Papers by Keyword: High Strength Steel Sheet

Abstract: In this work, the fracture prediction of aluminium alloy sheet, grade AA2024-T3 was investigated using the Fracture Forming Limit Curve (FFLC) and Analytical Fracture Forming Limit Stress Curve (FFLSC) as failure criteria. Initially, the FFLC was experimentally achieved by means of the Nakajima Stretch forming test coupled with GOM strain measurement system, providing the Digital Image Correlation (DIC) technique. Subsequently, the analytical FFLSC was plastically computed using experimental FFLC data combination with Swift hardening model and anisotropic Hill’48 yield model for representing anisotropic plastic deformation behaviour on examined sheet. Finally, Fukui stretch drawing and Three point bending tests were conducted both experiment and numerical simulation to evaluate the fracture state and verifying an applicability of obtained FFLC and FFLSC. In the simulation, the ABAQUS 2017 element deletion function was employed for directly implementing the fracture criteria regarding the FFLC and FFLSC. It was concluded that the analytical FFLSC could more realistically predict fracture behaviour better than the FFLC based on strain. In term of percentage error, the analytical FFLSC generated an error less than the FFLC.

Abstract: The finite element simulation technology can provide strong support for the optimization of processing technology and the treatment of detailed problems in the processing process. Two finite element methods applied to hot forming of high-strength steel plates are introduced, namely the incremental method and the deformation method. Two methods are used for simulation calculations. The finite element simulation based on incremental theory has high accuracy and requires more complete mold and process information. It is mainly used in the middle and late stages of product and mold design. And the finite element simulation based on deformation theory have fast calculation speeds and are mainly used in the early stages of product and mold design. Both types of methods have high practical value.

Abstract: The influence of different thickness combinations was investigated on the strength of the lap joint of dissimilar steels. In this study, lap joints of dissimilar steels were welded by laser welding. The tensile shear test was conducted for the lap joints. Rotational deformation process around the weld bead of the lap joint was observed by a digital video camera during the test. Motion analysis from the video of the tensile shear test indicated that the rotation angle around the weld bead was reduced by overlapping higher strength grade steel. Three-dimensional elastic-plastic finite element analysis was performed for the tensile shear test of the lap joint. The numerically calculated deformation behavior of the lap joint subjected to tensile shear loading showed reasonable agreement with the experimental record. It was found that the rotation angle was reduced and tensile shear strength of the lap joint increase by overlapping higher strength grade steel sheet.

Abstract: To reduce springback in U-shape of high strength steel sheet, the present paper proposes a new process parameter in U-bending technique. This paper aims to present the effect of clearance between die and pressure pad on springback behavior. The process in this work consists of four steps: (1) clamping of a sheet between a punch and a pressure pad, (2) bending with constant clamping force, (3) pushing-up at bottom of the part by using the pressure pad, and (4) final release tool. From the experimental results, decreasing of bending moment by bottom pushing-up resulted in the springback reduction. An appropriate of the clearance between die and pressure pad combined with bottom pushing-up force can be reduced springback. Our results suggested that the Y-U model, an advanced kinematic hardening, is essential for accurate numerical simulation of springback behavior.

Abstract: It is strongly desirable for the vehicle to improve passenger safety and at the same time to reduce the weight of the vehicles. A hollow section for the body structure of automobiles is studied. A high strength steel sheet is used to make the hollow sections in this studied, which are typically joined by resistance spot welding have insufficient energy absorption because the joins are not continuous. Thus, to overcome this problem, the hollow section is joined using the hemming process. The high strength steel hollow sections joined by hemming and resistance spot welding were then examined by tensile and fatigue tests. The hollow section with hemmed joins showed better performance in both tests. The overlapping joins of the hemmed hollow section have greater strength as compared to the resistance of spot welding joins.

Abstract: A servo-controlled tension-internal pressure testing machine with an optical 3D deformation analysis system (ARAMIS®, GOM) was used to measure the multiaxial plastic deformation behavior of a 590MPa high strength steel sheet for a range of strain from initial yield to fracture. Tubular specimens were fabricated from the sheet sample by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the forming limit curve (FLC) and forming limit stress curve (FLSC), in addition to the contours of plastic work and the directions of plastic strain rates. It was found that the shapes of the measured work contours changed with the increase of work hardening (plastic work). The observed differential work hardening (DWH) behavior was approximated by changing the material parameters and the exponent of the Yld2000-2d yield function (Barlat et al, 2003) as a function of the equivalent plastic strain. The FLC and FLSC calculated using the Marciniak-Kuczyński-type (M-K) approach with the DWH model were in good agreement with the measurement.

Abstract: Stiffness improvement of stamping die without weight increase 780MPa grade high strength steel sheet was achieved by using Finite Element (FE) simulation combined with topology optimization. In the first step of the procedure, FE simulation of a stamping process was conducted in order to evaluate the contact force on a die surface at the end of stamping. Subsequently, topology optimization was carried out subject to the prescribed weight constraint, in which the stiffest die structure for the given force and displacement boundary conditions was obtained. Maximum displacements during stamping between thus optimized die and the one designed by the experienced engineer were compared. It was found that the optimized die showed less deformation than the other even if the total weights of both dies were almost the same.

Abstract: First, with a view of raising the operation rate of the press used in hot stamping and press quenching process, the influence of completion timing of press quench on shape quality and hardness property of the product was examined by an arc-shaped hot bending test with practical hot stamping steel sheet, and recommendable measures were suggested. In addition, by comparing the test results with those of austenitic stainless steel sheet without martensitic transformation, it was implied that good forming performance in the practical steel sheet was closely relating to its martensitic transformation. Then, in this connection, the influence of shape retaining force in the press quench operation on the shape quality was examined and the relevant features were revealed. Finally, with regard to the relation of the good shape quality in hot stamped and press-quenched steel sheet part with martensitic transformation, its mechanism was considered from the point of view of minimization of the internal work accompanying the anisotropic martensitic transformation with volume expansion.

Abstract: To reduce springback in U-bending the present paper proposes a new technique where the bottom of a U-bent part is clamped between a punch and a counter-punch during bending and it is pushed up with the counter punch at the final stage. The effect of counter punch pressure, both in sheet clamping and bottoming, was investigated by performing experiments on dual phase 590MPa and TRIP 780MPa HSS sheets. From the experiment, an appropriate combination of the clamping force and the final pushing-up force was found where springback was reduced to almost zero. To investigate the mechanism of the reduction of springback in the above three-step U-bending process, FE simulation of the bending with PAM-STAMP 2G was also conducted where the advanced kinematic hardening Yoshida-Uemori model was employed.

Abstract: Stress-strain responses of a high strength steel sheet of 980MPa grade under uniaxial tension and its springback in V- and U-bending were investigated at elevated temperatures ranging from 573-973K. The flow stress decreased drastically with the increase of temperature, from which it was expected that springback is reduced by warm forming. In V-bending test, however, the temperature effect on springback was not so clear, while in U-bending springback decreased with temperature rise. It was found that such difference in temperature dependent springback behavior between V- and U-bending was caused by stress relaxation which took place during unloading process.