Improvements in analytical design procedures developed over the past ten years have led to a better understanding of the significance of calculated stresses as described in "Section l - Design Considerations" of the "PVRC Interpretive Report of Pressure Vessel Research." On the basis of those stress considerations, the present report estimates, insofar as possible, the factors insuring against the various stress-dependent modes of pressure-vessel failures. For these modes of failure, safety indexes are estimated for present Code allowable design stresses and for higher allowable design stresses with particular emphasis on high strength steels (tensile strengths up to 130,000 psi) and on designs based on yield strength rather than tensile strength. Although the expected change in safety indexes for higher design stresses are presented, no changes in the allowable design stresses are proposed or recommended because such changes are the prerogative and responsibility of Code-making bodies.

An analysis of the index of safety against bursting indicates that, when the allowable design stress is based on tensile strength, the safety index increases with increased tensile strength in the range 60,000 to 130,000 psi. However, for an allowable design stress based on yield strength, the safety index decreases with increased strength.

Ductility safety indexes are difficult to estimate because they depend on the mode of failure, the degree of biaxiality or triaxiality, and the magnitude of strain concentration. For static failures occurring by a shear mechanism, the ductility index of safety decreases with increased tensile strength and is lower for designs based on yield strength than for designs based on tensile strength. Nevertheless, the safety indexes appear adequate for primary, secondary, and peak strains for high strength-steel pressure vessels designed to operate at a stress as high as one-half the yield strength.