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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.

The conventional forming limit curve (FLC) has been widely and successfully used as a failure criterion to detect localized necking in stamping. However, in stamping advanced high strength steels (AHSS), under certain circumstances such as stretching-bending over a small die radius, the sheet metal fails much earlier than predicted by the FLC. This type of failure on the die radius is commonly called “shear fracture.” In this paper, the laboratory Stretch-Forming Simulator (SFS) and the Bending under Tension (BUT) tester are used to study shear fracture occurring during both early and later stages of stamping. Results demonstrate that the occurrence of shear fracture depends on the combination of the radius-to-thickness (R/T) ratio and the tension/stretch level applied to the sheet during stretching or drawing. Based on numerous experimental results, an empirical shear fracture limit curve or criterion is obtained.

One of the issues in stamping of advanced high strength steels (AHSS) is the stretch bending fracture on a sharp radius (commonly referred to as shear fracture). Shear fracture typically occurs at a strain level below the conventional forming limit curve (FLC). Therefore it is difficult to predict in computer simulations using the FLC as the failure criterion. A modified Mohr-Coulomb (M-C) fracture criterion has been developed to predict shear fracture. The model parameters for several AHSS have been calibrated using various tests including the butter-fly shaped shear test. In this paper, validation simulations are conducted using the modified (M-C) fracture criterion for a dual phase (DP) 780 steel to predict fracture in the stretch forming simulator (SFS) test and the bending under tension (BUT) test. Various deformation fracture modes are analyzed, and the range of usability of the criterion is identified.

Prephosphated steels have been developed by applying the phosphate coating on zinc coated sheet steels to increase the lubricity in the automotive stamping process and adding extra corrosion protection. The prephosphate coating was also found to be able to further absorb the lubricant, which can reduce the oil migration and excessive amount of lubricant dripping on the die surface and the press floor. Due to its enhanced lubricity characteristic, the applications have been expanded to more-recently developed advanced high strength steels (AHSS). Because of the higher strength of AHSS, it is crucial to understand their performance under more extreme forming conditions such as higher die temperature, contact pressure and sliding speed, etc. The intent of this study is to investigate the tribological performance and die wear behavior of prephosphated AHSS in the die tryout and production conditions.

The thermal deflection associated with the conventional die heat treating procedure usually requires extra die grinding process to fine-tune the die surface. Due to the size of the production die, the grinding is time consuming and is not cost effective. The goal of the study is to develop a new die heat treating process utilizing the flexible laser heat treatment, which could serve the same purpose as the conventional die heat treating and avoid the thermal deflection. The unique look of the developed zebra pattern laser heat treating process is defined as the Zebra Line. The heat-treating parameters and processes were developed and calibrated to produce the laser heat treating on laboratory size dies, which were subjected to the die wear test in the laboratory condition. The USS HDGI 980 XG3TM steel was selected to be carried out on the developmental dies in the cyclic bend die wear test due to its high strength and coating characteristic.

Gas Metal Arc Welding (GMAW) is widely employed for joining relatively thick sheet steels in automotive body-in-white structures and frames. The GMAW process is very flexible for various joint geometries and has relatively high welding speed. However, fatigue failures can occur at welded joints subjected to various types of loads. Thus, vehicle design engineers need to understand the fatigue characteristics of welded joints produced by GMAW. Currently, automotive structures employ various advanced high strength steels (AHSS) such as dual-phase (DP) and transformation-induced plasticity (TRIP) steels to produce lighter vehicle structures with improved safety performance and fuel economy, and reduced harmful emissions. Relatively thick gages of AHSS are commonly joined to conventional high strength steels and/or mild steels using GMAW in current body-in-white structures and frames.

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.

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).

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.

