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Influence of reactor irradiation on the mechanical behavior of ITER TF coil candidate insulation systems
Fusion Engineering and Design 66 /68 (2003) 1201 /1207 www.elsevier.com/locate/fusengdes
Influence of reactor irradiation on the mechanical behavior of ITER TF coil candidate insulation systems K. Bittner-Rohrhofer *, K. Humer, H. Fillunger, R.K. Maix, Z.D. Wang, H.W. Weber Atomic Institute of the Austrian Universities, Stadionallee 2, A-1020 Vienna, Austria
Abstract Extensive material tests have to be performed in order to obtain information on the radiation induced change in the mechanical behavior of insulating materials for the ITER Toroidal Field (TF) coil. The investigated insulation systems are R-glass fiber reinforced tapes, vacuum impregnated with a DGEBA epoxy resin and interleafed with Kapton Hfoils. According to the actual operating conditions of ITER /FEAT, the systems were irradiated in the TRIGA reactor (Vienna, Austria) to neutron fluences of 5 /1021 and 1 /1022 m 2 (E /0.1 MeV). Static tensile, short-beam-shear (SBS) as well as double-lap-shear (DLS) tests were carried out at 77 K prior to and after irradiation. Furthermore, results on swelling and weight loss as well as on the material properties under tension /tension fatigue loading conditions are presented. # 2003 Elsevier B.V. All rights reserved. Keywords: Reactor irradiation; Mechanical behavior; ITER TF coil
1. Introduction The superconducting magnets of ITER [1,2] require a high-quality material performance of the insulation due to the exposure to low temperature neutron and gamma irradiation. In principle, these insulation systems are glass fiber reinforced plastics (GFRPs) consisting of boron-free glass fibers and matrix materials (e.g. multifunctional epoxy resins). In addition, the ITER specifications
* Corresponding author. Tel.: /43-1-58801-14188; fax: / 43-1-58801-14199. E-mail address: [email protected] (K. Bittner-Rohrhofer).
require multiple electrical barrier layers, such as Kapton foils. However, the stability and the lifetime of such insulation systems change under these demanding conditions. Thus, the performance of materials suggested for the ITER fusion device has to be assessed under sequential irradiation conditions [3 /5]. In the present contribution, we report on two insulation systems primarily applied as turn and ground insulation for the Toroidal Field Model Coil (TFMC) [6] of ITER. Static and dynamic measurements in tension and under interlaminar shear loading conditions were done at 77 K prior to and after reactor irradiation to fluences of 5 / 1021 and 1 /1022 m 2 (ITER design fluence level factor 2 above the maximal expected one).
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2. Materials and test procedures
3. Results and discussion
The insulation systems were manufactured by Ansaldo (Italy) and by Alstom-MSA (France). Both laminates consist of combined Kapton-H foil/R-glass-fiber reinforcement tapes, which are vacuum pressure impregnated (‘‘VPI’’-process) with epoxy DGEBA systems (Araldite F type for Ansaldo and MY 745 for Alstom) fabricated by Vantico (Switzerland). The dimensions of the Kapton tapes are 20/25 /0.05 /0.075 mm (width /thickness). The Ansaldo tapes are provided with an additional adhesive to fix the position of the Kapton on the glass fabric for better handling. A detailed description of the manufacturing procedure can be found in [7]. The same procedure was also used for the Alstom plates without Kapton foils (designated as ‘‘A’’)1. Because of the anisotropic material properties, the tensile and DLS-specimens [8] (outer dimensions for Ansaldo: 70 /10 /2 mm and for Alstom: 70 /10 /4 mm) and the SBS-specimens (Ansaldo: 2.5 mm and Alstom: 4 mm thickness) were loaded under two different directions, i.e. parallel (08) and perpendicular (908) to the winding direction of the reinforcement tapes. All static and dynamic experiments were carried out at 77 K using a servo-hydraulic MTS 810 testing device. The ultimate tensile strength (UTS) was measured according to DIN 53455 and ASTM D638. The interlaminar shear strength (ILSS) was determined from the DLS-test and SBS-test according to the ASTM D2344 standard with a span-to-thickness ratio of 4:1 for Ansaldo and 5:1 for both Alstom systems. To simulate the pulsed conditions of ITER /FEAT, tension/tension fatigue experiments (ASTM D 3479) at 10 Hz and at a minimum-to-peak ratio of R /0.1 were done up to 106 cycles. Four or five samples were measured for each load level. A standard deviation of /20% was observed.
