TESTING-METHODS AND CRACK RESISTANCE BEHAVIOUR OF Al2O3

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TESTING-METHODS AND CRACK RESISTANCE BEHAVIOUR OF Al2O3

F. Deuerler, R. Knehans, R. Steinbrech

To cite this version:

F. Deuerler, R. Knehans, R. Steinbrech. TESTING-METHODS AND CRACK RESISTANCE BEHAVIOUR OF Al2O3. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-617-C1-621.

�10.1051/jphyscol:1986194�. �jpa-00225625�

JOURNAL D E PHYSIQUE

Colloque C1, suppl6ment au n 0 2 , Tome 47, f6vrier 1 9 8 6 page c1-617

TESTING-METHODS AND CRACK RESISTANCE BEHAVIOUR OF A1,0,

F. DEUERLER, R. K N E H A N S ( ~ ) and R. STEINBRECH

Universitdt Dortmund, Abt. Chemietechnik, Postfach 500500, 0-4600 Dortmund 50, F.R.G.

R'esum'e

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A b s t r a c t - The R-curves o f 3 alumina m a t e r i a l s , d i f f e r e n t i n g r a i n s i z e , were measured u s i n g t h e s i n g l e edge-notched beam (SENB), t h e double-canti- l e v e r beam (DCB), and t h e double t o r s i o n (DT) techniques. The increase o f crack r e s i s t a n c e d u r i n g crack propagation i s discussed i n terms o f crack s u r f a c e i n t e r a c t i o n .

I

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F r a c t u r e mechanics parameters obtained by d i f f e r e n t t e s t i n g techniques o f t e n show a considerable disagreement. Moreover t h e toughness data do n o t always reveal c l e a r t r e n d s about t h e i n f l u e n c e o f m i c r o s t r u c t u r e /I/. Munz /2/ has suggested t h a t t h i s disagreement may stem from t h e R-curve behaviour o f alumina. Thus i t was t h e aim o f t h e present work t o i n v e s t i g a t e t h e i n f l u e n c e o f b o t h specimen geometry and m i c r o s t r u c t u r e on t h e R-curve behaviour i n more d e t a i l . Extending t h e e x p e r i - mental schedule o f D a l g l e i s h e t a l . /3/, t h e R-curves o f t h r e e alumina batches, d i f f e r e n t i n g r a i n s i z e , were measured using t h e s i n g l e edge-notched beam (SENB), t h e d o u b l e - c a n t i l e v e r beam (DCB) , and t h e double t o r s i o n (DT) techniques.

11 - EXPERIMENTAL METHODS

Three batches o f alumina w i t h d i f f e r e n t g r a i n s i z e were s i n t e r e d from t h e same f i n e grained s t a r t i n g powder (ALCOA CT 8000 SG). To achieve d i f f e r e n t g r a i n s i z e s an isothermal s i n t e r i n g t i m e o f 1, 3 and 12 hours a t 1700°C r e s p e c t i v e l y was chosen. The specimens were hard machined from l a r g e r disc-shaped s i n t e r bodies t o t h e f i n a l dimension given i n Fig. 1. A l l specimens were notched w i t h a diamond saw blade, 0.1 mm i n width. I n a d d i t i o n a one- sided crack-leading groove (0.5 mm i n w i d t h ) was c u t i n t h e DCB and DT specimens.

The specimens were loaded under c o n d i t i o n s o f c o n t r o l l e d crack growth a t constant crosshead speed m/sec) and t h e crack r e s i s t a n c e (R) data were evaluated from t h e load- displacement curves u s i n g a compliance method:

(I) now w i t h Krupp Widia GmbH, 4300 Essen, FRG

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986194

JOURNAL DE PHYSIQUE

(P: load, B: crack width, a: crack length, C: Compliance).

The dC/da-functions f o r t h e d i f f e r e n t t e s t techniques were d e r i v e d from t h e toughness equations i n / 3 / .

Fig. 1: Specimen geometries ( u n i t s i n mm)

S E N B

PI

I11

-

?

A - Crack Resistance Curves

1 4

2 w ,

The dependence o f crack r e s i s t a n c e (R) on crack propagation (ha), i.e. t h e R-curve behaviour, i s shown i n Fig. 2 f o r t h e t h r e e t e s t i n g techniques. For a b e t t e r survey o n l y R-curve data o f i n d i v i d u a l specimens a r e p l o t t e d .

The r e s u l t s o f the SENB- (Fig. 2a) and t h e DCB-specimens ( F i g . 2b) a l l o w t o deduce a general type o f R-curve which i s c h a r a c t e r i z e d by an i n i t i a l increase o f crack r e s i s t a n c e f o l l o w e d by a gradual t r a n s i t i o n i n t o a f i n a l plateau. Compared t o t h i s general curve i t seems t h a t t h e constant R-values o f t h e DT-specimens ( F i g . 2c) have t o be appointed t o t h e p l a t e a u regime. R e l i a b l e R-values f o r t h e beginning o f crack propagation can n o t be determined w i t h DT-specimens /4/.

A l l t h r e e t e s t i n g geometries e x h i b i t a s i g n i f i c a n t g r a i n s i z e dependence. I n t h e course o f crack extension t h e coarse grained m a t e r i a l always has a h i g h e r crack r e s i s t a n c e than t h e f i n e grained m a t e r i a l .

