Wakako Araki

THERMAL STRESS ANALYSIS OF THERMOVISCOELASTIC HOLLOW CYLINDER WITH TEMPERATURE-DEPENDENT THERMAL PROPERTIES

ABSTRACT We analyzed the thermal stress on a thermoviscoelastic hollow cylinder with temperature-dependent thermal properties with the finite difference method. It was gradually heated at the inner surface and the outer surface was kept at the initial temperature. The cylinder material was thermorheologically simple and had a temperature-dependent coefficient of linear thermal expansion, thermal conductivity, and thermal diffusivity (and/or specific heat). A bisphenol A–type epoxy resin was chosen as the thermoviscoelastic material of the cylinder for numerical analysis. Based on these results, we discuss the effects of thermoviscoelasticity and temperature-dependent thermal properties on the stress field.

Effect of Particle Size on Fracture Toughness of Spherical-Silica Particle Filled Epoxy Composites

We investigated the particle size effects on the fracture toughness of epoxy resin composites reinforced with spherical-silica particles. The silica particles had different mean particle diameters of between 1.56 and 0.24µm and were filled with bisphenol A-type epoxy resin under different mixture ratios of small and large particles and a constant volume fraction for all particles of 0.30. As the content with the added smaller particle increased, the viscosity of each composite before curing remarkably increased. We conducted the single edge notched bending test (SENB) to measure the mode I fracture toughness of each composite. The fracture surface with the small particle content exhibited more rough areas than the surface with larger particles. The fracture toughness increased below the small particle content of 0.8 and saturated above it. Therefore, near the small particle content of 0.8, the composite had a relatively low viscosity and a high fracture toughness.

Time-temperature dependence of fracture toughness for bisphenol a epoxy resin

Abstract The relationship between the curing conditions and the time-temperature dependence of fracture toughness was investigated for bisphenol A epoxy resin. The glass transition temperature and Angells fragility parameter, which are obtained from thermoviscoelasticity measurements, were used to characterize epoxy resins cured under various conditions. Examination of the fracture toughness at various temperatures and displacement rates showed that it depends on both temperature and time, and that it follows the time-temperature equivalence principle. The time-temperature dependence of the fracture toughness was greatly affected by the fragility parameter. The fracture toughness of the resin with a smaller fragility parameter increased from lower temperatures to the brittle-ductile transition temperature than that of the resin with a larger fragility parameter when their glass transition temperatures were approximately 400 K. It was also found that the brittle-ductile transition temperature did not depend on the fragility parameter. This means that epoxy resin with a smaller fragility parameter has better fracture characteristics than epoxy resin with a larger fragility parameter if their glass transition temperatures are approximately 400 K.

Stabilization of a Zirconia System and Evaluation of Its Electrolyte Characteristics for a Fuel Cell: Based on Electrical and Mechanical Considerations

The purpose of this study is to clarify the relationship between ionic conductivity and phase transformation of zirconia system codoped with scandium oxide Sc 2 O 3 and ytterbium oxide Yb 2 O 3 . Aiming to achieve high ionic conductivity as well as high mechanical strength, the authors have also investigated the relationship between phase transformation and mechanical strength. The results have been discussed with respect to both the conductivity and the mechanical strength. The Sc- and Yb-codoped zirconia (ZrO 2 ) used as samples in this study were prepared by a standard solid-state reaction. X-ray powder diffraction (XRD) method was used to determine the crystal structures of the sintered samples. To detect any phase change between room temperature and 1273 K, thermal mechanical analysis (TMA) was conducted. To determine oxygen-ion conductivity in a temperature range from 873 to 1273 K in air, impedance measurements were performed with alternating current (ac). Single-cell performance was confirmed under the condition of 26.2 Pa partial hydrogen pressure. Finally, to measure bending strength, three-point bending tests were performed with a universal testing machine. The results of XRD and TMA showed that codoping of Sc 2 O 3 and Yb 2 O 3 into ZrO 2 successfully stabilized the cubic phase when the average radius ratio of these two dopants in total was close to the ideal one for the eight-coordinate. The ac impedance measurement demonstrated that the cubic-phase stabilization achieved a high conductivity. Adequate amounts of dopants produced oxygen vacancies for high conductivity without complex defects: ZrO 2 system doped with 1 mol % of Yb 2 O 3 and 8 mol % of Sc 2 O 3 showed the highest conductivity at 1273 K and 0.30 S/cm. The bending strength decreased with increasing the content of doped Sc 2 O 3 from 7 mol % to 11 mol %, depending on the amount of the tetragonal phase, which contributes to strengthen materials. In the performance test, the ZrO 2 system stabilized with doping 1 mol % Yb 2 O 3 and 8 mol % Sc 2 O 3 with thickness of 2.16 mm showed maximum power density at 1273 K, that is, 210 mW/cm 2 . From all the above tests, we recommend that, based on electrical and mechanical considerations, 1Yb8ScSZ is the present best option for an electrolyte material for a solid oxide fuel cell.

