Tadaharu Adachi

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.

Controlling of Distribution of Mechanical Properties in Functionally-Graded Syntactic Foams for Impact Energy Absorption

In the study, novel fabrication processes of functionally-graded (FG) syntactic foams were developed to control distribution of the mechanical properties in the FG foams for highly impact energy absorption. In order to control mechanical properties, the density distributions in FG foams were graded by floating phenomenon of the light-weight micro-balloons in matrix resin during curing process. The density distribution in the foam could be controlled by adjusting the average volume fraction and the turning procedure of the mold before grading the micro-balloons in the foam. The compression tests of the fabricated FG foams suggested that the foams had high absorption of impact energy since the foams collapsed progressively due to the grading of the density distribution.

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.

Measurements of local elastic moduli by amplitude and phase acoustic microscope

Abstract A new method is suggested for the nondestructive measurement of elastic moduli in a localized area, 100–400 μm in diameter, by the complex V ( z ) curve using an amplitude and phase acoustic microscope. The inverse Fourier transform of the complex V ( z ) curve contains the reflectance function of a liquid-specimen interface. Therefore, the longitudinal, transverse and Rayleigh wave velocities for the specimen are simultaneously obtained by the inversion of the complex V ( z ) curve. The elastic moduli for glass obtained from wave velocities by acoustic microscope agree fairly well with those by other methods. The present method is applied to aluminium alloy, and it is shown that this method is useful in measuring the microscopic characteristics in inhomogeneous materials.

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.

Development of Integral Molding of Functionally-Graded Syntactic Foams

In this paper, we suggested a novel process to fabricate bulk functionally-graded (FG) syntactic plastic foams and evaluated mechanical properties of the fabricated FG foams. The density distribution in the foams was graded due to floating phenomena of the micro-balloons in the matrix resin before gelling in the fabrication process. The distribution of the density could be predicted by finite difference analysis with Richardson and Zaki’s formula for Stoke’s velocity. The density distribution was found to be controlled by average density, micro-balloons size, and temperature and duration of the fabrication process and so on. The progressive collapse of the FG foam due to grading mechanical properties was confirmed to be effective to improve energy absorption in the compression test.

Magneto-Thermo-Elastic Stresses Induced by a Transient Magnetic Field in a Conducting Hollow Circular Cylinder

We investigate the dynamic and quasi-static behavior of magneto-thermo-elastic stresses induced by a transient magnetic field in a conducting hollow circular cylinder. A transient magnetic field defined by an arbitrary function of time acts on the outer surface of the hollow cylinder and parallel to it. The fundamental equations of plane axisymmetrical electromagnetic, temperature and elastic fields are formulated, and solutions for the magnetic field, eddy current, temperature change and dynamic and quasi-static solutions for stresses and deformations are analytically derived in terms of the arbitrary function. The stress solutions are determined to be sums of a thermal stress component caused by eddy current loss and a magnetic stress component caused by the Lorentz force. The case of a magnetic field defined by a smoothed ramp function with a sine-function profile is examined in particular, and the dynamic and quasi-static behavior of the stresses and deformations are numerically calculated.

Fabrication of Bulk Functionally-Graded Syntactic Foams for Impact Energy Absorption

Function of functionally-graded (FG) foams as energy absorption material for impact was discussed on the basis of theoretical analysis, and fabrication process of the foams was proposed in the paper. The FG foams were found to be useful as impact absorber due to progressively local fracture or cushion in the theoretical analysis. Next the fabrication process of the FG foams was suggested. The graded dispersion of the micro-balloons was conducted before curing the matrix resin in the process. The density distributions in the FG foams were confirmed to be predicted by the numerical analysis on the basis of floating the micro-balloons. Finally, compression tests were carried out to evaluate mechanical properties.

Fracture energy of nano- and micro-silica particle-filled epoxy composites

The effects of particle size on the surface energy of the fracture surface and elastic moduli of nano- and micro-spherical silica-particle-filled epoxy composites were investigated experimentally. The Youngs modulus and shear modulus of the composites agreed well with the results evaluated by Lewis and Nielsens equation and were shown to be dependent on only the volume fraction of the particle. Surface energies of the fracture surface were evaluated from the critical energy release rates and the surface areas including the unevenness. The surface energies were shown to be constant regardless of the volume fraction of the particle.