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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.

The simulation of a crash event in the design stage of a vehicle facilitates the optimization of crashworthiness and significantly reduces the design cost and time. The development of a fracture material card used in crash simulation is heavily dependent on laboratory testing data. In this paper, the experimental characterization process to generate fracture data for fracture model calibration is discussed. A third-generation advanced high strength steel (AHSS), namely the XG3TM steel, is selected as the example material. For fracture model calibration, fracture locus and load-displacement data are obtained using mechanical testing coupled with digital image correlation (DIC) technique. Test coupons with designed geometries are deformed under different deformation modes including shear, uniaxial tension, plane strain and biaxial stretch conditions. Mini-shear, sub-sized tensile, and Marciniak cup tests are employed to achieve these strain conditions.

For the past decades, the adoption of empirical equations in the forming limit curve (FLC) calculation for conventional steels has greatly simplified the forming severity assessment in both forming simulations and on the stamping shop floor. Keeler’s equation based on the n-value and sheet thickness is the most popular one used in North America. However, challenges have been encountered on the validity of the equation for advanced high strength steels (AHSS) since Keeler’s equation was developed based on the FLC data mostly from mild steels and conventional high strength steels. In this study, forming limits of various AHSS grades under different strain conditions are experimentally determined using digital image correlation technique. Both Marciniak cup and Nakazima dome tests are exercised to demonstrate the differences in the resultant forming limits determined with different test methods.

Advanced high-strength steels (AHSS), due to their significantly higher strength than the conventional high-strength steels, are increasingly used in the automotive industry to meet future safety and fuel economy requirements. Unlike conventional steels, the properties of AHSS can vary significantly due to the different steelmaking processes and their fracture behaviors should be characterized. In crash analysis, a fracture model is often integrated in the simulations to predict fracture during crash events. In this article, crash simulations including a fracture criterion are conducted for a third-generation AHSS, that is, 980GEN3. A generalized incremental stress state dependent damage model (GISSMO) in LS-DYNA is employed to evaluate the fracture predictability in the crash simulations.

Today’s automotive industry is witnessing increasing applications of advanced high strength steels (AHSS) combined with innovative manufacturing techniques to satisfy fuel economy requirements of stringent environmental regulations. The integration of AHSS in novel automotive structure design has introduced huge advantages in mass reduction while maintaining their structural performances, yet several concerns have been raised for this relatively new family of steels. One of those concerns is their potentially high springback after forming, which can lead to geometrical deviation of the final product from its designed geometry and cause difficulties during assembly. From the perspective of accurate prediction, control and compensation of springback, further understanding on the effect of residual stress in AHSS parts is urged. In this work, the residual stress distribution in a 980GEN3 steel part after hydroforming is investigated via experimental and numerical approaches.

Digital image correlation (DIC) technique has been proved as a potent tool to determine the forming limit curve (FLC) of sheet metal. One of the major technical challenges using the DIC to generate FLC is to accurately pinpoint the onset of localized necking from the DIC data. In addition to the commonly applied ISO 12004-2 standard, a plethora of other DIC data analysis approaches have been developed and used by various users and researchers. In this study, different approaches, including spatial, temporal and hybrid approaches, have been practiced to determine the limit strains at the onset of localized necking. The formability of a 980GEN3 sheet steel was studied in this work using the Marciniak cup test coupled with a DIC system. The resulting forming limits determined by different approaches were compared. Strengths and limitations of each approach were discussed. In addition, the conventional finger-touch approach was excised using specimens with perceivable localized necks.

Fracture strain data provide essential information for material selection and serve as an important failure criterion in computer simulations of crash events. Traditionally, the fracture strain was measured by evaluating the thinning at fracture using tools such as a microscope or a point micrometer. In the recent decades, digital image correlation (DIC) has evolved as an advanced optical technique to record full-field strain history of materials during deformation. Using this technique, a complete set of the fracture strains (including major, minor, and thickness strains) can be approximated for the material. However, results directly obtained from the DIC can be dependent on the experiment setup and evaluation parameters, which potentially introduce errors to the reported values.

Advanced high strength steels (AHSS) have been widely accepted as a material of choice in the automotive industry to balance overall vehicle weight and stringent vehicle crash test performance targets. Combined with efficient use of geometry and load paths through shape and topology optimization, AHSS has enabled vehicle manufacturers to obtain the highest possible ratings in safety evaluations by the Insurance Institute for Highway Safety (IIHS) and the National Highway Traffic Safety Administration (NHTSA). In this study, vehicle CAE side impact models were used to evaluate three side impact crash test conditions (IIHS side impact, NHTSA LINCAP and FMVSS 214 side pole) and the IIHS roof strength test condition and to identify several key components affecting the side impact test performance. HyperStudy® optimization software and LS-DYNA® nonlinear finite element software were utilized for shape and gauge optimization.

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 improve the energy absorption capacity of front-end structures during a vehicle crash, a novel 12-sided cross-section was developed and tested. Computer-aided engineering (CAE) studies showed superior axial crash performance of the 12-sided component over more conventional cross-sections. When produced from advanced high strength steels (AHSS), the 12-sided cross-section offers opportunities for significant mass-savings for crash energy absorbing components such as front or rear rails and crush tips. In this study, physical crash tests and CAE modeling were conducted on tapered 12-sided samples fabricated from AHSS. The effects of crash trigger holes, different steel grades and bake hardening on crash behavior were examined. Crash sensitivity was also studied by using two different part fabrication methods and two crash test methods. The 12-sided components showed regular folding mode and excellent energy absorption capacity in axial crash tests.

