Plasma and Fusion Research: Regular Articles

Volume 11, 2405065 (2016)

Evaluation of Shear Strength in Soldered and Mechanical Lap
Joints of High-Temperature Superconducting Tapes Intendedfor a Remountable Magnet∗)
Luis APARICIO, Satoshi ITO and Hidetoshi HASHIZUME
Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University,
6-6-01-2 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan(Received 3 December 2015 / Accepted 9 March 2016)
This study describes the electro-mechanical response under tensile and shear stresses of the mechanical andsoldered lap-joint methods proposed for joining Rare-Earth Barium Copper Oxide (REBCO) high-temperature
superconducting (HTS) coated conductors (CCs) in designs of a “remountable” (demountable) HTS magnet andjoint-winding of an HTS magnet intended for the helical fusion reactor FFHR-d1. Proper joint of REBCO CCs
for the HTS magnets not only requires suﬃciently low joint resistance to avoid quench phenomena, but also
involves mechanical stability against large electromagnetic forces. Results obtained from tensile test to apply
shear stress namely tensile shear tests performed on mechanically jointed REBCO CCs showed that the linear
proportionality between shear strength and contact conductivity of the joint is conserved for larger joint section
area. An attempt to describe the failure mechanism of mechanical lap-joint revealed that a bending moment isoriginated at the joint section. Furthermore, results from tensile shear test on soldered joint between REBCO CCs
with copper stabilizer showed failure of the REBCO CCs with small axial loads while joint remained intact.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: segment-fabrication, demountable magnet, remountable magnet, high-temperature superconductor,
REBCO coated conductors, mechanical lap-joint, soldered lap-joint
DOI: 10.1585/pfr.11.2405065

1. Introduction
The concept of a superconducting magnet being assembled from several segments has been proposed for
Tokamak and Heliotron fusion reactors since 1980 [1, 2].
Since the development of Rare-earth Barium Copper Oxide (REBCO) high-temperature superconducting (HTS)
coated conductors (CCs) with critical parameters far exceeding that of low-temperature superconductors (LTS),
research and development activities related with the design and fabrication of superconducting magnets for nuclear fusion reactors have gained more interest from research institutes around the globe. HTS coils can be operated at relatively high temperatures (>20 K) in which range
their robustness against heat generation is high (larger specific heat), and the refrigeration energy is lower than LTS
coils. Under these considerations, resistive joints on thecoils can be acceptable, allowing the design of demountable HTS magnets [3, 4]. Now, demountable HTS toroidal
field magnets are considered to be applied to conceptual
designs of small tokamaks at fusion nuclear science facilities such as the Vulcan [5] and the ARC [6] in the UnitedStates. In Japan, the Large Helical Device type helical fusion reactor, FFHR-d1 [7], featuring HTS technology isauthor’s e-mail: lapa@karma.qse.tohoku.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

