Papers by Author: Hidehiko Kimura

Abstract: The internal stress in solid-oxide fuel cells (SOFCs) was evaluated during the thermal, reduction and re-oxidation cycles by using high-energy X-ray synchrotron radiation of about 70 keV at Beam line BL02B1 of SPring-8. The oxidized cell has a compression of about 400 MPa in the c-ScSZ electrolyte and a tension of 50-100 MPa in the NiO-YSZ anode at room temperature. In-situ measurement during the thermal cycle in an air atmosphere, the internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. After the thermal cycle, the internal stress returned to its initial value. In the measurement during the reduction cycle, the internal stress was smaller than that measured during the cooling cycle after the anode was reduced from NiO-YSZ to Ni-YSZ. In the re-oxidation cycle of a reduced cell, the internal stress in the electrolyte went into tension above 800 K when the anode was re-oxidized from Ni-YSZ to NiO-YSZ. This tensile stress is responsible for possible fracture of unit cells in SOFCs.

Abstract: The compressive stress distribution below the specimen surface of a nanocrystalline medium carbon steel was investigated nondestructively by using high-energy X-rays from a synchrotron radiation source, SPring-8 (Super Photon ring-8 GeV) in the Japan Synchrotron Radiation Research Institute. A medium carbon steel plate was shot-peened with fine cast iron particles of the size of 50 μm. By using the monochromatic X-ray beam with three energy levels of 10, 30 and 72 keV, the stress values at the arbitrary depth were measured by the constant penetration depth method. The stress was calculated from the slope of the sin2ψ diagram. Measured stress corresponds to the weighted average associated with the attenuation of the X-rays in the material. The real stress distribution was estimated by using the optimization technique. The stress distribution was assumed by the third order polynomial in the near surface layer and the second order polynomial. The coefficients of the polynomials were determined by the conjugate gradient iteration. The predicted stress distribution agreed well with that measured by the conventional surface removal method.

Abstract: Electron backscattering diffraction, EBSD, technique as well as atomic force microscopy, AFM, was employed to investigate fatigue damage mechanism in ultrafine-grained copper processed by equal channel angular pressing, ECAP. The fatigue damage evolution under axial tension compression was investigated. The results show that linearly shaped fatigue damage was introduced in the scale of micrometers in spite of the average grain size of 300 nm. The linear damage was randomly oriented when the shear direction of the last ECAP-pressing in perpendicular to the loading axis. The orientation analysis by EBSD revealed that the linear damage is introduced in the area with the same crystallographic orientation in the direction of the maximum Schmid factor as in the slip deformation in coarse-grained materials. The comparison before and after fatigue tests shows the grain coarsening in the area where large linear fatigue damage was formed. It is considered that strain concentration at the edge of the slips introduced in a relatively coarse ultrafine grain causes the grain rotation and deformation in the adjacent nano-sized grains, resulting in the grain coarsening and subsequent propagation of the slips in the order of micrometers.

Abstract: Both EBSD and AFM methods were used to investigate the active slip systems and fatigue crack initiation behavior in face-centered cubic polycrystalline metal, austenitic stainless steel, SUS316NG, under cyclic torsional loading. Most active slip planes are the primary slip planes having the largest Schmid factor. Grains with slip band cracks or transcrystalline cracks have larger Taylor's factors. On the basis of EBSD and AFM observations, h, the depth of intrusion vertical to the surface, S, and the component of the slip displacement perpendicular to the surface trace, SB, showed a sharp increase at the onset of crack initiation. The critical value of SB at crack initiation was 170 nm.

Abstract: Single-edge-notched specimens of ultrafine-grained steel were fatigued. The mean grain size of the steel is about 2 micrometers. Propagation behavior of fatigue cracks was observed with the crack closure. The resistance of the crack propagation of ultrafine-grained steel was larger than that of conventional steels. The crack closure acted as an important role for the larger resistance of fatigue crack propagation. After fatigue tests, stress distribution near the fatigue crack was measured by monochromatic X-rays from synchrotron radiation. The irradiated area was 100 µm x 100 µm. Residual and loading stress distributions ahead of the crack tip and on the crack wake was measured at the maximum stress intensity factor and zero applied load. The stress was determined by sin2ψ method. The measured stress was compared with the value calculated by FEM and the fatigue crack propagation model. The stress distribution at the maximum load and residual stresses agreed very well with the calculated results. The crack opening stress calculated by the residual stresses agreed with the experimental result.