6. 7. 8.

A . N . Dremin and G. A. A d a d u r o v , "Behavior of glassunder dynamicloading, " Fiz. Tverd. Tela, 6, No. 6, 1781-1784 (1964). A . M . Molodets andG. I. Kanel', "Dynamicproperty ofplexiglass underwave loadtngs, " Fiz. Goreniya Vzryva, No. 4, 628-631 (1976). G . V . Stepanov and V. V. A s t a n i n , " I m p a c t c o m p r e s s i b i l i t y of c e r t a i n s t r u c t u r a l m a t e r i a l s to a r a t e of 1 0 0 0 m / s e e , " P r o b . P r o c h n . , No. 9, 42-46(1975).

EFFECT

OF

TEMPERATURE

LOADING

ON S T R E N G T H

A REFRACTORY A. Y a .

AND

TYPE

VARIATION

OF

IN

MATERIAL UDC

Peras

620.171.3+666.3.017

Much work has been devoted to studying r e g u l a r i t i e s ha s t r e n g t h v a r i a t i o n of r e f r a c t o r y c e r a m i c m a t e r i a l s . H o w e v e r , i n v e s t i g a t i o n of the l i t e r a t u r e shows that the m a j o r i t y of these s t u d i e s w e r e c a r r i e d o u t a t r o o m t e m p e r a t u r e . In addition, i t i s n o t a l t o g e t h e r c l e a r w h e t h e r the r e g u l a r i t i e s and p a r a m e t e r s e s t a b l i s h e d f o r v a r i a tion o v e r a wide r a n g e of t e m p e r a t u r e a r e c h a r a c t e r i s t i c for r e f r a c t o r y m a t e r i a l o p e r a t i n g c o n d i t i o n s . A n o t h e r question, h a r d l y studied in the field of s t r e n g t h s t a t i s t i c s , is the e f f e c t of type of loading on s t r e n g t h v a r i a t i o n . Studies w e r e g e n e r a l l y e a r r i e d o u t with a s i n g l e type of loading, m o s t l y bending. In the p r e s e n t work a high g r a d e , high a l u m i n a , m a s s p r o d u c e d c e r a m i c S i n o k s a l (S-49) [1] was e x a m i n e d for the e f f e c t of t e m p e r a t u r e and type of loading on s t r e n g t h v a r i a t i o n . T e s t s w e r e c a r r i e d o u t with five types of loading; t h r e e - p o i n t and f o u r - p o i n t ( p u r e ) b e n d i n g , u n i a x i a l t e n s i o n , uniaxia[ and d i a m e t r a l c o m p r e s s i o n [2]. T e s t p i e c e s f o r a l l types of t e s t w e r e cut f r o m a s i n g l e type of block, a b a r 5mn~ in d i a m e t e r and 80 mm long p r e p a r e d by c a s t i n g u n d e r p r e s s u r e . T e s t p i e c e d e n s i t y w a s 3 . 6 9 g / c m 3 and the a p p a r e n t p o r o s i t y was in the r e g i o n of 0.1%. B a r s 05 x 60 m m w e r e u s e d f o r bending t e s t s with 50 m m between the s u p p o r t s . T e n s i l e t e s t s w e r e c a r r i e d outon the s a m e t e s t p i e c e s by m e a n s of g r i p s with T e x t o l i t e b u s h e s . T e s t p i e c e s ~5 • mm w e r e used for u n i a x i a l e o m p r e s s t o n t e s t s . L o a d i n g w a s a c e o m p l t s h e d b y m e a n s of h a r d a l l o y p l u n g e r s and r e n e w a b l e s t e e l l i n e r s . A h a r d a l l o y loading s u r f a c e was u s e d f o r d i a m e t r a l c o m p r e s s i o n t e s t s 05 • 3 . 2 r a m . The u l t i m a t e t e n s i l e s t r e n g t h was d e t e r m i n e d by the r e l a t i o n s h i p given in [2]. The e f f e c t of t e m p e r a t u r e on s t r e n g t h v a r i a t i o n was s t u d i e d under the s h n p l e s t f o r m of loading, t h r e e - p o i n t bending. Seven t e s t p i e c e s w e r e t e s t e d in s p e c i a l e q u i p m e n t fitted with a loading d e v i c e at r o o m and five e l e v a t e d t e m p e r a t u r e s (527, 897, 927, 1027, and1177~ I n o r d e r t o c o n f t r m the t e m p e r a t u r e dependence t e s t s w e r e a l s o c a r r i e d out a t 877 and 977~ T e m p e r a t u r e dependence of the s t r e n g t h v a r i a t i o n c o e f f i c i e n t and m e a n s t r e n g t h v a l u e s of the t e s t m a t e r i a l a r e given in F i g . 1. As c a n b e s e e n f r o m Figo l a , with i n c r e a s i n g t e m p e r a t u r e the s t r e n g t h v a r i a t i o n coefficient TABLE 1. C o m p a r i s o n of T a b u l a t e d F and C a l c u l a t e d F c Values of F i s h e r ' s C r i t e r i o n E s t a b l i s h e d by C o m p a r i n g S t r e n g t h V a r i a t i o n at Room and E l e v a t e d T e m p e r a t u r e Temperature, oc Characteristic No. of testpieces Coeff. of variation, % Value of F corresponding to significance levels

