MECHANICAL HYSTERESIS IN MATERIALS USED IN SUPERCONDUCTING ELECTRICAL MACHINES

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MECHANICAL HYSTERESIS IN MATERIALS USED IN SUPERCONDUCTING ELECTRICAL MACHINES

R. Adams, V. Coveney

To cite this version:

R. Adams, V. Coveney. MECHANICAL HYSTERESIS IN MATERIALS USED IN SUPERCON-

DUCTING ELECTRICAL MACHINES. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-291-

C9-296. �10.1051/jphyscol:1983940�. �jpa-00223387�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n°12, Tome 44, decembre 1983 page C9-291

MECHANICAL HYSTERESIS IN MATERIALS USED IN SUPERCONDUCTING ELECTRICAL MACHINES

R.D. Adams and V.A. Coveney

University of Bristol, Dept. of Mechanical Engineering, Queen's Building, Bristol BS8 1TR, U.K.

Resunfe - Nous presentons i o i l e s r e s u l t a t s pour quelques materiaux qui seront u t i l i s e s dans l e s machines e^lectriques supraconductrices ( c r y o g e n e r a t e u r s ) . Abstract - We present damping r e s u l t s for a v a r i e t y of m a t e r i a l s which w i l l be used in superconducting e l e c t r i c a l machines ( c r y o a l t e r n a t o r s ) .

I - INTRODUCTION

There are several practical applications for which knowledge of the dynamic behaviour of materials at temperatures near and below 20 K is important. These applications include liquid hydrogen technology and superconductor applications.

At present, most of the liquid hydrogen applications are in the area of rocket propulsion although there are tentative plans for hydrogen powered aircraft.

Superconductor applications include NMR, particle accelerators, plasma confinement and nuclear fusion, and superconducting AC generators. The purpose of this work was to study the materials used in superconducting AC generators so that losses due to mechanical hysteresis can be established reliably.

The problem is the inverse of that which is usually encountered in engineering.

Here, the interest is in low energy dissipation since the problem is not that of resonance, but rather of heat generation within the AC generator which can result in the conductors losing their superconducting properties. While much is known of the damping properties of constructional materials at room temperature, less is known of these properties at very low temperatures, and even less when cyclic stresses of engineering significance are considered. The objective of this work is to obtain results for several structural materials, such as copper, superconducting alloys, stainless steel and glass fibre reinforced plastics. The unit of damping used here is the specific damping capacity, IJJ, defined as ty = bJJ/U where U is the maximum energy stored per cycle, and At/ is the energy dissipated.

Some predictions will be made for the power dissipation per unit volume of material assuming a cyclic strain amplitude of 10 4 and a frequency of 50 Hz.

II - EXPERIMENTAL PROGRAMME

The specimen was in the form of a free-free beam and was driven in its first

flexural mode of vibration by a coil/magnet pair. The damping and natural frequency were recorded automatically as the temperature changed over the range from M-K to 293 K. A programmed microcomputer was used to control the frequency and amplitude of the excitation via a digital frequency synthesiser and a digital voltmeter. The microcompter controlled up to twenty information channels, so that readings were also taken of temperature as well as calculating the damping values. In this way, a print-out was given of damping and natural frequency at different cyclic stress amplitudes over a range of temperatures.

The specimen consists of a beam in bending and it is known that there is a linear variation of stress across the section in addition to the lengthwise variation in

Resunfe - Nous présentons ici les résultats pour quelques matériaux qui seront utilisas dans les machines 'électriques supraconductrices (cryogênerateurs).

Abstract - We present damping results for a variety of materials which will be used in superconducting electrical machines (cryoalternators).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983940

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bending moment. However, t h i s can be accounted f o r by measuring t h e v a r i a t i o n of damping with displacement amplitude and t h e n u s i n g a f u n c t i o n of t h e form

t o g e t h e r with t h e v a r i a t i o n of s t r e s s , o, w i t h p o s i t i o n i n t h e specimen. This t e c h n i q u e is f u l l y d e s c r i b e d i n r e f e r e n c e s 1 and 2 .

