The fracture behavior of a polyurethane elastomer in a hydrostatic tensile field has been experimentally studied via the ``poker-chip test.'' Microscopic examination of the material revealed residual gaseous bubbles, which were taken as fracture nucleation points. A study of the cavity behavior up through fracture initiation and propagation led to a determination of the fracture stress level, the identification of two regimes, or modes, of fracture, and the associated velocities of propagation.

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Observations are made concerning comparative fracture behavior of cracked and uncracked specimens under combined loading conditions. A theory is proposed which provides a general fracture criterion for mixed mode crack extension and which unifies the treatment of elastic fracture problems in initially cracked and uncracked geometries. All observations are supported by experimental fracture results taken from the literature on polymethyl-methacrylate and glass.