Experimental investigation of cyclic deformation of superelastic NiTi wire
Tan, Yu Xuan
Date of Issue2016
School of Mechanical and Aerospace Engineering
The cyclic deformation properties of the superelastic NiTi wire subjected to strain control were investigated and the results were documented in this report. Three experiments were conducted in total. Experiment 1 was focused on the strain rate dependence by subjecting the material to different strain rates of 10^(-2)/s, 10^(-3)/s and 10^(-4)/s. From this experiment, transformation peaks can be observed at strain rates of 10^(-4)/s and 10^(-3)/s, as an indication of the energy that is essential for the creation of a nucleus within the induced phase along the movement of the interface. The peaks disappeared when the strain rate is increased to 10^(-2)/s. As the temperature increases, an increase and decrease in transformation stresses is resulted during forward and reversed transformation, respectively. The slopes at forward and reversed martensitic transformation appears to be steeper due to the change in temperature as the material’s local temperature increases with increasing velocity at higher strain rates. Dissipated work can be seen to increase accordingly with strain rate because the energy dissipation or absorption is higher at higher strain rates. Experiment 2 was carried out to study the cyclic partial-loop behaviour of the wire under one cycle of loading and unloading from 2% to 12% strains. Overlapping responses can be observed during the forward transformation which serves as an indication of the material displaying good agreement and test repeatability. Experiment 3 was conducted to analyse the cyclic full-loop behaviour of the NiTi wire under 100 cycles of loading and unloading of individual strains from 1% up to 11%. An increase in residual strain is observed after subsequent cycles which indicates there is accumulation of dislocations around the infinitesimal defects within the material. In addition, the rate of decrease in residual strain can be seen to be rapid with increasing strain percentages. A reduction in critical and yield stresses can be observed at the early cycles of the test and eventually approached a constant at the 30th cycle, which can be resulted from the formation and accumulation of stabilised martensite as well as the indication of a heterogeneous structure. A perfect elastic recovery at 1% strain can be seen upon unloading. The complete recovery at 1st cycle of the 2%, 3% 5% and 7% strains are different from 1% as this recovery is resulted from the transformation of stress-induced martensite exhibiting a superelastic effect upon unloading. For strains above 1%, a decreasing trend starts to occur after the 1st cycle which remains constant at the end of 100 cycles. This reduction of superelastic response is known to be caused by an increase in dislocation fraction and stabilised martensite. For all strain percentages, a decrease in dissipated work at the first few cycles is the result of the residual deformation development. The decrease in dissipated work per cycle becomes rapid at higher strain percentage. This rapid decline in dissipated work is commonly associated with the material beginning to operate out of the pseudoelastic range. The young’s modulus of strains lower than 3% appeared to be constant throughout 100 cycles during loading and unloading. However, a decrease can be observed when the strain is set above 9%. It can be due to the formation of heterogeneous microstructure resulted from various microstructural mechanisms during stabilisation that has to do with the formation of stabilised martensite, detwinning, grain reorientation and slips deformation.
Final Year Project (FYP)
Nanyang Technological University