Search Results

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.

A novel performance classification system has been developed for advanced high-strength steel (AHSS). This system considers intrinsic global and local formability parameters derived from standard uniaxial tension tests and is applicable to all current and future AHSS materials. The overall AHSS performance index (P.I.) is defined herein as the product of the ultimate tensile strength (UTS) and the formability index (F.I.), where F.I. is an intermediate strain value between the true uniform strain and the true fracture strain (TFS). Target P.I. values are defined for First Generation AHSS (GEN1), Improved First Generation AHSS (GEN1+), Third Generation AHSS (GEN3), and AHSS Future. Performance is further distinguished by local, balanced, and global formability characteristics and by relative yield strength (yield-to-tensile ratio). Additionally, the influence of tension test specimen geometry and fracture area measurement method on the TFS value was explored.

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.

Developing lightweight, stiff and crash-resistant vehicle body structures requires a balance between part geometry and material properties. High strength materials suitable for crash resistance impose geometry limitations on depth of draw, radii and wall angles that reduce geometric efficiency. The introduction of 3rd generation Advanced High Strength Steels (AHSS) can potentially change the relationship between strength and geometry and enable simultaneous improvements in both. This paper will demonstrate applicability of 3rd generation AHSS with higher strength and ductility to replace the 780 MPa Dual Phase steel in a sill reinforcement on the current Jeep Cherokee. The focus will be on formability, beginning with virtual simulation and continuing through a demonstration run on the current production stamping tools and press.

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.

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.

The use of advanced high strength steels (AHSS) such as dual phase (DP), transformation induced plasticity (TRIP) and stretch flanging (SF) steels of the tensile strength of 600 MPa range are well established in automotive components production. This is due to their superior crash energy absorption ability and vehicle weight reduction potential. Recent trends show rapid growth in applications of even higher strength grades such as 800 MPa and 1000 MPa tensile strength and above. They are mostly used for fabrication of crash sensitive components to meet much higher safety requirements in side impact and roll-over accidents. One of the few concerns during the fabrication of AHSS components is the formability limit in flanging and hole expansion operations. Questions have been raised about the applicability of existing manufacturing experience with conventional high strength low alloy steels (HSLA) to new generations of AHSS.

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.

More and more advanced high strength steels (AHSS) such as dual phase steels and TRIP steels are implemented in automotive components due to their superior crash performance and vehicle weight reduction capabilities. Recent trends show increased applications of higher strength grades such as 780/800 MPa and 980/1000 MPa tensile strength for crash sensitive components to meet more stringent safety regulations in front crash, side impact and roll-over situations. Several issues related to AHSS stamping have been raised during implementation such as springback, stretch bending fracture with a small radius to thickness ratio, edge cracking, etc. It has been shown that the failure strains in the stretch bending fracture and edge cracking can be significantly lower than the predicted forming limits, and no failure criteria are currently available to predict these failures.

Forming Limit Diagrams (FLD) have been widely and successfully used in sheet metal stamping as a failure criterion to detect localized necking, which is the most common failure mechanism for conventional steels during forming. However, recent experience from stamping Dual-Phase steels found that, under certain circumstances such as stretching-bend over a small die radius, the sheet metal fails earlier than that predicted by the FLD based on the initiation of a localized neck. It appears that a different failure mechanism and mode are in effect, commonly referred to as “shear fracture” in the sheet metal stamping community. In this paper, experimental and numerical analysis is used to investigate the shear fracture mechanism. Numerical models are established for a stretch-bend test on DP780 steel with a wide range of bend radii for various failure modes. The occurrences of shear fracture are identified by correlating numerical simulation results with test data.

As a result of the increasing usage of high strength steels in automotive body structures, a number of formability issues, particularly bend and edge stretch failures, have come to the forefront of attention of both automotive OEMs and steel makers. This investigation reviews these stamping problems and attempts to identify how certain material properties and microstructural features relate to forming behavior. Various types of dual phase steels were evaluated in terms of tensile, bending, hole expansion, limiting dome height, and impact properties. In addition, the key microstructural differences of each grade were characterized. In order to understand the material behavior under practical conditions, stamping trials were conducted using actual part shapes. It was concluded that material properties can be optimized to maximize local formability in stamping applications. The results also emphasize that the dual phase classification can encompass a broad range of property variations.

Springback is a major concern in stamping of advanced high strength steels (AHSS). The existing computer simulation technology has difficulty predicting this phenomenon accurately even though it is well developed for formability simulations. Great efforts made in recent years to improve springback predictions have achieved noticeable progress in the computational capability and accuracy. In this work, springback simulation studies are conducted using FEA software LS-DYNA®. Various parametric sensitivity studies are carried out and key variables affecting the springback prediction accuracy are identified. Recently developed simulation technologies in LS-DYNA® are implemented including dynamic effect minimization, smooth tool contact and newly developed nonlinear isotropic/kinematic hardening material models. Case studies on lab-scale and full-scale industrial parts are provided and the predicted springback results are compared to the experimental data.

Advanced high strength steels (AHSS), such as dual phase (DP) and transformation induced plasticity (TRIP) steels, have been used successfully for making light weight vehicles and their usage is growing. Now, the automotive industry is expanding the use of AHSS to higher strength levels for further mass reduction. In a 2003 SAE paper, the material and formability characteristics for such steels were presented for steel grades of DP980, high yield type DP780 (780YM), low yield type DP780 (780YL), TRIP780, and TRIP590. In this study, experiments were conducted to assess the formability of these high strength steels using a T-channel, which incorporates several different forming modes in automotive stamping. The feasibility of computer simulation technology for the formability analyses of AHSS is also addressed.

In the past ten year period, due primarily to government mandates for fuel economy improvement, alternate materials have replaced steel on many closure applications at American OEMs (hoods, decklids, and liftgates). But due to recent cost reduction initiatives set by automakers and the advent of newly developed high strength steels, this trend has been challenged by lighter weight, less costly steel alternatives, with near equal or superior performance. This paper, through case studies undertaken at several North American OEM facilities, examines the cost differential, material property options, manufacturing differences, and performance characteristics between the application of aluminum and steel for common hood, lift gate, and deck lid assemblies for both current and future production parts.

Advanced high strength steels were a key enabling factor in achieving the remarkable results of the ULSAB-AVC (Advanced Vehicle Concepts) Program. The complete body structure consists of high strength steels with over 80% being advanced high strength steel grades. Vehicle weight reduction, reduced costs and improved safety performance are the main driving forces behind material selection for automotive applications. High strength steels (HSS) have demonstrated their ability to meet these demands and consequently have been the fastest growing light-weighting material in vehicle structures for the past decade. The evolution in steel technology in recent years has produced new grades of highly formable, advanced high strength steel (AHSS) grades that will continue to meet these automotive demands into the next decade.

A typical forming operation of chassis components (control arms, cross members, etc.) often involves edge stretching and/or hole expansion. As a result, the edge split is a common forming failure mode. To overcome this problem, Japanese and European automakers use stretch flange high strength (SFHS) steel due to its high strength and excellent edge stretch capability. Recently, SFHS steel has gained greater attention in North America and is currently being used for upper and lower control arm applications. This paper includes a discussion on general edge stretch issues in forming operations, including material data that demonstrate the higher stretch limit of SFHS steel as compared to other high strength steels. In a case study, SFHS steel is applied to a control arm and finite element analysis (FEA) is conducted to evaluate forming and structural performance.

The issue of cost and weight reduction at optimum car crash safety is a driving force behind the growing use of advanced high strength steels, particularly in Europe and Japan. Recent developments in the availability of high strength steel (HSS) sheets in North America; in particular transformation induced plasticity (TRIP) steels, offer an attractive option to the automotive designer for weight reduction and improved safety performance. For example, the use of TRIP steels, as opposed to more conventional steel products such as high strength low alloy (HSLA), in some applications may result in up to 40% part weight reduction at similar vehicle crash performance. When the excellent formability of TRIP steel is considered at product design stage, it may also lead to reducing part count and tooling cost. In this paper the formability of TRIP steels of various gauges is assessed. Experimental forming limit curves (FLCs) are determined for T600 grade.

Advanced high strength steels (AHSS) have been extensively used in the automotive industry for vehicle weight reduction. Although AHSS show better parent metal fatigue performance, the influence of material strength on spot weld fatigue is insignificant. To overcome this drawback, structural adhesive can been used along with spot weld to form weld-bond joints. These joints significantly improve spot weld fatigue performance and provide high joint stiffness enabling down-gauge of AHSS structures. However, modeling the adhesive joints using finite element methods is a challenge due to the nonlinear behavior of the material. In this study, the formulation of cohesive element based on the traction-separation constitutive law was applied to predict the initiation and propagation of the failure mode in the adhesively bonded joints for lap shear and coach peel specimens subjected to quasi-static loadings. The predicted load versus displacement relations correlated well with the test results.

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.