|M.Sc Student||Weber Yarden Ben-Zio|
|Subject||Real-Time Health and Stress monitoring of Composite|
Materials Using Magnetostrictive Fillers
|Department||Department of Mechanical Engineering||Supervisor||Professor Doron Shilo|
|Full Thesis text|
Structural health monitoring is an emerging approach for non-destructive inspection in which the distribution of stresses in a load bearing structure are monitored in real-time by embedded or attached sensors. Current approaches for Structural health monitoring are based on sensors such as fiber-optic Bragg-gratings, piezoelectrics and strain-gauges, which suffer from two major limitations: (1) they require wiring and constant power supply, and (2) they measure only local strains in one direction. In this research, we develop and characterize a novel method for health and stress monitoring in polymeric and composite materials by incorporating magnetostrictive Terfenol-D filler into the material. Magnetostrictive materials induce a change in their surrounding magnetic field when subjected to stress; thus, they serve as natural stress sensors, requiring neither power nor wiring.
A crucial characteristic for the application of this new stress monitoring method is the level of stress induced magnetic field changes. We show that an initial alignment of the particle magnetization, by an application of a magnetic field during specimen polymerization, enhances the level of magnetic field changes by an order of magnitude. We present a qualitative model that explains the effect of the poling field and suggests a method for choosing the direction of the poling field with respect to the loading conditions. An additional enhancement of the magnetic field changes, by almost an order of magnitude is obtained by decreasing the Terfenol-D particle size from ~300mm to ~38mm.
Measurements of the magnetic field as a function of stress under compression and shear loads demonstrate the possibility of monitoring the average stress in a specimen by a single magnetic sensor mounted at a distance of about 50 mm from the specimen. Furthermore, complete 3D mapping of the magnetic field reveals asymmetric field patterns due to an asymmetry of the stress distribution, pointing on the potential for distinction between different loading conditions by using multiple sensors.
We also developed a quantitative model which allows calculating the magnetic field changes under uniform compressive stress. Comparison of this model with experimental results indicates on significant differences, which are explained to be a result of different types of Terfenol-D particle microstructure and particle arrangements at the micro scale. This analysis shows that the magnetic field changes can be further increased by an order of magnitude by optimizing specimen preparation processes. An elaborated model that takes into account these microstructural effects demonstrates a fair agreement with the experimental results.