The effects of fracture origin and microstructure on bending strength have been investigated with four kinds of Si3N4 ceramics. The mean grain sizes of three kinds of Si3N4 ceramics were 0.2μm and the other one was 0.4μm. The average bending strength of the weakest one was 1000MPa and the strongest one was 1500MPa. Three kinds of defects, pores, anomalously grown grains and inclusions (Fe) were identified as fracture origins. The stresses applied at these fracture origins were calculated from the bending strength and the locations of fracture origins. As the parameter to describe the size of fracture origin, the equivalent crack length was estimated. The strength of the high-strength Si3N4 ceramics with more than 1500MPa depended on both the grain size and the defect size same as conventional-strength Si3N4 and agreed with the Kishimoto's equation, from which the fracture strength could be estimated by using defect size, grain size and fracture toughness.
The purpose of this study is to examine the effects of some oxide additives on the grain growth and transformation behaviors of ceria doped tetragonal zirconia (Ce-TZP) to improve its mechanical properties. Monolithic Ce-TZP and some alkaline earth oxide (CaO, SrO and BaO) doped Ce-TZP were fabricated by the pressureless sintering technique at 1500 to 1700°C in air atmosphere. The grain growth of ZrO2 matrix was suppressed by doping alkaline earth oxide. BaO doping induced the transformation of Ce-TZP, whereas CaO doping was effective in stabilizing the tetragonal phase of CeTZP. CaO-doped Ce-TZP exhibited the high strength over 1000MPa. BaO doped Ce-TZP increased the fracture toughness of monolithic Ce-TZP. There was no effect of SrO-doping on transformation behavior and mechanical properties for Ce-TZP. The relationship between the microstructure and mechanical properties of oxide doped Ce-TZP were discussed.
Raman spectra of NaF-Na2O-B2O3 glasses with B2O3 content=65, 80 and 85mol% were measured from ambient to 800°C. Tg and Td of the glasses were measured. The results are summarized as follows. (1) The Raman spectra of glasses in the NaF-B2O3 system are similar to those in the Na2O-B2O3 system. The line width and the peak intenisity ratio of 770cm-1 to 806cm-1, I770/I806, are almost same in both systems. (2) The peak positions of the Raman spectra shift toward lower frequency as temperature increases. (3) The line width increases as temperature increases. (4) The peak intensity ratio of 770cm-1 to 806cm-1, I770/I806, increases slightly with increasing temperature. (5) Tg and Td decrease as the NaF content increases. From the above mentioned results it is inferred that the similar structural units are present both in the NaF-B2O3 and the Na2O-B2O3 systems, and these structural units are mostly maintained at elevated temperatures. However, some structural rearrangement is implied above Tg among the borate groups containing four-fold coordinated boron atoms. The differences in Tg and Td between the two systems are attributed to the fluoline atoms connecting the borate groups.
Precipitation of LaBO3 on the surface of a glass of composition 0.1Cr2O3·25Li2O·10La2O3·65B2O3 (in mol%) was enhanced due to ultrasonic surface treatment (UST) with aqueous and ethanol suspensions of LaBO3 particles. The aqueous suspension gave a higher efficiency than the ethanol suspension. The crystallites were almost uniform in size distribution, and grew irregularly: increase-decrease-increase in size when heated at 580°C. Preferred growth of (200) was detected. The mechanism of UST enhanced crystallization is proposed.
It is expected that persistent spectral hole burning (PHB) materials will be used as a high density memory disk in future. For the application of PHB materials to memory disk, both high operation temperature and high optical multiplicity (inhomogeneous linewidth/homogeneous linewidth) at high temperature are required. Recently, our research group has discovered persistent spectral hole burning in Sm2+-doped alkali borate glasses at room temperature. These materials have high optical multiplicity and chemical stability, and can be made easier than fluoride PHB glasses or single crystals. In this paper, the effect of alkali ions on the hole burning behavior is studied. The glasses were made by melting boron oxide, alkali carbonate, and samarium oxide at 1400°C for 30minutes in Ar(98)/H2(2) atmosphere. Holes were burnt by 600mW/mm2 DCM dye laser at the vicinity of 683nm, which is tuned to 5D0-7F0 transition, and detected by 5D0←7F0 excitation spectrum excited by 6mW/mm2 DCM dye laser scanning and monitored at 5D0→7F2 emission. The width and depth of holes were measured as functions of alkali content and alkali ion size in alkali borate glasses. It was found that the hole width and depth increased with increasing the size and quantity of alkali ions. This alkali effect was discussed from the view point of glass structure.
