Design and characterization of acoustic pulse shape memory alloy actuators

Abstract

Single crystal Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs) are active materials which produce strain when a magnetic field is applied. The large saturation strain (6%) of Ni-Mn-Ga, and material energy density comparable to piezoelectric ceramics make Ni- Mn-Ga an interesting active material. However, their usefulness is limited by the bulky electromagnet required to produce a magnetic field. In this thesis, a novel actuation method is developed for shape memory alloys in their martensitic phase, whereby asymmetric acoustic pulses are used to drive twin boundary motion. Experimental actuators were developed using a combination of Ni-Mn-Ga FSMA single crystals and a piezoelectric stack actuator. In bi-directional actuation without load, strains of over 3% were achieved using repeated pulses (at 100 Hz) over a 30 s interval, while 1% strain was achieved in under 1 s. The maximum strains achieved are comparable to the strains achieved using bi-directional magnetic actuation, although the time required for actuation is longer. No-load actuation also showed a nearly linear relationship between the magnitude of the asymmetric stress pulse and the strain achieved during actuation, and a positive correlation between pulse repetition rate and output strain rate, up to at least 100 Hz. Acoustic actuation against a spring load showed a maximum output energy density for the actuator of about 1000 J/m³, with a peak-to-peak stress and strain of 100 kPa and 2%, respectively.