To improve the damage evaluation methods in the design code for Fast Breeder Reactors (FBRs), a series of creep—fatigue tests of structural models under thermal transient loadings are going on at Oarai Engineering Center of the Power Reactor and Nuclear Fuel Development Corporation (PNC). Test models are designed to incorporate representative structures of components and pipings used in FBRs and are subjected to severer cyclic thermal transients than those experienced in FBRs. The test is planned to be continued until failure occurs. This paper describes the creep—fatigue test results and their damage evaluation for the first test model.A 40 mm thick vessel model made of SUS304 austenitic stainless steel was subjected to cyclic thermal transients, in which sodium at 600°C and 250°C flowed repeatedly. The period of each transient was 2 h. Cracks were observed at seven test portions in the model after 1002 cycles of the thermal transients.Elastic and inelastic analyses were performed to evaluate creep—fatigue damage and crack propagation. The safety margins included in the creep—fatigue design methods based on elastic analysis as well as those based on inelastic analysis are discussed. Finally fracture mechanics analyses were performed to explain the observed crack growth.
Ultrafine silicon carbide powders of high purity have been prepared by pyrolysis of tetramethyldisilane at temperatures 700° to 1400°C. The diameter of the particles changed from 5 to 200nm depending on the pyrolysis conditions. The ultrafine particles consist of small crystallites of β-type SiC, arranged randomly in the particles. The molar ratio (C/Si) of the powders can be also controlled within the range 0.9 to 1.2 by pyrolysis conditions such as pyrolysic temperature and reactant concentration. No admixtures of carbon such as carbon soot or film on the SiC particles were observed with a high-resolution electronmicroscope even in the powders having (C/Si) ratio more than 1.0. The formation mechanism of the ultraf ine SiC powders is discussed in relation to the structure.
An overcut fly-milling test with inclined workpiece was performed to accurately evaluate the cutting and wear characteristics of diamond and CBN grains, Namely, a quantitative evaluation of the chipping and tip wear of a single grain was made by tracing across the grooves with a stylus instrument. The milling experiment was carried out on a surface grinder, in dry, at cutting speeds 12.7 and 38.0m/s, and feeds 1.3 and 4.0m/min. The work materials used were hardened SUJ 2 steel, P 20 carbide, alumina, Si3N4, and silicon. The test grains were natural diamond, CBN, green SiC, and fused alumina, where grit size was #12 with an exception of CBN grain (#14-20). The CBN grain had the most high resistance to chipping and attritious wear for cutting SUJ 2 steel. However, the CBN grain as well as A and GC grains could not cut the other work materials due to grain fracture. The high chipping probability of the diamond grains was found on a Si3N4 ceramic. Also, the most fine chipping appeared on this work material. The attritious wear of the diamond grain was most remarkable on the P20 carbide.
Void size dependency of the fracture strength of silicon nitride ceramics was studied to develop a failure model from a void. Fracture strength of silicon nitride ceramics including spherical voids generally decreased with increasing void size. A model of a spherical void with a circumferential crack was applied to estimate the crack extension. The length of the circumferential crack was calculated to be about 30μm, which corresponded to 10-15 times of the average grain diameter. Thus, it was assumed that the microcrack extension from a void occurred during loading before the maximum load was reached. Considering the transition of the fracture toughness from single-crystal to polycrystalline, it was concluded that the critical crach extension from the void was attributed to the dependence of the fracture toughness on crack size/grain size ratio.
This review examines the stages of the sol-gel process, including hydrolysis, condensation, gelation, aging, drying, and sintering. The species produced in the sol are polymeric, rather than being dense glass-like colloidal particles. Considerable control over the structure of the polymer is possible through an understanding of the chemistry of hydrolysis and condensation. The properties of the gel and its response to heat treatment are sensitive to the structure created in the sol stage. Solvent must be removed slowly to prevent the high capillary stresses from causing cracking. A model of drying is presented that explains the relationship between cracking, drying rate, gel size, and permeability. Heat treatment causes densif ication of the solid phase, as well as collapse of the pores. The sintering behavior of gels is complex, because the viscosity of the gel is affected by concurrent structural relaxation and changes in hydroxyl content.
Fine amorphous silicon nitride powders were prepared by vapor phase reaction between nitrogen-containing, chloride-free organosilicon compound and NH3 at the production rate of-200g/h. Crystallization was carried out by heat treating the resulting amorphous powders in Ar atmosphere at 1500°C. High purity crystalline silicon nitride powders could be synthesized with metallic impurities Fe, Al, Ca 95wt%, respectively. Mechanical properties of hot-pressed silicon nitride from the resulting crystalline powders were comparable to hot-pressed bodies fabricated from high purity silicon nitride powders by imide decomposition and carbothermic reduction of silica.
