Effect Of Bending Strain Research Articles

Strain Sensitivity of Electric-Magnetic Coupling in Flexible Terahertz Metamaterials

In order to investigate the effects of bending strain on electric-magnetic coupling, we fabricated and characterized two types of flexible terahertz (THz) metamaterials on polyethylene naphthalate (PEN) substrates, which had either asymmetric or symmetric configuration. The asymmetric flexible THz metamaterials showed a plasmon-induced transparency originating from electric-magnetic coupling. The transparency was rather robust and insensitive to strain. The symmetric metamaterials demonstrated a transmission dip at a frequency of 1.35 THz without applied strain due to electric resonance. However, if strain gradually varied, a continuously tunable transmission dip was observed at a frequency of 1.1 THz, which could be ascribed to electric-magnetic coupling induced by symmetry breaking. The promising results suggested that the asymmetric and the symmetric flexible THz metamaterials could find potential applications in curved devices and remote stress sensors, respectively.

The Contradiction Behavior Between Mechanical Performance and Degradation Mechanism of Electric at Cryogenic Temperature in Bi-2222/Ag Tapes with Different Protective Layer

The mechanical performance at cryogenic temperature and the degradation mechanism of electric behaviors of Bi-2222/Ag tapes with different protective layers as well as their contradiction laws are discussed. A variable temperature cryostat system is constructed to provide the successive cooling environment from room temperature (RT) to the liquid nitrogen temperature (LNT), and a cryogenic-type extensometer is also used to measure strain behavior of the superconducting tapes. And, the effects of bending strain on the critical current of Bi-2222/Ag tapes with different protective layer were measured using arched abrasives with different radius. Experimented results have shown that the protective layer of the tapes could have strong positive effects on the measured mechanical performance at room temperature and cryogenic temperature. The irreversible degradation on strain indicates that the I c reduction is caused mainly by crack formation and propagation in the brittle Bi-2222/Ag tapes. In particular, the degradation mechanism of multifilamentary sample with protective layer was also elaborated, and protective layer has some negative effect on electric behaviors. In addition, the behaviors of the n value with strains on standard Bi2222/Ag tape was also argued to predict the damage process in Bi-2222 tapes indirectly during bending tests.

Bending Strain Effects on the Critical Current in Cu and Cu–Nb–Stabilized YBCO-Coated Conductor Tape

High strength and high electrical conductivity Cu-Nb tape was tested as the new type of stabilizing layer for YBCO-coated conductor. For electrical interconnection of stabilizing layer to coated conductor the soldering process was used and no degradation in critical current, Ic, of laminated coated conductor after soldering process was observed. The dependence of normalized transport critical current, Ic/Ic0, on the bending radius of coated conductors with and without Cu-Nb stabilizing layer was investigated using the bending test probe providing the uniform bend process at 77 K and under self-field conditions. The reversible and irreversible variations of Ic/I0 with inverse bending radius were found and the irreversible bending radius limit was observed to depend on the neutral axis shift. The generation of the cracks appears to be the primary reason for the Ic/Ic0 degradation that was supported by ScanHall technique investigations.

Homogeneous performance and strain tolerance of long Bi-2223 HTS conductors under hoop stress

Two types of high-strength industrial Bi-2223 conductor, one laminated by copper alloy and the other laminated by stainless steel, have been tested to examine the effect of hoop stress on the transport property. The specimens (∼2 m long) were prepared by winding one layer around a GFRP mandrel and the measurements were made in a liquid helium bath with the hoop stress calculated from the BJR product applied by external magnetic field. A careful measurement wire configuration was necessary to cancel the noise pick-up from the environment for more accurate determination of Ic and n-value. We show for the first time that both conductors showed homogeneous voltage–current characteristics over a long length and degradations with hoop stress occurred uniformly, which is crucial information for the development of HTS magnet technology. The onset of degradation occurred at 200 MPa and 220 MPa, with additional bending stress present from the winding diameter of 108 mm, for copper alloy laminated and stainless steel laminated conductors, respectively. After considering the effect of bending strain, our result agrees well with the previously measured data.

