Effect of low temperature irradiation on the mechanical strength of organic insulators for superconducting magnets

Organic insulators, such as polyimide, epoxy resins and fibre reinforced epoxies, are irradiated in the fission reactor at about 5 K and the mechanical properties are measured in liquid helium and in liquid nitrogen without warm up. A t very low temperatures, the materials are completely brittle. After irradiation of the absorbed dose of 1. lx 109rad at about 5 K, the breaking stress of epoxy resins reduces by 30 ~ 40% and FRP and polyimide exhibit a slight decrease in the mechanical strength. FRP and polyimide have good radiation resistance with respect to the mechanical behaviour after a dose o f ~ l x 109 rad as compared with other selected organic materials.

Effect of low temperature irradiation on the mechanical strength of organic insulators for superconducting magnets S. Takamura and T. Kato Superconducting magnets for fusion reactors will be exposed to fast neutron and gamma irradiation produced by a fusion reaction. The insulators expected to be used in fusion reactor magnets also meet these levels of irradiation and the high mechanical stress produced by electromagnetic forces. Inorganic insulators can withstand high irradiation, but they are very brittle. Since ductile materials are required for coil windings, the materials used for superconducting magnet insulation are normally thought to be organic materials. Abdou 1 recently reported that the maximum neutron fluence at the magnets is about 5x1018 nvt for a life time of a fusion reactor at the optimum shield thickness. This fluence corresponds to an absorbed dose of about 101° rad. Very few experiments which study the effects of low temperature irradiation on the physical and the mechanical properties of organic insulators at cryogenic temperature have been reported 2~s. It must be determined whether organic materials would make suitable insulators and sufficient data on the effect of irradiation should be obtained in order to design such a magnet system. This paper reports the results of compression tests in liquid helium and liquid nitrogen after fission reactor irradiation at about 5 K.

made at 77K or at 4.2K using an Instron type testing machine. Crosshead speed was 0.5 mm min "I corresponding to a strain rate of 2 x 10-3s -1. Loadcell

Mc~m9 ~:~ crosshead

p~

Po~thy~ne sheet

steel plate

Experimental procedures Specimens used were Vespel (polyimide), Epoxy resins and fibre reinforced composites (FRP). The specimens of vespel and composites were pillar shaped with dimensions of 2x2x4 mm 3 and the Epoxy resins were cylindrical measuring, 2 mm in diameter and 4mm long. The specimens were set in holes in an aluminium plate and were enclosed in aluminium foil. This specimen holder having multiple specimens rode on the hardened steel plate in liquid helium or in liquid nitrogen without any warm up after irradiation. For the compression tests, the first specimen in the specimen holder was positioned between the upper plate and the lower one as shown schematically in Fig. la and b. After testing the ftrst specimen, further movement of the crosshead induced fracture. The next specimen was then brought into a position for testing and the tests continued sequentially until all the specimens had been examined. Compression tests were The authors are at The Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan. Paper received 16 January 1980.

Specimen

l

holder (Aluminum)ll

l"

......\~

Specimen /

S ~ : ~ .

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Ix\\\ \\~x~Koptontape Hardenedsteel plate b Fig. 1 Cryostat used for compression tests in liquid helium. When the first specimen has been deformed and fractured, further forward movement of the hardened steel plate brings the next specimen into a position ready for testing and tests take place sequentially

0011-2275/80/080441-04/:~02.00 © 1980 I PC Business Press CRYOGENICS. AUGUST 1980

441

ones tested at 4.2K, without any warm up after irradiation at about 5K, are given in Fig. 2. The appearance of the serrations will be due to either the fracture formation or the local rise in temperature from adiabatic deformation. After the occurrence of several stress drops, an approximately 45 ° crack was observed through the specimen. Therefore, it is considered that deformation and cleavage take place by the shear yielding.

LHe test

60C

5

Fig. 3 shows the temperature dependence of the yield stress of unirradiated specimens. The yield stress is defined as the stress at the intercept of the stress-strain curve with a straight line parallel to the slope of elastic region and offset 1% exten. sion. The serrations were observed characteristically in the stress-strain curves below 18K.

