The Effect of Hydrostatic Pressure on the Fracture of Brittle Substances

An arrangement is described permitting the fracture of brittle materials under the action of tensile stress superposed on hydrostatic pressure. The hydrostatic pressures range up to 30,000 kg/cm². The tensile stress superposed on the pressure required to break Pyrex glass is a strong function of the material by which pressure is transmitted to the lateral surface of the glass, but in all cases of fracture produced at pressures above 25,000 kg/cm² the net stress at fracture was compressive, that is, tensile fracture takes place against the direction of the stress. Beryllium and phosphor bronze lose the brittleness which characterize them at atmospheric pressure and fracture in tension under pressure after marked plastic deformation. Carboloy remains brittle in tension under pressure, but its tensile strength may increase by a factor of three. NaCl elongates plastically in tension under pressure with no obvious disturbance of the optical homogeneity. Pipestone remains completely brittle under pressure and of very low strength. In simple compression combined with hydrostatic pressure Pyrex glass increases greatly in strength. Single crystal Al₂O₃ under the same conditions exhibits plastic slip on the basal plane without fracture. In the discussion the importance of the condition of energy release as a factor controlling fracture is emphasized. The Griffith conception of fracture as resulting from stress concentration at the ends of microscopic cracks is consistent with many qualitative aspects of the situation, but probably cannot be carried through quantitatively.