Papers by Author: Yoshiaki Akiniwa

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 railway carbody structure for commuter and suburban train services in Japan is often made of austenitic stainless steel, which is used in the form of thin metal sheets manufactured by cold rolling. Spot welding and laser welding at lapped joints are used in the construction of such carbodies, but it is difficult to observe the strain distribution around these weld zones, which represent the critical area of the body structure’s strength. The objective of this study is to ascertain the strain distribution in the stainless steel around the weld zone of the carbody structure. To enable observation of this distribution, a strain scanning method using high-energy synchrotron radiation was applied to the strain measurement of austenitic stainless steel. The transmission method was applied in order to observe the internal weld zone. Using this method, we can measure the strain distribution from the surface to the inside of the weld zone. A lapped joint specimen, prepared by welding 2-mm-thick plates using the laser welding method, was used for measurement. Austenitic stainless steel generally poses problems in the measurement of strain due to its coarse grain and crystal texture. The gage volume in this measurement had a width of 3 mm and a height of 0.15 mm. The measurement provides the strain distribution of both residual strain and strain under loading, and the results obtained successfully show the distribution of strain in the weld zone. In addition, the differing tendency between the distribution of residual strain and that of strain under loading is clarified. The full width at half maximum (FWHM) value shows a difference between the tendency of the measured value of the weld zone and that of the base material.

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: Monotonic and cyclic loadings were subjected to electrodeposited copper foils (thickness is 8 and 20 μm), and the deformation behavior was observed. In-situ X-ray stress measurement was carried out under monotonic loading. The tensile strength of 8 μm foil was higher than that of 20 μm foil. On the other hand, the elongation of 8 μm foil was smaller. When the plastic deformation occurred, difference between the X-ray stress and the applied stress became large. The difference of 20 μm foil was larger than that of 8 μm foil. Fatigue strength of 8 μm foil was also higher than that of 20 μm foil. The value of the full width at half maximum, FWHM, increased dramatically at the first cycle, and then the value became nearly constant. Just before fracture, the value increased again. The change in FWHM corresponded to the change in the accumulated ratchet strain.

Abstract: Molecular dynamics simulation is conducted to investigate the effect of notch depth on the deformation and fracture behavior of a single crystal copper which is expected to a conductive material of micro-devices. In the stress – strain relationship, a normal stress increases with increasing applied strain. Then, the normal stress decreases rapidly. When the stress decreases, the dislocation emits from a notch root and the stacking fault is formed on the {111} plane, which is slip plane of the fcc crystalline structure. The maximum stress decreases with notch depth. The non-damaging defect size is quite small. The shear stress in the slip direction at dislocation emission is constant irrespective of the notch depth. The criterion of the dislocation emission is given by the critical value of the resolved shear stress in the sliding direction.

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: Tension-compression fatigue tests under various mean stress conditions were conducted with round bar specimens of short glass fiber reinforced polybuthyleneterephthalate made by injection molding. Under cyclic loading with high mean stresses, the creep phenomenon became predominant and the ratcheting deformation increased with the number of cycles. This phenomenon is characteristic of plastics including short glass fiber reinforced plastics. The experimental data of the fatigue strength at the stress ratios above 0.7 were lower than the prediction based on the modified Goodman diagram. We propose to use the creep rupture strength, σc, instead of the tensile strength, σB, as the strength without mean stress and the parabolic equation for a constant life in the amplitude-mean stress (σa-σm) diagram. Our new design equation for the mean-stress effect on the fatigue strength on plastics is as follows: σa = σw – (σw / σc 2) σm 2, where σw is the fatigue strength at the stress ratio R=-1 and σa is the stress amplitude under a mean stress of σm. We also proposed a method to obtain the constant-life relation from limited experimental data.

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: Two kinds of electrodeposited copper foils (thickness is 8 and 20 μm) were loaded statically, and the deformation behavior was observed. In-situ X-ray stress measurement was carried out under tensile loading. Fatigue tests were also conducted to observe the effect of the thickness on the fatigue strength. Change in the line broadening with stress cycles was observed to evaluate the fatigue damage. The tensile strength of 8 μm foil was higher than that of 20 μm foil. When the foils were loaded within elastic region, the stress measured by the X-ray method agreed with applied stress. When the plastic deformation occurred, difference between the measured stress and the applied stress became large. The difference of 20 μm foil was larger than that of 8 μm foil. Fatigue strength of 8 μm foil was also higher than that of 20 μm foil. The value of the full width at half maximum, FWHM, increased rapidly at the first cycle, and then the value became nearly constant. Just before fracture, the value increased again. The change in FWHM corresponded to the change in the accumulated ratchet strain.

Abstract: The strain scanning method was applied to the evaluation of the subsurface distribution of the residual stress beneath the shot-peened surface of an austenitic stainless steel SUS304L which had coarse grains and preferred orientation. The experiment was performed at beam line BL22XU at SPring-8 using monochromatic X-rays of 70.14 keV and a Ge (111) analyzer. The sizes of both incident and receiving slits were 2 × 0.2 mm2. The specimens were annealed or shot-peened and had the dimensions of 20 × 20 × 5 mm3. The grain size was about 37 μm. In order to obtain the diffractions from an enough number of grains, various types of oscillation methods, which were translation, rotation and tilting of the specimen, were examined. The translational oscillation was found to be enough to obtain the accurate strain distribution. By combining the translational oscillation method with the correction to the surface aberration, the subsurface distribution of the residual stress of shot-peened austenitic stainless steel was successfully determined.