101

9,8 0,I0 0,02

527

897

927

56 10,1

56 10,5

56 7,0

1,47 1,06

rc

1,47

1,15

1027

56 6,5

1177

56 6,9

m

1,78 1,96

1,78 2,27

1,78 2,02

P h y s i c o t e c h n i c a l E n e r g y P r o b l e m s I n s t i t u t e , A c a d e m y of S c i e n c e s of the Lithuanian SSR, Kaunas. T r a n s l a t e d f r o m P r o b l e m y P r o c h n o s t i , No. 6, pp. 105-107, J u n e , 1978. Ortghnal a r t i c l e s u b m i t t e d June 1, 1977.

720

0039-2316/78/1006-0720507.50

9 1979 Plenum Publishing C o r p o r a t i o n

K.v' o

/

~

~i' kgf/mmz F i g . 1. T e m p e r a t u r e d e p e n d e n c e of v a r i a t i o n c o e f f i c i e n t (a) a n d m e a n s t r e n g t h v a l u e (b) of c e r a mic S-49. o

a

~oo

b

8,~ ~,~

K v v a r i e s l i t t l e up to 900~ then f a i l s s h a r p l y a n d a g a i n r e m a i n s m o r e o r l e s s c o n s t a n t a t a b o u t 70% of the i n i t i a l v a l u e . The sig~nifieance of the c h a n g e in v a r i a t i o n t h r o u g h the t e m p e r a t u r e r a n g e 900-920~ and its i n v a r i a n c e a b o v e t h i s r a n g e w a s c o n f i r m e d b y F i s h e r ' s c r i t e r i o n f o r c o m p a r i n g s c a t t e r [3]. [ [ s h o u l d be noted that F i s h e r ' s c r i t e r i o n is d e s igned f o r c o m p a r i n g the n a t u r a l d i s p e r s i o n s 2. H o w e v e r , f o r c o n d i t i o n s when the m e a n m a t e r i a l s t r e n g t h ( ~ i v a r i e s m a r k e d l y with t e m p e r a t u r e , c o m p a r i s o n of d i s p e r s i o n s ~ c h a r a c t e r i z i n g the a b s o l u t e v a l u e of v a r i a t i o n e o u l d b e e r r o n e o u s . T h e r e f o r e , in the p r e s e n t w o r k F i s h e r ' s c r i t e r i o n is u s e d to c o m p a r e v a l u e s of K2v =s~/~r~, e v a l u a t i n g the r e l a t i v e m a g n i t u d e of v a r i a t i o n . T h i s c h a n g e ove r is c o n n e c t e d with the e r r o r e x p l a ined by the a b s e n c e of e [ va rues f r o m the t r u e m e a n (ma t h e m a t i c a l e x p e c ration of c r i v a l u e s ). H o w e v e r , s i n c e the c o n f i d e n c e r a n g e forcr i v a l u e s is v e r y n a r r o w , the e r r o r a r i s i n g b y c h a n g i n g f r o m c o m p a r i s o n of d i s p e r s i o n to c o m p a r i s o n of v a r i a t i o n c o e f f i c i e n t s is i n s i g n i f i c a n t and has no p r a c tical importance. T a b u l a t e d F and c a l c u l a t e d F c v a l u e s of F i s h e r ' s c r i t e r i o n o b t a i n e d by c o m p a r i n g s t r e n g t h v a r i a t i o n c o e f f i c i e n t s , e s t a b l i s h e d a t r o o m and e l e v a t e d t e m p e r a t u r e , a r e shown in T a b l e 1. A s can be s e e n . the n u l l h y p o t h e s i s a b o u t the p a r i t y of v a r i a t i o n at r o o m and e l e v a t e d t e r n p e r a t u r e is a c c e p t a b l e f o r 527 and 897~ and a t the 98% c o n f i d e n c e l e v e l is r e j e c t e d f o r 927. i 0 2 7 , and 1177~ T h u s , s t r e n g t h v a r i a t i o n c h a n g e s m a r k e d l y in p a s s ing t h r o u g h the t e m p e r a t u r e inte rva 1 900- 920(~C. I t f o l l o w s f r o m F i g . i b t h a t o v e r the s a m e t e m p e r a t u r e r a n g e t h e r e is a s h a r p c h a n g e in the i n c l i n a t i o n a n g l e of the t e m p e r a t u r e d e p e n d e n c e of s t r e n g t h . T h i s p h e n o m e n o n is e x p l a i n e d by the change in c h a r a c t e r of f a i l u r e fcom t r a n s c r y s t a l l i n e a t low t e m p e r a t u r e to i n t e r c r y s t a l l i n e a t e l e v a t e d t e m p e r a t u r e [4]. T h e c h a n g e in f a i l u r e type was c o n f i r m e d by p h o t o m i c r o g r a p h y of the f a i l u r e s u r f a c e u s i n g a scanning" m i c r o s c o p e ( F i g . 2). T h e c h a n g e in f a i l u r e p r o c e s s e v i d e n t l y p r e d e t e r m i n e s the s h a r p f a l l in s t r e n g t h v a r i a t i o n in e h a n g i n g t h r o u g h the t e m p e r a t u r e T t ( s e e F i g . 1). S t r e n g t h v a r i a t i o n a t t e m p e r a t u r e s below T t is d e t e r m i n e d by v a r i a t i o n of the c r i t i c a l s t r e s s g c r , c o r r e s p o n d i n g to a v e r y s e v e r e m i e r o e r a c k in the t e s t p i e c e , and is n a t u r a l l y r e f l e c t e d in[he f e a t u r e s of t h e b r i t t l e f r a c t u r e p r o c e s s . A t h i g h t e m p e r a t u r e s , a s is w e l l k n o w n [5], s h e a r p r o c e s s e s a l o n g g r a i n b o u n d a r i e s c o m m e n c i n g at s t r e s s a s h p r o v i d e s o m e i m p e t u s to the f a i l u r e p r o c e s s . The value of a s h is m o r e s t a b l e t h a n c r e r s i n c e it i s d e t e r m i n e d b y the b a s i c e o m p o s i t i o n o f t h e m a t e r i a l . The s t r e n g t h v a r i a t i o n o b s e r v e d ( m a r k e d l y s m a l l e r than a t low t e m p e r a t u r e ) is e v i d e n t l y e x p l a ined by f l u c t u a t i o n s in c h e m i c a l c o m p o s i t i o n , and v a r i a t i o n s in g r a i n g e o m e t r y and o r i e n t a t i o n . T h u s , the c h a n g e i n f a i l u r e p r o c e s s l e a d s t o a c r i t i c a l e h a n g e in s t r e n g t h v a r i a t i o n . It is a p p a r e n t [ h a t t e m p e r a t u r e T t m u s t be c o n s i d e r e d to be the l i m i t of a p p l i c a b i l i t y of s t a t i s t i c a l s t r e n g t h t h e o r i e s b a s e d o n the h y p o t h e s i s of the w e a k e s t g r a i n , in p a r t i c u l a r the W iebuIl t h e o r y . R e s u l t s f o r the i n v e s t i g a t i o n of the e f f e c t of l o a d i n g type on s t r e n g t h v a r i a t i o n a r e shown in T a b l e 2 a n d F i g . 3. It is e v i d e n t f r o m the data t h a t the l e a s t s t r e n g t h v a r i a t i o n is o b t a i n e d with t h r e e - p o i n t a n d p u r e b e n d i n g , s o m e w h a t g r e a t e r v a r i a t i o n with uniax ial and d i a m e t r a l c o m p r e s s ion, and the g r e a t e s t in t e n s i o n . T A B L E 2. B a s i c i n d i c e s f o r S t r e n g t h V a r i a t i o n in S - 4 9 C e r a m i e w i t h D i f f e r e n t T y p e s of L o a d i n g Type of loading

NO. of tests

]Mean .... a~ [Varmnon I tim teul-]Isquare strength d.eviaIcoeff., va/ue, ~Itton~ I~ kgf/mm z ]kgf/mmZ['I,