SiC powder has been synthesized using CO2 laser-heated vapor phase reactions in the silane-methane gas system. SiC particles form via a two-step reaction: silicon particle formation and subsequent carburization of the silicon particles. It is shown that the silicon particles form at the lower edge of the reaction zone, and carburization occurs in the hottest region, through which the CO2 laser beam passes. The formation of silicon particles can be explained by silane pyrolysis and subsequent collisions and coagulation of the silicon particles. The carburization mechanism of the silicon particles is discussed in terms of diffusion through a SiC product layer and methane pyrolysis. The rate-controlling step is believed to depend on the reaction conditions such as temperature and gas stoichiometry.
Eight kinds of commercial SiC powders were cold pressed isostatically, encapsulated in a silica glass tube, and HIPed in 150MPa-argon gas at 1900°-2000°C. The density, microstructure, hardness and KIC were studied. The results are summarized as follows:(1) The density of sintered specimens depends strongly on the mean particle size of starting powders and HIP temperature. High density (>99.7% T. D.) was obtained by HIPing above 1950°C using powders with the mean particle size less than 0.6μm. With 0.8μm α-SiC powder, the sintered density was about 95% T. D. with slight dependence on the HIP temperature.(2) The microstructure of highly dense sintered materials was uniform with fine grains, but that of 95% T. D. material was characterized by coarse grains and intra-granular isolated pores.(3) α-SiC powder sinters better than β-SiC powder for the same mean particle size.(4) Fine SiC powders gave high Vickers hardness and low KIC value in comparison to coarse powders.
Hot-pressed silicon nitride test bars were oxidized in air flow at 1200°C for 480h. The observations by composition and topography image of SEM indicate that cristobalite crystals of 1-4μm are formed (appears as black spots) and cracks run in all directions all over the oxidized surface, and Y2O3⋅2SiO2 crystals are clearly convex and the SEM image of cristobalites indicate that they are concave to the surface and this aspect is correspond to the COMPO image. The concentration of Al in the glass phase on the grain boundary is presumed to be high from the characteristic X-ray analysis. EDS analysis showed that a large amount of SiO2 and a small amount of alkali were found in the cristobalite phase and a large amount of Fe2O3, CaO and K2O are dissolved in the glass phase.
In this work, we have developed a nonlocal geometrically-exact shell theory and its computational formulation, which can model fracture and crack growth in shell structures under finite deformations. We have derived the local form of the nonlocal balance laws for a nonlocal continuum, which are instrumental in developing the nonlocal stress-resultant based geometrically-exact shell theory. This approach is particularly efficient for modeling material damage and structural failure process with strong discontinuities. A meshfree Galerkin weak formulation is developed for the nonlocal geometrically-exact shell theory by using nonlocal differential operators. Several numerical examples, including finite deformation and fractures, are conducted to verify the effectiveness of the present method. The numerical results demonstrate that the proposed nonlocal shell theory is an accurate and robust method to model the failure of shell structures.
The R-curve evaluation was discussed for “Fractometer”, and was applied to the short bar specimens of sintered silicon nitride. The values of KR were increased slowly with crack extension. The KR value of crack extension less than about 2mm could not be determined, because of the “pop in” phenomenon prior to the initiation of the stable fracture. When the increment of the applied displacement was stopped, the load relaxation due to subcritical crack growth was observed. Thus, it is concluded that the measured KR values are likely to be the KI values of subcritical crack growth corresponding to the crack opening displacement rate of the specimens.
Precursors of carbon-containing SiC fibers (SiC-C fiber) were synthesized with high yield by the copyrolysis of poly (dimethylsilane) and 3.9-80wt% pitch. During copyrolysis the decomposition product of polysilane reacts with pitch molecule with no significant condensation among pitch molecules. SiC-C fibers were obtained by melt spinning of the precursors and heat-treatment up to 1400°C in vacuum. The tensile strength and Young's modulus of the SiC-C fiber decreased with increasing carbon content. The electric conductivity is two orders of magnitude larger than that of SiC fibers obtained from pure polycarbosilane.
Young's modulus and internal friction of engineering ceramics such as silicon nitride, silicon carbide and tetragonal zirconia polycrystals were measured by the resonance method up to 1400°C. Effect of hanging position on the apparent value of Young's modulus and internal friction was investigated. The apparent value of internal friction increased when the specimen is hung near the edge of the specimen because of the loss of vibration by the equipments. The error of apparent value of internal friction was negligibly small if the shift of hanging position from the node is less than 2.4% of the total length of the specimen. Measured value of Young's modulus was independent of hanging position. Internal friction increased abruptly above 1150°C in hot-pressed silicon nitride, above 1000°C in pressureless sintered silicon nitride and tetragonal zirconia polycrystals with increasing temperature. Internal friction showed peaks at 1080°C in hot-pressed silicon nitride, at 600°C in silicon carbide and at 180°C in tetragonal zirconia polycrystals. All of these temperatures corresponded to the temperatures at which Young's modulus decreased.
