In this paper, the damping and dynamic stiffness of UHN125C carbon fiber/epoxy composite beam was experimentally measured. The effect of fiber orientation angle and stacking sequences on damping, resonance frequency, and dynamic stiffness was discussed with a focus on the effect of moisture absorption. Dried specimens were immersed in distilled water for a certain period to absorb water for 8, 16, and 24 d, respectively, and the moisture content absorbed in the specimen was measured. Furthermore, using the impact hammer technique, the measurements of dynamic responses were conducted on a cantilever beam specimen with one end clamped by bolts and metal plates. The damping properties in terms of loss factor were approximated by half-power bandwidth technique. The dynamic stiffness was evaluated using resonance frequency as a function of moisture content. The damping increased with the increase of moisture content; however, the dynamic stiffness reduced with the reduction of resonance frequency. The results of the dynamic stiffness were aided by measuring the dynamic strain using DBU-120A strain-indicating software. The increment in the dynamic strain strengthened the results obtained for dynamic stiffness.
Electric strain gauges have been the sensor of choice in structure monitoring applications. Recently, with the invention of fiber optic Bragg gratings and the rapid advance oftelecommunication industry, optical stiain sensors based on fiber Bragg grating is increasingly challenging the dominating position of the electric gauges. In this paper, we first discuss the performance and cost issues of typical electric strain sensing systems. This is followed by a description ofthe operating principles of fiber optic Bragg grating based strain sensors and a low-cost digital wavelength domain multiplexing technology for the interrogation of Bragg grating based sensor arrays. By comparing the performances and costs of electric and fiber Bragg grating based strain sensing technology at system level, we conclude that fiber optic strain sensing system is not only superior in performance but also provide lower cost per sensor for applications that requires more than 20 sensing points. Finally, a number of structure monitoring applications currently under investigation in Smart Material and Structures Research Laboratory at University of Maryland are presented.
In this article, low-velocity impact characteristics of UHN125C carbon fiber/epoxy composite, including unidirectional (0°), cross-directional (0°/90°), and quasiisotropic layups, were experimentally measured. The effect of the fiber orientation angle and stacking sequences on impact force and induced strain were measured via an instrumented drop-weight apparatus with special concern for the moisture absorption effect. Dried specimens were immersed in distilled water for a certain period of time to absorb water for intermediate and saturated moisture content. It was observed that the impulse was reduced with the increase in moisture content; on the other hand, strain increased with moisture, as measured by DBU-120A strainindicating software (MADSER Corp., El Paso, TX). Impact damage is widely recognized as one of the most detrimental damage forms in composite laminates because it dissipates the incident energy by a combination of matrix damage, fiber fracture, and fiber-matrix debonding. Therefore, it is extremely important to know the impact strength of a structure, especially for applications in industries such as aerospace, ship design, and some other commercial applications. The use of composite materials in engineering applications is increasing rapidly because they have higher strength-to-weight ratios than metals. The strength, stiffness, and, eventually, the life of composite materials are affected more than conventional materials by the presence of moisture and temperature. Thus, it is necessary to analyze the response of composites in a hydrothermal environment.
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