Forming limit diagram of auto-body steel sheets for high-speed sheet metal forming

Abstract This paper is concerned with the uniaxial tensile properties and formability of steel sheets in relation to the strain rate effect. The elongation at fracture for CQ increases at a high strain rate while the elongation at fracture for DP590 decreases slightly in relation to the corresponding value for a quasi-static strain rate. The uniform elongation and the strain hardening coefficient decrease gradually when the strain rate increases. The r-value of CQ and DP590 was measured with a high-speed camera in relation to the strain rate. The r-value is slightly sensitive to the strain rate. Static forming limit curves (FLCs) and high-speed FLCs were constructed with the aid of punch-stretch tests with arc-shaped and square-shaped specimens. In addition, a high-speed crash testing machine with a specially designed high-speed forming jig was used for the high-speed punch-stretch tests. Compared with the static FLC, the high-speed FLC of CQ is higher in a simple tension region and lower in a biaxial stretch forming region. The high-speed FLC for DP590 decreases in relation to the static FLC throughout the entire region. The elongation at fracture appears to be closely related to the simple tension region of the FLC. The shear fracture is observed from SEM images of specimens tested in the biaxial stretch forming region under the high-speed forming condition. The dimples indicating the shear fracture have elongated horseshoe shape. The high-speed FLC is lower than the static FLC in the biaxial stretch forming region because the shear fracture induces the decrease of ductility. The results confirm that the strain rate has a noticeably influence on the formability of steel sheets. Thus, the forming limit diagram of high-speed tests should be considered in the design of high-speed sheet metal forming processes.

[1]  Woei-Shyan Lee,et al.  Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2—Microstructural study , 2002 .

[2]  Tetsuo Naka,et al.  The effects of temperature and forming speed on the forming limit diagram for type 5083 aluminum-magnesium alloy sheet , 2001 .

[3]  P. Hu.  Rate-dependent quasi-flow corner theory for elastic/visco-plastic materials , 2004 .

[4]  Hoon Huh,et al.  Dynamic tensile characteristics of TRIP-type and DP-type steel sheets for an auto-body , 2008 .

[5]  M. L. Wenner,et al.  Strain and strain-rate hardening effects in punch stretching of 5182-0 aluminum at elevated temperatures , 1979 .

[6]  Lihui Lang,et al.  Multi-layer sheet hydroforming: Experimental and numerical investigation into the very thin layer in the middle , 2005 .

[7]  Chung-Ho Lee,et al.  Identification of Forming Limits of Sheet Metals for Automobile Parts by Asymmetric Deep-drawing Experiments , 1998 .

[8]  Kwansoo Chung,et al.  Spring-back evaluation of automotive sheets based on isotropic-kinematic hardening laws and non-quadratic anisotropic yield functions: Part II: characterization of material properties , 2005 .

[9]  Fahrettin Ozturk,et al.  Experimental and numerical analysis of out-of-plane formability test , 2005 .

[10]  On the determination of Hill’s plastic strain ratio , 1983 .

[11]  A. Korhonen,et al.  Forming and fracture limits of austenitic stainless steel sheets , 2008 .

[12]  K. S. Raghavan,et al.  Recent Progress in the Development of Forming Limit Curves for Automotive Sheet Steels , 1992 .

[13]  Akhtar S. Khan,et al.  Behaviors of three BCC metal over a wide range of strain rates and temperatures: experiments and modeling , 1999 .

[14]  Gorton M. Goodwin,et al.  Application of Strain Analysis to Sheet Metal Forming Problems in the Press Shop , 1968 .

[15]  Hoon Huh,et al.  High speed tensile test of steel sheets for the stress-strain curve at the intermediate strain rate , 2009 .

[16]  Hoon Huh,et al.  Investigation of Elongation at Fracture in a High Speed Sheet Metal Forming Process , 2009 .

[17]  S. P. Keeler.  Plastic instability and fracture in sheets stretched over rigid punches , 1961 .

[18]  R. Hill.  The mathematical theory of plasticity , 1950 .

[19]  Glenn S. Daehn,et al.  Formability of steel sheet in high velocity impact , 2005 .

[20]  Joachim Danckert,et al.  Determination of the plastic anisotropy r in sheet metal using automatic tensile test equipment , 1996 .

[21]  Influence of strain-rate sensitivity on necking and instability in sheet metal forming , 1999 .

[22]  Kwansoo Chung,et al.  Spring-back evaluation of automotive sheets based on isotropic–kinematic hardening laws and non-quadratic anisotropic yield functions, part III: applications , 2005 .

[23]  John W. Hutchinson,et al.  Influence of strain-rate sensitivity on necking under uniaxial tension , 1977 .

[24]  G. Daehn,et al.  Effect of sample size on ductility in electromagnetic ring expansion , 1996 .

[25]  Yongnam Kwon,et al.  Forming limit of AZ31 alloy sheet and strain rate on warm sheet metal forming , 2008 .

[26]  Z. Marciniak,et al.  Limit strains in the processes of stretch-forming sheet metal , 1967 .

[27]  Amit K. Ghosh.  Tensile instability and necking in materials with strain hardening and strain-rate hardening , 1977 .

[28]  Glenn S. Daehn,et al.  Hyperplasticity: Increased forming limits at high workpiece velocity , 1994 .