Strength of Structural Ceramics with Micro Cracks

Abstract

This paper describes the physical meaning of the representative crack length, 2c_s, and the effectiveness of the strength evaluation by fracture toughness and the representative crack length for specimens with different shapes and sizes. Four point bending tests were carried out for pressureless sintered silicon nitride specimens with different sizes having smooth surfaces or notches. The relation between fracture stresses and the Knoop indented crack lengths showed that the fracture stresses were constant in the region less than 2c_s. Also, the representative crack length 2c_s was constant for specimens with different shapes and sizes. From the fact that the fracture toughness evaluated for the real cracks of specimens was nearly equal to the fracture toughness of the ceramics, 2c_s is the crack length of the semi-circular surface crack, for which the real cracks in the ceramics are substituted.