Advanced Materials Manufacturing & Characterization Vol 3 Issue 1 (2013)

Advanced Materials
Manufacturing & Characterization
journal home page: www.ijammc-griet.com

Ybco Superconductor Characterization under Shear Strain

Ziauddin Khan, AnanyaKundu, YuvakiranParavastu and SubrataPradhan
Institute for Plasma Research, Near Indriabridge, Bhat, Gandhinagar – 382428, India.

ARTICLE INFO A B S T R A C T

Article history: YBCO based high temperature superconductors in practical applications are subjected to shear strain.
Received 19 Nov 2012 `Critical current characteristics’ of such high temperature superconductors are known to get degraded
Accepted 26 Dec 2012 in strain state. In this work, shear stresses resulting from the finite twisting of HTS tape having varying
widths have been modeled using FEA analysis for different twisting angles. Supporting the findings of
this model, an American Superconductor Corporation (AMSC) 2G YBCO tape of a given width has been
Keywords: twisted and experimentally investigated in self field for a given current ramp-rate. Under uniform twist
Coated conductor, of the YBCO tape at 77 K, the degradation in the current carrying ability up to 30% was observed. The
critical current, irreversibility in the current carrying ability of HTS tape was also observed beyond the twisting angle
shear stress, per unit length of 25 degree/cm. The superconductor to resistive transition index, `n’ is found to
FEA analysis. behave in an identical manner to the critical current as a function of twisting angle. Such degradation is
largely attributed to the torsional shear strain resulting from the twisting.

Introduction
High temperature second generation (2G) commercial In this paper, the FEA analysis is discussed in section-II. The
grade YBCO coated conductors are available now-a-days in long experimental results and conclusion are discussed in section-III &
length for extensive uses in electrical power cable (Rutherford section-IV respectively.
cable) in the form of stacked twisted HTS conductors [1-3]. Even
the proposal of coated conductor in conduit cable (CCICC) employs FEA Analysis
the idea of stacked HTS conductor twisted over different
The finite element analysis was carried out considering a
transposition lengths [4]. In such applications individual HTS tape
300 mm length YBCO tapes having 4 mm, 8 mm and 12 mm widths.
subjects to twist induced shear stress/strain. The strain beyond a
The parameters considered for the analysis along with tape
certain limit can critically affect the transport property as well as
geometry are given in the table 1.
can modify the super-current carrying path in the coated
conductor [5]. Hence, it has become necessary to ensure the Table 1: Properties of AMSC make YBCO tape.
maximum tolerable shear stress/strain applied to HTS which could
induce recoverable damage. Previous works on the torsoinal strain Quantity Value
dependence of Bi-2223, YBCO coated conductor shows Dimension, L × t 300 × 0.2 mm2
experimental findings on Ic degradation behavior. Finite element Widths, w 4, 8, 12 mm
analyses along with theoretical co-relation also have been YBCO film thickness 0.8 µm
surveyed on torsion experiments of high temperature Substrate thickness 75 µm
superconducting tapes [6-8]. In case of pure torsion, the strain Young’s modulus 133.3 GPa
induced on the tape cross-section is non-uniform and complex. Poisson’s ratio 0.28
However, the maximum torsional shear strain could be determined Co-efficient of thermal
at the midpoint of the width side of the cross-section using the 1.5 × 10–5 m/mK
expansion
formula (εt = tθ/L) [8-9]. Temperature 80 K
In the present study, FEA modeling for shear stress developed on
2G YBCO tape of different widths has been considered. Further, the Static structural type analysis is adopted for determining
shear strain dependence of the critical current of coated conductor the longitudinal elongation and the shear stress developed due to
has been investigated experimentally for a high current ramp-rate. twist at different angles. ANSYS workbench is used for simulation
in which the element type is chosen by the software itself
________________
according to the type of analysis. 3-D model is used and the
 Corresponding author: Ziauddin Khan
meshing is carried out using the same platform. Twisting moment
 E-mail address: ziauddin@ipr.res.in
 Doi: http://dx.doi.org/10.11127/ijammc.2013.02.022 127
Copyright@GRIET Publications. All rights reserved.
from 90° to 1530° was applied at both ends of the tape in opposite
directions with a step increment of 90°. For a given tape width, the
maximum shear stress and the elastic strain for the full rotation of
1530° are shown in figure 1 and figure 2 along with the meshing.

Similarly, for each tape width at different rotational

increment, the shear stress are calculated and presented in figure
3. The percentage of elongation in length due to shear stress and
the percentage of the shear strain developed for maximum twist
are tabulated in table 2 which indicates that the shear strain is
more dominant as compared the shear stress.

Figure 3: Shear stress calculated by FEA analysis for YBCO tape of different
widths of different twist angle per unit length.