Size-Dependent Correlations between Strain and Phonon Frequency in Individual ZnO Nanowires

The effect of uniaxial tensile strain on individual ZnO nanowires with diameters ranging from 500 nm to 2.7 μm and the effect of pure bending strain on ZnO microwires are systematically investigated by Raman spectroscopy. It is found for the first time that the tensile and compressive strains result in a linear downshift and upshift of the phonon frequencies of the E2L, E2H, E1TO, and second-order modes compared with the strain-free state, respectively, while the A1TO mode is not influenced by the strain. Furthermore, the strain modulation on phonons depends strongly on the nanowire diameter. The E2H phonon deformation potential is ~3 cm(-1)/% for the 500 nm nanowire, while 1% tensile strain results only in ~1 cm(-1) downward frequency shift for the 2.7 μm ZnO wire. The results provide a versatile "local-self-calibration" and nondestructive method to measure and monitor the local strains in ZnO micro/nanostructures.

Improvement of ultraviolet photoresponse of bent ZnO microwires by coupling piezoelectric and surface oxygen adsorption/desorption effects

Localized ultraviolet photoresponse properties of bent ZnO microwires bridging two perfect Ohmic contacts in both atmospheric and high vacuum (8 × 10(-6) torr) environments have been investigated for the first time to explore the bending strain effect on the photoelectrical properties of ZnO. It is found that the ZnO microwire has higher photoconductivity and faster rising speed when photo-excitation is localized at the bending region in an atmospheric environment, while the rising speeds are almost the same when photo-excitations are localized at the bending and straight regions under vacuum. The bending strain induced improvement of the UV photoresponse in air was well explained by considering the coupling of piezoelectric effects and the surface oxygen adsorption/desorption procedure on the bent ZnO microwire. Our results are valuable for designing and fabricating strain modulated photoelectrical micro/nano-devices.

Enhancement of bending strain tolerance and current carrying property of MgB2 based multifilamentary wires

The effect of bending strain on current carrying capacity of MgB2 multifilamentary wires was studied with 4, 8 and 16 multifilamentary wires. The critical current density (JC) of straight wires and bent wires with 5, 10, and 15cm diameter was measured. Both annealed & bent and bent & annealed wires were used for measurement. The JC of annealed & bent wires were found to decrease with decrease in bent diameter and the rate of degradation of JC decreased with increasing number of filaments, while bent & annealed wires almost retained its JC at all diameters studied.

Experimental characterization and modeling of the bending strain effect on flexible microwave diodes and switches on plastic substrate

In this letter, comprehensive experimental characterization and modeling of the bending strain effect on flexible microwave diodes and switches are conducted. The flexible microwave devices/circuits indicate different performance dependence with bending strains under different bias conditions. It is observed that individual diodes and switch circuits have the same dominant factors (series resistance and inductance) for radio frequency properties under bending conditions. More importantly, variations of the dominant factors are almost only dependent on the strains and regardless of diode areas or connection topology. The study provides guidelines for designing and using high-speed diodes/switches for flexible monolithic microwave integrated circuits.

Comparison of the Bending Strain Effect on Transport Property in Lap- and Butt-Jointed Coated Conductor Tapes

Although long-length superconducting wires are now being developed and manufactured, there is still a need for jointing in coil manufacturing and power cables for power distribution which requires miles in length. Therefore, it is important to develop an easy and practical jointing that has an acceptable electrical and mechanical integrity. Depending upon the applications, jointed coated conductor (CC) tapes will experience different stresses and strains during manufacturing and operation that could affect its transport property. Bending strain characterization is necessary since CC tapes are being wound into coils for magnet applications and other electrical devices. In this study, two joint types soldered by mechanical controlled jointing with uniform pressure application have been adopted. Critical currents of the jointed tapes have been measured at 77 K and self field. In the case of easy bending test, a new test procedure has been adopted considering the non-uniform flexural stiffness between jointed and unjointed parts of the sample. Relationship between the critical current and bending strain in easy bending mode has been investigated.

