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Finite Element Analysis (FEA) has been successfully used in the simulation of sheet metal forming process. The accurate prediction of the springback is still a major challenge due to its sensitivity to the geometric modeling of the tools, strain hardening model, yield criterion, contact algorithm, loading pattern, element formulation, mesh size and number of through-thickness integration points, etc. The objective of this paper is to discuss the effect of numerical parameters on springback prediction using dynamic explicit FEA codes. The example used in the study is from the Auto/Steel Partnership High Strength Steel Rail Springback Project. The modeling techniques are discussed and the guidelines are provided for choosing numerical parameters, which influence the accuracy of the springback prediction and the computation cost.

Limitations in the commonly used anisotropic yield criteria of sheet metals, such as Hill's 1948 and Hosford's 1979, are discussed in this paper. Analyses demonstrate that the commonly used yield criteria do not satisfy simultaneously the uniaxial tensile test data in rolling, transverse and diagonal directions and equibiaxial tension data. In order for the existing anisotropic yield criteria to fit experimental data in all the orientations, an enhanced method and an associated yield criterion are proposed. The method proposed can also be used to modify various anisotropic yield criteria in order to fit more material test data.

Forming and strain rate effects on crashworthiness of automotive body components were investigated in this study. Dynamic tensile tests were carried out to establish the stress-strain relationships at elevated strain rates. Dynamic tests of bending and axial crashing at various speeds were conducted using a stamped hat square column. The experimental results indicate that the absorbed energy of the hat square column decreased with the increase of material thinning in case of high strength steels. FEM analyses using material models with both strain rate sensitivity and forming effects were carried out to evaluate the computer prediction accuracy of crashworthiness.

Forming of sheet metal structures induces pre-strains, thickness variations, and residual stresses. Pre-strains in the formed structures introduce work hardening effects and change material fatigue properties such as stress-life or strain-life. In the past, crashworthiness and durability analyses have been carried out using uniform sheet thickness and stress- and strain-free initial conditions. In this paper, crashworthiness and durability analyses of hydroformed front rails, stamped engine rails and shock towers on a full vehicle and a Body-In-White structure are performed considering the residual forming effects. The forming effects on the crash performance and fatigue life are evaluated.

Laminated steel sheets sandwiched with a polymer core are increasingly used for automotive applications due to their vibration and sound damping properties. However, it has become a major challenge in finite element modeling of laminated steel structures and forming processes due to the extremely large differences in mechanical properties and in the gauges of the polymer core and the steel skins. In this study, circular cup deep drawing and V-bending experiments using laminated steels were conducted in order to develop a modeling technique for laminate forming processes. The effectiveness of several finite element modeling techniques was investigated using the commercial FEM code LS-Dyna. Furthermore, two production parts were selected to verify the modeling techniques in real world applications.

Sheet hydro-forming was applied to hydro-form a door outer panel using different steel grades. The effect of mechanical properties and the forming conditions on panel properties such as thickness profile and cross-sectional shape accuracy were investigated by both experimental sheet hydro-forming and FEM forming analysis. 590MPa T.S. steel grade was successfully formed with improved dent resistance compared to the conventional 340MPa T.S. steel grade. On the other hand, the results of the FEM forming process analysis showed that the pre-forming conditions were important in controlling the fracture formation during forming and to improve dent resistance, which successfully led to the best forming condition.

It has been widely recognized that the accurate estimation of spot welded joint fracture are needed to improve the accuracy of car crash FE analyses. The criteria model must be developed to realize the consideration of spot welded joint fracture under various kinds of load conditions. Conventionally, only tensile shear test and cross-tensile test have been usually performed for estimation of the strength characteristic of spot-welded joints. However, the spot-welded strength under complex loads can not be adequately estimated by using the conventional test methods. In this study, newly developed tests, “Inclined Cross Tensile Tests”, loading both shear and axial force on spot welded joint were conducted to understand fracture strength and build up its criteria. Especially, in the inclination of cross-tensile test, special attachment for the specimen to prevent bending deformation was developed.

Pickup box drum drop test is critical in vehicle development to determine the impact strength of the floor panels. Physical hardware tests on prototypes have been used to assess whether the performance of the future pickup box meets design requirements. In order to reduce costs and shorten development cycle, CAE methodology was developed to accurately model the drum drop test. In this paper, a CAE procedure for modeling the drum drop test is proposed. Dynamic explicit finite element code LS-Dyna was used to simulate the non-linear impact process of a drum onto the box floor. The permanent plastic damages on the floor panel were recorded in both simulation and experiments. Very good correlation between the simulation results and the physical hardware tests was achieved. It indicates that the methodology developed is an effective tool in evaluating the performances of the pickup box floor panels.

A Draw Bead Simulator (DBS), with modified draw beads, was employed in this study to understand the springback behavior of sheet metal subjected to multiple bending-unbending cycles. The investigations were carried out in both the rolling and the transverse rolling directions on four types of materials: Electro-Galvanized DQ steel, light and heavy gauge Hot-Dip Galvanealed High Strength Steels, and Aluminum alloy AL6111. The sheet geometries, thickness strains, pulling forces and clamping forces were measured and analyzed for the purpose of establishing a benchmark database for numerical predictions of springback. The results indicate that the springback curvature changes dramatically with the die holding force. The conditions at which the springback is minimized was observed and found to depend on the material properties and the sheet thickness. Analysis with an implicit FEM showed that the predicted and the experimental results are in very good agreement.

The fatigue strength of the high strength steel sheet for truck frame use was investigated. The fatigue strength of the steel sheet with scale increases with the decrease in the roughness of the steel sheet surface. The fatigue strength of the sheared edge increases with the decrease in the roughness of the fractured surface. In case of the microstructure consisting of pearlite and coarse carbides, the fatigue strength of the sheared edge decreases. Based on these findings, two types of the 780MPa grade high strength hot rolled steel sheet with high fatigue strength as well as good formability have been developed.

From the viewpoint of applying ERW (Electric Resistance Welded) steel tubes for automotive structural parts by means of hydroforming technology, a series of free-bulge experiments is carried out. Four kinds of mild and high tensile strength steel tubes are provided. Initial stress ratio, αm that represents an axial force level is adopted as an experimental parameter. Expansion limit increases with decrease of αm. The bulge shape and the expansion limit are also affected by the mechanical properties of tubes. The strain hardening property in bulge deformation is approximated by a parabolic equation.