Constitutive Description Of Shape Memory Alloys With Phase Transformation Microstructure

Shape memory alloys (SMAs) have been receiving increasing attention in recent years, due to their particular properties under thermomechanical loading, such as ferroelasticity, shape memory effect and pseudoelastisity. These properties are related to the martensitic phase transformation and are extensively used in many fields, such as industry, aviation, national defense, instruments and medical devices, etc. The rapid increasing applications of shape memory alloys require better understanding and more accurate description of the thermomechanical behavior of SMAs, especially the behavior of SMAs subjected to multi-dimensional loading complex thermomechanical loading histories. It has been recognized that the macroscopic property of a material is strongly dependent on its microstructures. The experiments on the NiTi SMAs show significant difference in the pseudoelastic behavior as well as the microstructures of the SMA under pure torsion and under pure tension, especially a sharp descent after the onset of martensitic phase transformation and the corresponding distinct martensitic band during a tensile process. However, such significant behavior has not been successfully described.In this dissertation, the differences in the responses of a NiTi SMA microtube subjected to pure tension and pure torsion are systematically investigated. A finite element analysis is performed for the significant stress drop phenomenon, taking into account the phase-transformation microstructures and their evolution observed during tension. It is found that the physical and mechanical mechanism of the distinct stress drop can be attributed to the variation of the microstructure, i.e., the initiation and growth of the martensite band that strongly reduce the constraint between phases. A constitutive model is proposed based on that a shape memory alloy is the mixture of martensite and austenite with laminated microstructure, taking the thickness of martensitic band as an important parameter. The corresponding incremental form of constitutive model is developed, and the constitutive behavior of a NiTi microtube is analyzed and compared with experimental results. The main work and conclusions in this dissertation are as follows:(1) The progress on the research of the pseudoelastic behavior of shape memory alloys is reviewed. The physical mechanism of the difference in the macroscopic behavior during tension and torsion, as well as the evolvement of microstructure, is...