Effect of geometrical defects in forming sheet steel by biaxial stretching

Abstract

In-plane, equibiaxial punch stretching tests have been performed to examine the influence of defects on the formability of two grades of ferritic, high-strength, low-alloy steel. The defects are long nar-row grooves installed by local electrolytic dissolution normal to the rolling direction. The results are examined in the context of the Marciniak-Kuczynski (MK) imperfection analysis, and the homo-geneity index,f. The variation of the measured forming limit strain with relative groove depth, ex-pressed by the indexf, is compared with calculations employing Hill’s nonquadratic yield function. For the deeply grooved samples, which experience localization and fracture within the groove, the measured dependence corresponds with the one calculated for an exponentM = 1.9. For the most shallow grooves, localization and fracture proceed outside the groove, along the rolling direction, and the forming limit strains are identical to the values ε* = 0.38 and ε* = 0.040, for the un-grooved samples of the two steels. These forming limits correspond to control by inhomogeneities off ≃ 0.990 andf ≃ 0.992, respectively. For the range of defect sizes studied, the severity of de-fects decreases for shorter lengths and narrower widths. It is estimated that, by increasing the homo-geneity index tof = 0.999, an improvement in biaxial forming limit strain to approximately ε* = 0.59 could be obtained.