Instability (maximum pressure) criteria for materials obeying a parabolic stress-strain law are investigated. Two special solutions are formulated, accompanied by the instability condition obtained from the author’s approximate theory. Bursting pressure is most accurately predicted by the approximate theory. In turn, the polar deflection and strain at instability are more closely predicted by special solution II. It is shown that neither the “pureness” nor the annealed condition of a metal gives adequate assurance of reliability in frangible disk service. The way to insure this requirement is to confirm experimentally that, in the uniaxial test, fracture will not occur before the logarithmic strain exceeds roughly (0.36 + 0.73 m), or that in a bulge test the applied hydrostatic pressure passes through a maximum prior to bursting.