Fusion Engineering and Design 75–79 (2005) 189–192

Mechanical performance of magnet insulation materialsfabricated by the “Insulate-Wind-and-React” technique

K. Bittner-Rohrhofer, K. Humer∗, H. Fillunger, R.K. Maix, H.W. Weber

Atomic Institute of the Austrian Universities, Vienna University of Technology, 1020 Vienna, Austria

Available online 20 July 2005

Abstract

Superconducting magnet coils are usually fabricated according to the “Wind-React-Insulate-and-Transfer” technique. How-ever, the alternative “Insulate-Wind-and-React” technique could simplify the coil manufacturing process considerably. Aninsulation system designed for this process has been investigated. It consists of a R-glass fiber reinforcement heat treated at650◦C and impregnated afterwards with epoxy. For the mechanical material characterization, tensile, short-beam shear (SBS)and tension–tension fatigue tests were employed at 77 K. In addition, half of the SBS samples were reactor irradiated to a neutronfluence of 1× 1022 m−2 (E > 0.1 MeV) to check for radiation induced material degradation.© 2005 Elsevier B.V. All rights reserved.

Keywords: Fatigue; Tension; Interlaminar shear; Low temperatures; Reactor irradiation

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. Introduction

The high magnetic fields needed for several lowemperature applications, e.g. the magnets for fusionevices, require the application of Nb3Sn as the con-uctor material of superconducting coils. However, theiobium–tin composite requires a final heat treatmentt elevated temperatures (650–800◦C) to form the brit-

le superconducting intermetallic compound Nb3Sn.sually, such coils[1] are fabricated according to thetandard “Wind-React-Insulate-and-Transfer” (W-R-I-) technique, where the superconductor is wound and

∗ Corresponding author. Tel.: +43 1 58801 14156;ax: +43 1 58801 14199.

E-mail address: khumer@ati.ac.at (K. Humer).

heat treated first, before the insulation is appliedthe coil vacuum-impregnated with epoxy. In ordesimplify the manufacturing process of such coils anlower the costs, an alternative procedure, the “InsuWind-and-React” (I-W-R) technique is consideredthis case, the heat treatment is carried out on thecoil system, i.e. the turn and pancake (or layer) instion is applied prior to the reaction heat treatment.superconducting coil system is afterwards impregnwith epoxy resin to form a rigid block, i.e. the “transfestep can be avoided.

Nb3Sn high field coils for fusion devices neto withstand high electromagnetic forces and hvoltages in the case of a quench and therefore reexcellent mechanical and dielectric properties ofinsulation, even in the presence of a radiation e

920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.fusengdes.2005.06.025

190 K. Bittner-Rohrhofer et al. / Fusion Engineering and Design 75–79 (2005) 189–192

ronment [2,3]. Furthermore, the insulating materialhas to be highly heat resistant in order to withstandthe reaction heat treatment at∼650◦C. Bruzzone etal. [4] investigated various superconducting magnetinsulation systems for the “Insulate-Wind-and-React”technique. The materials were fabricated by impreg-nating various R-glass, quartz-glass or ceramic fiberreinforcements in epoxy resin. Especially for the puretapes manufactured from quartz fibers, they found apoor breaking strength combined with an extreme brit-tleness after the heat treatment due to some structuraltransformation of the fibers. Quartz fibers, therefore, donot meet the reinforcement requirements. Concerningthe fiber/epoxy composites, the experiments[4] con-firmed the feasibility to insulate coils according to thistechnique, when special care is taken with the choiceof the fibers. R-glass and ceramic fibers seem to besuitable.

In the present study, an insulation system for the“I-W-R” technique fabricated by European industry(Ansaldo, Italy) has been investigated. It consists ofa two-dimensional R-glass fiber reinforcement, heattreated first and impregnated afterwards with epoxy. Inorder to characterize the mechanical material perfor-mance under static load, tensile and short-beam shear(SBS) tests were carried out at 77 K. Furthermore,tension–tension fatigue tests were made to simulate thedynamic load conditions caused by the Lorentz forces.In addition, a set of SBS samples was irradiated toa fast neutron fluence of 1× 1022 m−2 (E > 0.1 MeV),i byr

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beams were applied for pressing the insulation dur-ing impregnation. This procedure was followed by theVPI process. The final laminate was then cut along thetwo longitudinal sides of the steel plate and two plateswere obtained. Because of this “wrapping-technique”the behavior of the laminate is significantly anisotropic.Therefore, the specimens were loaded under two differ-ent directions, i.e. parallel (0◦) and perpendicular (90◦)to the winding direction of the tapes[5].

