Relation between glass-forming regions and amorphous-forming regions were discussed among the various multi-component systems according to the preparation methods and experimental conditions. It was concluded that melt-quenching technique can yield comparatively small range of amorphous composition since obtainable quenching rate is limited. Sputtering technique, on the other hand, is regarded as a most powerful technique to produce amorphous materials. Thus sol-gel technique is another most promising technique to obtain wide range of amorphous materials as long as the gels formed are water resistant. Sol-gel method can transform high melting point compounds such as Al2O3 and TiO2 into amorphous. TC/TL ratio can be regarded useful for the evaluation of amorphous-forming ability despite the different preparation techniques. In conclusion it can be stated that amorphous-forming ability is intrinsic to the constituents, only slightly dependent on the preparation technique.
Elemental boron and boron nitride powders were mixed and pressureless-sintered in nitrogen. The structure and mechanical parameters of the ceramics were investigated and compared with those obtained from boron nitride powder free of the elemental boron. A positive effect of the addition of boron was found in increasing density, mechanical strength and refractoriness, and in suppressing growth of boron nitride grains. The reason of these changes has been explained by the formation of a refractory suboxide with a formula B6O as a consequence of a boron-controlled extraction of boric oxide (B2O3). The extraction of the sesquioxide being originally adhered to the surface of boron nitride grains allowing a more effective formation of boron nitride ceramic skeleton. From the morphology of the ceramics, it has been suggested that the formation of volatile boron suboxide (B2O2) precedes to localization of oxygen in B6O grains.
An attempt was made to examine the validity of a wake-dilatation model of microcrack toughening in ceramic matrix composites. Model two-phase, particulate composite system was chosen for study. Spheroidized alumina particles with average size 25 and 12μm, respectively, were dispersed in a soda lime silica glass having thermal expansion coefficient greater than the expansion coefficient of alumina. The composite containing 25-μm alumina particles was formulated by selecting a combination of differential thermal expansion and particle size, whereby microcrack toughening is expected to occur most effectively. Another composite containing 12-μm alumina particles was designed as a reference material, in which the condition for the occurrence of microcracking is not met. For both composites, specimens containing 30vol% of alumina particles were prepared by hot-pressing technique. Fracture toughness and R-curve measurements were carried out on each of the composites. Glass-alumina(25μm) composite exhibited a rising R-curve behavior. The experimental results were interpreted on the basis of a wake-dilatation model of microcracking. By evaluating physical parameters involved in the model, its validity was discussed.
The authors have already made clear the deterioration mechanism of durability of concrete structures due to the meteorological action, especially, the fundamental mechanism of deterioration due to the dry and wet action and the freezing and thawing action, and the effect of the concrete factors on them. This paper furthermore deals with the effect of the initial moisture content of concrete on the freezing and thawing resistance and analyzes mathematically the concrete factors dominating the elastic modulus of elasticity.
Carbon fiber reinforced facing/aramid honeycomb core sandwich beams were studied through three point bending test. In order to clarify the influences of water on the stiffness and strength, the dry and wet specimens were used. At the same time, the fractured specimens were inspected by a scanning electron microscope (SEM) and a scanning acoustic microscope (SAM) to observe the fracture surfaces and interior damages of the specimens, respectively, and the fracture mechanism was discussed. The experimental results showed the following. (1) With 6wt.% water absorption the flexural stiffness of the sandwich composites rises a little while the shearing stiffness falls by 11%. (2) The core shearing strength is more sensitive to water absorption than the facing one. For a 3wt.% absorption ratio, the facing strength and core shearing strength are 9% and 18% lower, respectively. However, the decrease has a tendency to level off over 4wt.% absorption ratio. (3) The bonding strength between the facing and the core is larger than the matrix. (4) The buckling axes observed in the compressive fracture surfaces are more seen in wet specimens than dry ones, which shows that the fibers are liable to buckle in wet specimens. (5) At a small span length (<200mm), the wet specimens deflect more than the dry specimens, which is due to the degradation of the aramid fiber paper of the cell wall.
Fiber reinforced composite materials are widely used in many structures. It is well known that composite materials inelastically deform. Although there are some researches on fatigue failure, damage and crack propagation of composite materials, cyclic inelastic behavior of composite materials subjected to cyclic loading whose direction is different from that of the fiber has not been investigated in detail. In this paper, in order to investigate the cyclic inelastic behavior and the fatigue failure of composite materials, cyclic tension-compression loading under several conditions are carried out using laminated graphite/epoxy tubular specimens. The definition of fatigue failure is also discussed based on the test results. It is found that the CFRP subjected to cyclic loading shows the characteristic cyclic inelastic deformation. Moreover, using the concept of the plastic strain energy density and the concerned definition of fatigue failure, the relationship between the number of fatigue failure and the plastic strain energy density can be expressed by a formula.
The cycle-dependent pure fatigue crack propagation and the time-dependent creep-fatigue one, in general, were scarcely accompanied with microcracks and voids or cavities in the vicinity of the tip of propagating cracks inside the material when subjected to relatively high strain rate cycles at intermediate temperatures. In very low strain rate and/or high temperature conditions, however, diffusion-controlled creep cavities and microcracks tend to be generated along grain boundaries at the mid-thickness. These cavities and microcracks affect the macrocrack propagation in high temperature fatigue in two ways; one is the decrease in the crack propagation resistance and the other is the decrease in the crack driving force, i.e., the J-integral. Consequently, the creep-fatigue crack propagation is accelerated by several times and the succeeding fatigue crack propagation in the pre-creep damaged material by ten times at most for a given J-integral range, ΔJ.
A constitutive model of cyclic plasticity is formulated to describe proportional and nonproportional hardening of type 316 stainless steel in the range from room temperature to 973K. For this purpose, we first examine the evolution of the size of isotropic and kinematic hardening range based on the experimental results performed previously by the present authors. It is elucidated that the complicated behavior of cyclic hardening under proportional cycles is induced by the evolution of kinematic hardening variable. Then, new evolution equations of kinematic hardening variables are established by incorporating this information into the nonlinear kinematic hardening rule. In particular, the dependence of cyclic hardening on the history of strain amplitude is taken into account. The final model is established by incorporating these evolution equations into the nonproportional viscoplastic model proposed by one of the present authors. It is elucidated that the proposed model can describe the behavior of proportional and nonproportional cyclic hardening under various loading conditions including strain cycles with amplitude variation in the range from room temperature to 973K.
Plane-strain dynamic flow localization in tension blocks under a wide range of deformation rates has been investigated numerically. The material is characterized by a thermo-elasto-viscoplastic constitutive equation with strain-gradient-dependent flow stress. The mixed type variational principle for the corresponding materials is developed to satisfy this continuity requirement of the higher-order derivative of velocity. In this variational principle, the velocity and representative strain rate are regarded as independent variables. The finite element method developed using the variational principle has been employed to explore the effects of deformation rate applied, and shape and size of the blocks on the flow localization behavior.
This paper reports several interesting features in the shape memory alloy observed experimentally under multi-axial complex loading conditions including some temperature changes (the general loading condition). The experiments were performed systematically by applying the combined loads of axial force and torque to thin-walled tubular specimen made of a Cu-based polycrystalline shape memory alloy. In these systematic experiments, the strong path dependency of pseudo-elastic phenomenon was observed, and moreover this dependency was disappeared completely only when the materirals went back to the stress free state. This kind of unique behaviour of shape memory alloy may be quite interesting from a view point of new engineering applications of shape memory alloy.