Amorphous Al2O3-ZrO2 composite powders with 5-30mol% ZrO2 have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2-propanol. Simultaneous crystallization of γ-Al2O3 and t-ZrO2 occurs at 870°-980°C. The γ-Al2O3 transforms to α-Al2O3 at 1160°-1220°C. Hot isostatic pressing has been performed for 1h at 1400°C under 196MPa using α-Al2O3-t-ZrO2 composite powders. Dense ZrO2-toughened Al2O3 (ZTA) ceramics with homogeneous-dispersed ZrO2, particles shows excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t/m ratios are examined, with emphasis on the relation between strength and fracture toughness.
Dense manganese zinc ferrites (MnZn ferrites) and lead lanthanum zirconate titanate (PLZT) with fine grains were fabricated by post-hot isostatic pressing (HIP) technique using finely ground starting powders prepared by a media agitation milling method. The average particle size of MnZn ferrites milled by media agitation milling was 0.03μm, and the average grain size of the HIP'ed body was 1.2μm. The ferrites showed low power-loss at high frequencies and initial permeability of 500 at 10MHz. HIPing treatment of PLZT ceramics improved the transmittance without significant increase in grain size. The optical transmittance and grain size of HIP'ed PLZT were 69% at the wavelength of 750nm and 2μm, respectively. PLZT with fine grains showed microscopically uniform birefringence. At the wavelength below 500nm, the transmittance of Ar-HIP'ed PLZT was higher than that of O2-HIP'ed PLZT. The highly transparent PLZT is feasible for the fabrication of optical shutter arrays having high dimensional accuracy and optical uniformity for electrophotography.
Densification of glass is an interesting method to control optical properties of glass, such as refractive index, wavelength dispersion of refractive index, optical durability and so on, without changing its chemical composition. In this paper, densification of silica glass by means of hot isostatic pressing (HIP) and structural relaxation of the densified glass at elevated temperatures have been investigated. Two types of synthetic silica glass which have high and low OH contents were used in this experiment. The density of glass increased by about 1% after HIP treatment of 2000kgf/cm2, 1200°C, 2hrs. The refractive index (n) increased with increasing density. The wavelength dispersion of index (dn/dλ) also increased by the densification. X-ray induced absorptions at 4.8eV and 5.8eV were suppressed by the densification. The structure of the high- and low-OH glasses was stable up to about 600 and 800°C, respectively. These results suggest that the densified silica glass can be widely applied to optical use.
Self-propagating high-temperature synthesis (SHS), also called as combustion synthesis, has advantages such as energy efficiency, simplicity, high reaction temperature and productivity. The TiAl-based composite materials containing TiB2 and Ti2AlN were combusion-synthesised within few seconds from the mixed reactants of Ti+(0.7-0.95)Al+(0.05-0.50)B under nitrogen gas atmosphere of 30-100atm. The combustion temperature exceeded the melting points of TiAl and Ti2AlN, and hence most parts of the composite was fused due to high reaction heat. The molten product was isostatically pressed by surrounding nitrogen gas. The products solidified were fully dense without cracks, which consist of homogeneous grains of -10μm due to rapid cooling after the SHS reaction. The use of pressurized reactive gas such as nitrogen was found to be very effective for simultaneous synthesis and consolidation of nitride ceramics-dispersed TiAl intermetallic compound.
The low cycle push-pull loading fatigue test was carried out on medium carbon steel. It is well known that the low cycle fatigue life can be evaluated by the Coffin-Manson relationship expressed by the equation ΔεPαNf=c, where ΔεP is the plastic strain range, Nf the cycles to fracture and α and c are material constants. The exponent α ranges from 1.4 to 2.0 in most metals, but was determined to be 2.2 for the particular steel used here. We investigated the crack initiation and propagation in an attempt to explain this value, based on the conclusion of previous investigations that the Coffin-Manson relationship is equivalent to the propagation law for small cracks. The present material has inclusions of various sizes, some of them larger than the mean pearlite size. When a crack initiated from an inclusion, the initial crack length measured from the surface depended on the inclusion size. This is the reason for the high value of α. Since the crack propagation rate for a given stress amplitude is almost independent of the initial crack length, the low cycle fatigue life can be calibrated as if the crack initiated from a pearlite. α then becomes 1.66 which is within the range of general cases. The fatigue life of metals in which cracks initiate from an inclusion can then be predicted using the adjusted values of α and c, if the inclusion size is known.
Titanium, Aluminum, and fine SiC or Al2O3 powders were used to synthesize Ti-Al intermetallicmatrix composites by hot press reactive sintering. In the SiC reinforced composites, the exothermic heat during sintering caused a reaction between SiC powders and Ti, producing Ti-Si compounds. The compounds increased the hardness of the composites, and existed mainly between the Ti3Al grain and the TiAl lamella. The fractured surface of the SiC reinforced composites showed much smaller cleavage than that found in the non-reinforced Ti-Al intermetallic. The change in size of microstructure was correlated with the size of fracture unit by the result of microbeam X-ray analysis.