An overview on the static UTS and ILSS as well as on swelling (¥) and weight loss (9) of the insulation systems is given in Tables 1 and 2.
1
Alstom samples provided with Kapton-foils are designated as ‘‘AK’’.
3.1. Swelling and change of mass The Ansaldo samples show significant swelling and weight loss (Table 1) after irradiation to a neutron fluence of 5 /1021 and 1 /1022 m 2 (E / 0.1 MeV). In particular, swelling is indicated by the formation of bubbles inside the laminate (Fig. 1) after irradiation to the highest dose level. However, this effect was only observed in the clamping area of those samples cut parallel to the tape winding direction. The formation of bubbles is presumably caused by the radiation-induced degradation of the stability of the additional adhesive (for more details see [7]). No systematic change in dimensions in the through-thickness direction or in mass was found for both Alstom systems. 3.2. SBS-tests on 08 and 908 Ansaldo specimens The measurements (Table 1) do not show a significant influence of the tape winding direction on the ILSSSBS. Irradiation to a fast neutron fluence of 5 /1021 m 2 (E/0.1 MeV) leads to a degradation of the ILSSSBS by 15% for 08 and by 10% for 908 samples, respectively. Further irradiation to the highest dose level leads to a decrease of the ILSSSBS to /35 MPa. 3.3. Static and fatigue tensile tests on Ansaldo specimens Table 1 shows the results on the UTS at 77 K before and after irradiation to a neutron fluence of 1 /1022 m 2 (E /0.1 MeV). It is obvious that the UTS of samples loaded parallel to the winding direction (08) is significantly higher (by a factor of /3). However, the material strength in both directions decreases considerably after irradiation to 544 MPa (08) and to 131 MPa (908), respectively. Further, an adverse influence of the adhesive is observed for 908 samples, which break
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Table 1 Results of the static measurements at 77 K of the Ansaldo system before and after irradiation Ansaldo 08
Ansaldo 908
Fluence (E/0.1 MeV) (m 2)
UTS (MPa)
ILSSSBS (MPa)
UTS (MPa)
ILSSSBS (MPa)
Unirradiated 5 /1021 1 /1022
8449/41 / 5449/54
459/7 389/6 349/4
2869/11 / 1319/10
419/5 379/4 359/2
¥ (%)
9 (%)
/ /2.49/0.5 /3.09/0.23
/ /1.29/0.29 /1.49/0.3
remains constant up to 104 cycles and then predecreases continuously up to more than 106 fercycles (Fig. 2a) at a load limit of 60% (79 ablMPa). Accordingly, the material fails without y at reaching a well defined life endurance limit. this Tension /tension fatigue measurements on 08 losamples show a similar behavior before irradiacation, i.e. a rapid decrease of the Wo¨hler-curve in tion the range from 80 to 20% of the static UTS (Fig. pri2b). A shift to higher numbers of cycles is found or Fig. 1. Formation of bubbles inside the Ansaldo system after for load levels B/0.6 (B/326 MPa). A well to irradiation to a fast neutron fluence of 1 /1022 m 2 (E/0.1 defined fatigue resistance could not be found at as MeV) as indicated by the arrows. a load limit of 30% (163 MPa), even though more well than 106 cycles were run. as after irradiation. A detailed discussion of possible reasons for the remarkably (70 /75%) lower UTS of 908 samples and the consequences 3.4. SBS-tests on 08 and 908 Alstom specimens of the adhesive for practical applications in the TF coils can be found in [7]. No systematic influence of the Kapton foil on A life endurance limit of 35% (100 MPa) was the ILSSSBS was observed for the 08 and 908 observed for unirradiated 908 samples. The fatigue samples, respectively (cf. Table 2). Both Alstom behavior changes drastically after irradiation. It systems loaded parallel to the tape winding direcTable 2 Results of the static measurements at 77 K of both Alstom systems before and after irradiation ‘‘A’’ 08
‘‘A’’ 908
Fluence (E/0.1 MeV) (m 2)
UTS (MPa) ILSSSBS (MPa) ILSSDLSa (MPa)
UTS (MPa) ILSSSBS (MPa) ILSSDLSa (MPa)
Unirradiated 5 /1021 1 /1022
8459/43 7499/79 6959/24 ‘‘AK’’ 08
3459/14 / 1149/20 ‘‘AK’’ 908
Fluence (E/0.1 MeV) (m 2)
UTS (MPa) ILSSSBS (MPa) ILSSDLS (MPa)
UTS (MPa) ILSSSBS (MPa) ILSSDLS (MPa)
Unirradiated 5 /1021 1 /1022
7569/51 6659/15 6129/45
3879/21 / 2149/16
a
No FE correction employed.
