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Accurate description of the plastic yield locus is important for accurate prediction of sheet metal formability and springback using FEM. This paper presents experimental results obtained for the initial plastic yield locus and its evolution for some selected Advanced High Strength Steels (AHSS). A review of available experimental methods was conducted to select appropriate techniques for testing. For loading in tension-shear, the Arcan test was selected, however because of lack of uniformity of the stress distribution, the test was not included in the final series of tests. Shear testing, uniaxial tensile testing, plane strain testing and stacked compression testing were used to determine the yield locus. From the test results and analysis for the selected AHSS, it seems that the onset of initial yielding and its isotropic evolution to 4% plastic strain is best described by the von Mises yield function.

Forming Limit Curves (FLCs) have been used in press shops for decades during die development and more recently as failure criteria when used in conjunction with FEA for part feasibility analysis. Around the world there are different techniques used to determine the FLC. The differences between the techniques lie in tooling, specimen geometry and in the method used to determine the critical strains. A comprehensive study on FLCs of selected AHSS was carried out at ArcelorMittal Global R&D, where the different commonly used techniques and a new technique employing Digital Image Correlation (DIC) were employed to determine the FLCs. This paper presents results of these comparisons.

The component being formed experiences some type of prestrain that may have an effect on its fatigue strength. This study investigated the forming effects on material fatigue strength of dual phase sheet steel (DP600) subjected to various uniaxial prestrains. In the as-received condition, DP600 specimens were tested for tensile properties to determine the prestraining level based on the uniform elongation corresponding to the maximum strength of DP600 on the stress-strain curve. Three different levels of prestrain at 90%, 70% and 50% of the uniform elongation were applied to uniaxial prestrain specimens for tensile tests and fatigue tests. Fatigue tests were conducted with strain controlled to obtain fatigue properties and compare them with the as-received DP600. The fatigue test results were presented with strain amplitude and Neuber's factor.

Tensile properties of sheet steels at dynamic conditions are becoming more important for automotives in recent years due to the positive strain rate effect of steels which significantly improves energy absorption capability during crash events. However, several testing techniques are used by different testing laboratories, no testing standards are available, and the quality of data generated by different laboratories is often not comparable. In order to improve the data quality at high strain rate testing conditions and thus to improve the accuracy of crash simulation results, The International Iron and Steel Institute (IISI) initiated a project to develop the “Recommendations for Dynamic Tensile Testing of Sheet Steels”. The document provides guidelines for key elements of high strain rate testing, testing techniques, input methods, specimen geometry and stress/strain measurement instrumentations.

Application of Advanced High Strength Steels (AHSS) to closure and structural parts in the automotive industry is increasing in future models. In addition to weight reduction, the other primary motivation to consider these products is the improvement of structural performance that is needed to meet future stringent safety standards. AHSS products have a combination of unique microstructures and mechanical behavior. It is important to develop basic knowledge and understanding of all the manufacturing aspects of forming these products, so that robust forming processes can be engineered to successfully form parts in a production environment. The edge condition obtained after post-draw operations such as trimming has a significant influence on processes such as stretch flanging. A study to investigate the influence of punching clearance on the edge characteristics of various AHSS products has been initiated at Mittal Steel R&D.

To aid in understanding die wear when stamping AHSS, a study to characterize the contact pressure distribution in drawbeads during stamping had been undertaken. As direct measurement of contact pressure for a drawbead is not feasible during metal flow, a combination of experimental and Finite Element (FE) simulation techniques were used to determine the contact pressure distributions and the maximum contact pressure for a number of different conditions. Testing was conducted using the Drawbead Simulator (DBS) for two different bead configurations. The materials in this investigation were 0.7mm and 0.8mm EG BH210 and EG DP500. Static Implicit FE analyses were conducted with ABAQUS Standard using 2D plane strain continuum elements. A combined hardening model in conjunction with strain rate effects was used to describe material behavior as it flows through the drawbeads.

Drop tower axial crush testing was performed on hat section samples of various steel grades ranging in minimum tensile strength from 410 MPa to 1300 MPa. It was demonstrated that the energy absorption capability increases with the tensile strength of the steel. However, steels of very high strength, greater than 980 MPa tensile strength, exhibited a greater tendency for weld button pullout or material fracture, and thus limited energy the absorption capability. The effect of the closeout plate and the yield strength of the steel on energy absorption were also investigated. FEA simulations were performed and correlated to the experimental results. A flow stress based material criterion is introduced based on the analytical approach to compare the crush performance of steels.

