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Assessment of Embedded Fiber Bragg Gratings for Structural Health Monitoring of Composites

M. TODD, M. YEAGER, C. KEY, W. GREGORY

Abstract


The use of composite materials has expanded significantly across civil, aerospace, and marine structures in both new and retrofit designs. The performance benefits from composites—typically, weight reduction with increased strength, corrosion resistance, and improved thermal and acoustic properties—are potentially offset by a variety of failure modes (delamination, disbonding, fiber breakage, matrix cracking, etc.) whose genesis and progression aren’t as well understood as traditional (often metallic alloy) materials. Structural health monitoring (SHM) necessarily involves acquisition of inservice raw data, with one attractive sensing solution for SHM systems being based on fiber Bragg gratings (FBGs). FBGs are photorefractive elements directly encoded into the optical fiber (via refractive index modulation) that function as strain gages, and their small footprint, minimal weight. conformability, lack of a spark source, and electromagnetic immunity enhance their performance over many other strain sensing architectures. More importantly, optical fibers may be embedded directly into composite structures at the time of their manufacture, allowing for integrated, internal sensing of material strain/load state. This paper considers the practical aspects of embedding FBG arrays in CRFP and GRFP composite structures manufactured by OVB and VARTM processes. In the first phase, the embedded specimens were subject to long-term water submersion to verify performance in this environment and compared to unwetted performance. Static load testing of the specimens was performed to both cross-compare results and compare to structural models. In the second phase, dynamic hammer testing was performed to investigate response performance in impact scenarios, and a number of features extracted from the responses were compared for their ability to characterize the damage induced. In the third phase, a larger, more complex jointed specimen was tested for bearing damage at the connections with a fully embedded FBG array interrogated by commerciallyavailable hardware.

doi: 10.12783/SHM2015/206


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