TECHNOLOGY AREA(S): Air Platform, Ground/Sea Vehicles, Materials/Processes

ACQUISITION PROGRAM: PEO Ships + SEA21

OBJECTIVE: Develop an intelligent, adaptable, distributed fiber optic (FO) network for Condition Based Management by Continuous Based Monitoring (CBM2) of Naval systems. The distributed FO network should be capable of monitoring in real-time the temperature, load, structural fatigue, vibration, impact events and acoustic emissions (AE) at many points along each optical fiber of the network with sampling rates as high as 1 MHz (such as for acoustic emissions monitoring). The same Fiber Optic Condition Based Management (FO-CBM) network should be capable of generating ultrasonic waves, tunable from 10 kHz to 500 kHz, at multiple points along the same FO network for the purpose of actively probing the system for damage near hot spots.

DESCRIPTION: The Navy is currently developing concepts to expand the size of the fleet. A major consideration in this planning is extending the service life of current operational assets beyond their original design lives. As an alternative to mid-life and beyond structural modification or refurbishment of Navy vessels, continuous hull and structural health monitoring combined with prognostics can provide estimates of remaining service life, time to failure, and time to maintenance. CBM2 plus prognostics can also provide: real-time operational situational awareness; validate new structural design or structural repairs under real loading environments; accelerate the incorporation new materials while lowering the risk; help with the validation of new structural models; improve maintenance planning; and increase asset availability while providing an overall cost saving to the Navy.

FO sensors have become an important class of sensors under consideration for CBM2 applications. This is in part due to the high static and dynamic strain sensitivity that they afford, which are comparable to their state-of-the-art electrical counterparts. In other respects, such as immunity to electromagnetic interference (EMI), corrosion resistance, a high dynamic range, large strain-to-failure, sensing linearity, reliability, and a small footprint, FO sensors are superior to their electrical counterparts. Not only are these sensors physically small (~ 0.1 mm3) and lightweight (~ mgrams/meter), but many sensors (and sensor modalities) can be incorporated into a single optical fiber, each with very large sensing bandwidth. Despite these advantages, FO sensors have mostly operated in a quasi-static sensing mode requiring external energy sources to mechanically excite the structure, such as from sea wave motion, ship maneuvering, and take-off and landing events. Unfortunately, some damage mechanisms and defect types have a high-frequency vibration signature (e.g., ship wave slamming or acoustic emissions from crack growth) while other types of damage have a weak or nonexistent acoustic response to typical operational loads (e.g., corrosion and plastic deformation). A highly desirable feature of any CBM2 system is the ability to operate simultaneously in passive and active interrogation modes over a broad range of frequencies. For example, strain and temperature could be monitored quasi-statically (below 100 Hz typically) while impact events and acoustic emissions would be monitored at higher frequencies (above 1 kHz). Active interrogation by generating ultrasonic waves in the range of 10 kHz to 500 kHz would increase the signal-to-noise ratio for detecting the presence of damage such as cracks, corrosion, and other defects.

This SBIR topic seeks innovative approaches to develop an intelligent, adaptable, distributed passive and active FO CBM2 system. The FO system should be capable of passively monitoring: 1) static and quasi-static (0 ~ 100 Hz) strains and temperature for fatigue or usage monitoring, 2) vibrations (100 Hz ~ 10 kHz) for impact or mechanical subcomponent monitoring, and 3) acoustic emissions (10 kHz ~ 1 MHz) for crack localization and growth monitoring on a mechanical or structural system at many points along each optical fiber of the network. The same FO network should be capable of incorporating ultrasonic transducers at multiple locations along the fiber network and should be expandable to accommodate more transducers as the number of hot spots grows over time. Direct conversion of electromagnetic energy into ultrasonic energy [Ref 2] is just one of several approaches for generating ultrasonic waves. Approaches that provide the most control of the ultrasonic wave parameters (amplitude, frequency, duration, etc.) will be given higher preference for continued development. The sensors and ultrasound transducers (UTs) in each fiber of the network could be addressed independently or collectively depending on the operational conditions, monitoring requirements, or system architecture. The FO-CBM system should be capable of operating autonomously, be able to triangulate the location of cracks or impact events, and be capable of actively tracking the progression of damage. All data recordings should be time stamped with a local system clock which should be periodically synchronized with the platform master clock. If the technology proves to be reliable, during the Phase II Option period (if exercised) and for purposes of data analysis, diagnostics, and prognostics, data formats should be made compatible with other platform data streams such as from machinery, platform operations, and environmental parameters.