In order to improve sinterability and dielectric properties of Ba(Zn1/3Ta2/3)O3-Ba(Zn1/3Nb2/3)O3 ceramics, the effects of additives on the sintered density and dielectric properties of the (1-x)Ba(Zn1/3Ta2/3)O3-xBa(Zn1/3Nb2/3)O3 ceramics modified with MnO2 and SiO2 additives were investigated. Solid solution ceramics of the system (1-x)Ba(Zn1/3Ta2/3)O3-xBa(Zn1/3Nb2/3)O3+wwt% additive were prepared by the solid state reaction of powder materials, where x=0-1 mole fraction and w=0-0.9 weight%. Ceramic and dielectric properties at low frequency (1MHz) and at microwave frequency (10-13GHz) of the system were studied. The sintered density and dielectric properties of the base composition were improved markedly through the selection of additives in proper amounts. The addition of relatively large amounts (0.7wt%) of MnO2 yielded high density (99% theoretical density) and high relative dielectric constant (42) for the base composition of x=1. High density (99% theoretical density) and high relative dielectric constant (28.9 at 1MHz) were obtained by addition of a small amount (0.1wt%) of SiO2 to the base composition of x=0. This ceramics also showed superior dielectric properties at microwave frequency (the relative dielectric constant of 26.2 and Q of 7500 at 12.948GHz) and very low temperature coefficient of resonant frequency (τf) of -3.3ppm/°C from -40 to 80°C. The microstructure indicated that the ceramics modified with a small amount (0.1-0.2wt%) of SiO2 had a fine, uniform grain structure.
The effect of structural anisotropy on sliding wear behavior was studied on uniaxially hot-pressed silicon nitride. Frictional coefficient μ and wear rate ΔV/L under a constant load were measured by a pin-on-disk tester. Under low applied load, the abrasion wear was predominant, in which wear tracks were observed in parallel with the sliding direction and the wear rate depended on the applied load. Under high applied load, the wear rate was constant and independent of load. Adhesion behavior was observed on the sliding surface. The applied load, at which the adhesion behavior was observed, depended on the microstructure of silicon nitride. This transition load became low for the specimen in which many pillar-shaped particles were observed.
The effects of temperature and sliding speed on the tribological behavior of Cr3C2 with addition of TiC sliding on itself in deoxygenated water were investigated from room temperature to 300°C under the corresponding saturated vapor pressures. The friction coefficient increased at elevated temperatures at all sliding speeds, but decreased with increasing sliding speed. The specific wear rate of the disk was larger than that of the plate, and the dependence of the specific wear rate of the disk on sliding speed was different from that of the plate. The friction and wear properties of the material are considered to be controlled by tribochemical reactions to produce H2, C, CH4, C2H6 and CO2 and by microfracture. The relative molar ratios (H2/CH4, C2H6/CH4) where H2, CH4 and C2H6 were produced by the sliding friction were constant for all the wear tests. The relative molar ratio (H2/CH4) for the wear test at 300°C was smaller than that at the static corrosion of the specimens under the same hydrothermal condition as the wear test.
A fracture criterion based on the concept of linear notch mechanics (LNM) for predicting the failure load of composites containing stress concentrations is subjected to further experimental scrutinization. An experimental program is presented which examines the effect of notch geometry on the fracture of FRP plates. This is accomplished by obtaining experimental data on tension tests and fourpoint bending tests of short-glass-fiber reinforced polycarbonate plates having U-shape notches for a wide range of notch-root radii and depths. The concept of LNM is based on the similarity of the severity near the notch root. The severity near the notch root can be expressed by both the maximum elastic stress σmax and the notch-root radius ρ. On the basis of the concept of LNM the fracture criterion for notched plates is expressed as: σmax=σmax, c(ρ), where σmax, c is the material constant, which is governed by the notch-root radius ρ only and independent of notch geometry and specimen size. The experimental results can be clearly explained on the basis of the concept of LNM, and show that the fracture criterion for notched plates mentioned above is applicable to the material used.
