Composite materials offer several advantages over metals for structural applications, such as weight saving, tailoring, etc. However, they can be subject to visible and invisible damage leading to catastrophic failure.

Lamb Wave-based Structure Health Monitoring (SHM) Systems have been developed in recent years for such structures. These systems use multiple sensors embedded or surface mounted directly into the composite. The basic principle is that they can transmit a pulse of energy with a PZT transducer in the ultrasonic frequency and receive it with another PZT transducer or delay and receive the echo to detect damage.

In addition, hybrid Fiber Bragg Grating (FBG) acousto-optic Lamb wave systems are being tested. The advantage of these systems is that they can be operated in a passive or active mode and provide weight saving advantage while maintaining good sensitivity.

A hybrid acousto-optic lamb wave system that uses Polarization Maintaining (PM) FBG is investigated in this project. The PM-FBG can measure either axial or perpendicular allowing area strain. This system can be operated in both a passive as well as active mode. In the passive mode, axial and torsional loads can be measured. In the active mode, the type of damage, its size and location can be determined. Finite element analysis is used for sensitivity analysis and optimum placement of the gratings.

Three composite structures (honeycomb, hat stiffening, and fiber reinforced foam) without flaws are fabricated (two of them with PM-FBGs). Impact measurements are performed on each specimen with various energies.

The testing allows both transverse and longitudinal stress characterization; whereas, this should also help in mapping the location of damage and rank each structure that propagates the acoustic waves with each impact loads.

This study is supported by funds from the National Aeronautics Space Administration (NASA) in collaboration with the Vanderbilt University IGERT program on Risk and Reliability Engineering.

REFERENCES

1. Lee B C, and Staszewski, "Modeling of Lamb waves for damage detection in metallic structures: Part I. Wave propagation," Smart Mater. Struct., 12 (2003) 804-814.

2. Coppola, G., Minardo, A., Cusano, A., Breglio, G., Zeni, G., Cutolo, A., Calabro, A., Giurdano, M. and Nicolais, L., "Analysis of feasibility on the use of fiber Bragg grating sensors as ultrasound detectors," Smart Structures and Materials 2001: Sensory Phenomena and Measurement Instrumentation for Smart Structures and Materials, Proceedings SPIE vol. 4328, 2001, Bellingham WA: SPIE Optical Engineering Press, pp. 224-232.

3. Takeda N., Okabe Y., Tsuji R., Takeda S-i., "Application of chirped fiber Bragg grating sensors for damage identification in composites," Smart Structures and Materials 2002.

4. Mendez A., Morse T.F., "Specialty Optical Fibers Handbook First Edition," Academic Press 2007.

ACKNOWLEDGEMENTS

This study is supported by funds from the National Aeronautics Space Administration (NASA) in collaboration with the Vanderbilt University IGERT program on Risk and Reliability Engineering.