Dynamic Fracture in Aerospace High Polymers - American Chemical

27 Dynamic Fracture in Aerospace High Polymers AKIRA KOBAYASHI and NOBUO OHTANI

Downloaded by TUFTS UNIV on October 16, 2016 | http://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch027

Institute of Space a n d A e r o n a u t i c a l Science, U n i v e r s i t y of T o k y o , K o m a b a , M e g u r o - k u , T o k y o 153; J a p a n

High polymers are widely used i n aerospace v e h i c l e s nowadays. One widely used high polymer f o r aircraft windshields i s polymethyl methacrylate (PMMA). Suppose the aircraft encounters v i o l e n t air turbulence r e s u l t i n g i n heavy gust load during flight. The aircraft w i n d s h i e l d m a t e r i a l i s then subjected to loads l e a d ing toward p o s s i b l e dynamic f r a c t u r e . Perhaps the w i n d s h i e l d is one o f the weakest p o r t i o n s throughout the e n t i r e aircraft structure a g a i n s t such dynamic l o a d i n g . In the present paper, both macroscopic and microscopic aspects of dynamic f r a c t u r e i n PMMA are reported. F i r s t , the running crack propagation v e l o c i t y p r o f i l e s a t room temperatures subjected to various s t r a i n r a t e s were i n v e s t i g a t e d from the macroscopic p o i n t of view, and the strain r a t e dependency e s s e n t i a l to viscoelastic s o l i d s were s t u d i e d . Then, another t o p i c r e l a t e d to microscopic b a s i s is t r e a t e d . That is, the r e l e a s e d energy r a t e s during dynamic f r a c t u r e were measured, and the c o r r e l a t i o n s between f r a c t u r e surface patterns observed m i c r o s c o p i c a l l y and corresponding characteristic f r a c t u r e mechanics parameter such as energy r e l e a s e r a t e s during dynamic crack propagation were surveyed a t d i f f e r e n t t e s t temperatures under s t a b l e crack propagation with the a i d o f l i n e a r f r a c t u r e mechanics and microscopic t o o l s . Running Crack Propagation V e l o c i t y P r o f i l e s Measurement o f Crack Propagation V e l o c i t y . For years the present authors have been engaged i n the experimental study on dynamic crack propagation i n high polymers. There are s e v e r a l techniques to measure the running crack propagation v e l o c i t y (1), i . e . , the v e l o c i t y gage method, the e l e c t r i c p o t e n t i a l method, the u l t r a s o n i c s method and the high speed photography method. Among these methods the v e l o c i t y gage method i s most f a v o r a b l e e s p e c i a l l y f o r p l a s t i c s because o f t h e i r e l e c t r i c i n s u l a t i o n p r o p e r t i e s .

0-8412-0567-l/80/47-132~367$05.00/0 © 1980 American Chemical Society May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

RESINS F O R A E R O S P A C E

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Figure 1.

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LSilver coating A material 9 AO (Du Pont No-4817)

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10 101 10 10

Velocity gauge arrangement

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To synchroscope

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Figure 2.

Electronic circuit

Re

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Initial crack tip radius : 0 0 1 R

Strain gage center —

Figure 3. Test specimen for macroscopic study

0

Figure 4. 48 sec , ( 1

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8

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Strain-rate-dependent crack propagation velocity profiles: ( ; — 2.6 X JO" sec- , ( ) e = 2.6 X 10 sec' ( £ = 2.6 χ 10~ sec 2

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May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

) e = )

27.

