The Effective Utilization of Yield Strength

In many structural and constructional applications (such as pressure vessels), steels, particularly high-yield-strength steels, are not being utilized as effectively as may be possible. This occurs because the design stress permitted by many specifications and codes is effectively based only on the tensile strength. Thus, the increase in yield-to-tensile-strength ratio with increasing tensile strength is not recognized, and no design-stress credit is given for the proportionately higher yield strength of high-strength steels. Because increased utilization of yield strength will probably require demonstration of the satisfactory fabricability and service performance of high-yield-strength steels, the present paper summarizes the general effects of increased yield strength on formability and weldability and on resistance to failure by stress-dependent modes—overload, brittle fracture, fatigue, and stress corrosion. The present state of knowledge indicates that fabrication does not significantly limit the use of high-strength steels. Although high-yield-strength steels are more difficult to form and weld than lower-strength steels, appropriate forming and welding practices that are not unduly restrictive are in common use for such steels. If design stress is based directly on yield strength, the safety factor against failure by simple overload or by unstable propagation of a crack decreases with increasing yield strength. However, increasing fracture toughness can significantly reduce susceptibility to failure by unstable crack propagation that is the result of low shear energy absorption or the growth of cracks to critical size by fatigue or stress corrosion. In recent years, the fracture toughness of steel has been continuously rising because control of metallurgical factors is continuously improving. For this and other reasons that suggest beneficial effects of yield strength, the possibility of more effectively utilizing the yield strength of steel should be reexamined.