The densities of melts in the systems Li2O-B2O3, Na2O-B2O3 and K2O-B2O3 were measured by a counterbalanced sphere method in the temperature range from 850°to 1250°C. The room temperature densities of alkali borate glasses, i.e., Li2O-B2O3, Na2O-B2O3, K2O-B2O3, Rb2O-B2O3, and Cs2O-B2 glasses were also determined. The results obtained show that the addition of alkali oxides to boric oxide causes the contraction of the B-O network. The results was interpreted in terms of the molar volume theory on the assumption that the atomic groups of borate form in the glasses.
The mechanical properties of polystyrene composites filled with hollow, spherical Silas balloons were investigated. Three different grades of Silas balloons were used in this study: Grade S with the diameter ranging from 74 to 149μ, Grade M from 149 to 259μ and Grade L from 259 to 590μ. The tensile, flexural and compressive strengths as well as moduli were determined as a function of filler content by using an universal tensile testing instrument. The tensile, compressive and flexural strengths of the composites decreased with increasing the filler content. However, since the density of the composites also decreased with increasing the filler content, the specific strengths of the composites with Grade S Silas balloons showed little change from their original strengths even though the filler content was varied. As for the moduli of elasticity under tension, compression and flexure, the composites containing Grade S Silas balloons with or without silan-treatment showed increase in these moduli with increasing the filler content. The glass microballoons were also used to make composites with polystyrene resin and the mechanical properties of these composites were compared with those of the composites containing Grade S Silas balloons with or without silan-treatment. The ones with Silas balloons had the mechanical properties equivalent or superior to the ones with glass microballoons, when compared on a volume % basis. The effect of adhesion between polystyrene and Silas balloons was studied by using both untreated and silan-treated (coupling agent: UCC A-1120) Grade S Silas balloons. The nature of the fracture surface was examined with a scanning electron microscope. The tensile and flexural strengths as well as elastic moduli of the composites were improved when the adhesive force of fillers became stronger.
In the previous paper, a Nonlineer-Defect-Model of the crack initiation by tensile deformations was proposed. Based on the assumption that there is a defect of mechanically nonlinear character in a one-dimensional crystal, at which Young's modulus E veries with the strain ε as E=Eo(1-βε), where Eo and β are constants, it has been found that the strain at the defect ε is given in terms of the stress σ as ε=1/2β(1±√1-4βσ/E0) and also β governs the fructure stress σc=E0/4β. In this paper, the above model is developed further in order to explain explicitly the crack propagation. When it is assumed that nonlinear character of E depends on ε with a small time lag ξ as E=E0(1-βε+βξ(dε/dt)), then ε is defined as the solution of the following equation εdε/ε2-1/βε+σ/βE0=dt/ξ. The analytical solution has been obtained for each case of σ_??_σc. Of special interest is the solution for σ>σc, which is the formulated expression of the crack propagation when ε increases with time to infinity. Lastly, the applicability of the present model on the actual crack propagation in a three-dimensional body has been discussed.
The Nonlinear-Defect-Model, proposed in the previous paper to explain analytically the macroscopic fracture of one-dimensional body, was developed further in order to establish relations with the structure of steel. In this paper, the physical characteristics of local fracture at a macroscopic as well as microscopic nonlinear-defect (ND) in a one-dimensional body was first discussed as compared with the actual macroscopic fracture. Then, the residual strain in the body having various kinds of microscopic NDs was expressed analytically. The above discussion suggests that three types of ND approximation (Single-, Many- and Equivalent-ND) can be used to express the local fracture and residual strain. In the Many-ND approximation, the following three regions of strain have to be discussed separately: (1) Initial region where small strain is led from all of the different kinds of ND, distributed homogeneously in the body, (2) Final region where fracture is led only from ND distributed in the fracture plane, and (3) Transient region where intermediate strain is caused by the transition from (1) to (2).
Sharply notched specimens of 0.03%C annealed steel were fatigued under completely reversed in-plane bending. Microstructural change in the vicinity of the notch root during fatigue process preceding crack initiation was examined by means of the X-ray microbeam diffraction technique and optical microscopy. The following are the summary of the results obtained. (1) The distribution of the excess dislocation density near the notch root just before crack initiation was measured with the X-ray microbeam technique and was used to evaluate the stress amplitude at the notch root, termed as the effective stress amplitude σeff. The relation between σeff and the number of stress cycles at crack initiation near the notch root Nnc was equal to that between the stress amplitude σ and the number of stress cycles at crack initiation Nc for plain specimens. A method of estimating Nnc of notched specimens from σ-Nc relation for plain specimens was proposed. (2) The formation of slip bands for notched specimens took place when the average value of elastic stress amplitude over one grain at the notch root was higher than the lower yield stress of the test material. (3) The initiation of fatigue cracks for notched specimens was thought to occur when the stress amplitude was high enough for the linked micro cracks to grow beyond the grain boundary, which were formed along the slip bands within a grain at the notch root. The critical stress amplitude can be correlated to the threshold value of the stress intensity factor in fatigue crack propagation laws proposed by Paris et al.