KOBAYASHi AND O H T A N i

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I n t h e p r e s e n t s t u d y , v e l o c i t y gages w e r e e m p l o y e d , o f w h i c h d e t a i l s a r e a s shown i n F i g u r e 1. I n F i g u r e 1 t h e c o n d u c t i v e s i l v e r c o a t i n g m a t e r i a l , DuPont No. 4 8 1 7 , f o r m i n g a s e r i e s o f c o n d u c t i n g w i r e s , i s p a i n t e d p e r p e n d i c u l a r t o the expected crack p r o p a g a t i o n p a s s a g e s o a s t o be b r o k e n a s t h e r u n n i n g c r a c k f r o n t passes across the conducting w i r e s , r e s u l t i n g i n the e l e c t r i c r e s i s t a n c e change i n t h e e l e c t r o n i c c i r c u i t shown i n F i g u r e 2. Thus t h e r u n n i n g c r a c k p r o p a g a t i o n v e l o c i t i e s between p a i n t e d c o n d u c t i v e w i r e s a r e measured c o m b i n e d w i t h t h e i n t r o d u c t i o n o f t i m e s c a l e from t h e t r a c e on a synchroscope. S p e c i m e n s . PMMA., m a n u f a c t u r e d by Sumitomo C h e m i c a l Co. L t d . , J a p a n , was u s e d a n d t h e s p e c i m e n c o n f i g u r a t i o n i s a s shown i n F i g u r e 3. F o i l gages o f e l e c t r i c r e s i s t a n c e t y p e a r e p l a c e d on the specimen t o check t h e a p p l i e d s t r a i n r a t e s . Experimental Results o f Crack V e l o c i t y P r o f i l e s . I n perform i n g t h e e x p e r i m e n t a c o n v e n t i o n a l I n s t r o n - t y p e t e n s i l e t e s t e r was u s e d t o r e a l i z e t h e mode I c o n s t a n t s t r a i n - r a t e t e n s i o n l o a d i n g on a s p e c i m e n f o r t h e s t r a i n r a t e s b e t w e e n 2.6 χ 10 ^ / s t o 2.6 χ 10 ^ / s , w h i l e t h e s p e c i a l l y d e s i g n e d h i g h s p e e d t e n s i l e t e s t e r , UTM-5, made b y T o y o - B a l d w i n , J a p a n , was e m p l o y e d f o r t h e s t r a i n r a t e o f 48/s. A l l t h e t e s t s w e r e done a t room t e m p e r a t u r e . These t e s t r e s u l t s a r e shown i n F i g u r e 4, w h e r e c = t h e a r b i t r a r y c r a c k l e n g t h a n d C Q = t h e i n i t i a l c r a c k l e n g t h o f 5 mm. Discussions. I ti s quite evident that there e x i s t s the s t r a i n r a t e dependency i n dynamic c r a c k p r o p a g a t i o n v e l o c i t y p r o f i l e s a s s e e n f r o m F i g u r e 4. T h i s i s v e r y i n t e r e s t i n g , b e c a u s e PMMA h a s l o n g been t r e a t e d r a t h e r e l a s t i c a t l e a s t a t room temper a t u r e s so f a r . However, t h e o b t a i n e d s t r a i n r a t e d e p e n d e n t c r a c k p r o p a g a t i o n v e l o c i t y p r o f i l e s t e l l u s t h a t PMMA s h o u l d be t r e a t e d as v i s c o e l a s t i c r a t h e r t h a n e l a s t i c , s i n c e t h e s t r a i n r a t e depend e n c y emerges more o r l e s s f r o m t h e v i s c o u s component i n h e r e n t t o such v i s c o e l a s t i c s o l i d s c o r r e s p o n d i n g t o t h e a p p l i e d s t r a i n r a t e s F r a c t u r e Surfaces and Energy Release

Rates

Next, f r a c t u r e s u r f a c e s were observed m i c r o s c o p i c a l l y and t h e i r c o r r e l a t i o n s w i t h energy r e l e a s e r a t e s were s t u d i e d . Energy Release Rates. As i s well-known, energy i s r e l e a s e d a s a c r a c k a d v a n c e s when t h e e x t e r n a l l o a d i s a p p l i e d on a s p e c i men. T h e r e a r e v a r i o u s methods o f c h a r a c t e r i z i n g r e s i s t a n c e t o c r a c k i n g o r r e l e a s e d e n e r g y i n m a t e r i a l s . Among m e t h o d s , one c a n measure t h e s p e c i f i c w o r k o f f r a c t u r e o f a q u a s i - s t a t i c a l l y p r o p a gating crack. C o n v e n t i o n a l t e c h n i q u e s t o o b t a i n t o u g h n e s s i n met als a r e r a t h e r complicated i n case a p p l i e d t o v i s c o e l a s t i c s o l i d s , since thecompliance c a l i b r a t i o n s vary w i t h d i f f e r e n t t e s t i n g crosshead speeds and d i f f e r e n t c r a c k p r o p a g a t i o n v e l o c i t i e s d u r i n g



370


Figure 5. Load-displacement relation employed for the sector area method

Ai Bi Displacement

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^ S i l v e r coating material (Du Pont — ^ No. 4817)

Figure

6. Compact tension specimen for microscopic study

H=120 mm

h=55 mm

L=125 mm

W=100 mm

C«=33 mm

D=25

Thickness: t=8

mm

mm



27.

