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Advanced High Strength Steels (AHSS) have been implemented in the automotive industry to balance the requirements for vehicle crash safety, emissions, and fuel economy. With lower ductility compared to conventional steels, the fracture behavior of AHSS components has to be considered in vehicle crash simulations to achieve a reliable crashworthiness prediction. Without considering the fracture behavior, component fracture cannot be predicted and subsequently the crash energy absorbed by the fractured component can be over-estimated. In full vehicle simulations, failure to predict component fracture sometimes leads to less predicted intrusion. In this paper, the feasibility of using computer simulations in predicting fracture during crash deformation is studied.

To achieve optimal axial and bending crush performance using dual phase steels for components designed for crash energy absorption and/or intrusion resistance applications, the cross sections of the components need to be optimized. In this study, Altair HyperMorph™ and HyperStudy® optimization software were used in defining the shape design variables and the optimization problem setup, and non-linear finite element code LS-DYNA® software was used in crush simulations. Correlated crash simulation models were utilized and the square cross-section was selected as the baseline. The optimized cross-sections for bending and axial crush performance resulted in significant mass and cost savings, particularly with the application of dual phase steels.

In previous investigations, it has been shown that the dent resistance of an auto body panel depends upon the yield strength of the material. However, it is known that the yield strength of steel increases with prestrain due to strain hardening. Panel design and material selection based on the material properties obtained from unstrained sheet steels may lead to inaccurate prediction of the dent resistance of the formed panel. In this study, the effect of prestrain on the static dent resistance of auto body panels was investigated. Using existing empirical relationships between dent resistance and panel properties, it was found that the static dent resistance of an auto panel depends not only on the part geometry and material properties but also on the strain level in the panel. The improvement in dent resistance resulting from a material change from an AKDQ steel to a bake hardenable steel or a high strength steel was determined at different strain levels.

Because of their excellent crash energy absorption capacity, dual phase (DP) steels are gradually replacing conventional High Strength Low Alloy (HSLA) steels for critical crash components in order to meet the more stringent vehicle crash safety regulations. To achieve optimal axial and bending crush performance using DP steels for crash components designed for crash energy absorption and/or intrusion resistance applications, the cross sections need to be optimized. Correlated crush simulation models were employed for the cross-section study. The models were developed using non-linear finite element code LS-DYNA and correlated to dynamic and quasi-static axial and bending crush tests on hexagonal and octagonal cross-sections made of DP590 steel. Several design concepts were proposed, the axial and bending crush performance in DP780 and DP980 were compared, and the potential mass savings were discussed.

Interface friction between sheet metal and tooling in sheet metal forming is examined in different forming modes using laboratory simulative tests. Stretchability is studied by the limiting dome height test; drawability is investigated by a four inch Swift cup draw test and the coefficient of friction is measured by the draw bead simulator under bending and unbending deformation. The responses of the interface friction in six different coated and uncoated steel sheets are studied using seven different lubricants. It is found that the interface friction between sheet metal and tooling is very sensitive to the forming mode and the type of coating. For the same lubricant and coated material, two different forming modes may produce very different results in interface friction. However, overall good and bad lubricants for all forming modes can be determined for a given coated material using these three tests.

The front rail plays a very important role in vehicle crash. Trigger holes are commonly used to control frame crush mode due to their simple manufacturing process and flexibility for late changes in the product development phase. Therefore, a study, including CAE and testing, was conducted on a production front rail to understand the effects of trigger hole shape, size and orientation. The trigger hole location in the front rail also affects crash performance. Therefore, the effect of trigger hole location on front rail crash behavior was studied, and understanding these effects is the main objective of this study. A tapered front rail produced from 1.7 mm thick DP600 steel was used for the trigger hole location investigation. Front rails with different trigger spacing and sizes were tested using VIA sled test facility and the crash progress was simulated using a commercial code RADIOSS. The strain rate, welding and forming effects were incorporated in the front rail modeling.

Body panel performance properties such as denting force, oil canning/critical buckling load, initial and secondary stiffnesses under dynamic loading (drop weight test) were measured for different strength steels and two aluminum alloys using both flat and curved sheets. It was found that all these properties varied with the drop velocity. For the steels, the denting force steadily increased with the increase in drop velocity. For the aluminum alloys, the denting force increased with the drop velocity at lower velocities and decreased or remained unchanged at higher velocities. The oil canning/critical buckling load increased with the increase in drop velocity and initial and secondary stiffnesses decreased with the increase in drop velocity for both steel and aluminum. The dent resistance performance for some aluminum alloys with thicker gauge is comparable to steels dent tested at lower velocities.

The dent resistance of an automotive body panel has been used as one of key design parameters for automotive body panels. Quasi-static dent testing procedures have been well documented in North America using A/SP Standard Dent Resistance Test Procedures and numerous publications in static denting are also available. However, test procedures under dynamic denting are not very well documented and limited data exist on dynamic denting performance of automotive body panels. In this paper, dynamic dent tests are carried out using different impact velocities and different test procedures. The advantages and disadvantages of test procedures are discussed. Different ways to characterize the dynamic dent test results are investigated and discussed. Due to higher impact velocity during the dynamic dent testing, the acceleration effect must be considered in the data analysis. Experiments were carried out on a hydraulic controlled dynamic dent tester.

The formability window of a material depends upon the forming limit and its strain distribution ability. For two materials with the same forming limit, the formability performance is governed by their strain distribution ability. In this study, strain hardening behavior of different strength steels was investigated using a uniaxial tension test and the forming limit was studied using both Marciniak cup and dome tests. The n-value of steels varies with strain. Different strain hardening behaviors are found between mild steels and high strength steels. Strain distribution ability of steels increases with the increase in the overall n-value. The peak n-value at a low strain level enhances the strain distribution ability of the steel. It has been shown from both theoretical and experimental studies that a constant thickness strain line exists on the left side of the forming limit curve.

The effects of sheet thickness, yield strength, strain aging and prestrain on the dynamic dent resistance of sheet steel are investigated using an instrumented drop-weight test. It is found that the dynamic dent resistance is less dependent on the sheet thickness and the yield strength of the material than the static dent resistance. The dent resistance of automotive steels under dynamic loading conditions increases with prestrain in a manner similar to static denting. The relative ranking of different strength steels in the performance of dynamic denting is provided at various strain levels. An empirical relation of dynamic denting force and energy with the sheet thickness and the yield strength of the material is derived for a flat panel. Data confirm that dynamic dent resistance is improved using high strength steels including bake hardenable steels and rephosphorized steels.