ORIGINAL ARTICLE Effect of mechanical and fractographic properties on hole expandability of various automobile steels during hole expansion test R. Narayanasamy & C. Sathiya Narayanan & Palani Padmanabhan & T. Venugopalan Received: 17 March 2009 / Accepted: 7 July 2009 / Published online: 23 July 2009 # Springer-Verlag London Limited 2009 Abstract Hole expandability is a vital formability parameter for automobile body parts that are subjected to deep drawing conditions. In this paper, the influence of mechanical and fractographic properties on hole expansion ratio has been studied and reported. The hole expansion test has been carried out for seven different automotive steel sheets of varying thicknesses. Hole expansion ratio expressed in terms of hole expansion percentage is strongly influenced by the microstructure of the sheet metal. Hole expansion test experiments were performed on flat circular plates with a hole in the center to investigate the fracture behaviors of various automobile steels such as microalloyed, C–Mn, high- strength I.F. (SPRC-35) of three grades, extra galvannealed I.F., and HSLA (E-36) steel sheets. In the hole expansion test, deformation by lip is caused when the punch expands the hole. Fracture by petalling occurs when the holes in the sheets are completely pierced by the punch. Large circum- ferential strains are accommodated in the deforming sheet material. The mechanical properties, namely, strain harde- ning exponent (n), normal anisotropy ( r), formability parameter (n r), and other properties, namely, Mohr's circle shear strains (γ 31 and γ 12 ), strain triaxiality factor (T), and stress triaxiality factor (T o ), affected the hole expansion ratio of different steels tested. Similarly, the fractographic factors, such as void size in micrometers, void area fraction, and d- factor, affect the hole expansion ratio. Among the steel sheets tested, extra galvannealed I.F. steel possesses the highest hole expansion ratio. Keywords Hole expansion test . Automobile steels . Microstructure . Mechanical and fractographic properties 1 Introduction Sheet metal forming is a near net shape manufacturing process by which various components used for automotive industry are made from sheet metals. The increasing competition in the automotive industry is forcing compa- nies to constantly look for improvements in material design and performance. The trend of applying high-strength steels such as microalloyed, C–Mn, high-strength I.F. (SPRC-35) of three grades, extra galvannealed I.F., and HSLA (E-36) steel sheets, due to their low weight-to-strength ratio, which leads to better fuel economy and formability [1, 2] has developed in recent years. Each of these benefits improves the overall performance and quality of the vehicle. Furthermore, these steels have the potential to affect cost while improving performance. Concerning materials, re- search has been aimed at understanding the effect of material composition and behavior on sheet formability. This necessitates evaluation of formability of the sheets. Considerable attention has been paid by researchers to studying sheet metal formability through hole expansion ratio. The hole expansion ratio is assessed by the hole expandability test. The hole expansion ratio is influenced R. Narayanasamy (*) : C. S. Narayanan Department of Production Engineering, National Institute of Technology, Tiruchirappalli 620015 Tamil Nadu, India e-mail: narayan@nitt.edu C. S. Narayanan e-mail: csathiyanarayanan@yahoo.co.in P. Padmanabhan Department of Mechanical Engineering, R.V.S College of Engineering and Technology, Dindigul 624005 Tamil Nadu, India e-mail: ajaypalani@yahoo.co.in T. Venugopalan The Tata Iron and Steel Company Limited, Jamshedpur 831001 Jharkhand, India e-mail: tvenugopalan@crm.tatasteel.com Int J Adv Manuf Technol (2010) 47:365–380 DOI 10.1007/s00170-009-2201-x