The essential problems of structural ceramics are low fracture toughness, considerable size effect of fracture stress and large scatter of both values.
In this study, various silcon nitride (Si₃N₄) ceramics are manufactured by hot pressing and N₂ gas pressure heat treatments. The effects of sintering condition on the grain size and mechanical properties of these materials are examined in detail. And, the relation between critical process zone size and decrease rate of bending strength due to grinding is also investigated.
In result, the grain size of Si₃N₄ is clearly varied with the sintering condition. The fracture toughness of Si₃N₄ is increased with the increase of grain size. On the other hand, the shape parameter of fracture toughness is decreased. The highest fracture toughness measured is 9.73 MPa√m. IF (Indentation Fracture) test results revealed a distinct anisotropic behaviour. Plane-strain fracture toughness K_IC is decreased as the fracture stress σ_o of a plain specimen increase. The dependence of fracture stress on the crack length for these sintered materials is also fairly well expressed by using the process zone size fracture criterion. It is also cleared that the critical process zone size D_c is also a useful index for the sensitivity to grinding.

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At elevated temperature which is still below the creep operating region in air or water environment, characteristics of fatigue crack growth threshold for alloy-and carbon-steels are not well clarified. For the structural integrity operating at those temperature ranges, such as nuclear reactors, it is an urgent task to make clear them up.
In the previous study, the fatigue crack growth rates and the crack closure were measured for a low alloy steel, JIS SFVV3 (A508-3), used for the pressure vessel of the nuclear reactor. The temperature range examined was from room temperature up to 350°C which covers the operating temperature at 288°C of the nuclear reactors. Further detailed studies were attempted by accumulating the threshold data for various alloy- and carbon-steels. Influencing factors are made clear and a simple method for predicting thresholds is developed by accounting for the above effects.
The temperature dependence on the growth rate and the crack closure and the stress ratio effect are studied. Especially near the fatigue threshold where the temperature effect is significant, it is found that the oxidation produced on the crack surfaces at elevated temperature have an important role. Their contribution is examined quantitatively. At room temperature, the fretting oxide debris (Fe₂O₃) cause the crack closure. Higher the temperature, the oxidation (Fe₃O₄) protects the crack surfaces from the occurrence of fretting oxide. Thus the fatigue threshold decreases as increasing temperature up to around 100°C. Hereafter, it increases due to the increasing in thickness of oxidation which promotes the occurrence of crack closure. The dependencies of the yield stress and the cyclic frequency on the growth rates can be explained from the same view point.

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Computational study on the hypervelocity- impact phenomena of projectiles on double-layered bumpers consisting of different materials was conducted by using of the PISCES-2DELK code. In the present work, the effect of the impedance mismatching in the layered bumper on the characteristics of the damage capability of the solid fragments behind bumper plates was discussed. It was evaluated that the predominant factor to determine the effectiveness of the bumpers on solid fragmentation was the intensity of the concentrated load acting on the mainwall. It was also found that the trace of the motion of the elements forming the surface of a spherical projectile was useful to estimate the deformation of the projectile with high-strain-rate.

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In recent years gas holders have become larger in capacity and higher in pressure in order to use the gases which are accompanied in the steel manufacturing processes effectively. In these trends a new type gas holder which is characterised by a rational structure and perfect sealing has developed. It includes the following major improvements:
(1) Cylindrical or spherical structures are adopted in side wall, piston and roof in order to be applicable to relatively high gas pressure.
(2) Perfect gas sealing system is developed by using hydrostatic oil pressure whose level is kept stable by durable packing rings composed with fiber reinforced rubber.
This paper shows the structural characteristics of this new type gas holder, the durability of packing ring for oil dipping, the wear properties for sliding and the performance test of the gas holder with the new sealing device by using the model, approx.1/10 in scale. In addition the operation data of new type gas holders are described in comparison with the fundamental test results.
The following conclusions are obtained.
(1) NBR, the main composition of packing ring, is proved stable for the long term oil dipping, even if gas impurities are included. Packing ring is also verified to have mechanical durability and wear speed is assumed to be abt. 1×10^-3mm/km under the operation of gas holder.
(2) In the performance test the piston with new gas sealing device slides smoothly according to the change of gas volume and performs the perfect gas sealing. And the friction force of piston during the slide can be analysed by the pressures of packing ring and stored gas.
(3) The gas pressure fluctuation of new type gas holder can be assumed from the results of the model gas holder. And these assumed values are in good accordance with the actually measured ones.
(4) Six new type gas holders have been constructed and have been working well. These facts show that the gas holder with new sealing device is developed successfully.
Structural strength and displacement of gas holder will be discussed based on their analyses under wind load and seismic load in the following papers.

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In manufacturing, cold and hot plastic working and welding a stainless clad steel, the important considerations on heat-treatment were described. In most cases, the selection of heat-treating conditions for clad steels was made clear to be considerably difficult.
Therefore, it was perferable provided that the heat-treatment of the clad steel could be avoided. However, if heat treating is required, the deterioration of properties of both cladding metal and base metal was needed to be minimarized by the adequete heat-treating condition and by using the materials being designed the chemical compositions.

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