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formability limit diagram

Sheet metal formability refers to a material's ability to undergo shaping without failures like necking or tearing. Three main factors influence formability: 1) the metal's properties, 2) friction levels during forming, and 3) the equipment used. Common tests to evaluate formability include cupping, tension, bulge, and forming limit diagrams. The earliest test was the Erichsen cupping test which measures the depth a ball can press into a clamped sheet before cracking. The bulge test applies hydraulic pressure to a clamped circular blank to measure its limit before failure from biaxial stretching. Forming limit diagrams map major and minor strains measured from grids printed on sheets after stretching tests.

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YoucaN 4.4- Formability of sheet metal
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Sheet metal formability is generally defined as the ability of a sheet to undergo the desired shape change without failure such as necking and tearing. Three factors have a major influence on formability: 1. Properties of the sheet metal as discussed before 2. Friction and lubrication at various interfaces in the operation 3. Characteristics of the equipment, tools, and dies used
Several techniques have been developed to test the formability of sheet metals, including the ability to predict formability by modeling the particular forming operation 1. Cupping test 2. Tension test 3. Bulge test 4. Forming-limit diagrams Formability of sheet metals
The earliest tests developed to predict formability of sheet metal were cupping tests, namely, the Erichsen tests 1. In the Erichesn test, a sheet-metal specimen is clamped over a flat die with a circular opening and a load of 1000 kg 2. A 20 mm diameter steel ball is then hydraulically pressed into the sheet until a crack appears on the specimen 3. The distance d, in mm, is the Erichsen number Erichsen test The greater the value of d the greater is the formability d = Erichsen number Because the stretching under the ball is axisymmetric, they do not at all simulate the exact conditions of actual forming operations
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In this test, a circular blank is clamped at its periphery and is bulged by hydraulic pressure, thus replacing the punch. The operation is pure biaxial stretching, and no friction is involved, as would be the case in using a punch. The bulge limit (depth penetrated prior to failure) is a measure of formability Bulge-test results on steel sheets of various widths. The specimen farthest left is subjected to, basically, simple tension. The specimen farthest right is subjected to equal biaxial stretching.
Procedure:  Blank sheet is marked with a grid of circles (2.5-5mm).  The blank is then stretched over a punch, until the grid pattern deforms where necking and tearing occur.  The deformed circles are measured in the failed region, that is the major strain, and miner strain are obtained: that is after stretching, the original circle has deformed into ellipse shape  typically 10 data points taken.  A series of tests on a certain metal produces the FLD. Major Strain: (5-4)/4 * 100 = 25% Minor strain: (3.2-4)/4 * 100 = -20% Example: Before punch stretch test: Original circle diameter: 4mm After punch stretch test: Major ellipse axis : 5mm Minor ellipse axis :3.2mm
• During stretching in shee metal, volume is constant l + w + t = 0 • Major strain always large than minor strain If the surface area of ellipse after stretching is larger than the original circle, we know the thickness of the sheet has changed, its thinner due to stretching. Although the major strain is always positive (because forming sheet metal takes place by stretching in at least one direction), the minor strain may be either positive [strain occur in the transvers direction greater than the original] or negative or shrinking [strain occur in the transvers direction smaller than the original] or zero [ No strain occur in the transvers direction ]in the transverse direction
#1 +ε1 -ε2 +ε1 +ε2 +ε1 ε2=0 #3 #4 #2 Courtesy of Roy A. Lindberg, 1983 After sheet metal deformation, the major and minor axes of the circles on the grid pattern are used to determine the coordinates on the forming limit diagram. ε1 Major strain ε2 Miner strain because the minor planar strain is zero When normal isotropy R=1 ( that is, the width and thickness strains are equal. ϵw = -05. ϵl )
The most basic and common test used to evaluate formability. It determines important properties of the sheet metal such as: - total elongation of the sheet specimen at fracture. - strain hardening exponent, n. - the normal anisotropy (R) and -the planar anisotropy (delta R).
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YoucaN Ironing?
If the thickness of the sheet as it enters the die cavity is more than the clearance between the punch and die, it has to be reduced by a deformation called ironing. By controlling the clearance, C, ironing produce a cup with a constant wall thickness. Because of the volume constancy, an ironed cup will be longer than a cup produced with a large clearance. Thus, ironing can correct earing that occurs in deep drawing. Ironing Ironing to achieve a more uniform wall thickness in a drawn cylindrical cup.
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