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Drop tower crash tests in a three-point bending configuration were carried out on spot welded box sections, adhesive bonded box sections, and laser welded cylindrical tubes made from a variety of advanced high strength steels. In the tests, a 147-kg indenter with a 28-cm diameter impacts the specimen at approximately 6 m/s, and the bending loads and energy absorption are determined. The results show that the maximum bending loads best correlate to the product of yield strength and thickness-squared, while the energy absorbed over 10-cm displacement best correlates to ultimate tensile strength times thickness-squared. As such, higher strength steels can be used to improve crash performance without increasing weight or to maintain crash performance with weight reduction. Other significant findings of the study are as follows. Bake hardening alone may improve bending crash performance slightly, while cold rolling and baking does not.

Material formability is a very important aspect in the automotive stamping, which must be tested for the success of manufacturing. One of the most important sheet metal formability parameters for the stamping is the edge tear-ability. In this paper, a novel test method has been present to test the aluminum sheet edge tear-ability with 3D digital image correlation (DIC) system. The newly developed test specimen and fixture design are also presented. In order to capture the edge deformation and strain, sample's edge surface has been sprayed with artificial speckle. A standard MTS tensile machine was used to record the tearing load and displacement. Through the data processing and evaluation of sequence image, testing results are found valid and reliable. The results show that the 3D DIC system with double CCD can effectively carry out sheet edge tear deformation. The edge tearing test method is found to be a simple, reliable, high precision, and able to provide useful results.

Edge flanging represents one of the forming modes employed in multistage forming, and advanced high strength steels (AHSS) are more prone to edge cracking during sheared edge flanging than the conventional high strength steels (HSS) and mild steels. The performance of the sheared edge in flanging operation depends on the remaining ductility of the material in the sheared edge after the work hardening (WH) and damage produced by blanking and subsequent forming operations. Therefore, it is important to analyze the effect of work hardening produced by blanking and subsequent forming operations prior to edge flanging on the edge flanging performance. In this study, the effect of different forming operation sequences prior to edge flanging on the edge flanging performance was analyzed for a dual phase 780 steel.

This paper introduces an industrial application of digital image correlation technique on the measurement of aluminum edge stretching limit. In this study, notch-shape aluminum coupons with three different pre-strain conditions are tested. The edge stretching is proceeded by standard MTS machine. A dual-camera 3D Digital Image Correlation (DIC) system is used for the full field measurement of strain distribution in the thickness direction. Selected air brush is utilized to form a random distributed speckle pattern on the edge of sheet metal. A pair of special optical lens systems are used to observe the small measurement edge area. From the test results, it demonstrate that refer to the notched coupon thickness, pre-tension does not affect the fracture limit; refer to the virgin sheet thickness, the average edge stretch thinning limits show a consistent increasing trend as the pre-stretch strain increased.

Accurate determination of the forming limit strain of aluminum sheet metal is an important topic which has not been fully solved by industry. Also, the effects of draw beads (enhanced forming limit behaviors), normally reported on steel sheet metals, on aluminum sheet metal is not fully understood. This paper introduces an experimental study on draw bead effects on aluminum sheet metals by measuring the forming limit strain zero (FLD0) of the sheet metal. Two kinds of aluminum, AL 6016-T4 and AL 5754-0, are used. Virgin material, 40% draw bead material and 60% draw bead material conditions are tested for each kind of aluminum. Marciniak punch tests were performed to create a plane strain condition. A dual camera Digital Image Correlation (DIC) system was used to record and measure the deformation distribution history during the punch test. The on-set necking timing is determined directly from surface shape change. The FLD0 of each test situation is reported in this article.

Steel-plastic-steel (SPS) laminated sheet materials can be utilized in certain automotive applications to achieve significant weight savings over “conventional” sheet steels. Three SPS laminates were produced using various combinations of light gage steel skins and polypropylene cores. Compared to homogeneous steels, density reductions of 35 to 46 percent were achieved. Benefiting from the ductility of their steel skins, SPS laminates can posses adequate formability for typical automotive sheet applications. Furthermore, the forming limit curves can be predicted using the work hardening exponent and thickness of the composite laminate. In three-point bending, the elastic stiffness of SPS laminates is nearly equivalent to that of monolithic steels of the same thickness. Thus, weight reductions similar to those of aluminum alloys can be achieved utilizing laminates in stiffness-critical applications.

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.

The influence of strain rate on work hardening behavior has been determined for a variety of high strength steels including high-strength low-alloy (HSLA), dual phase (DP), and transformation-induced plasticity (TRIP) steels. Tensile testing was performed at true strain rates of 10-3 s-1 and 1.0 s-1 to represent laboratory testing conditions and dynamic press-forming operations, respectively. Work hardening behavior is described by the conventional strain hardening exponent (n-value), the work hardening rate (dσ/dε), and the Shape-Tilt-Strength (STS) equation as an alternative approach. The effects of deformation temperature and temperature rise during deformation (adiabatic heating) on work hardening are also evaluated. Increasing the strain rate generally increases the work hardening rate at smaller strains, which may contribute to a broader initial strain distribution in press forming.

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.

Axial drop tower crash tests were carried out on a variety of 70-mm outer-diameter continuous-welded cylindrical steel tubes with several thicknesses (t). Ultimate tensile strength (UTS) ranged from less than 300 MPa for a fully stabilized steel to greater than 800 MPa for the advanced high strength steels (AHSS). In the tests, a 520-kg weight is dropped from a height of 3.3 meters to achieve impact velocities of 6.1 to 6.7 m/s (14 to 15 mph). Load and acceleration data are recorded as a function of time as the tube is crushed axially. The results show that, for a given impact condition, the peak and average crush loads of a steel tube is directly proportional to UTS × t2, while axial crush distance is inversely proportional to UTS × t2. As such, crash deformation can be reduced by substituting higher strength steels of the same thickness, or existing crash deformation can be maintained and weight reduction achieved by substituting higher strength steels with reduced thickness.

In Aluminum Alloy, AA, sheet metal forming, the through thickness cracking at the edge of cut out is one of the major fracture modes. In order to prevent the edge cracking in production forming process, practical edge stretch limit criteria are needed for virtual forming prediction and early stamping trial evaluations. This paper proposes new methods for determining the edge stretching limit of the sheet coupons, with and without pre-stretching, based on the Digital Image Correlation (DIC) technique. A numbers of sets of notch-shaped smaller coupons with three different pre-stretching conditions (near 5%, 10% and fractured) are cut from the prestretched large specimens. Then the notch-shaped smaller coupons are stretched by uniaxial tension up to through edge cracking observed. A dual-camera 3D-DIC system is utilized to measure both coupon face strain and thickness strain in the notch area at the same time.

Studies in an Angular Stretch Bend Test (ASBT) have demonstrated that the failure location moves from the side wall to punch nose area. This occurs as the R/T ratio decreases below a certain limit and applies to most low carbon steels with the exception of Dual Phase (DP) steels. Such behavior in DP steels indicates that bending effects have a severe impact on the formability of DP materials. Therefore, the traditional criterion using the forming limit curve (FLC) is not suitable to assess the formability at punch radius areas for DP steels due in part to its uniqueness of unconventional microstructures. In this paper, a new failure criterion, ‘Bending-modified’ FLC (BFLC), is proposed by extending the traditional FLC using the “Stretch Bendability Index” (SBI) concept for the stretch bendability assessment.