Solid and hollow cylinders of Pyrex glass were twisted under hydrostatic pressure up to 1000 kg/cm2. The torsional strength of specimens coated with thin rubber films was increased by superposed hydrostatic pressure, while that of naked specimens remained constant. It is shown that rupture commences at a surface flaw and that the effect of hydrostatic pressure on the torsional strength of glass may be explained on the basis that fracture occurs when a critical stress-intensity factor is reached at a part-through surface crack.

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