Fusion Engineering and Design 58–59 (2001) 289–293

Irradiation effects on the fatigue behaviour of an ITERcandidate magnet insulation material in tension and

interlaminar shear

P. Rosenkranz a, K. Humer a,*, H.W. Weber a, E.R. Hodgson b

a Atominstitut der osterreichischen Uni�ersitaten, Stadionallee 2, 1020 Vienna, Austriab Fusion Materials Research Unit, Euratom/CIEMAT Fusion Association, A�da. Complutense 22, 28040 Madrid, Spain

Abstract

The magnets of ITER and future fusion plants will be subjected to fast neutron and �-radiation over their lifetime,which influences the mechanical properties of the magnet insulation materials. The ITER candidate materialCTD-112P, a two-dimensionally woven S2-glass-fibre reinforced epoxy, was investigated at 77 K in the static andfatigue modes in tension and interlaminar shear. The material was irradiated in the TRIGA reactor (Vienna) withseveral neutron fluences and by a 60Co-source (Madrid) up to 107 Gy. The results of the tensile tests show a minorreduction (�5%) in the ultimate tensile strength caused by irradiation and a reduction in the tensile fatigue lifetimeonly at the highest neutron fluence (2×1022 m−2, E�0.1 MeV). The shear properties of the material are affectedneither in the static nor in the fatigue mode up to a fast neutron fluence of 5×1021 m−2, i.e. a total absorbed doseof 2.8×107 Gy. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Irradiation effects; Fatigue behaviour; ITER magnet insulation material

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1. Introduction

The mechanical properties of insulation materi-als for the superconducting magnets in ITER andfuture fusion plants, i.e. glass-fibre reinforcedplastics, have been identified as an area of concernfor the long-term operation of such magnets [1].The ultimate tensile strength (UTS) and above allthe interlaminar shear strength (ILSS) and theirperformance under dynamic load, corresponding

to the pulsed operation of a TOKAMAK-confi-nement system [2,3], are sensitive indicators ofmaterial failure in fibre reinforced laminates espe-cially at cryogenic temperatures. To simulatethese conditions, low frequency fatigue measure-ments at 10 Hz were made up to one millioncycles. However, due to the space limitations inall irradiation facilities, the tests have to be doneon samples that are considerably smaller thanthose required for standard test conditions. Theinfluence of the specimen geometry on the UTSand the ILSS under static and dynamic loadconditions are discussed in Refs. [4,5]. Refs [6,7]give a good overview about existing ILSS testmethods and their disadvantages.

* Corresponding author. Tel.: +43-15880114156; fax: +43-15880114199.

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

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0920 -3796 (01 )00573 -7

P. Rosenkranz et al. / Fusion Engineering and Design 58–59 (2001) 289–293290

2. Test and irradiation procedures

CTD-112P is a tetrafunctional, TGDM, epoxyresin that contains 15 wt% alumina (Al2O3) fillerin the resin and is used as a hot-melt pre-pregsystem (solvent-less). The laminates contain anominal 51% fibre volume fraction of boron-freeS-2 glass fabric; weave style 6580 with a silanefinish. Samples for both the double lap shear (cf.Fig. 1B) and the tensile tests were cut in such away that the load was applied in warp direction.The dimensions of the samples are shown in Fig.1. The tension– tension fatigue tests were madeaccording to the ASTM-standard D3479 and thesame test procedure was also used for the staticand dynamic double lap shear tests. The ILSS iscalculated by simply dividing the failure load bythe shear area on both sides between the notchand the hole (see Fig. 1B). More experimentaldetails are given in Refs. [4,5].