809/4 449/3 319/4
819/4 509/4 359/5
479/4 379/4 139/1
349/1 299/2 179/2
779/4 379/4 249/3
759/4 459/6 279/4
449/7 / 129/3
389/2 / 159/2
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Fig. 2. (a, b) Tension /tension stress lifetime diagrams for 08 (a) and 908 (b) loaded Ansaldo specimens before and after reactor irradiation to a fast neutron fluence of 1 /1022 m 2 (E/0.1 MeV) measured at 77 K. The measurements were stopped manually above 106 cycles, as indicated by the arrows.
tion show a value of /80 MPa prior to irradiation, which degrades after irradiation to the ITER design fluence level by 61% for ‘‘A’’ and by 57% for ‘‘AK’’, respectively. The results on the unirradiated 908-samples are lower by 4 /7%. Irradiation to 1 /1022 m 2 leads to a considerable decrease of the ILSSSBS to 24 /27 MPa.
m 2. The radiation-induced degradation by / 22% for ‘‘A’’ and 15% for ‘‘AK’’ does not allow further fatigue measurements due to the end limit of the test system. A detailed discussion of these results is given in [10]. 3.6. Static and fatigue tensile tests on Alstom specimens
3.5. Shear fatigue tests on 08 Alstom specimens In comparison to the SBS results, the static ILSS obtained from the DLS-test was lower by /50%, as expected from FE calculations [9]. The fatigue life in the unirradiated state is about 106 cycles at a maximum load level of 0.5 ILSSDLS (/24 MPa) for ‘‘A’’ and 0.6 (/21 MPa) ILSSDLS for ‘‘AK’’ (Fig. 3a, b). However, a fatigue life endurance limit was not found for both GFRPs after irradiation to a neutron fluence of 5 /1021
The static tests (Table 2) show a significant difference by up to /60% between the UTS of 08 and 908 samples before irradiation to a neutron fluence of 1 /1022 m 2. The material strength of ‘‘A’’ as well as of ‘‘AK’’ degrades drastically after irradiation in both directions by /19% (08) and by up to 67% (908). A poor fatigue behavior, indicated by a rapid decrease in the range from 80 to 20% of the UTS before irradiation, was observed in all Wo¨hler-
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Fig. 3. (a, b) Shear fatigue behavior of DLS Alstom specimens without Kapton (a) and with Kapton (b) before and after irradiation to a neutron fluence of 5 /1021 m 2 (E/0.1 MeV). The test was interrupted manually after 1 million cycles, as indicated by the arrows.
curves (Fig. 4a, b) measured on a subset of 08 samples. However, neutron irradiation to fluences of 5 /1021 m 2 and of 1 /1022 m 2 (E /0.1 MeV) leads to an unexpected shift to a higher number of cycles for stress levels B/0.6 UTS (Fig. 4a, b), as also observed for the Ansaldo system. Further, the fatigue process is still in progress at a load limit of 20/25% ( /140 /150 MPa) in both Alstom systems after irradiation to 1 /1022 m 2 (for more details see [10]).
4. Summary The turn and ground insulation of the TFMC were assessed under static and dynamic load at 77 K prior to and after irradiation to neutron fluences
of 5 /1021 and 1 /1022 m 2 (E /0.1 MeV). The force was applied parallel and perpendicular to the wrapped tape. The samples were further investigated under tension /tension fatigue load. For comparison, the ground insulation was also tested without Kapton foil. The main results can be summarized as follows: . Significant swelling (3%) and weight loss (1.4%) were observed in the Ansaldo material. Bubble formation was found inside the laminate. No systematic dimensional change in the throughthickness direction and in weight was found for the Alstom material. . Both tape winding directions show a similar interlaminar shear strength (ILSSSBS, shortbeam-shear (SBS) test) in the range from 75 to
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Fig. 4. (a, b) Tension /tension stress lifetime diagrams for 08 loaded Alstom specimens without Kapton (a) and with Kapton (b) before and after reactor irradiation to a fast neutron fluence of 5 /1021 and 1 /1022 m 2 (E/0.1 MeV) measured at 77 K. The measurements were stopped manually above 106 cycles, as indicated by the arrows.