The twist angle  is increased up to 1530 with constant

rotational angle interval. Two voltage tap method was adapted to
check the homogeneity of the critical current ‘Ic’ degradation
behavior along the gauge length. These voltage taps with sufficient
lengths soldered at the centre of the tape width at 5.0 cm and 10.0
cm apart from the middle node. The experiment was repeated with
Figure 1: Shear stress after twisting the YBCO tape of 4 mm width at an reversing the twisting angle at lower value in order to check the
angle of 1530°.
reversibility of ‘Ic’. Data acquisition schematic is shown in figure 4.

Figure 2: Elastic strain after twisting the YBCO tape of 4 mm width at an

angle of 1530°.
Figure 4: Data acquisition schematic for Icmeasurement.
Table 2: Maximum change in length in % along with maximum shear strain
in % for different widths YBCO tape. For a particular rotational angle ‘’, the transport
current ‘I’ is driven by unipolar AMI XFR (0-200A, 0-12V) DC power
Strip width Longitudinal elongation Shear supply. Current ‘I’ is ramped through AMI programmer (Model
(mm) (%) strain (%) 430) up to its maximum amplitude and then ramp down to zero.
4 0.648 1.78 Voltage limit in the programmer is kept greater than the voltage
6 0.753 1.78 calculated using V = L (dI/dt) + Vo, The voltage drop `Vo’ is dropped
across the power lead which is measured to be less than 2V. During
8 0.806 1.78
the ramp-up and ramp-down phases, the voltage drops at the two
junctions and transport current are acquired in KeithleyMultimeter
Experimental and results
(Model 2750 Integra Series). The sampling frequency of this
AMSC make YBCO tape of 300 mm length and 4 mm KeithleyMultimeter is 84 ms per data point. The multimeter is
width was clamped tightly to hylum supports on both sides using connected to LABVIEW installed PC via GPIB interface. One
nut-bolt arrangement in order to avoid the free shrinking along its precision signal conditioning card is used to amplify the voltage
longitudinal length during twisting. Then, it was fixed to rotating signals corresponding to two voltage taps into measurable level. I-
knob arrangements of the cold bath provided at both ends for V measurement is pursued using standard four probe method. The
twisting. These knobs rotated in either direction to provide typical electric field criterion of 1.0 µV/cm for critical voltage (Vc)
prefixed twist induced strain. The sample tape is connected to is employed to determine ‘Ic’ value. The critical current ‘Ic0’ for the
current leads via flexible copper braids which also get twisted virgin untwisted tape at  = 0° was found to be 105 A. The ‘Ic’
along with the sample tape and hence the stress concentration at value was measured for each twisting angle ‘’ and then
the edge of the tape could be eliminated. The entire arrangement normalized by the virgin critical current ‘Ic0’. The normalized
was dipped into LN2 bath to avoid evolution of local hot spots. critical current values Ic/ Ic0 for the twist angles per unit length was
plotted as shown in figure 5.

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 using SME image of a nodal portion of the twisted sample as shown
in figure 7. It indicates the torsion induced damage in the
microstructure of the tape which could have resulted in the
degradation of the superconductor film and the current is shared in
the resistive alloy material causing Ic and ‘n’-value deterioration.

Conclusion
From FEA analysis and the experiment, we observed
that the critical current Ic degradation of YBCO tape occurrs due
to the torsional strain only after a definite strain value.
Thereafter, degradation is gradual as the strain increases. Finally,
the Ic value falls suddenly indicating the deterioration in the
transport property YBCO tape. In similar fashion, ‘n’-valve
behavior of YBCO tape follows Ic degradation characteristic. Also,
Figure 5: Behavior of normalized critical current (Ic/Ic0) with respect to it is observed from FEA analysis that the degradation does not
twist angle per unit length. depend on the tape width. Since the elongation does not have
significant contribution as compared to shear strain, the wider
Figure 5 indicates the absence of substantial Ic/Ic0 tape is not too much sensitive to the twisting.
degradation corresponding to /L between 0 to 24 (/cm) whereas
a gradual Ic/Ic0 degradation was observed between 27 to 36
(/cm). A rapid degradation of Ic/Ic0 occurred beyond 36 (/cm).
After exposure the tape to such high stain, irreversible phenomena
are observed in the tape, when untwisted. The reversible
phenomenon in the critical current of the tape was observed when
it is exposed to the twist angle per unit length up to 25 (/cm).
The ‘n’ value behavior of the coated conductor as function of
torsional strain was also investigated. It is estimated from the
empirical power law relation V/Vc = (I/Ic)n. As the exponent ‘n’ is
linked with the current flowing through the superconducting core,
it gets affected by the imposed torsional strain on the tape. The
gradual yet significant transition from superconducting state to
resistive state of the tape is reflected in the normalized ‘n/n0’ value
versus thetwist angle per unit length plot as shown in figure 6. Figure 7: SEM images of the twisted sample showing crack at the
interlayer portion of the twisted sample.

Acknowledgement
We acknowledge Mr. KalpeshDosi, Mr. Yohan Christi, and Mr.
DashrathSonara for extending their valuable time and support for
experimental preparation.

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