At temperatures ranging from -50° to 80°C polymethyl methacrylate (PMMA) plates of 5mm thickness were impacted with PMMA flyer plates of 1mm thickness, which were accelerated by the foil explosion method. Spall cracks of various growth stages produced in specimens were examined in detail by the aid of an optical microscope. It was found that in general the spall in PMMA plates was not an extension of a single crack but consisted of many small cracks of disc shape. (Each of them shall be called as a unit crack.) The diameter of the unit cracks in a specimen, which extended perpendicularly to the direction of tensile stress, varied from 30 to 700μ. Most of the unit cracks had circularly symmetrical patterns with discernible origins at their centers. This point of origin was surrounded by a region of mirror-like smoothness and a number of fine cracks spreaded radially from the periphery of the mirror region. The distance of crack separation at the mirror region was comparable to the wave length of visible light. When the amplitude of stress waves applied to specimen plates was not large enough, such mirror region was not distinguishable and the whole pattern of the unit crack became similar to that of the well-known“parabola-mark”in the vicinity of its focus. When the amplitude was sufficiently high, the separation of fracture surfaces became very distinct all over the area of well-grown unit cracks. It was characteristic of this case that another region of mirror-like smoothness appeared near the outer circumference of the crack disc. As an intermediate case between the above two extremes, very fine cracks with a cloud-like appearance sometimes appeared radially outside a rough area of an irregular shape with a shell-like appearance adjacent to the inner mirror region. Neither specimen temperatures nor annealing conditions affected the pattern of spall cracks. Their sizes were also independent of specimen temperatures in the range of 23°-9°C. The present results suggest that the initiation of spalling in PMMA is substantially dominated by the location and properties of internal nuclei such as flaws and that both the configuration of the nuclei and the time required for them to form origins of unit cracks determine the final crack growth.
The delayed crack growth rate dc/dt is generally expressed as a function of stress intensity factor K as follows: dc/dt=a1Km where a1 and m are constants that depend on several external factors. From the nucleation theory, Yokobori suggested that these constants were functions of temperature, surface energy, etc. However, there have been few experimental studies about the dependence of these constants upon external factors, although such a study is important to clarify the delayed crack growth mechanism. The present experiment was carried out to investigate the effect of environment on a1 and m by using a high polymeric solid, namely polycarbonate. It is hoped that the results are applicable to the study of metals because of the similarity in delayed crack growth rate between high polymers and metals. The main results obtained are summarized as follows: (1) There are two types of delayed crack growth; one is characteristic of the crack growth in air and another is that in the environment. (2) The environmental delayed crack growth process consists of three stages. (3) At the stage of stable crack growth, the growth rate is expressed as the power function of stress intensity factor. The power varies depending upon the kind of environment.
Concrete can be regarded as a composite material composed of cement paste or mortar as a matrix and gravel as an aggregate. Thermal diffusivity is a measure of material's ability to undergo a temperature change. In this study, the thermal diffusivity of concrete was measured and the results were examined from the viewpoint of a composite material. When both of the fine and the coarse aggregates are of the same rock type, the diffusivity of the concrete shows a tendency to increase with increasing the aggregate content. When they are different, however, such tendency no longer exists. Hence the rock type is one of the most important factors which determine the diffusivity of concrete. Since the exact value of diffusivity of aggregate is generally unknown, a model coarse aggregate was made of the mortar of known diffusivity and was used to make concrete samples. The measurement of the diffusivity on these samples showed that the diffusivity of concrete can be expressed in terms of the diffusivities and the volume fractions of the constituent materials. This result can be used to estimate an unknown diffusivity of aggregate, mortar or concrete when two of these three quantities are known.
This paper describes the fatigue failure of rock samples under the pulsating tensile stress. The specimens used were prepared with the procedure developed by one of authors, using the alignment jig and grinding lathe. On the assumption that the fatigue failure takes place in stochastic process, the wide fluctuation of fatigue life is discussed. The statistical analysis of the test results shows that the probability of survival is given as a linear combination of two exponential functions, each of which represents a Poisson process of 1st order. This result agrees with that of the compression fatigue test reported previously. Finally, the effects of the mean stress and stress amplitude on the rate constants of fatigue failure are discussed and compared with those in the compression fatigue test.
The static mechanical properties of vascular walls were determined from the change in external radius due to distending pressure as a series of the biomechanical studies of the blood circulation system. Segments of abdominal aorta, carotid artery and femoral artery excised from mongrel dogs were used as the test specimens. The arrangement and interrelation of the structural components were also examined on the transverse and longitudinal sections of these walls fixed at various distending pressures. The results obtained are summarized as follows: (1) In the low pressure range below 100mmHg, the wall radius increases rapidly with increasing pressure. However, vascular walls become much stiffer as they are distended in the pressure range between 100mmHg and 150mmHg. And above 150mmHg they almost lose their distensibility. (2) Distensibility of vascular walls decreases in the following order: abdominal aorta, carotid artery and femoral artery. It corresponds to the order of their distance from heart or of the volumetric fraction of elastin component existing in their walls. (3) The distension ratio (λθ)0, which is the ratio of external radius at each pressure to that at 20mmHg, can be related to the distending pressure p by the following equation in and below the physiological pressure range; logp=a+b(λθ)0. The constants a and b in this equation can be used as parameters representing the mechanical properties of vascular walls. (4) The tangential stress induced in the wall of abdominal aorta, which is calculated by Lame's equation, can be related to the tangential strain by a logarithmic function in the low stress range below 10g/mm2 and by an exponential one in the higher range. (5) Waviness of the elastin lamellae and interlamellar distances decrease markedly with increasing pressure between 100mmHg and 150mmHg. At and adove 150mmHg elastin lamellae become almost straight and interlamellar distances become uniform. These microscopic observations correspond to the unique change in distensibility of vascular walls stated in (1).