In-pile instrumentation systems in present-day light water reactors (LWR) are indispensable to monitor all situations during reactor operation and reactor shut down. However, the systems did not work sufficiently well in situations like the severe accident (SA) at the Fukushima Dai-ichi nuclear power station. Thus, it is necessary to develop monitoring systems for the prevention of injuries in the event of an SA. Mineral-insulated (MI) cables with radiation and heat resistance are exposed to a mixture gas including nitrogen, oxygen, hydrogen, water vapor, and fission products at high temperature under SA conditions. In this study, corrosion tests under the simulated SA conditions (air, air/H2O, and I2/CO/O2/H2O) for temperatures up to 1015°C were performed for candidate sheath materials of type 316 stainless steel (316SS) and nickel-based alloy (NCF600). As a result, a uniform oxide film was formed on the surface of both the 316SS and NCF600 specimens in the air or air/H2O environments from 720°C to 1015°C, and the fracture time of the MI cable was evaluated by the degree of corrosion and the parabolic law. Conversely, when the gas mixture environment contained I2, each surface of the specimen showed complicated corrosion behavior that caused not only local corrosion but also the formation of a uniform oxide film at 800°C.
A dynamic double torsion testing machine and measurement system were developed on the basis of the split Hopkinson pressure bar test and the theoretical one-dimensional wave propagation in the input and output bars, and their validity was confirmed by measuring the dynamic fracture toughnesses of non-stoichiometrically cured epoxy resins. The applicable range of the stress intensity factor, as formulated by Fuller was determined from the specimen shape and loading conditions for the dynamic double torsion test. The available crack length was found to be below 70% of the specimen length by measuring the static fracture toughness and analyzing the natural frequency of the specimen. The duration of the impact force until dynamic fracture in the dynamic double torsion test was found to be longer than the reciprocal of the lowest natural frequency of the out-of-plane bending mode of the specimen, as expressed by an approximate equation. Because the mechanical properties of the epoxy resin had little dependency on time in the experiments at room temperature, the validity of the testing machine and measuring system were able to be confirmed by comparing the dynamic and static fracture toughnesses of the epoxy resins and observing the fracture surfaces.
In order to investigate a self-excited oscillatory phenomenon of a two-dimensional confined jet with a cylinder as a downstream target, the authors conduct two-dimensional numerical analyses based on vorticity ζ and stream function ψ using a finite-difference discretisation method. At first, confirming the reliability and accuracy of the present analyses, the authors examine two kinds of preliminary tests prior to the main test. Aa a result, the authors reveal (1) the two-dimensionality of the phenomenon by comparing computations with experiments, and (2) the importance and complexity of the upstream of the downstream target.
Effect of the microstructure of a metal-core piezoelectric fiber/aluminum composite on the characteristics of its output voltage was investigated. The metal-core piezoelectric fiber/aluminum composite was developed to overcome the problems associated with piezoelectric ceramics, such as has poor mechanical properties, reliability; brittleness and low fracture strain. This composite contains piezoelectric fiber embedded in aluminum matrix by using interphase forming/bonding method, which significantly improved the fracture strain of the metal-core piezoelectric fiber. The composite is expected to be used in long-term, high reliability sensors and energy harvesting devices. However, it was observed that the output voltage variation is caused by the residual Al-Cu eutectic alloy from the embedding process and contributes to the eccentricity of the core. These drawbacks, interfere with the practical use of this composite. In this study, the influence of the composite microstructure, such as the presence of the residual eutectic alloy and the eccentricity of the metal-core on the output voltage characteristics, is evaluated using finite element analysis. First, it was succeeded establishing a method considering that the radial direction of polarization, and confirmed the validity of the proposed method by reproducing the output voltage anisotropy of the composite and comparing the experimental values obtained in conventional studies with analysis value. Using this method, it was shown that the decrease of the output voltage is caused by the following; 1) The eutectic alloy having a high Young's modulus inhibits the stress transmission between the matrix and the piezoelectric ceramics, the stress of the piezoelectric ceramics decreases, 2) Eccentricity of the core lowers the stress in the portion where the piezoelectric constant is high near the core. From these results, possibility of design and suppress variations of the output voltage characteristics by adjust the structure of the composite was suggested.
The integrity of flange joints is critical in all pipe systems. To test this integrity, an ultrasonic method characterizing the condition of the flange joint would be a vital tool. In this study, we analyzed the contact conditions on a metal/gasket/metal connection. The instantaneous frequency profile, i.e., the phase change of the wave with time for longitudinal waves transmitted in an Al alloy/gasket/Al alloy system, was evaluated under various contact pressures. The instantaneous frequency (IF) was calculated with a complex continuous wavelet transform with a modified Morlet function as a mother wavelet. The maximum IF of the waves monotonically increased with contact pressure, showing similar properties as the amplitude of the waves. A one-dimensional numerical calculation with a finite difference time domain method with a spring condition for expressing various contact conditions or contact stiffness revealed that a large phase delay of the transmission wave was generated at a low contact stiffness interface, and the amount of delay was correlated with the contact stiffness. On the other hand, the delay in the reflected wave was very small.
Surface texture is an important parameter which affects functions and performance of industrial components. Although stylus and optical techniques are commonly used for evaluating the surface topology, they are applicable only to accessible surfaces. In practice, the geometrical features measurement of inaccessible surfaces from back side is sometimes demanded, for example, in inspection of safety-critical parts such as inner surfaces of pipes. For evaluating such internal surfaces, ultrasonic technique is one of the most effective among others. However, little attention has been paid to the evaluation of inaccessible periodic surfaces so far. In this paper, an ultrasonic pulse-echo technique, namely, master curve technique is developed for evaluating the pitch and the height of periodic triangular surfaces which is inaccessible or hidden on the back side. It is found that 60° of incident angle is appropriate for the development of the master curve equation to compromise between the resolution of measurement and the measurable range of the height-to-pitch ratio. By using P-wave at 60° of incidence angle, the pitch of the surface profile is evaluated from the classical diffraction grating equation, and then the height is evaluated by the master curve equation built from numerical simulation. The validity of the proposed method was verified by both numerical simulation and experiment. It was confirmed that the pitch is accurately measured in most cases. The height was also evaluated with good accuracy when it is smaller than a half of the pitch.
Three silicon-nitride (Si3N4) ceramic specimens (differing in terms of sintering additive and bending strength) were test corroded in 3-mol/L-HCl aqueous solutions at 80°C for 1500 h. The corrosion resistance of each specimen was evaluated by measuring weight loss and bending strength of the specimens before and after the immersion tests. The corroded and fractured surfaces of the specimens were observed by SEM. When the specimens were soaked in the aqueous HCl solutions, weight loss and bending strength decreased with immersion time. Moreover, after immersion, the color of a layer in the fractured section (called “discolored layer” hereafter) changed, and the layer became thicker with immersion time. The discolored layer included a corroded layer at the point of contact with the corrosion solution (where grain boundaries were eluted). The correlation between corroded-layer thickness and weight loss and that between bending strength and weight loss were both found to be linear. However, the gradients of those correlations for each test specimen were found to differ, so it is difficult to summarize these correlations with one linear mathematical expression. Measured bending strength and bending strength predicted using fracture toughness (KIC) of the Si3N4-ceramic specimens and crack length were found to be closely related. In this prediction, the corroded-layer thickness of the specimens immersed in HCl solutions was considered to be equivalent to the diameter of semi-circular surface cracks.
Determination and prediction of the dynamic properties of an O-ring for bearing support were performed. Utilizing O-rings as supporters of bearing is a promising way to suppress severe vibrations such as resonance and self-excited whirl experienced in high-speed turbo machinery. However, analytical prediction of the dynamic properties of O-rings has not been very successful so far because of its non-linear dependence on many parameters. In this study, focusing on the incompressibility of rubber materials, the isochoric shear viscoelasticity of an O-ring material was measured for high frequencies of up to 1 kHz. In measuring the viscoelasticity, a testing method developed by the authors was used. This method enables obtaining high-frequency shear viscoelasticity directly without assuming the temperature-frequency superposition principle. The obtained dynamic shear properties were modeled as functions of the frequency and hydrostatic pressure. Finite element models of squeezed O-rings were constructed with the material model assuming uniform property distribution, and dynamic analyses were conducted. The dynamic properties of O-rings were determined from the time-series data for the applied force and displacement. The data agreed with the experimental results of an actual O-ring. It was found that the dynamic properties of rubber components can be analytically predicted by considering the frequency and hydrostatic pressure dependence on the viscoelasticity.
Galling and wear have been a tribology problem in sheet metal forming of stainless steel. Although lubrication oil with chlorinated extreme pressure (EP) additives have been used, environmental and safety issues have demanded not to use chlorinated EP additives. For developing chloride-free oil for cold ironing of stainless steels, some commercial sulfur-based EP additives were evaluated by a cup internal ironing test. Moreover, the superior sulfur-based EP additive was combined with calcium- and zinc-base type additives in order to improve anti-galling performance. The mixture oil, as shown the high performance in the cup internal ironing test, successively passed a 10,000 shots practical process with severe ironing of stainless steel. After the 10,000 shots, it was found that no galling was observed on these 10,000 products. Based on the X-ray photoelectron spectroscopy (XPS) results, sulfide and calcium carbonate were formed on the surface of the products. This lubricating film seems to prevent galling in the practical process. The developed oil is an example of a chloride-free oil to replace the conventional chloride-containing oil.
In recent years, due to its high specific stiffness and strength, fiber reinforced plastics (FRP) are being used in aerospace components, automobile components, sports equipment, and in various other applications. Especially since composite laminates have superior mechanical properties, there is a high demand for their use as structural materials. However, mechanical properties in the out-of-plane direction of composite laminates, specifically the interlaminar strength and fracture toughness, are much weaker than those in the in-plane direction. This study focuses on the needle punch techniques that aim to improve composite properties in the out-of-plane direction. This technique is typically used for fabricating non-woven fabrics. Fiber webs were punched by a plate containing many special needles with many barbs. A portion of fibers in the in-plane direction were aligned in the out-of-plane direction. In this study, the needle punch process is applied on chopped strand mats. Static tensile tests, fatigue loading tests, and residual strength tests are performed. Tensile properties, residual strength properties and fracture mechanisms of FRPs with needle-punched chopped strand mats are investigated.