Abstract
Biaxial tensile tests of 6000–type sheet aluminum alloy were carried out using newly designed cruciform specimens. The specimens were deformed under linear loading paths in a servo-controlled biaxial tensile testing machine. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. On the other hand, the successive contours of equal plastic work in biaxial stress space changed their shapes progressively, exemplifying differential work hardening. The geometry of the entire family of the work contours was compared with the yield loci calculated from existing yield criteria. The work contour at 0.0005 equivalent plastic strain almost coincided with the von Mises ellipse, while its curvature decreased as work-hardening progressed and became close to the Tresca-type yield locus. Moreover, it was observed that the directions of the measured plastic strain vectors were almost coincided with the associated normals to the current work contour in stress space; accordingly it appears that the work contours act as instantaneous plastic potentials at least under linear loading paths. The effect of age-hardening on the geometry of work contours was small.