40C U~

200

Epoxy resins. Figs 4a and b are the stress-strain curves I

obtained at 77K for unirradiated and irradiated Epikote 828 hardened by aromatic amine and acid anhydride, respectively

J

0

Strain, % Fig. 2 Stress - strain curves of Vespel (SP-1) (polyimide) in compression test at 4.2 K before and after reactor irradiation at about 5Kwhen: 1-0rad;2-4.5x10 Brad ( n f = 1 . 2 x 1 0 1 7 n v t a n d "), = 3.3 x 108 R); 3 - 1.1 x 109 tad) nf = 3.0 x 1017 nvt and 3, = 8.0 x 108 R)

300_:

FRP. The stress-strain curves tested at 4.2 K for glass fibre reinforced epoxy and carbon fibre reinforced epoxy which are hardened by acid anhydride are given, in Figs 6a and b

Venoel (SP-t) (Polyimide)

•

The stress-strain curves of Epikote 828 hardened by aliphatic amine, aromatic amine and acid anhydride tested at 4.2 K are shown in Figs 5a, b and c, respectively, together with the curves for irradiated specimens which are tested without warm up after irradiation at about 5 K. The brittleness was observed at 4.2 K even in unirradiated specimens.

a 800

Epikote 828 Horder~r aromatic

Irradiated at 5K

LN~ test

200

e

°

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I

200

t

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Ternperoture, K Fig. 3 Temperature dependence of yield stress for Vespel obtained from compression tests, where the yield stress is determined as the stress at the intercept of the stress-strain curve with a straight line parallel to the slope of elastic region and offset 1% extension. Insert: the typical stress-strain curves at various temperatures

80C

0

Strain, %

_ h E~kote 828 u (Hardener ocid~ ~onhydride /

60C

Irradiation was performed at about 5 K in LHTL of JAERI. The fast neutron flux was 1.15 x 1012 ncm'2s "1 (>(3.1 MeV) with fission neutron spectrum which were measured by

0

LN 2 test

2,5 I

40C

/~j..2~

~Wormup

to room

means of foil. The 3' dose rate was estimated to be 1.1x 10 7 R h "1. The total absorbed dose after fast neutron irradiation of 3.3x1017 nvt accompanied by y rays of 8.8xl0SR,

was 1.1 × 1 0 9 rad using the relations that to 1 rad and 1 R to 1 rad3.

10 9

20C

nvt corresponds

10%

+.,e--ll,.l 0

0

0

Results

Strain, %

Vespel. Vespel is one of the polyimides. The stress-strain

Fig. 4 Stress - strain curves of Epikote 8 2 8 hardened by aromatic amine and acid anhydride, a - 1 : 0 rad; 2, 3 = 1.1 x 109 rad,nf:

curves o f unirradiated specimens tested at 4.2K and irradiated

3 x 1017 nvt 3,: 8 x 108 R

442

CRYOGENICS

. AUGUST

1980

ool ,Z

2O Epikote 828 Hardener oliphatic amine

Discussion

Irradiated at 5K

At very low temperatures, the inter-molecular and intramolecular mobility in organic materials cannot respond to the applied stress so that the chain scission occurs and thus crack onset is brought about. In Vespel, the dip in the yield stress below about 18 K is probably attributed to the occurrence of crack introduced from the decrease of molecular mobility. The serrations in stress-strain curve are observed below about 18 K. The deformation mode at 4.2 K seems to be appreciably different from that at 77 K. The brittleness at 4.2 K is observed in most organic materials as shown

LHe test

400 I

2

20C -'l

10%

0

800

0 Slrcin, %

b

z2

Epik01e 828 Hardener aromatic amine

Irradiated ol 5K

i

60O

LHe test

FRP Gloss fibre Epikote 828 Hardener acid anhydride

LHe test

/

i

I

'600

radiated 01 5K to 400

no

400 20C

03

200

2

I0% 0

0

--I

Strain, %

0

b

I

Strain,% C Epikote 828 Hardener acid anhydride/

krodioted at 5K

60(:

FRP Carbon fibre Epikote 828 Hardener acid anhydride

LHe test

LHe test

/,

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n° 40C

I 60(:

20C

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40C

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Fig. 5 Stress - strain curves of epoxy resins hardened by aliphatic amine a - and aromatic amine b - and acid anhydrice c - at 4.2 K before and after reactor irradiation at about 5 K. 1 - 0 rad; 2 - 1.0 x 1017 nvt;3": 8 x 10 8 R

L F 0

respectively. In this figure, the stress-strain curves at 4.2 K in irradiated specimens which are tested without any warm up after irradiation at about 5 K are also shown. The effect of irradiation on breaking stress in various materials tested at 77 and 4.2 K is summarized in Figs 7a and b, respectively.

