adhesive bonding of fibre composite

7/27/2019 Adhesive Bonding of Fibre Composite

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A dh esive bo nd ing of f ibre-reinforced compositesB . M . P a r k e r( D e fe n c e R e s e a r c h A g e n c y , U K )

T h e a p p l i c a b i l i t y o f t e s t m e t h o d s f o r a d h e s i v e - b o n d e d m e t a l a d h e r e n d s t o fi b re -r e i n f o rc e d c o m p o s i t e s i s co n s id e r e d . B o n d s t r e n g th s o b t a i n e d b y m e t h o d s c h o s e n a ss u i t a b l e a r e c o m p a r e d .

T h e m o s t c r i ti c a l f a c t o r i n m e t a l jo i n t s i s t h e p r e t r e a t m e n t o f th e a d h e r e n d b e f o r eb o n d i n g , a s t h is c o n t r o l s t h e s t r u c tu r e a n d c h e m i s t r y o f t h e o x i d e s u rf a c e a n d h e n c ed u r a b i l i t y i n h o t - h u m i d c o n d i t i o n s , o f t e n w i t h l i t tl e e f fe c t o n i n it i a l s t r e n g th s . F o r f ib r e -r e i n f o r c e d e p o x y r e s i n a d h e r e n d s , t h e c r it i ca l f a c t o r i s t h e i n i t ia l b o n d . I n i ti a l b o n ds t r e n g th , m e a s u r e d b y a n y m e t h o d , i s r e l a t e d t o th e p r e s e n c e o f c o n t a m i n a n t s o n t h ea d h e r e n d s u r f a ce . L o s s o f s t r e n g t h i n h o t - h u m i d c o n d i t i o n s i s r e l a t e d t o w a t e ra b s o r p t i o n b y a d h e s i v e a n d m a t r i x a n d n o t t o r e d u c e d a d h e s io n .T h e r m o p l a s t i c - m a t r i x c o m p o s i t e s r e q u i r e p r e t r e a t m e n t s w h i c h in c r e a s e t h e i rs u r f a c e e n e r g y a n d w e t t a b i l i t y b y a d h e s iv e s .Key words: a d h e s i v e - b o n d e d j o i n ts ; c o m p o s i t e a d h e r e n d s ; d u r a b i l it y ; t es t m e t h o d s ;s u r fa c e c o n t a m i n a t i o n

A n y s t r u c tu r e , w h e t h e r i t is a m o t o r c a r , a n a e r o p l a n eo r a l a w n m o w e r , n e e d s t o b e s u i ta b l e f o r t h e j o b t h a ti t i s e x p e c t e d t o d o a n d t o l a s t . T h a t i s , i t h a s t o b es t r o n g e n o u g h i n it ia l ly t o m e e t s o m e s t a n d a r d a n d i tm u s t r e t a i n i t s s t r e n g t h f o r i t s e x p e c t e d l if e t im e ; i t m u s tb e d u r a b l e .A n a i r f r a m e i s f i rs t a n d f o r e m o s t e x p o s e d t ov i b r a t i o n a n d t h e s t r e s s e s o f t a k e - o f f a n d l a n d i n g w h i c hr e s u l t i n f a t i g u e d a m a g e . T h i s r e s u l t s i n c r a c k i n g a n d i sw e ll u n d e r s t o o d f o r m e t a ls . C o r r o s i o n o f m e t a ls i s a ls oa p r o b l e m .W h e n o r g a n i c m a t e r i a l s s u c h a s f i b r e - r e i n f o r c e dc o m p o s i t e s a n d a d h e s iv e s a r e i n t r o d u c e d i n t os t r u c t u r e s , a f u r t h e r p r o b l e m i s a l s o i n t r o d u c e d , t h a t o ft h e e ff e c t o f a b s o r b e d a t m o s p h e r i c m o i s t u r e o n t h eo r g a n i c m a t e r i a l . I n c o m p o s i t e s , t h e r e s i n - m a t r i xd o m i n a t e d p r o p e r t i e s a r e r e d u c e d . T h i s a l s o a p p l i e s t ot h e p r o p e r t i e s o f a d h e si v es . B u t o f m u c h g r e a t e rc o n c e r n i n t h e p a s t h a s b e e n t h e e f f e c t o f m o i s t u r e o nt h e a d h e s iv e - t o - m e t a l b o n d . T h i s p r o b l e m h a s l o n gb e e n r e c o g n i z e d a n d h a s r e s u l t e d , i n t h e a i r c r a f ti n d u s t ry , i n a n e n o r m o u s v o l u m e o f w o r k o n t h es u r f a ce p r e t r e a t m e n t o f m e t a l s, p a r t i c u la r l y a l u m i n i u m ,

