A two-level micromechanical theory for a shape-memory alloy reinforced composite

A two-level micromechanical theory is developed to study the influence of t he shape and volume concentration of shape-memory alloy (SMA) inclusions on the overall stress-strain behavior of a SMA-reinforced composite. The firs t level exists on the smaller SMA level, in which, under the action of stre ss, parent austenite map transform into martensite. The second level is on the larger scale consisting of the metastable SMA inclusions and an inactiv e polymer matrix. The evolution of martensite microstructure is evaluated f rom the irreversible thermodynamics, in conjunction with the micromechanics and physics of martensitic transformation. By taking martensite to exist i n the form of thill plates on the micro scale and assuming SMA inclusions t o be homogeneously aligned spheroids on the macro scale, the overall stress -strain behaviors of a NiTi-reinforced composite are calculated for various SMA shapes and concentrations. The results indicate that, under a tensile axial loading, martensitic transformation is easier to take place when SMA inclusions exist in the form of long fibers, but most difficult to occur wh en they are in the form Of flat discs. In general the levels of the applied stress at which martensite transformation commences, finishes, and austeni tic transformation starts, and finishes, are found to decrease with increas ing aspect ratio of the SMA inclusions while the damping capacity increases with it; these properties point to the advantage of using fibrous composit es for actuators ol sensors under a tensile loading. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.