EXPERIMENTAL IDENTIFICATION OF PLASTICITY AND FAILURE LAWS OF ANISOTROPIC LOW-CARBON SHEET STEEL FOR COLD FORMING MODELING

To formulate a mathematical model of cold sheet forming of thin-walled products, an experimental method is proposed for identifying defining relations of plastic yield of anisotropic sheet low-carbon steel DC04EK. The experimental studies were done on a Zwick Z100/SN5A multipurpose floor-mounted test stand with a maximal testing force of 100 kN at a temperature of 20°С. Longitudinal strain was registered with a Multisens long-run longitudinal strain gage. To measure transversal strain, a manual DMS strain gage was used, which was located between the test rods of a Multisens extensionmeter. To determine the elasticity modulus, yield strength, plastic anisotropy coefficient, to construct a deformational hardening curve and to determine points of a limiting deformation curve, specimens in the form of straps with parallel edges, cut out of a sheet at angles of 0, 45 and 90° relative to the rolling direction, were tested in uniaxial tension at room temperature. Based on the obtained data, parameters of Barlat's yield function Yld 2000-2d were determined. To describe a deformational hardening curve with a yield plateau, a six-parametric approximation is proposed. It is found that plastic anisotropy coefficients are practically independent of deformation up to elongation of 30%. The limiting strain curve is constructed using Martsinyak - Kudzinski method relating the limiting state with a critical growth of localization of deformation under biaxial tension; to this end, an experimentally certified model of plasticity was used, as well as experimental data on limiting strains in a specimen loaded in uniaxial tension.