Thermo-mechanically Coupled Macro- And Micro-scopic Cyclic Constitutive Models Of NiTi Shape Memory Alloy

NiTi shape memory alloy (SMA) has been widely used in aerospace, bio-medical and civil engineering fileds due to its excellent super-elasticity, shape memory effect, bio-compatibility and damping capability. In practical applications, the components and devices made by the NiTi SMA are often subjected to a cyclic thermo-mechanical loading. During the cyclic deformation of NiTi SMA, functional degeneration (degeneration of super-elasticity and shape memory effect) and thermo-mechanical coupling effect are two important issues which should be taken into account. Thus, in order to simulate and predict the cyclic deformation of the structures and devices made by NiTi SMA under complex thermo-mechanical loading condition, it is very necessary to construct a constitutive model considering the functional degeneration and thermo-mechanical coupling effect simultaneously. In recent years, based on experimental observations, many constituve models have been constructed in different scales. However, in the macro-scale the existing models only considered one of the two issues mentioned above, and the coupling effect of the two issues has not been reasonably involved yet. Moreover, all the macro-scopic models adopted a phenomenological approach to describe the functional degeneration of NiTi SMA, the physical nature of the functional degeneration has not been considered yet. In the micro-scale, the existing models foucsed only on the thermo-mechanical deformation in one loading-unloading cycle. The functional degeneration can not be described reasonably since its micro-mechanism has not been considered yet.It is seen that the the existing constitutive models (including macro-phenomenological and micromechanical ones) are still insufficient. Therefore, this thesis will construct thermo-mechanical coupled cyclic constitutive models of NiTi SMA in macroscopic and microscopic scales. The specific contents are listed as follows:(1) In the macroscopic sacle, by summarizing the existing macro-microscopic experimental observations on NiTi SMA, the physical nature of functional degeneration is proposed, i.e., the interaction between martensite transformation and defects. Based on the framework of thermodynamics, a macroscopic thermo-mechanical coupled constitutive model is constructed to describe the rate-dependent functional degeneration of NiTi SMA. The proposed model is verified by simulating and predicting the cyclic deformation of super-elastic NiTi SMA at different loading rates. Furthermore, the temperature-dependent functional degeneration is predicted by the model.(2) In the representative volume element (RVE) of single crystal, different mechanisms of inelastic deformation, i.e., martensite transformation, martensite reorientation, martensite detwinning, austenite plasticity and martensite plasticity are introduced by considering certain crystallographic orientation relationships. Based on the framework of thermodynamics, a thermo-mechanical coupled constitutive model is constructed in the single crystal scale. By using the explicit scale-transition rule and the assumption of uniform temperature field, the single crystal model is extended to the polycrystalline version. The proposed model is verified by simulating and predicting the uniaxial and non-proportional multiaxial thermo-mechanical deformation of NiTi SMA at different temperatures, loading rates and stress levels.(3) By summarizing the existing macro-microscopic experimental observations, a new mechanism of inelastic deformation is proposed, i.e., martensite reorientation induced plasticity. Based on the established crystal plasticity constitutive model in section (2), the mechanisms of inelastic deformation contributing to the functional degeneration of NiTi SMA, i.e., transformation induced plasticity, reorientation induced plasticity and accumulation of residual martensite are quantitatively introduced into the RVE of single crystal. Based on the framework of thermodynamics, a thermo-mechanical coupled cyclic constitutive model is constructed in the single crystal scale and extended to the polycrystalline version by using the explicit scale-transition rule and the assumption of uniform temperature field. The proposed model is verified by simulating and predicting the uniaxial, non-proportional multiaxial and rate-dependent cyclic thermo-mechanical deformation of NiTi SMA.(4) In the RVE of single crystal, two mechanisms of inelastic deformation, i.e., martensite transformation and transformation induced plasticity are considered.24 martensite variants are regarded as the ellipsoidal inclusions with the same geometrical morphology but different orientations and embedded in the austenite matrix with anisotropic elasticity. The mean stress fields in austenite phase and each martensite variant are obtained by the Mori-Tanaka homogenization method. Based on the assumption of instantaneous domain growth, the forward and reverse inheritances for the transformation induced plastic strain and dislocation density are proposed. Based on the framework of thermodynamics, a cyclic micromechanical constitutive model is constructed for NiTi SMA single crystal. The proposed model is verified by simulating and predicting the anisotropic cyclic deformation of NiTi SMA single crystal.