|M.Sc Student||Adelsberg Nehara|
|Subject||Wireless Thin Layer Force Sensors Based on|
Magnetostrictive Composite Material
|Department||Department of Design and Manufacturing Management||Supervisor||Professor Doron Shilo|
|Full Thesis text|
Ensuring that bolted joints and compressively fastened assemblies are fastened for long times under service conditions, which may include shock loads, high accelerations, or strong vibrations, is a major challenge. There are two approaches to addressing this problem: simulation experiments and structural health monitoring (SHM). Both approaches are based on measurements of compressive loads between the assembled parts and currently both of them are hampered by the lack of suitable force sensors.
Ferromagnetic magnetostrictive materials offer a unique method for stress and strain sensing and are of considerable interest for real-time remote force sensing and SHM. Magnetostrictive materials change their magnetic field in response to strain and stress changes. When a magnetostrictive material is loaded mechanically its magnetization state undergoes changes which can be measured by a magnetic sensor and inversely translated into the force value. The magnetic field can be monitored continuously and remotely, rendering an indicative signal as to the level of stress in the material.
In this study, we produced wireless thin-layer force sensors based on washers made of magnetostrictive composite materials featuring Terfenol-D particles embedded in an epoxy matrix. The sensor has a washer-like shape, which makes it suitable for sensing forces in bolted joints and compressively fastened assemblies. The small washer thickness (1.5 mm) is necessary for providing small compliance and low mass, which are essential for simulation experiments in order to have minor influence on the mass and compliance of the system. Another advantage is the simplicity of forming sensors with different shapes.
The magnetostrictive composite materials were characterized by a combination of characterization techniques, including X-ray diffraction, magnetic hysteresis measurements, and magneto-mechanical tests. For the later, a dedicated experimental setup and a flexible loading frame were designed and implemented to allow for the measurement of small magnetic field changes during precisely controlled mechanical tests. Several different test procedures were applied to study the effects of repeatability, hysteresis, loading rate, and stress relaxation, which are crucial characteristics of force sensors. The results reveal an operation range in which the magneto-mechanical response is linear, repeatable, have a minor amount of hysteresis and demonstrate no relaxation and rate effects.
Special effort was dedicated to increasing the magnetic field induced by the sample and accordingly increasing the sensitivity and the signal-to-noise ratio. For this purpose, we verified experimentally the sequence of processes that occur during the curing and poling stages of the composite material and identified the critical (hardest) process that eventually determines the magnetic field induced by the sample. Based on these insights, we demonstrated ways for producing force sensors with higher sensitivity and signal-to-noise ratio, by increasing the poling magnetic field and the temperature during the curing of the epoxy.