The constitutive behavior and failure of ductile materials are described in the present work for a general case of\nloading in terms of the secant moduli, which depend on the first (dilatational) and second (deviatoric) strain\ninvariants. This approach exposes the distinct behavior of materials to the equivalent normal and shear stresses. The\nsecant moduli enable the establishment of two (instead of one) constitutive equations necessary for the complete\ndescription of these materials. Emphasis is given in the accuracy of the resulting constitutive equations in terms of\ntheir predictions relative to actual experimental data for two materials systems. Failure predictions, according to\nT-criterion, are derived for two materials under combined torsion and tension, which are in good agreement\nwith experimental data. Finally, the associated failure surfaces in a stress space are presented as well.
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