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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).

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.

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.

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.

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.

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.

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.

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).

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 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.

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.

In this study fatigue tests of GMAW (Gas Metal Arc Welding) welded joints were conducted on both 1.6mm body sheet (DQSK-GA, DP590-GA, DP780-GI, and TRIP 780-GI) and 3.4mm frame materials (SAE1008 HR 240MPa, HSLA420 HR, DP600 HR, and uncoated Boron). Further, mixed thickness joints were tested which combined 3.4mm SAE1008 HR with each of the 1.6mm separately – with the exception of DQSK. A number of different joint configurations were tested including single and double lap-shear, start-stop lap shear, butt weld, and perch mount. Great care was taken in this study to ensure that the geometry of the welds was consistent, not only within a given material lay-up, but between all of the specimens of a given type – this effort was made in order to substantially reduce life scatter and provide a better understanding of the role base material plays in the fatigue life of GMAW joints.

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.

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.

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.

In the first part of this paper, a previously published acceleration compensation methodology for dynamic dent testing [1] was successfully applied to calculate dent loads and applied energy in dynamic dent testing. This procedure was validated utilizing a hydraulic controlled dynamic dent tester on a number of low carbon and bake hardenable steels. In the second part of this study, the impact of strain rate on material bending and hardening in high-speed dynamic dent resistance testing was studied. Previous work [2] investigated these factors in static dent resistance. The procedure utilized in that research was further developed and adapted for high speed testing and used as a basis for a new, single loading incremental dynamic dent test. This new test was used to investigate the effects of material bending and hardening in high-speed dynamic dent resistance. Testing incorporated laboratory produced stretch dome panels with 2% biaxial strains as test specimens.

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.

Different die surface polish conditions result in a noticeable effect on material flow in stamping, which can lead to splitting, wrinkling, or other surface stretching issues associated with different friction conditions. These occurrences are not only limited to the non-coated dies, but also nitrided and chrome plated dies. To ensure quality control of the stamped parts, the die conditions corresponding to different polishing procedures need to be developed based on measurable parameters such as surface roughness (Ra). The intent of this study is to investigate the effects of nitrided and chrome plated die surface roughness on friction. The Bending-Under-Tension (BUT) test was conducted to simulate the stamping process due to the test’s versatility and flexibility in changing test parameters. The test involves moving sheet metal across a 3/8-inch diameter pin, which substitutes for a die surface. The pin can be modified by material, heat treatment, coating, and surface roughness.

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.

The hole piercing process is a simple but important task in manufacturing processes. The quality requirement of the pierced hole varies between different applications. It can be either the size or the edge quality of the hole. Furthermore, the pierced hole is often subject to a secondary forming process, in which the edge stretchability is of a main concern. The recently developed advanced high strength steels (AHSS) and ultra high strength steels (UHSS) have been widely used for vehicle weight reduction and safety performance improvements. Due to the higher strength nature of these specially developed sheet steels, the hole piercing conditions are more extreme and challenging, and the quality of the pierced hole can be critical due to their relatively lower edge stretching limits than those for the conventional low and medium strength steels. The stretchability of the as-sheared edge inside the hole can be influenced by the material property, die condition and processing parameters.