AUTHORS: Natsumi Miyazaki, Toshihiko Hoshide, Daichi Itaya

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ABSTRACT: In applications of porous ceramics to elements in energy related components, their strength properties should be appropriately evaluated to guarantee in-service durability. In this work, bending strength properties of porous alumina specimens with four different shapes were investigated to clarify geometrical size effects depending on porosity. 3-point bending tests were conducted using notched specimens as well as smooth specimens of three distinct sizes. It was revealed that the bending strength decreased drastically as increasing porosity. An effective volume concept was introduced in discussing geometric size effect. Average strength was correlated with effective volume for specimens with respective porosities. A little bit of dispersion was seen, though the average strength for all materials was well correlated with the effective volume independently of specimen geometry. Cross-sections were observed through a laser scanning microscope to characterize spatial and size distributions of pores. In this study, based on the observed pore characteristics, a fracture mechanics procedure was proposed by presuming pores to be surrounded by virtual cracks. Monte Carlo simulations based on the proposed procedure were carried out by assuming the same characteristics of crack distribution as those of pore distribution in a material. Strength simulated by using the proposed procedure almost coincided with experimentally observed one. Consequently, the proposed procedure was confirmed to be applicable to evaluation of porosity and geometric size effects on strength.

KEYWORDS: alumina, porosity, bending strength, size effect, effective volume, fracture mechanics

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