-

4

Cyclic s t r a i n s of t h e o r d e r of 10 % could be a t t a i n e d i n t h e a p p a r a t u s used and t h i s was n e c e s s a r y i n o r d e r t o cover t h e r a n g e of amplitudes l i k e l y t o be achieved i n c r y o a l t e r n a t o r s .

I11 RESULTS AND DISCUSSION Annealed cryogenic grade copper

We were r a t h e r s u r p r i s e d a t first a t f i n d i n g a Bordoni peak i n annealed copper ( s e e Fig. l ) , a s t h i s r e s u l t appeared t o c o n t r a d i c t work p u b l i s h e d by N i b l e t t

& Wilks ( 3 ) who found a ~ d r d o n i peak only f o r cold-worked copper. I n o r d e r t o

check t h a t t h e v i b r a t i o n t e s t i n g had n o t cold-worked t h e copper, t h e Vickers h a r d n e s s (KV10 = 5052) of a specimen t h a t had been t e s t e d a t l o w t e m p e r a t u r e was compared with a s i m i l a r , u n t e s t e d annealed specimen,

Temperature

Fig. 1 V a r i a t i o n of s p e c i f i c damping c a p a c i t y ,

6,

with temperature f o r copper, two Nb T i s u p e r c o n d u c t o r s , a composite c a b l e and GlOCR g l a s s c l o t h i n eP oxy

and with a n o t h e r s i m i l a r u n t e s t e d a n n e a l e d specimen which had s u b s e q u e n t l y been extended c o l d by 1.4% (HV10 = 56+2). These h a r d n e s s t e s t s i n d i c a t e d t h a t t h e amount of cold-work i n t r o d u c e d by t e s t i n g was e q u i v a l e n t <1% e x t e n s i o n , whereas t h e curve of damping a g a i n s t temperature c l o s e l y ressembled N i b l e t t and Wilks' c u r v e s f o r >2%. I t i s now b e l i e v e d t h a t t h e Bordoni peak o c c u r s f o r annealed specimens i n very pure f . c . c . m e t a l s when t h e a p p l i e d c y c l i c s t r e s s i s s u f f i - c i e n t l y high ( 4 , 5 ) .

Superconducting a l l o y s

S e v e r a l superconducting a l l o y s were t e s t e d , b o t h i n t h e pure form and f a b r i c a t e d a s f i l a m e n t s i n a copper m a t r i x . There was a s t r o n g maximum i n damping (I) ² 1 . 9 % ) i n t h e as-received NbTi and a s s o c i a t e d w i t h t h i s damping peak was a minumum i n r e s o n a n t frequency and, t h e r e f o r e , i n Y o u n s modulus. If changes i n l e n g t h can be n e g l e c t e d , a t t h e minimum t h e modulus i s 3% below t h e room temperature v a l u e . I n a d d i t i o n , t h e r e is a secondary damping peak (I) 2 0.95%) a t 25K. A f t e r 300%

c o l d work, t h e major damping peak h a s diminished t o approximately t h e same l e v e l a s t h e secondary peak, which h a s been much l e s s e f f e c t e d .

Not s u r p r i s i n g l y , t h e behaviour of t h e Cu/NbTi composite ( t y p e A61) r e f l e c t s t h a t of both t h e copper and t h e NbTi. A s f o r copper, t h e Bordoni peak is t h e dominant f e a t u r e a t 70K ( I ) = 2 . 6 % ) . But it should be n o t e d t h a t t h i s peak i s s t r o n g e r t h a n t h e ("law of mixtures") combination of r e s u l t s f o r copper and NbTi. I t a p p e a r s t h a t t h e r e i s a r e s i d u a l e f f e c t of cold-work on t h e copper i n s p i t e of t h e anneal- i n g a t 350 C t o one hour a f t e r each draw.

