CH3 - 3. Technical approach As described above, the goal of...

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3. Technical approach As described above, the goal of this work is to develop a miniature instrument based on optical fiber sensors to perform DMA in situ during composite manufacturing. The approach is to attach a fiber optic strain gage to a miniature actuator, as illustrated in Figure 3-1. Candidate materials for the actuator include piezoelectric, magnetostrictive, and shape memory alloy materials. Piezoelectric actuators were chosen due to their ability to produce large forces and measurable strains, and due to their compatibility with typical composite manufacturing methods, including autoclaves, RTM, and compression molding. By immersing the sensor/actuator assembly into a curing thermoset resin and applying a steady-state sinusoidally varying excitation to the actuator, a time-varying shear stress will be transferred from the actuator into the adjacent resin. The mechanical response of the resin and the sensor/actuator assembly will be constrained by the changing mechanical impedance of the resin. Therefore, the rheological properties of the resin, which can be related to the mechanical impedance by the sensor geometry, can be determined by monitoring the strain response of the sensor assembly through the optical fiber strain gage. PZT piezoelectric actuator EFPI strain gage silicone elastomer epoxy epoxy input fiber conductive epoxy electrical leads Figure 3-1. Prototype rheometer sensor. 3.1 Fiber Optic Strain Gage An optical fiber sensor design was chosen for the strain gage element of the rheometer because the small size of the fiber presents a good potential for miniaturization. Optical fibers may be embedded in fiber reinforced composites with little or no change in the
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15 Russell G. May, Jr. Chapter 3. Technical Approach 15 mechanical properties of the composite if the sensor is aligned with the reinforcing fibers. 27 In addition, the use of optical fiber strain gages improves immunity to electromagnetic interference, which may plague resistive strain gages in a typical manufacturing environment. Other attributes of optical fibers that commend them for this application include elimination of ground loops, excellent strain resolution, and an ability to operate at high temperatures. In principle, the embedded viscoelasticity sensor can be used as a strain gage after fabrication of the composite part, in order to monitor internal strains. The optical fiber sensor used for measuring strain in the viscoelasticity sensor is a modification of the extrinsic Fabry-Perot interferometer (EFPI) design. 28 The standard EFPI sensor is a phase-modulated sensor that uses optical path length difference in an interferometric cavity to measure strain, as illustrated in Figure 3-2. A single-mode fiber, used as an input/output fiber, and a multimode fiber, used purely as a reflector, form an air gap that acts as a low-finesse Fabry-Perot cavity. The hollow core fiber, a glass capillary tube, serves to align the two fibers collinearly. As the sensor is strained, the silica tube and
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CH3 - 3. Technical approach As described above, the goal of...

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