Some physical properties of 40SiO2·10Al2O3·(50-x)CaO·xZnO glasses and their glass structure were investigated by measuring glass transition temperature, softening temperature, thermal expansion coefficient, density, viscosity, IR absorption and crystallization behavior. The crystallization behavior was examined with the aid of DTA and X-ray diffraction. The structure of glass and the role of Zn2+ ions, which were elucidated from crystallization behavior and IR absorption, are as follows. (1) ZnO combines with SiO2 to form a willemite (2ZnO·SiO2)-like structure in which Zn2+ is in 4-fold coordination. (2) As the ZnO content increases, a wollastonite (CaO·SiO2)-like and a gehlenite (2CaO·Al2O3·SiO2)-like units transform into a willemite-like and an anorthite (CaO·Al2O3·2SiO2)-like units. (3) The AlO4 tetrahedra coupled with Ca2+ are formed. (4) The substitution of ZnO for CaO lowers viscosity and reduces thermal expansion coefficient. This is explained by the lower dissociation energy of ZnO, compared with CaO, and lowering in the average coordination number of oxide ions with the replacement of the 6-fold coordinated Ca2+ ions by the 4-fold coordinated Zn2+ ions.
The 720 and 780cm-1 components of the deconvoluted Raman spectra of glasses in the systems RnO-TeO2 (R=Li, Na, K, Ba, Zn) were attributed to TeO3 groups and the 665cm-1 band was to TeO4 groups. The theoretical values of the molar ratio TeO3/TeO4 derived from an assumed vitrification reaction were compared with the ratio derived from the intensity ratios of the Raman bands, I(720)/I(665) and I(780)/I(665). It was indicated that [O2/2Te=O] and [O1/2Te(=O)-O]- units were induced by the addition of network modifying oxides in the composition range of lower atomic ratio O/Te. It was proposed that the decrease in the rate of increase of the ratio TeO3/TeO4 in the range of larger O/Te was due to the equilibrium TeO4/2↔[O2/2Te=O] favoring the former unit.
Crystallization behavior of Bi-Ba-Cu-O glasses has been studied. Glass-transition temperature Tg and crystallization onset temperature Tx are slightly lower than those of Bi-Sr-Ca-Cu-O glasses. (Tx-Tg)'s of the present glasses are, however, similar to those of Bi-Sr-Ca-Cu-O glasses. The temperatures of the first endothermic peaks in the DSC curves TEndo are about 700°C when BiO1.5/BaO>1 and about 850°C when BiO1.5/BaO<1, which are assigned to the melting of CuBi2O4 and BaCuO2 crystals formed in the products, respectively. BaBiO2.55 or BaBiO2.77 crystal is formed in the first stage of crystallization and transformed to BaBiO3 crystal at higher temperatures. No crystal containing all the cation species of Bi, Ba and Cu is formed from the present glasses, which can be attributed to the extremely high stability of BaBiO3-x crystal. The activation energies for crystallization Ea's are 382 and 325kJ/mol for the first and second peaks which are assigned to the formation of BaBiO2.77 crystal in the DSC curve for 50BiO1.5·25BaO·25CuO glass. It has been found that the Ea's for Bi-Ca-Cu-O and Bi-Sr-Cu-O glasses are higher than those of Bi-Sr-Ca-Cu-O glasses.
The microstructure and optical properties of transparent glass-ceramics containing ZnAl2O4: Cr3+ have been examined by using X-ray diffraction, transmission electron microscopy and optical absorption measurement. The glass-ceramics were prepared by the heat treatment of glass with 20ZnO·20Al2O3·50SiO2·6TiO2·4ZrO2·0.1Cr2O3 composition (mol%). ZnAl2O4 and ZrTiO4 precipitated from the glass when the heat treatment temperature was above 850°C. The heat treatment below 1100°C successfully yielded the transparent glass-ceramics. The crystallite size of ZnAl2O4 increased from 5nm to 20nm as the heat treatment temperature was increased from 850°C to 1100°C. At 1100°C, the ZnAl2O4 crystallites precipitated as a cubic form. The optical absorption spectra at room temperature indicate that the absorption due to the 4A2→4T2 transition, which corresponds to the 10Dq, takes place at the wavelength of 650nm for the glass and at 557 to 530nm for the glass-ceramics. This fact suggests that the ligand field strength for the Cr3+ ion sites increases as the Cr3+ ions are incorporated into ZnAl2O4. The wavelength of the absorption band due to the 4A2→4T2 transition increases monotonically with an increase in crystallite size. This phenomenon is explaned by taking into account the surface state of fine. ZnAl2O4: Cr3+ particles and the variation of the concentration of Cr3+ ions in ZnAl2O4: Cr3+ with heat treatment temperature.
