A Damage Constitutive Model Of Coupled Plasticity-Creep For Casting Magnesium Alloy And Applications

As one of the best light-weight structural materials with high strength-to-weight ratio and excellent mechanical properties, magnesium alloys have been receiving extensively attention of researchers of mechanics and materials. However, casting magnesium alloys usually contain many voids and inclusions and microstructures induced during loading processes, which may result in reductions of creep resistance and the life of the materials. Because the deformation of an magnesium alloy usually involves plasticity, rheology, damage and their interaction, either the experimental or theoretical investigation to the constitutive behavior encounters more complicity. The uniaxial creep magnesium alloy AM50 under elevated temperature and different cyclic tensile stress is investigated with an MTS810 material testing system. Experiment shows that the creep rate strongly depends on applied stress and temperature. The damage of the magnesium alloy and its mechanism is analyzed. Making use of irreversible thermodynamics with internal variables, a visco-elastoplastic constitutive model is proposed by introducing generalized time defined in the space of irreversible strain and Newtonian time. A damage evolution based on a spherical void model for mixed hardening materials is obtained by means of Gurson's model. The damage evolution law is embedded in the adopted constitutive equation, and a visco-elastoplastic damage constitutive model is obtained for coupled plasticity and creep of casting magnesium alloy. The proposed model does not use the concept of yield surface and the corresponding loading/unloading rule, and describe unitedly the elastic, inelastic and viscous behavior of materials, which may bring convenience to the corresponding numerical analysis. The proposed model is also a united description for the coupled viscous and plastic deformation of materials, in which both viscous and plastic deformation are regarded as irreversible deformation related to thermal activation. The corresponding numerical algorithm and computational code are developed, and the response of AM50 subjected to cyclic stress at elevated temperature is analyzed. It shows that the computed results agree well with the experimental observation. The phenomena of creep acceleration and reversion could be well described with the model. In order to verify the validity of the developed approach in the case of multiaxial nonproportional loading, the coupled plasticity and creep of 304 stainless steel subjected to biaxial stress at elevated temperature is analyzed. Comparison between the computed and experimental results shows satisfactory agreement, which also demonstrates the validity of the proposed model and the corresponding numerical approach.