Development of viscoplastic constitutive equation through biaxial material testing

Internal-state variables have been used to represent the deformation historyin the recently proposed viscoplastic constitutive equations. In the current study, creep tests under nonproportional loadings were used to study the relative roles played by the internal-state varaibles in the constitutive equation by tracing the strain trajectory in strain space for a given stress trajectory in stress space. An experimental approach to studying the evolution rule for the tensorial state variable is also proposed. The experimental results on 2618-T61 aluminum alloy suggest that the scalarstate variable should play a much more dominant role than the tensorial state varaible in the constitutive modeling of 2618-T61 aluminum alloy.