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Tuesday, May 18, 2010 - 2:15 PM

Modeling and Experimental Study of Simultaneous Creep, Plasticity and Transformation of High Temperature Shape Memory Alloys During Cyclic Actuation

U. Desai, J. Monroe, P. Kumar, G. Chatzigeorgiou, D. C. lagoudas, Texas A & M University, College Station, TX; I. Karaman, Texas A&M University, College Station, TX; R. Noebe, G. Bigelow, NASA Glenn Research Center, Cleveland, OH

High Temperature Shape Memory Alloys represent a class of Shape Memory Alloys (SMAs) with transformation temperatures greater than 100C. Thermally induced transformation behavior of these alloys under load has been widely studied to understand their actuation performance. More recent efforts have focused on improving the formability of these alloys. As a consequence of their high transformation temperatures, the HTSMAs can be exposed to a temperature regime where creep behavior can occur simultaneously during the transformation. The creep behavior of ternary NiTiPd and the interaction between transformation and creep occurring simultaneously has been recently studied and a thermodynamic model was developed to capture this interaction. However the experimental and modeling effort were limited to one actuation cycle. The present work extends the effort to capture cycling actuation behavior of HTSMAs undergoing simultaneous creep and transformation. For the thermomechanical testing, a high temperature test setup was assembled on a MTS frame. Preliminary calorimetry and uniaxial tests were conducted in austenite and martensite phases and temperatures and stress levels for the creep tests were chosen based on the test results. Standard creep tests and constant stress thermal cycling tests for multiple cycles at different heating/cooling rates were conducted. The model proposed in is extended in order to take into account a) the effect of multiple thermal cycling on the generation of plastic strains due to transformation (TRIP strains) and b) both primary and secondary creep effects. The model calibration is based on the test results. The creep tests and the uniaxial tests are used to identify the viscoplastic behavior of the material. The parameters for the SMA properties, regarding the transformation and transformation induced plastic strain evolutions, are obtained from the material phase diagram and the thermomechanical tests. The model is validated by predicting the material behavior at different thermomechanical test conditions.

Summary: The present work extends the effort to capture cycling actuation behavior of HTSMAs undergoing simultaneous creep and transformation. For the thermomechanical testing, a high temperature test setup was assembled on a MTS frame with the capability to test up to temperatures of 600C. Preliminary calorimetry and uniaxial tests were conducted in austenite and martensite phases and temperatures and stress levels for the creep tests were chosen based on the test results. Standard creep tests and constant stress thermal cycling tests for multiple cycles at different heating/cooling rates were conducted. The model proposed in [10] is extended in order to take into account a) the effect of multiple thermal cycling on the generation of plastic strains due to transformation (TRIP strains) and b) both primary and secondary creep effects. The model calibration is based on the test results. The creep tests and the uniaxial tests are used to identify the viscoplastic behavior of the material. The parameters for the SMA properties, regarding the transformation and transformation induced plastic strain evolutions, are obtained from the material phase diagram and the thermomechanical tests. The model is validated by predicting the material behavior at different thermomechanical test conditions.