On the evolution of yield surfaces in three-dimensional stress space and some results of aluminum alloy

This thesis studies the evolution of yield surfaces for Al6061 under strain control in the axial and torsional directions and stress control in the hoop direction. A new method of automated yield stress probing is based on proof strain and used in
the determination of yield surfaces. The experimental determination of the subsequent yield surfaces in (σ_θz-σ_θθ), (σ_zz-σ_θθ) and (σ_zz-σ_θz) spaces after proportional and non-proportional loading paths were conducted to investigate the evolution of the yield surfaces. The experimental result showed that the rotation of the subsequent yield surfaces in the (σ_θz-σ_θθ) space was dependent on torsional pre-loading paths, and in the (σ_zz-σ_θθ) space, the subsequent yield surfaces of axial pre-loading paths showed obvious rotation. Finally, 3D yield surfaces of different pre-loading paths constructed by using the data of the yield points in the (σ_θz-σ_θθ), (σ_zz-σ_θθ) and (σ_zz-σ_θz) spaces were conducted to investigate the evolution of the yield surfaces in the 3D stress space. The result showed that the subsequent yield surfaces of torsional pre-loading paths rotated around the σ_zz axis , and the subsequent yield surfaces of axial pre-loading paths rotated around the σ_θz axis in the 3D stress space.