b e f o r e a d h e s i v e b o n d i n g . W i t h t h e s e r v i c e l iv e s o fa i r c r af t b e in g e x t e n d e d b e y o n d w h a t w a s p r o b a b l yi n i t i a l l y e n v i s a g e d , ' a g e i n g a i r c r a f t ' a r e n o w a m a j o rc o n c e r n .F i b r e - r e i n f o rc e d c o m p o s i t e s a r e b e i n g m o r e a n dm o r e w i d e l y u s e d i n c i v il a s w e ll a s m i l i t a r y a i r f r a m e sa n d i t i s n e c e s s a r y t o k n o w w h a t t h e i r d u r a b i l i t y i sl i k e ly t o b e a n d w h e t h e r a n y s p e c i a l m e a s u r e s a r er e q u i r e d t o i m p r o v e i t .T h i s p a p e r c o n s i d e r s t h e a p p l i c a b i l i t y t o c o m p o s i t e so f m e t h o d s w h i c h a r e u s e d t o t e st th e s t r e n g t h o fb o n d e d m e t a l jo i n t s , u s i n g c a r b o n f i b r e -r e i n f o r c edc o m p o s i t e ; t e s t s a r e t h e n c o m p a r e d t o s e e w h e t h e r t h e yg i ve th e s a m e i n f o r m a t i o n o n b o n d i n g . L a p j o i n ts a n dw e d g e s p e c i m e n s w e r e c h o s e n f o r a s t u d y o f t h e e f f e c to f a b s o r b e d m o i s t u r e o n j o i n t s t r e n g t h . T h e r e su l tso b t a i n e d a r e c o m p a r e d .

Test metho ds for composite jo intsT h e m a i n t e s t s u s e d f o r m e t a l - t o - m e t a l j o i n t s a r e t h el a p s h e a r j o i n t ( s in g l e o r d o u b l e ) , t e n s il e b u t t j o i n t ,f l o a t i n g r o l l e r p ee l , T - p e e l a n d w e d g e t e s t . A l l h a v e

0143-7496/94/02/0137-07 ~;) 1994 Bu tterw orth-H eine ma nn Ltd~'~ Crown Cop yr ight , 1993 INT.J.ADHESION AN D ADHESIVES VO L. 14 NO. 2 APRIL 1994 137


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b e e n u s e d t o a s s e s s d u r a b i l i t y b y e x p o s u r e t o h o th u m i d c o n d i t i o n s , a n d s o m e h a v e a l s o b e e n s u b j e c t e dto na tu ra l , s t a t i c , w e a the r ing .L a p s h e a r j o i n t sT h e r e a r e n o p r o b l e m s i n t r a n s l a t i n g s i n g l e o r d o u b l el a p j o i n t s f r o m m e t a l t o c o m p o s i t e . T h e d i m e n s i o n ss h o w n i n F i g. 1 a r e t h o s e r e c o m m e n d e d b y C R A G ,M e t h o d 1 02 ( s p e c im e n A ) i.F lo a t in g r o l l e r p e e lThi s spe c ime n (F ig . 2 ) (EN 2243 Pa r t 02 , s imi l a r t oA S T M D 3 1 6 7 ) c o ns i s ts o f a t h i ck b a s e m e m b e r a n d at h i n m e m b e r , w h i c h h a s t o b e f le x i b le e n o u g h t o b e n dr o u n d t h e r o l l e r . T h e s a m e r e s t r i c t i o n o n t h e t h i nm e m b e r a p p l i e s a s f o r T - p e e l. T h i s t e s t c a n b e u s e d t ot e s t t h e b o n d a b i l i t y o f u n a g e d c o m p o s i t e b y u s i n g at h i n m e t a l m e m b e r a n d a t h i c k c o m p o s i t e b a s e .H o w e v e r , t h e u s e o f a t h i n m e t a l m e m b e r w o u l d c a u s ec o m p l i c a t i o n s i n d u r a b i l i t y s t u d i e s d u e t o t h e e f f e c t o fm o i s t u r e o n t h e a d h e s i v e / m e t a l i n t e r f a c e . U n l e s s i t i sa c t u a l l y i n t e n d e d t o s t u d y m e t a l - t o - c o m p o s i t e j o i n t s ,t h i s spe c ime n i s no t su i t a b l e fo r du ra b i l i t y s tud ie s .W e d g e t e s tT h i s t e st , to A S T M D 3 7 6 2 f o r m e t a l s , h a s b e e na d a p t e d f o r c o m p o s i t e s a s s h o w n i n F i g . 3 . T h e l e n g t ho f 1 0 0 r a m w a s t h e r e s u l t o f u s in g t h i s a s th e s t a n d a r d~_~ h e s i v e fillet

