Fracture Analysis of Ceramics with Small Defects Based on R-Curve Method.

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  • セラミックスの微小欠陥からの破壊強度のR曲線法による評価
  • セラミックス ノ ビショウ ケッカン カラ ノ ハカイ キョウド ノ R キョ

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Since the critical size of defects in ceramics is normally very small, the conventional method of fracture mechanics often becomes inappropriate. In the present paper, the fracture strength of silicon nitride with small defects is evaluated based on the R-curve method. Parameters of the R-curve of silicon nitride are determined from the experimental relationship between the fracture strength and the defect size. For penny-shaped cracks, the fracture strength is constant for small defects and the critical stress intensity factor is constant for large defects. The equivalent defect size is an excellent parameter for correlating the fracture strength with the size of elliptical cracks with an aspect ratio between 0.3 and 1. For small voids, the fracture strength is nearly the same as that for a penny-shaped crack. On the other hand, for large voids, it approaches a constant value which is equal to the strength of materials without voids divided by the elastic stress concentration factor. When the tip radius of long deep notches is small, the critical stress intensity factor for the fracture is independent of the tip radius. A Monte-Carlo simulation of bending fracture of silicon nitride was performed by random distribution of penny-shaped cracks with random sizes in a specimen by a computer. On the basis of twenty five runs of fracture simulation, the relationship between the bending strength and the defect size, and the cumulative distribution function of the fracture strength were successfully derived.


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