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Research Papers

Investigation of Strain Gradients and Magnitudes During Microbending

[+] Author and Article Information
Lijie Wang, Yannis Korkolis

Department of Mechanical Engineering,  University of New Hampshire, 33 Academic Way, Durham, NH 03824

Brad L. Kinsey1

Department of Mechanical Engineering,  University of New Hampshire, 33 Academic Way, Durham, NH 03824bkinsey@unh.edu

1

Corresponding author.

J. Manuf. Sci. Eng. 134(4), 041011 (Jul 24, 2012) (9 pages) doi:10.1115/1.4007066 History: Received July 14, 2011; Revised June 10, 2012; Published July 24, 2012; Online July 24, 2012

Sheet metal forming of parts with microscale dimensions is gaining importance due to the current trend toward miniaturization, especially in the electronics industry. In microforming, although the process dimensions are scaled down, the polycrystalline material stays the same (e.g., the grain size remains constant). When the specimen feature size approaches the grain size, the properties of individual grains begin to affect the overall deformation behavior. This results in inhomogeneous deformation and increased data scatter of the process parameters. In this research, the influence of the specimen size and the grain size on the distribution of plastic deformation through the thickness during a three-point microbending process is investigated via digital image correlation (DIC). Results showed that with miniaturization, a decrease in the strain gradient existed which matched previous research with respect to microhardness measurement.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Examples of microformed parts [1]

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Figure 2

Flow stress decreases with decreasing specimen size due to increase in share of surface grains [6]

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Figure 3

Variation of tension and bending yield strength with varying thickness to grain size ratio (ϕ) [7]

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Figure 4

Loading stage with specimen and tooling shown

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Figure 5

Schematic of microbending tooling

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Figure 6

Setup of camera, lens, and loading stage on a steel board

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Figure 7

Full-field, strain distribution data from DIC analysis

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Figure 8

Strain distribution at the bend area directly under the punch through the thickness of (a) 1.588 mm, (b) 0.5 mm, and (c) 0.25 mm sheets

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Figure 9

Absolute strain distribution through the thickness for (a) fine, (b) medium, and (c) coarse grained structures. The normalized axis is with respect to the sheet thickness.

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Figure 10

Absolute strain difference between 1.588 mm and 0.25 mm specimens at outer surface for varying grain structures

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Figure 11

Strain gradient (i.e., slope) changes for 1.588 mm, 0.5 mm, and 0.25 mm thickness specimens with varying grain structure

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Figure 12

No shape change with miniaturization

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Figure 13

Normalized peak bending force versus specimen size curves for (a) fine, (b) medium, and (c) coarse grained structures. The displacement was normalized by the sheet thickness and the force as described in the text. Note the scales on the ordinate axes are different for each plot.

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Figure 14

Normalized peak bending force versus specimen size curves for fine, medium, and coarse grained structures

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Figure 15

Bending force versus displacement data for 0.25 mm thickness specimen showing Hall-Petch effect

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Figure 16

Previous results [11] obtained by hardness measurement for (a) 1.625 mm and (b) 0.25 mm thickness specimens with various grained structures

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Figure 17

Previous results [11] obtained by hardness measurement for specimens with (a) fine and (b) coarse grained structures

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Figure 18

Strain distribution of (a) 20 μm and (b) 65 μm with various specimen sizes

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Figure 19

Strain distribution data for (a) 1.588 mm thickness specimen with fine grain structure and (b) 0.5 mm thickness specimen with coarse grain structure

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Figure 20

Maximum strain on tension surface for various specimen thicknesses and grain structures

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