The crack growth resistance under thermal shock loading on silicon nitride was characterized using Disc-on-Rod test which was developed by the authors. The microdamage during thermal shock fracture was monitored by acoustic emission (AE) technique. Specimens were composed of β-Si3N4 with an acicular structure. Cylinders of silicon nitride with various microstructures were sintered at different temperatures. A pre-crack was introduced using Knoop indentation. In the Disc-on-Rod test, the specimens were uniformly heated to 850 ºC, and only the center of circular specimen was quenched by contacting with a copper rod. Within a fraction of a second after contacting, crack propagated in an unstable manner, at the same time, a high amplitude AE signal was detected. Subsequently, stable crack propagation was observed, and several low amplitude AE signals were generated corresponding to crack propagation. The specimen sintered at higher temperature showed lower crack growth resistance. From the results of microstructure observation and the fracture mechanical consideration, it is suggested that larger grains with > 1.5 μm in minor axes contribute to the toughening of materials. Consequently, the results of this study provide fundamental insights for the development of ceramic materials with high resistance to thermal shock fracture.
Natural convection heat transfer from a vertical cylinder in liquid sodium was experimentally studied. Two test cylinders of different dimensions were used. They were 7.62 and 17.51 mm in diameter, and 186 and 257 mm in heated length, respectively. The surface heat flux was ranged from 2×104 to 2×106 W/m2 at the bulk liquid temperatures of 673, 773 and 873 K. The local heat transfer coefficients on the cylinders were obtained systematically at various heights, x, from the leading edge of the heated section. On the other hand, theoretical equations for laminar natural convection heat transfer from a vertical cylinder were numerically solved by using PHOENICS code for the same conditions as the experimental ones considering the temperature dependence of thermo-physical properties concerned. The local Nusselt numbers, Nux, on the vertical cylinders obtained experimentally were compared with the corresponding theoretical values on the Nux versus modified local Rayleigh number, Rf [=Grx*Pr2/(4+9Pr1/2+10Pr)], graph. The experimental values of Nux are almost in agreement with the corresponding theoretical values of Nux with the deviations less than 20 % for the range of Rf tested here. The Nux on the rod diameter of a heat exchanger for a power plant, D=31.8 mm, were numerically analyzed by using this code. A correlation, which can describe the effects of the cylinder diameter and the cylinder height, was given based on the experimental and theoretical values. This correlation can describe the experimental and theoretical values of Nux for Rf ranging from 1.5×102 to 4.7×106 within 20 % difference.
Active vibration control devices are extensively used in various industrial fields. These devices are categorized according to their mechanisms into the following two types: (A) The vibration is controlled by actuators whose ends are connected to the controlled object and to “a fixed floor or a reaction wall”. (B) The vibration is controlled by actuators whose ends are connected to the controlled object and to “movable mass”. Typical types (A) devices are active vibration isolation devices. The advantage of a type (A) device is its excellent vibration control performance. However, it is difficult to downsize these devices because the actuator has to support the controlled object. In contrast, typical type (B) devices are active mass dampers (AMD). They do not need to support the controlled object; therefore, it is possible to realize compact systems. However, the control system design tends to be complicated, especially for multi-axis plants. In this study, we propose a new vibration control system design concept called “Direct Inertia Force Control (DIFC)”. By using DIFC, we can achieve the above mentioned advantages of both types (A) and (B), as well as circumvent the disadvantages. Furthermore, the effectiveness of DIFC is verified via experiments on a newly designed single-degree-of-freedom active vibration control device.
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