DETERMINING THE EFFECTIVE STRESS - EFFECTIVE STRAIN CURVE OF A HIGH-STRENGTH LOW-ALLOY STEEL SHEET FROM THE VISCOUS PRESSURE BULGE TEST

Abstract

The accuracy of the stress - strain relation that defines the work-hardening characteristic of sheet metals is an important factor for the aspect of accurate prediction of the sheet material behavior in forming operations from the analyses. In order to obtain that relation, there are studies suggesting bulge test in which balanced biaxial stress state forms, instead of tensile tests in which the stress state is uniaxial. Determining flow stresses at the strains higher than ones reached in uniaxial tension without any extrapolation as implemented to the uniaxial data is emphasized as the advantage of bulge test. The tests in which the bulge is formed under viscous pressure were applied to the high-strength low-alloy (HSLA) steel sheet selected (1.01 mm - H 320 LA). Grease is used as viscous medium in the tests carried out with a die with inside diameter of 275 mm. The effective stress - effective strain curve in viscous pressure bulge test was determined by a technique based on the database obtained from finite-element analysis. That curve was compared with the flow stress curves observed in the tensile tests at 00, 450, and 900 to the rolling direction and the flow stress curve at sheet thickness direction calculated from uniaxial tensile data at the three directions according to the approach of average plastic strain ratio based on the Hill-1948 anisotropic yield criterion using the equivalency of the balanced biaxial tension to the through-thickness compression.