Ferromagnetic Shape Memory Alloys (FSMAs) in the Ni-Mn-Ga system are a recent class of active materials that can generate magnetic field induced strains of 10% by twin-variant rearrangement. This work details an extensive analytical and experimental investigation of commercial single-crystal Ni-Mn-Ga under quasi-static and dynamic conditions with a view to exploring the material's sensing and actuation applications. The sensing effect of Ni-Mn-Ga is experimentally characterized by measuring the flux density and stress as a function of quasi-static strain loading at various fixed magnetic fields. The bias field is shown to mark the transition from irreversible to reversible (pseudoelastic) stress-strain behavior. A constitutive model based on continuum thermodynamics is developed to describe the coupled magnetomechanical behavior of Ni-Mn-Ga. Mechanical dissipation and the microstructure of Ni-Mn-Ga are incorporated through internal state variables. The constitutive response of the material is obtained by restricting the process through the second law of thermodynamics. The model is further modified to describe the actuation and blocked-force behavior under a unified framework.
The behavior of Ni-Mn-Ga under dynamic mechanical and magnetic excitations is addressed. First, a new approach is presented for modeling dynamic actuators with Ni-Mn-Ga as a drive element. The constitutive material model is used in conjunction with models for eddy current loss and lumped actuator dynamics to quantify the frequency dependent strain-field hysteresis. Second, the magnetization response of Ni-Mn-Ga to dynamic strain loading of up to 160 Hz is characterized, which shows the response of Ni-Mn-Ga as a broadband sensor. A linear constitutive equation is used along with magnetic diffusion to model the dynamic behavior. Finally, the effect of changing magnetic field on the stiffness of Ni-Mn-Ga is characterized by conducting mechanical base excitation tests. The measured field induced stiffness shift of 61% indicates that Ni-Mn-Ga is well suited for vibration absorption applications requiring electrically-tunable stiffness. Ni-Mn-Ga FMSA is thus demonstrated as a multi-functional smart material with possible applications in sensing, actuation, and vibration control which require large deformation, low force, tunable stiffness and fast response. The magnetomechanical models provide a thorough understanding of the material behavior in quasi-static and dynamic conditions.