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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.

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.

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.

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.

Advanced High Strength Steels (AHSS) are replacing conventional high strength low-alloyed steels (HSLA) in crash sensitive body in white (BIW) applications. Along with innovative product design, they offer superior crash energy management and vehicle weight reduction potential. However, Controlling springback and dimensional accuracy is one of the major concerns in manufacturing AHSS parts. One of the most effective springback control techniques is to design a part with added geometric features such as side stiffening beads, state beads, top hat beads, and embossments, etc. at the product design stage. On the other hand, product design communities tend to believe that the above listed features may result in premature crash initiation in the part. This paper uses an innovative and experimentally verified finite element method (FEM) for crash sensitive component design and optimization.

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.

The issue of cost and weight reduction at optimum car crash safety is a driving force behind the growing use of advanced high strength steels, particularly in Europe and Japan. Recent developments in the availability of high strength steel (HSS) sheets in North America; in particular transformation induced plasticity (TRIP) steels, offer an attractive option to the automotive designer for weight reduction and improved safety performance. For example, the use of TRIP steels, as opposed to more conventional steel products such as high strength low alloy (HSLA), in some applications may result in up to 40% part weight reduction at similar vehicle crash performance. When the excellent formability of TRIP steel is considered at product design stage, it may also lead to reducing part count and tooling cost. In this paper the formability of TRIP steels of various gauges is assessed. Experimental forming limit curves (FLCs) are determined for T600 grade.

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 issue of improving car crash energy management and maintaining cost and weight reduction are the driving forces behind the growing use of advanced high strength steels, particularly in Europe and Japan. Recent developments in the manufacture of high strength steel (HSS) sheets in North America, in particular Dual Phase (DP) steels, offers an attractive option to the automotive designer for weight reduction and improved safety performance. For example, the use of dual phase steels, as opposed to more conventional steel products such as high strength low alloys (HSLA), in some cases may result in up to 40% part weight reduction at similar vehicle crash performance. In this paper the formability of commercially produced hot-dipped dual phase steels of various gages and grades is assessed. Forming Limit Curves (FLCs) are determined for commercial DP590 and DP780 grades. These FLCs were compared to the conventional ASM FLCs calculated from n-value and sheet thickness.