In the previous work, isocyanate modification of epoxy acrylate resin (EPA) was studied to improve the mechanical properties of CF/GF hybrid composite materials. In this work, some chemical reactions of diphenyl methan diisocyanate (MDI) as the third component with surface treatment agents of CF and GF were studied to improve the mechanical properties of the laminates. Epoxy silane and amino silane coupling agents were applied to GF surface in order to improve the interlaminar adhesion bonding. As the results, interlaminar shear strength and flexural bending strength were markedly improved, which indicated that some covalent bonds such as ester, urethan and urea bonds partially linked fiber surfaces with resin matrix.
Fatigue fracture mechanism was investigated for polycarbonate plates with few contents of short glass fibers fabricated by an injection molding method. In this paper, fatigue fracture behavior under zero-tension cyclic loading was observed. It was found that (1) at a low stress amplitude, fracture occurred due to the initiation of surface crazes and the growth of shear cracks at their tips and (2) at a high stress amplitude, the shear cracking from glass fibers in the inside of the specimen, which were inclined at 45° to the loading axis, became a fracture nucleus.
The stress corrosion properties of GFRP, made of glass cloth or glass chopped strand mat and unsaturated polyester resins, have been evaluated under acidic conditions. Four types of unsaturated polyester resins were used as the matrix. Each matrix resin has a different fracture toughness level. The retention strength of the cloth-GFRP and mat-GFRP under acidic conditions were 20-40% and 40-80% of the original strength in air, respectively, at a very slow tensile rate. The retention strength vs. fracture toughness curve showed the existence of an optimum value in matrix toughness for the high performance of stress corrosion. The fatigue induced cracks did not give any degradation in the tensile strength in air, but decreased the tensile strength of GFRP significantly under acidic conditions. The results show that the fracture toughness of matrix resin is a critical parameter in determining the rate of fatigue crack propagation.
The influence of aging treatment on the fatigue strength of 18 Ni-maraging steels has been investigated in relation to the behavior of age hardening and the formation of reversion austenite in an 18Ni-8Co-5Mo steel. In the present study, fatigue test was performed on two series of specimens in order to make the effect of age hardening phases on fatigue property clear; in the first series, the strength of test pieces was controlled to have the same proof strength by varying aging temperature and time, and in the second series, it was changed by varying aging time at 750K, which is usually applied as the aging temperature in the commercial 18Ni-maraging steel. Fractography was also examined to study the fracture mode. The main results obtained are as follows. (1) The fatigue limit is almost independent of the aging condition of specimens as far as they have the same proof strength. The computed fatigue strength, however, decreases when specimens have been hardened through the precipitation of low temperature phases. (2) In the case of 750K aging, specimens strengthened to around the maximum hardness (Hv 450) have the almost same value in the fatigue limit independent of small difference in the aging time, although the fatigue limit of under-aged specimens was much lower. (3) A small amount of reversion austenite, which is formed in an over-aging condition, is very effective in increasing fatigue resistance. An amount of 3 vol% austenite was enough for improving the fatigue limit of a maraging steel used.
The effects of addition of Zr on both microstructures and mechanical properties of four advanced Al-8wt.% Zn-1 wt.% Mg-0-2.4wt.% La alloys were investigated, which had excellent stress corrosion cracking (SCC) resistance. The main results are as follows; (1) The peripheral zone of all the extruded Al-Zn-Mg-Zr-La alloys were recrystallized with coarse grained structures, and the regions within the core were fibrous and unrecrystallized on macroscale. The core region was consisted of subgrains with low angle orientation, which were situated in layer-like colonies. For the alloys with La, the primary phase particles including elements of La and Zn dispersed in the matrix. (2) Tensile properties of Al-8wt.% Zn-1wt.% Mg-0.1wt.% Zr with various La content were much superior to Al-8wt.% Zn-1 wt.% Mg alloy (the master alloy). Values in tensile strength were from 460 to 490MPa, and a maximum in elongation was about 11%. (3) The crack nucleation life on the SCC tests increased with increasing La content. The core region including the fibrous structures with fine subgrains exhibited more resistant to SCC than the peripheral one with the recrystallized coarse structure.
Cracks sometimes occur at the underhead fillet of a bolt in the hot dip galvanizing process. In the preceeding paper, the effect of Hot-dip galvanizing condition in this process on crack nucleation was investigated experimentally. As a result, it was found that crack nucleation easily occurs in the cooling process. In this study, to investigated the effect of cooling condition on crack nucleation in the hot dip galvanizing process, some experiments were performed. The main results obtained are as follows. (1) In the cases of water cooling method and time water cooling method, cracks occurred at the underhead fillet of a bolt. But, in the cases of shower cooling method and air cooling method, no crack occurred. (2) In the case of air cooling method, the whole plating layer became Fe-Zn layer and the plating surface became very rough.
