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7.1.1 Initial Ring Sensor Concept

This chapter outlines the development of an ultrasonic sensor based on the concept of a ring resonator (Figure 7.1). Its principle of operation is based on the localization of a fundamental resonance mode at a frequency which is tunable by design. This resonance mode is preferentially excited by out-of-plane motion. The ring deformation is sensed by an FBG and PWAS which are bonded to the ring. This concept has been termed a piezo- optical ring sensor (Giurgiutiu 2014) due to its ability to use a piezoelectric or fiber-optic sensing element, and is called a ring sensor in this work for conciseness. Contributions to the ring sensor development including design, evaluation, modeling, and refinement have been published in two conference proceedings (Frankforter, Lin, and Giurgiutiu 2014; Frankforter et al. 2015) and two journal articles (Giurgiutiu et al. 2014; Frankforter, Lin, and Giurgiutiu 2016).

The initial concept of the ring sensor was presented in a 2013 dissertation by Roman (2013); a patent was awarded (Giurgiutiu et al. 2013). The objective was to develop a resonator which could use an FBG sensor to detect out-of-plane motion associated with AE events in a manner similar to conventional AE sensors. This dissertation entered

been developed. Although the ring sensor was designed for fiber-optic sensing, its capability to excite an FBG had not yet been demonstrated. Similarly, its capabilities to serve as an ultrasonic wave sensor had also not been established. In this chapter, the proof of concept for the ring sensor to detect guided waves using an FBG sensing element is demonstrated.

Figure 7.1 (a) 100 kHz ring sensor geometry and (b) fundamental resonance mode shape (Roman, 2013)

7.1.2 Initial Refinement of the Ring Sensor

Development of the ring sensor geometry by Roman (2013) occurred in multiple stages. Initially, a ring-shaped geometry was used (a circular outer profile and circular inner hole). This allowed the resonance frequency f_i to be approximated using a formula from the work of Blevins (Blevins 1980; Roman 2013):

2 2 2 ( 1) 2 ( 1) i i i EI f m R i     (7.1)

where i is the mode number, R is the radius of the midline of the ring, E is the Young’s Modulus, I is the moment of inertia, and

m

Roman (2013) then refined the ring geometry in an attempt to maximize the response and provide better bonding characteristics (Figure 7.1a). Instead of a circular hole for the ring, an elliptical hole was tested with the idea of forcing the maximal displacement preferentially along the major axis of the ellipse on which the FBG was placed. The bottom surface was flattened to provide better bonding characteristics, with the top surface flattened as well for symmetry; the top surface also permitted the bonding of a PWAS as a second sensing element. A small hole was incorporated along the major axis of the ellipse to bond an FBG sensing element. These changes gave the ring sensor a fundamental “breathing”-type vibrational mode (Figure 7.1b) analogous to the motion of a breathing crack. The two lateral sides of the ring sensor move 180° out of phase from each other, tensioning and compressing an FBG stretching across the hole and exciting a PWAS on the top surface.

Roman (2013) used the commercially available FEM software ANSYS Workbench to perform 3D modal analysis and created designs with resonance frequencies targeted at 100 kHz, 200 kHz, and 300 kHz. For reference, the 100-kHz ring sensor was 8.00 mm in diameter and the 300-kHz ring sensor was 4.35 in diameter. These are roughly the size of the smallest commercially available AE sensors (for reference, the MISTRAS Pico sensor is 4.78 mm in diameter and 3.94 mm in height). The 100 kHz and 300 kHz ring sensors

frequencies were experimentally validated using the EMIS method and chirp excitation, yielding resonance frequencies at 113 kHz for the nominally 100 kHz ring sensor, and 270 kHz for the nominally 300 kHz ring sensor respectively.

7.1.3 Ring Sensor Proof of Concept – Scope and Approach

From a review of AE, fiber-optic, and ultrasonic sensor design literature, it was found that the scope needed to be expanded for the development of a sensor which could be practical for SHM application. The initial concept was to develop a sensor which detected out-of-plane motion from AE events using resonance amplification principles (Roman, 2013). Two additional goals were added which could be readily tested:

• Create a sensor whose FBG detects a higher strain than a plate-bonded FBG. Previously, it was assumed that sensing about a resonance frequency would be sufficient for amplification; however, there are losses that need to be minimized in the transfer of energy to the optical fiber.

• Create a sensor which imparts omnidirectional sensing capabilities to an FBG. Plate-bonded FBGs have the major limitation that they only sense along the optical fiber’s longitudinal axis.

A third goal: “create a sensor that isolates its FBG from quasi-static strain” required the use of additional facilities, and was thus tested later (Chapter 9): This third goal ensured that the FBG would not cease operation when modest structural loads provided produced strains which exceeded the FBG strain range allotted for dynamic sensing.

To develop a proof-of-concept for the use of the ring sensor as an in situ FBG ultrasonic wave sensor, the following experimental work was performed:

• Testing an FBG sensing element on a free (unbonded) ring sensor

• Experimental validation of a plate-bonded ring sensor’s capability to detect guided waves

• Characterizing the sensor’s mechanical performance, including bonding effects, strain amplification, and directional response

The rest of this chapter focuses on this proof of concept and characterization of the initial ring sensor prototype. The focus was on the 100 kHz prototype as this was larger and easier to handle. It also served as a good basis for miniaturization and sensitivity improvements as discussed later in Chapter 9.