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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.

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.

To evaluate vehicle crash performance in the early design stages, a reliable fracture model is needed in crash simulations to predict material fracture initiation and propagation. In this paper, a generalized incremental stress state dependent damage model (GISSMO) in LS-DYNA® was calibrated and validated for a 780-MPa third generation advanced high strength steels (AHSS), namely 780 XG3TM steel that combines high strength and ductility. The fracture locus of the 780 XG3TM steel was experimentally characterized under various stress states including uniaxial tension, shear, plane strain and equi-biaxial stretch conditions. A process to calibrate the parameters in the GISSMO model was developed and successfully applied to the 780 XG3TM steel using the fracture test data for these stress states.

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).

Drop tower crash tests in a three-point bending configuration were carried out on spot welded box sections, adhesive bonded box sections, and laser welded cylindrical tubes made from a variety of advanced high strength steels. In the tests, a 147-kg indenter with a 28-cm diameter impacts the specimen at approximately 6 m/s, and the bending loads and energy absorption are determined. The results show that the maximum bending loads best correlate to the product of yield strength and thickness-squared, while the energy absorbed over 10-cm displacement best correlates to ultimate tensile strength times thickness-squared. As such, higher strength steels can be used to improve crash performance without increasing weight or to maintain crash performance with weight reduction. Other significant findings of the study are as follows. Bake hardening alone may improve bending crash performance slightly, while cold rolling and baking does not.

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.

Steel-plastic-steel (SPS) laminated sheet materials can be utilized in certain automotive applications to achieve significant weight savings over “conventional” sheet steels. Three SPS laminates were produced using various combinations of light gage steel skins and polypropylene cores. Compared to homogeneous steels, density reductions of 35 to 46 percent were achieved. Benefiting from the ductility of their steel skins, SPS laminates can posses adequate formability for typical automotive sheet applications. Furthermore, the forming limit curves can be predicted using the work hardening exponent and thickness of the composite laminate. In three-point bending, the elastic stiffness of SPS laminates is nearly equivalent to that of monolithic steels of the same thickness. Thus, weight reductions similar to those of aluminum alloys can be achieved utilizing laminates in stiffness-critical applications.

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.

Edge flanging represents one of the forming modes employed in multistage forming, and advanced high strength steels (AHSS) are more prone to edge cracking during sheared edge flanging than the conventional high strength steels (HSS) and mild steels. The performance of the sheared edge in flanging operation depends on the remaining ductility of the material in the sheared edge after the work hardening (WH) and damage produced by blanking and subsequent forming operations. Therefore, it is important to analyze the effect of work hardening produced by blanking and subsequent forming operations prior to edge flanging on the edge flanging performance. In this study, the effect of different forming operation sequences prior to edge flanging on the edge flanging performance was analyzed for a dual phase 780 steel.

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.

Fatigue data for a new microalloyed steel have been generated and compared with SAE 9259. The steel is designed to be used for suspension coil springs at high strength levels (~2100 MPa) in order to reduce weight. Both stress-controlled and strain-controlled tension-compression fatigue tests were conducted. The high strength microalloyed steel showed a 17 to 25% increase in endurance limit over SAE 9259 for polished specimens. Strain-controlled fatigue tests showed comparable strain resistance for these two steels. However, the microalloyed steel has higher stress carrying capability and less cyclic softening than SAE 9259 in the constant strain condition. Shot-peening is proved to improve its fatigue resistance under uniaxial tension-compression loading in stress amplitudes above 724 MPa (105 ksi).

Axial drop tower crash tests were carried out on a variety of 70-mm outer-diameter continuous-welded cylindrical steel tubes with several thicknesses (t). Ultimate tensile strength (UTS) ranged from less than 300 MPa for a fully stabilized steel to greater than 800 MPa for the advanced high strength steels (AHSS). In the tests, a 520-kg weight is dropped from a height of 3.3 meters to achieve impact velocities of 6.1 to 6.7 m/s (14 to 15 mph). Load and acceleration data are recorded as a function of time as the tube is crushed axially. The results show that, for a given impact condition, the peak and average crush loads of a steel tube is directly proportional to UTS × t2, while axial crush distance is inversely proportional to UTS × t2. As such, crash deformation can be reduced by substituting higher strength steels of the same thickness, or existing crash deformation can be maintained and weight reduction achieved by substituting higher strength steels with reduced thickness.

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.

The effect of forming strains on the fatigue behavior of an automotive mild steel, interstitial free steel, was studied after being prestrained by balanced biaxial stretch and plane strain. In the long life region, higher than 5×105 reversals, prestrain improves fatigue resistance. In the short life region, prestrain reduces fatigue resistance. At even shorter fatigue lives, the detrimental effect of prestrain diminishes. For plane strains, the fatigue behavior is anisotropic. In the direction perpendicular to the major strain, the steel exhibits much better fatigue resistance than in the direction parallel to the major strain.

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.