An automated procedure for the determination of the material constants in a constitutive equation which is used to model the multiaxial nonlinear material behavior of isotropic materials is discussed. The material model used in this research is a unified theory where the time-dependent strain (creep) and time-independent strain (plasticity) are treated as one (unified) inelastic strain. The flow equation considers the inelastic rate of strain and it is assumed that inelastic strain is present at all levels of stress. Application of the model to proportional and nonproportional biaxial loadings is presented.

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