Abstract
The influence of yield function parameters on hole-expansion (HE) predictions are investigated for an anisotropic AA6022-T4 aluminum sheet. The HE experiment is performed in a fully-instrumented double-action hydraulic press with a flat-headed punch. Full strain fields are measured by a stereo-type digital image correlation (DIC) system. The stress state gradually changes from uniaxial to plane-strain tension to biaxial tension in the radial direction. Besides HE, the plastic anisotropy of AA6022-T4 is characterized by uniaxial tension and plane-strain tension experiments. Uniaxial tension is considered as the most important, since it is the stress state along the hoop direction in the hole. For the finite element (FE) simulation, the Yld2000-2d non-quadratic anisotropic yield function is used with two different parameter sets, calibrated by: (1) uniaxial tension only (termed Calib1) and, (2) both uniaxial and plane-strain tension (Calib2). The strain field predictions show a good agreement with the experiments only for Calib2, which takes into account plane-strain as well uniaxial tension. This indicates the importance of biaxial modes, and in particular plane-strain tension, for the adopted yield function to produce accurate HE simulations.
Highlights
The hole-expansion (HE) of a thin sheet using a flat-headed punch is akin to the expansion of a circular hole in an equibiaxial tensile field
Perhaps the first investigation of the anisotropic case was by [6], who used the yield function proposed by Parmar and Mellor [7] and discovered that the strain field predictions were very sensitive to the anisotropic parameters
|s1 − s2 |m + |2s1 + s2 |m + |s1 + 2s2 |m = 2σm where si=1∼3 are the principal components of deviatoric stress s of Cauchy stress σ, σ is equivalent stress, and m is an exponent based on the crystal structure, i.e., m = 6 for bodycentered cubic (BCC) and m = 8 for face-centered cubic (FCC) materials [25]
Summary
The hole-expansion (HE) of a thin sheet using a flat-headed punch is akin to the expansion of a circular hole in an equibiaxial tensile field. A variety of anisotropic yield criteria, e.g., Hill ’48 [14], Barlat ’89 [15], and Yld2000-2D [16], or crystal plasticity, have been used in numerical simulations of HE [3,17,18,19,20,21,22,23,24] In some of these works, anisotropy was calibrated based on uniaxial tension tests at angles to the rolling direction (RD). Comparing the predictions to the HE experiment reveals the ability of the yield function to capture the resulting strain fields, including their local features. This work can be used as a 1st step in assessing the forming limits that can be expected in hole stretching and flanging
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