Glass f i b r e r e i n f o r c e d p l a s t i c s

R e s u l t s a r e g i v e n here f o r t y p e GlOCR GRP composite which i s a c r y o g e n i c g r a d e m a t e r i a l made from g l a s s c l o t h i n a n epoxy m a t r i x . I t can be s e e n from F i g . 1 t h a t t h e r e is a l a r g e damping peak

(6 =

6.6%) a t 240K. There was a l s o a l a r g e i n c r e a s e i n Young's modulus (about 20%) a s t h e temperature was decreased from 30GK t o 4K.

I t i s b e l i e v e d t h a t t h e main damping peaks i n t h e e p o x i e s a r e due t o (main c h a i n ) segmental motion and it is t h i s behaviour t h a t g i v e s e p o x i e s t h e i r r e l a t i v e l y high damping c h a r a c t e r i s t i c s n e a r room t e m p e r a t u r e ; a t low t e m p e r a t u r e s t h e y t a k e on a d i f f e r e n t ( g l a s s y ) c h a r a c t e r .

Since GlOCR u s e s a s o l i d - t y p e epoxy r e s i n t h e manufacturers were unable t o manu- f a c t u r e a s u i t a b l e p i e c e o f r e a c t e d r e s i n f o r o u r t e s t s . However, a specimen made from a s i m i l a r epoxy (Ciba MY750) gave a damping peak of I)

=

20% a t 210K, con- f i r m i n g t h a t t h e peak i n t h e G l O C R m a t e r i a l was m a t r i x dominated.

S t a i n l e s s s t e e l s

Three s t a i n l e s s s t e e l s o f t h e AISI 316 f a m i l y were t e s t e d . These a r e c a n d i d a t e m a t e r i a l s f o r t h e r o t o r of a c r y o a l t e r n a t o r and comprise t h e bulk of m a t e r i a l . A l l of t h e s e m a t e r i a l s gave v e r y much lower damping v a l u e s ( s e e F i g . 2) t h a n t h e copper, superconductors and GRP, t h e lowest being 316 LN. The anomalies i n t h e r e s u l t s f o r 316 LN were r e p e a t a b l e , b u t not e x p l a i n a b l e .

A l l t h e s t a i n l e s s s t e e l s t e s t e d showed a p p a r e n t l y anomalous behaviour i n Young's modulus below a c e r t a i n t e m p e r a t u r e (40-70K). T h i s t y p e of anomalous behaviour h a s been r e p o r t e d ( a t v e r y much h i g h e r f r e q u e n c i e s ) by L e d b e t t e r

st

( 6 ) and L e d b e t t e r ( 7 ) , who have suggested t h a t it was due t o magnetic t r a n s i t i o n s ( 6 ) and probably s p i n - g l a s s t r a n s i t i o n s ( 8 ) . However, it h a s a l s o been suggested t h a t t h e anomalies might be due t o p a r t i a l A u s t e n i t i c - M a r t e n s i t i c t r a n s i t i o n s , which could have important i m p l i c a t i o n s f o r t h e use of t h e s e s t e e l s . A simple experiment h a s been c a r r i e d o u t by u s which involved comparing t h e inductance of a c o r e l e s s c o i l and a s i m i l a r c o i l w i t h EN58B (304) s t e e l specimen a s t h e c o r e . The two c o i l s which formed two limbs of an o u t of b a l a n c e , 2kHz a . c . b r i d g e were t a k e n through

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Temperature

K

F i g . 2 V a r i a t i o n of s p e c i f i c damping c a p a c i t y , $, w i t h temperature f o r t h r e e s t a i n l e s s s t e e l s

t h e temperature range 4K - 300K. There was some p o s s i b l e i n d i c a t i o n of "anomalous"

magnetic behaviour ( i e : r e v e r s a l i n dL/dT) a t and below l o O K , whereas t h e anomaly i n e l a s t i c modulus occurred a t and below 70K. The magnitude of t h e anomaly i n inductance was not c o n s i s t e n t with an A u s t e n i t i c t o M a r t e n s i t i c t r a n s i t i o n of more t h a n I%, and was t h e r e f o r e u n l i k e l y t o have caused t h e l a r g e modulus anomaly observed.

