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The “adequate” number of integration points (NIP) required to achieve accurate springback simulation results is studied in this paper in an effort to clarify confusions reported in the literature and shed light on the origin of the confusion. A bending-under-tension model is adopted where springback solution can be obtained with analytical integration through metal thickness. Numerical integrations are then performed and compared with analytical solution to assess associated errors. A crucial distinction is made in the paper that, the model can be posed either as a displacement-value problem where both tension strain and bending radius are prescribed or as a mixed-value problem where the tension force and bending radius are prescribed. Although they are physically equivalent due to the uniqueness of solution, the numerical solutions are different. The associated errors in springback respond differently to the number of integration points employed.

Flanging is a secondary operation in sheet metal forming processes. Traditionally, the design of flange shape and trim line is based on an engineer's experience. It takes several iterations to achieve the desired flange geometry because of potential splits. In this paper, an efficient CAE-based tool is developed to quickly predict the formability of a given flange design and enable the optimization of trim lines. A numerical algorithm is formulated in this CAE tool to convert the 3D flanging process into an equivalent in-plane deformation problem. The developed CAE tool is also integrated with the optimization software LS-OPT for trim line design.

Die wear is a significant issue in sheet metal forming particularly for stamping Advanced High-Strength Steels (AHSS) because of their higher strength and microstructure composition. Reliable predictions of the magnitude and distribution of die wear are essential if cost-effective wear-protection strategies are desired in the early stages of tooling development. A die Wear Severity Index (WSI) is introduced in this paper to quantify the magnitude of die wear, which in essence characterizes the frictional energy dissipation per unit area on the die surface throughout the entire forming cycle. It can be readily obtained as part of any finite element simulation of stamping process utilizing incremental solution techniques.

Forming Limit Diagrams (FLD) have been widely and successfully used in sheet metal stamping as a failure criterion to detect localized necking, which is the most common failure mechanism for conventional steels during forming. However, recent experience from stamping Dual-Phase steels found that, under certain circumstances such as stretching-bend over a small die radius, the sheet metal fails earlier than that predicted by the FLD based on the initiation of a localized neck. It appears that a different failure mechanism and mode are in effect, commonly referred to as “shear fracture” in the sheet metal stamping community. In this paper, experimental and numerical analysis is used to investigate the shear fracture mechanism. Numerical models are established for a stretch-bend test on DP780 steel with a wide range of bend radii for various failure modes. The occurrences of shear fracture are identified by correlating numerical simulation results with test data.

Advanced High Strength Steels (AHSS) such as DP590 HDGI are helping automakers meet increasingly higher structural performance requirements while maintaining or reducing weight of the vehicle body structure [7]. One of the issues facing design engineers implementing new materials such as AHSS is the lack of understanding the expected material variability within a steel supplier and also from one steel supplier to another; and how the variability affects product attribute performances. In this paper, we present an analysis of the aggregated mechanical property variability data obtained from several steel suppliers for a popular AHSS grade and also present studies related to the effect of material variability on structural responses.

A new strain-based forming limit criterion is proposed to assess the localized necking failure for sheet metal forming simulations when deformation paths deviate significantly from linearity. Different from the traditional strain-based Forming Limit Diagrams (FLD) in terms of major and minor strains, the new FLD is constructed based on effective strains and material flow direction at the end of forming. This new criterion combines the advantages of stress-based FLD for its path-independence and the traditional linear strain path FLD for its easy interpretation. The proposed FLD is validated through both theoretical prediction with Marciniak-Kuczynski (M-K) model and available experimental data in literature, and its relationship with stress-based FLDs is discussed.

Dent resistance is an important attribute in the automotive panel design, and the ability to accurately predict a panel's dentability requires careful considerations of sheet metal properties, including property changes from stamping process. The material is often work-hardened significantly during forming, and its thickness is reduced somewhat. With increased demand for weight reduction, vehicle designers are seriously pushing to use thinner-gauged advanced high-strength steels (AHSS) as outer body panels such as fenders, hoods and decklids, with the expectation that its higher strength will offset reduced thickness in its dentability. A comparative study is conducted in this paper for a BH210 steel fender as baseline design and thinner DP500 steel as the new design.

Experimental procedures and results of a benchmark test for springback are reported and a complete suite of obtained data is provided for the validation of forming and springback simulation software. The test is usually referred as the Slit-Ring test where a cylindrical cup is first formed by deep drawing and then a ring is cut from the mid-section of the cup. The opening of the ring upon slitting releases the residual stresses in the formed cup and provides a valuable set of easy-to-measure, easy-to-characterize springback data. The test represents a realistic deep draw stamping operation with stretching and bending deformation, and is highly repeatable in a laboratory environment. In this study, six different automotive materials are evaluated.