Fine alumina powder prepared by an ammonium alum method was encapsulated in a stainless steel mold with the dimensions of O.D. 22φ∼I.D. 10φ×30hmm. The mold containing alumina was preheated at 1200 or 1300°C for about 8 minutes, and then immediately press-forged under uniaxial compressive load. The particle size of the starting powder (0.15μm) grew to about 0.2 or 0.3μm during the preheating, and a modarately dense polycrystal with the grain size of 0.2 or 0.3μm was obtained by the masstransport mechanism promoted during the deformation. The hot-forged polycrystalline alumina, however, did not show satisfactory hardness, because of some stored residual stress around the grain boundary. A polycrystalline alumina with excellent hardness was obtained after annealing the hot-forged body at a relatively law temperature of 1000 to 1100°C for 24 hours. The densification mechanisms of alumina powder during the hot-forging and annealing processes were investigated through microstructural observations by TEM and SEM.
Glass ceramic specimens having a controlled surface flaw introduced by Knoop indentation were tested under dynamic, static and cyclic loads by four-point bending of plates (uniaxial tension) and by diametral-compression of disks (tension-compression). All specimens showed a susceptibility to dynamic, static and cyclic fatigue failure. Equivalent time-to-failure obtained from the dynamic and cyclic fatigue data, using the assumption that there is no enhanced effect of cycle on the rate of subcritical crack growth, was plotted in the diagram between applied stress and time-to-failure. The crack propagation parameter, n, was estimated from the relation of time-to-failure and the maximum applied stress in the static and cyclic fatigue tests and the stressing rate dependence of the fracture strength in the dynamic fatigue tests. Consequently, it was found that the n value for diametral-compression loading became somewhat higher than that for four-point bending. The higher n value in diametral-compression could be mainly explained by the fact that a crack in a tension-compression stress state has a less freedom of motion than that in a uniaxial tension stress state, and such a strong orientation dependence for crack growth in a tension-compression stress state restrains the crack to grow around obstacles or grains.
The purpose of this paper is to investigate in detail the relation between the microstructure and high-temperature hardness for Al2O3/SiC micro-and nanocomposites. Two types of Al2O3/SiC composites were fabricated by hot-pressing the mixtures obtained by a conventional ceramic process. Al2O3/finer-SiC composites showed a peculiar microstructure with nanometer-size SiC particles located inside the matrix Al2O3 grains, yielding “nanocomposite”, whereas Al2O3/2μm-SiC composites had the 2μm-SiC particles dispersed at the grain boundary, yielding “microcomposite”. The addition of SiC was found to be very effective in improving the high-temperature mechanical properties of Al2O3 for both of the above Al2O3/SiC composites. Especially, Al2O3/finer-SiC nanocomposites exhibited a better deformation resistance and anti-creep behavior at high-temperatures.
In order to apply the maximum thermal stress equation to water-quenching thermal shock tests of ceramics in practice, a novel method for evaluating the heat transmission behaviour and measuring the effective heat transfer coefficient was proposed. Zirconia ceramic, whose heat conductivity does not change over a wide temperature range, was used for the study. In this experiment, the change of temperature at two different positions in the specimen was measured after water quenching. The dependences of heat transmission behaviour on time and surface temperature were discussed. The present method made it possible to obtain the effective heat transfer coefficient data which clarify the relationship between the intrinsic properties of ceramics and the critical quenching temperature difference.