Fig. 1 Single lap shear composite test specimen, all dimensions in mm

pa ne l l e ng th f o r l a p jo in t s ; a l onge r spe c ime n i s ne e de di f i t i s r e a l ly ne c e ssa ry to kno w c r a c k l e ng ths l i ar "ba d ', j o in t s . Spe c ime n e dge s arc po l i she d to a t l e a s t 400 g r i t .T - p e e lT h i s i s s h o w n i n F i g . 4 . T h e r e a r e p r o b l e m s i n a d a p t i n gt h i s f o r c a r b o n f i b r e - r e i n f o r c e d c o m p o s i t e d u e t oa n i s o t r o p y o f t h e b a s e m a t e r i a l . A s w i t h t h e f l o a t i n gro l l e r pe e l, a ve ry th in l a m ina te i s r e qu i r e d to be a b l et o b e n d t h e u n b o n d e d e n d s e n o u g h t o g r i p t h e m . T h i si s a m i n i m u m o f a 4 - p l y l a m i n a t e l b r u n i d i r e c t i o n a lma te r i a l , o r tw o l a ye r s o f c lo th . I t i s poss ib l e t ha t a( 4 5 )~ l a m i n a t e c o u l d b e u s e d t o r e d u c e m o d u l u s i n th ed i r e c t i o n o f t h e t e s t. A 2 - p l y c l o t h l a m i n a t e h a s b e e nuse d in su r fa c e s tud ie s : , bu t , i f t he bon d i s goo d , t he rei s a p r o b l e m w i t h f a i l u r e o f t h e a d h e r e n d a c r o s s t h ee n d o f t h e b o n d l i n e .T e n s i l e b u t t j o i n tT h e m e t a l s p e c i m e n u s e s b a r s t o c k ( F i g . 5 ) . T h e r e a r ep r a c t i c a l d i f fi c u l ti e s in a d a p t i n g t h is f o r c o m p o s i t e s . I fa b a r i s m a c h i n e d w i t h t h e b o n d l i n e i n t h e s a m e p l a n ea s t h e f i b r e a t t h e c o m p o s i t e s u r f a c e , t e s ti n g i n t e n s i o nw o u l d b e t e s ti n g t r a n s v e r s e t o t h e f i b r e p l a n e a n d t h ec o m p o s i t e s t r e n g t h w o u l d b e d e p e n d e n t o n t h e m a t r ix ,w h i c h i s u s u a l l y m o r e b r i t t l e t h a n m o s t s t r u c t u r a la d h e s i v e s . T h e r e i s p r o b a b l y n o r e a s o n w h y t h i ss p e c i m e n c o u l d n o t b e u s e d f o r b r i t t l e a d h e s i v e s .C o m p a r i s o n o f t e s t m e t h o d s o n u n a g e d j o in t sE x p e r i m e n t a lA l l t h e s p e c im e n s d e s c r i b e d i n t h is p a r t o f t h e w o r kw e r e m a d e f r o m u n i d i re c t i o n al C i b a - G e i g y F i b r e d u xX A S / 9 1 4 C e p o x y - m a t r i x co m p o s i t e , n o m i n a l ly 2 m mt h i c k . T h e s u r f a c e w a s v a r i e d b y c u r i n g a g a i n s t a r a n g eo f r e l e a s e a g e n t s 3, w h i c h l e f t p o l y t e t r a f l u o r o e t h y l e n e(PTFE), s i l i c o n e o r p o l y a m i d e r e s i d u e s o n t h e c o m p o s i t e

