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Health Monitoring of Composites with Embedded Fiber Bragg Gratings

  • Author(s): Yeager, Michael J.
  • Advisor(s): Todd, Michael D
  • et al.
Abstract

The aim of this dissertation is to investigate a systems-level approach to the structural health monitoring (SHM) of composite laminate material systems with embedded fiber Bragg gratings (FBG). Although applicable across many structural applications, this research focuses on damage assessments for submarine components. Some of the pragmatic issues surrounding embedded FBGs are discussed including the embedment process, robustness, and egress of the embedded fiber. A design solution is presented for the egress of embedded fibers that could potentially break down barriers towards eventual commercialization of embedded FBGs. The survivability of the design is explored through mechanical shock testing.

The detectability of connection damage is explored in the form of both bearing damage and mechanical fastener loosening. Both time and frequency domain features are used for damage detection in a bolted specimen subjected to incremental bearing damage. A novel approach to mechanical fastener preload monitoring is presented using a specialized washer design that uses a spectral shape feature for damage detection.

Impact damage assessment is performed on a representative test article. The statistical framework for final diagnosis is presented within a Mahalanobis distance based discrimination framework, and outlier analysis is then performed via a sensor-level voting scheme in the context of a binary hypothesis test. Some of the statistical inefficiencies of the traditional use of the Mahalanobis distance are addressed using robust estimations of locations and scatter as the inputs in the outlier detection. The performance of several robust distance calculations are compared to the traditional formulation. Finally, impact localization is performed with an event-agnostic algorithm and a novel imaging process as an alternative to conventional triangulation.

Current implementations of embedded FBGs in composites for SHM are limited to laboratory scenarios that address only certain components of the damage assessment process. This work provides a systems level investigation from specimen manufacturing to near field-ready damage assessment capabilities that is a step towards industrialization of embedded FBG networks for the SHM of composites.

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