Understanding the tensile strength of ceramics in the presence of small critical flaws

The objective of this paper is to analyze the effect of small critical flaws on the strength of polycrystalline ceramic materials. For this purpose, a finite fracture approach based on the coupled criterion (CC) is used to describe the initiation of a crack near a stress concentrator. The initiation criterion combines both a stress and an energy condition. The required input fracture-mechanics parameters are the tensile strength and the fracture toughness. Both a blunt and a sharp geometry are studied. The size of the starter crack developing near the stress concentrator can be easily estimated in each case when the CC is fulfilled. Based on the calculations, if the size of the defect is smaller than the characteristic material length, numerical predictions reveal that the defect (either sharp or blunt) has no effect on the strength, reaching the intrinsic tensile strength of the material. This result is in a good agreement with experimental results obtained from the strength measurements of ceramic materials with controlled flaws. It is also shown that combining two fracture tests after introducing flaws with controlled sizes enables to identify the fracture parameters of the ceramic material