Shigeru Akiyama

A new method to evaluate the thermal shock resistance of ceramics by laser pulse irradiation

Structural ceramics are attracting attention in the development of nuclear fusion reactors because they have excellent wear- and heat-resistant characteristics. However, in some applications, they will be exposed to very high temperature and high-heat-flux environments. These ceramics are also subjected to thermal loadings that change rapidly with time. Therefore, it is important to investigate their thermal shock characteristics. A new approach to evaluate the thermal shock resistance of structural ceramics is based on laser pulse irradiation on the ceramic surface. The temperature and thermal stress distributions of cylindrical ceramics under irradiation by laser beams are discussed by using the MARC finite element computer code with arbitrary quadrilateral axisymmetric ring elements. The relationship between the spot diameter of the laser beam and the maximum compressive thermal stress is derived for various power densities of the laser beams. A critical fracture curve is obtained from these relationships that can specify a critical power density for a given laser beam spot diameter. The irradiation experiments are done on a machinable ceramic by using a CO[sub 2] laser. Finally, theoretical results are compared with experimental ones. Both results show good agreements. Consequently, this method can be a new standard thermal shock test instead of morexa0» the water quench test that has been used widely. 7 refs., 18 figs., 2 tabs. «xa0less

Thermal shock strength of Al2O3 by laser irradiation method

Abstract Various structural ceramics have been developed as heat-resistant materials. It is very important to investigate their thermal shock characteristics. This report presents a newly developed, laser irradiation method to evaluate thermal shock strength and proposes that a critical power density can be a new measure to evaluate heat resistant materials. The temperature and thermal stress distributions of the cylindrical shaped Al 2 O 3 ceramics under irradiation by CO 2 laser beams are analyzed using FEM. The maximum tensile and compressive stresses are obtained with respect to beam diameters for various power densities. These relations lead to critical power densities at which materials are fractured. The relationships between the critical power density and beam diameters derive critical fracture curves which gives the fracture criterion of ceramics. Finally, the irradiation experiments were carried out and their results got a good agreement with the theoretical fracture criterion. It was concluded that the critical fracture curve can be a new measure to evaluate thermal shock resistance of ceramics.

Thermal shock resistance of carbon-carbon (C/C) composite by laser irradiation technique

Abstract Thermal shock strength of carbon–carbon composite has not been evaluated yet. This paper presents it by laser irradiation technique newly developed for ceramics. Traditionally adapted quenching test has a big difficulty in which heat transfer coefficient is very unstable under the quenching process. To avoid this problem, laser irradiation method was proposed and applied effectively to several ceramics. Thermal shock strength of carbon–carbon composite was evaluated by laser irradiation method and the critical laser power density which causes the material’s fracture was obtained. It was concluded that the critical power density can be a new measure to evaluate thermal shock strength.

Observation of Microstructural Deformation Behavior in Metals Caused by Cavitation Impact

To clarify the erosion mechanism of cavitation, we pay attention to the internal changes in grains caused by cavitation impact. The vibratory cavitation test based on ASTM G32 was carried out and the cross section of directly under the eroded surface of a specimen was observed using the electron backscattering diffraction (EBSD) technique. The situations of grain boundaries and the changes in the crystal orientations of grains in aluminum, copper, and steel were analyzed. As a result, the following observations were made. Cavitation impact causes grain refining of sub-micrometer order near the eroded surface. There exist small changes in crystal orientation in grains. In steel, grain boundaries were generated on the eroded surface and were growing inward in the grain. Finally, the mechanism of internal deformation of grains caused by cavitation impact was discussed.

Thermal Shock Strength of Carbon-Carbon Composite by Laser Irradiation Technique Based on Shear Strength Criterion

Abstract The laminated carbon-carbon (C/C) composite is candidate material for plasma-facing components in fusion reactors. Its thermal shock strength must be evaluated for an application to the components facing the high-temperature plasma field. However, few studies have been done. A laser irradiation method newly proposed was applied effectively to lamination of C/C composite for the evaluation of thermal shock strength by subjecting a heat flux up to 27 J/mm2 on the specimen surface. By introducing the critical power density at which the material fracture occurs, thermal shock strength was obtained. It was concluded that the critical power density could be a new measure to evaluate thermal shock strength for laminated C/C composite.