The effect of sintering treatment on the creep rupture life of a 316 stainless steel has been investigated. Creep rupture tests were carried out at 750°C and 37MPa, and three kinds of sintering treatments were used: (a) stress-free annealing at 750°C; (b) hot isostatic pressing at 750°C and 37MPa; and (c) compressive creep at 750°C and 37MPa. These sintering treatments were conducted in order to remove creep-induced grain boundary cavities by interrupting the creep, and the treated specimens were then retested at the original temperature and stress. The repetitive creep/sintering cycles were continued to the rupture of the specimens. The results obtained were as follows; (1) The stress-free annealing had no significant effect the rupture properties. Considerable amounts of grain boundary cavities and cracks were observed in the specimens given the repetitive creep/annealing cycles. It is thought that the limited rupture life extension is attributable to this insufficient sintering of cavities as well as overaged microstructure due to annealing. (2) The hot isostatic pressing and the compressive creep could remove the grain boundary cavities almost completely, and no developed cavity was observed in the ruptured specimens. The rupture was caused not by linkage of grain boundary cavities but by growth of oxidized surface nucleated cracks which could not be healed by the sintering treatments. It is suggested that the growth of surface cracks is the reason why the rupture life extension was not significant.
In order to clarify the influence of moisture in air on fatigue crack propagation characteristics at stage 2 of high strength steel (SNCM439) whose tensile strength was about 2000MPa, fatigue tests were conducted under stress ratio, R, of 0.1-0.8 in wet air (relative humidity, RH, of 20-80%) and dry air (RH=0%), and the fracture surfaces were observed by SEM. The following results were obtained: (1) The acceleration of crack growth with moisture in air (RH=20-80%) was recognized in the high ΔK region under R=0.1-0.8. This phenomenon was due to the hydrogen embrittlement. (2) The degree of crack growth acceleration was influenced by stress ratio and depended on both ΔK and Kmax. (3) The thickness effect of specimen was not recognized on the crack growth acceleration. (4) The crack growth acceleration was different between two charges. This difference might be caused by the amount of penetrating hydrogen and the diffusivity of penetrating hydrogen. (5) The fatigue fracture toughness was influenced by the pre-austenitic grain size.
The effects of shot peening on the axial fatigue properties of Ti-6Al-4V were examined in connection with residual stress and microstructure. Double edge notched plate specimens with the stress concentration factor 1.63 were prepared. By shot peening, compressive residual stress was introduced into the surface layer of approximately 70μm in depth, which showed the maximum value of 600MPa. Fatigue tests were carried out at R=0.05. The fatigue strength of the shot peened specimens increased by about 1.3 times that of the unpeening specimens. The fatigue crack initiaion site in the shot peened specimens was at about 150 to 180μm below the specimen surface. In the unpeened specimens, however, a fatigue crack was initiated within α phase microstructure at the specimen surface of the notch root. The increase of fatigue strength in the shot peened specimens might be due to a shift of the initiation site from the surface to the subsurface. The surface residual stress in the shot peened specimen measured by an X-ray diffraction method slightly decreased during cyclic loading.
The actual load on mechnical structures does not always have a constant stress amplitude. Therefore, it is very important for the prediction of fatigue life under complex load to investigate whether Miner's rule is valid or not. In this paper, rotating-bending fatigue tests for short carbon-fiber reiforced poly-ether-ether-ketone (CFRPEEK) under two-step loading were carried out to investigate the applicability of Miner's rule. In case of a short fiber reinforced composite like CFRPEEK, the fatigue process should be treated by dividing it into two stages, that is, the one where the fatigue damage concentrates on the edge of the short carbon fiber, and the other where the fatigue damage concentrates on near the fatigue crack's tip. From the result of the experiments, it was concluded that Miner's rule can be applied to the both stages in the short fiber reinforced composites as in the case of metals, and thus Miner's rule holds well under two step loading in CFRPEEK.
