This article presents the results of a parametric study using the finite element method of a full dome hemispherical punch stretching process that is commonly used in the experimental determination of forming limit diagrams (FLDs) of sheet metal. The simulations show the formation of necks, which appear to be a localized neck rather than diffuse necks. With the aid of the parametric study, the roles of friction, strain hardening exponent, strain rate sensitivity, and anisotropy are analyzed for the given punch geometry and their effect on the FLDs explained. The results of finite element simulations done with an imperfection in the element of the sheet where the neck forms are reported and compared with Marciniak-Kuczynski simulations. The simulations are also compared with some experimental results available in the literature. © 2012 Taylor and Francis Group, LLC.

Ramarathnam Krishna Kumar

Department of Engineering Design

Experimental determination

Finite element simulations

FORMING LIMIT DIAGRAMS

Parametric study

Punch stretching

Strain rate sensitivity

Strain-hardening exponent

STRETCHABILITY

finite element method

Sheet metal

Strain hardening

Thermal barrier coatings

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IIT Madras

Finite element simulation of sheet stretched over a hemispherical punch: A parametric study

Mechanics of Advanced Materials and Structures

Forming process in sheet metal working refers to the manufacturing of products by deforming the material in the required die cavity without being broken or local necking. Defects such as wrinkling, local necking, and cracking may occur during the process. These are the results of either reduction or increase in sheet thickness while the material is being stretched inside die cavity. Prevention of defects is important during forming and is challenging if the thicknesses of forming sheets is less than 100 microns. These sheets are applied in the manufacturing of components having two dimensions of less than millimeter size, an operation known as micro-forming. Miniaturization has attracted many fields such as medical instrumentation, mobile manufacturing, automotive, and aerospace. Appropriate strain distribution in forming ensures products with better quality. Forming limit diagram (FLD) has been invented to plot major and minor strains along with forming limit curves. In other words, FLD shows wrinkling, localized necking as well as safe zones with various strain paths, and it proves to be a basic tool for understanding rupture characteristics of the material. There are three approaches for plotting FLD such as empirical (analytical), numerical, and experimental. The research work presents experimental micro-forming investigations on an ultra-thin brass sheet of 90 microns, to plot FLD, using the limit dome height test according to ASTM E2218-02 standard. The experimental setup was designed and developed to micro-form standard specimens applying 15 mm hemispherical punch. FLD plotted with experimental and empirical approaches shows agreement on failure limit curve.

Materials and Manufacturing Processes

Micro Forming Analysis of Ultra-Thin Brass Foil

Journal	Mechanics of Advanced Materials and Structures
ISSN	15376494
Open Access	No