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Body panel performance properties such as denting force, oil canning/critical buckling load, initial and secondary stiffnesses under dynamic loading (drop weight test) were measured for different strength steels and two aluminum alloys using both flat and curved sheets. It was found that all these properties varied with the drop velocity. For the steels, the denting force steadily increased with the increase in drop velocity. For the aluminum alloys, the denting force increased with the drop velocity at lower velocities and decreased or remained unchanged at higher velocities. The oil canning/critical buckling load increased with the increase in drop velocity and initial and secondary stiffnesses decreased with the increase in drop velocity for both steel and aluminum. The dent resistance performance for some aluminum alloys with thicker gauge is comparable to steels dent tested at lower velocities.

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.

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.

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.

An experimental study of springback was conducted for a hat channel section with varying cross sections and controlled gap between punch and die. The channel section was formed in a single step forming process with upper pressure pad. DP590 steel was compared to a group of high strength steels (HSS), e.g. HSLA270, 340 and 420. In addition, sidewall curl phenomenon was studied utilizing bending under tension test. This paper describes methodology of experiment and discusses springback related results. It also offers recommendations that can be applied to die-punch gap control or material substitution situations. The results of this investigation can be used to verify accuracy of springback predictions in finite element analysis (FEA).

The effects of sheet thickness, yield strength, strain aging and prestrain on the dynamic dent resistance of sheet steel are investigated using an instrumented drop-weight test. It is found that the dynamic dent resistance is less dependent on the sheet thickness and the yield strength of the material than the static dent resistance. The dent resistance of automotive steels under dynamic loading conditions increases with prestrain in a manner similar to static denting. The relative ranking of different strength steels in the performance of dynamic denting is provided at various strain levels. An empirical relation of dynamic denting force and energy with the sheet thickness and the yield strength of the material is derived for a flat panel. Data confirm that dynamic dent resistance is improved using high strength steels including bake hardenable steels and rephosphorized steels.

Interface friction between sheet metal and tooling in sheet metal forming is examined in different forming modes using laboratory simulative tests. Stretchability is studied by the limiting dome height test; drawability is investigated by a four inch Swift cup draw test and the coefficient of friction is measured by the draw bead simulator under bending and unbending deformation. The responses of the interface friction in six different coated and uncoated steel sheets are studied using seven different lubricants. It is found that the interface friction between sheet metal and tooling is very sensitive to the forming mode and the type of coating. For the same lubricant and coated material, two different forming modes may produce very different results in interface friction. However, overall good and bad lubricants for all forming modes can be determined for a given coated material using these three tests.

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.

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.

Continuing pressure to reduce mass and cost of vehicles is driving the development of new high strength steel products with improved combinations of strength and formability. Galvanized, cold rolled dual phase steel products are new alternatives to conventional high strength low alloy (HSLA) steel for strength limited applications in vehicles. These steels have higher tensile strengths than HSLA products with nearly equivalent formability. This paper compares the performance of HSLA and dual phase sheet steel products in a series of drop tower tests. Samples were prepared by stamping the steel sheets into typical rail-type parts using a production-intent die process. The parts were sectioned, and subsequently fabricated into hat-shaped assemblies that were then dynamically crushed by a drop weight. The experiments were designed such that the entire energy input by the drop weight was absorbed by the samples.

Experimental procedures and results of a benchmark test for springback are reported and a complete suite of obtained data is provided for the validation of forming and springback simulation software. The test is usually referred as the Slit-Ring test where a cylindrical cup is first formed by deep drawing and then a ring is cut from the mid-section of the cup. The opening of the ring upon slitting releases the residual stresses in the formed cup and provides a valuable set of easy-to-measure, easy-to-characterize springback data. The test represents a realistic deep draw stamping operation with stretching and bending deformation, and is highly repeatable in a laboratory environment. In this study, six different automotive materials are evaluated.