being designed by the National Institute for Fusion Science. The design takes the advantageous option of thesegment-fabrication [8, 9] to solve the engineering issue ofits huge and complex helical coils. There are two options:the joint-winding method [8, 10, 11] in which the helical
HTS coils are wound by joining half-pitch or one-pitchof the conductor segments with permanent joints, and theremountable (demountable) segmented coil method [4, 9].
The latter one is currently considered as the advanced option.
The main issue resides in choosing the proper joint
method applicable for REBCO CCs in order to satisfy thedesired operational parameters [8, 11, 12] (lower joint resistance and high mechanical stability) while maintaining practicality during joint fabrication process. Lap-joint
between REBCO CCs is one of the candidates for thesegment-fabrication: the mechanical lap-joint where theCCs are pressed together with an indium foil between joint
surfaces [10–14] or the soldered lap-joint [8]. The former
has been subject of exhaustive research due to its advantages regarding easy fabrication and proficient joint resistance. In a recent study [12], fundamental investigation on
tensile characteristics of the mechanical lap-joint between
Gadolinium Barium Copper Oxide (GdBCO) CCs having
silver and copper stabilizers with a joint surface area of 25mm2 showed a linear tendency between contact conduc-2405065-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 11, 2405065 (2016)tance (the reciprocal of contact resistance) and achieved
maximum tensile load (shear strength of the joint). It
was discussed that as the number of contact asperities increases, so does the contact conductance along with an increase with the shear strength. Moreover, improvement on
the shear strength of the joint was achieved when increasing the joint pressure. However, the consistency of theseresults when joint area becomes larger has yet to be confirmed.
In other previous studies, tensile characteristics of soldered lap-joint between two Yttrium Barium Copper Oxide(YBCO) CCs with only silver stabilizer [15] showed thatfor higher loads it was observed that delamination between
YBCO and buﬀer layers due to crack formation at the edgeof the joint occurred before any visible damage to the integrity of the joint. However, tensile characteristics of soldered joint between REBCO CCs with silver and copperstabilizers, which has higher strength than the YBCO CCwith only silver stabilizer have yet to be reported.
Therefore in this study we focused first on the description of the tensile characteristics of mechanical lap-joint
between GdBCO CCs having silver and copper stabilizers with a joint area of 50 mm2 in order to define properly
the proportionality between contact conductance and shear
strength of the joint. Next, we performed tensile test toapply shear stress at the joint, namely tensile shear test ofsoldered lap-joint using the same GdBCO CCs with silver
and copper stabilizer. A brief analysis of the joint fracture mechanism for the mechanical lap joint was attempted
based on structural adhesive joint theory [16]. The procedures and results for tensile shear test of the mechanicaland soldered lap-joint are discussed in section 2.

2. Tensile Shear Tests of Mechanicaland Soldered Lap-Joints
2.1

Sample fabrication

The copper stabilized GdBCO CC (FYSC-SC05, Fujikura Ltd.) was chosen for this study. The GdBCO
CC is 5 mm wide and has a layered structure composedby a Hastelloy substrate (100 µm and 75 µm for previous [12] and present study, respectively)/Al2 O3 (100 nm)/
Y2 O3 (30 nm)/MgO (5 nm)/CeO2 (0.5 µm)/GdBa2 Cu3 O x(1.8 µm)/silver (6.4 µm)/tin (2 - 4 µm) and copper (100 µm
and 75 µm for previous [12] and present study, respectively). The GdBCO has critical current of ∼265 A at 77 K,
self-magnetic field.
For the mechanical lap-joint samples used in thisstudy, the copper stabilizer was chosen as the joint surface.
The surface was polished using a #400 grit size sandpaper(particle diameter: 40 µm) and then cleaned using ethanol.
The selected joint area was 50 mm2 (5 mm × 10 mm). An
indium foil with a thickness of 100 µm and a size corresponding to the selected joint area was used as interlayermaterial located between the GdBCO CCs. Figure 1 (a)shows the jig used to apply a joint pressure of 100 MPa

Fig. 1 (a) Mechanical lap-joint fabrication set-up, (b) Soldered
lap-joint fabrication set-up.

Fig. 2 Experimental set-up of the tensile shear test.

during the fabrication of the mechanical lap-joint. Finally,
the joint pressure was released by removing the jig.
For the soldered lap-joint samples, surface preparation
was the same as with the mechanical lap-joint samples. After cleaning the surfaces, they were coated with Tin-Lead(Sn63Pb37) solder with a melting point of 183 ◦ C. The
joint area for all soldered lap-joint samples was 25 mm2(5 mm × 5 mm). Figure 1 (b) shows soldered lap-joint samples fabrication set-up. Both GdBCO CC were placed overa copper plate. Then, the copper plate was heated up to∼230 ◦ C using a hot plate. After the solder became softenough, small joint pressure (<10 MPa) was applied to thejoint section using a brass block bolted to the copper plate
during the heat treatment. After cooling down, the jointpressure was released.

2.2 Experimental set-up and procedure
Figure 2 shows the experimental set-up used for thetensile shear test. The samples were placed in a customized
tensile shear test machine designed for low temperature experiments. The whole test section is submerged in liquid
nitrogen (77 K) and a current of 50 A is supplied. Tensile load is provided by a stepper motor with fixed rotational speed and its amplitude was monitored through aload cell. The potential diﬀerence across the joint section
was also monitored to evaluate the joint resistance (dividing the voltage by the current). In addition, strain gauges
were attached to each GdBCO CC in order to determine irreversible strain [17] and to verify that the specimens wereindeed being pulled.

