|M.Sc Student||Oren Kanner|
|Subject||Ferromagnetic Shape Memory Alloy Actuators|
|Department||Department of Mechanical Engineering||Supervisors||Professor Shilo Doron|
|Dr. Ganor Yaniv|
|Full Thesis text|
Ferromagnetic Shape Memory Alloys (FSMA) are a relatively new class of active materials that have received significant attention due to the fact that they exhibit large strains, nearly 10%, while operating at fairly high frequencies, over 1 kHz. They show particular promise for remote actuation applications as they can be actuated using an external magnetic field. Despite their large actuation strains, FSMAs are limited by their relatively low blocking stress of roughly 4 MPa above which they exhibit no field-induced strain. Recent experiments have shown that micro-scale FSMA specimens exhibit much higher blocking stresses, suggesting that they have exceptional potential for use in Micro ElectroMechanical Systems (MEMS).
In this research, the problem of FSMA actuation was approached from two independent directions, one experimental and one theoretical. On the experimental side, the behavior of a flapping actuator designed to generate propulsion in fluid was characterized. This flapper made use of a novel shear mode of actuation that occurs in specimens cut along the  crystallographic axis. A dynamic experimental system was developed that was capable of subjecting it to an alternating magnetic field of 0.7 T and measuring thrust forces with a resolution of 30 μN. In addition, a high-speed camera was used to analyze the flapper’s actuation motion. The flapper exhibited an engineering shear strain of 12% and produced a transient thrust of up to 40 mN, showing great potential for use in swimming devices.
On the theoretical side, a magnetoelastic model of FSMA behavior was developed, presenting a new fundamental concept for modeling FSMAs that takes into account the processes through which magnetization alignment occurs and their kinetics. Two specimen microstructures were introduced to ensure magnetic compatibility at the twin boundaries between martensitic variants despite magnetization rotation. The first compensates for the rotation with an internal magnetic field and results in a relatively high free energy that suggests a higher blocking stress than commonly observed in FSMA specimens. The second introduces an internal magnetic domain structure within the disfavored variant to preserve compatibility on average and results in a lower free energy overall. However, there exists an energy barrier at small magnetization rotation angles whose magnitude is dependent on the specimen thickness; this leads to a fundamental size effect in FSMAs relating a specimen's actuation properties and microstructure to its dimensions. This model explains experimental results showing unusually high blocking stresses in small FSMA specimens.