Ferroelasticity and spin-state transitions of LaCoO3

A uniaxial compression test of polycrystalline lanthanum cobaltite (LCO) was performed to investigate mechanical behavior of LCO in the temperature range of 83–553 K. Prepared by solid-state reaction, the electrical resistivity and the linear expansion coefficient of polycrystalline LCO, measured between 80 and 1273 K, exhibit distinct changes attributed to spin-state transitions of cobalt ions around 100–200 K and 400–600 K, but are relatively constant between 200 and 400 K. The stress-strain curve obtained under uniaxial compression shows strong nonlinearity due to ferroelastic domain switching process between 83 and 553 K. Initial Youngs moduli, critical stress, and dissipated energy evaluated from the stress-strain curves decrease by about a half with increasing the temperature, whereas there was no drastic changes even around the spin-state transition temperatures. The initial modulus agrees with the temperature dependence of the apparent Youngs modulus measured under low-stress cyclic loading.

Fatigue crack growth mechanism in cast hybrid metal matrix composite reinforced with SiC particles and Al2O3 whiskers

Abstract The fatigue crack growth (FCG) mechanism of a cast hybrid metal matrix composite (MMC) reinforced with SiC particles and Al 2 O 3 whiskers was investigated. For comparison, the FCG mechanisms of a cast MMC with Al 2 O 3 whiskers and a cast Al alloy were also investigated. The results show that the FCG mechanism is observed in the near-threshold and stable-crack-growth regions. The hybrid MMC shows a higher threshold stress intensity factor range, Δ K th , than the MMC with Al 2 O 3 and Al alloy, indicating better resistance to crack growth in a lower stress intensity factor range, Δ K . In the near-threshold region with decreasing Δ K , the two composite materials exhibit similar FCG mechanism that is dominated by debonding of the reinforcement–matrix interface, and followed by void nucleation and coalescence in the Al matrix. At higher Δ K in the stable- or mid-crack-growth region, in addition to the debonding of the particle–matrix and whisker–matrix interface caused by cycle-by-cycle crack growth at the interface, the FCG is affected predominantly by striation formation in the Al matrix. Moreover, void nucleation and coalescence in the Al matrix and transgranular fracture of SiC particles and Al 2 O 3 whiskers at high Δ K are also observed as the local unstable fracture mechanisms. However, the FCG of the monolithic Al alloy is dominated by void nucleation and coalescence at lower Δ K , whereas the FCG at higher Δ K is controlled mainly by striation formation in the Al grains, and followed by void nucleation and coalescence in the Si clusters.

Mechanical Properties of Nano/Micro-Silica Particles Bidispersed Epoxy Composites

Mechanical properties of nano/micro-silica particles bidispersed epoxy composites were investigated based on experimental results. The composite specimens varied with different compositions of nano and micro-silica particles (240 nm and 1.56

Use of ultrasonic back-reflection intensity for predicting the onset of crack growth due to low-cycle fatigue in stainless steel under block loading.

m) were prepared with the constant volume fraction, 0.30. The thermo-viscoelastic properties for the composites and the neat epoxy measured in the temperature ranges from 123 K to 523 K and compared to theoretical results according to Lewis and Nielsen’s law with the maximum particle packing given by Ouchiyama and Tanaka’s model. In addition, fragility derived from the thermo-viscoelasticity measurements was used to characterize the strength and fracture toughness of the composites. From results, we found that the thermo-viscoelasticity of the composite was dependent on nano and micro-particles packing, and its strength and fracture toughness were effectively evaluated by fragility.

Oxygen Diffusion in Yttria-Stabilized Zirconia Subjected to Mechanical Stress

The present study proposes the use of ultrasonic back-reflected waves for evaluating low cycle fatigue crack growth from persistent slip bands (PSBs) of stainless steel under block loading. Fatigue under high-low block loading changes the back-reflected intensity of the ultrasonic wave that emanates from the surface. Measuring the change in ultrasonic intensity can predict the start of crack growth with reasonable accuracy. The present study also proposes a modified constant cumulative plastic strain method and a PSB damage evolution model to predict the onset of crack growth under block loads.

PREDICTION OF FRACTURE INITIATION IN THERMO-VISCOELASTIC MATERIAL

The oxygen diffusion in 4, 8, and 14 mol%-yttria-stabilized zirconia subjected to uniaxial stresses in the [100], [110], and [111] directions is investigated. In the case of uniaxial stress in the [100] direction, the oxygen diffusion in 4YSZ and 8YSZ is facilitated in the tensile direction and deteriorated in the compressive direction without changing the total diffusion property, which could be caused by the elastic recovery force. For 14YSZ, however, the oxygen diffusion remains unchanged regardless of the stress, probably due to high yttria concentration. In the case of tensile stress in the [110] and [111] directions, the oxygen diffusion is also increased in the tensile direction, which could be attributed to the improvement of the oxygen diffusion in the <100> direction caused by the stress component in the <100> direction.