The Constitutive Model And Experimental Study On Shape Memory Alloys

Because of various disasters, such as earthquakes and Hurricane, etc., concrete structures often appear cracks and other damage phenomena in long-term use. How to improve the seismic capacity of concrete structures and how to repair and reinforce these structures become a much-needed engineering problem. Shape memory alloys (SMAs) have many characteristics such as shape memory effect, super-elasticity, high damping and resistance, etc., reflecting the properties of self-perception, self-diagnostics and adaptive capacity, and these properties are bound to make SMAs play a significant role in the field of vibration control and structural health monitoring. However, there are still many problems to be studied further about the SMAs, for instance, vibration control and concrete structure repair as well as constitutive models, especially the multi-dimensional constitutive model. In this paper, some basic researches about the constitutive model and applications in civil engineering for SMAs are conducted, and the main works of the paper are as follows:(1) The basic mechanical properties of Austenite phase SMA wires with different diameters, different cycles, strain amplitudes, loading rates, temperature and other factors were studied by experimental method, and the resistance of the SMA wire at different imposed conditions were also studied. The results showed that loading magnitude, loading rate and temperature were the important parameters, which will affect the mechanical properties of SMA wires.(2) Based on Kazuhiko Arai's theory for which the method of energy analysis was used, one-dimensional continuous dynamic model for the SMAs was established by taking into account the affects of the transformation of SMA and the variables of strain rate. In recent decades, scholars at home and abroad had made a great deal of research and analysis for the constitutive model of the SMA material. However, the most of the researched models were not pay attention to the influence of the strain rate, and use different relationship to express the process of loading and unloading. In order to prove the usefulness of the proposed model, the results calculated by using the proposed model were compared with the results by experiments, and the results were in good agreement.(3) An improved three-dimensional constitutive model of the SMAs was established by using the yield surface equation of plasticity, combining with the idea of Brinsion's one-dimensional constitutive model and the concept of Boyd and Lagoudas model. The proposed constitutive model can be used to calculate the deformation of the SMAs under different temperatures, different stresses and different loading paths. In order to verify the validity of the three-dimensional model, the MATLAB software was used for programming and the simulation of three different examples, and the simulation results showed that the improved model can not only well describe the SMA's mechanical behavior under one-dimensional situation, but also be very well to qualitatively describe the SMA shape memory effect and superelasticity of two-dimensional sheet.(4) A validation test for repairing cracks and deformation of concrete beams by using shape memory effect was conducted in a laboratory. The SMA smart concrete beams were made by embedding the SMA wires and SMA strands in the concrete beams, which all the SMAs were under martensite phase at room temperature. The specimens used for the four-point bending experiment were experimentally developed. After completely unloading, the embedded SMAs were drived by Joule heating to repair the cracks width and deflection in concrete beams by using the restoring force which generated in the process of phase transformation. The experimental results showed that the repairing effect for the damaged concrete beams without bond strength was better than those with bond strength because of the easy heat transmission from the SMA wires to the surrounding concrete. The repairing effect of the concrete beam with the SMA strands was better than that with a single SMA wire for the different contact areas. The repairing effect of the concrete beams without tension reinforcement was better than that with tension reinforcement for the tension reinforcement hindered the recovery of the SMAs.(5) A novel type of SMA-viscous damper was designed and manufactured and the corresponding mechanical property test was conducted. At present, the damper has only a single form of energy consumption and no self-centrical function. The developed damper by using the superelastic of the austenitic SMAs has not only highly energy-consuming but also the performance of self-reset. The proposed damper was tested by loading and unloading, and experimental results showed that the energy consumption capacity of the damper could increase significantly by increasing the loading frequency and load displacement amplitude. A dynamic time-history analysis for a model of simple eight storey steel frame structure installed the proposed SMA dampers under earthquake excitations was carried out and the simulation results showed that the new SMA damper can well reduce the displacement and maximum acceleration at the top of the structure. This can be proved that the developed SMA damper can significantly reduce the vibration of this structure.