Half of the SBS samples were irradiated at ambi-ent temperature (∼340 K) in the TRIGA reactor(Vienna) to a fast neutron fluence of 1× 1022 m−2

(E > 0.1 MeV), i.e. the ITER design fluence. Thereactor operates at a�-dose rate of 1× 106 Gy h−1,a fast neutron flux density of 7.6× 1016 m−2 s−1

(E > 0.1 MeV) and a total neutron flux density of2.1× 1017 m−2 s−1, respectively.

Both the static and dynamic tensile as well asthe SBS tests were done at 77 K with a servo-hydraulic MTS 810 Material Testing System. Theapparent interlaminar shear strength (ILSS) was mea-sured according to the ASTM D2399 standard. Thestatic ultimate tensile strength (UTS) was assessedon small specimens[6] with outer dimensions of70 mm× 10 mm× 4 mm (length× width× thickness)according to the standards DIN 53455 and ASTMD638. The tension–tension fatigue tests were doneaccording to ASTM D3479 in the load control modeat R = 0.1 using a sinusoidal load function with a fre-quency of 10 Hz. In order to obtain stress-lifetime dia-g lsr andi esw

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s int test)a -so o thew( v-i e.g.f lse-w ns

n order to check for material degradation inducedadiation.

. Experimental

The insulation system was manufacturednsaldo, Genova, Italy. The laminate consists of a-glass fiber reinforcing tape[5] vacuum-impregnateith an epoxy DGEBA system. Prior to the impreation process, the glass fiber tape was heat trea

nert atmosphere at 650◦C for 24 h. It was then wrappehalf-overlapped) around a steel plate. In order to gntee safe wrapping, both sides of the longitudteel plate were well rounded. In addition, Mylar wrapped around the plate to enable easy detachf the laminate after the vacuum pressure impre

ion (VPI) process. Two steel plates and a set of bo

rams (S/N curves, Wohler curves), various load leveanging from 80 to 40% of the UTS were chosennvestigated up to 106 cycles to failure. Four samplere measured at each data point.

. Results and discussion

The results obtained under static load conditionension and interlaminar shear (short-beam sheart 77 K are summarized inTable 1. Regarding the tenile tests, the UTS parallel (0◦) to the winding directionf the tapes is 492 MPa. Specimens perpendicular tinding direction of the tape (90◦) show a lower UTS

248 MPa) by∼50% than the 0◦ samples. This behaor is of great relevance for practical applications,or the TF-coils of ITER, as discussed in detail ehere [5]. Furthermore, fractographic investigatio

K. Bittner-Rohrhofer et al. / Fusion Engineering and Design 75–79 (2005) 189–192 191

Table 1Ultimate tensile strength (UTS) and interlaminar shear strength(ILSS) measured parallel (0◦) and perpendicular (90◦) to the wind-ing direction of the reinforcement tapes for the unirradiated and theirradiated state at 77 K

Neutron fluence m−2

(E > 0.1 MeV)Fiberdirection (◦)

UTS (MPa) ILSS (MPa)

0 0 492± 16 76± 80 90 248± 40 53± 6

1× 1022 0 – 26± 71× 1022 90 – 26± 4

of the tensile specimens show very smooth fracturesurfaces without any pull-out of fibers or rovings. Weassume that the heat treatment leads to some structuraltransformation of the fibers resulting in an extreme brit-tleness and, thus, a poor breaking strength comparedto the material fabricated by the usual “W-R-I-T” tech-nique, which was investigated in an earlier study (cf.Fig. 9 in Ref.[5]). This W-R-I-T material was addition-ally reinforced with a Kapton film, in order to improveits dielectric performance. A comparison of the earlierW-R-I-T data with the present data obtained on the I-W-R material, shows that the UTS parallel (0◦) to thewinding direction of the tapes was 844 MPa instead of492 MPa, i.e. by about 70% higher, and perpendicu-lar to the winding direction (90◦) 286 MPa instead of248 MPa, i.e. by about 15% higher.

Concerning the tension–tension fatigue experi-ments, the S/N curves assessed on the unirradiatedmaterial loaded parallel (0◦) and perpendicular (90◦)to the winding direction of the tape at 77 K are shownin Fig. 1. In 0◦ direction, the residual strength decreasescontinuously up to 106 cycles, without reaching the lifeendurance limit. For specimens loaded in 90◦ direction,both the initial and all the residual strength data arequite poor, i.e. in a range between 248 and 149 MPa.A comparison of the residual strength data in theITER relevant range and beyond (i.e. in the range from3× 104 to 106 cycles) between the W-R-I-T (cf. Fig.9 in Ref. [5]) and the I-W-R material shows, that theresidual strength parallel (0◦) to the winding directiono eci-m thet out3 m-p e oft ate-

Fig. 1. Normalized and absolute tension–tension stress-lifetime dia-grams for 0◦ (�) and 90◦ (©) loaded specimens obtained at 77 K.

rial. Especially under dynamic load in 90◦ direction,these Kapton layers lead to an additional reduction ofthe material performance and, consequently, to lowerresidual strength. In view of the ITER specifications[1,7,8], the I-W-R materials do not fulfill the designcriteria, even without irradiation to the ITER designfluence.

