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Submitted on 1 Jan 1986
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RESISTANCE TO THERMAL FATIGUE AND STANDARDS
J. Glandus, P. Boch, C. Jouin
To cite this version:
J. Glandus, P. Boch, C. Jouin. RESISTANCE TO THERMAL FATIGUE AND STANDARDS.
Journal de Physique Colloques, 1986, 47 (C1), pp.C1-643-C1-647. �10.1051/jphyscol:1986198�. �jpa- 00225629�
RESISTANCE TO THERMAL FATIGUE AND STANDARDS
J . C . GLANDUS, P. BOCH a n d C. JOUIN*
E.N.S.C.I.. U.A. C.N.R.S. 3 2 0 , F - 8 7 0 6 5 L i m o g e s , F r a n c e
"P.s.A., C e n t r e T e c h n i q u e C i t r o g n , C h e m i n V i c i n a l n 0 2 , E t u d e s e t
R e c h e r c h e s , F - 7 8 1 4 0 V e l i z y , F r a n c e
Rksumk La rgsistance a m chocs thermiques d l u n e p i k e mkanique ne dgpend pas que des c a r a c t k r i s t i q u e s du matgriau, mais a u s s i de donnkes e x t r i n s k u e s (ggomktrie, &changes thermiques ... ) . I1 semble cependant u t i l e de normaliser l e s conditions d'kvaluation de grandeurs usuellement c i t k e s , en p a r t i c u l i e r ATc, e t de dkvelopper des t e s t s de contr6le des e f f e t s de l a f a t i g u e ther- mique.
Akstract Thermal shock r e s i s t a n c e of ceramic p a r t s does not depend on mate- r i a l s c h a r a c t e r i s t i c s only, b u t a l s o on external parameters (piece geometry, heat exchange ... ) . However, it seems u s e f u l t o standardize the conditions of measuring of some usual q u a n t i t i e s , p a r t i c u l a r l y ATc, and t o develop some tests t o control thermal f a t i g u e damage.
IWJXO DUCTION
B r i t t l e n e s s of ceramics leads t o various drawbacks, p a r t i c u l a r l y a poor r e s i s t a n c e t o thermal shocks. This makes t h e use of ceramic p a r t s i n thermal engines d i f f i c u l t , and explains t h e e f f o r t s made t o improve t h e understanding of thermal shock featu- res.
VAMAS+ is involved i n "pre-standardization" s t u d i e s on thermal shock behaviour of ceramics, i n t h e s p i r i t t h a t standards should be d e v e l o p d not only t o allow t h e ranking of abstruse properties lxlt a l s o t o help users' choice. An i l l u s t r a t i o n of t h i s p i n t of view, chosen f o r t h e case of strength, would be t o standardize f a t i g u e l i m i t / 1 / , instead of " i n e r t " s t r e n g t h which is generally not a parameter of r e a l a p p l i c a b i l i t y . This paper considers the c r i t i c a l temperature drop (ATc), and a l s o pints out the need f o r thermal f a t i g u e benches.
I THERMAL SHOCK RESISTANCE AND THERMAL SHCCK TESTS
-
The thermal shock behaviour of a p a r t of a given shape, made of a given material, and working a t a given set of heat exchange conditions, should be studied a t t h r e e
+ " J e r s a i l l e s Project on Advanced Materials and Standards", which is a m u l t i l a t e r a l collaboration on advanced materials research (Canada, CEC, FRG, France, I t a l y , J a p a n , UK, USA), s e t up a t t h e Economic Summit of Versailles i n June 1982.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986198
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sequential l e v e l s , namely ( i ) t h e temperature f i e l d , (ii) t h e s t r e s s / s t r a i n f i e l d , and (iii) t h e occurence and t h e extent of damwe /2/3/4/. It must be pointed out t h a t t h e knowledge of material parameters (e.g. R , R' . . . ) is not enough t o foresee t h e k h a v i o u r of a p a r t , more especially a s the main u n c e r t a i n t i e s concern heat ex- changes++. Hence, t h e r e a r e presently two complementary trends / 6 / :
. Either t h e experimental study of a piece a s s i m i l a r a s possible a s t h e actual p a r t , subjected t o thermal exchanges a s similar a s possible as the a c t u a l exchanges.
. O r t h e computer a s s i s t e d derivation of t h e q u a n t i t i e s of i n t e r e s t (e.g. stress) by using only "primary" experimental parameters (e.g. thermal expansion, thermal conductivity, e l a s t i c i t y constants, s t r e n g t h , etc.) and by mapping t h e data f o r an a r b i t r a r y range of thermal shock s e v e r i t i e s (from "mild" shocks t o "hard" ones, because here t h e r e is no more experimental l i m i t a t i o n ) . T h i s approach does not p a r t i c u l a r i z e "thermal s t r e s s e s " from "mechanical s t r e s s e s " thus u n i f i e s thermal f a t i g u e and dynamic mechanical f a t i g u e /7/.
