Investigations Of Superelastic Behavior Of NiTi Shape Memory Alloy Under Complex Loading

NiTi shape memory alloy is one of the most developed driving components in intelligent materials. They have wide applications due to their unique shape memory effect and superelastic properties. Up to now, a lot of work about NiTi has been undertaken, but most found in literatures has been performed only on wires and thus are one-dimension. In fact, because of the complexities of geometric configures and the recombination actions of loading, engineering components are often in multi-axial loading condition, therefore, it is necessary to study the mechanical properties of NiTi alloy under complex loading.In the thesis, theoretic analysis, numerical model, model verified by experiments were employed to carry out some researches on the superelasticity. They are as follows:1. Based on a series of experiments, the effects of testing temperature, cyclic loading-unloading, displacement rates and stress amplitude on the superelasticity and the fatigue life of the nearly equi-atomic NiTi alloy have been studied.2. The biaxial fatigue behaviors of NiTi thin-walled tubes under uniaxial tension, pure torsion, biaxial proportional loading and nonproportional loading with the different phase angles have been studied. With the same axial and shear strain level, the influences of different loading mode and phase angle on the biaxial fatigue life of NiTi alloy have been analyzed.3. Based on the model of Gall et al, a new modified model has been developed, and the model has been implemented into the finite-element program ABAQUS/standard. Comparing the experimental and numerical results, it can be concluded that the model can capture the essential features of our SMA. In addition, because the only parameters estimated from an analytical standpoint are the terms in interaction energy matrix, H^mn, the effect of varying H^mn on the predicted polycrystalline stress-strain behaviors has been studied. Through discussion, the values C and I are chosen. 4. Using the current model, the influence of crystallographic texture on the stress-strain asymmetric behavior of polycrystalline NiTi shape memory alloy under tension and compression has been researched. Furthermore, a series of calculations of biaxial tension (compression) and proportional tension (compression)-torsion for the NiTi polycrystalline has been performed to determine the initial surface of phase transformation in the stress space which determines the onset of stress induced martensitic phase transformation. The main purpose of this study is to numerically illustrate the effects of crystallographic texture.5. Based on the new developed model, the effect of surface roughness on the superelastic behaviors of NiTi alloy has been studied. Periodic surface notches with four different notch depths as surface roughness were introduced into the models. For comparison, simulations were also performed on smooth models and models with a single notch.6. The model has been used to predict the superelastic response of the thin-walled NiTi tubes under combined tension-torsion loading. In order to validate the model, thin-walled tube has been tested under biaxial loading. From the comparison between the simulated and experimental results, it is found that the model is quite good in predicting the overall loading-unloading process of the SMA. Both the experimental and simulated results demonstrate the significant differences in the superelastic responses under different biaxial loading paths.