Tadashi Shiota

Dependence of the particle emission during the fracture of silica glass on its mechanical property

The emission intensities of photons, negative and positive charges during the fracture of silica glass and its bending strength were measured to investigate a dependence of such particle emission on the mechanical property. The emission intensities of these particles were sharply increased and showed peaks at the moment of the fracture. In the negative and positive charge emissions, there was no clear correlation between the bending strength and the peak emission intensity. In contrast, there was a positive correlation between them in both of the UV/VIS (160 to 650 nm) and IR (640 to 850 nm) photon emissions. Moreover, the ratio of the UV/VIS photon emission intensity to the IR increased with the bending strength. The result in this study suggests that crack propagation behavior as well as the released energy at the fracture would influence the photon emission during the fracture of silica glass.

Sintering and mechanical properties of gadolinium-doped ceria ceramics

Gadolinium-doped ceria (GDC) ceramics were made by sintering at various temperatures from 1000°C to 1400°C in air. The true density and apparent density were measured to calculate the relative density of GDC ceramics. The change in relative density revealed that densification of GDC ceramics increased up to 1200°C, and thereafter turned downward. It was suggested that pores were formed at 1300°C and 1400°C due to non-stoichiometry of ceria. JIS-type specimens were cut from the sintered body and tested by 4-point bending. Youngs modulus and bending strength decreased with increasing the sintering temperature from 1200°C to 1400°C, corresponding to the change in the relative density.

Gas Pressure Dependence of Photon Emission Accompanying Fracture of Polycrystalline MgO in Nitrogen

The photon emission accompanying fracture of a polycrystalline MgO was investigated at room temperature under N2 gas pressures from 10-4 to 105 Pa. At fracture, the ultraviolet, visible and infrared photon emissions instantaneously increased, and then rapidly decreased in most of the experimental conditions. However, in a N2 gas pressure of around 100 Pa, their peak counts lasted for about 10 milliseconds, and the amount of the UV photon emission was fifteen times larger than those obtained in the other N2 gas pressures. This abrupt increment in the emission was explained by the luminescence due to N2 gas discharge according to the classical Townsend’s theory. In conclusion, the photon emission accompanying fracture of a polycrystalline MgO mainly originated from the excited defects as reported by the authors previously, but the N2 gas discharge had a supplementary effect on the emission around a specific N2 gas pressure.

Time-resolved measurement of photon emission during fast crack propagation in three-point bending fracture of silica glass and soda lime glass

Simultaneous time-resolved measurements of photon emission (PE) and fast crack propagation upon bending fracture were conducted in silica glass and soda lime glass. Observation of fracture surfaces revealed that macroscopic crack propagation behavior was similar between the silica glass and soda lime glass when fracture loads for these specimens were comparable and cracks propagated without branching. However, a large difference in the PE characteristics was found between the two glasses. In silica glass, PE (645–655 nm) was observed during the entire crack propagation process, whereas intense PE (430–490 nm and 500–600 nm) was observed during the initial stages of propagation. In contrast, only weak PE was detected in soda lime glass. These results show that there is a large difference in the atomic processes involved in fast crack propagation between these glasses, and that PE can be used to study brittle fracture on the atomic scale.

Estimation theory for random force exerted on grains during superplastic deformation of ceramics

This paper focuses on grain motion during superplastic deformation in TZP (Tetragonal Zirconia Polycrystal) ceramics. After estimation theory for random force was developed, the kinetic data of grains were stochastically analyzed, and random force acting on grains was estimated: At an earlier stage of superplastic deformation, distribution of random force differs between the loading direction and its normal direction. As superplastic deformation advances, however, it becomes identical in two directions. Such the change in the random force is discussed in terms of constraint by surrounding grains.

Random walk analysis of grain motion during superplastic deformation of TZP

This study focuses on grain motion in TZP (Tetragonal Zirconia Polycrystal) ceramics during superplastic deformation. The specimen was 16 times elongated repeatedly at 1400°C in air. The increment of true plastic strain was set to be 2%, and the specimen was deformed up to 30.3% true plastic strain finally. After each deformation, displacement vectors of specified 748 grains were measured from their position vectors determined by FE-SEM micrographs. As a result, the grains move to the tensile loading direction in zigzag way. And also, the zigzag motion changes with plastic strain: The grains move randomly (random walk motion) by the first 15% true plastic strain, and then grain motion becomes spatially uniform gradually. It is related to changes of constraint of surrounding matrix.

Detection of damage initiation and its evolution in insulating brick from non-linearity of stress/strain curve

This paper deals with experimental verification of the present authors theory relating to estimation of damage accumulation in materials showing non-linear stress/strain curves. The theory is based on parallel elements model and gives that the second order derivative of a stress/strain curve is proportional to a probability density function of strain at which each element is damaged. Uni-axial compression test was conducted for an insulating brick and its probability density function is obtained from the non-linear stress/strain curve. The function coincides well with acoustic emission data simultaneously measured in the compression test. This fact shows that the theory is useful for quick and brief estimation of damage initiation and its evolution in insulating bricks.

Estimation of Weibull parameters from parameters of initial distribution of flaw size

The distribution of the largest flaw size is derived from the initial distribution of flaw size based on extreme value statistics, and also the distribution of fracture origin size is given by transforming Weibull distribution by fracture mechanical relation. These two distributions are equivalent under uniaxial loading. By using this relation, their parameters are related each other and Weibull parameters are estimated from the parameters of the initial distribution of flaw size and the number of links.

Estimation of the number of cracks generated by compressive stress in polycrystalline graphite using ultrasonic method

A theory was developed to estimate the change in the apparent sonic velocity in porous ceramics by solving Lavrovs integral equation of the cylindrical inclusion problem. It was shown that the change in apparent sonic velocity was caused by phase shift of the sonic wave. To extend the theory to the case of multiple inclusions, the superposition principle was applied. The theory was verified by analyzing the apparent sonic velocity during a compression test of polycrystalline graphite, and the number of cracks that formed during loading was estimated to be 700, when the dimensions of crack length and width were assumed to be 1mm and 10μm, respectively.

The Role of Strength, Fracture Cause, and Fracture Location Data on the Reliability Analysis of Ceramics

Strength reliability of ceramics depends on accuracy of parameters involved in the probability distribution function for fracture. The parameters are usually estimated by use of strength data. However, one may have additional information in the experiment, such as fracture cause data, fracture location data, flaw-size data and flaw-orientation data. In this paper, we will incorporate these additional information in the parameter estimation to improve the accuracy of the reliability. A new theory on the asymptotic variances is presented.