Some Studies On Micromechanics Model For Shape Memory Alloy

Intelligent material is developed through a combined information and material, which is one of the important parts in the field of the material science. Shape memory alloys (SMAs) have been attracted interest in the field of composite materials and have been proposed as sensors and large strain actuators for use in intelligent composites and structures. This is because SMA has native ability to undergo reversible thermoelastic martensitic phase transformation under external thermomechanical loading. The concept of 'intelligent SMA composite' has been proposed in the field of composites.Magnetic shape memory alloy (MSMA) has emerged as an interesting addition to traditional smart materials. In addition to the strains originating from temperature or stress controlled conventional shape memory behavior, large strains can be produced in this alloy under the external application of moderate magnetic fields. It has high response time, and can transform magnetical energy into mechanical energy, which makes MSMA suitable for a wide range of applications.The constitutive relations of these materials have been a crucial issue for application of these smart materials. In this dissertation, combined the micromechanical and the thermodynamic theory, constitutive models for SMA composite and MSMA are developed by selecting proper internal variants. The main works are as follows.Based on the micromechanical and the thermodynamic theory, a three-phase model for the SMA composite is developed, in which the composite is considered as the austenitic phase, the product phase (martensite) and the matrix phase. In the present model, the interaction among the three phases is analyzed. From the micromechanical analysis, the macroscopic free energy function is found. Then macroscopic transformation strain, effective elastic compliance, macroscopic constitutive model are derived. Compared with the traditional two-phase method, non-linearity of SMA need not be considered. The method is not only simply but also the interaction among the three phases is considered. The theory analysis is approach to the fact. By comparing with references, it is shown that the results are credible. It is helpful to design the intelligent composite.The interaction energy between the matrix and the inclusions in shape memory alloy reinforced composite is one of the most important and complicated parts in thermodynamic constitutive theory. Although it could be a small number, it is a key factor to understand the transformation mechanism. The interaction energy is derived based on the classical theory of micromechanics by using a three-phase micromechanical model. The model can be used to analyze inclusions with various shapes. The effect of fiber volume fraction, fiber shape and material of matrix on the interaction energy was investigated. Many quantitative maps and important rules are obtained.Based on the micromechanical and the thermodynamic theory, a three-phase micromechanical model of composite with elastoplastic matrix and SMA reinforcement is developed. Compared with the traditional two-phase method, non-linearity of SMA need not be considered. The interaction among the austenitic phase, the martensite phase and the matrix phase is considered. It is more near to the fact. Primary attention is paid to the effect of shape, geometric ratio and volume fraction of fiber and temperature on the overall response of composite as well as on the internal stress and strain evolution. It is helpful to design the intelligent composite.Based on the micromechanical theory, a three-phase model for thermal expansion coefficient and transformation strain coefficient of shape memory alloy composite is developed. With the micromechanical analysis, the common expression for thermal expansion coefficient and transformation strain coefficient of shape memory alloy composite is derived. The expressions can be used in various shape fiber. The attentions are paid to study the effects of fiber geometric, shape and volume ratio of fiber in composite on the effective thermal expansion coefficient and transformation strain coefficient of composite.Based on the micromechanical and the thermodynamic theory, a constitutive model for magnetic shape memory alloy (MSMA) is developed. The kinetic equation is established in terms of the thermodynamic driving force derived from the reduction of Gibbs free energy of MSMA. The effective module and overall inelastic strain due to reorientation are obtained. The nonlinear and hysteretic strain response of MSMA is investigated under constant magnetic field and constant stress. The theoretical results are found to be in good agreement with experimental data.