The Influence of Bending Strain on the Critical Current of ${\rm Nb}_{3}{\rm Sn}$ Strands With Different Filament Twist Pitch

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The effect of bending strain on the transport properties of <formula formulatype="inline"><tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> strands has been the object of several investigations in the last years. We report on the performances of internal tin <formula formulatype="inline"><tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> strands with different filament twist pitches, pre-compressed by swaging into thin stainless steel tubes before the reaction heat treatment and subject to pure bending strain. We have previously reported on the comparison between the performances of the technological OST-Dipole strand (TW-strand) with those of the same strand, but with untwisted superconducting filaments (UNTW-strand). The critical current, as well as the n-index of the samples with twisted filaments gradually degraded with the applied bending strain, while an enhancement of the performances has been observed on UNTW-strands. Here, results on further experimental measurements carried out on OKSC internal tin strands with different filament twist pitches are discussed, in order to clarify some aspects of the current transfer process within the strands. Finally, we carried out an analysis through the second derivative of the V-I curve, which evidenced a peaked critical current distribution for the UNTW-strands, while TW-strands under bending showed a higher degree of non-homogeneity, proven by broader distributions. </para>

Bending-stress-driven phase transitions in pentacene thin films for flexible organic field-effect transistors

The effects of bending strain on the structure and electrical characteristics of pentacene films in flexible devices were investigated. It was found that the volume fraction of bulk phase in the pentacene film increases from 10.7% to 27.7% under 1.1% of tensile strain but decreases to 3.5% under 1.0% of compressive strain. These bending-stress-driven phase transitions between the bulk phase and the thin-film phase in the pentacene film resulted in the changes in field-effect mobility, and were driven by the differences between the in-plane dimensions of the crystal unit cells of the two phases to reduce the external bending stress.

Bending strain effects on the critical current in Cu-stabilized IBAD/EDDC processed SmBCO coated conductor tape

Influences of bending strain on the critical current, I c, in Cu-stabilized IBAD/EDDC processed SmBCO coated conductor (CC) tapes were investigated at 77 K and self-field using a small-scale Goldacker type bending test rig. Considering the location of the superconducting layer against the neutral axis, tensile and compressive bending strains were applied to the SmBCO coating layer and the strain effect on I c degradation was discussed. Under tensile bending strain, the I c decreased monotonically but recovered reversibly up to the irreversible strain limit. Under compressive bending strain, however, the I c increased but in some cases it suddenly dropped after straightening due to the delamination of the copper stabilizer. Also, repeated bending strains were applied to Cu-stabilized SmBCO CC samples in order to observe the behavior of I c under cyclic loading. I c Initially decreased with the application of 0.7% bending strain, but when the tape was subjected to repeated bending up to 100 cycles, no more degradation occurred. And finally, the I c almost recovered to I c0 when the tape was straightened.

Bending Strain Characteristics of Critical Current in Double-Layered Bi-2223 Superconducting Tapes

The evaluation of reliability of HTS tapes is necessary for practical applications such as magnet, power cables, motors, SMES and HTS coils. The bending strain effect on the critical current, Ic, in multifilamentary Bi-2223 HTS tapes have been investigated. Single and double layered tapes (for high current capacity) were subjected to thermal and pressurization cycles at pressurized liquid nitrogen (LN2) up to 1 MPa. After the pressurization test, the subsequent occurrence of damage like ballooning due to the diffusion and expansion of LN2 into the tapes were investigated and compared. The Bi-2223 tapes showed different Ic degradation behavior depending upon the geometry and ratio of reinforcement to superconductor adopted.