81 MPa for the Alstom systems. The ILSSSBS of the Ansaldo system is approximately two times lower than that of the Alstom insulation. Irradiation to 1 /1022 m 2 leads to a ILSSSBS for Alstom systems of 31/35 MPa (08 direction) and of 24/27 MPa (908 direction). The Ansaldo samples degrade at the same fluence by up to 24%. The fatigue life of the Alstom double-lapshear (DLS) samples is about 106 cycles at a load limit of 50/60%. However, no fatigue resistance could be obtained after irradiation to 5 /1021 m 2 (E /0.1 MeV) due to the rather low ILSSDLS of 29/37 MPa and experimental limitations in this range. . All GFRPs loaded parallel to the tape-winding direction (08 specimen) show the highest UTS, (756 /845 MPa), which decreases after irradiation to a neutron fluence of 1 /1022 m 2 by 35% (Ansaldo) and by 19% (both Alstom systems), respectively. Moreover, a poor fatigue life endurance limit at 20/25% was observed in
both unirradiated materials. After irradiation, all lifetime diagrams show a shift to a higher number of cycles for load levels B/0.6. Consequently, a defined fatigue resistance could not be established, even though more than 106 cycles were run. . In comparison to 08 specimens, a drastically lower (60 /70%) UTS was obtained in all 908 specimens. Especially, transverse loaded Ansaldo samples show a rather poor fatigue behavior, i.e. stress levels of 100 MPa before and of 79 MPa after irradiation. With respect to numerical investigations by Jong [11] of the ITER-team, the maximal tensile stress in the ground insulation is /35 MPa. Taking safety margins, the properties of the Ansaldo and the Alstom insulation are close to the limit of the ITER demands. Since all systems show a reduced mechanical integrity at a fluence of 5 /1021 m 2 and even more at the ITER design
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fluence of 1 /1022 m 2, the materials require basic improvements in order to ensure a safe performance at those parts of the coils, where maximal loads are expected. Improved radiation resistance of the epoxy resins and more uniform wrapping and compaction during the fabrication process would increase the interfacial bonding inside the laminates and thus, the stability of the insulation. Adhesives between the sandwich tape must be avoided because of their significant adverse influence on the material performance, especially in the 908 direction under static and dynamic load.
Acknowledgements Technical assistance by Mr H. Niedermaier and Mr H. Hartmann is acknowledged. This work has been carried out within the Association EURATOM-OEAW.
References [1] R. Aymar, ITER R&D: executive summary: design overview, Fusion Eng. Des. 55 (2001) 107 /118. [2] T. Mizoguchi, ITER R&D: executive summary: R&D overview, Fusion Eng. Des. 55 (2001) 119 /136.
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[3] H.W. Weber, E. Tschegg, Test program for mechanical strength measurements on fiber reinforced plastics exposed to radiation environments, Adv. Cryog. Eng. 36 (1990) 869 /875. [4] P. Rosenkranz, K. Humer, H.W. Weber, Reactor Irradiation effects on the fatigue behavior of CTD-112 P at 77 K in tension and interlaminar shear, Adv. Cryog. Eng. 46 (2000) 175 /180. [5] H. Fillunger, H. Gerstenberg, K. Humer, R.K. Maix, K. Okuno, P. Rosenkranz, H.W. Weber, N. Wiegner, Irradiation and mechanical testing of ITER relevant magnet insulation, Fusion Eng. Des. 58 /59 (2001) 135 /140. [6] R.K. Maix, H. Fillunger, F. Hurd, E. Salpietro, N. Mitchell, O. Libeyre, P. Decool, et al., Completion of the ITER toroidal field model coil, Fusion Eng. Des. 58 /59 (2001) 159 /164. [7] K. Bittner-Rohrhofer, K. Humer, H.W. Weber, Lowtemperature tensile strength of the ITER-TF model coil insulation system after reactor irradiation, Cryogenics 42 (2002) 265 /272. [8] P. Rosenkranz, K. Humer, H.W. Weber, Influence of the test geometry on the fatigue behavior of fiber reinforced plastics in tension and interlaminar shear at RT and 77 K, Adv. Cryog. Eng. 46 (2000) 181 /187. [9] D.H. Pahr, F.G. Rammersdorfer, P. Rosenkranz, K. Humer, H.W. Weber, A study of short-beam-shear and double-lap-shear specimens of glass fabric/epoxy composites, Composites: Part B 33 (2002) 125 /132. [10] K. Bittner-Rohrhofer, K. Humer, H. Fillunger, R.K. Maix, H.W. Weber, Mechanical behavior of the ITER TF-Model Coil ground insulation system after reactor irradiation, Cryogenics 42 (2002) 697 /704. [11] C.T.J. Jong, Private communication: ‘‘Stresses in insulation’’, 9 July 2002.
