This report discusses the guided Lamb wave sensing using polarization-maintaining (PM) fiber Bragg grating (PM-FBG) sensor. The goal is to apply the PM-FBG sensor system to composite structural health monitoring (SHM) applications in order to realize directivity and multi-axis strain sensing capabilities while reducing the number of sensors. Comprehensive experiments were conducted to evaluate the performance of the PM-FBG sensor attached to a composite panel structure under different actuation frequencies and locations. Three Macro-Fiber-Composite (MFC) piezoelectric actuators were used to generate guided Lamb waves that were oriented at 0, 45, and 90 degrees with respect to PMFBG axial direction, respectively. The actuation frequency was varied from 20 kHz to 200 kHz. It was shown that the PM-FBG sensor system was able to detect high-speed ultrasound waves and capture the characteristics under different actuation conditions. Both longitudinal and lateral strain components in the order of nano-strain were determined based on the reflective intensity measurement data from fast and slow axis of the PM fiber. It must be emphasized that this is the first attempt to investigate acouto-ultrasonic sensing using PM-FBG sensor. This could lead to a new sensing approach in the SHM applications.
Fiber Bragg Grating-based (FBG) strain sensors have been widely used in engineering applications requiring small size,
light weight, amenability to multiplexing, and very fast response times. State-of-the-art FBG interrogators are capable of measuring as low as sub micro strains and as high as 1% fiber strain in tension and higher still under compression. In this paper, we will discuss the development of an FBG based real-time instrumentation system to conduct highly
dynamic strain measurements during an impact. A high-speed FBG interrogation system was used along with an FBG
sensor data analysis software for efficient post-processing. In order to capture high strain data during an impact event, one needs to conduct measurements at very fast speeds and simultaneously to maintain FBG sensor survivability. A high strain FBG fixture was designed accordingly. Such high strain fixture allows the FBG strain sensor to measure the actual field strain with a reduction factor K in order to expand the strain measurement range. Numerical simulation results using finite element analysis (FEA) were used to validate the high strain fixture design analysis. Finally, a proof-ofconcept FBG-based high strain measurement system has been demonstrated to measure dynamic strain data under impact tests. Experimental strain reduction factors were determined from the strain data and correlated well with FEA predicted values.
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