For the past 10 years, Stephen Gedney, professor and chair of electrical engineering, has been involved in multiple programs funded by the Office of Naval Research (ONR) that are focused on the prediction and removal of magnetic signatures from naval vessels. Thanks to funds from the ONR Defense University Research Instrumentation Program (DURIP), Gedney has established a new lab focused on such research.
While at sea, naval vessels endure tremendous changes in magnetic or mechanical stress encountered during maneuvering, from changing wave states and temperature variations, as well from the large changes in hydrostatic pressures experienced by underwater vehicles. In fact, dynamic stresses can be the largest contributors to changes in magnetic susceptibility and permanent magnetization in a vessel’s steel members, which impact the vessel’s magnetic signature.
Magnetic signatures above a certain threshold must be removed or else the vessel is susceptible to detection by hostile forces. This is accomplished through the use of longitudinal, vertical and athwartship degaussing coils placed around the ship. Unfortunately, assessment of the magnetic state of a ship currently has inherent delays and uncertainty.
The most accurate assessment of a vessel’s magnetic state is through the use of off-board sensors; however, measurement cannot realistically be done in real time and requires an auxiliary measurement system or vehicle. There is uncertainty in predicting the signature using onboard sensors because of an inability to directly assess the magnetic and magnetostrictive hysteresis (or history) of the ship’s ferrous materials.
The goal of Gedney’s ONR-sponsored research is to be able to predict the magnetic state of a ship given the ship’s location, movement, stress and thermal history, with the objective of first being able to predict the ship’s magnetic signature in real time and second to use this capability as part of a closed-loop degaussing system.
The success of this research is strongly dependent on the ability to model the physical magnetic properties of the ship’s ferrous materials, typically high-tensile steel. Accurate models must represent the impact of the magnetic properties under the influence of varying magnetic fields; axial, biaxial and shear stresses; temperature; and eddy currents, as well as magnetic viscosity and creep. To date, the research into the fundamental physics of ferromagnetic hysteresis for each of these phenomena remains incomplete.
The new ONR DURIP-sponsored laboratory allows Gedney’s team to solidify the laws for these physical properties of ferromagnetic materials and to develop physics-based mathematical models that describe them. The new laboratory provides a controlled environment to carefully study the phenomena independently and in cooperation, thus enabling the accurate prediction of the changes in magnetization and magnetic properties due to changes in mechanical, thermal and magnetic stresses. Such nonlinear and hysteretic models can then be used by the advanced software developed by Gedney’s team to greatly enhance the impact of degaussing system design and the employment of closed-loop degaussing systems.