2軸引張試験による各種鋼板の塑性変形特性の測定と解析

概要

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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, (σ_<Yx'>σ_<Yy>), 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 (σ_<Yx'>σ_<Yy>) 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 (σ_<Yx'>σ_<Yy>) 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.
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