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f r a c t u r e because o f s t r a i n r a t e dependent m a t e r i a l m o d u l i . The s e c t o r a r e a method p r o p o s e d b y G u r n e y a n d Hunt ( 2 ) i s a p o w e r f u l t e c h n i q u e u n d e r t h e s e c i r c u m s t a n c e s , a s t h e v e r y method e l i m i n a t e s t e d i o u s and troublesome compliance c a l i b r a t i o n s . Furthermore, i t o f f e r s many i n c r e m e n t a l r e l e a s e d e n e r g y v a l u e s f r o m a s i n g l e t e s t . I f t h e specimen i s prepared such t h a t t h e c r a c k p r o p a g a t i o n v e l o c i t y v a r i e s d u r i n g q u a s i - s t a t i c crack propagation, t h i s might r e f l e c t s t r a i n r a t e dependency i n t h e work o f f r a c t u r e . V i a s e c t o r method, t h e e n e r g y r e l e a s e r a t e G p r e s c r i b e d b y two q u a n t i t i e s , a l o a d Ρ a n d a d i s p l a c e m e n t ô a t t h e l o a d i n g p i n , c a n be e x p e r i m e n t a l l y measured a s shown i n F i g u r e 5, w h e r e f o r example, Δ Ο Α Β denotes the v e r y energy r e l e a s e as t h e c r a c k f r o n t a d v a n c e s c o r r e s p o n d i n g t o t h e d i s p l a c e m e n t i n c r e m e n t Α^Βΐ· Ρ i s measured b y a l o a d c e l l a n d ô b y a c l i p gage. S p e c i m e n . To r e a l i z e t h e q u a s i - s t a t i c c r a c k p r o p a g a t i o n , a compact t e n s i o n t y p e s p e c i m e n a s shown i n F i g u r e 6 was u s e d , w h e r e v e l o c i t y gages a r e a l s o e m p l o y e d t o measure t h e c r a c k v e l o c i t y . Experimental Procedures. I n the present experiment, these e n e r g y r e l e a s e r a t e s were m e a s u r e d d u r i n g q u a s i - s t a t i c c r a c k p r o p a g a t i o n p r o d u c e d b y mode I t e n s i o n w i t h a n I n s t r o n t y p e t e n s i l e t e s t e r a t 0.5mm/minute c r o s s h e a d s p e e d . T h r e e t e s t t e m p e r a t u r e s (243°K, 285°K a n d 328°K) w e r e o b t a i n e d w i t h t h e a i d o f l i q u i d n i t r o g e n , room t e m p e r a t u r e s a n d h o t a i r b l o w e r , r e s p e c t i v e l y . As d e s c r i b e d b e f o r e , t h e r u n n i n g c r a c k p r o p a g a t i o n v e l o c i t y was measu r e d by v e l o c i t y gages, l o a d by a l o a d c e l l and t h e d i s p l a c e m e n t a t t h e l o a d i n g p i n b y a c l i p gage d u r i n g f r a c t u r e . A f t e r f r a c t u r e , f r a c t u r e s u r f a c e s w e r e o b s e r v e d b y a m i c r o s c o p e w i t h some 50 t o 400 t i m e s m a g n i f i c a t i o n s . E x p e r i m e n t a l R e s u l t s a n d D i s c u s s i o n s . Thus o b t a i n e d e n e r g y r e l e a s e r a t e s i n terms o f r u n n i n g c r a c k f r o n t p o s i t i o n d u r i n g s t a b l e c r a c k p r o p a g a t i o n a r e shown i n F i g u r e s 7 t o 9. I n t h e s e f i g u r e s , t h e energy r e l e a s e r a t e G i s e x p r e s s e d by t h e energy r e l e a s e d i v i d e d by t h e f r a c t u r e s u r f a c e a r e a c o r r e s p o n d i n g t o i n d i v i d u a l c r a c k f r o n t a d v a n c e . I n F i g u r e s 7 t o 9, e a c h c h a i n l i n e shows t h e w e i g h t e d a v e r a g e c u r v e , w h i l e t h e shaded r e g i o n d e n o t e s t h e s c a t t e r v a l u e s . I t i s r e c o g n i z e d t h a t G i s a t i t s maximum when t h e s t a b l e c r a c k p r o p a g a t i o n i n i t i a t e s a n d t h e n t h e monoton o u s l y d e c r e a s i n g tendency f o l l o w s h e r e a f t e r f o r a l l c a s e s . The maximum G v a l u e d e c r e a s e s w i t h t h e i n c r e a s e o f t e s t t e m p e r a t u r e . F u r t h e r , i n v i e w o f F i g u r e 10, dG/dc becomes l a r g e a s t h e d e c r e a s e i n t e s t temperature. Crack p r o p a g a t i o n v e l o c i t i e s i n terms o f a r u n n i n g c r a c k f r o n t a t i n d i v i d u a l t e s t t e m p e r a t u r e a r e shown i n F i g u r e 11. I n F i g u r e 1 1 , c r a c k v e l o c i t y p r o f i l e s f o r b o t h 328°K a n d 285°K a r e r a t h e r s i m i l a r , b u t t h a t f o r 243°K i s d i s t i n c t i v e a n d a l w a y s showi n g l o w e r v a l u e s compared t o t h e p r e v i o u s two h i g h e r t e m p e r a t u r e cases. A l l the curves a r e o f decreasing f u n c t i o n o f a running



