The yield curves of two types of copper powder preforms with different loading paths, i. e., the one compacted hydrostatically and the other compacted in a closed die, were determined by uniaxial, triaxial and hydrostatic compression experiments and were drawn in Rendulic stress plane.
For the both type preforms (hydrostatic and closed die preforms), the yield curves were similar to elliptic curves or lemniscates. The yield curve for the hydrostatic preform had a major axis in σm-axis (the axis of equal triaxial stresses σ1=σ2=σ3) direction, while the Curve for the closed die preform had a major axis in the pre-loading direction. For the same initial density of the both type preforms, their yield curves approximately coincided to each other if their major axes were superimposed by rotating about the origin in Rendulic plane. Therefore, the copper powder compacts (preforms) were found to obey to the kinematic hardening rule.
With the increase in initial density of the hydrostatic or closed die preform, the yield curve in Rendulic plane expanded without rotation of the respective major axis; the yield curves of the both type preforms with different initial densities were approximately similar, having their center at the origin in Rendulic plane.