Formability Evaluation of Sheet Metals Based on Global Strain Distribution
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Necking
Forming limit diagram
Plane stress
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Low carbon steel sheets have many applications in industries, especially in automotive parts, therefore it is necessary to study formability of these steel sheets. Forming limit diagram (FLD) is one of the strong pieces of equipment used to study the formability of sheet metals. In this study, FLDs have been determined experimentally for three grades ST12, ST14, and Interstitial free (IF) by conducting punch-stretching experiments using suitably designed and fabricated tools. Formability observed from FLDs has been correlated with mechanical properties and formability parameters like punch type, punch diameter, friction between punch and sheet, work hardening exponent (n) and plane-anisotropy (r) of the sheets. Results have indicated that forming strains in IF and ST14 steel sheets are higher than ST12 and higher n×r values and thickness are desirable to raise the forming strains. The sheet orientation can be effective and in addition, depends on the strain path .For example, forming limit strains in 45 angles with respect to the rolling direction are less than that for 0 and 90 within the right band of FLDs. Keywords– Forming limit diagram, formability, r, n values, low carbon steel
Forming limit diagram
Carbon steel
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Strain hardening exponent
Sheet steel
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Necking
Forming limit diagram
Incremental sheet forming
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Hardening (computing)
Deep drawing
Strain hardening exponent
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Forming limit diagram
Necking
Tola
Deep drawing
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Necking
Forming limit diagram
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Forming limit diagrams (FLD) are powerful and efficient tools to determine limit strains, plastic instability, and prediction of necking as well as tearing during various sheet metal forming processes. These diagrams illustrate the range of strains at which the sheet material can be drawn without fracture. One of the popular methods to determine forming limit diagrams is the hemispherical-punch stretch forming process. In this research study, the forming limit diagram of thick anisotropic sheet metals in warm conditions has been determined using finite element modeling of the hemispherical-punch stretch forming process. To take the anisotropy of the sheet metals into account, the anisotropic Hill-48 yield criterion has been used. To determine the necking point of the sheet metals the second derivative of equivalent plastic strain has also been used. Keeler’s theory has been selected to validate the finite element modeling results. The results indicate that finite element modeling can predict forming limit diagrams well.
Necking
Forming limit diagram
Tearing
Limit load
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The sheet metal forming process is the process of deforming the sheet metal into a desired shape without fracture or excessive localized necking. Variables in sheet metal forming process can be discussed together with formability and test methods. There are many defects occurring during sheet metal forming processes, such as cracking, wrinkling, local necking, buckling etc. The strain measurement in a deformed sheet metal is necessary for measurement comparison. As the thickness of sheet metal is very small as compare to other dimensions of the sheet metal, the sheet metal operation is usually considered as a plane stress problem. The Forming Limit Diagram (FLD) was also determined from surface strain measurement. The FLD is the graph between major strain (℮1) and minor strain (℮2). The Forming Limit Curve (FLC) or the Forming Limit Diagram (FLD) is useful concept for characterizing the formability of sheet metal, which reflects the maximum principal strains that can be sustained by sheet materials prior to the onset of localized necking. Generally there are three methods to establish FLD i.e. theoretical, numerical and experimental. In this paper experimental method is used to develop FLD. For experimental determination of FLD of Mild Carbon steel sheet Limit Dome Height testing is used according to the American Society of Testing Material (ASTM) as published in ASTM E 2218-02. In this paper the procedure of grid marking, punch stretching and strain measurement is used. For punch stretching operation the set up of spherical die and punch has developed on the hydraulic Universal Testing Machine (UTM). The material used for the die and punch is High Carbon High Chromium Steel (HCHCr). For printing the grids on sheet material chemical etching method is used. In this grid making process grids of 5 mm diameter circles printed. For trial experiment the sheet metal sample is stretched at the force of 23 KN. The deformed circles were converted into ellipse and from that deformed ellipse major and minor strains are to be calculated. After that the FLD will give the two different regions of safe and failure zone.
Necking
Forming limit diagram
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Deep drawing
Plane stress
Carbon steel
Hydroforming
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Forming limit diagram
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Abstract Sheet metal forming is a process widely used in the manufacturing industry. There are numerous sheet metal forming processes to evaluate and understand the formability. Among all formability tests, the basic formability can be formulated through tensile tests and followed with specialized tests. In the present paper, the formability of AA 6023-T6 sheet of 2mm thickness by modelling for stretching test namely limit dome height (LDH) test was performed using PAM STAMP 2G a commercial finite element software. For the simulation, input mechanical properties like yield strength (c), material strength coefficient (K), strain hardening exponent (n), plastic strain ratio (R) etc., were considered from the existing literature. For the simulation, two different conditioned sheet such as at room temperature and annealed sheet at 400°C. For all the simulations, four strain paths 100 × 200mm, 125 × 200mm, 150 × 200mm, 175 × 200mm and 200 × 200 mm were taken. Results are drawn based on the three localized necking criteria namely the effective strain rate-based criterion (ESRC – R1), major strain rate-based criterion (MSRC –R2), thickness strain rate-based criterion (TSRC – R3). Form the obtained results, forming limit diagrams are developed for the both condition of sheet metal. It is observed that, formability of AA 6023-T6 sheet in-plane condition (i.e. 100x100 mm) annealed sheet at 400°C is shown better forming whereas in bi-axial condition (i.e. 200x200 mm) got reduced compared to room temperature sheet. The same phenomenon is noted in all the necking criteria too.
Necking
Forming limit diagram
Deep drawing
Strain hardening exponent
Hardening (computing)
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