2001 Volume 87 Issue 4 Pages 198-204
Biaxial tensile tests of seven kinds of rolled sheet steels with different r-values were carried out. Cruciform specimens were deformed under linear stress paths in a servo-controlled biaxial tensile testing machine. The stress ratios (R.D.: T.D.) adopted were 1:0, 2:1, 1:1, 1:2 and 0:1. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. Stress points consisting of contours of equal plastic work, (σYx, σYy), were measured and compared with Hill's quadratic and Logan-Hosford yield loci in the principal stress space. The results obtained are as follows. (i) Logan-Hosford yield criterion can predict (σYx, σYy) with an error of several percent and the directions of incremental plastic strain rate vectors with an error of 5 degrees at most. (ii) The (σYx, σYy) of sheet steels with an average r-value larger than 1.5 exhibit a tendency to approach Hill's yield locus in the directions of stress ratios 1:1 and 2:1 as the material work-hardens. This phenomenon has been also observed in the past literature and seems to be the common differential hardening characteristics of sheet steels having rather high r-values. (iii) The differential hardening of sheet steels is not caused by texture development, but rather governed by the work-hardening characteristics of slip systems.
