Abstract

Formula to estimate the ideal strength of brittle material was derived from the consideration of the relationship between the fracture under uniform tensile stress and the transformations of the material, sublimation, melting and solid to solid transformation. The validity of the formula was examined by comparing the ideal strength with the fracture stress of whiskers. The accuracy of the formula was within ± 30% for the material whose ionicity is less than 50%. The fracture stress of the whisker whose ionicity is more than 50% is about 1/3 to 1/5 of the value given by the formula. The discrepancy in the ionic materials seems to come from the possible occurence of shear fracture in the materials. From the formula, mullite is concluded that it would have the fracture stress similar to silicon nitride.