TECHNOLOGY AREAS: Air Platform, Ground/Sea Vehicles, Materials/Processes

ACQUISITION PROGRAM: NAVSEA; NAVAIR

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), which controls the export and import of defense-related material and services. Offerors must disclose any proposed use of foreign nationals, their country of origin, and what tasks each would accomplish in the statement of work in accordance with section 3.5.b.(7) of the solicitation.

OBJECTIVE: To develop a multi-channel, distributed fiber optic (FO) acoustic emissions (AE) monitoring system for the detection of impact damage and cracks in structural components. The sensor system should be capable of detecting AE events from growing cracks even in the presence of a quasi-static background strain field (produced by quasi-static loads and/or a background temperature field). The system should have a small footprint, be able to operate unattended aboard a ship, submarine or aircraft and should be able to triangulate the location of cracks or of the impact location.

DESCRIPTION: A non-intrusive system for the reliable detection of cracks in aging DoD structures (ships, subs and aircraft) as well as in next generation weapon systems is critically needed. AE monitoring is the only proven method for detecting cracks in metals without having to place the sensor directly on top of the crack. However, present AE monitoring systems suffer from various limitations. The sensors are big and bulky, each sensor needs two wire leads (or a coaxial cable) to pick up the signal, the wire leads have to be heavily shielded to avoid EMI, each sensor needs a pre-amplifier and signal conditioner nearby, and two more wire leads are required for each amplifier. All this makes current technologies intrusive, heavy, susceptible to EMI, with many failure points and overly complicated. Techniques that use fiber optic Bragg gratings offer the opportunity of solving all these limitations. A single optical fiber will have embedded in it various Bragg grating sensors, all sensors will be interrogated using a single light source beam, and since there is little attenuation in the fiber there will be no need for pre-amplifiers or signal conditioners. Also, the system does not require EMI shielding since it is optical in nature. Finally, recent advances in fiber optic demodulation offer enhanced sensitivity for crack detection.

PHASE I: During the phase I the contractor will demonstrate the ability to monitor acoustic emissions in a loaded Aluminum panel by using the advanced fiber optic sensor concept. The system will have a minimum of four sensors in a single fiber optic sensing line. Acoustic emissions will be simulated by performing pencil break tests and/or by producing short burst of energy from an ultrasonic transducer centered around 300KHz. The loading level will range from 10% to 60% of the yield strength of the aluminum plate. The sensitivity of the system will be compared theoretically and experimentally to that of a standard single channel piezoelectric AE transducer.

PHASE II: During the Phase II the contractor will develop all the necessary optical and electronic components for a multi-channel (8 channels minimum), stand alone, dynamically reconfigurable, adaptive acoustic emission monitoring system with a small foot print and with a multiple-channel tunable laser source for enhanced signal to noise performance. By dynamically reconfigurable it is meant that if there are 64 FBGs in a single fiber then the system should be able to reconfigure itself so as to monitor at least 8 FBGs that are closest to the flaw or the impact point. By adaptive it is meant that the AE events can be detected in the presence of a quasi-static background strain. By stand alone it is meant that the system will collect, analyze, compress and store all AE events and strain history at the switch of a button for a period of at least one week.

PHASE III: A health monitoring system of this nature could be installed in many DoD platforms (including destroyers, cruiser, amphibious ships, submarines, fighter, patrol and transport aircraft) which have key structural components (such as pressurized bulkheads, wing attachment point, rudders and propellers) that require periodic inspections to ensure the lack of cracks. Significant cost savings could be achieved by the installation of such a system and therefore, performing maintenance at longer time intervals or only when the system indicates that it is required. The contractor, in collaboration with the Navy monitoring team, will seek a potential military application and/or demonstration during Phase III.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Commercial aviation would benefit significantly from a system of this nature as well. The same problems that we experience in our platforms (ships, aircraft and ground vehicles) are experienced by equivalent commercial platforms. For example, wide spread area fatigue damage has been determined to be a major source of problem for commercial aviation.

REFERENCES:
1. "Adaptive two-wave mixing wavelength demodulation of Fiber Bragg Grating dynamic strain sensors" Yi Qiao, Yi Zhou, and Sridhar Krishnaswamy, , Applied Optics, vol. 45, No. 21, pp 5132-5142.

2. "Structural Health Monitoring System for Detecting Impact Events and Acoustic Emissions," Yi Qiao and Sridhar Krishnaswamy, Proceedings of the Third European Workshop on Structural Health Monitoring, 2006, Ed. A. Guemes, DEStech Publications Inc.

3. "Multiplexed adaptive two-wave mixing wavelength demodulation of fiber Bragg grating sensor for monitoring both dynamic strains and quasi-static drifts," Yi Qiao and Sridhar Krishnaswamy, SPIE vol. 6167, Smart Structures and Materials 2006: Smart Sensor Technology and Measurement Systems; Daniele Inaudi et al; Eds., March 2006.

KEYWORDS: Optical Fibers, Bragg Gratings, Acoustic Emission, Health Monitoring, Condition based maintenance, CBM

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