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.