Maintaining ship propulsion shafting integrity is a critical concern for the US Navy in order to provide full mission capability of fleet assets. Fleet-wide maintenance experience presently dictates drydocking ships at ten year intervals for full shaft removal and inspection to determine whether the glass-reinforced plastic (GRP) covers protecting the shaft-sleeve interface have successfully prevented seawater intrusion and attendant shaft pitting/fatigue damage. This destructive type of inspection can reveal no water ingress or shaft damage, meaning that the sleeve was removed before the end of its useful operating life. Conversely, inspections have also revealed significant damage, evidencing cover failure well before the end of its typical remaining useful life. To meet this critical need, an embedded, multi-modal distributed sensing platform has been developed to provide an in situ assessment of shaft and barrier coating condition, which periodically and non-destructively measures shaft condition at shorter intervals. With this enhanced capability, shaft maintenance would be driven by shaft and GRP barrier condition instead of a calendar basis, thereby reducing the number of costly drydock cycles over a ship’s lifetime. Frequent monitoring would also reduce the risk of catastrophic shaft failure between inspection cycles, were the barrier properties to fail prematurely.

The sensing hardware has been designed for a conformal, low-profile form factor to permanently reside encased in the fairing compound in the region between the shaft, bearing sleeve, and GRP cover. A wireless power and signal transfer through the GRP cover, to eliminate the need for conductors penetrating the shaft, has been designed for the final application. Sensing elements to monitor the coating protection system and localized shaft damage severity have been developed. To detect the degradation of the protection system, novel ring shaped sensors have been developed to observe changes in local conductivity and overall barrier properties using electrochemical impedance spectroscopy (EIS) techniques. To further validate the seawater intrusion measurement, circumferential inductive sensors using an eddy-current type inspection measurement have been created for an independent indication of pitting or cracking damage surrounding the shaft. The results and conclusions of these efforts to develop the sensor and embedded electronic design will be presented.

Subject
Electric current, Coils, Test methods, Primers, Electrochemical impedance spectroscopy, Water, Sensors, Materials, Impedance, Defects, Steel, Electrodes, Damage
Keywords:
Corrosion sensors, coatings, pitting/cracking detection, coating breakdown, barrier properties, seawater ingress detection, materials evaluation, embedded systems
© 2014 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2014
Association for Materials Protection and Performance (AMPP)
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