Effects of Combined Stresses on Fracture of Alumina and Graphite

A unique method was developed for mechanical testing of brittle materials to create an unlimited number of stress ratios in the tension‐tension and tension‐compression quadrants. The stress states are achieved by internal and external pressurization of tubular specimens in a special pressure vessel. Failure envelopes were determined for polycrystalline alumina and fine‐grained isotropic graphite. The modified maximum strain energy and Coulomb‐Mohr theories fit the data best; these theories should be plotted as bands representing probability of failure. The biaxial tensile strengths of alumina and graphite are lower than their uniaxial tensile strengths; the tensile strengths decrease as the compressive stress in the orthogonal directions increases.