534 C. DeValve et al. structures such as the aforementioned helicopter rotors or wind turbine blades is essential to maximizing the performance and lifespan of these structures while minimizing their adverse effects on surrounding individuals. Currently, the majority of vibration suppression methods in rotating composite structures consist of complex active techniques that add weight and intricacy to the structure, and therefore a passive means of vibration suppression built into the composite material of these rotors would be beneficial. Previous research has shown that there is a weak interfacial bond at the CNT-matrix interface when CNT’s are incorporated into thermosetting composite matrices [9], which has been sought to be remedied by methods such as CNT surface functionalization [10, 11]. Although this weak interfacial bond may be seen as a material defect, this characteristic can alternatively be exploited since a stick-slip action resulting in energy dissipation can occur at the CNT-matrix interface as the material is put under strain [12, 13]. Experimental measurements have supported this theory by demonstrating that the damping in CNT-reinforced composites increases with increasing strain [6, 7, 14]. Considering that a rotating structure is subject to increased loads and strains due to rotation-induced forces, it is reasonable to infer that the CNT damping mechanism described above has the potential to provide a significant damping enhancement to rotating composite structures during operation when increased strains are present. Building upon this idea, the current work presents an experimental study which investigates the effects of matrix-embedded CNT’s on the modal parameters of rotating fiber-reinforced composite systems, using Operational Modal Analysis (OMA) techniques [15, 16] and the Eigensystem Realization Algorithm (ERA) [17] to analyze data measured from rotating cantilever composite beams. 52.2 Method The experimental investigation focuses on rotating fiber-reinforced CNT-infused composites in order to explore the influence of CNT’s on the modal properties of functional engineering composite structures. Single-Walled Carbon Nanotubes (SWCNT’s) are used in the present study, which were purchased from SES Research (Houston, TX) with >90% purity and dimensions of <2 nm in diameter and 5–15 m in length. The composite samples were made via hand layup and compression molding, and the cured three-part composites were tested using an in-house developed experimental test setup and a proprietary ERA code developed at the University of Bristol. 52.2.1 Composite Fabrication The thermosetting-matrix fiber-reinforced SWCNT-infused composite samples were fabricated using compression molding techniques in which the resin flows through the thickness of the part as it is being compressed to ensure uniformity of CNT dispersion throughout the sample. The CNT-epoxy mixture was prepared as follows: First, EPON 826 resin (Hexion Specialty Chemicals, Columbus OH) was mixed with Heloxy Modifier 68 (Hexion Specialty Chemicals, Columbus OH) with a ratio of 4:1 by weight in order to reduce the viscosity of the EPON 826 resin. Appropriate amounts of this resin mixture and CNT’s were then weighed into a beaker using an Ohaus Explorer Pro balance to achieve the appropriate weight percentage of the CNT’s in the final epoxy. The nanotubes were then delicately mixed into the resin by hand using a medical grade utensil in order to ensure that the CNT’s were integrated with the resin before transport from the balance. After this brief manual stirring, the resin-CNT combination was blended using a IKA T25 high-shear mixer for 90 s. A Cole-Parmer 500 W ultrasonic processor with the appropriate size probe was then used to process the sample using a pulsed on-off mode with 1 s pulses for a total processing time of 5 min. The appropriate amount of hardener was then added to the sample and immediately incorporated with the CNT-resin sample using manual stirring for 2 min and the high-shear mixer for an additional 90 s. The entire resin sample was then transferred to a larger diameter beaker to increase the exposed surface area of the resin and degassed for 30 min. The composite beams were fabricated using a hand-layup technique and compression molded using a three-part aluminum mold consisting of a separable top section, a middle spacer frame, and a bottom plate, presented in the left and center portions of Fig. 52.1, along with a finished composite plate shown on the far right-hand side of the figure. The top and bottom mold sections were sealed to the middle spacer using an O-ring fitted to a machined groove in each respective piece. A 3 K plain weave carbon fiber fabric (Fibre Glast Development Corp, Brookville OH), with 12.5 picks and wefts per inch, a nominal thickness of 0.3 mm, and an areal weight of 193g=m2 was used as the reinforcement fabric. Precise amounts of the epoxyCNT mixture was poured between every two layers of carbon fiber fabric during the hand layup to minimize any filtering of the CNTs from the epoxy during the resin-permeation stage of the compression molding process. The sample was then processed using a programmable hot press from Tetrahedron Associates (San Diego, CA) using the following cure cycle:
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