Hidehiko Kimura

In Situ Synchrotron Measurement of Internal Stresses in Solid-Oxide Fuel Cell during Red-Ox Cycle

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.

Effect of residual stresses on fatigue strength of severely surface deformed steels by shot peening

The compressive stress distribution below the specimen surface of severely surface deformed steels by shot peening was investigated by using laboratory X-rays and high-energy X-rays from a synchrotron radiation source, SPring-8 in the Japan Synchrotron Radiation Research Institute. Medium carbon steel plates were heat treated in two different conditions. The Vickers hardness of material-A and B after heat treatment is 408 and 617 HV, respectively. The specimens were shot-peened with fine cast iron particles of the size of 50 m. The coverage was selected to be 5000%. For the synchrotron radiation, by using the monochromatic X-ray beam with several energy levels, the stress values at the arbitrary penetration depth were measured by the constant penetration depth method. The shot-peened specimens were fatigued under four-point bending. The improvement of fatigue strength of material-A was not so large because of large surface roughness. On the other hand, for the material-B, the surface roughness was smaller and the fatigue strength was higher than that of ground specimens.

EBSD-AFM Hybrid Analysis of Crack Initiation in Stainless Steel under Fatigue Loading

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 Taylors 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.

Fatigue thresholds of discontinuously reinforced aluminum alloy correlated to tensile strength

Abstract The fatigue strength of reinforced and unreinforced aluminum alloys with silicon carbide particles or whiskers increases in proportion to the ultimate tensile strength. The proportional constant is slightly larger for composites than for unreinforced alloys. The threshold stress intensity range ΔKth tends to decrease with increasing tensile strength. When compared at the same tensile strength, ΔKth tends to be larger for composites than for unreinforced alloys. The effective threshold stress intensity range, ΔKeffth, is fairly constant irrespective of the tensile strength. The characteristic defect sizes, a′0 and a0, tend to be smaller with increasing tensile strength. When compared at the same tensile strength, a′0 is larger for composites, which indicates a low notch sensitivity of composites.

Degradation of a sintered Cu nanoparticle layer studied by synchrotron radiation computed laminography

Abstract The degradation process in a sintered Cu nanoparticle layer was studied by synchrotron radiation computed laminography (SRCL), which allows the high-resolution nondestructive observation of internal cracks in flat devices. A Cu layer was produced by sintering Cu nanoparticles between a Si chip and a direct bonded aluminum (DBA) substrate. This layer was then stressed by applying multiple thermal cycles, after which SRCL measurements were repeatedly performed. Images of the distributions of sintered Cu densities reconstructed from the SRCL observations show cracks with tortuous shapes generated around the dense Cu nanoparticles; these cracks propagated along the direction from the DBA substrate to the Si chip during thermal stress testing. Whereas crack propagation in typical soldering materials occurs along grain boundaries, the similar process in Cu nanoparticle layers is regulated by their sintered density distributions.

Fatigue Damage Mechanism of Nanocrystals in ECAP-Processed Copper Investigated by EBSD and AFM Hybrid Methods

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.

Determination of Residual Stress Distribution in Severe Surface Deformed Steel by Shot Peening

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.

Measurement of Stress Distribution Near Fatigue Crack in Ultra-Fine Grained Steel by Synchrotron Radiation

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.

Quantification of lead-free solder fatigue by EBSD analysis

Abstract Since lead-free solder was adopted for in-vehicle electronic components, a sufficient number of years have passed to allow examining the condition of lead-free solder joints in components used in the field. For this paper, using an EBSD analysis technique, solder joint degradation analysis was conducted on specimens subjected to accelerated testing and field-aged components. Degradation indicators of the level of grain refining and the amount of strain in the Sn phase in solder were obtained from specimens subjected to accelerated testing. The analysis results of field-aged components revealed increases in the degradation indicators commensurate with the age of components, thereby we were able to estimate field stress from the recovered components.

Strain Measurement Near Fatigue Crack in Ultrafine-Grained Steel by Polychromatic Synchrotron Radiation

For engineering components with cracks, it is very important to evaluate the reliability for fracture. The X-ray diffraction method is now widely used to measure non-destructively the loading and residual stresses in crystalline materials. Synchrotron radiation sources provide the X-rays with extremely high intensity as well as a narrow divergence. The high intensity X-rays with a narrow divergence enables stress measurements in a localized region. The strain distribution near the fatigue crack in the steel plate with ultrafine-grained surface layers, called SUF plates, was measured by the polychromatic X-ray from synchrotron radiation at SPring-8. The spatial resolution in the direction parallel to the crack propagation direction was 0.1 mm. The strain distributions at several applied stress levels were determined for six diffraction planes. The measured strain distribution was compared with the result calculated by the FE analysis. The average value of the measured strains for several diffraction planes agreed well with the calculated results.