저자명 박남수 
년도 2013 

Advanced high-strength steels (AHSS) for automotive applications generally exhibit an excellent combination of high strength and high formability resulting primarily from their high strain hardening capabilities. However, the material behavior of AHSS is still not fully identified and AHSS can show unpredictable and sudden failure as well as high and unpredictable spring back in sheet metal forming, which limits their usage in some applications. For the better application of the material, it is important to understand their plastic behavior and predict the onset of fracture in metals accurately. Especially in sheet metal forming, the anisotropy effect on the fracture strain should also be taken into consideration to consider the fracture behavior more properly since they show different mechanical properties resulting from their crystallographic structure and the characteristic of the rolling process.

The object of the study is to investigate the anisotropy effect of DP980 sheets on the fracture forming limit diagram (FFLD). In order to construct the FFLD, the history of equivalent plastic strain and strain path of the material point where the maximum equivalent plastic strain is observed just before the onset of fracture should be acquired at various loading paths. To utilize the data of various loading paths, three different types of tests are carried out to the facture including uniaxial tensile tests on classic dog-bone specimens, pure shear specimens and plane strain grooved specimens. Each specimen is made along three different orientations: the angle of 0˚ (rolling direction, RD); 45˚ (diagonal direction, DD); 90˚ (transverse direction, TD) with respect to the rolling direction of sheet metal to check the anisotropy effect on the fracture strain.

Strain measurements are performed using the 2-dimensional digital image correlation (DIC) system. DIC is an optical method which delivers whole-field noncontact measurement of strain providing the ability to accurately determine peak deformation as well as gradients over the entire area of interest. Fracture tests are carried out using Instron 5583 with dog-bone, pure shear and plane strain grooved specimen with deformation images captured by the high speed camera.

In order to reflect the influence of anisotropy on the fracture strain more, the Hill’48 yield function is applied to the Lou–Huh fracture model instead of von Mises yield function. And a modified Lou–Huh fracture model is represented in the space of strain path to use the experiment result directly. The history effect on the change of loading path, which should be considered for the damage accumulation in the material, is also applied to the modified Lou–Huh fracture model. Based on the measured data from the tests, modified Lou–Huh fracture loci are constructed and represented with respect to the loading direction. The loci are also transformed into the fracture forming limit diagrams which is of great importance in automotive applications.

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