S. Ochiai

Deformation parameters in single steel wire-copper matrix composites

By using the single carbon-steel fibre-copper matrix composites, the important parameters controlling dislocation motions in the fibre, matrix and composite, namely flow stress, internal stress, effective stress, change in flow stress due to change in strain-rate or temperature, stress exponent of strain-rate, effective stress exponent of dislocation velocity, activation volume and activation enthalpy were measured at the stage in which the mechanical interaction between the components was negligible. It was found that all the composite parameters were determined only by the properties of the components and for each parameter, a modified rule of mixtures was derived.

Internal residual strain and critical current maximum of a surrounded Cu stabilized YBCO coated conductor

The deformation behavior of the surrounded Cu stabilized YBCO coated conductor based on the Hastelloy substrate and its influence on the critical current were precisely investigated. The mechanical properties were assessed at room temperature and 77 K. The greatest contribution was brought by two metallic components of the Hastelloy substrate and Cu stabilized layers. The internal strain exerted on the superconducting YBCO layer was determined directly by using synchrotron radiation facilities. The thermally induced residual strain with compressive component decreased during the tensile loading and changed to a tensile component at the force free strain (Aff), at which the internal stress becomes zero in the YBCO layer. Beyond Aff, the increasing rate of internal strain slowed down, suggesting brittle behavior, that is, the formation of micro-cracks. The applied strain dependence of the critical current could be divided into two regions. In the reversible region, the strain dependence obeyed the intrinsic strain effect and was well expressed by the Ekin formula. Beyond the reversible limit, the critical current decreased rapidly with strain. The degradation is suggested to be attributed to the formation of cracks in the YBCO layer. The force free strain evaluated from the mechanical properties was 0.26%. On the other hand, the strain at the critical current maximum was observed to be 0.035–0.012%. These facts suggest re-examining the hypothesis supposing that the critical current maximum appears at the force free strain in YBCO coated conductors.

A computer simulation of strength of metal matrix

The deformation and fracture behavior of metal matrix composites with a reaction layer at the fiber-matrix interface was studied by means of a computer simulation experiment, using a two-dimensional model, and the results of the simulation experiment were compared with the predictions based on the single fiber model, which has been proposed to describe the reduction of strength of composites due to a reaction layer. In the simulation experiment, the composite was regarded as an assembly of single fiber elements, in which, for each element, the reaction layer introduces a notch on the fiber surface when it is broken, which reduces the strength of the fiber if the thickness of the layer is thinner than a critical value, as has been studied by using the single fiber model. The strength of composites was reduced with increasing thickness of the reaction layer and the fracture mode became catastrophic. The strength values obtained by the simulation were equal to those based on the single fiber model only when the fracture of the fiber was caused by the extension of the notch having been introduced by premature fracture of the reaction layer. In other cases, the strength values of the simulation were lower than those predicted by the single fiber model, although the single fiber model gave approximate values.

Residual and fracture strains of Bi2223 filaments and their relation to critical current under applied bending and tensile strains in Bi2223/Ag/Ag alloy composite superconductor

Mechanical and electromagnetic stresses are exerted on Bi2223∕Ag∕Ag alloy superconducting composite tapes during fabrication∕winding and operation, which cause reduction in critical current when the Bi2223 filaments are damaged. In the damage process, the thermally induced residual strain and fracture strain of the Bi2223 filaments play a dominant role. The aim of the present work was to propose a comprehensive method for estimation of these strain values and a quantitative description method of the relation of critical current to the applied bending∕tensile strain, and to examine the accuracy of the method in comparison with the experimental results. The residual strain of Bi2223 filaments in the composite tape was measured by the x-ray diffraction method. The measured residual strain value was used for analysis of the load-strain curve, from which the intrinsic fracture strain of filaments was estimated. The relation of critical current to applied bending∕tensile strain was predicted by the proposed cal...

Influences of cracking of coated superconducting layer on voltage-current curve, critical current, and n-value in DyBCO-coated conductor pulled in tension

Influences of cracking of coating layer under applied tensile strain on V(voltage)-I(current) curve, critical current, and n-value of DyBa2Cu3O7−δ coated conductor were studied experimentally and analytically. The experimentally measured variations in V-I curve, critical current, and n-value with increasing applied strain and the correlation of n-value to critical current were described well by the partial crack-current shunting model of Fang et al. Also, the variations in the ratio of shunting current to overall critical current and the ratio of voltage developed in the cracked region to overall voltage with extension of crack, and the variation in critical current with the ratio of noncracked area to overall cross-sectional area of superconducting layer were revealed.

