Structural Health Monitoring Using High-Density Fiber Optic Strain Sensor and Inverse Finite Element Methods

Abstract In an effort to mitigate accidents due to system and component failure, NASA’s Aviation Safety has partnered with industry, academia, and other governmental organizations to develop real-time, on-board monitoring capabilities and system performance models for early detection of airframe structure degradation. NASA Langley is investigating a structural health monitoring capability that uses a distributed fiber optic strain system and an inverse finite element method for measuring and modeling structural deformations. This report describes the constituant systems that enable this structural monitoring function and discusses results from laboratory tests using the fiber strain sensor system and the inverse finite element method to demonstrate structural deformation estimation on an instrumented test article. Introduction NASA Langley Research Center has developed instrumentation based upon principles of Optical Frequency-Domain Reflectometry (OFDR) for the provision of large-scale, densely distributed strain sensors using optical fiber embedded with Bragg gratings. Fiber Optic Bragg Grating technology enables the distribution of thousands of sensors that are immune to moisture and electromagnetic interference and have negligible weight penalty. The theory and development of this technology, called FOSS (Fiber Optic Strain System) is described in [1]. The algorithms and methods for deriving strain from an OFDR measurement system are described in [2]. At NASA Langley, this technology provides a key component for research and development relevant to comprehensive aerospace vehicle structural health monitoring (SHM). It has been lab tested in various applications. Notably, FOSS was used during structural testing of an advanced composite transport wing box [3]. An integrated prototype system has been developed that includes hardware and software for the acquisition of data from an optical network and conversion of the data into strain measurements. To enable a comprehensive structural health capability, an inverse finite element method (iFEM) has been developed that uses the measured strain data to compute the full-field displacements and, subsequently, internal loads experienced by the structure in real time. The technique is applicable to thin and moderately thick beams, plates, shells, and built-up structures. The mathematical foundation for iFEM is described in [5]. This report briefly describes the FOSS system and the inverse finite element method and demonstrates the capability of the approach using a simple structural test. The structural test was conducted on an instrumented beam test article subjected to tip deflection. The strain measurements were used by the inverse finite element model to reconstruct the deformed shape of the beam. Details of the configuration of the beam test article and the experimental results are described. The report concludes with a discussion of issues and potential areas of future research.