Throe-point bend ing Pure bending Uniaxia 1 c o m -

101 100

46,8 42,8

4,62 4,42

9,9 10,3

pression

100

235,3

28,57

12,1

200 180

22,8

2,80 3,61

12,3 15,2

Diamerral compression Tension

23,7

721

73

9

JJ4t~$57354,(~,q57t2P2,q2N2.~K 1822,Y6,,r /~ 18r

Strength distribution range kgf/mm

a

b

Fig. 2

c

d

'

e

Fig. 3

Fig. 2. Mierofraetograms and 977~ (b).

of S-49 ceramic

testpieees failedat 877~

(a)

Fig. 3. Strength distribution histograms forS-49 ceramic testpteces with three-point (a) and pure bending (bS, uniaxial (c) and diametral compression (d), andalso tension (e). Comparison of the strength variation coefficients for t h r e e - p o i n t a n d four-point bending by means of F i s h e r ' s criterion confirm ins ignifieant divergence, and do not require rejection of the null hypothesis about the parity of these f o r m s of loadh~g. However, it is interesting to note that for another test highalumina m a t e r i m 5M-4 the strength variation coefficient in pure bendlngappears to be g r e a t e r (9.6%) than in t h r e e - p o i n t

bending (8.9%5. The observed differenee may apparently be explained by the fact that with pure bending in the zone of m a x i mum m o m e n t there are often u n c h a r a c t e r i s t i c s t r u e t u r a l defects, atypieal for a g i v e n m a t e r i a l , and onty o c c u r ring in individual testpieces. The increase in variationwith uniaxial and d i a m e t r a l e o m p r e s s i o n is explained by the high sensitivity of these f o r m s o f t e s t to loading conditions. Nonuniform toad distribution over the area of eoataet, whiehevidently cannot be avoided in parts of the testpieee, is m o s t probably the basic reason for strength variation with these f o r m s of loading. Greatest strength variation oeeurs in tension. The main proble]n here is the practical unavo~dability of unstable loadingeonditions eonneetedwith eeeentrie loading. The origin of this instability is explainedby v a r i a tions (within perm issible Limits) of testpiece geometry, a n d a l s o inaccuracies a r i s i n g due to setting up the t e s t piece inthe grips [6]. Thus, the difference in strength variation with d i f f e r e n t f o r m s of loading is explained inthe f i r s t instance by the nature of load application to the testpiece, a n d m o r e a e e u r a t e l y by the level of reproducibility of this p r o c e s s . F r o m this point of view iris fully understood thatwith technically s i m i l a r f o r m s of ioading(three-poLnt and pure bending, uniaxial and diametral compression5 the strength variation indices are close together.

LITERATURE 1.

2. 3.

722

CITED

I. D. Abramson, Ceramins forAvtationProducts [in Russ [an], Oborongiz, Moscow (1963). I~:. M. Mikha [lovskH, "Determ ination of the ultimate strength of brittle m a t e r l a Is in tension, " P r o b t . P r o c h n . , No. l l , 55-56 (19765. N . V . Smirnov and [. V. Dunin-Barkovskii, Course in Probability T h e o r y and Mathemat[ealStatisties for Technical Applications [in Russian], Nauka, Moscow {19655.

4.

5. 6.

U . D . K i n g e r i , "High m e l t i n g point o x i d e s , " in: S t u d i e s a t High T e m p e r a t u r e [in R u s s i a n ] , Moscow (1962), pp. 126-147. R. W. Davidge and A. G. E v a n s , "The s t r e n g t h o f c e r a m i c s , " Mat. Sci. E n g . , 6, No. 5, 280-298 (1970), L. Mordfine a n d M . K e r p e r , "Strength t e s t i n g of c e r a m i c s , " in: M e c h a n i c a l and T h e r m a l P r o p e r t i e s of C e r a m i c s , NBS (1969), pp. 243-262.

FAILURE

OF

COMPOSITES

SUPERCONDUCTING AT

LOW

TEMPERATURES

T. A. Parkhomenko, and V. A. Stepanenko

V.V.

