THERMODYNAMICALLY CONSISTENT THERMOMECHANICAL MODELING OF KINETICS OF MACROSCOPIC PHASE TRANSITION IN SMA USING PHASE FIELD THEORY

Experimental observations have shown that polycrystalline NiTi wires, strips and tubes develop inelastic strain via nucleation and growth of macroscopic martensitic domains under mechanical loading. These domains consist of almost fully-transformed grains, which result from micro-domains that are formed at the grain-size level. Evolution of these macroscopic domains via transformation front propagation is accompanied by complex interactions between mechanical work, latent heat, heat transfer, and loading rates. These interactions could affect the performance reliability or controllability of the material when deployed. Therefore, modeling effort is necessary to describe these interactions so as to improve the design and application of SMA devices. A 3-D thermodynamically consistent thermomechanical macroscopic model, which is able to describe phase transition kinetics in shape memory alloys, is proposed in this work. The model employs a Ginzburg-Landau-type kinetic law resulting from the notion of configurational forces associated with the gradient of an order parameter (a field variable). As a first attempt to demonstrate the capability of the model, 1-D simplification of the model is implemented within a finite element framework. Kinetics of phase transition and the effects of heat production associated with the thermomechanical coupling on the stress-strain response of an SMA are examined. In particular, the roles of external loading rate and heat transfer boundary conditions on the stress-strain response are investigated for displacement-controlled loading. Results obtained are in good agreement with experimental trends.