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12-24-2013, 08:34 AM,
Composite materials are being increasingly used in automotive, civil, marine, and especially weight sensitive aerospace Application, primarily because of its specific strength and stiffness. This necessitates studies on vibration and buckling behaviour of the structures. Most of the analysis on vibration of composite plates is done either analytically or by different numerical methods. Very little is reported on the experimental investigation of laminated composite plates using the present state of the art instrumentation or measurement. The present research is mostly experimental study based on vibration measurement and buckling behaviour of industry driven woven fiber composite panels for different layer thickness. The effects of different geometry, boundary conditions, aspect ratio and type of fiber on the natural frequencies of vibration of woven fiber composite panels are studied in this investigation. The effects of variation in temperature and moisture concentration due to hygrothermal conditioning, on the natural frequencies are also investigated. Critical buckling load is determined for laminates with various thicknesses. Experiments have also been conducted to study the vibration and buckling characteristics of carbon/glass hybrid plates for different lamination sequence and percentage of carbon and glass fiber. A finite element package, ANSYS 13.0 was used to obtain the numerical results and plot the mode shapes for various modes of vibration.
The composite plates of different layers are manufactured using woven carbon fiber by hand lay-up method followed by cutting to required dimension. The free vibration characteristics are studied with First Fourier Transform (FFT) analyzer, accelerometer using impact hammer excitation. The Frequency Response Function (FRF) is studied using Pulse Lab Shop to obtain a clear understanding of the vibration characteristics of the specimen. The critical buckling load is determined using INSTRON 1195.
From the results obtained it was observed that, the frequencies of vibration as well as critical buckling load increased with increase in thickness. For different boundary conditions, the modal frequencies were determined to be highest in case of fully clamped condition in comparison to all other boundary conditions. It was also observed that with increasing aspect ratio there is a gradual increase in the modal frequencies obtained, due to higher stiffness. As the conditioning temperature deviates from the manufacturing temperature, the natural frequencies decrease gradually. The increase in moisture concentration of the laminate results in decrease in the modal frequencies. When compared to Glass fiber reinforced polymer (GFRP) the natural frequencies of vibration obtained from Carbon fiber reinforced polymer (CFRP) plates were found to be significantly higher which is representative of their higher specific strength. The results of buckling tests showed that the buckling load increases with increase in thickness of the laminate.
The studies concluded that the samples when subjected to thermal conditioning for ample time lose their stiffness and so the modal frequencies decrease. The decrease in the frequencies is proportional to the temperature difference between the conditioning temperature and the manufacturing temperature. Absorption of moisture at temperatures well above room temperature also leads to damage of the laminate and so the modal frequencies decrease. The fatigue testing done by repeatedly exposing the sample to a particular temperature, attain a constant value of stiffness after reduction in initial few iterations.
The studies on hybrid plates show that they possess the advantages of both their constituent fibres and have properties intermediate to the properties of individual fibres. The effect of percentage composition and sequence of lamination of the fibres on vibrational and buckling characteristics of the composite plates were observed.
It was observed that the failure due to tensile load in hybrids is governed by delamination between layers. The values of vibrational analysis present similar conclusions with regards to stiffness of plates as obtained from the tensile tests. The buckling results show that stiffer materials on outermost layer give maximum buckling strength compared to those with carbon fibers in inner layers.

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