The high temperature oxidation behaviors of gas-pressure-sintered (GPS) Si3N4 containing small amounts of impurities in addition to 5wt% Y2O3 and 0, 2 or 5wt% Al2O3 (A0, A2, A5) as the sintering aids were examined at 1400°C for 15h in pure oxygen. For comparison, the GPS Si3N4 obtained from highly pure Si3N4 powder (B5) was examined. The oxidation reaction was followed every minute by measuring the evolved N2 and NO using a quadrupole mass spectrometer. The weight changes of the bodies before and after the oxidation were measured and their oxidized surfaces were analyzed by SEM and EDX. The oxidation behavior of GPS Si3N4 with Y2O3 and Al2O3 additives obeyed an asymptotic kinetics possibly due to the formation of protective scales. The N2 evolution rate, the parabolic rate constant of the initial oxidation stage, the amount of N2 evolved, and the weight gain increased with an increase in the amount of Al2O3. The SEM observation of the oxidized surface revealed that the proportion of a crystalline phase identified as Y2O3·2SiO2 decreased and a glassy phase increased with increasing Al2O3 content. The oxidation behavior of B5 was fairly fitted with two stage parabolic kinetics. B5 exhibited a larger amount of N2 evolved and larger weight gain than A5.
A Malvern type constitutive equation is sometimes used to analyze the dynamic response of an elastic/viscoplastic body at strain rates of 103-104s-1. The value of material constant K in the equation has sometimes been determined by comparing the experimental results with the numerical ones. On the other hand, a constitutive equation which is called the modified Malvern constitutive equation which covers a wide range of strain at a high strain rate, has been proposed previously by comparing the experimental results with the numerical ones using one dimensional elastic/viscoplastic wave propagation in a lead bar. The modified type is considered to be one of the Malvern type containing a weak flow stress dependence on strain rate up to about 103s-1. In this paper, it is predicted that the coefficient of the Malvern type coincides with that of the constitutive equation obtained by considering the viscous drug region of dislocation so that the constant Ks in the Malvern and modified types can be basically determined using the viscous drug coefficient of dislocation. In the case of the Malvern type, the constant K determined by using this coefficient is not in agreement with the constant found by comparing the numerical and experimental results because the weak dependence of the stress on the strain rate up to about 103s-1 is not taken into consideration. However, it appears that the constant K determined by using viscous drug coefficient is in good agreement with the constant by comparing experimental results with the numerical ones in the case of the modified type.
Multiaxial stress tests were carried out on thin-walled cylindrical specimens of the A2024-T6 aluminum alloys strengthened by two different precipitates. One of the A2024-T6 alloys has the (GP+GPB) structure obtained by aging for 10min at 200°C; the other has the (θ'+S') structure obtained by prolonged aging of 5.5hr at 200°C. The effect of aging time was examined by comparing two types of plastic deformation behavior. From the proportional combined loading tests of axial load, internal pressure and torsion, it was found that the flow stresses changed with a rotation of the principal stress axes and a difference arose between the directions of the principal stress and principal strain increment. Under tension-internal pressure and tension-torsion, the equi-strain surfaces and strain behavior were also determined. Both alloys exhibited axisymmetric anisotropy. The degree of anisotropy was larger in the 2024-T6 (10min) than in the 2024-T6 (5.5hr). In addition, the plastic deformation behavior could be expressed precisely by the constitutive equation derived from the proposed yield function, hardening law and associated flow rule.
This paper is concerned with the effect of physical aging on flexural creep behavior of epoxy resin. Epoxy resins were subjected to various degrees of physical aging by different thermal history, i.e., rapidly cooled, slowly cooled and aged epoxy resin. Long term flexural creep tests on these resins were carried out at a constant temperature, and short term flexural creep tests were simultaneously carried out at various temperatures. The long term creep behavior of epoxy resin was obviously influenced by the degree of physical aging. The creep deformation decreased as the physical aging proceeded. Each long term creep behavior was estimated by applying the modified reciprocation law of time and temperature, which was proposed by a previous paper, to the short term creep behaviors. These estimated long term creep behaviors agree fairly well with the measured creep behaviors, in particular for physical aging proceeded epoxy resin. Furthermore, the time-temperature shift factor (horizontal shift factor) and temperature shift factor (vertical shift factor) of the modified reciprocation law of time and temperature decrease as the physical aging proceeds.
