Shape memory alloys (SMAs) are well known for their substantial power density in shape memory mode and their large recoverable strains in superelastic mode. NiTi, the most popular SMA, has been studied extensively in pure tension, but studies of more complex structural forms and other deformation modes are rare in the published literature. Therefore, the first purpose of this research was to characterize and understand the superelastic thermomechanical behavior of one such structural form, cables (or wire ropes). The second purpose was to understand the superelastic tension, compression, and bending behavior of cylindrical NiTi tubes.
Cables made from SMA wires are relatively new and unexplored structural elements that combine many of the advantages of conventional cables with the unique properties of SMAs, leading to a number of potential applications. An extensive set of uniaxial tension experiments were performed on two SMA cable constructions, a 7x7 right regular lay, and a 1x27 alternating lay, to characterize their superelastic behavior in room temperature air. Details of the evolution of strain and temperature fields were captured by simultaneous stereo digital image correlation (DIC) and infrared imaging, respectively.
Different aspects of the SMA cable responses were considered. First, the nearly isothermal, yet quite different, superelastic responses of the two cable designs were examined. Second, selected subcomponents excised from the two cable constructions were studied to determine the individual contributions of the cables hierarchical construction. Third, the
elongation rate sensitivity of the cables and their subcomponents were inspected to compare and quantify their thermomechanical coupling.
The tube experiments in the second part of this research should serve to calibrate and validate material models used to simulate SMA cables in the future. Tubes were studied instead of wires to avoid experimental difficulties, but even using tubes, custom built fixtures were required to avoid buckling during uniaxial compression and to avoid axial loads during large-rotation bending. Stereo DIC measurements during the tube experiments revealed that the material instability, which leads to propagating transformation fronts in pure tension, also leads to highly heterogeneous strain fields during bending.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91514/1/breedlun_1.pd