Dent resistance has become one of the key performance criteria of automotive outer body panels due to the increased use of lightweight sheet steels for vehicle weight reduction. In order to sustain or improve the dent resistance of body panels with reduced metal thickness, commercial patches have been attached to the inner surface of exposed panels. The actual patch effects on dent resistance are not evident. Hence, the understanding of benefits of using a patch to improve dent resistance is also limited. In this study, the effects of a patch on dent resistance are investigated. A special fiberglass patch is developed to compare with commercial patches. Fixed load [12] and fixed speed, single loading conditions are carefully designed and incorporated into the quasi-static and dynamic dent test. For comparison purposes, the incremental quasi-static loading condition is also examined.

A standardized quasi-static dent test has been used in evaluating the dent resistance of automotive body panels for many years. In order to predict the onset of a visible dent, cyclic loading with small load increments was adopted into standard automotive test procedures. Although researchers have investigated the effects of sheet thickness, yield strength, strain aging and prestrain on the static dent resistance of sheet steel in the past, material bending and hardening effects on static dent resistance were assumed to be inconsequential, and were therefore largely ignored. In this study, the impacts of material bending and hardening on static dent resistance are investigated. A fixed load, single loading condition was carefully designed for different materials and incorporated into the quasi-static dent test. For comparison, the incremental quasi-static loading condition is also examined.

In the North American automotive industry, various advanced high strength steels (AHSS) are used to lighten vehicle structures, improve safety performance and fuel economy, and reduce harmful emissions. Relatively thick gages of AHSS are commonly joined to conventional high strength steels and/or mild steels using Gas Metal Arc Welding (GMAW) in the current generation body-in-white structures. Additionally, fatigue failures are most likely to occur at joints subjected to a variety of different loadings. It is therefore critical that automotive engineers need to understand the fatigue characteristics of welded joints. The Sheet Steel Fatigue Committee of the Auto/Steel Partnership (A/S-P) completed a comprehensive fatigue study on GMAW joints of both AHSS and conventional sheet steels including: DP590 GA, SAE 1008, HSLA HR 420, DP 600 HR, Boron, DQSK, TRIP 780 GI, and DP780 GI steels.

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).

Press hardened steels (PHS) are commonly used in automotive structural applications because of their combination of extremely high strength, load carrying capacity and the ability to form complex shapes in the press hardening process. Recent adoption of increased roof crush standards, side impact requirements and the increased focus on CO2 emissions and mass reduction have led autmotive manufacturers to significantly increase the amount of PHS being designed into future vehicle designs. As a way to further optimize the use of these steels, multi-gauge welded blanks of PHS and multi-material blanks of PHS to microalloyed steels of various thickness have been developed to help achieve these requirements. More recently, tailor rolled PHS, whereby the steel is rolled such that the thickness changes across the width of the sheet, have been developed.

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 fracture of advanced high strength steels (AHSS) can occur in both the stamping process and the crash event. Fracture due to poor sheared edge conditions in the stamping process was reduced with a recently developed optimal shearing process for AHSS. Currently, the improvement in the energy absorption due to the improved edge condition during crashes performed under different loading conditions had not been closely verified. The purpose of this study is to design and build a miniature component of AHSS and a three-point bending test for investigating the influence of various conditions of the sheared edge on the energy absorption in crashes. AHSS including DP600, TRIP780, DP980 and DP1180 were selected in the study. A small channel component was developed and fabricated using DP980 to simulate key features of the B-pillar. The exposed non-constrained, as-sheared edge was subject to stretch bending forces in three-dimensional space during the three-point bending test.

Low carbon steels are being replaced by advanced high strength steels (AHSS) due to high demand of the future lighter weight vehicle, while still maintaining good or even better crash performance. However, sidewall curl and springback (section opening) have been found to increase as the strength of the sheet metal increases. Experiments have been conducted on the bending under tension (BUT) test to seek an effective control methodology regarding the applications of the advanced high strength steels (AHSS) in this study. Steels that were studied included a low carbon steel (DQSK), two dual phase steels (DP) and a transformation induced plasticity (TRIP) steel. Two different gauges of each AHSS were also included for a gauge sensitivity study. Different processing variables (four different diameter pins combining with five different back tension forces) were applied to the tests, and the springback angle and sidewall curl were measured for bend and bend-unbend areas of the specimen.

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.

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.