No one of t h e two approaches focuses on t h e c r i t i c a l temperature drop (Arc), because even i n t h e simple case of hard shocks (ATc = R/\y(fi)) ATc is reduced t o t h e combi- nation of a material parameter ( R ) and of a material and heat exchange dependent function 413)/4/. However, ATc is frequently given by ceramics s u p p l i e r s a s a material parameter and continues t o be considered by u s e r s a s a s i g n i f i c a n t c h a r a c t e r i s t i c , although t h e v a r i a b i l i t y i n its determination deprives it of physi- c a l meaning. This c a l l s f o r t h e necessity of standardize t h e ATc measurements... but i t must be remembred t h a t ATc is only a reference, which cannot be used t o design a piece.
I1 A STANDARDIZABLE TEST TO MEASURE ATc
The proposal (Fig. 1) corresponds t o a technique used by Hasselman /8/. Samples a r e cylinders (-60 mm i n length and -6 mm in diameter), with a Vickers indentation i n t h e i r middle. Both ends are covered by thermally i n s u l a t i n g caps. A system of pneu- matic actuating allows t h e quick displacement, a t a f i x e d r a t e , from a furnace a t temperature T1 i n t o a bath a t temperature T2, t h e sample being gripped i n a low
thermal i n e r t i a f i x t u r e . The indentation i n i t i a t e s thermal cracks and s o decreases t h e Weihll s c a t t e r i n g which occurs when cracks a r e i n i t i a t e d by n a t u r a l flaws /9/10/. The end caps prevent t h e i n i t i a t i o n of cracks a t t h e edges, and help t o a p proximate t h e system a s an i n f i n i t e bar.
I PNEUIIATIC ACTUATING pr
QUENCHING BATH Temp. 1 2
T
IERTIA FIXTW~E
Various quenching s e v e r i t i e s , from hard shocks ( a R ) t o mild ones (a R ' ) , may be obtained by varying quenching media (molten metal, water, o i l s . . . ) . The control of damage can be performed by t h e c l a s s i c a l method of breaking the shocked samples -
F& . I A p o n n i b l c hXandatid tat do& ATc meanu- nemenix.
- -
++ Even f o r a parameter a s simple a s B i o t o s number ( D ) , and f o r heat exchange conditions a s t y p i c a l a s water quenching, the disagreement between the l i t e r a t u r e d a t a ranges on an order of magnitude /5/.
ATc and could allow t h e study of cumulative shocks on t h e same sample.
I11 A THERMAL FATIGUE BENCH
The simplicity (of t h e samples and of t h e equipment), and gocd q u a l i t i e s of repro- d u c i b i l i t y , argue in favour of t h e previous t e s t a s a standardizable test. However, it s u f f e r s of two drawbacks, namely (i) i t is b e t t e r s u i t e d f o r shocks a t decreasing temperature than f o r shocks a t increasing temperature and it is not convenient t o run i n t h e high temperature range, and (ii) it is not adapted t o thermal f a t i g u e s t u d i e s , because even i f non d e s t r u c t i v e evaluation is used it cannot be applied i n r e a l time, i.e. it cannot allow a continuous r e g i s t r a t i o n of t h e damage. For t h i s
-- reason, w e have b u i l t an automated bench with a r a d i a t i v e heating; acoustic emission allows us t o control t h e i n i t i a t i o n and t h e propagation of cracks.
Fig. 2 G e n d v i m od .the bench
06 .them& ~a..t.igue.
Figure 2 g i v e s a general view of t h i s device and Figure 3 i l l u s t r a t e s t h e b l o c k d i a - gram of t h e whole experimental set. It can be observed t h a t t h e lower p a r t of t h e bench c o n s i s t s of a r o t a t i v e i r o n t a b l e on which up t o f i v e samples in t h e form of d i s c s ( @ (30 mm) a r e gripped by means of cooled m e t a l l i c c l i p s . The use of such a holding methcd allows us t o superimpose a mechanical induced s t r e s s f i e l d on thermal induced stress f i e l d . The upper p a r t of t h e k n c h involves a horizontal heating pla- t e i n nimonic, t h e dimensions of which a r e c l o s e t o those of t h e specimen. A pneu- matic actuating moves t h i s heating p a r t v e r t i c a l l y s o t h a t it can be s e t a t about 0.1 t o 0.3 mm of t h e sample f o r t h e r a d i a t i o n heating step. This actuating a l s o enables t h e heating p l a t e t o return rapidly during t h e cooling/rotation stages.
TRANSDUCER ,-+=I
PLDTTER
/ REGULATOR
DISPLACEMENT ROTATION
Fig.3 Block-diagfimnrn 06 -the bench 06 t h a u n d ,JaA&ue.