PHASE I: Define and develop a concept for an intelligent, adaptable, distributed, active and passive FO-CBM system that can meet the capabilities outlined in the description above. Design the software and hardware of the FO-CBM system to be capable of monitoring temperature, quasi-static strain, vibration, and acoustic emissions in each fiber of the network in at least 64 points along each fiber (target fiber length is 100 m). Ensure the system is capable of generating ultrasonic waves in at least eight locations along each optical fiber. Demonstrate a benchtop FO-CBM system consisting of a single fiber with a minimum of one ultrasound transducer and a minimum of four of each temperature, strain, vibration, and AE sensors, all in the same optical fiber and bonded to one or several 3�x3�x1/8� Aluminum plates. Develop a Phase II plan.

PHASE II: During Phase II effort, the contractor will complete the purchase of all the components necessary for the development of a prototype FO-CBM network, consisting of a minimum of 4 optical fibers, each fiber with a minimum of 4 ultrasound transducers and 64 FO sensor for monitoring temperature, quasi-static strain, vibration and acoustic emissions.� As part of the final validation, the contractor will install the system in 4 aluminum panels, each measuring 3�x3�x1/8�, with a single optical fiber per panel and demonstrate that the system is capable of generating ultrasonic waves and monitoring temperature, strain, vibration and acoustic emission events in each panel.

PHASE III DUAL USE APPLICATIONS: The company will be expected to support the Navy in transitioning the technology for Navy use.� The company will further refine the prototype for production and determine its effectiveness in an operationally relevant environment.� The company will support the Navy for test and validation to qualify and certify the system for Navy use.

REFERENCES:

1. Wang, G., Pran, K., Sagvolden, G., Havsgard, G.B., Jensen, A.E., Johnson, G.A. and Vohra, S.T. �Ship hull structure monitoring using fibre optic sensors�, Smart Materials and Structures. Vol. 10, pp. 472�478, (2001). http://iopscience.iop.org/article/10.1088/0964-1726/10/3/308/pdf

2. Tian, J., Zhang, Q., and Han, M. �Distributed fiber-optic laser-ultrasound generation based on ghost-mode of tilted fiber Bragg gratings�, Optics Express, Vol. 21, No. 5, pp 6109- 6114, (2013). http://digitalcommons.unl.edu/electricalengineeringfacpub

3. R�ger, M., B�ttger, G., Dreschmann, M., Klamouris, C., Huebner, M., Bett, A.W., Becker, J., Freude, W., and Leuthold, J. �Optically powered fiber networks�, Optics Express, Vol. 16, Issue 26, pp. 21821-21834, (2008). https://www.researchgate.net/publication/23686186_Optically_powered_fiber_networks

4. Frankenstein, B., Fischer, D., Weihnacht, B. and Rieske, R. �Lightning Safe Rotor Blade Monitoring Using an Optical Power Supply for Ultrasonic Techniques�, 6th European Workshop on Structural Health Monitoring - Fr.2.C.3. http://www.ecphm2012.com/Portals/98/BB/fr2c3.pdf

KEYWORDS: Condition Based Management; Continuous Based Monitoring; Structural Health Monitoring; Optical Fibers; Bragg Gratings; Acoustic Emission; Ultrasound Generation; Sensors and Actuators; Crack Detection