When superelastic shape memory alloy is under external loading, austenite would transform into martensite upon critical stresses, via nucleation and growth of macroscopic domains. Domain boundary propagation is one of the key issues associated, since latent heat is generated at the fronts and they act as moving heat sources or sinks, resulting in strong thermo-mechanical coupling. The motivation of the thesis is to investigate the domain front kinetics, in NiTi wide plate and tube respectively, through a series of experiments. Constant-rate uniaxial tensile tests within a wide strain rate range have been conducted, with high speed camera recording the surface morphology or DIC strain fields, and infrared camera recording the temperature fields at the same time. It was found that...[ Read more ]

When superelastic shape memory alloy is under external loading, austenite would transform into martensite upon critical stresses, via nucleation and growth of macroscopic domains. Domain boundary propagation is one of the key issues associated, since latent heat is generated at the fronts and they act as moving heat sources or sinks, resulting in strong thermo-mechanical coupling. The motivation of the thesis is to investigate the domain front kinetics, in NiTi wide plate and tube respectively, through a series of experiments. Constant-rate uniaxial tensile tests within a wide strain rate range have been conducted, with high speed camera recording the surface morphology or DIC strain fields, and infrared camera recording the temperature fields at the same time. It was found that, the stress-strain responses, the front velocities and temperature distributions are significantly dependent on the applied strain rate. The tip velocity can be hundreds of times larger than the front mid-point velocity. It monotonically increases with the rate, and determines the temperature jump amount within the course of tip propagation. It was found by using DIC technique on NiTi plate configuration, that the time scale and length scale of strain evolution within the martensite domain are also very sensitive to the strain rate, under the influence of interaction between the local temperature and macroscopic stress. An implicit form of kinetic relation is assumed, indicating the leading factor determining the front moving velocity in non-thermodynamic-equilibrium case is the front velocity. The evolution of transformation patterns, temperature fields, and extent of stress hardening are determined by the competition of different length scales and variation of local driving forces. Further, the simulation results of a FEM numerical study, based on non-local model with non-convex Helmholtz free energy function, also well support the experimental observations, and imply that the kinetic relation follows as a consequence of balance principles.

Key words: Phase transformation; Domain front propagation; Kinetic relation; Rate dependence; Tip velocity; DIC (Digital Image Correlation); Driving force; Thermo-mechanical coupling.