Determination of mechanical ve modal parameters of composit materials using vibration analysis

This thesis presents the application of vibration analysis in the determination of dynamic modulus of elasticity, shear modulus, and modal damping ratios of a unidirectional composite beam in tension, bending, and torsion. Frequency domain approach is used for the determination of both modulus of elasticity and shear modulus, whereas modal damping ratios are predicted by the use of short-time Fourier transform in the longitudinal, flexural, and torsional vibrations. It has been found that the results for the dynamic modulus of elasticities and shear modulus obtained by the vibration analysis and mechanical tests show a very good agreement. An analytical expression of the STFT for the free vibration response of a viscously damped mdof system has been derived using Hanning window. Moreover, analysis of a simulated signal for a three-degree of freedom system has revealed that the short time Fourier transform is capable of not only decoupling vibration into its natural modes, but predicting the modal damping ratios with high accuracy even they are considerably large. Furthermore, impact response of a unidirectional composite beam is analysed by the STFT for the determination of modal damping ratios in longitudinal, flexural, and torsional vibrations. It has been found that the modal damping ratios of the unidirectional composite beam are closer to each other in longitudinal vibration, whereas the modal damping ratios in the flexural vibration is changing, the higher the modal frequency, the larger the value of modal damping ratio. Besides, modal damping ratio for the first mode of the torsional vibration exhibits relatively larger value compared to those in longitudinal and flexural vibrations.