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.

The frame rail has an impact on the crash performance of body-on-frame (BOF) and uni-body vehicles. Recent developments in materials and forming technology have prompted research into improving the energy absorption and deformation mode of the frame rail design. It is worthwhile from a timing and cost standpoint to predict the behavior of the front rail in a crash situation through finite element techniques. This study focuses on improving the correlation of the frame component Finite Element model to physical test data through sensitivity analysis. The first part of the study concentrated on predicting and improving the performance of the front rail in a frontal crash [1]. However, frame rails in an offset crash or side crash undergo a large amount of bending. This paper discusses appropriate modeling and testing procedures for front rails in a bending situation.

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.

The conversion between cast aluminum lower control arms (LCAs) and stamped steel LCAs has prompted the need for new LCA designs to achieve parallel levels of performance. Component tests procedures and CAE modeling methodologies need to be utilized to assess future LCA designs across a variety of vehicle lines to meet or exceed performance criteria. Therefore the overall goal of this study was to develop a standardized test procedure to test the stiffness, deformation and strength of LCAs. In addition, CAE modeling methodologies to better model LCAs will be developed. The test procedures and CAE modeling methodologies would then be used to set performance targets for future LCA designs. To standardize the LCA test procedure, component test fixtures were developed in this work. The objective of the fixtures is to test LCAs with similar boundary conditions they would experience in vehicle crash. Three different test modes are examined in this project.

Finite element models of cast aluminum and stamped steel lower control arms (LCAs) were created to simulate subsystem tests of LCA with bushings and brackets. Several modeling methods were used to simulate the dynamic responses of cast aluminum LCAs, and the advantages and disadvantages of each method are discussed. Factors that are essential for modeling stamped steel components found in previous studies [1, 2] including strain rate, forming, and welding effects are incorporated in the stamped steel LCA models. Difficulties in modeling LCAs subsystem, possible remedies, and further improvements are also discussed in this paper.

Advanced High Strength Steels (AHSS) along with innovative design and manufacturing processes are effective ways to improve crash energy management. Crash trigger hole is another technology which can been used on front rails for controlling crash buckling mode, avoiding crash mode instability and minimizing variations in crash mode due to imperfections in materials, part geometry, manufacturing, and assembly processes etc. In this study, prototyped crash columns with different trigger hole shapes, sizes and locations were physically tested in frontal crash impact tests. A corresponding crash computer simulation model was then created to perform the correlation study. The testing data, such as crash force-displacement curves and dynamic crash modes, were used to verify the FEA crash model and to study the trigger sensitivity and effects on front rail crash performance.

Spot welding is the primary joining method used for the construction of the automotive body structure made of steel. A major challenge in the crash simulation today is the lack of a simple yet reliable modeling approach to characterize spot weld separation. In this paper, an attempt has been made to develop a spot weld modeling methodology to characterize spot weld separation in crash simulation. A generalized two-node spring element with 6 DOF at each node is used to characterize the spot weld nugget. To represent the connection of the nugget with the surrounding plates, tied contacts are defined between the spring element nodes and the shell elements of the plate. Three general separation criteria are proposed for the spot weld that include the effects of speed and coupled loading conditions. The separation criteria are implemented into a commercially available explicit finite element code.

This paper presents an investigation into the behavior of spot-welded steel connections based on a DOE approach. This work is a part of spot-weld modeling methodology development work being performed at Ford. Control factors such as material, coating, gage size, and noise factors such as loading direction (angle), and speed are considered in this study. Different levels of each variable are included to cover a wide range of practical applications. The test methodology used to generate the responses for the spot-weld coupons have been discussed in a companion paper [1]. From the force-displacement curves obtained from the test, the responses such as peak force, displacement at peak force, and rupture displacement are identified. These responses are then statistically analyzed to identify the relative importance and effect of the design factors. Finally, response surface models are developed to determine responses across different levels of each variable.

Spot-welds are the primary joining methods for steel sheet metals used in the manufacturing of automobile body structure. Often the impact responses are significantly affected by the characteristic properties, such as stiffness, failure strength, etc of spot-welds. In view of this, understanding the behavior and the properties of spot-welds under static and impact loadings are critical for accurate CAE analysis of vehicle impact events. To this end, a comprehensive DOE based spot-weld testing has been undertaken by considering a wide variety of variables. The test data thus obtained were analyzed to determine the requisite mechanical properties of spot-welds as a function of the key variables such as gage, yield strengths, speed, etc. Spot-weld connections have been tested for gages ranging from 0.7 to 3.0 mm using a unique specimen configuration developed at Ford.

Resistance to dents and dings, caused by plant handling and in-service use, is generally recognized as an important performance requirement for automotive outer body panels. This paper examines the dent resistance improvements that can be achieved by maximizing surface stretch, through adjustments to the press settings, and substitution of a higher strength steel grade. Initially, the stamping process was optimized using the steel supplied for production: a Ti/Nb-stabilized, ultra low carbon (ULC) grade. The stamping process was subsequently optimized with a Nb-stabilized, rephosphorized ULC steel, at various thicknesses. The formed panels were evaluated for percent surface stretch, percent thinning, in-panel yield strength after forming, and dent performance. The results showed that dent resistance can be significantly improved, even at a reduced steel thickness, thus demonstrating a potential for weight savings.