The brittle fracture locus for crack initiation in several brittle materials under biaxial states of plane stress is determined experimentally and compared with the classic theory due to Griffith. While good qualitative agreement is observed, the quantitative agreement is poor. It is found that replacing the two-dimensional hairline crack assumed by Griffith by a similar circular one leads to much better agreement between theory and experiment. The plane stress disk test is considered in terms of the new theory and some previously misunderstood aspects of this test are explained. Finally, the relation between crack initiation and crack propagation is briefly discussed.

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