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surface. The level of contamination was reduced eitherby hand abrasion with an abrasive cloth or by blastingwith dry alumina grit. The nature and level ofcontaminat ion was estimated by X-ray photoelectronspectroscopy (xes) which showed3 that surfacecontaminat ion is partly removed by hand abrasionwith carborundum and, except in cases of very heavycontamination, virtually completely removed by drygrit-blasting.All surfaces were wiped clean o f loose debris with anacetone-soaked tissue before bonding. The adhesivesused were all modifi ed structural epoxies. E was a two-part paste cured for 1 h at 60C, B and J were filmadhesives cured for 1 h at 120C, and P and R werefilms cured for 1 h at 175C. L, cured for 1 h at 175C,was apparently untoughened and far more brittle thanall the others.Wedge joints were made individually from pieces ofcomposite 100mm long by 25mm wide; using astandard 25 mm long wedge, this limited the maximumcrack length to about 80 mm. Wedges were insertedand crack lengths measured using a travellingmicroscope before and after exposure to 50C/96%relative humidi ty (RH) for up to 900 h. Joints were thenbroken open.Single overlap joints were made with a 12.5 x 25 mmoverlap. Joint strengths in tension were measuredbefore and afte r exposure to 50C/96% RH for 6 weeks.The mode of failure of broken joints was estimatedvisually. For wedge joints, the initial failure mode wasexamined. The three modes that could be differentiatedwere interracial failure between adhesive andcomposite, delamination of the composite and cohesivefailure in the adhesive layer. Frequently, all three werepresent in a single joint.

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J o i n t s t r e n g t h sPrevious work has shown a correlation between thelevel of contamination on the composite surface, theunaged strength of adhesive-bonded joints 3 and initialcrack length*, and between floating roller peel strengthand lap joint strength.A series of tests was performed in which lap shear,floating roller peel and wedge join ts were all madefrom the same laminate panel, so that the extent ofsurface contamination transferred from the mouldrelease agent to the laminate surface could be assumedto be about the same in each specimen. All thesecomparisons were made using adhesive E. Strengthdata for unaged lap shear and wedge joints are given inTable 1, together with the major mode of failure andits estimated percentage. As can be seen, compositeswhich gave interfacial failure in lap joints gave thesame failure in wedge joints, and composites givingfailure in the adhesive gave the same in wedge joints.However, composites for which the lap joints failed inthe composite gave wedge joints which failed in theadhesive. This was also the case for peel joints. It istherefore legitimate to compare the different types ofjoint.The results for lap shear strength are compared withthose for peel strength in Fig. 6 and with initial wedgecrack length in Fig. 7. It is clear from Fig. 6 that jointstrength and floating roller peel strength (note that thepeel strength scale is logarithmic) are related to eachother. As increasing severity of treatment reducessurface contamination, the strengths of bo th jointsincrease and the mode of failure changes from being atthe interface between composite and adhesive to failureaway from the interface.The data in Fig. 7 tell much the same story: cracklength decreases as joint strength increases and themode of failure changes. However, in this case, jointstrength appears to have reached a maximum for cracklengths below about 55 mm, which can be achievedwith only partial removal of contamination. Note that,because of the specimen dimensions, the maximumcrack length was 80 mm, at which the specimen fell

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a p a r t . U s e o f lo n g e r a d h e r e n d s c o u l d r e s u l t i n a r a n g eo f c r a c k l e n g t h s > 8 0 m m f o r t h e p o i n t s s h o w n a t t h i sl eng th .T e s t i n g j o i n t d u r a b i l i t yF o r t h e r e a s o n s g i ve n a b o v e , o n l y l a p a n d w e d g e j o i n t ss e e m t o b e s u i t a b l e f o r s t u d y i n g t h e d u r a b i l i t y o fc o m p o s i t e - to - c o m p o s i t e j o i n t s. B o t h h a v e b e e n u s e dw i d e l y f o r d u r a b i l i t y s t u d i e s o n m e t a l - t o - m e t a l j o i n t s .R e c e n t a n a l y si s o f o u t d o o r w e a t h e r i n g d a t a o n l a pj o i n t s a n d l a b o r a t o r y e x p o s u r e o f w e d g e s p e ci m e n ss h o w s t h a t a c o r r e l a t i o n c a n b e m a d e b e t w e e n t h e m s.L a p j o i n t sT h e e f f ec t o f p r o l o n g e d e x p o s u r e t o h o t - h u m i dc o n d i t i o n s h a s b e e n s t u d i e d f o r s i n g l e l a p c o m p o s i t ej o i n t s 6, v . J o i n t s m a d e w i t h g r i t - b l a s t e d a d h e r e n d s w e r ee x p o s e d u n s t r e s s e d t o 5 0 C / 9 6 % R H f o r u p t o 3 y e a r s .R e s i d u a l s t re n g t h w a s m e a s u r e d a s a f u n c t i o n o ft e m p e r a t u r e . D i f f e r e n t a d h e s i v e s g i v e j o i n t s w i t hd i f f e r e n t i n i t ia l s t r e n g t h s , s o r e s u l ts h a v e b e e nn o r m a l i z e d b y t a k i n g a l l re s i d u a l s t r e n g t h s a s ap e r c e n t ag e o f i n it ia l r o o m t e m p e r a t u r e s t r en g t h .T h e e f f e c t o f a b s o r b e d m o i s t u r e a s a f u n c t i o n o ft e m p e r a t u r e i s s h o w n i n F i g . 8 ; o p e n s y m b o l s a r eu n a g e d j o i n t s a n d c l o se d s y m b o l s a g e d o n e s . W i t h t h ee x c e p t i o n o f u n a g e d j o i n t s o f t h e b r i t t le a d h e s i v e L ,s t r e n g t h d e c r e a s e d w i t h i n c r e a s i n g t e s t t e m p e r a t u r e .H o t - w e t e x p o s u r e g e n e r a l ly r e d u c e d jo i n t s t r en g t h .A f t e r a g e i n g , j o i n t r e t e n t io n s f o l lo w e d a p p r o x i m a t e l yp a r a l l e l c u r v e s w i t h t h e 1 2 0 C - c u r e d a d h e s i v e s h o w i n gl o w e r r e t e n t i o n t h a n t w o m o d i f i e d 1 7 5 C - c u r e d o n e s ,a n d a ll s h o w i n g m u c h p o o r e r r e t e n t i o n t h a n a d h e s i v eL . H o w e v e r , i t m u s t b e n o t e d h e r e t h a t t h e i n i t i a l

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F ig . 8 S u m m a r y o f t e s t t e m p e r a t u r e a n d e x p o s u r e o n j o i n t st r e n g thr e d u c t i o n f o r j o i n t s b o n d e d w i t h a d h e s i v e s B ( O , I l L J ( ~ ) , L ( [C ], I I ) , P( A , A ) a n d R ( V , V ) . O p e n s y m b o l s , u n a g e d j o i n ts ; cl o s e d s y m b o l s ,j o i n t s a g e d f o r 2 5 0 0 0 h