The samples were irradiated at ambient temper-ature (�340 K) in the TRIGA reactor in Vienna(�-dose rate: 1×106 Gy h−1, total neutron fluxdensity: 2.1×1017 m−2 s−1, fast neutron fluxdensity: 7.6×1016 m−2 s−1 (E�0.1 MeV)) withfast neutron fluences of 5×1021 m−2, 1×1022

m−2 and 2×1022 m−2, respectively. According to

Fig. 2. Swelling and weight loss of CTD-112P at several totalabsorbed dose levels (�: swelling; �: weight loss).

the computer-code SPECTER [8], this corre-sponds to a total absorbed dose in the material of2.8×107, 5.6×107 and 11.2×107 Gy, respec-tively. In addition, some double lap shear speci-mens were exposed to a dose of 107 Gy from a60Co-source. Swelling and weight loss of the sam-ples were measured prior to testing at 77 K. Theinfluence of the irradiation temperature on themechanical properties of various fibre reinforcedplastics was discussed in Ref. [9].

3. Results

Fig. 2 shows the swelling and the weight loss ofthe material caused by the irradiation. It shouldbe noted, that significant changes occur onlyabove the total absorbed dose that corresponds toa fast neutron fluence of 5×1021 m−2 (E�0.1MeV). The effects are mainly related to the pro-duction of hydrogen during irradiation. Similarvalues have been obtained by Reed et al. [10].

The UTS (Table 1) decreases to a minor extent(�5%) after fast neutron irradiation, and remainsunchanged at both dose levels. The UTS is deter-mined by fibre-specific failure mechanisms, whichare not affected at these neutron dose levels.Splitting and fraying of fibre bundles (‘brush’)characterize the failure in the static mode.

Fig. 1. Sample geometries and dimensions (mm) for (A) ten-sile; and (B) double lap shear tests.

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Table 1Mechanical properties of CTD-112P in the unirradiated stateand after several irradiations in the TRIGA reactor (Vienna)and with the 60Co-source (Madrid)

Ultimate tensileTotal absorbed Interlaminar sheardose (Gy) strength (ILSS)strength (UTS)

(MPa) (MPa)

Unirradiated 33�3938�3332�4–1×107 (Madrid)

2.8×107 (Vienna) 890�34 32�227�4–5.6×107 (Vienna)

889�2811.2×107 (Vienna) 22�3

maximum stress of 310 MPa in the unirradiatedand of 295 MPa in the irradiated state at onemillion cycles, which meets the conditions set forITER (104–106 cycles, 100–400 MPa) [2]. At adose level of 11.2×107 Gy, the material sustainsone million cycles at 0.25 UTS or a maximum of220 MPa.

In Table 1, the dependence of the ILSS, ob-tained with the double lap shear test, on the doseis listed. A problem with the double lap shear testlies in a certain dependence of the obtained ILSSon the ratio of the shear length L to the samplethickness t [4,11]. Becker [12] suggested an extrap-olation of the measured ILSS for the single lapshear test to L/t=0 to obtain the ‘true’ ILSS.This assumption may also hold for double lapshear specimens. The initial results of a scalingprogram [4] showed an increasing ILSS with de-creasing shear length L, as obtained also by Evanset al. [11], while the fatigue behaviour was almostunaffected by the variation of the shear length.The first results of an FE-analysis for our testgeometry indicate that the ILSS at L/t=1 isunderestimated by up to 50% when employing ourpresent calculation method. This would also agreewith the results obtained by Reed et al. [10] fromthe shear compression tests of the same material.Nevertheless, the ILSS (Table 1) obtained fromthe present method of calculation shows the de-pendence on the total absorbed dose nicely, sincethe results of the FE-analysis and of additionalacoustic emission measurements show that delam-ination is indeed the failure mode of this testmethod. At doses above 2.8×107 Gy, the epoxymatrix gets seriously damaged, which leads to adecrease of the ILSS. This also agrees with theresults obtained by Reed et al. [10] at a fastneutron fluence of 1.8×1022 m−2.

