Sohei Iwamoto

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...

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.

Thermally and mechanically induced residual strain and strain tolerance of critical current in stainless steel-laminated Bi2223/Ag/Ag alloy composite superconductors

It is known that the tensile strain tolerance of the critical current of Bi2223 composite superconductors is improved when the compressive residual strain of Bi2223 filaments in the current transport direction (sample length direction) is enhanced. In the present work, the thermally and mechanically induced residual strain accumulation process of a stainless steel-laminated Bi2223/Ag/Ag alloy superconducting composite tape fabricated at American Superconductor Corporation was studied. A calculation procedure is presented for a description of the strain change of the constituents (Bi2223, Ag, Ag alloy and stainless steel) during cooling and heating, and during lamination, followed by the stress relaxation. As the input values, the mechanical property values of the constituents and the residual strain of Bi2223 filaments in the laminated composite tape at room temperature, measured with x-rays, were used. Such an approach revealed the change of residual strain of each constituent in the thermal and mechanical process in the laminated composite tape. Then, from a comparison of the residual strain accumulation in the laminated tape with that in the insert tape, the effects of the stainless steel lamination on the enhancement of the compressive residual strain of Bi2223 filaments and the improvement of the tensile strain tolerance and strain tolerance window of critical current at 77 K of the composite tape are discussed.

Thermal stress-induced fracture of coating layer of galvannealed steel

The thermal stress-induced multiple fracture of the Fe-Zn intermetallic compound coating layer of galvannealed steel was investigated for samples with IF (interstitial free steel) and SPCC (steel plate cold commercial, Japanese Industrial Standard) as substrates. The analysis of the measured length of the multiply fractured coating layer showed that the fracture strength of the coating layer was 260–280 MPa and the residual stress around 720–740 MPa of the coating layer was reduced to around 130–140 MPa through the multiple fracture. The influences of the coating layer thickness and the stress–strain curve of the substrate on the multiple fracture behavior of the coating layer were also clarified.

Fracture Behavior of Brittle Coating Layer on Metal Substrate

The compressive fracture and spalling of multiply-cracked brittle coating layer on metal substrate under tensile stress was studied for the anodic-oxidized aluminum wire with the Al2O3 coating layer and galvannealed IF steel plate with the intermetallic compound coating layer consisting of ζ, δ1 and Γ1 and Γ phases. The thin Al2O3 coating layer on Al wire was fractured by buckling in the circumferential direction by the compressive hoop stress, but thick layer was debonded by the tensile radial stress at interface. The thickness of the coating layer at the transition from the compressive fracture to interfacial debonding was around 30 µm. In the galvannealed steel plate sample, the (ζ+δ1) phases were fractured by buckling in the width direction, resulting spalling of the (ζ+δ1) phases in the first stage, and the remained (δ1+Γ1+ Γ) phases or (Γ1+Γ) phase was again fractured by buckling, followed by the spalling of the remained phase.

An Image-Based Microscale Simulation of Thermal Residual Stresses in DSE Oxide Ceramic Composite

An image-based microscale analysis was conducted by using finite element method (FEM) to simulate the thermal residual stresses in directionally solidified eutectic (DSE) oxide ceramic composites from the thermal expansion and elastic properties and the microstructure features of the constituent phases. This microscale analysis allows a real simulation of morphologies of constituent phases such as size, array and shape. Meanwhile, this model can be applied not only for the calculation of thermal residual stresses, but also for the calculation of mechanical properties. In this work, simulations focus on the distribution of thermal residual stresses in the directionally solidified eutectic (DSE) Al2O3/ Y3Al5O12 (YAG) ceramic composite. A good agreement between simulated and measured thermal residual stresses was observed.