1 4 0 I N T . J . A D H E S I O N A N D A D H E S I V E S A P R I L 1 9 9 4


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j o i n t s t e s t e d a t t h e s a m e t e m p e r a t u r e , w a s c a l c u l a t e d f o re a c h t e s t t e m p e r a t u r e a n d e x p o s u r e t i m e . S t r e n g t hd e c r e m e n t s f o r t h e b r i t tl e a d h e s i v e L w e r e t e m p e r a t u r e -d e p e n d e n t b u t a p p e a r e d t o b e t e m p e r a t u r e - i n d e p e n d e n tf o r o t h e r , m o d i f i e d o r t o u g h e n e d a d h e s i v es ; f o r th e s e , a na v e r a g e d e c r e m e n t w a s c a l c u l a t e d o v e r a ll t es tt e m p e r a t u r e s . T h e a v e r a g e l o s se s in s t r e n g t h a r e s h o w na s a f u n c t i o n o f w a t e r c o n t e n t i n F i g . 9 . A t l o w w a t e rc o n t e n t s , a v e r a g e l o s se s l ay w i t h i n a n a r r o w b a n d f o r a lla d h e s i v e s , b u t d i f f e r e n c e s b e t w e e n a d h e s i v e s b e g a n t oa p p e a r a t > 2 % w a t e r c o n t en t . T h i s m a y i n d i c a t e t h a t , a ts h o r t t i m e s / l o w w a t e r c o n t e n t , c h a n g e s i n j o i n t s t r e n g t ha r e c o n t r o l l e d b y t h e l a m i n a t e , b u t t h a t a t l o n g e r t i m e st h e a d h e s i v e b e c o m e s t h e c o n t r o l l i n g f a c t o r .J o i n t s w e r e a ls o m a d e w i t h u n t r e a t e d a d h e r e n d s , b u tw e r e o n l y a g e d f o r 6 w e e k s . T h e r e s u lt s , e x p r e s s e d i nt e r m s o f p e r c e n t a g e s t r e n g t h r e t e n t i o n , a r e g i v e n inT a b l e 1 t o g e t h e r w i t h t h e m a j o r m o d e s o f f a il u re .U n t r e a t e d j o i n t s a r e n o t s p e c i fi c a ll y i d e n t i f ie d b u t a r eg e n e r a l l y t h e o n e s w i t h l o w i n i t ia l s t r e n g t h s w h i c hf a i l e d a l o n g t h e i n t e r f a c e . A s c a n b e s e e n f r o m T a b l e 1o n l y i n o n e c a s e w a s t h e r e a c h a n g e i n t h e m o d e o ff a i l u r e , a n d i t w a s e x c e p t i o n a l f o r t h e r e t o b e a na p p r e c i a b le c h a n g e i n t h e p r o p o r t i o n o f t h a t m o d e .W e d g e j o i n t sT h e w e d g e t e s t w a s d e v e l o p e d s p e c i f ic a l ly to s t u d y t h e8d u r a b i l it y o f m e t a l a d h e r e n d j o i n t s , b y e x p o s in gs t r e s s e d s p e c i m e n s t o a h o t - h u m i d e n v i r o n m e n t a n df o l lo w i n g c r a c k g r o w t h .C o m p o s i t e w e d g e s p e c im e n s w e r e a l s o e x p o s e d t o5 0 C / 9 6 % R H a n d c r a c k g r o w t h f o l l o w e d f o r u p t o1 68 h . T h e 4 8 h c r a c k l e n g t h r a t i o s a r e g i v e n i n T a b l e 1 .T h e c h a n g e i n c r a c k l e n g t h o v e r 4 8 h f o r a s e l e c t i o n o fspe c ime ns i s show n in F ig . 10 . The t ime i s f a i r l ya r b i t r a r y b u t e x p e r i e n c e h a s s h o w n t h a t m o s t c r a c k

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g r o w t h t a k e s p l a c e w i t h i n t h i s t i m e . O n l yc o n t a m i n a t i o n t y p e a n d m o d e o f f a il u r e a r e i d e n ti f ie d .F r o m t h i s i t a p p e a r s t h a t , i n d e p e n d e n t o fc o n t a m i n a t i o n a n d i n it ia l c r a c k l e n gt h , c r a c k g r o w t hu s u a l l y w a s g r e a t e r f o r s p e c i m e n s w h i c h f a i l e d a t t h ei n t e r f a c e t h a n f o r t h o s e w h i c h f a i l e d i n t h e a d h e s i v e o ra d h e r e n d . C r a c k g r o w t h a l so t e n d e d t o b e g r e a t e r f o rs p e c i m e n s w i t h l o n g e r i n i t i a l c ra c k s .F r o m c l o s er c o n s i d e r a ti o n o f t h e n a t u r e o f t h es u r f a c e c o n t a m i n a t i o n f o r a w i d e r a n g e o f s p e c i m e n s , itc a n b e c o n c l u d e d t h a t c r a c k g r o w t h i s g e n e r a l l y l o w

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F ig . 10 C r ac k g r ow t h as a f unc t i on o f s u r f ac e c on t am ina t i on ( O , P T FE ;* , s i l i c one ; O , po l y am ide ) and f a i l u r e m o de ( I , f a i l u r e be t ween adhe s i v eand c om p os i t e ; A , fa i l u r e w i t h i n a dhes i v e ; C , f a i l u r e w i t h i n c o m pos i t e )