It is well known that ferroelectric (non-centrosymmetric) crystals are capable of revealing “bulk photovoltaic effect” i.e. production of greater-than-band-gap voltages under uniform illumination. While having piezoelectricity at the same time, the ferroelectrics are considered to cause the photostrictive effect as the superposition of these two phenomena. This paper describes the photovoltaic and photostrictive effects in the (Pb, La) (Zr, Ti)O3 (PLZT) system. The composition PLZT (3/52/48) was found to reveal the largest product value of the photovoltaic voltage and piezoelectric coefficient. The photostriction and photovoltaic voltage are strongly dependent on the preparation method such as grain size and remanent polarization even with the same composition. Using PLZT ceramics, a photodriven relay has been fabricated and a primitive relay function controllable by the optical irradiation has been developed.
Six kinds of Si3N4 starting powders which were mixed with 6wt% Y2O3, 2wt% Al2O3 and 3wt% MgO as densification aids, were pressed and normally sintered at 1550°-1650°C. The sintered compacts were HIP'ed without capsule in a nitrogen atmosphere of 100MPa at 1700°C. Density, flexural strength and α-to β-phase transformation were studied. The results obtained are summarized as follows:(1) Density and strength of HIP'ed Si3N4 compacts were strongly affected by characteristics of starting powders such as specific surface area, green density and amount of α-phase.(2) The mean strength of sintered compacts of which starting powder had a specific surface area of 14.2m2/g and contained 97% α-phase, increased from 370 to 870MPa by HIP treatment.(3) Improvement of strength depends on α-to β-phase transformation and the formation of fibrous grain in Si3N4 compacts during HIP treatment.
Zr-based thin film metallic glass (TFMG), exhibiting the unique properties of good glass forming ability (GFA), corrosion resistance, and biocompatibility, can be applied in various novel fields of industries. An ultra-smooth surface is obtained with the TFMG coatings, which is beneficial to modify the work surface. Thus, TFMG can be extended to medical appliances, such as surgical blades and micro-surgery scissors. The aims of this study are to fabricate the Cu-containing Zr-based TFMG onto SUS304 plates and to investigate the surface physical properties and their antimicrobial effects. The chemical compositions of Zr–Cu–Ni–Al coatings are examined by a field emission electron probe micro-analyzer (FE-EPMA). The amorphous structures of all TFMG are characterized by the X-ray diffractometry. The surface properties are analyzed by an atomic force microscope (AFM) and a water contact angle goniometer for the 304 stainless steel substrate and Zr–Cu–Ni–Al TFMG. Liquid culture methods and plate counting methods are used to assess the antimicrobial performance of specimens. The antimicrobial rate against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) under the Japanese Industrial Standard JIS Z2801: 2000 is over 99%. The release of copper ion from TFMG specimen is determined by the inductively coupled plasma-mass spectrometer (ICP-MS) to evaluate the antimicrobial activity of Cu-containing Zr-based TFMG. The results show that the surface of SUS 304 stainless steel substrate can be modified with deposited Zr–Cu–Ni–Al TFMG, and their improved antimicrobial efficacy against those bacteria is attributed to their amorphous rough surface, hydrophobic properties and released copper ion. The TFMG developed in this study with adequate hardness, good adhesion ability and antimicrobial efficiencies can be used as a promising candidate to improve the surface properties of the medical appliances and also to reduce the possibility of nosocomial infection.
The deterioration of concrete containing dolomite and marine sand was studied mainly by SEM and EPMA. The SEM observation revealed that minute cracks were present in and around the coarse grain of dolomite in concrete. The EPMA line analysis showed that the Cl- ion was combined with Mg-containing minerals such as dolomite. It was assumed that partially chlorinated dolomite was expansive and thus responsible for the deterioration of the concrete.
A new method for preparing ultra-fine alumina powders consisting of spherical particles based on combustion has been developed. A mixture of fuel gas, H2+CH4, and aluminum powder with particle size of several micrometers is burnt in a conventional burner for glass blowing. Examination of obtained products by electron microscopy and X-ray diffraction showed that they were alumina with two types of structures, θ- and δ-Al2O3. The present note describes an economical and efficient method for the production of alumina spherical particles.