Influence of reactor irradiation on the mechanical behavior of ITER TF coil candidate insulation systems K. Bittner-Rohrhofer *, K. Humer, H. Fillunger, R.K. Maix, Z.D. Wang, H.W. Weber Atomic Institute of the Austrian Universities, Stadionallee 2, A-1020 Vienna, Austria
Abstract Extensive material tests have to be performed in order to obtain information on the radiation induced change in the mechanical behavior of insulating materials for the ITER Toroidal Field (TF) coil. The investigated insulation systems are R-glass fiber reinforced tapes, vacuum impregnated with a DGEBA epoxy resin and interleafed with Kapton Hfoils. According to the actual operating conditions of ITER /FEAT, the systems were irradiated in the TRIGA reactor (Vienna, Austria) to neutron fluences of 5 /1021 and 1 /1022 m 2 (E /0.1 MeV). Static tensile, short-beam-shear (SBS) as well as double-lap-shear (DLS) tests were carried out at 77 K prior to and after irradiation. Furthermore, results on swelling and weight loss as well as on the material properties under tension /tension fatigue loading conditions are presented. # 2003 Elsevier B.V. All rights reserved. Keywords: Reactor irradiation; Mechanical behavior; ITER TF coil
1. Introduction The superconducting magnets of ITER [1,2] require a high-quality material performance of the insulation due to the exposure to low temperature neutron and gamma irradiation. In principle, these insulation systems are glass fiber reinforced plastics (GFRPs) consisting of boron-free glass fibers and matrix materials (e.g. multifunctional epoxy resins). In addition, the ITER specifications
* Corresponding author. Tel.: /43-1-58801-14188; fax: / 43-1-58801-14199. E-mail address: [email protected] (K. Bittner-Rohrhofer).
require multiple electrical barrier layers, such as Kapton foils. However, the stability and the lifetime of such insulation systems change under these demanding conditions. Thus, the performance of materials suggested for the ITER fusion device has to be assessed under sequential irradiation conditions [3 /5]. In the present contribution, we report on two insulation systems primarily applied as turn and ground insulation for the Toroidal Field Model Coil (TFMC) [6] of ITER. Static and dynamic measurements in tension and under interlaminar shear loading conditions were done at 77 K prior to and after reactor irradiation to fluences of 5 / 1021 and 1 /1022 m 2 (ITER design fluence level factor 2 above the maximal expected one).
0920-3796/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0920-3796(03)00340-5
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2. Materials and test procedures
3. Results and discussion
The insulation systems were manufactured by Ansaldo (Italy) and by Alstom-MSA (France). Both laminates consist of combined Kapton-H foil/R-glass-fiber reinforcement tapes, which are vacuum pressure impregnated (‘‘VPI’’-process) with epoxy DGEBA systems (Araldite F type for Ansaldo and MY 745 for Alstom) fabricated by Vantico (Switzerland). The dimensions of the Kapton tapes are 20/25 /0.05 /0.075 mm (width /thickness). The Ansaldo tapes are provided with an additional adhesive to fix the position of the Kapton on the glass fabric for better handling. A detailed description of the manufacturing procedure can be found in [7]. The same procedure was also used for the Alstom plates without Kapton foils (designated as ‘‘A’’)1. Because of the anisotropic material properties, the tensile and DLS-specimens [8] (outer dimensions for Ansaldo: 70 /10 /2 mm and for Alstom: 70 /10 /4 mm) and the SBS-specimens (Ansaldo: 2.5 mm and Alstom: 4 mm thickness) were loaded under two different directions, i.e. parallel (08) and perpendicular (908) to the winding direction of the reinforcement tapes. All static and dynamic experiments were carried out at 77 K using a servo-hydraulic MTS 810 testing device. The ultimate tensile strength (UTS) was measured according to DIN 53455 and ASTM D638. The interlaminar shear strength (ILSS) was determined from the DLS-test and SBS-test according to the ASTM D2344 standard with a span-to-thickness ratio of 4:1 for Ansaldo and 5:1 for both Alstom systems. To simulate the pulsed conditions of ITER /FEAT, tension/tension fatigue experiments (ASTM D 3479) at 10 Hz and at a minimum-to-peak ratio of R /0.1 were done up to 106 cycles. Four or five samples were measured for each load level. A standard deviation of /20% was observed.
An overview on the static UTS and ILSS as well as on swelling (¥) and weight loss (9) of the insulation systems is given in Tables 1 and 2.
