Search Results

A detailed investigation of the influence of anisotropic hardening models on drawbead restraining force is presented in this paper. The recently modified Yoshida model is adopted to characterize the anisotropic hardening behavior for steels. A two-dimensional drawbead model is used and the restraining forces corresponding to several different bead penetrations are obtained and compared against experimental results. The comparison of the predicted results for the Modified Yoshida Model with isotropic hardening models indicates that the anisotropic hardening gives lower drawbead restraining forces in general. The impact of hardening models on springback is also presented, and it's demonstrated that the springback amount predicted by the modified Yoshida model is much closer to the experimental data than that predicted by conventional isotropic hardening model.

Incremental Sheet Forming (ISF) is an emerging sheet metal prototyping technology where a part is formed as one or more stylus tools are moving in a pre-determined path and deforming the sheet metal locally while the sheet blank is clamped along its periphery. A deformation analysis of incremental forming process is presented in this paper. The analysis includes the development of an analytical model for strain distributions based on part geometry and tool paths; and numerical simulations of the forming process with LS-DYNA. A skew cone is constructed and used as an example for the study. Analytical and numerical results are compared, and excellent correlations are found. It is demonstrated that the analytical model developed in this paper is reliable and efficient in the prediction of strain distributions for incremental forming process.

Die wear is a significant issue in sheet metal forming particularly for stamping Advanced High-Strength Steels (AHSS) because of their higher strength and microstructure composition. Reliable predictions of the magnitude and distribution of die wear are essential if cost-effective wear-protection strategies are desired in the early stages of tooling development. A die Wear Severity Index (WSI) is introduced in this paper to quantify the magnitude of die wear, which in essence characterizes the frictional energy dissipation per unit area on the die surface throughout the entire forming cycle. It can be readily obtained as part of any finite element simulation of stamping process utilizing incremental solution techniques.

Material constitutive modeling is an important aspect in the continuous improvement process for sheet metal forming simulation and analysis. In this study, a sensitivity study of material constitutive parameters on forming simulation results is carried out for two different sheet metal parts. Three different yield criteria are evaluated in the simulation including isotropic yield criterion, Hill's 1948 anisotropic yield criterion and Barlat's non-quadratic anisotropic yield criterion. It is found that the forming results depend upon the yield criterion used in the simulation. Both thinning and stress in the formed part are very sensitive to the change in anisotropy value (r-value). Effects of strain rate sensitivity of a material on forming results are demostrated through the use of two different stress and strain data from different tensile speeds. It is also found that forming results such as thinning are very sensitive to the strain hardening behavior of the material.

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.

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.

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.

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.

Prediction of springback has become a major focus in sheet metal forming. Validation of finite element codes that are being developed to predict springback require accurate property data and a more complete understanding of the residual stresses that are involved. To provide experimental data for these calculations, neutron and synchrotron X-ray diffraction measurements were carried out to determine the through-thickness distribution of axial and hoop (or tangential) residual stresses in deep-drawn steel and aluminum cups. The techniques are able to provide true spatial resolutions as low as 0.05 mm for a strain measurement on cups with ≤ 1 mm wall thickness. It was found that the stresses exhibit non-linear gradients through the thickness that also depend on the axial position.

The front rail plays a very important role in vehicle crash. Trigger holes are commonly used to control frame crush mode due to their simple manufacturing process and flexibility for late changes in the product development phase. Therefore, a study, including CAE and testing, was conducted on a production front rail to understand the effects of trigger hole shape, size and orientation. The trigger hole location in the front rail also affects crash performance. Therefore, the effect of trigger hole location on front rail crash behavior was studied, and understanding these effects is the main objective of this study. A tapered front rail produced from 1.7 mm thick DP600 steel was used for the trigger hole location investigation. Front rails with different trigger spacing and sizes were tested using VIA sled test facility and the crash progress was simulated using a commercial code RADIOSS. The strain rate, welding and forming effects were incorporated in the front rail modeling.