A fracture criterion based on the concept of linear notch mechanics for predicting the tensile strength of composites containing stress concentrations is subjected to further experimental scrutinization. An experimental program is presented which examines the effect of hole size on the fracture of FRP plates. This is accomplished by obtaining experimental data on tension tests of FRP plates containing through-the-thickness circular holes for a wide range of sizes. Two kinds of FRP plates were used in the tests: a woven roving glass fabric epoxy laminate and a short glass fiber/polycarbonate laminate. The experiment shows that the maximum elastic stress at the edge of the hole when the specimen fails, σmax, c, is governed by the radius, of hole, ρ, with the exception of very small holes. The characteristic curve of σmax, c against ρ for the plates containing a circular hole agrees with the curve for the notched plates. On the basis of the concept of linear notch mechanics, the experimental results mentioned above can be clearly explained. For very small holes the situation is more complex. The maximum elastic stress at fracture, σmax, c is determined predominantly by the tensile strength of unnotched bars and is independent of the hole size and the width of the plate. It seems likely that the limiting value of the diameter of hole is a characteristic value of the material.
A computer program of Finite Element Method using three-dimensional beam elements has been developed in order to analyze the behavior of delamination of laminated composites. Two types of beam elements which consist of fiber reinforced plastics and resin matrix were used in the numerical model of laminated composites. The progress of delamination and relation of load vs. displacement when some elements were fractured were simulated by decreasing Young's modulus and shearing modulus of fractured elements. As the examples, the mechanical behavior and the delaminated region of the laminate plate under concentrated load were analyzed. The computational results had a qualitative agreement with the experimental ones. Therefore, it has been revealed that the mechanical behavior of laminated composites can be analyzed by the developed program.
The low-pressure-plasma spray (LPPS) process is used to overlay coatings of MCrAlY alloy for protection against high temperature corrosion and oxidation. This coating process has been found to be very effective for gas turbine components. However, mechanical properties of LPPS MCrAlY alloy, which have an important effect on coating life, have not always been clarified. Five kinds of freestanding MCrAlY specimens (CoCrAlY, CoNiCrAlY, CoNiCrAlY+Ta, NiCrAlY, NiCoCrAlY) were machined from the thick LPPS coatings. Also, the heat treated MCrAlY specimens (1393K, 2h, Ar atmosphere) were used in experiments. The Vickers hardness, Young's modulus, Poisson's ratio and four point bending strength of the LPPS MCrAlY alloy were measured at room temperature in comparison with a nickel-base super-alloy, IN738LC. The experimental results suggest that the volume percentage of precipitated aluminum composite in the LPPS MCrAlY alloy has an important effect on the mechanical properties. Namely, there is a tendency that the Vickers hardness and Young's modulus increase with increasing the volume percentage of aluminum composite, and conversely the bending strength decreases with increasing the volume percentage of aluminum composite.
This paper presents a method to measure the plastic strain of steel specimens with no contact using the laser speckle method. This method is based on a phenomenon that the intensity distribution of the laser speckle changes depending on surface profile change occurred by plastic deformation. The intensity distribution of the laser speckle was analyzed quantitatively using an image processing system. The laser speckle pattern and surface profile were observed for specimens of three kinds of steels plastically deformed by static tension. The distribution of the speckle intensity was evaluated after smoothing by calculating correlation function. A new parameter to express the width of the distribution was derived. It was clarified that there is a fixed relation between speckle distribution and the magnitude of plastic strain and it is possible to estimate plastic strain using the speckle distribution. It was also found that the relation between speckle distribution and plastic strain is almost identical for three kinds of steels treated in this experiment.
This paper describes the precision three-points bending machine developed for measuring Young's modulus of thin films used for electronic devices. The developed machine has 0.05μm displacement and 9.8×10-6N load resolution. The Young's modulus of 200μm glass plate in thickness measured by the machine agreed with that of the published value within 3% difference, which shows that the machine has a sufficient performance and reproducibility of the data. The Young's moduli of sputtered Co-Ta-Zr amorphous thin films, of which thickness were ranged from 1μm to 10μm, were measured. The Young's moduli of the thin films thicker than 4.5μm showed a constant value of 150GPa, but those smaller than 4.5μm in thickness increased with decreasing film thickness. The Young's modulus of 1μm in thickness showed a value of 250GPa which is about 70% larger than that of 10μm in thickness. The Young's moduli measured for the films more than 4.5μm in thickness well agreed with the bulk Young's modulus of amorphous Co alloy.