Research On Shape Memory Alloy And Shape Memory Alloy Reinforced Intelligent Concrete Structure

As a material of novel fimction, shape memory alloy (SMA) used as an integrated sensor and actuator has drawn much attention in civil engineering in recent years. The utilization of SMA to improve earthquake resistance and the control the vibration of structures has become an important research topic. However, some areas of the study are still in an exploratory or developing stage, such as the constitutive modeling and sensing properties of SMAs and the properties of intelligent concrete structures reinforced with SMAs. Therefore, it is necessary to continue and further the knowledge base in this area. Targeting the existing problems in the application of SMA in civil engineering, this work will discuss the following:(1) Considering the advantages in mechanical properties of steel strands, a new form of NiTi SMA wire, SMA strands, is proposed in this thesis. The mechanical properties of SMA strands and SMA wires are fully investigated. The superelastic mechanical properties of SMA strands subjected to cyclic loading were investigated experimentally. The relationship between recovery stress and temperature of martensite SMA strands was also studied by experiment. By comparing the test results of SMA strands and SMA wires, it was found that the form of SMAs affects certain aspects of its mechanical properties.(2) Utilizing the nonlinear mapping capability of artificial neural networks, a constitutive model for the superelasticity of shape memory alloy is proposed according to the experimental results of SMA wire under different strains. The results show that the numerical results agree well with experimental observations. This model can describe the nonlinear relationship between stress and strain of SMA and has good forecasting capability. Meanwhile, this model addresses the shortcomings of classic phenomenological models in describing the residual strain and the mismatching phenomenon of loading and unloading curves in the stress-induced, martensite stage.(3) The electrical properties of SMA can be used to realize health monitoring and self-diagnosis of structures or make the SMA into sensor elements. Therefore, the sensing properties model of SMA was built by combining the relationship between electrical resistivity and martensite fraction of SMA and the one-dimensional constitutive model developed by Brinson. The relationship between relative resistance and strain were analyzed at different temperature and different initial phase by this model. The electrical properties of SMA were investigated by experiment. The results show that results of the analytical model agree well with the experimental results, indicating that the model is reasonable.(4) The self-rehabilitation properties of intelligent concrete structures reinforced with SMA strands were investigated by experiment. Three-point bending tests were conducted on the beams reinforced with SMA strands and on ordinary reinforced concrete beams to compare their bearing capability, residual deflection and residual crack width. The influence of pre-stress and bonding configurations were also analyzed. The beams with martensite SMA strands were loaded once and activated by current after cracks were generated. The test results show that SMA strands can improve the mechanical properties of concrete beams and control the crack width while ensuring the bearing capacity, indicating that the intelligent concrete beams have good self-rehabilitation capabilities compared to ordinarily reinforced concrete beams.(5) The influence of SMA on the vibration performances of concrete columns were studied through initial displacement method under free vibration and vibration with current activation. For the free vibration test, the active property tuning (APT) principle was utilized to change the vibration performance of the structures. For vibration with current activation, the active strain energy tuning (ASET) principle was applied to alternate the vibration performance of structures. The test results indicate that these two principles changed the vibration properties of the. concrete columns. Their influence on the damping ratio of concrete columns was almost the same while for the frequency of the concrete columns, the influence of the ASET principle was larger than that of the APT principle.(6) The self-damping and self re-centering structural system was built by replacing the rebar of reinforced concrete structures with superelastic SMA because of the damping and recovery capabilities of superelastic SMA. The seismic responses of SMA concrete structures under one-way and two-way earthquake motions were investigated. The period of the structures, the site types and the intensity of earthquake motions were considered and a reinforced concrete structures with the same dimensions and the same reinforcements was used as a reference. The results show that SMA concrete structures have smaller horizontal and torsional responses and good re-centering capabilities compared to ordinary reinforced concrete beams.