טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentYoffe Alexander
SubjectMagnetostrictive Composites for Wireless Stress Sensing
Applications
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Doron Shilo
Full Thesis textFull thesis text - English Version


Abstract

Magnetostrictive composites are multifunctional materials that incorporate magnetostrictive material in a softer matrix. High magneto-mechanical coupling in the magnetostrictive composites makes them promising candidates for transducing and sensing purposes. In particular, load sensing based on magnetostrictive composites offers two unique advantages over other sensing methods. First, the magnetic field sensor can be mounted remotely from the magnetostrictive material and second, a variety of miniature and complicated sensor shapes can be easily formed.

The main fundamental challenge in the way of developing a wireless stress sensing method based on magnetostrictive composites is the non-linear and history dependent behavior of magnetostrictive materials. There are no constitutive laws for the relations between stress, strain, and magnetization in magnetostrictive materials on the macroscopic level. Therefore, any model for their response has to be based on knowledge of the sequence of micro-scale physical processes that occur during the magneto-mechanical response. 

In this work we introduce a new procedure for modeling the magnetic field induced by an external load applied on a magnetostrictive composite material. This model is based on an assumed sequence of physical processes that occur at the microscopic scale, and it includes both domain switching and magnetization rotation. The modeling procedure is demonstrated on a problem relevant for load sensing applications in which the magnetostrictive composite is subjected to a uniaxial compression. Moreover, a set of compression test and magnetic hysteresis measurement validated the predicted sequence of microscopic physical processes and provided clarity regarding certain influential assumptions made in the model such as perfect bonding between the bonding matrix and the magnetostrictive filler and 180° domain switching.

Further, we investigated the relationships between the stress, strain, and magnetic field emitted from epoxy-based Terfenol-D composite materials. Several key factors that are crucial to the performance of stress sensors were studied, including the reversibility, hysteresis, strain rate effects and strain amplitude effects. The experimental results were compared with simulations based on the physically based model that accounts for the inherent hysteretic and non-linear mechanical behavior of the epoxy.

In addition, the magneto-mechanical behavior at different temperatures was examined.

Finally, a novel hybrid composite that combines particles of soft and hard ferromagnetic materials was produced and demonstrated significantly stronger magnetic signal.