Studies On Transfomation Mechanisms And Mechanical Properties Of Porous Shape Memory Alloys

As one of the important parts of the material science, intelligent material is developed by the combination of information and material. Shape memory alloy (SMA) has attracted great interest in the field of composite materials for the native abilities including pseudoelasticity and shape memory effect. What’s more, the concept of ’intelligent SMA composite’ has been proposed recently.The constitutive relations of these materials have been a crucial issue for application of these smart materials. In this dissertation, combined the composite mechanics and the micromechanical theory, transformation behaviors and mechanical properties for functionally graded SMA and porous SMA are developed. The main works are as follows:Some of the recent efforts have focused on incorporating the metal-ceramic FGM utilizing SMA as the metallic phase. By developing such a composite, the transformation capabilities of the SMA can be combined with the heat resisitant of ceramic and have great potential to be used in new sensor technology, information technology and the emerging field of smart materials systems. An analytical methodology combining averaging technique of composites and an SMA constitutive model is developed to determine the transformation properties of the FG-SMA composite. The results obtained from the analyses of such a composite show that after transformation the stress in the SMA composite is lower than in the case of pure elastic composite under the same thermal loading. This decrease stress can result in an increase in temperature resistance and improved mechanical properties of SMA composites. Considering the Tresca yield function and the constitutive model of SMA materials, Analytical solutions are derived for the pseudoelastic re-sponse of a functionally graded SMA thick-walled cylinder subjected to internal pressure. This work will be explored through a parametric study on SMA composites design.It is well known that the phase transformation characteristics of dense SMA are independent of hydrostatic stress, while the macroscopic behavior of porous SMAs are significantly affected by hydrostatic stress because of stress concentrations caused by the existence of voids. And the phase transformation for porous SMAs occurs earlier than that of dense SMAs. Therefore, it is necessary to develop a model accounting for hydrostatic stresses to describe the constitutive relationship of porous SMAs. The cases under pure hydrostatic stress and pure deviatoric stress are analyzed respectively, and the relationships between martensite volume fraction and different stresses are then obtained. Base on the J2-I1theory, the approximate yield function and constitutive model for porous SMA are obtained. Based on Gurson’s theory and dilatational plasticity theory, the analytical solution of the yield function and constutive model for porous SMA are finally obtained. Numerical results are compared with the experimental data and both show good agreement with the published experimental data. Importantly, the transformation initiation stress is much closer to the experiment data.