In this paper, a systematic study on the effect of flaws on flexural strength of ultra-high temperature ceramics (UHTCs) was carried out by using three typical materials (i.e., ZrB2, ZrB2-SiC and ZrB2-SiC-G ceramics) containing micro-sized flaws with controllable sizes, shapes as well as orientations. Depending on the flaw sizes and orientations, different fracture modes were analyzed by SEM observation and theoretical calculation. Based on Emmerich’s hole/strength model and the maximum strain-energy release-rate mixed-mode fracture criterion, a universal flaw/strength model was developed to describe the flexural strength of flawed UHTCs after taking both material properties and flaw’s geometries into account, and compared well with the experimental data in both mode I and mixed-mode conditions. Prediction results of the proposed model have further verified that substantially consistent with the flexural strength reports of UHTCs in the literature as well as the theoretical strengths calculated from the elastic modulus, which means that the proposed model has a great potential in guiding the design of high-strength ceramics in full-scale (from atomic-scale to macro-scale).