/ ' b g

b

W2 PI2

b = 2 an=13 b = 2 an ^% 19

w =25 bg = 1.5 w =12,5 bg = 1.5

1 = 7 5 y,,=g 1 =75

1 7

/ r ,

b

Fig. 2: Crack resistance curves of d i f f e r e n t t e s t i n g techniques 150-

-

E

\

DCB

o 16 u m

A 9 v m n l v m

0 0 0 ⁰

0 O

0 A A A A A A A A

z P

;50fn ^q q n o ^{0 0 0} q

0

0

0 ^{. . . . . . . . . . . . . . . . . . . . . . . .}

0 5 10 15 20

Aa [ m m l

C1-620 JOURNAL DE PHYSIQUE

A comparison o f t h e p l a t e a u v a l u e s o f t h e d i f f e r e n t geometries r e v e a l s good agreement o n l y f o r t h e 9 Dm m a t e r i a l (R = 7524 N/m). F o r t h e o t h e r g r a i n s i z e s , s i g n i f i c a n t d e v i a t i o n s e x i s t between DT-curves and SENB- and DCB-curves

.

B

-

Renotch experiments r e c e n t l y enabled t o r a t i o n a l i z e t h e R-curve b e h a v i o u r o f SENB-specimens f r o m an a d d i t i o n a l energy d i s s i p a t i o n a l o n g t h e c r a c k s u r f a c e s /5/.

I n a q u a l i t a t i v e model f o r SENB-specimens /6/ i t i s assumed t h a t a "sharp" i n t e r - c r i s t a l l i n e crack does n o t l e a d i m m e d i a t e l y t o s t r e s s f r e e c r a c k s u r f a c e s . Because o f t h e macroscopic rough ( i n t e r c r y s t a l l i n e ) f r a c t u r e s u r f a c e t h e r e a r e s t i l l i n t e r a c t i o n s i t e s b e h i n d t h e c r a c k t i p , e.g. energy can be d i s s i p a t e d by b r e a k i n g o u t p r o t r u d e d g r a i n s and f o r m i n g f u r t h e r s i d e cracks. Such an i n t e r a c t i o n zone i n - creases t o a f i n a l l e n g t h ( z ) which depends on both, t h e amount o f c r a c k opening displacement and t h e roughness 6 f t h e c r a c k surfaces. Then, w i t h f u r t h e r i n c r e a s - i n g c r a c k l e n g t h t h e i n t e r a c t i o n zone o f c o n s t a n t l e n g t h z i s t r a i l e d i n t h e wake o f t h e c r a c k t i p r e s u l t i n g i n t h e p l a t e a u v a l u e s o f t h e R-curves.

20 30 40 50

CRACK LENGTH [mml

j5'

F i g . 3: Crack r e s i s t a n c e b e h a v i o u r i n r e n o t c h t e s t . Hatched a r e a f e a t u r e s t h e normal R-curve-shape.

I n s e t o u t l i n e s t h e r e l a t i o n between c r a c k t i p and r e n o t c h depth.

DCB 16um o original notch

renotched I

e renotched

II

As t h e r e s u l t s o f t h e SENB and DCB-experiments show s i m i l a r R-curve behaviour, i t i s necessary t o p r o o f t h a t the model a l s o a p p l i e s f o r f r a c t u r e i n DCB-specimens and hence can be generalized. Thus renotch t e s t s were performed w i t h DCB-specimens too.

S t a r t i n g f r o m t h e notch t h e crack was propagated u n t i l t h e crack r e s i s t a n c e reached t h e beginning o f t h e p l a t e a u (Fig. 3). A f t e r unloading the o r i g i n a l notch now was elongated up t o ( c l o s e behind) t h e crack t i p . New l o a d i n g r e v e a l s a s i g n i f i c a n t e f f e c t o f t h e r e n o t c h i n g procedure. I n s t e a d o f c o n t i n u i n g w i t h R-values i n t h e p l a t e a u regime t h e specimen s t a r t s again w i t h t h e t y p i c a l R-curve increase. Repeating t h e experiment f o r a second t i m e y i e l d s t h e same r e s u l t s . The crack r e s i s t a n c e drop i s i n e x c e l l e n t agreement w i t h experimental observations o f Swain ( p r i v a t e communications) who renotched DCB-specimens o f alumina j u s t beyond t h e crack t i p .

Thus i t has t o be concluded t h a t t h e mechanisms o f energy d i s s i p a t i o n , which cause R-curve behaviour i n DCB-specimens a r e again l o c a l i z e d a t t h e crack surfaces. This i n d i c a t e s t h a t t h e i d e a o f i n t e r a c t i n g crack surfaces i n A1203 may be generalized.

The l e s s c o n s i s t e n t DT-values mean no r e s t r i c t i o n t o t h e model, because a q u a n t i - t a t i v e treatment o f R-curve behaviour has t o consider how crack opening d i s - placement and macroscopic roughness o f t h e crack surfaces i n t e r a c t i n each i n d i - v i d u a l t e s t technique. I t seems p o s s i b l e t h a t t h e n o n - s t r a i g h t crack f r o n t i s r e s p o n s i b l e f o r t h e d e v i a t i n g r e s u l t s o f DT-specimens as compared t o SENB- and DCB-specimens.

ACKNOWLEDGEMENTS

The authors would l i k e t o thank Mr. A. Reichl f o r h i s assistance w i t h t h e e x p e r i - mental work.

REFERENCES

/1/ E. Dorre, H. Hubner; i n "Alumina

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/2/ D. Munz; i n "Fracture Mechanics o f Ceramicsii, Vol. 6, e d i t e d by R.C. Bradt, D.P.H. Hasselman and F.F. Lange, Plenum Press, New York 1983, p. 1

/3/ B.J. D a l g l e i s h , P.L. P r a t t , J. Sandford; Science o f Ceramics 8, 1976, p. 225 /4/ L. L i , J.M. Weick, R.F. Pabst; Ber. Dt. Keram. Ges. 57 (1980) 5

/5/ R. Knehans, R. Steinbrech; Journ. Mat. Sci. L e t t e r s

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