Yield locus and work hardening behavior of a thin-walled steel tube subjected to combined tension-internal pressure

Abstract

A servo-controlled tension-internal pressure testing machine for metal tubes is newly developed. The testing machine is capable of giving arbitrary stress or strain paths to a tubular specimen using an electrical, closed-loop feedback control system. Biaxial stress experiments are carried out for low carbon steel tubes. Contours of equal plastic work are determined in the principal stress space for linear stress paths. The work contours are found to keep a geometry almost identical to Hosford's yield locus. The directions of measured incremental plastic strain vectors are in good agreement with those of local outward normals to Hosford's yield locus. Moreover, a subsequent yield locus following equibiaxial tension is measured without unloading using the abrupt strain path change method proposed by Kuroda and Tvergaard [Acta Mater. 47 (1999) 3879-3890]. A yield vertex is successfully observed at the point of loading, and non-normality behavior of the plastic strain rate vector for non-proportional loading is confirmed.