1
Alstom samples provided with Kapton-foils are designated as ‘‘AK’’.
3.1. Swelling and change of mass The Ansaldo samples show significant swelling and weight loss (Table 1) after irradiation to a neutron fluence of 5 /1021 and 1 /1022 m 2 (E / 0.1 MeV). In particular, swelling is indicated by the formation of bubbles inside the laminate (Fig. 1) after irradiation to the highest dose level. However, this effect was only observed in the clamping area of those samples cut parallel to the tape winding direction. The formation of bubbles is presumably caused by the radiation-induced degradation of the stability of the additional adhesive (for more details see [7]). No systematic change in dimensions in the through-thickness direction or in mass was found for both Alstom systems. 3.2. SBS-tests on 08 and 908 Ansaldo specimens The measurements (Table 1) do not show a significant influence of the tape winding direction on the ILSSSBS. Irradiation to a fast neutron fluence of 5 /1021 m 2 (E/0.1 MeV) leads to a degradation of the ILSSSBS by 15% for 08 and by 10% for 908 samples, respectively. Further irradiation to the highest dose level leads to a decrease of the ILSSSBS to /35 MPa. 3.3. Static and fatigue tensile tests on Ansaldo specimens Table 1 shows the results on the UTS at 77 K before and after irradiation to a neutron fluence of 1 /1022 m 2 (E /0.1 MeV). It is obvious that the UTS of samples loaded parallel to the winding direction (08) is significantly higher (by a factor of /3). However, the material strength in both directions decreases considerably after irradiation to 544 MPa (08) and to 131 MPa (908), respectively. Further, an adverse influence of the adhesive is observed for 908 samples, which break
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Table 1 Results of the static measurements at 77 K of the Ansaldo system before and after irradiation Ansaldo 08
Ansaldo 908
Fluence (E/0.1 MeV) (m 2)
UTS (MPa)
ILSSSBS (MPa)
UTS (MPa)
ILSSSBS (MPa)
Unirradiated 5 /1021 1 /1022
8449/41 / 5449/54
459/7 389/6 349/4
2869/11 / 1319/10
419/5 379/4 359/2
¥ (%)
9 (%)
/ /2.49/0.5 /3.09/0.23
/ /1.29/0.29 /1.49/0.3
remains constant up to 104 cycles and then predecreases continuously up to more than 106 fercycles (Fig. 2a) at a load limit of 60% (79 ablMPa). Accordingly, the material fails without y at reaching a well defined life endurance limit. this Tension /tension fatigue measurements on 08 losamples show a similar behavior before irradiacation, i.e. a rapid decrease of the Wo¨hler-curve in tion the range from 80 to 20% of the static UTS (Fig. pri2b). A shift to higher numbers of cycles is found or Fig. 1. Formation of bubbles inside the Ansaldo system after for load levels B/0.6 (B/326 MPa). A well to irradiation to a fast neutron fluence of 1 /1022 m 2 (E/0.1 defined fatigue resistance could not be found at as MeV) as indicated by the arrows. a load limit of 30% (163 MPa), even though more well than 106 cycles were run. as after irradiation. A detailed discussion of possible reasons for the remarkably (70 /75%) lower UTS of 908 samples and the consequences 3.4. SBS-tests on 08 and 908 Alstom specimens of the adhesive for practical applications in the TF coils can be found in [7]. No systematic influence of the Kapton foil on A life endurance limit of 35% (100 MPa) was the ILSSSBS was observed for the 08 and 908 observed for unirradiated 908 samples. The fatigue samples, respectively (cf. Table 2). Both Alstom behavior changes drastically after irradiation. It systems loaded parallel to the tape winding direcTable 2 Results of the static measurements at 77 K of both Alstom systems before and after irradiation ‘‘A’’ 08
‘‘A’’ 908
Fluence (E/0.1 MeV) (m 2)
UTS (MPa) ILSSSBS (MPa) ILSSDLSa (MPa)
UTS (MPa) ILSSSBS (MPa) ILSSDLSa (MPa)
Unirradiated 5 /1021 1 /1022
8459/43 7499/79 6959/24 ‘‘AK’’ 08
3459/14 / 1149/20 ‘‘AK’’ 908
Fluence (E/0.1 MeV) (m 2)
UTS (MPa) ILSSSBS (MPa) ILSSDLS (MPa)
UTS (MPa) ILSSSBS (MPa) ILSSDLS (MPa)
Unirradiated 5 /1021 1 /1022
7569/51 6659/15 6129/45
3879/21 / 2149/16
a
No FE correction employed.
