Strain gradient-induced size effect of Nickel-Titanium shape memory alloys

This study investigates size effect in nickel-titanium (NiTi) shape memory alloys (SMAs), focusing on their elastic deformation and phase transformation behaviors. A series of experiments, including bulk-scale tension tests, micro-scale tension, compression, and cantilever bending tests, were conducted to observe the effect of specimen dimensions on SMA behavior. Micro-scale tension and compression tests unveiled a notable asymmetry in the phase transformation stress, irrespective of specimen dimensions. Moreover, micro-cantilever bending tests, spanning a thickness range from $1.9$ to $21.0\mu m$, revealed a significant increase in both the effective elastic modulus and phase transformation stress as the beam thickness decreased. A constitutive model has been developed to address the tension/compression asymmetry and size effect based on couple stress theory, and implemented in finite element analysis of beam structures. Finally, experimental results were compared with simulation outcomes, and the deformation mechanisms responsible for size effect were discussed. The growing prominence of SMAs in micro/nano-scale applications highlights the necessity of understanding and accounting for size effect. Therefore, developing the capability to measure and simulate size effect is crucial for ensuring the effective utilization of SMAs in these scales.