimen. For this reason, compliance calibration methods are suggested for both the DCB Mode I test [9] and the ENF Mode II test [8]. The type of compliance calibration used with the DCB specimen does not work with the MMB specimen because the delamination growth is not always stable.…”

Generation of R-Curve from 4ENF Specimens: An Experimental Study

“…Moreover the effect of friction increases with span ratio [27,28]. Kageyama et al [12] have shown that the experimental compliance does not agree well with the theoretical one.…”

“…One of the most accurate data reduction methods to determine delamination toughness from 4ENF test data is compliance calibration technique (as compared to beam theory and finite element analysis if the crack length is measured accurately). Because of that, it was shown that beam theory and finite element based data reduction techniques can produce errors in toughness values [27,28] due to the fact that it is difficult to accurately determine the bending stiffness of each test specimen. But compliance calibration data reduction is a direct method that assumes linear elastic behaviour and self-similar crack advance.…”