809/4 449/3 319/4
819/4 509/4 359/5
479/4 379/4 139/1
349/1 299/2 179/2
779/4 379/4 249/3
759/4 459/6 279/4
449/7 / 129/3
389/2 / 159/2
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Fig. 2. (a, b) Tension /tension stress lifetime diagrams for 08 (a) and 908 (b) loaded Ansaldo specimens before and after reactor irradiation to a fast neutron fluence of 1 /1022 m 2 (E/0.1 MeV) measured at 77 K. The measurements were stopped manually above 106 cycles, as indicated by the arrows.
tion show a value of /80 MPa prior to irradiation, which degrades after irradiation to the ITER design fluence level by 61% for ‘‘A’’ and by 57% for ‘‘AK’’, respectively. The results on the unirradiated 908-samples are lower by 4 /7%. Irradiation to 1 /1022 m 2 leads to a considerable decrease of the ILSSSBS to 24 /27 MPa.
m 2. The radiation-induced degradation by / 22% for ‘‘A’’ and 15% for ‘‘AK’’ does not allow further fatigue measurements due to the end limit of the test system. A detailed discussion of these results is given in [10]. 3.6. Static and fatigue tensile tests on Alstom specimens
3.5. Shear fatigue tests on 08 Alstom specimens In comparison to the SBS results, the static ILSS obtained from the DLS-test was lower by /50%, as expected from FE calculations [9]. The fatigue life in the unirradiated state is about 106 cycles at a maximum load level of 0.5 ILSSDLS (/24 MPa) for ‘‘A’’ and 0.6 (/21 MPa) ILSSDLS for ‘‘AK’’ (Fig. 3a, b). However, a fatigue life endurance limit was not found for both GFRPs after irradiation to a neutron fluence of 5 /1021
The static tests (Table 2) show a significant difference by up to /60% between the UTS of 08 and 908 samples before irradiation to a neutron fluence of 1 /1022 m 2. The material strength of ‘‘A’’ as well as of ‘‘AK’’ degrades drastically after irradiation in both directions by /19% (08) and by up to 67% (908). A poor fatigue behavior, indicated by a rapid decrease in the range from 80 to 20% of the UTS before irradiation, was observed in all Wo¨hler-
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Fig. 3. (a, b) Shear fatigue behavior of DLS Alstom specimens without Kapton (a) and with Kapton (b) before and after irradiation to a neutron fluence of 5 /1021 m 2 (E/0.1 MeV). The test was interrupted manually after 1 million cycles, as indicated by the arrows.
curves (Fig. 4a, b) measured on a subset of 08 samples. However, neutron irradiation to fluences of 5 /1021 m 2 and of 1 /1022 m 2 (E /0.1 MeV) leads to an unexpected shift to a higher number of cycles for stress levels B/0.6 UTS (Fig. 4a, b), as also observed for the Ansaldo system. Further, the fatigue process is still in progress at a load limit of 20/25% ( /140 /150 MPa) in both Alstom systems after irradiation to 1 /1022 m 2 (for more details see [10]).
4. Summary The turn and ground insulation of the TFMC were assessed under static and dynamic load at 77 K prior to and after irradiation to neutron fluences
of 5 /1021 and 1 /1022 m 2 (E /0.1 MeV). The force was applied parallel and perpendicular to the wrapped tape. The samples were further investigated under tension /tension fatigue load. For comparison, the ground insulation was also tested without Kapton foil. The main results can be summarized as follows: . Significant swelling (3%) and weight loss (1.4%) were observed in the Ansaldo material. Bubble formation was found inside the laminate. No systematic dimensional change in the throughthickness direction and in weight was found for the Alstom material. . Both tape winding directions show a similar interlaminar shear strength (ILSSSBS, shortbeam-shear (SBS) test) in the range from 75 to
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Fig. 4. (a, b) Tension /tension stress lifetime diagrams for 08 loaded Alstom specimens without Kapton (a) and with Kapton (b) before and after reactor irradiation to a fast neutron fluence of 5 /1021 and 1 /1022 m 2 (E/0.1 MeV) measured at 77 K. The measurements were stopped manually above 106 cycles, as indicated by the arrows.
