Abstract—In this research, Forming Limit Diagrams for super- tension sheet metals which are using in automobile industry have been obtained. The exerted strains to sheet metals have been measured with four different methods and the errors of each method have also been represented. These methods have been compared with together and the most efficient and economic way of extracting of the exerted strains to sheet metals has been introduced. In this paper total error and uncertainty of FLD extraction procedures have been derived. Determination of the measurement uncertainty in extracting of FLD has a great importance in design and analysis of the sheet metal forming process. Keywords—Forming Limit Diagram, Major and Minor Strain, Measurement Uncertainty. I. INTRODUCTION ESPITE the automation that has been implemented in the various techniques for manufacturing sheet metal parts, die and part in the majority of cases is still an advanced art. The variables that enter in to the forming of sheet metal parts are numerous and their interactions are extremely complex. All of these factors (Material flow properties and ductility, die geometry and material, lubrication, press speed, blank holder pressure, etc) contribute to the success or failure of sheet metal forming process [1]. Calculation of the stress and strain contribution on the sheet metal surface by analytical or numerical methods and comparison of their results with permissible quantities lead to get away from the necking problems. The Forming Limit Diagram (FLD) represents the acceptable limits of strain in a curve with two principle surface strains. Any combination of the two surface strains and falling below the FLD curves is considered acceptable and any combination falling above it will produce failure. Failure is defined as the appearance of localized thinning or necking in sheet metals. The using of the FLD concept in predicting failures in sheet forming operations was pioneered by Keeler but in subsequent work Keeler and Goodwin used production stampings to establish the FLD for low-carbon steels, which has been called Keeler-Goodwin curve. Since the FLD represents the first local plastic M.Mahboubkhah is with Manufacturing Engineering Group, Mechanical Engineering Department, Tabriz University, Tabriz, Iran (corresponding author to provide phone: +984113392485; fax: +984113356026; e-mail: mahboobkhah@tabrizu.ac.ir). H. Fayazfar is with Metallurgy Department Sharif University of Technology, Tehran, Iran (e-mail: fayazfar@gmail.com). instability [2-5]. Thus precise construction of the FLD curve has a key importance in sheet metal forming techniques. Various methods have been proposed for measuring of the major and minor strains of sheet metal surface [6]. In this paper for measurement of the deformed circle diameters, four different methods have been employed and the error and uncertainty of measurements have been compared with together. II. PREPARATION OF THE DIE AND SHEET METAL SPECIMENS FOR IMPLEMENTATION OF DEEP DRAWING TEST To obtain the limits of major and minor strains, deep drawing test, by means of a hemispherical punch has been done. The punch-stretch apparatus and sheet metal specimens have been prepared according to ASTM standard [7]. A. The Punch-Stretch Apparatus Preparation According to ASTM standard the hemispherical punch radius amounts to 50 mm. A single stroke press is applied for deep drawing test. Another additional stroke for stripping the formed sheet metal is provided by means of four standard springs according to Fig. 1. The hemisphere bead on the upper part of the die is used to reduce required blank holder force. (a) 3D drawing of deep drawing die Determination of Measurement Uncertainty in Extracting of Forming Limit Diagrams M. Mahboubkhah, H. Fayazfar D World Academy of Science, Engineering and Technology International Journal of Industrial and Manufacturing Engineering Vol:3, No:4, 2009 438 International Scholarly and Scientific Research & Innovation 3(4) 2009 scholar.waset.org/1307-6892/14390 International Science Index, Industrial and Manufacturing Engineering Vol:3, No:4, 2009 waset.org/Publication/14390