Sheet Metal Stamping Performance and Test -- Thickness Direction Anisotropy Coefficient
Sheet metal stamping is an important forming technology in the automotive industry and other key industries. Thickness direction anisotropy coefficient (TDA) is a constantly anisotropy parameter indicating the degree of anisotropy of sheet material in the stamping direction. The research of this parameter can help to set up the stamping die and model the forming process accurately, whereas the anisotropic characteristics of material determine the forming limits in practice. Therefore, it is essential to measure and analyze the anisotropic coefficient in the each grade sheets.
In general, the TDA coefficient of certain grade of steel is usually lower than 0.8. As the TDA coefficient is higher than 0.8, it means that the material has high anisotropy. The material featuring high anisotropy will cause some issues during stamping such as spring back, cracking, wrinkling, or other forms of distortion. The anisotropy coefficient of different grades of sheets is required to be measured. Different tests such as tensile testing, plane anisotropy testing, three-point bending tests, etc. are conducted to evaluate the TDA coefficient.
In tensile testing, the tensile modulus of the metal sheet under test is usually the average value of the longitudinal modulus (E₁) and the transverse modulus (E₂). It characterizes the metal sheets mechanical properties, especially its anisotropy. The TDA coefficient is calculated as
TDA=E₁/E₂
When the TDA value is close to 1, it shows that the metal sheet has low anisotropy. When the TDA value is 0.7 or lower, it shows that there is severe material anisotropy.
In plane anisotropy test, the metal sheet sample is clamped in the testing machine, and the testing force is applied in two directions at the same time. It is observed if the metal sheet sample has bending deformation under the testing force applied in the two directions. Then, the TDA coefficient is calculated by
TDA=Fr/Fs
where, Fr is the bending angle of the sample deformed under the force in the longitudinal direction, and Fs is the bending angle of the sample deformed under the force in the lateral direction.
In the three-point bending test, the metal sheet sample is clamped between the testing machine roll and an anvil which is fixed on the testing machine, and the testing force is applied in the longitudinal direction. The bending deformation of the sample is then observed. The bending angle of the sample strains under the testing force is measured. Meanwhile, the bending angle and the loading direction are also noted down. The TDA value is calculated by
TDA= (Fr-F2)/Fs
where Fr is the bending angle of the sample strain under the force in the longitudinal direction, and Fs is the bending angle of the sample strain under the force in the lateral direction.
In conclusion, TDA coefficient is an important parameter to measure the anisotropy of sheet material in the stamping direction. Various tests like tensile testing, plane anisotropy testing, three-point bending tests are performed to evaluate it accurately. The TDA value is required to be lower than 0.8, if not, it will face some problems such as spring back, cracking and wrinkling during stamping process. The research of TDA coefficient helps to set up the stamping die, model the forming process and determine the forming limits in practice.
