Vibration control of composite structural elements with constrained layer damping treatments

This dissertation examines the feasibility of using viscoelastic damping materials in the form of constrained layer tapes for vibration control of composite structural elements so as to reduce resonant displacements and noise levels. The research involved the analytical and experimental investigations of the dynamic behavior of unidirectional and off-axis glass/epoxy graphite/epoxy composite laminated beams treated with such damping tapes. Particular interest is paid to the effects of partial coverage so that instead of distributing the treatment uniformly over the entire beam surface, substantial penalties in terms of increased structural weight can be offset by length optimizations. This requirement stems from a need (as in helicopter rotor blade applications) for a tradeoff between the added weight of the viscoelastic layer and the resultant changes in the dynamic characteristics of the structure.; A Fast Fourier Transform based impulse-frequency response vibration technique was used for experimentally identifying optimal lengths of damping tape to be applied for maximizing the structural loss factor (a measure of damping). The experimental results are compared with analytical results obtained by the Ross-Kerwin-Ungar analysis (generally used for predicting the dynamic behavior of conventional metallic structures treated with such surface damping treatments) and a more versatile modal strain energy/three dimensional finite element method. Measurement and predictions show that, for a given composite beam vibrating in a given mode, there is an optimum damping tape distribution which produces maximum shear deformation of the intermediate viscoelastic damping layer, thus providing effective vibration control in severe dynamic environments. Significant improvements in damping where predicted and measured when the constraining layer was clamped at the fixed end of the beam. The three-dimensional finite element implementation of the modal strain energy method proved to be a powerful analytical tool for predicting damping of such complex systems. The impulse-frequency response vibration technique has also proven to be a fast, reliable and accurate method for measuring damping in such structures.; Preliminary investigations on the effects of axial and transverse pre-loads (using specially designed loading fixtures to simulate the effects of rotating blades) on the damping and resonant frequencies of composite beams treated with such damping tapes are also presented.