Fig. 4 shows the shear fatigue behaviour of thematerial at several dose levels. For the unirradi-ated material, after pure �-irradiation of 107 Gyand also after a total absorbed dose of 2.8×107

Gy of combined neutron and �-irradiation, thefatigue life of the material is about one millioncycles at a maximum stress level of 0.75 ILSS.This corresponds to a maximum stress of 25 MPain both the unirradiated and the irradiated stateat one million cycles. Keeping in mind that the

Fig. 3 shows the influence of the absorbed doseon the fatigue behaviour in tension. The tensilefatigue lifetime remains unchanged at a total ab-sorbed dose of 2.8×107 Gy, while the highestdose causes a rapid degradation of the fatiguelifetime. This corresponds to the observed be-haviour of swelling and weight loss. The tensilefatigue lifetime is a matrix-dominated property,which degrades significantly when the matrix be-comes brittles under irradiation. In the dynamicmode, the specimens fail with a transverse frac-ture line. For the unirradiated material and after atotal absorbed dose of 2.8×107 Gy, the materialsustains about one million cycles at a maximumstress level of 0.33 UTS. This corresponds to a

Fig. 3. Tension– tension fatigue results for CTD-112P at sev-eral total absorbed dose levels. The S–N curves are normal-ized by the UTS, cf. Table 1 (�: unirradiated; � : 2.8×107

Gy; �: 11.2×107 Gy).

P. Rosenkranz et al. / Fusion Engineering and Design 58–59 (2001) 289–293292

Fig. 4. Shear fatigue behaviour of CTD-112P at several totalabsorbed dose levels. The S–N curves are normalized by theILSS, cf. Table 1 (�: unirradiated; � : 1×107 Gy; �: 2.8×107 Gy; �: 5.6×107 Gy; : 11.2×107 Gy).

� The UTS shows only minor reductions (�5%)caused by fast neutron irradiation, and remainsunchanged by increasing the dose level to11.2×107 Gy.

� The tensile fatigue behaviour remains un-changed up to a total absorbed dose of 2.8×107 Gy, while a four-times higher dose levelleads to a rapid degradation of the fatiguelifetime.

� Above 2.8×107 Gy, the material damagecaused by the irradiation leads to a decrease ofthe ILSS (�20–30%).

� Up to 2.8×107 Gy the shear fatigue behaviourof the material is unchanged, while at higherdose levels the fatigue lifetime is difficult toassess properly.These results demonstrate that the material sus-

tains a fast neutron fluence of 5×1021 m−2 (E�0.1 MeV), i.e. a total absorbed dose of 2.8×107

Gy, without serious damage. Further, the fatiguelifetime in tension and in interlaminar shear arenot affected at that dose level. Thus, the materialproperties seem to meet the ITER operating con-ditions in the static as well as in the fatigue mode.

Acknowledgements

The authors are greatly indebted to CompositeTechnology Development Inc., Boulder, USA, forproviding us with the test samples. Technical sup-port by H. Niedermaier and E. Tischler is ac-knowledged. This work has been carried outwithin the Association EURATOM–OEAW.

References

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present calculation of the ILSS underestimates theactual shear strength of the material, the ITERconditions (104–106 cycles, 10–30 MPa [2], �35MPa [10]) are again satisfied. At higher absorbeddose levels, many samples became so brittle thatthey failed prematurely during the fatigue testprocedure. It was therefore hardly possible to getinformation about the fatigue lifetime of the ma-terial at high doses, but the few results are alsoshown in Fig. 4. The arrows indicate that the testwas terminated, because the specimen did not fail.

4. Summary

Mechanical tests on irradiated CTD-112P, anITER candidate magnet insulation material, werecarried out in tension and interlaminar shear at 77K both under static and dynamic load. The mate-rial was irradiated with several dose levels of pure�-radiation and combined neutron and �-radia-tion to assess their influence on the mechanicalproperties, which represent an area of concern forthe long-term operation of such magnets. Theresults can be summarized as follows:� Significant swelling and weight loss of the ma-

terial occur only above a fast neutron fluenceof 5×1021 m−2 (E�0.1 MeV), i.e. a totalabsorbed dose of 2.8×107 Gy.

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