a n d i s o n l y a p p r e c i a b l e w h e n t h e c o m p o s i t e s u r f a c e i sc o n t a m i n a t e d w i t h P T F E o r s i l ic o n e , b u t n o t w i t hp o l y a m i d e , a n d t h e j o i n t f a i ls a l o n g t h e a d h e s i v e /c o m p o s i t e i n t e r f a c e . A d u r a b l e j o i n t i n a m e t a l w e d g et e s t i s o n e w h e r e t h e c r a c k g r o w t h i s l o w , a n d f a i l u r e sd o n o t r u n a l o n g t h e m e t a l / a d h e s i v e i n t e r f a c e . I n t h e s et e r m s , c o m p o s i t e j o i n t s w h i c h f a i l a lo n g t h e i n t e r f a c ei n i ti a l ly d u e t o c o n t a m i n a t i o n a r e l i k e ly t o b e l e ssd u r a b l e . I n a d u r a b l e jo i n t , a n y c r a c k g r o w t h t h a t t a k e sp l a c e re s u l t s f r o m t h e e f f e c t o f a b s o r b e d w a t e r o n t h ea d h e s i v e. C r a c k t ip s a r e o f t e n v i s i b l y b l u n t e d a s t h ea dhe s ive y i e ld s .B y a n a l o g y w i t h s t r e n g t h r e t e n t i o n o f l a p j o i n t s ,w e d g e t e s t s c a n b e a s s e s s e d b y t a k i n g e x p o s e d c r a c kl e n g t h c o m p a r e d w i t h i n i t ia l c r a c k l e n g t h . S i n c e c r a c kle ng th inc re a se s w i th t ime , i t i s c onve n ie n t t o d iv idein i t i a l c ra c k l e ng th , a 0 , by c ra c k l e ng th a t t he c hose nt i m e s o t h a t t h e s m a l l e r t h e r a t i o , t h e l e s s d u r a b l e t h ej o i n t . T h e ao/a4~ r a t i o s a r e g i v e n in T a b l e 1 .C o m p a r i s o n o f d u r a b i l i t y te s t sT h e ' d u r a b i li t y f a c t o r ' d a t a f o r b o t h t y p es o f e x p o s e djo in t a re p lo t t e d in F ig . 11 . G ive n the sc a t t e r , i t i sd i f f i c u l t t o r e a c h a n y c o n c l u s i o n a s t o w h e t h e r t h e t e s t sc o r r e l a t e . I f t h e t w o p o i n t s a t 7 4 % a n d 1 4 6 % r e t e n t i o na r e i g n o r e d , i t m i g h t b e c o n c l u d e d t h a t j o i n t s t r e n g t hr e t e n t i o n i s d e c r e a s e d o n l y w h e n t h e c r a c k g r o w t h r a t i oi s a pp roa c h i ng 1 .00 , t ha t i s w h e n ve ry l i t tl e c ra c kg r o w t h i s t a k i n g p l a c e . T h i s c o u l d b e t h e l o g i c a l