It should be p o i n t e d o u t t h a t both we and L e d b e t t e r measure E from t h e v e l o c i t y of sound ( ~ / p ) + and t h e dimensions of t h e specimen. I f t h e dimensions show some anomalous v a r i a t i o n w i t h t e m p e r a t u r e , t h e n it i s d i f f i c u l t by dynamic t e s t s t o s e p a r a t e t h e two p a r a m e t e r s .

A d d i t i o n a l work on t h e expansion of m a t e r i a l s ( e s p e c i a l l y s t a i n l e s s s t e e l s ) i n t h e range 4

-

300K is t h e r e f o r e b e i n g c a r r i e d o u t u s i n g a very s e n s i t i v e d i l a t o m e t e r . The purpose of t h i s work i s t o s e e i f t h e r e a r e any anomalies i n expansion which can

be r e l a t e d t o t h e anomalies i n r e s o n a n t frequency.

Energy d i s s i p a t i o n

To convert t h e v a l u e s of damping i n t o energy d i s s i p a t i o n , we use t h e equation:

AU

=

^$j'EE2lJ,

where

f =

frequency of v i b r a t i o n , E = Young's modulus

and E

=

s t r a i n amplitude.

Using v a l u e s of

f

= 50hz and E = 10 -4

,

we a r r i v e a t t h e v a l u e s given i n Table 1 f o r a temperature of 4K.

Table 1 Energy d i s s i p a t i o n i n c r y o a l t e r n a t o r m a t e r i a l s

M a t e r i a l 3

A l l (Watts/m )

Cryogenic grade copper >350 NbTi a s r e c e i v e d 103

NbTi cold worked 300% 8 8

A61 superconductor -200

GlOCR GRP 20

316 annealed 33

316 LN annealed 1 7

316 High C 1 7

It is c l e a r from t h e s e r e s u l t s t h a t t h e superconductor i s a s i g n i f i c a n t s o u r c e of energy d i s s i p a t i o n , owing t o t h e high damping of b o t h t h e NbTi and t h e copper.

Under c y c l i c c o n d i t i o n s of l o a d i n g , it i s t h e r e f o r e e s s e n t i a l t h a t t h e h e a t gen- e r a t e d be conducted away e f f i c i e n t l y .

I V CONCLUSIONS

Damping r e s u l t s have been given f o r a v a r i e t y of c r y o a l t e r n a t o r m a t e r i a l s . Under t h e c y c l i c l o a d i n g c o n d i t i o n s which may be experienced i n p r a c t i c e , t h e h i g h e s t energy d i s s i p a t i o n w i l l occur i n t h e superconductors and t h e s e must t h e r e f o r e be a d e q u a t e l y cooled.

V REFERENCES

1. Adams R.D.& Bacon D.G.C., J. Phys. D: Appl. Phys,

6,

(1973), 27.

2. Guild F.J.& Adams R.D., J . Phys. E: S c i . I n s t r . ,

14,

(1981), 355.

3 . N i b l e t t D.H. & Wilks J . , P h i l . Mag.,

2,

(1957), 1427.

4. N i b l e t t D .H., P r i v a t e communication.

5 . Stadelmann P. & Benoit W . , S c r i p t a M e t a l l u r g i c a ,

11,

(1977), 645.

6. L e d b e t t e r H . M . , Weston W.F. & Naimon E.R., J . Appl. Phys.,

5 ,

(1975), 3855.

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7. L e d b e t t e r H.M., J . Appl. P h y s . ,

52,

(19811, 1587.

8. C o l l i n g s E.W. & L e d b e t t e r H.M., P h y s i c s L e t t e r s ,

c,

^(1979), 53.