A three-phase SMA model based on transformation surfaces

Self-sensing is one of the key features of intelligent materials. Thermal Shape Memory Alloy (SMA) wires change their electrical resistance during phase transformation from martensite to austenite and vice versa. During cooling, the intermediate R-Phase may form, which has a significant impact on electrical resistance. SMA models incorporating all three phase fractions are required for the design and control of advanced SMA actuators. This paper investigates the feasibility of a three-phase SMA model that accurately captures both strain and resistance behavior. Based on experimental data from a commercial SmartFlex ® wire, the transformation processes between the three material phases are identified. The proposed model uses phase transformation surfaces to accurately represent stress-dependent changes in transformation characteristics. It is shown how parallel transformation processes that compete for one phase fraction can be modeled. A parameterization process is suggested that uses the measured mechanical contraction and electrical resistance at different stress levels as well as the measured transformation enthalpies of the SMA wire. Model data are compared with experimental results. It is analyzed up to which stress level the R-phase has an impact on electrical resistance, and what role it plays in today’s self-sensing schemes.