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The r-value is a very important parameter in the forming simulations of high strength steels, especially for steels with prominent anisotropy. R-values for sheet steels conventionally measured by extensometers were found neither consistent nor accurate due to difficulties in measuring the width strain. In this study, the Digital Image Correlation (DIC) technique was applied to determine r-values in Longitudinal (L), Transverse (T) and Diagonal (D) directions for cold rolled DP980 GI, DP780 GI, DP600 GI and BH250 GI sheet steels. The r-values measured from DIC were validated by finite element analysis (FEA) of a uniaxial tensile test for BH250. The simulation results of the load-displacement for two plasticity models were compared to experimental data, with one being the isotropic yield (von-Mises) and the other being an anisotropic model (Hill-48) using the r-value measured from DIC.

In vehicle crash events there is the potential for fracture to occur at the processed edges of structural components. The ability to avoid these types of fractures is desired in order to minimize intrusion and optimize energy absorption. However, the prediction of edge cracking is complicated by the fact that conventional tensile testing can provide insufficient data in regards to the local fracture behavior of advanced high strength steels. Fracture prediction is also made difficult because there can be inadequate data on how the cutting processes used for hole piercing and blanking affect the edge condition. In order to address these challenges, research was undertaken to analyze edge fracture in simple test pieces configured with side notches and center holes. Test specimens were made from a number of advanced high strength steels including 590R (C-Mn), 780T (TRIP), 980Y (dual phase) and hot stamp 1500 (martensitic).

Advanced high strength steels (AHSS) are becoming major enablers for vehicle light weighting in the automotive industry. Crash resistant and fracture-toughened structural adhesives have shown potential to improve vehicle stiffness, noise, vibration, and harshness (NVH), and crashworthiness. They provide weight reduction opportunity while maintaining crash performance or weight increase avoidance while meeting the increasing crash requirement. Unfortunately, the adhesive bonding of galvanneal (GA)-coated steels has generally yielded adhesive failures with the GA coating peeling from the steel substrate resulting in poor bond strength. A limited study conducted by ArcelorMittal and Dow Automotive in 2008 showed that GA-coated AHSS exhibited cohesive failure, and good bond strength and crash performance. In order to confirm the reliable performance, a project focusing on the consistency of the adhesive bond performance of GA-coated steels of 590 MPa strength level was initiated.

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.

Springback prediction is one of the roadblocks for using advanced high strength steel in the automotive industry. Accurate characterization and modeling of the mechanical behavior of AHSS is recognized as one of the critical factors for successful prediction of springback. Conventional tensile test based material characterization and constitutive modeling may lead to poor springback simulation accuracy. Aiming to accurately predict springback, a series of advanced material characterizations including bi-axial material testing, large-strain loading path reversal testing, unloading tests at large strain, stress-strain behavior beyond uniform elongation, were performed for selected AHSS and associated constitutive models were developed to incorporate these characterizations. Validations through lab samples and industrial parts show that the AHSS springback prediction accuracy is significantly improved with these improved material models.

Adhesive bonding technology is rapidly gaining acceptance as an alternative to spot welding. This technology is helping automobile manufacturers reduce vehicle weight by letting them use lighter but stronger advanced high strength steels (AHSS's). This can make cars safer and more fuel efficient at the same time. The other benefits of this technology include its flexibility, ability to join dissimilar materials, distribute stress uniformly, provide sealing characteristics and sound dampening, and provide a moisture barrier, thus minimizing the chance for corrosion. The lap shear work reported in the late 1980s and early 1990s has led to the prevalent perception that the galvannealed (GA) coating can delaminate from the steels, resulting in poor joint performance. However, the above work was carried out on steels used primarily in automobile outer body panels.

A new method has been developed to measure full stress-strain curves using Digital Image Correlation (DIC) for Advanced High Strength Steels (AHSS). With the post-necking strain measured by the built in-house DIC system during tensile tests, stress-strain data for AHSS beyond uniform elongation up to fracture can be determined. In this paper, the technique to generate full stress-strain curves by DIC is introduced. The measured stress-strain curves are compared with those obtained by extrapolation methods. The measured stress-strain data generated by the new method is validated by finite element analysis (FEA).

Predicting fracture behavior of Advanced High Strength Steels (AHSS) on both manufacturing and crash simulations is becoming more and more important with the wide use of AHSS in automotive industry. The accurate measurement of fracture strains is a critical input for predicting failure in FEA simulations. It is well known that fracture is a highly localized behavior and fracture strain is gauge or size dependent. In this paper, a full field measurement technique, Digital Image Correlation (DIC), is employed to measure gauge-dependent fracture strains for several Advanced High Strength Steels (AHSS) under tensile test conditions and Limit Dome Height (LDH) tests. Applications of the fracture strains for FEA simulation are discussed.

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.

Rollover crash is one of the important fatal crash modes in highway accidents. To protect occupant safety, the National Highway Traffic Safety Administration (NHTSA) has proposed a higher roof strength requirement in the upcoming new federal regulation. Meanwhile fuel efficiency and environmental friendliness demands that the safety cage design should have the minimum weight while providing the sufficient roof crush strength. These requirements pose a challenge to automotive design engineers. In this paper, Advanced High Strength Steels (AHSS) are introduced as an enabler to support this challenging task. The advantages of different types of AHSS for vehicle crashworthiness are presented. The criteria to select materials to improve the roof crush performance are discussed in detail.

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.

Because of increasing fuel costs and environmental concerns, the automotive industry is under enormous pressure to reduce vehicle weight. One strategy, downgaging, substitutes a reduced gage (thickness) steel in place of a thicker one, and is usually accompanied by a material grade change to a higher strength steel. Thus, Advanced High Strength Steels (AHSS) are increasingly used for lightweight automotive body structures. The critical durability concern with steels is the spot welds used to join them, since fatigue cracks in body structures preferentially initiate at spot welds. Hence, the Auto/Steel Partnership (A/SP) Sheet Steel Fatigue Taskforce undertook an investigation both to study the fatigue performance of AHSS spot welds, and to generate data for OEM durability analysis. The study included seven AHSS grades and, for comparison, mild steels and a conventional High Strength Low Alloy grade, HSLA340.

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.

This paper investigates the characteristics of GMAW of various sheet steels grades ranging from HSLA, dual phase, to martensitic. From the arc welding point of view, the dual phase and martensitic steels behave similarly to conventional high strength steels. Regarding the properties of GMAW joints, the static and dynamic mechanical testing were conducted and compared along with the weld metal microhardness and microstructure. Results show that while the strength of the sheet steel weld, in general increases with the base material strength, Joint Efficiency, defined as the ratio of the strength of joint to the strength of the base metal, decreases with the increase of martensite fraction in the sheet steel. Martensitic steels, especially, exhibits reduced weld strength due to softening of the HAZ. However, fatigue strength of these steels is not adversely affected by the softened HAZ, and is insensitive to the strength of the steel.

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.

Traditional constitutive models can only describe a parallel or divergent stress strain response at different strain rates. This paper presents a new constitutive model that can describe convergent, divergent or parallel stress strain patterns. The new model is a modification to the popular Johnson-Cook model. By comparison with the Johnson-Cook model using high strain rate data of seven high strength steels, the new model is evaluated. The results showed that the new model could adequately describe the stress strain relation at high strain rates for the seven steels. In addition, an empirical relationship between the parameters in the new constitutive model and quasi-static tensile data has been developed based on the analysis of several high strength steels. The equation requires only quasi-static data as the input and is capable of estimating flow stresses at high strain rates.