372


Figure 7. Variation of energy release rate caused by crack propagation

Ο

Figure 8. Variation of energy release rate caused by crack propagation (285 K)

σο4

04

0-6

c/w

Figure 9. Variation of energy release rate caused by crack propagation


Î-0



Dynamic

02

04

06 C/W

0-8

10

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0-4

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1-0

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Fracture

373

Figure 10. Comparison of nondimensional energy release rates in terms of nondimensional crack fronts

Figure 11. Crack propagation velocities at various temperatures: ( ) 328 K, (— j 285 K, ( )243K




374

Figure 12. Microscopically observed fracture surface at 243 Κ (G = 0.05 Kg · mm/mm ) 2

Figure 13. Microscopically observed fracture surface at 285 Κ (G = 0.05 Kg · mm/mm ) 2




Dynamic

375

Fracture

Figure 14. Microscopically observed fracture surface at 328 K(G = 0.05 Kg · mm/mm ) 2




376





27.

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OHTANi

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377

Fracture

crack f r o n t as t h e crack advances. Microscopic inspection o f fracture surfaces reveals that t h e r e e x i s t s two c h a r a c t e r i s t i c p a t t e r n s i r r e s p e c t i v e o f t e s t t e m p e r a t u r e s . One i s a c o m b i n e d h a c k l e a n d r i v e r p a t t e r n a n d t h e other i s represented by r i v e r p a t t e r n o n l y . F u r t h e r m o r e , i t was f o u n d t h a t t h e e n e r g y r e l e a s e r a t e s measured w e r e a l m o s t i d e n t i c a l a t those c h a r a c t e r i s t i c p a t t e r n r e g i o n s though t h e t e s t temperat u r e s were d i f f e r e n t . I n F i g u r e s 12 t o 14 i t i s o b s e r v e d t h a t there e x i s t hackle l i n e s and r i v e r p a t t e r n s c o n c u r r e n t l y , and G i s e q u a l t o a b o u t o.05 Kg-mm/mm i r r e s p e c t i v e o f t e s t t e m p e r a t u r e . N e x t , a s t h e r u n n i n g c r a c k f r o n t advances f u r t h e r , once appeared h a c k l e l i n e s v a n i s h and t h e f r a c t u r e s u r f a c e i s t o be covered w i t h r i v e r p a t t e r n s a n d smooth s u r f a c e s , h a v i n g G = 0.02 Kg-mm/mm o r s o , f o r a l l t e s t t e m p e r a t u r e c a s e s a s shown i n F i g u r e s 15 t o 17. I t i s i n t e r e s t i n g t o f i n d the above-mentioned, f o r which f u r t h e r e f f o r t s h o u l d be t r i e d . 2


2

Conclusions Two a s p e c t s o f d y n a m i c f r a c t u r e b e h a v i o r , m a c r o s c o p i c a n d m i c r o s c o p i c , w e r e s t u d i e d f o r PMMA.. M a c r o s c o p i c a l l y t h e s t r a i n r a t e d e p e n d e n t c r a c k p r o p a g a t i o n v e l o c i t y p r o f i l e s w e r e phenomen o l o g i c a l l y o b s e r v e d a t room t e m p e r a t u r e , c a l l i n g a t t e n t i o n t o t h e f a c t t h a t PMMA. s h o u l d be t r e a t e d a s v i s c o e l a s t i c r a t h e r t h a n e l a s t i c e v e n a t room t e m p e r a t u r e l e v e l . I n m i c r o s c o p i c a s p e c t two c h a r a c t e r i s t i c f r a c t u r e s u r f a c e p a t t e r n s w e r e f o u n d , one w i t h a combined h a c k l e and r i v e r p a t t e r n , and the o t h e r w i t h r i v e r p a t t e r n o n l y , w h i l e t h e f o r m e r h a s G o f 0.05 Kg-mm/mm a n d t h e l a t t e r 0.02 Kg-mm/mm , a l l i r r e s p e c t i v e o f t e s t t e m p e r a t u r e . 2

Acknowledgements The a u t h o r s a r e g r a t e f u l t o Mr. M a s a y u k i Munemura f o r h i s e f f o r t devoted t o the present study. Mr. J u n N a g a s h i m a a n d Mr. H i d e o H a n a n o i a r e a l s o a c k n o w l e d g e d f o r t h e i r a s s i s t a n c e i n experiment.

Literature Cited 1. Liebowitz, H. ed., "Fracture", Academic Press New Y o r k , 1968, Vol. II, p . 5 4 5 . 2.

G u r n e y , C., H u n t , J., P r o c . A229, 5 0 8

Roy. Soc.

London

(1967)

RECEIVED February 15, 1980.


Dynamic Fracture in Aerospace High Polymers - American Chemical

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