Research Papers

Analysis of Micro/Mesoscale Sheet Forming Process by Strain Gradient Plasticity and Its Characterization of Tool Feature Size Effects

[+] Author and Article Information
Linfa Peng

Shanghai Key Laboratory of Digital Manufacture
for Thin-walled Structures,
Shanghai Jiao Tong University,
Shanghai 200240, China
e-mail: penglinfa@sjtu.edu.cn

Peiyun Yi, Peng Hu

Shanghai Key Laboratory of Digital Manufacture
for Thin-walled Structures,
Shanghai Jiao Tong University,
Shanghai 200240, China

Xinmin Lai

Shanghai Key Laboratory of Digital Manufacture
for Thin-walled Structures,
Shanghai Jiao Tong University,
Shanghai 200240, China
State Key Laboratory
of Mechanical System and Vibration,
Shanghai Jiao Tong University,
Shanghai 200240, China
e-mail: xmlai@sjtu.edu.cn

Jun Ni

Department of Mechanical Engineering
and Applied Mechanics,
University of Michigan,
Ann Arbor, MI 48109

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received April 13, 2014; final manuscript received November 25, 2014; published online December 19, 2014. Assoc. Editor: Chengying Xu.

J. Micro Nano-Manuf 3(1), 011006 (Mar 01, 2015) (9 pages) Paper No: JMNM-14-1027; doi: 10.1115/1.4029262 History: Received April 13, 2014; Revised November 25, 2014; Online December 19, 2014

Conventional material models cannot describe material behaviors precisely in micro/mesoscale due to the size/scale effects. In micro/mesoscale forming process, the reaction force, localized stress concentration, and formability are not only dependent on the strain distribution and strain path but also on the strain gradient and strain gradient path caused by decreased scale. This study presented an analytical model based on the conventional mechanism of strain gradient (CMSG) plasticity. Finite element (FE) simulations were performed to study the effects of the width of microchannel features. Die sets were fabricated and micro/mesoscale sheet forming experiments were conducted. The results indicated that the CMSG plastic theory achieves better agreements compared to the conventional plastic theory. It was also found that the influence of strain gradient on the forming process increases with the decrease of the geometrical parameters of tools. Furthermore, the feature size effects in the forming process were evaluated and quantitated by the similarity difference and the similarity accuracy. Various tool geometrical parameters were designed based on the Taguchi method to explore the influence of the strain gradient caused by the decrease of tool dimension. According to the scale law, the difference and accuracy of similarity were calculated. Greater equivalent strain gradient was revealed with the decrease of tool dimension, which led to the greater maximum reaction force error due to the increasing size effects. The main effect plots for equivalent strain gradient and reaction force indicated that the influence of tools clearance is greater than those of punch radius, die radius, and die width.

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Fig. 1

Diagram of the experiment setup

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Fig. 2

Die inserts used in experiments

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Fig. 3

Reaction force–displacement curves

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Fig. 4

The flow chart of the UMAT

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Fig. 5

Schematic diagram of micro/mesosheet forming process

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Fig. 6

Comparison of the force–displacement response for numerical and experimental results

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Fig. 7

Equivalent plastic strain and equivalent plastic strain gradient

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Fig. 8

The contact point and the strain gradient contribution to the flow stress

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Fig. 9

Diagram of geometric parameters for sheet forming process

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Fig. 10

von Mises stress distribution and reaction force base on conventional method

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Fig. 11

Results based on strain gradient theory

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Fig. 12

Main effect diagram of the equivalent strain gradient

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Fig. 13

Main effect diagram of maximum reaction force error



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