Performance enhancement under bending of Nb3Sn strands with untwisted filaments

We report on the effect of bending strain on the transport properties of multifilamentary Nb3Sn strands having twisted or untwisted superconducting filaments in order to clarify some aspects of the current transfer process within the wire cross section. Critical current measurements were performed on internal tin Nb3Sn strands, which were inserted and compacted inside thin stainless steel tubes before the heat treatment. With the application of a maximum bending strain of 0.5%, the strands with twisted filaments present a degradation of the critical current and of the n-index, while an enhancement of the performances is observed for the untwisted strands. The experimental results are interpreted in relation to the predictions of Ekin’s model.

High-strength CuNb/Nb3Sn strand cables with residual strain controlled by the repeated bending treatment

We have developed practical multifilamentary Nb3Sn wires with CuNb composite reinforcement (CuNb/Nb3Sn). In order to fabricate a CuNb/Nb3Sn superconducting magnet by a react-and-wind method, the bending strain effect was investigated in detail for CuNb/Nb3Sn wires. We found that the repeated bending treatment enhances Tc from 17.4 to 17.9 K and Bc2 from 24.0 to 25.3 T at 4.2 K for CuNb/Nb3Sn wire. As a result, the repeated bending treatment for CuNb/Nb3Sn wire outstandingly enhances the critical current in high magnetic fields. A Nb3Sn filament was prepared by chemically solving practical multifilamentary CuNb/Nb3Sn wires. After removing Cu stabilizer, CuNb reinforcement, Nb barrier, and Bronze, Bc2 of a Nb3Sn filament was measured, and the Bc2 value of 25.9 T at 4.2 K was obtained. This means that the bending treatment for CuNb/Nb3Sn wires extremely reduces the residual strain close to the strain free state. To decrease the residual strain, it is important to control the three-dimensional strain distribution of CuNb/Nb3Sn wires.We intended to apply the bending effect to the cabling process of CuNb/Nb3Sn strands by a react-and-wind method. High strength Nb3Sn cables consisting of CuNb/Nb3Sn strands and stainless steel reinforcement strands were developed for a performance test of a next phase superconductor with a large critical current and a strong mechanical property at a high magnetic field of 20 T.

Simultaneously bending and tensile strain effect on critical current in YBCO coated conductors

YBCO coated conductors have been expected for the application to a coil for superconducting magnetic energy storage (SMES). In the application to a superconducting coil, the coated conductors experience bending, uniaxial tensile strain and their combined strain. Therefore, the influence of simultaneous bending and tensile strain on critical current should be revealed. In this work, we developed the test method of critical current under such combined strain state. As a result, it was confirmed that compressive pre-bending can improve the stress tolerance of the YBCO coated conductors. On the other hand, compressive bending strain suppresses the initial critical current by the intrinsic strain effect. These results indicate that optimal bending radius should be selected in order to realize superior stress tolerance and high current capacity simultaneously.

Bending Strain Effects on the Critical Current of Bi-2223 Superconducting Tapes by Pure Bending Method

The effects of bending strain on the critical current of 19-filament Ag/Bi-2223 and 121-filament AgMg/Bi-2223 tapes produced by powder-in-tube method have been studied by a pure-bending method at 77K. The I cn (e) curves are of two regimes, I cn does not decrease until bending strain increases to a limit e irr . The e 0.9 values of 19-filament Ag/Bi-2223 and 121-filament AgMg/Bi-2223 tapes are 0.32% (negative bending), 0.64% (positive bending) and 0.30%, respectively. The I cn (e) curves are fitted by the formulae I cn =1-(e/a) b , where a and b is the material parameters. The strain dependences of the I cn values for the pure-bending, modified RRT and bending-rig methods were nearly identical up to e irr .

