International Journal of Scientific & Engineering Research, Volume 4, Issue 5, May-2013 734 ISSN 2229-5518 IJSER © 2013 http://www.ijser.org Stress Based Forming Limit Krunal K. Rathod, Mehul D. Gohil, Prof. D. R. Shah Abstract— Metal forming processes is finding increasing acceptance as a manufacturing process for various engineering components. In need of higher performance and due to economic and ecological reasons, lightweight construction is the key factor to success, mainly not only in the transportation sector, but also in general engineering, machine tools and architecture. This seminar is deals with the stress based forming limit diagram. With the use of forming limit diagram one can predict the ability of metal to form in various shapes. Here in this seminar comparison between strain and stress based diagram is done. And in addition with this, finding the advantage of stress based FLD to Strain based FLD. In processes like Hydroforming and flanging or multistage processes the stress based forming limit diagram is given better prediction of onset of necking compare to strain based forming limit diagram. In case of tube Hydroforming of the square cross section extended stress based forming limit diagram is introduced. Index Terms— Failure Criterion, Finite Element Analysis, Forming Limit Diagram, Forming Limit Stress Diagram. ——————————  —————————— 1 INTRODUCTION IE engineers mostly use finite element analyses in the metal forming industry to find the formability of sheet- metal products prior before the dies are built in order to save money in die build and tryout costs, as well as to address manufacturability issues early in the product design cycle. One of the most important objectives in this assessment is to avoid necking and fracture of the sheet metal. Although the finite element method FEM does not directly predict whether or not the sheet metal will neck or tear during the forming of the product, it does predict the metal flow and the develop- ment of stress and strains throughout the forming process. The engineer determines the forming severity by comparing the predictions of the FEM to a forming limit criterion, which is a function of the sheet-metal properties and the forming history. Obviously, a critical factor in the success of FEM analysis is the reliability of this forming limit criterion. The most commonly used method of gauging form- ing severity with respect to necking is based on the forming limit diagram FLD developed by Keeler and Goodwin. The diagram is composed of a curve in strain-space defined to characterize the forming limit of the material. As long as all strains on the part fall below this forming limit curve FLC, that part will be free from necks. The forming limit is determined by forcing the material to follow linear strain paths, and measure the strain on the material just before a neck appears. As the application of the FEM was extended to analysis of hydro-forming, redraws and flanging operations, where the total strain path is significantly nonlinear, the limi- tations of the conventional FLD could no longer be ignored. Furthermore, nonlinear strain paths have been found to be much more common in the first draw die than first believed, resulting in costly errors in the assessment of forming severity [1]. Stoughton proposed a method through which, under a suitable set of constitutive assumptions, the strain-based FLC can be transferred to principal stress space [2]. It is also shown that within the scope of the constitutive assumptions, there exists a single curve in principal stress space that represents the formability limit of the sheet. Therefore, the stress-based FLC appears to be attractive to predict the onset of necking when the sheet is subjected to nonlinear load paths [3]. 2 DETERMINATION OF FLD The FLD is based upon the work of KEELER and GOODWIN where the plane strain limit is given as follows: (1) Where n is the strain hardening coefficient, and n＜0.21, t is the metal thickness in mm. The key feature of the FLD is an experimentally determined forming limit curve (FLC). The shape and location of the FLC, which define the boundary between strain states that are al- ways free of necks from those states that are prone to necking, are a characteristic of the metal that is independent of the forming process or work piece shape. Therefore, the distance between the FLC and all of the measured or predicted strain. The material properties and stress strain diagram is shown below. The strain states used to determine the strain limit are com- monly obtained via the dome test procedure, where grid markings are etched onto the surface of specimens. In these tests, various strain states are achieved by adjusting different parameters like the lubrication conditions between the sheet metal and the specimen width. The width varies at 180, 160, 140, 120, 100, 80, 60, 40 and 20 mm. Length of all specimens is D ———————————————— • Krunal K. Rathod is currently pursuing masters degree program in Me- chanical engineering in Gujarat Technological University, India, Mo. No.- +91-9725005851. E-mail: Krunal_07786@yahoo.com • Mehul D. Gohil is currently pursuing masters degree program in Mechani- cal engineering in Gujarat Technological University, India, Mo. No.-+91- 9925988434. E-mail: mehuldgohil@gmail.com • Prof. D. R. Shah is currently assistant professor in Mecahnical engineering in Gujarat Technological University, India, Mo. No.-+91-9925237030. E- mail: darshit99@yahoo.com IJSER