Improvement of Reversible Strain Limit for Critical Current of DI-BSCCO Due to Lamination Technique

The DI (dynamically innovative)-BSCCO-Bi2223 tapes achieved high critical current as well as high modulus of elasticity. Further the reversible strain limit and the corresponding stress for critical current have been remarkably increased by means of lamination technique. During the course of development, their optimized architecture has been designed based on the principle of the rule of mixture for maximizing the force free strain exerted on the superconducting component. The reversible strain/stress limit (A rev/R rev) was defined as a strain, at which the critical current recovers to the level of 99% I co. Selecting several kinds of laminating materials and changing condition of the fabrication, the excellent Cu alloy-3ply tape with I co of 311 A/cm was realized of which A rev and R rev reached 0.42% and 300 MPa, respectively. Further during the theoretical analysis, the increase of reversible strain limit were made clear to be attributed to the increase of thermally induced residual strain as well as the compensation effect of laminated layers against a local fracture mode.

Thermally induced residual strain accumulation in Bi2223/Ag/Ag alloy composite superconductor

A method to estimate the thermally induced residual strain accumulation under varying temperature in a Bi2223/Ag/Ag alloy composite superconductor was presented, in which the mechanical property values measured from the stress?strain curves of the samples with different residual strain states, the residual strain value of Bi2223 filaments in the composite tape at room temperature measured by x-ray diffraction and the reported coefficients of thermal expansion of the constituents (Bi2223, Ag and Ag alloy) in the relevant temperature range were incorporated. This method was applied to estimate the change of the residual strain of all constituents of the high critical current type composite tape fabricated by American Superconductor Corporation as a function of temperature. The residual strain value at 77?K estimated by this method and the reported fracture strain of Bi2223 filaments accounted well for the measured strain tolerance of the critical current at 77?K.

Prediction of variation in critical current with applied tensile/bending strain of Bi2223 composite tape from tensile stress-strain curve

An approach to predict the variation in critical current with applied tensile/bending strain of Bi2223/Ag/Ag alloy composite tapes from the tensile stress-strain curves was presented. Three different fabrication-route samples were used to examine the applicability of the present approach. The damage strain parameter, referring to the difference between the tensile fracture strain and residual strain of Bi2223 filaments along the sample length direction (current transport direction), was estimated from the variation in the slope of the tensile stress-strain curve. With the estimated damage strain parameter, the irreversible tensile strain for critical current was predicted, which agreed well with the experimental result in all samples. Also by substituting the estimated damage strain parameter into the core shape—incorporated model, the critical current-bending strain curve was predicted, which described satisfactorily the experimental result in all samples. The present approach could be a useful tool for ...

Distribution of normalized critical current of bent multifilamentary Bi2223 composite tape

The distribution of the normalized critical transport current (critical current normalized with respect to the original value) of Bi2223/Ag/Ag alloy composite tape under bending strain of 0%–0.833% was studied experimentally and analytically. The experimental results were analyzed by a modeling approach based on the relation of the heterogeneous damage evolution to the distribution of the critical current. The main results are summarized as follows. (1) The measured distribution of the critical current values varying with bending strain was described well by the present approach. (2) When all specimens were damaged at high bending strains (0.338%–0.833% in the present work), the distribution of the critical current of the bent-damaged specimens was expressed by the three-parameter Weibull distribution function, the reason for which was revealed. (3) The distribution of the irreversible strain was estimated, with which the influence of the increase in the fraction of damaged specimens on the variation of c...

Analysis of mesoscopic stress states with delamination and their relation to critical current under bending deformation in Bi2223/Ag superconducting composite tapes

The mesoscopic stress and strain states of Bi2223/Ag/Ag-alloy superconducting composite tapes have been studied both analytically and experimentally under bending deformation. The tapes used in the present study were supplied as the standard samples for the VAMAS round-robin program (classified as VAM1 and VAM3). Detailed tape bending analysis was completed based on a damage-free initial state, and the calculated decrease of critical current, Ic, due to Bi2223 filament fracture was compared to the experimental Ic decrease. The calculated Ic was much lower than that obtained in the experiments for both tapes. Metallography indicated the presence of delamination in as-received as well as bend-tested tapes. The analysis was therefore modified to include delamination and it was completed for the case where delamination occupied the full width of the tape mid-plane. The calculated Ic with delamination was higher than the experimental results for both tapes. Delamination occupying partial width of the mid-plane explained this difference. Finally, the width ratio where delamination exists was calculated by comparing the analytical results with delamination and experimental results. This ratio increased with increasing curvature of the tape.