Fracture behavior of Si3N4 ceramic was investigated under cyclic mode I/mode II mixed-mode loading in various environments. The crack propagation rate was higher than that for cyclic mode I loading, and it was higher in water than in nitrogen gas. The fretting and hydration reaction seemed more active than those for cyclic mode I loading, and therfore even if the stress shielding effects by particle bridging and wedging exist, those effects were easily decreased, and the effective stress intensity factor range ΔKtip was increased. When the debris was accumlated by the fretting between the crack surfaces, the substantial effective stress intensity factor for mode II was increased. Therefore, it was suggested that in the case that KI was decreased by the particle bridging and KII was increased by the fretting the fracture morphology became transgranular fracture rather than interglanular fracture.
Experiments and axisymmetric laminate analysis have been carried out to evaluate the progress of failure in thick-wall filament wound glass/epoxy pipes under internal pressure. The initial failure of laminates was controlled by matrix cracking perpendicular to the glass fiber direction. The laminated pipes wound with ±89.1/±5° showed the initial failure at low pressure and the failure progressed sequentially accompanying with sounds. On the other hands, for ±58° laminated pipes the final failure which gives leakage took place at the same time as the initial failure. These experiments coincided with the results of axisymmetric laminate analysis. In order to accomplish the resistance against high pressure failure, a flexible matrix which prevents matrix cracking was introduced for ±55° laminated pipes. The final failure pressure of the flexible matrix pipe was about three times of that of the original matrix pipe.
This paper describes the theoretical aspect of rapid crack propagation in gas-pressurized plastic pipes. The rapid crack propagation shows brittle fracture and has about the same velocity as decompression. The method applied here is to compare two characteristic curves, i.e., the crack velocity and the gas decompression curve. This method has been proposed by Mimura and adopted for analysis of shear crack propagation in steel pipelines. The behavior of crack propagation and arrest is well explained by this analysis. Finally, drop weight tear test (DWTT) is proposed as a simple test method to evaluate the brittle fracture of plastic pipes.
Cutting of brittle materials such as glass is usually made with a diamond scriber or a rotary roller chisel. It is said that the mechanical cutting of glass using the scriber or the chisel leads to lower cutting quality. A non-contacting cutting method using a CO2 laser is recently being studied enthusiastically. Precision breaking of glass and other brittle materials, however, has not been made. Curviform breaking of transparent glass plates by YAG laser was studied on the soda-lime glass specimens of which surface was painted marker ink (magic ink) for absorber of YAG laser beam. Influence of a laser tracing curvature on the deviation of broken line was examined. It was found from the experiments that smaller curvatures provide larger deviation of breaking location from the laser tracing locus, which reflects directly to lower quality breaking of glass.
In order to prevent the failure of petro-chemical plants, safety assessment based on fracture mechanics is required. However, the analytical condition and the material properties are not always clear or known. For an unfavorable condition, rational assumptions with empirical background are required to be introduced. In this paper, the procedure to determine the hot start temperature and the allowable crack size for actual reactors in the lack of data is explained and the analyzed results are shown. In the case that some defects are detected during operation, the defects are usally removed and repaired with weld. However, as welding at the operating site may cause damage in some cases, repairing by grinding and smoothing is recommended in the paper. The procedure to determine the optimum shape in smoothing was proposed and is applied to an actual spherical storage tank.
It is a hard work to produce the bending strength test specimen of extremely stiff consistency concrete for roller compacted concrete pavement. The purpose of this study is to reduce the work time of the specimen production. In order to carry out this purpose, superplasticizer was added to the base concrete after mixing. The consistency was measured by a Swedish-type Vebe apparatus and the vibrating compaction value of the base concrete was 50±5 seconds. The suitable superplasticizer content was found to be 1.2% by cement content. By adding superplasticizer suitably to the base concrete, the vibrating compaction value and air content were decreased considerably and the compaction ratio was increased slightly. Furthermore, the surface vibrating compaction time to produce test specimens and the time to finish concrete surface by trowel could be reduced to half and one-eighth respectively, the surface finish condition was well and smooth enough, and the bending strength of the superplasticized concrete was equal to that of the base concrete at the age of 28 days. From these results, the work time to produce test specimens is reduced by adding superplasticizer.
The main aim of this paper is to establish a hardness evaluation method for a ultra-thin surface layer of sub-micron order, and to apply the method to determine the surface hardness of ion implanted 13Cr steel, of which fatigue strength properties were examined previously. First, the microhardness of fully-annealed S25C steel (0.25%C carbon steel) was evaluated to find the optimum loading method and the dispersion of the data was statistically characterized. The controlling condition of the loading process was varied in two ways; constant loading rate and constant loading time. The result reveals that the constant loading time method is preferable for the measurement of a thin surface layer of sub-micron order. The data of the microhardness measurements with 50 repeats under a same condition shows that the indentation depths distribute normally, and that more than 30 measurements are necessary to eliminate the possible measurement errors. Second, the microhardnesses of both ionimplanted and non-implanted 13Cr steel specimens were evaluated using the method established above. The surface layer of sub-micron order is hardened by the ion implantation, and the further increase in hardness can be attained by the post-heat treatment. The hardness-increased depths are consistent with the diffused depths of the ions during the heat-treatment, which is estimated by ion micro analyzation (IMA) in a previous study.