Prediction of dent resistance of automotive closure panels using Finite Element Analysis (FEA) has increased in popularity as an effective screening tool to determine the optimum combination of material properties and thickness for a given exposed panel application. The analysis process typically includes formability analysis, the results of which are mapped onto a structural model for analysis of dent resistance. Although this method is general, allowing for consideration of the entire panel assembly and boundary conditions, the approach can be very time consuming especially when evaluating several combinations of material grade and thicknesses. This paper presents an alternative approach for the prediction of dent resistance. Using a number of screening simulations the most influential variables affecting dent resistance were determined. A rectangular bi-curved plate was used to approximate the panel assembly.

To further the application of Advanced High Strength Steels (AHSS) in automotive body and structural parts, a good knowledge and experience base must be developed regarding the press formability of these materials. As a first step towards accomplishing this goal, the American Iron and Steel Institute, in collaboration with the United States Department of Energy, jointly funded under the Technology Roadmap Program, a study by Ispat Inland Research Laboratories to characterize the formability of AHSS using simulative laboratory tests. Splitting limits under different conditions and springback behavior of several grades of conventional high strength steels (HSS) such as bake-hardenable and HSLA steels, advanced high strength steels (AHSS) such as dual-phase and TRIP steels, and ultra-high strength steels (UHSS) such as recovery-annealed and tempered martensitic steels were characterized.

Spot weld fatigue performance of dual phase steels is of great interest due to much higher fatigue strength of its base steel. In this study, 0.8mm DP500-EG and 1.4mm DP600-GI were tested for both tensile shear and cross tension conditions. For comparison, tensile shear test was also conducted for 1.6mm HSLA350-GI and 0.8mm DQSK-GI. Although fatigue strength was different due to their different gages, by using the stress index, Ki, a parameter to describe the local stress condition, fatigue strength of all four steels merged to a narrow scatter band, indicating very little dependence of spot weld fatigue on the strength of the base steel. In addition, the effect of weld surface cracking on fatigue strength of dual phase steels is of concern due to their high strength, despite the fact that it can occur to any steels under conditions of high current or electrode misalignment.

Bending under tension is an important deformation mode during stamping and has been observed to limit achievable ductility for high strength steels. This paper presents experimental results from Angular Stretch Bend (ASB) testing, which has been used to characterize bending under tension behavior for several conventional, advanced high strength steels and ultra-high strength steels. Steels that were studied include Bake Hardenable steels, High Strength Low Alloy (HSLA) steels, Dual Phase (DP) steels, Transformation Induced Plasticity (TRIP) steels, and tempered martensitic steels. Failure heights were determined under sample lockout conditions for different punch radii. By comparing absolute formability measured by the failure height, the results can be used to provide material formability ranking for different R/t ratios. In addition, strain distributions were analyzed to provide bending under tension forming limits for the different steel grades.

In an effort to optimize outer body panel steel utilization with respect to dent resistance performance and weight reduction, the automotive industry continues to investigate the application of higher strength steels. Most recently, dual phase steel has been recognized as a very promising material substrate for outer body panel application, due to its inherent formability and final part performance attributes. This paper presents a comprehensive study of Ispat Inland's new electrogalvanized dual phase “DI-FORM 500” product, which was specifically designed to meet automotive exposed quality standards. It reviews the mechanical properties, aging characteristics, formability, dent resistance, weldability and fatigue strength of this product, along with a representation of its application advantages to the automotive industry, in terms of part performance, weight savings and cost avoidance.

With interest in improving vehicle quality and customer satisfaction, Ford Motor Company initiated an effort aimed at improving dent resistance of closure panels. An investigation of various means of product improvement led to the recognition of dual phase steels, due to their inherent formability and strain hardening attributes, as the most appropriate steel panel for outer panel applications. Ispat Inland's new Electro-galvanized dual phase steel DI-FORM 500 (henceforth referred to by the generic designation, DP500), which meets 500 MPa minimum tensile strength, was specifically designed to meet automotive exposed quality standards. This paper compares the dent resistance performance of automotive door assemblies manufactured with both Bake Hardenable 210 (BH210) and DP500 door outer panels. Results indicate the achievement of significantly improved outer panel dent resistance through the use of the DP500 product.

The quasi-static denting behavior of sheet steels has been analyzed in a systematic FEA study, which considers material properties, panel radius, sheet thickness, panel length and boundary conditions. Results from a full factorial experimental matrix have been analyzed statistically to identify those variables and variable interactions that influence dent performance. The primary factors which control dent performance are material properties, panel radius and sheet thickness, while panel length and boundary conditions are not significant. Based on the results of this study, two commonly used dent criteria (loading energy and visible dent load) are analyzed, and previously reported opposite effects of radius of curvature on dent performance are clarified.