CRYOGENICS.

at 5K

03 10%

0

I

A U G U S T 1980

0

10%

j q

Strain, %

Fig. 6 a - Stress - strain curves of glass fibre reinforced b - and carbon fibre reinforced at 4.2 K before and after reactor irradiation at a b o u t 5 K . 1 - O r a d ; 2 - 4 . 5 x 1 0 8 r a d n f : l . 2 x 1 0 1 7 n v t 3 ' : 3 . 3 x 108R3-1.1 x 109radnf3x1017nvt3":8x108 R

443

in Figs 2,5 and 6. However, the mechanical assessment of radiation resistance at 4.2 K could roughly be made from the data from mechanical tests at 77 K, comparing Figs 7a and b, Van der Voorde’ investigated the effect of 77 and 20 K irradiation on the mechanical properties of various organic materials. He also reported that the strength at 77 K of epoxy resin decreases sharply after 1O9rad. In the present experiments, a strength for epoxy resins decreases sharply in both tests at 4.2 and 77 K after low temperature irradiation of about lo9 rad. The powder filled epoxy resin hardened by aliphatic amine had the high modulus, but a relatively low elongation up to fracture was observed and the powder filler did not seem to be effective for radiation resistance in this epoxy resin.

kpel (polyimide) Epwe 828 olphotic amine 1 ( hordened

2

Epikote 828 oromotic omine I t hardened

2

I

’

1

lrrodioted ot 5K

I 0 rcid I I

Epikote 828 acid onhydride ( hordened I GFRP epikote 828, glass, ( acid onhydride J CFRP

:

(gZi,h;;;.)

a

I

I

I

200

0

‘I

’

I

I

I

I

400

I

600

800

Breok~ng stress, MPo

lrmdioted at 5K

LHe test

V-1

I Orod 2

I

1.1x10e rod n

2

(Pdyimide)

I

I 2

Epihote 828 acid anhydride ( hordened )

I 2

R

I

1

2

From the present work, it is evident that polyimide and fibre reinforced epoxies exhibit a good resistance to radiation after irradiation of lo9 rad.

:3x IO” nvt

7‘ :8x10e Epikote 828 olitic amine ) ( tmrdened Epikote 828 orcmotic omine ( hordened )

The authors would like to thank the member of LHTL group and JRR3, Japan Atomic Energy Research Institute, for their invaluable help. They would also like to thank Dr. S. Mori of Japan Atomic Energy Research Institute for encourage. ment in this study.

I

1

I

1

GFRP epikote 828, gbss, > ( acid onhydride

:

References

I

I

1 2

CFRP 2

b

I

0

I 200

I

Breaking

I I 400

I

I 600

I

3 I

4

stress, MPo

Fig, 7 a -Average breaking stressesat 77 K b -4.2 K before and after reactor irradiation of 3 x 10” nvt and +ydose of 8 x lo8 R at about 5 K

444

The mechanical properties of epoxy resins are temperature dependent. On the other hand, the mechanical properties of FRP are temperature independent since the stress-strain curves at 77 and 4.2 K are very similar. Therefore, the mechanical strength of FRP seems to be determined mainly by that of fibre glass. The radiation-induced failure of the strength of epoxy resins as a component of FRF’,results in only a slight degradation in the overall strength of FRF’,as shown in Fig. 6.

5

Abdou,M.A. JNuclMaterials 72 (1978) 147 van dex Voorde, M.H. IEEE Trans Nucl Sci 18 (1970) 784 ibid. 20 (1973) 693 Niih@ma, S., Okada, T. 6th International Conference on Magnet Technology (MT-6)) Bratislavia, Czechoslovakia (1977) 121 Takamura, S., Kato, T. Nonmetallic materials and composites at low temperatures, Clark, A.F. et al. (ed) Plenum Pub Corp New York, (1979) 1.55 Long, V.J., Kemohan, R.H., Coltman, Jr. R.R. Nonmetallic materials and composites at low temperatures, Clark, A.F. et al. ed. Plenum Pub Corp, New York, (1979) 141

CRYOGENICS.

AUGUST

1980