2405065-2

Plasma and Fusion Research: Regular Articles

Volume 11, 2405065 (2016)

Fig. 5 Picture of one of the soldered lap-joint samples after tensile shear test was performed.

Fig. 3 Maximum tensile load as a function of contact conductance characteristics for mechanical and soldered lapjoint of GdBCO CCs at 77 K.

Fig. 6 Shear strength as a function of contact conductivity formechanical lap-joint of GdBCO CCs at 77 K.
Fig. 4 Mechanical lap-joint sample with a joint area of 50 mm2
after being pulled apart.

2.3

Results and discussion

Figure 3 shows a graph of the maximum tensile load
as a function of contact conductance for mechanical lapjoints without joint pressure and joint areas of 25 mm2 and50 mm2 , and soldered lap-joint with a joint area of 25 mm2
and also without applied joint pressure during the tensile
shear test. The contact resistance is obtained by subtracting
the layer material’s resistance at the joint section from thejoint resistance [18].
Results obtained from the tensile shear test of mechanical lap-joint of GdBCO CCs with a joint area of 50 mm2
showed an increase in maximum applied tensile load. As
the tensile load increases during the test, the CCs reachstrain values above irreversible strain when load reaches
the purple region. The same is true for the green region
corresponding to past results [12] where thickness of theCC is 4/3 times larger than that of CCs used in the presentstudy. At these regions, the critical current of the GdBCO
CCs has decreased below 50 A, and this was observed as asudden increase in the voltage measured across the joint.
Figure 4 shows a picture of one of the mechanical lapjoint samples after being pulled apart. The failure mechanism of the mechanical joint is related to the shear strengthof the interface between the indium and copper stabilizer
which is closely related to the real contact area; on a microscale, when two metal surfaces are brought together, thereal load bearing area is distributed along contact between

sharp, rough or rugged projections called asperities which
deform through elastic and plastic regimes when subjectedto compression. Addition of all the contributions from theasperity contacts gives the real contact area between twosmooth surfaces [20]. However, it is often diﬃcult to determine this value experimentally.
On the other hand, the failure mechanism for the caseof the soldered lap-joint specimens was not due to the joint
breaking apart but rather that of the GdBCO CCs deforming until delamination of the CC and further rupture occur.
Joint condition of the soldered lap-joint samples after tensile shear test coincided with the one described in [15] ascan be seen in Fig. 5 where the joint remained undamaged
for all specimen. Failure may be attributed to fracture atthe interface between silver layer and GdBCO due to stressconcentration induced during tensile shear test. It is also
believed that the solder stiﬀness has also some influenceon the joint failure mechanism.
A more specific view of the joint electro-mechanicalperformance of mechanical lap-joint samples can be obtained through dividing the maximum tensile load and thecontact conductance by the joint apparent area which will
give the shear strength and contact conductivity. Figure 6
shows these results for all mechanical lap-joint samples
with diﬀerent values of joint pressure. The samples withjoint pressures of >0 MPa during the tensile shear test [12]were installed to the tensile machine having the jig shownin Fig. 1 (a) and those with a joint pressure of 0 MPa did
not have the jig as is described in Section 2.1. It can

2405065-3

Plasma and Fusion Research: Regular Articles

Volume 11, 2405065 (2016)tion growth is also expected.

3. Conclusion

Fig. 7 The Goland and Reissner bending moment factor (geometric illustration) [16].

be confirmed that the linear proportionality between shear
strength and contact conductivity of the joint region canbe applied for a diﬀerent joint area. Moreover, joint shear
strength for the case of 50 mm2 fitted with past results linear tendency line for 0 MPa joint pressure [12], meaningthat the number of real contact spots per unit area is almost constant regardless of joint length. The reason why
the red-dashed and black-dashed linear trending lines differ for the samples with joint pressures of > 0 MPa and for
those with 0 MPa is discussed in the next sub-section.