Pustovalov,

UDC 539.4

Modern s u p e r c o n d u c t i n g m a t e r i a l in the form of w i r e o r b u s b a r , as used in c r y o g e n i c e n g i n e e r i n g , is a c o m p o s i t e , it c o n s i s t s of the a c t u a l s u p e r c o n d u c t i n g wire and a st~rrounding m a t r i x of a n o r m a l m e t a l which m u s t have high t h e r m a l and e l e c t r i c a l conductivity. Solid solutions of titanium and niobium have been used with s u c c e s s as s u p e r c o n d u c t i n g m a t e r i a l s and c o p p e r as the m a t r i x . At room t e m p e r a t u r e and above the m a t r i x and s u p e r c o n d u c t i n g m a t e r i a l a r e r e a s o n a b l y ductile and this is o f s o m e e n g i n e e r i n g convenience i n t h e i r p r o d u c t i o n . However, during cooling tothe o p e r a t i n g t e m p e r a t u r e s the ductile b e h a v i o r s of the wire ( s u p e r c o n d u c t o r ) and m a t r i x ( n o r m a l m e t a l ) a r e opposite due to their d i f f e r e n t types of l a t t i c e s . A l l o y s o f titanium and n i o b i u m l i k e o t h e r m e t a l s a n d a l l o y s having a bcc lattice b e c o m e e m b r i t tied as the t e m p e r a t u r e is r e d u c e d w h i l e the d u c t i l i t y o f f c c m e t a l s i n c r e a s e s as the t e m p e r a t u r e f a l l s . This m a y lead to a complex t e m p e r a t u r e r e l a t i o n for the d u c t i l i t y and s t r e n g t h and i n c e r t a i n c a s e s it b e c o m e s difficult to use a s u p e r c o n d u c t o r in c r y o g e n i c e q u i p m e n t o p e r a t i n g under conditions such that the m e c h a n i c a l s t r e s s e s a r e v a r i a b l e in magnitude and sign. In addition, during cooling to the o p e r a t i n g t e m p e r a t u r e the s u p e r c o n d u c t o r changes to the s u p e r c o n d u c t i n g state and it is known [1] that this m a y cause changes of d u c t i l i t y . The f i r s t i n v e s t i g a t i o n s of the s u p e r c o n d u c t i n g c o m p o s i t e s showed that p l a t i n g w i t h c o p p e r which is ductile a t l o w t e m p e r a t u r e s i n c r e a s e s the d u c t i l i t y of the cable b y m o r e t h a n a n o r d e r of magnitude in c o m p a r i s o n with the d u c t i l i t y of the s u p e r c o n d u c t o r b e e o m i n g e m b r i t t l e d [2-4]. Such a d i f f e r e n c e m u s t be r e f l e c t e d in the f r a c t o g r a p h y of the c o m p o s i t e and the s u p e r c o n d u c t o r . In r e l a t i o n to this in the p r e s e n t work an e l e c t r o n - m i c r o s c o p i c investigation has been made of the f r a c t u r e s u r f a c e with the aim of studying the n a t u r e of the f a i l u r e of the s u p e r c o n d u c t o r , the c o p p e r m a t r i x , and the c o m p o s i t e as a whole o v e r a wide t e m p e r a t u r e r a n g e . Experimental

Method

The e x p e r i m e n t s w e r e conducted on t e s t p i e c e s made from i n d u s t r i a l l y produced s u p e r c o n d u c t i n g m a t e r i a l s : a c o m p o s i t e cable of 1 . 5 - m m d i a m e t e r c o m p o s e d of 24 w i r e s of 0 . 1 5 - r a m d i a m e t e r of the NT-50 alloy (50 at.% Nb, 50% Ti) p l a t e d with c o p p e r , and a l s o a cable of 0 . 8 2 - m m d i a m e t e r c o m p o s e d of six twisted w i r e s of 0.17mm d i a m e t e r of the T - 6 0 alloy (40% Nb, 60% Ti) p l a t e d w i t h c o p p e r . The t e s t p i e c e s w e r e made in the following m a n n e r . Onto the ends of p i e c e s of w i r e of length 25 mm w e r e s o l d e r e d caps of s o l d e r by m e a n s of which the

a

F i g . 1. T e m p e r a t u r e r e l a t i o n s for the s t r e n g t h p a r a m e t e r s of the individual w i r e s u s e d in the s u p e r conducting c a b l e .

c

P h y s i c o t e c h n i c a l Institute, A c a d e m y of S c i e n c e s of the U k r a i n i a n SSR, K h a r k o v . institute of Strength P r o b l e m s , A c a d e m y of S c i e n c e s o f t h e U k r a i n i a n SSR, Kiev. T r a n s l a t e d f r o m P r o b l e m y P r o c h n o s t i No. 6, pp. 1 0 8 - 1 1 3 , J u n e , I978. O r i g i n a l a r t i c l e s u b m i t t e d J u n e 1 5 , 1977.

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