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o u t c o m e o f t h e e f f ec t o f w a t e r o n t h e e p o x y r e s i ns i fp l a s t i c i za t i o n r e d u c e s t h e s t r e n g t h o f th e a d h e s i v e a n da l s o t o u g h e n s i t . T h e f a c t t h a t a w e d g e t e s t c r a c kg r o w s , t h e r e f o r e , d o e s n o t g i v e r e l i a b l e i n f o r m a t i o n o nw h a t m i g h t h a p p e n t o l a p j o i n t s t r e n g t h , w h i c h iss t r e s s e d i n a d i f f e r e n t w a y , o n e x p o s u r e .T h e r m o p l a s t i c - m a t r i x c o m p o s i t e sI n a l l t h e w o r k d e s c r i b e d a b o v e , e p o x y - m a t r i xc o m p o s i t e w a s b o n d e d w i t h a n e p o x y a d h e si v e . C u r e de p o x y r e s i n s w o u l d b e e x p e c t e d t o b e e a s i l y w e t t e d b ye p o x y r e s i n s a s t h e y s h o u l d b e c o m p a t i b l e c h e m i c a l l ya n d t h e r m o d y n a m i c a l l y . H o w e v e r , m o s t t h e r m o p l a s t i csh a v e l o w s u r f a c e e n e r g i e s a n d a r e n o t r e a d i l y w e t t e d b ya d h e s i v es . T h e r m o p l a s t i c s s u c h a s p o l y e t h y l e n e a n dp o l y p r o p y l e n e r e q u i re p r e t r e a t m e n t b e f o r e b o n d i n g i no r d e r t o r a i s e t h e i r s u r f a c e e n e r g i e s a n d m a k e t h e mw e t t a b l e b y a d h e s iv e s . C o n s i d e r a b l e e f f o r t h a s b e e n p u ti n t o t h e p r e t r e a t m e n t o f t h e e n g i n e e r i n g t h e r m o p l a s t i cP E EK ( p o l y e t h e r e t h e r k e t o n e ) , u s e d i n c o m b i n a t i o n w i t hc a r b o n f i br e s a s A P C - 2 c o m p o s i t e 9, w i th c o r o n ad i s c h a r g e p r e t r e a t m e n t b e i n g t h e c u r r e n t f a v o u r i t e .C o n c l u s i o n sTh e re su l t s o f t e s t i ng s ing le l a p , f l oa t ing ro l l e r pe e l a ndw e d g e j o i n t s m a d e w i t h e p o x y c o m p o s i t e w i t h d i f f er e n ta m o u n t s a n d t y p e s o f su r f a ce c o n t a m i n a t i o n s h o w t h a ta c o r r e l a t i o n e x i s ts b e t w e e n t h e d i f f e r e n t t e s ts .H o w e v e r , e x p o s u r e t e s ts f o r d u r a b i l i t y i n h o t h u m i dc o n d i t i o n s a d d n o t h i n g t o t h e a r g u m e n t : i f t h e s u r fa c eh a s a l r e a d y b e e n s h o w n t o b e b a d b y t h e i n i t i a l t e s t s ,j o i n t s t r e n g t h i s n o t g e n e r a l l y f u r t h e r r e d u c e d , a n d l o n gc r a c k s g r o w l o n g e r . I f j o i n t s t r e n g t h s a r e b e t t e r a n dc r a c k s s h o r t , c h a n g e s o n d u r a b i l i t y t e s t i n g a r e d u e t oc h a n g e s i n t h e a d h e s i v e a n d n o t t o c h a n g e s i n t h e b o n db e t w e e n a d h e s iv e a n d c o m p o s i t e . T h e r e f o r e n oa d d i t i o n a l i n f o r m a t i o n i s g a i n e d f r o m d u r a b i l i t yt e s t i n g , a n d i n i t i a l s t r e n g t h o r c r a c k l e n g t h d a t a a n dm o d e o f f a i l u r e ar e s u f f i c i e n t t o t e ll w h e t h e r a s u r f a c eis su i t ab l e f o r b o n d i n g .T h e s e r e s u l t s a r e s p e c i f i c t o e p o x y - m a t r i xc o m p o s i t e s b o n d e d w i t h e p o x y a d h e si v e s. T h e t e s tm e t h o d s s h o u l d a p p l y e q u a l l y t o t h e r m o p l a s t i c - m a t r i xc o m p o s i t e s , b u t i t s h o u l d b e n o t e d t h a t t h e y d or e q u i r e s u r f a c e p r e t r e a t m e n t t o i n c r e a s e t h e i rw e t t a b i l i t y b y a d h e s i v e s .

1 42 I N T . J. A D H E S I O N A N D A D H E S I V E S A P R I L 1 9 94


7/7

AcknowledgementsThis work has been carried out with the support ofProcurement Execut ive , M in is t ry of D ef ence .References

1 Cur t is , P.T. (Ed) Technical Re port TR 88012 (Roya l AerospaceEstabl ishment, F arnb orou gh, 1988)2 Hamer , J . BAe Report AL/MAT/3985 (Br i t ish Aerospace, 1986)3 Parker , B.M. and Wagho rne , R.M. Sur f In te rface Ana l 17 (1991) p

4714 Parker, B.M. "Bond ing and Repa i r o f Composi tes" (Bu t te rwor ths,London, 1989) p 51

5 Parker , B.M. unpub l ished resu l ts6 Parker, B.M. Technical Re port TR 89001 (Royal Aerospac e

Establ ishment, Farn boro ugh, 1989)7 Parker, B.M. I n t J A d h e s i o n a n d A d h e s i ve s 10 (1990) p 1878 Marceau, J .A., Mo j i , Y. and McM i l lan , J .C. Adhesives Age 20 (1987)

p 289 Kodok ian , G.K.A. and Kin loch , A.J . J M ater Sc i Le tt 7 (1988) p 625

A u t h o rThe au thor i s w i th the Mate r i a l s and S t ruc turesD e p a r t m e n t , D e f e n c e R e s e a r c h A g e n c y , F a r n b o r o u g h ,H a n t s G U 1 4 6 T D , U K .

I N T . J . A D H E S l O N A N D A D H E S I V E S A P R I L 1 9 94 1 43

adhesive bonding of fibre composite

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