The Effect of Stress State on Ductility of ASME Pressure Vessel Tubular Steel Grades

Abstract

Ductile failure in steels is highly controlled by the stress state characterized by the stress triaxiality (T) and the Lode parameter (L). The stress triaxiality sets the rate at which the voids in a material grow and the Lode parameter indicates the type of stress state, i.e. tension, shear or compression. Recently it has been shown that the sensitivity to the Lode parameter increases with increase in material's strength. The ASME Boiler and Pressure Vessel Code, which is the most widely used standard for the design of such pressure equipment, requires pressure vessels to be designed to resist local ductile failure. However, the standard does not account for the Lode parameter dependence. The aim of this research was to develop an experimental procedure to determine the ductile failure locus in terms of T and L for tubular materials used in manufacturing pressure vessels. This suggested improvements for the ductile failure criterion widely used in many codes and standards such as ASME B&PV. Since tubular materials were considered in the present study, an optimization study to develop a standard experimental procedure to determine the tangential ductile behavior was carried out. Two specimens' configurations were used to obtain failure loci of two tubular materials; seamless medium strength carbon steel tubular (SA-106 Gr. B) and super duplex high strength stainless steel (SA-790). Results from experiments and simulations were used to determine the ductile failure loci of both materials in terms of T and L. Failure loci revealed negligible sensitivity to the Lode parameter for SA-106 carbon steel. However, noticeable sensitivity was observed in SA-790 high strength stainless steel. Fractographs of both SA-106 and SA-790 verified the macromechanical observations.

Date of Award	Jan 2016
Original language	American English
Supervisor	Imad Barsoum (Supervisor)