Figure 4 shows the temperature vs time dependence f o r t h e hot and t h e cold faces of a sample of zirconia. It may be pointed o u t t h a t a heating r a t e of 20°C/s is main-
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tained during the f i r s t s t a g e s (t<40 s ) of heating and t h a t , when t h e steady s t a t e is reached, a gradient of 100°c is obtained through t h e thickness (e=2mm). The radial. temperature vs time dependence is i l l u s t r a t e d i n Figure 5 f o r t h e same material. Besides t h e previous a x i a l gradient, a maximum r a d i a l gradient of 4O0C/mm is obtained which submits t h e sample t o a t r i a x i a l s t a t e of stress. This r e s u l t s
£ran t h e heat conduction induced a t t h e periphery of t h e sample by t h e cooling e f f e c t s due t o t h e m e t a l l i c clips.The control of t h e induced damage is performed " i n s i t u " by means of acoustic emission measurements. The m e t a l l i c gripping c l i p s a r e used a s vaweguides, s o t h e i r cooling is e s s e n t i a l t o avoid t h e excessive heating of t h e transducer. Figures 6 and 7 show t h e number of acoustic emission counts recorded versus t h e imposed temperature d i f f e r e n c e (&TI f o r two commercial q u a l i t i e s of
zirconia (£ran ~ k h i n e y - ~ e s m a r q u e s t , batches r e f . ZFME & TZP).
F i g . 4 A x i d ,them& g x a d i e n t . F i g . 5 Radial X h m d g h a d i e n t . For cumulative shocks of increasing s e v e r i t i e s , t h e acoustic emission remains a t a r a t h e r low l e v e l up t o a c r i t i c a l value ATc f o r which t h e crack propagation occurs and gives rise t o a s i g n i f i c a n t increase i n t h e measured values ; thus one obtains ATc (ZFME) = 750°C and ATc (TzP) = l l O O O ~ .
TZP y SAMPLE 1
+ S A M P L E 2
LIJ S A M P L E 3
I
, .-
F i g . 6 A.E. kecond 06 t h m d Fig.? A.E. kecokd 0 6 ,them&
damage dotr ZFME. damage doh TZP.
ZFnE y SAMPLE 1
+ S A M P L E I
- CI S A M P L E 3
The samples being submitted t o a s i n g l e thermal shock f o r each AT value, i t may be thought t h a t t h e t o t a l cumulative e f f e c t is negligeable /ll/ and t h a t t h e c r i t i c a l temperature differences measured a r e c o n s i s t e n t whith t h e thermoelastic a n a l y s i s r e s u l t s /12/. Apart from the M.0.R (cxR), a l l t h e thermomechanical parameters are
W 2 - -
1- -
C ' , " ' " "
Z P S i B E 5 3
T.inp.r.kur. (.C>
Indeed : ATcl/ATcZ = 1100/750 = 1.45 which is c l o s e t o CR, /c& = 750/450 = 1.65
These two r a t i o s a r e i n r a t h e r good agreement, and t h i s experiment confirms t h e a p p l i c a b i l i t y of a c o u s t i c emission f o r t h e non d e s t r u c t i v e e v a l u a t i o n of thermal damage.
REFER ENCES
--
/1/ R i t t e r , J.E., e t al., Am. Ceram. Soc. B u l l . 63 (1984) 1517 /2/ Hasselman, D.P.H., J. Am. Ceram. Soc. 52 (1969) 600 /3/ Hasselman, D.P.H., Am. Ceram. Soc. Bull. 49 (1970) 1033
/4/ Hasselman, D.P.H. I n t . Conf. on Engineering Ceramics,Dec. 16-20 (1984) Jerusalem, I s r a e l .
/5/ Becher, P.F., J. Am. Ceram. Soc. Com. 64 (1981) C17
/6/ Kamo, R. e t dl ; C a r r u t h e r s , W.D. e t a l . , Am. Ceram. Soc. P r m e e d i n g s 5 , 5-6 (1984) 312 ; 350.
/7/ Kamiya, N. and Kamigaito, 0. J. Mat. Sc. 17 (1982)
/ 8 / Hasselman , D .P .H. , College of ~ n g i n e e r i n g r v ~ ~ , Blacksburg, VA 24061 ,USA /9/ F a k e r , K.T. e t al., J. Am. Ceram. Soc. 64 (1981) 296
/lo/ Gonzalez, A. e t dl., ASTM Spec. Techn. T e s t . P u b l i c . 844 (1984) 43 /11/ Glandus, J.C. and Boch, P., Interceram 5 (1984) 33
/12/ Glandus, J.C. and Boch, P . , Mat. Sc. Monographs v o l 6 (Energy and Ceramics) E l s e v i e r (1980) 661
/13/ Kingery, W.D., J. Am. Ceram. Soc. 38 (1955) 3
ACKNOWLEDGEMENTS
The a u t h o r s a r e g r a t e f u l t o Prof. D.P .H. Hasselman, f o r v a l u a b l e d i s c u s s i o n s , and t h e y thank Peugeot s o c i 6 t 6 Automobiles f o r its f i n a n c i a l s u p p o r t .
