Studies On The Thermal-Mechanical Properties Of Functionally Graded Shape Memory Alloy

Functionally Graded Shape Memory Alloy (FG-SMA) is a new kind of functional materials which possesses the excellent properties of both Functionally Graded Materials (FGM) and Shape Memory Alloys (SMA), such as the property of FGM to eliminate the stress singularity and the shape memory effect and pseudoelasticity of SMA. Due to its particular properties of physics and mechanics, FG-SMA has attracted wide attention and investigations. FG-SMA has potential applications in many fields, and all these applications should base on the mechanical properties of this material. In this article, the mechanical behaviors of FG-SMA with different structures under different loads are investigated in detail, and the main works are as follows:The elastoplastic behavior of a FGM simply supported beam consisting of elastic and elastoplastic materials under uniformly distributed load is investigated, and the theoretical solutions are obtained. A power function is used to describe the volume fractions of constituent materials, and the average property of the FGM is obtained by using the averaging method of micromechanics. This method can avoid the assumption of the varing properties of the whole material, and can consider the different Possion's ratios of different constituent materials. What's more, only the elastoplastic material in the FGM beam will yield, and the yield function is determined by the stress of the elastoplastic material only, rather than the average stress of the FGM. The method used in this work is closer to real material, while the method by assuming the variation of the whole properties of FGM can not describe this status really.Considering the constitutive relations of each constituent, theoretical results of the mechanical behavior of a FG-SMA beam under pure bending are given out. The constitutive relation of SMA is described by a piecewise linear model. This method can simplify the calculation effectively by avoiding directly considering the complex form of the martensite volume fraction. It can be studied that compared to common FGM, FG-SMA can decrease the maximum stress greatly. Compared to experimental results, it can also be learned that this method can describe the mechanical behaviors of FG-SMA very well.By combining the heat conduction theory with the theory of the mechanics of composite materials, the thermomechanical properties of a FG-SMA plate under graded temperature load is investigated, and a transformation function which can be used to determine the transformation status of a position in the plate is given out, as well as the method to calculate the thermal stress in the plate. The numerical results show that under different surface temperatures, the temperature in the plate distributes nonlinearly along the thickness direction. The position at which the martensite transformation takes place first is not fixed due to the difference of the temperature and material properties at different position of the plate, so it needs to consider the transformation function under certain surface temperatures to determine the transformation areas. Compared to the existing results, it can be learned that the method provided in this work is valid to study the thermal properties of FG-SMA.Functionally Graded Porous Shape Memory Alloy (FGP-SMA) is a new kind of porous SMA, in which the porosity varies along the gradient direction. According to the theory of mesomechanics, considering the interaction of the components, a mesomechanical constitutive model of FGP-SMA is established, which can be used to describe the mechanical behavior of FGP-SMA under complex loads, including stress and temperature. With this model, the mechanical behavior of a FGP-SMA cylinder under uniaxial compression is investigated.By investigating the transformation mechanism of porous SMA, a new transformation function considering the effect of hydrostatic stress is provided, as well as a macro phenomenological model of FGP-SMA. What's more, a new method for creating the finite element model of FGP-SMA is also given out. It can be studied from the numerical results that similar to common porous SMA, the average stress of FGP-SMA increases smoothly with the increase of strain, without an obvious turning point of transformation. Furthermore, the finite element results show that the transformation starts from the areas around the porous where the stress singularity is high, and then expand to other areas.The mesomechanical model, the macro phenomenological model and the finite element model provided in this work all can describe the mechanical behaviors of FGP-SMA very well, and the results of the mesomechanical model have more in common with the experimental data than the results of the macro model, while the macro model can simplify the calculation, avoiding iteration.