81 MPa for the Alstom systems. The ILSSSBS of the Ansaldo system is approximately two times lower than that of the Alstom insulation. Irradiation to 1 /1022 m 2 leads to a ILSSSBS for Alstom systems of 31/35 MPa (08 direction) and of 24/27 MPa (908 direction). The Ansaldo samples degrade at the same fluence by up to 24%. The fatigue life of the Alstom double-lapshear (DLS) samples is about 106 cycles at a load limit of 50/60%. However, no fatigue resistance could be obtained after irradiation to 5 /1021 m 2 (E /0.1 MeV) due to the rather low ILSSDLS of 29/37 MPa and experimental limitations in this range. . All GFRPs loaded parallel to the tape-winding direction (08 specimen) show the highest UTS, (756 /845 MPa), which decreases after irradiation to a neutron fluence of 1 /1022 m 2 by 35% (Ansaldo) and by 19% (both Alstom systems), respectively. Moreover, a poor fatigue life endurance limit at 20/25% was observed in
both unirradiated materials. After irradiation, all lifetime diagrams show a shift to a higher number of cycles for load levels B/0.6. Consequently, a defined fatigue resistance could not be established, even though more than 106 cycles were run. . In comparison to 08 specimens, a drastically lower (60 /70%) UTS was obtained in all 908 specimens. Especially, transverse loaded Ansaldo samples show a rather poor fatigue behavior, i.e. stress levels of 100 MPa before and of 79 MPa after irradiation. With respect to numerical investigations by Jong [11] of the ITER-team, the maximal tensile stress in the ground insulation is /35 MPa. Taking safety margins, the properties of the Ansaldo and the Alstom insulation are close to the limit of the ITER demands. Since all systems show a reduced mechanical integrity at a fluence of 5 /1021 m 2 and even more at the ITER design
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fluence of 1 /1022 m 2, the materials require basic improvements in order to ensure a safe performance at those parts of the coils, where maximal loads are expected. Improved radiation resistance of the epoxy resins and more uniform wrapping and compaction during the fabrication process would increase the interfacial bonding inside the laminates and thus, the stability of the insulation. Adhesives between the sandwich tape must be avoided because of their significant adverse influence on the material performance, especially in the 908 direction under static and dynamic load.
Acknowledgements Technical assistance by Mr H. Niedermaier and Mr H. Hartmann is acknowledged. This work has been carried out within the Association EURATOM-OEAW.
References [1] R. Aymar, ITER R&D: executive summary: design overview, Fusion Eng. Des. 55 (2001) 107 /118. [2] T. Mizoguchi, ITER R&D: executive summary: R&D overview, Fusion Eng. Des. 55 (2001) 119 /136.
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[3] H.W. Weber, E. Tschegg, Test program for mechanical strength measurements on fiber reinforced plastics exposed to radiation environments, Adv. Cryog. Eng. 36 (1990) 869 /875. [4] P. Rosenkranz, K. Humer, H.W. Weber, Reactor Irradiation effects on the fatigue behavior of CTD-112 P at 77 K in tension and interlaminar shear, Adv. Cryog. Eng. 46 (2000) 175 /180. [5] H. Fillunger, H. Gerstenberg, K. Humer, R.K. Maix, K. Okuno, P. Rosenkranz, H.W. Weber, N. Wiegner, Irradiation and mechanical testing of ITER relevant magnet insulation, Fusion Eng. Des. 58 /59 (2001) 135 /140. [6] R.K. Maix, H. Fillunger, F. Hurd, E. Salpietro, N. Mitchell, O. Libeyre, P. Decool, et al., Completion of the ITER toroidal field model coil, Fusion Eng. Des. 58 /59 (2001) 159 /164. [7] K. Bittner-Rohrhofer, K. Humer, H.W. Weber, Lowtemperature tensile strength of the ITER-TF model coil insulation system after reactor irradiation, Cryogenics 42 (2002) 265 /272. [8] P. Rosenkranz, K. Humer, H.W. Weber, Influence of the test geometry on the fatigue behavior of fiber reinforced plastics in tension and interlaminar shear at RT and 77 K, Adv. Cryog. Eng. 46 (2000) 181 /187. [9] D.H. Pahr, F.G. Rammersdorfer, P. Rosenkranz, K. Humer, H.W. Weber, A study of short-beam-shear and double-lap-shear specimens of glass fabric/epoxy composites, Composites: Part B 33 (2002) 125 /132. [10] K. Bittner-Rohrhofer, K. Humer, H. Fillunger, R.K. Maix, H.W. Weber, Mechanical behavior of the ITER TF-Model Coil ground insulation system after reactor irradiation, Cryogenics 42 (2002) 697 /704. [11] C.T.J. Jong, Private communication: ‘‘Stresses in insulation’’, 9 July 2002.