Concerning the investigations under shear load(short-beam shear test), the ILSS is found to amountto 76 MPa (0◦) and 53 MPa (90◦) in the unirradi-ated state, respectively (cf.Table 1). After irradiationto 1× 1022 m−2 (E > 0.1 MeV), the ILSS degrades toabout 25 MPa for both load directions, which is unac-ceptable for the ITER coils[8]. New types of resinsand fibers should be combined and tested, in order todevelop materials for the “Insulate-Wind-and-React”technique, which fulfill the ITER design criteria forthe magnet insulation.

4. Summary and conclusion

An insulation system for the “Insulate-Wind-and-React” technique has been investigated. A two-dimensional R-glass fiber reinforcing tape was heattreated first and impregnated afterwards with epoxy.In order to characterize the material performanceunder appropriate load conditions, both static tensile

f the tapes is about equal, whereas the W-R-I-T spens cut perpendicular to the winding direction of

ape (90◦) show a lower residual strength (by ab5%) than the I-W-R material. However, such a coarison is not completely straightforward becaus

he absence of the Kapton layers in the I-W-R m

192 K. Bittner-Rohrhofer et al. / Fusion Engineering and Design 75–79 (2005) 189–192

and short-beam shear tests as well as tension–tensionfatigue tests were carried out at 77 K. In addition, a setof SBS samples was irradiated to a fast neutron fluenceof 1× 1022 m−2 (E > 0.1 MeV), in order to check formaterial degradation induced by radiation. The resultscan be summarized as follows:

• The ultimate tensile strength in the winding directionof the tape (0◦) is about 500 MPa. A reduction of theUTS by about 50% was found on samples testedperpendicular to this direction (90◦).

• Smooth fracture surfaces without any pull-out offibers or rovings were observed. A structural trans-formation of the fibers during their heat treatmentmight lead to their extreme brittleness resulting in apoor breaking strength.

• The fatigue performance of the material is poor. At106 cycles, the residual strengths are 230 MPa (0◦)and 150 MPa (90◦) in the unirradiated state.

• The interlaminar shear strength is about 75 MPa (0◦)and 50 MPa (90◦) in the unirradiated state, respec-tively. After irradiation, about 25 MPa were mea-sured for both load directions.

With respect to ITER, the observed fiber brittle-ness and the resulting low breaking strength of thefibers lead to a poor dynamic material performanceunder the pulsed operating conditions of the ITERcoils, even without irradiation. Furthermore, the inter-laminar shear strength assessed after irradiation to theITER design fluence is unacceptably low and does notm d inv ndt ent

for the “Insulate-Wind-and-React” technique and per-tinent test programs are strongly recommended.

Acknowledgements

Technical assistance by Mr. E. Tischler and Mr. H.Hartmann is acknowledged. This work has been carriedout within the Association EURATOM-OEAW.

References

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[3] K. Humer, H.W. Weber, R. Hastig, H. Hauser, H. Gerstenberg,Dielectric strength, swelling and weight loss of the ITER toroidalfield model coil insulation after low temperature reactor irradia-tion, Cryogenics 40 (2000) 295–301.

[4] P. Bruzzone, K. Nylund, W.J. Muster, Electrical insulation systemfor superconducting magnets according to the wind and reacttechnique, Adv. Cryog. Eng. 36 (1990) 999–1006.

[5] K. Bittner-Rohrhofer, K. Humer, H.W. Weber, Low-temperaturetensile strength of the ITER-TF model coil insulation sys-tem after reactor irradiation, Cryogenics 42 (2002) 265–272.

[6] P. Rosenkranz, K. Humer, H.W. Weber, Influence of specimensize on the tension-tension fatigue behaviour of fibre-reinforcedplastics at room temperature and at 77 K, Cryogenics 40 (2000)155–158.

[ . 24,

[ for-

eet the ITER EDA design criteria. Therefore, aniew of the simpler coil manufacturing process ahe resulting lower costs, further material developm

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8] ITER Design Description Document DDD 11 Magnet, Permance Analysis/ Structural Analysis, pp. 20, 45, 2001.