Namsu Park

Material properties of the nugget zone in resistance spot-welded DP980 steel joint at various strain rates

ABSTRACT This paper suggests an approach to evaluate the material properties of the nugget zone in resistance spot-welded DP980 steel joints at various strain rates ranging from 0.001 to 100 s−1. Two kinds of micro-specimens are prepared for the tests: one cut from the centre of the welded nugget zone; and the other cut at 1 mm away from the welded nugget centre. Test results show that the welded nugget zone has little discrepancy in the material properties depending on sampling location and that the flow stress of the nugget zone is relatively high compared with that of the base metal at each strain rate tested. It is confirmed from microscopic observation that the differences are mainly caused by the formation of coarse martensite phase in the welded nugget zone due to the rapid cooling after the resistance spot-welding process.

Prediction of fracture initiation in square cup drawing of DP980 using an anisotropic ductile fracture criterion

This paper deals with the prediction of fracture initiation in square cup drawing of DP980 steel sheet with the thickness of 1.2 mm. In an attempt to consider the influence of material anisotropy on the fracture initiation, an uncoupled anisotropic ductile fracture criterion is developed based on the Lou - Huh ductile fracture criterion. Tensile tests are carried out at different loading directions of 0°, 45°, and 90° to the rolling direction of the sheet using various specimen geometries including pure shear, dog-bone, and flat grooved specimens so as to calibrate the parameters of the proposed fracture criterion. Equivalent plastic strain distribution on the specimen surface is computed using Digital Image Correlation (DIC) method until surface crack initiates. The proposed fracture criterion is implemented into the commercial finite element code ABAQUS/Explicit by developing the Vectorized User-defined MATerial (VUMAT) subroutine which features the non-associated flow rule. Simulation results of the square cup drawing test clearly show that the proposed fracture criterion is capable of predicting the fracture initiation with sufficient accuracy considering the material anisotropy.

Prediction of Fracture Strains for DP980 Steel Sheets Using a Modified Lou–Huh Ductile Fracture Criterion

This paper is concerned with the prediction of fracture strains for DP980 steel sheets using a modified Lou–Huh ductile fracture criterion. The usage of DP980 steel is significantly increasing in the automotive industry for weight reduction, enhancement of crashworthiness and safety of car body. The material behavior of AHSS show unpredictable and sudden fracture during sheet metal forming process. A modified Lou–Huh ductile fracture criterion is utilized to predict the formability of AHSS because the conventional FLD constructed based on necking is unable to evaluate the formability of AHSS. Fracture loci were extracted from 3D fracture envelopes by assuming the plane stress condition to evaluate equivalent plastic strain up to the point of fracture at a wide range of loading paths. Three different types of specimens such as pure shear, dog-bone and plane strain grooved specimens were used for tensile tests to construct 3D fracture envelopes of DP980. Fracture strain of each loading path was evaluated to show that there is little deviation between predicted fracture strains and experimentally acquired ones. From the comparison, it is concluded that the 3D fracture envelopes can accurately predict the onset of the fracture of DP980 steel sheets in complicated loading conditions including the pure shear condition.

Anisotropy effect on the stress-based fracture forming limit diagram using a modified Lou-Huh ductile fracture criterion

This paper is concerned with the anisotropy effect on the stress-based fracture forming limit diagram using a modified Lou-Huh ductile fracture criterion. In sheet metal forming process, a usage of advanced high-strength steels (AHSSs) has been remarkably increasing for the lightweight car body and good formability. It is, however, unable to evaluate the formability of AHSS with the conventional forming limit diagram during complex forming processes since it is obtained by assuming the proportional loading path and AHSS shows sudden fracture involving little amount of necking. The stress-based fracture forming limit diagram was constructed using a modified Lou-Huh ductile fracture criterion in order to evaluate the formability of AHSS accurately. The anisotropy effect on the fracture strain is also evaluated to reflect the material behavior of sheet metals in constructing the criterion for the prediction of onset of the fracture. The constructed stress-based fracture forming limit diagrams deal with the s...