This paper contains experimental results for mechanical characteristics of a mild steel sheet. The determination of the forming limit stress curve for solicitation along two direction is drawn. Study of prestrain influence from the limit stress diagram is presented. | Vietnam Journal of Mechanics, NCST of Vietnam Vol. 21 1 1999, No 1 {57- 64} STUDY OF PRESTRAIN INFLUENCE FOR THE PREDICTION OF THE NECKING OCCURRENCE NGUYEN NHAT THANG Hanoi University of Technology ABSTRACT. This paper contains experimental results for mechanical characteristics of a mild steel sheet. The determination of the forming limit stress curve for solicitation along two direction is drawn. Study of prestrain influence from the limit stress diagram is presented. - §1. Introduction The industrial blank usually show complex strain-paths, so the use of forming limits diagram, draw for rectilinear strain-paths can not allow to predict the success of a drawing operation in press shop. In some works we have seen the existence of a single forming limits stress curve which is independent on the strain path shape, while the classical forming limit curves are dependent on them !or an orthotropic sheet-steel (2], (3], (6]. In previous work [11], we have shown the strain path influence on the forming limit diagram. Here, we determine the stress states at the onset of necking for rolling and transverse directions of sheet-steel. Then we present the forming limit diagrams calculated for different prestrains. §2. Determination of the forming limits stress curves for solicitation along rolling and transverse directions The experimental results have been obtained from a mild steel sheet named SE DDR-T700 of thickness . - Elastic limits along the rolling and transverse directions O'Q = 166; u 90 = 170 (MPa) - Landkford's coefficients along two directions: ro = ; rgo = 57 Variation of two coefficients is presented on Fig. 1 r 2 0 0 r0 = 1,5 0,5 ·o ~----+-----~----~-----+----~~----~--- 5 1D f5 20 25 30 % f . Variation of anisotropic coeficient - Hill'~ coefficients [5] F = ; G = ; H = (MPa- 112 ). The forming limit diagrams are determined by means of a ¥arciniak's device. The strain at the onset of necking are