2.4

Failure mechanism of the joints

Following an increase in the applied axial load, abending moment will appear at the edge of the joint thatwill make the overlap section rotate until the line of actionof the load coincides with the center line of each GdBCO
CC as can be seen in Fig. 7. This eﬀect is called the Goland
and Reissner bending moment factor [16]. It can be further
concluded that the indium layer will no longer be solely inshear, but will have peeling force at the ends of the joint.
Moreover, this eﬀect may also have some influence on thecrack propagation at the edge of the joint in the case ofsoldered lap-joint samples. In addition, the upward shifting of the linear tendency (increase in joint shear strength)
observed in Fig. 5 when joint pressure is being applied tomechanical lap-joint samples during the tensile shear testis because the use of the jig will not allow the peeling force
to develop at the edge of the joint.
Another explanation for the upward shifting may be
due to a phenomena called junction growth [19, 20]. According to Tabor [19], when a tangential load is applied
simultaneously with a normal load, plastic flow will occur at a lower applied normal pressure. Even if it is a smalltangential stress, it produces a small but finite displacement
and causes an increase in the real contact area [19,20]. Thisphenomena is called “junction growth” and it gives rise to
the shear strength of the interface layer between the indium
and the copper stabilizer. However, the tribological eﬀects
on metallic adhesion are beyond the scope of this paper, sothey will be treated in future works.
Nevertheless, the current design for the HTS conductors proposed for the FFHR-d1 [11] will include joint pressure being applied by copper and stainless steel jackets
around a HTS CCs region. In such a case, the Goland andReissner bending moment can be neglected and the junc-

In this study we described the electro-mechanical behavior of both mechanical and soldered lap-joint between
GdBCO CCs with copper stabilizer under uniaxial tensile
load at 77 K. Results indicated that the linear tendency between joint shear strength and contact conductivity canalso be applied for the case of mechanical lap-joint with
diﬀerent joint areas. On the other hand, the results for thesoldered lap-joint showed that even with diﬀerent mechanical stabilizer like copper in the case of the REBCO CCs
used in this study, the joint section remained undamaged
while the REBCO CCs broke in the process at a constantload value. In addition, a brief consideration of the fracture mechanism for both types of joints revealed that forthe case of lap-joints with no joint pressure, a bending moment that makes the joint section rotate is generated due tonon-collinear loading.

Acknowledgments
This work was supported in part by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for
Scientific Research (C) under Grant 26420849; and by theNational Institute for Fusion Science (NIFS) Collaboration
Research Program under Grant NIFS13KECF010.
[1] J.R. Powell et al., Cryogenics 20, 59 (1980).
[2] K. Uo et al., IEEE Trans. Mag. 23, 580 (1987).
[3] Bromberg et al., Fusion Eng. Des. 54, 167 (2001).
[4] H. Hashizume et al., J. Plasma Fusion Res. SERIES. 5, 532(2002).
[5] Z.S. Hartwig et al., Fusion Eng. Des. 87, 201 (2012).
[6] B.N. Sorbom et al., Fusion Eng. Des. to be published.
[7] A. Sagara et al., Fusion Eng. Des. 89, 2114 (2014).
[8] N. Yanagi et al., Fusion Sci. Techol. 60, 648 (2011).
[9] H. Hashizume and S. Ito, Fusion Eng. Des. 89, 2241
(2014).
[10] N. Yanagi et al., Nucl. Fusion 55, 053021 (2015).
[11] S. Ito et al., IEEE Trans. Appl. Supercond. 26, 4201510(2016).
[12] S. Ito et al., IEEE Trans. Appl. Supercond. 25, 4201025(2015).
[13] K. Kawai et al., IEEE Trans. Appl. Supercond. 23, 6409408(2013).
[14] S. Ito et al., Plasma Fusion Res. 9, 3405086 (2014).
[15] M. Sugano et al., IEEE Trans. Appl. Supercond. 17,9606863 (2007).
[16] R.D. Adamas and W.C. Wake, Structural Adhesive Joints
in Engineering (Elsevier Appl. Sci. Publishers, England
1984) p.16.
[17] M. Sugano et al., Supercond. Sci. Technol. 25, 054014(2012).
[18] Y. Seino et al., IEEE Trans. Appl. Supercond. 25, 6603405(2015).
[19] D. Tabor, The Royal Society Publishing Proceedings A251, 1266 (1959).
[20] C. Mathew, Tribology on the Small Scale (Oxford University Press. 2008) pp.70-72.

2405065-4