Evaluation of Fracture Strength of Notched Porous Ceramics Based on R-Curve Method

Notched specimens of two kinds of porous ceramics with porosity 40% whose mean grain sizes were 200 and 500 μm were fractured under four-point bending. A single edge notch with nine depths ranging from 0.1 to 4.8 mm was introduced to the rectangular specimen with a height of 8 mm. Small notches were non-damaging and fracture did not start from the notches. The size of the non-damaging notch was larger for larger grain-sized material. The load versus strain record showed nonlinearity before reaching the maximum load for all specimens fractured from the notch. The critical stress intensity factor for crack initiation from the notch was nearly constant. The resistance curve was constructed by estimating the crack length from the compliance change of the specimen, and was successfully used for determining the maximum load point in bending tests. Fractographic observations showed that fracture occurred at the binder phase or in the particle containing many tiny holes.