The effect of fiber orientation on viscoelastic properties of CFRP (carbon fiber reinforced plastic) was examined by means of a wave propagation testing. Specimens used in the present study were CFRP hollow cylinders, which were made in various fiber orientation angles (winding angles) by filament-winding technique. Longitudinal impact tests were performed and strain wave histories were resolved into Fourier components in order to determine the complex compliance. The experimental results revealed that both the dynamic compliance (real part of complex compliance) and the loss compliance (imaginary part of complex compliance) remarkably increase with fiber orientation angle at about 45° and show the maximum values at about 70°. Furthermore, based on the correspondence principle between elasticity and viscoelasticity, the laminate theory for elastic constants was modified to treat viscoelastic properties and the effect of fiber orientation angle on the complex compliance was theoretically discussed. It was found that the modified theory provides reasonable estimations on the change of viscoelastic properties with fiber orientation angle.
For carbon fiber reinforced plastics (CFRP) using 6 kinds of carbon fibers with different surface conditions, ILSS was measured from 25°C to 200°C at various cross head speeds (CHS), and the influence of surface structure of carbon fiber on the high temperature ILSS of CFRP was investigated. The following results were obtained. (1) ILSS measured at high temperatures was higher for CFRP with surface treated fibers. (2) The logarithmic values of shift factor aT by adopting time-temperature superposition principle were plotted against 1/T. These Arrhenius' plots showed differences among kinds of fibers and those with or without surface treatment. In every case, plots are aligned on a line that bends at a point. (3) In the lower temperature range than the bending point on Arrhenius' plot, the activation energy were from 140 to 230kJ/mol and it increased with the increment of the amount of COOH group obtained from C1s peak of X-ray photoelectron spectroscopy.
Fracture toughness testing of two transparent materials, fused silica and PMMA (polymethyl methacrylate), has been carried out according to a method suggested by ISRM (International Society for Rock Mechnics) in order to know the crack front evolution and also to examine the unloading compliance method for determining the crack length, and a K-resistance curve has been evaluated by the extended ISRM suggested method for these materials. The main results obtained in this study are as follows: (1) The crack front in brittle fused silica preceeds at the ends when the crack length is small, and it approaches to be a straigth line as the crack grows. On the contrary, the crack front in the ductile PMMA always preceeds at the center. Thus, the shape of the crack front depends on the material characteristics. (2) The unloading compliance method according to the ISRM suggested method can evaluate the crack length almost accurately for fused silica while it underestimates the crack length for PMMA, since the time-dependent crack growth occurs in the early stage of unloading. The gradient of a straigth portion in a load-displacement curve during unloading should be used to evaluate the crack length accurately for the materials such as PMMA. (3) A K-resistance curve can be determined almost accurately for brittle fused silica by the extended ISRM suggested method.
Fracture toughness tests of W-Ni-Fe sintered tungsten alloys with various volume fractions of tungsten particles were carried out under various loading rates. Fracture toughness of the sintered tungsten alloys decreased with increasing volume fraction of tungsten particles. An increase in loading rate resulted in a significant decrease in fracture toughness. From the fractographic observations, the facets of cleavage fracture of tungsten particle decreased and the facets of ductile fracture of matrix increased with an increase in fracture toughness. When the volume fraction of tungsten particle is low, a stable crack dominantly grows in the matrix phase, resulting in high toughness. On the contrary, when the volume fraction of tungsten particle is high, the matrix phase exists only in the triple points among tungsten particles. So the deformation of matrix phase is restricted and consequently an unstable crack grows in tungsten particles in cleavage fracture manner.
The fracture behavior of SFC materials containing a single carbon fiber emedded in epoxy or nylon 6 matrix which were loaded in tension was examined by AE monitoring. It was confirmed that AE signals resulted from fiber breakage and the number of AE events was in a good agreement with the fiber fragments produced during the fracture process. The mean fragment length and interfacial shear strength were calculated by using the number of AE events, and they were comparable with those obtained from the fragment length distribution and the micromechanical model. The specimen thickness influenced the mean fragment length and the interfacial shear strength, but the tensile speed in the experimental conditions did not. It was shown that the interfacial shear strength for the SFC with epoxy matrix was higher than that with nylon 6 matrix in the same treatment of a fiber surface. The difference in fracture behavior between two SFC materials with epoxy or nylon 6 matrix was also observed by AE spectrum analysis.
By using slanted pre-crack specimens, fatigue crack growth under a mixed- mode condition was discussed within the scope of linear fracture mechanics. The discontinuous displacement measured along the pre-crack with an angle β=30 deg. between the crack and the axis of tension was smaller than the theoretical value. This discrepancy suggests the contact of the crack surfaces under a large relative slip displacement. The measured fatigue fracture angles agreed well with the values calculated based on the maximum tangential stress criterion, using the effective stress intensity factors (Mode I, II) estimated from the discontinuous displacement along the pre-crack. For the bent crack formed after fatigue crack initiation from the pre-crack, the path of the fatigue crack growth was mainly dominated by a mode I discontinuous displacement component on the measurement. A method was described in which the bent crack was converted into a straigth crack in a vector form of the discontinuous displacement along the crack. Using this method, the stress intensity factors (Mode I) of the bent crack were evaluated. They were in good agreement with the theoretical values except the case of the heavy contacts between the crack surfaces.
The cyclic effect on fatigue of a SiC-Whisker/Si3N4 composite (CMC) was investigated. Three-point bending specimens were used for both static fatigue and cyclic fatigue tests at room and an elevated (1400°C) temperatures. The following conclusions were obtained from the experiment. The cyclic effect on fatigue of CMC seems to exist at the both temperatures when the fatigue life under cyclic loading is compared with the static fatigue life in terms of time from a viewpoint of slow crack growth (SCG). However, the observed cyclic effect is not much appreciable. The fatigue of CMC is mostly dominated by SCG. The cyclic effect at room temperature is larger than that at 1400°C. At 1400°C, the cyclic effect becomes large with an increase in applied stress. SCG area in the fracture surface of the specimen under sustained loading is smaller than that under cyclic loading.
Recently, a relatively high molecular weight methacrylate monomer (HMWM monomer) has been developed. Since the monomer hardly evaporates under an ambient condition, it is possible to apply a thermal polymerization process under the atmospheric pressure in the production of polymer impregnated concrete or mortar (PIC). Furthermore, since it polymerizes at room temperature, it is possible to eliminate the thermal polymerization process. This paper describes the compressive strength of PIC using the mixtures of HMWM monomer and multifunctional acrylic monomers. The factors in the experiments were as follows: the number of functional groups of the multifunctional acrylic monomers and their contents, the water-cement ratio of base mortars, polymerization temperature and the polymerization time of PIC polymerized at room temperature (20°C). Compressive strength tests were made on 16×30mm cylindrical specimens. The water-cement ratio of the base mortars for PIC was varied from 30 to 60%, and their sand-cement weight ratio was 1.0. PIC using HMWM monomer or the mixtures of HMWM monomer and the multifunctional acrylic monomers were prepared as follows: (1) The base mortars were dried at 110°C for 48 hours, (2) evacuated at 8.0kPa for 1 hour, (3) impregnated with the monomers, (4) pressurized at 0.20MPa for 30 to 40 minutes, and (5) subjected to thermal polymerization at 70°C for 12 hours or storage at room temperature for 14 and 28 days. The principal conclusions obtained from the test results are summarized as follows: (1) The compressive strength of the base mortars is extremely improved by using mixed HMWM monomers, even though the compressive strength of PIC using mixed HMWM monomers is lower than that of PIC using MMA monomer. (2) When the number of functional groups of the multifunctional acrylic monomers increases 2 to 6, the compressive strength of PIC increases. In the range of the content of the multifunctional acrylic monomers 10 to 30%, the highest compressive strength of PIC is obtained when the content is 20%. (3) The compressive strength of PIC increases with the glass transition temperature of polymers. (4) The water-cement ratio of the base mortars influences the compressive strength of PIC, and the smaller the water-cement ratio becomes, the higher the compressive strength of PIC does. (5) In the use of a mixed HMWM monomer with a number of functional groups of multifunctional acrylic monomer of 6 and a multifunctional acrylic monomer content of 20%, the compressive strength of PIC polymerized at room temperature for 14 days is nearly equal to that of PIC polymerized at 70°C for 12 hours.