X-ray Fractographic Approach to the Brittle Fracture of Ceramics and Ceramic Composites

Abstract

Fracture toughness tests were conducted on pre-cracked and notched specimens of Al2O3 and Si3N4 ceramics and on a TiC-Cr3C2 ceramic composite. Pre-cracking was carried out on single-edge notched specimens by longitudinal compression, and the effect of notch-tip radius on the toughness value was studied. The distribution of the residual stress near the fracture surface was measured by the X-ray diffraction method. This new method for introducing a pre-crack in a single-edge notched specimen by eccentric longitudinal compression was effective, the pre-crack length being controlled by the contact length of compressive loading. The fracture toughness was proportional to the square root of the notch-tip radius for cases with a notch root radius larger than a certain value, ρ0. For cases with sharper notches and pre-cracks, the fracture toughness values were nearly constant. The residual stress on the fracture surface measured by X-ray was tensile. This tensile residual stress gradually diminished and became compressive as the depth increased. At a certain depth, the residual stress was almost constant. The size of the plastic zone (ωy) was determined from the residual stress distribution beneath the fracture surface. This size was related to the fracture toughness (Kρ) and the bending strength (σB) by the following equation: