Search Results

The objective of this study was to assess the formability of two 3rd generation advanced high strength steels (3rd Gen AHSS) with ultimate strengths of 980 and 1180 MPa and evaluate their applicability to a structural B-Pillar for a mid-sized sport utility vehicle. The constitutive behavior including strain-rate effects and formability were characterized to generate the material models for use within AutoForm R8 software to design the B-pillar tooling and forming process. An extended Bressan-Williams instability model was able to deterministically predict the forming limit curves obtained using Marciniak tests. The tooling for the representative B-pillar was designed and fabricated with Bowman Precision Tooling and forming trials conducted for both 3rd Gen steels that had a thickness of 1.4 mm.

The superior formability and local ductility of the emerging class of third generation of advanced high-strength steels (3rd Gen AHSS) compared to their conventional counterparts of the same strength level offer significant advantages for automotive lightweighting and enhanced crash performance. Nevertheless, studies on the material behavior of 3rd Gen AHSS have been limited and there is some uncertainty surrounding the applicability of developed methodologies for conventional dual-phase (DP) steels to this new class of AHSS. The present paper provides a comprehensive study on the quasi-static and dynamic constitutive behavior, formability characterization and prediction, and the fracture behavior of two commercial 3rd Gen AHSS with an ultimate strength of 1180 MPa that will be contrasted with a conventional DP1180. The hardening response to large strain levels was determined experimentally using tensile and shear tests and then validated with 3-D simulations of tensile tests.

The development and calibration of stress state-dependent failure criteria for advanced high-strength steel (AHSS) and aluminum alloys requires characterization under proportional loading conditions. Traditional tests to construct a forming limit diagram (FLD), such as Marciniak or Nakazima tests, are based upon identifying the onset of strain localization or a tensile instability (neck). However, the onset of localization is strongly dependent on the through-thickness strain gradient that can delay or suppress the formation of a tensile instability so that cracking may occur before localization. As a result, the material fracture limit becomes the effective forming limit in deformation modes with severe through-thickness strain gradients, and this is not considered in the traditional FLD. In this study, a novel bending test apparatus was developed based upon the VDA 238-100 specification to characterize fracture in plane strain bending using digital image correlation (DIC).

Practical evaluation and reduction of edge cracking are two challenging issues in stamping AHSS for automotive body structures. In this paper, the effects of the shear clearance and shear rake angle on edge cracking were investigated with three different grades of AHSS; TRIP780, DP 980, and DP 1180. Five different shear clearances, between 5% and 25% of material thickness, were applied to the flexible shearing machine to generate samples for the half specimen dome test (HSDT). The shear loads and the shear edge quality were thoroughly characterized and compared. The HSDT created the edge forming limits as compared to the base material forming limit diagram. The load-displacement curve was acquired by the load-cell and the strain distribution was measured using a digital image correlation (DIC) system during the dome test.

Practical evaluation and accurate prediction of edge cracking are challenging issues in stamping AHSS for automotive body structures. This paper introduces a new hole-expansion testing method that could be more relevant to the edge cracking problem observed in stamping AHSS. A new testing method adopted a large hole diameter of 75 mm compared to the ISO standard hole diameter of 10 mm. A larger hole diameter was determined to be sensitive to edge cracking using the finite element method (FEM) based sensitivity analyses with various hole sizes. A die punching tool was developed to replicate typical production blanking conditions. An inline monitoring system was developed to visually monitor the hole edge cracking during the test and synchronize with the load-displacement data. Two AHSS materials, DP980 and TRIP780, and an aluminum alloy, A1 5182-O, were experimentally evaluated.

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).

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.

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.

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.

A highly ductile TRIP steel sheet has been developed and put into practical use to achieve weight savings in automotive Bodies-in-White. During deformation, TRIP steels experience a strain-induced transformation of retained austenite to hard martensite. This TRIP-effect increases the strength of the material and improves the formability by distributing strain over a larger area. In comparison to conventional high strength steel (HSS), TRIP steels have a superior balance between strength and ductility. The TRIP steels newly put into practical use at 590MPa and 780 MPa strength levels had equivalent formability to conventional HSS at the 440MPa and 590 MPa levels, respectively. Also it achieved equivalent coating quality, paintability and spot weldability to conventional HSS. Consequently, it was found that these TRIP steels can feasibly be applied to a wide variety of vehicle parts.

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.

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).