The strain sensing principle of shape memory alloy (SMA) based on its electrical resistance (ER) value is analyzed in this paper. A mathematical model to describe this principle is developed based on the relationship between the electrical resistivity and the martensite fraction of an SMA wire. The proposed model can be used at different temperatures and different initial conditions. To verify the validity of the proposed model, various experiments were conducted. The numerical results based on the proposed model agree well with experimental results, indicating that an SMA wire with a proper model can be utilized as a sensing element.

To investigate the influence of shape memory alloys (SMAs) on the dynamic properties (mainly refers to the frequency and the damping ratio) of concrete columns, the vibration curves of the concrete column reinforced with steel wire and SMA wires with different phase were measured through initial displacement method. In this study, the experiment included two parts. One is the free vibration test, which utilized the active property tuning (APT) principle, i.e. the passive damping, of SMAs to change the vibration performance of the concrete columns. The results show that SMAs can increase the damping ratio of the concrete column due to its high damping and decrease the frequency owing to its lower stiffness. The other is the vibration test with electrical current activation, the active strain energy tuning (ASET) principle, i.e. the active damping, was applied to alternate the vibration performance of the concrete columns. In this test, martensitic SMA wires were strained up to 4% and were heated with a current of 40A. The test results show that with increasing of the temperature, the frequency of the concrete column decreases, while its damping ratio increases. Comparing the results of these two tests, the influence of two tuning principles, APT principle and ASET principle, on the damping ratio of concrete columns is almost the same. However, for the frequency of the concrete columns, the influence of ASET principle is larger than that of APT principle.

The rehabilitation properties of intelligent concrete beam reinforced with shape memory alloys (SMA) is investigated. In this study, SMA strands, made of 7 single wires, are used to replace the steel bars in ordinary concrete beam. The test of single point loading at the mid-span is conducted on the beam. The experiment has three stages including loading, unloading and heating the SMA strands. During heating the SMA strands by electrical current, the crack width can be reduced by taking the advantage of the shape memory effect of SMA strands. The results illustrate that SMA is a candidate material for rehabilitation of reinforced concrete structures.

Considering the advantages of steel strands in mechanical properties, a new form of shape memory alloy (SMA) wire, SMA strands, is designed in this study. To compare the recovery properties of martensitic SMA strands and SMA wire, the recovery stress of which with different prestrains were tested by using a material testing machine with a temperature-controlled cabinet. In this experiment, the SMA strands and the SMA wire have the same composition and the same cross-sectional area. The experimental results show that the maximum recovery stresses of the SMA strands with different prestrain are almost the same, which illustrates that the prestrain has no much effect on the maximum recovery stress of the SMA strands. Moreover, the recovery stress curve in heating stage does not coincide with that in cooling stage. Furthermore, the residual recovery stress of the SMA strands in cooling stage is also smaller than that of the SMA wire. This demonstrates the SMA strands can recovery initial state better.