The use of tailor welded blank sheets for body-in-white applications has brought many new forming challenges. Mechanical properties of tailor welded blanks measured from the tension test showed significantly higher strengths and lower elongations than in the base metal. The forming limit of tailor welded blanks was significantly reduced when compared to that in the base metal. For the weld made from materials with dissimilar thicknesses/strengths, additional formability issues were also raised in addition to those for the weld made from materials with similar thickness/strength. These issues increase the potential for failure in the lesser thickness/lower strength material in the area immediately adjacent to the weld when the difference in thicknesses or strengths of the two materials increases.

The formability window of a material depends upon the forming limit and its strain distribution ability. For two materials with the same forming limit, the formability performance is governed by their strain distribution ability. In this study, strain hardening behavior of different strength steels was investigated using a uniaxial tension test and the forming limit was studied using both Marciniak cup and dome tests. The n-value of steels varies with strain. Different strain hardening behaviors are found between mild steels and high strength steels. Strain distribution ability of steels increases with the increase in the overall n-value. The peak n-value at a low strain level enhances the strain distribution ability of the steel. It has been shown from both theoretical and experimental studies that a constant thickness strain line exists on the left side of the forming limit curve.

Flanging is a secondary operation in sheet metal forming processes. Traditionally, the design of flange shape and trim line is based on an engineer's experience. It takes several iterations to achieve the desired flange geometry because of potential splits. In this paper, an efficient CAE-based tool is developed to quickly predict the formability of a given flange design and enable the optimization of trim lines. A numerical algorithm is formulated in this CAE tool to convert the 3D flanging process into an equivalent in-plane deformation problem. The developed CAE tool is also integrated with the optimization software LS-OPT for trim line design.

Dent resistance is an important attribute in the automotive panel design, and the ability to accurately predict a panel's dentability requires careful considerations of sheet metal properties, including property changes from stamping process. The material is often work-hardened significantly during forming, and its thickness is reduced somewhat. With increased demand for weight reduction, vehicle designers are seriously pushing to use thinner-gauged advanced high-strength steels (AHSS) as outer body panels such as fenders, hoods and decklids, with the expectation that its higher strength will offset reduced thickness in its dentability. A comparative study is conducted in this paper for a BH210 steel fender as baseline design and thinner DP500 steel as the new design.

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.

This paper is concerned with the use of solid elements in sheet metal forming simulation, particularly springback prediction for flanging when the flanging radii are comparable with the metal thickness. It is demonstrated that appropriate solid elements must be used instead of shell elements in order to obtain adequate results. Numerical difficulties associated with development of suitable solid elements are discussed in detail, with emphasis on the volumetric locking and transverse shear locking phenomena respectively. The transverse shear locking arises from the incompatible deformation modes when the element is used for thin structure bending analysis. A four point bending testing problem is used to study the performances of different solid elements. A locking-free solid element based on assumed strain formulation is developed in Ford in-house program MTLFRM for accurate springback prediction, and a flanging example is given to demonstrate its application.

A new strain-based forming limit criterion is proposed to assess the localized necking failure for sheet metal forming simulations when deformation paths deviate significantly from linearity. Different from the traditional strain-based Forming Limit Diagrams (FLD) in terms of major and minor strains, the new FLD is constructed based on effective strains and material flow direction at the end of forming. This new criterion combines the advantages of stress-based FLD for its path-independence and the traditional linear strain path FLD for its easy interpretation. The proposed FLD is validated through both theoretical prediction with Marciniak-Kuczynski (M-K) model and available experimental data in literature, and its relationship with stress-based FLDs is discussed.

Many uncertainties exist in the sheet metal stamping such as the variation of incoming material properties, die and press setup conditions, long-term tool wear and degradations. They are interacting in a way to make the process less robust, thus contributing to increased scrap rates and more unscheduled downtime. This paper presents a new approach for the die design optimization where these uncertainties are taken into account. A Tailor-Welded B-pillar consisting of 1.65mm DP600 and 0.9mm DDQ is selected as the focal part to demonstrate the new design process. The study is divided into two phases. The focus of the first phase is to understand the complexity of the formability window and determine effective optimization techniques under deterministic conditions. It is found that the formability window is highly nonlinear, or even discontinuous if a global objective function such as the Maximum Failure Factor is used.