Structural health monitoring (SHM) is one of the most important tools available for the maintenance, safety, and integrity of aerospace structural systems. Lightweight, electromagnetic-interference- immune, fiber-optic sensor-based SHM will play an increasing role in more secure air transportation systems. Manufacturers and maintenance personnel have pressing needs for significantly improving safety and reliability while providing for lower inspection and maintenance costs. Undetected or untreated damage may grow and lead to catastrophic structural failure.

The Fiber Bragg Grating/Piezo Array sensor-actuator system as a concept demonstration for structural health monitoring.
Damage can originate from the strain/stress history of the material, imperfections of domain boundaries in metals, delamination in multi-layer materials, or the impact of machine tools in the manufacturing process. Damage can likewise develop during service life from wear and tear, or under extraordinary circumstances such as with unusual forces, temperature cycling, or impact of flying objects. Monitoring and early detection are key to preventing a catastrophic failure of structures, especially when these are expected to perform near their limit conditions.

The ultimate goal of SHM technology is to develop autonomous (preventive) maintenance systems for continuous monitoring, inspection, and damage detection of structures with minimum labor involvement in real time, and in order to prevent catastrophic structural failure with timely service/maintenance. The ultimate solution will include both advanced hardware and advanced mathematical algorithms.

On the hardware side, a high-speed, high-channel-count fiber-optic sensor interrogation system was developed. On the SHM algorithmic side, algorithmic methods were developed for characterizing the damage from sensory data collected over several strategically placed sensors.

A dynamic response-based damage detection technique is relatively easy to implement and offers a wealth of differential diagnostic capabilities. The basic assumptions of this technique are that the dynamic parameters such as natural frequencies, mode shapes, transfer functions, or response functions depend on the physical properties across the structures. Therefore, the changes in these dynamic characteristics can be used to locate and assess problem areas. Smart optical fiber Bragg grating (FBG) sensors have been increasingly used in SHM, and they could be either surface-bonded or embedded within the structures, and form an array of sensors for dynamic response measurement. For a small-scale demonstration, Lamb waves are excited by a single piezoelectric actuator and captured by three FBG sensors whose response is in turn captured by a parallel processing FBG interrogator capable of sampling each sensor simultaneously at hundreds of kilohertz. The number of sensors required for damage detection is fewer than low-frequency techniques that can also use FBGs, such as those based on the mode shapes of the structure.

This work was done by Richard J. Black, Ferey Faridian, Behzad Moslehi, and Vahid Sotoudeh of Intelligent Fiber Optic Systems Corporation (IFOS) for Dryden Flight Research Center. DRC-010-015

NASA Tech Briefs Magazine

This article first appeared in the October, 2012 issue of NASA Tech Briefs Magazine.

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