Evaluation of bending strain dependence of critical current of Nb 3Al conductor for coils with react-and-wind method

In order to clarify the limitation of bending strain in conductor to fabricate the toroidal field coils of DEMO reactor by react-and-wind (R&W) method, the effect of bending strain on the critical currents ( I c) of Nb 3Al cable-in-conduit (CIC) conductor was investigated using the newly developed apparatus which can bend cooled sample from outside of cryostat. The Nb 3Al strand manufactured by jelly roll process was 0.74 mm in diameter with the copper/non-copper ratio of 4.0. The CIC conductor sample which consisted of 54 Nb 3Al strands and 27 copper wires inserted into a stainless steel conduit was 12.4 mm in diameter and 2.3 m in length. The CIC conductor was wound into a torsion coil spring shape with 4.5 turns and 150 mm in diameter. The bending strain and external magnetic field ranges for I c measurement were from 0% to 0.6% and from 9.2 T to 11.3 T, respectively. It was clearly shown that the I c decrease of the CIC conductor by bending is considerably low compared with that of strand, indicating the relaxation of intrinsic strain in Nb 3Al filaments in CIC conductor due to the cabling effect. The I c was not decreased up to 0.4% in bending strain and slightly decreased to 96% of initial I c at 0.55% in bending strain. This indicates that the R&W method with bending strain of 0.6% is fully applied to the coil fabrication method. The furnace size for heat-treatment for R&W method becomes half of wind-and-react (W&R) method.

The Effect of Deformation Stress-Strain and Temperature on The I&lt;sub&gt;c&lt;/sub&gt; Degradation of Bi-2223/Ag Tapes

Bi-2223/Ag tapes are usually used for superconducting power cables and magnets. Bi- 2223 ceramic superconducting core can be damaged under complicated stress and strain conditions such as winding tension, bending and twisting. In this study, we have presented the effect of axial stress and bending strain on the superconducting properties of Bi-2223/Ag tapes. In order to establish the value for 95% retained Ic of the Bi-2223/Ag tapes under various stress-strain conditions, the tension apparatus with bending former was used to apply the tension and bending stress-strain. Tension and bending stress-strain simultaneously applied to the tapes could reduce the critical current even though each applied stress and strain values were not higher than that of 95% retained Ic of the tapes. Complex stress-strain conditions including the thermal stress-strain have accelerated the degradation of the Bi-223/Ag tapes. The deformation temperature was important to maintain the 95% retained Ic of the Bi-2223/Ag tapes after bending or tension deformation because mechanical strength of the tapes can be changed drastically between room temperature and 77 K.

Comparison of bending strain effect on the critical current degradation of Bi-2223 tapes through different measurement techniques

Unlike in the tests under tension, transverse compression and torsion, the bending test of HTS tapes requires lots of time and effort since the sample should be bent or mounted successively onto sample holders having different bending radius at room temperature, and then cooled down to measure the critical current, Ic, up to 77K at each step. In this process, the effect of repeated thermal cycle on the Ic degradation can not be ignored. The establishment of a practical and effective measurement method of the critical current as a function of bending strain for HTS tapes should be considered. A ρ-shaped sample holder which provides a series of bending strains to HTS tapes was newly devised. In this case, the connection of Bi-2223 tapes to current terminal blocks was done mechanically. Using this sample holder, the bending strain effect on the Ic degradation behavior in Bi-2223 tapes in the easy bending mode was investigated, and discussed them comparing with other data obtained by different testing methods, namely, the conventional bending method using FRP sample holders and the Goldacker-type continuous bending test rig. Commercially available Bi-2223 tapes which have different reinforcing structures were supplied for this study. By using the newly devised ρ-shaped sample holder, it was possible to obtain a bending strain characteristic of Ic in Bi-2223 tapes at one time cooling which lessened the testing time significantly when compared with other testing methods and supply good reproducible data. The Ic degradation behavior in Bi-2223 tapes was similar to the cases using FRP sample holders although it showed slightly higher Ic values.