With large recoverable deformation and damping capacity, NiTi SMA has been widely used as actuator, energy absorber and vibration isolation device. However, jump phenomena due to nonlinearity of the material are not desired in these systems. Therefore this thesis investigates the jump phenomena of thermomechanical responses in a nonlinear vibration system with phase transformable NiTi SMA in multiple domains and further studies the grain size effects of the nanocrystalline NiTi SMA on the jump phenomena.

Through synchronized measurement of rotational angle and temperature of the NiTi SMA in a torsional vibration system under external sinusoidal excitation, oscillations of these two physical quantities are acquired in both transient and steady states. By monotonically increasing...[ Read more ]

With large recoverable deformation and damping capacity, NiTi SMA has been widely used as actuator, energy absorber and vibration isolation device. However, jump phenomena due to nonlinearity of the material are not desired in these systems. Therefore this thesis investigates the jump phenomena of thermomechanical responses in a nonlinear vibration system with phase transformable NiTi SMA in multiple domains and further studies the grain size effects of the nanocrystalline NiTi SMA on the jump phenomena.

Through synchronized measurement of rotational angle and temperature of the NiTi SMA in a torsional vibration system under external sinusoidal excitation, oscillations of these two physical quantities are acquired in both transient and steady states. By monotonically increasing/decreasing the excitation frequency with small intervals around the primary resonance frequency, FRCs of the thermomechanical responses are obtained and jump phenomena are identified in the frequency domain. Similarly, by monotonically changing the excitation amplitude at a fixed frequency, ARCs are also quantified and the jumps are captured in the amplitude domain. In the time domain, spontaneous jump phenomena of thermomechanical responses from the lower branch of solution to the alternative upper branch are identified at critical excitation without any external interference and the characteristic evolution patterns are revealed by experiment. At the excitations within the metastable region, a large negative disturbance can also trigger the jump phenomena in the time domain. To control the undesired jump phenomena, nanocrystalline NiTi SMA bars with average grain size ranging from 10 nm to 1000 nm are fabricated via severe plastic deformation and heat treatment. As the grain size goes down to nano scale, nonlinearity, damping capacity and latent heat of the nanocrystalline material are greatly reduced in torsion. Consequently, dynamic responses of the nonlinear vibration system become more stable and the jump phenomena become smaller or even vanish. To quantify the thermomechanical responses, an effective Duffing oscillation model and a lumped heat transfer analysis are introduced to determine the thermomechanical responses of the nonlinear vibration system and to further identify the jump phenomena in multiple domains.

For a nonlinear vibration system with NiTi SMA, the jump of mechanical response is always accompanied by a simultaneous jump of the thermal response due to the thermomechanical coupling effect. Simultaneous jump phenomena of the thermomechanical responses exist in both frequency and amplitude domains and are significantly affected by the external excitation amplitude and frequency respectively. The jump phenomena are also identified in the time domain and such jumps are naturally associated with the excitation and disturbance. At critical excitations, the jump phenomena occur spontaneously in the time domain without any external interference. At excitations within the metastable region, an external disturbance with proper direction and magnitude can also trigger such jump phenomena in the time domain. Grain size of nc NiTi SMA significantly influences the jump phenomena of the nonlinear vibration system with the nanocrystalline material. Grain refinement is an effective method to control the thermomechanical properties of the nanocrystalline NiTi SMA and to further manipulate the nonlinear instability of the dynamic system.

Keywords: NiTi shape memory alloy, phase transition, thermomechanical coupling effect, nonlinear vibration, jump phenomenon, frequency response curve, amplitude response curve, Duffing oscillator, lumped heat transfer analysis, gain size effects.