Book Description

ABSTRACT Free Vibration Analyses of Stationary and Rotating Tapered Composite Beams with Delamination Puneet Jagpaul The exceptional engineering properties and customizability of the laminated composites have enabled their use in the design of the stationary and rotating tapered structures in the aerospace and energy sectors. The tailoring capabilities of the composite material can be used to stiffen the structure at one location while being flexible at other location and consequently reduce the weight, as required in specific applications such as helicopter rotor blade, windmill blade and turbine blade. The vibration characteristics (natural frequencies and mode shapes) of the stationary and rotating structures differ substantially and must be well identified in the design stage. The composite structures are prone to failures such as delamination and fiber-matrix debonding caused during their fabrication or in service, especially when used as blades and beams in various stationary and rotating applications. Delamination reduces the overall stiffness and the strength of the laminates, which may lead to local or sudden structural failures. The delaminated structure has reduced natural frequencies and exhibits different mode shapes than that of the intact structure. In the present thesis, the free vibration analyses of stationary and rotating tapered composite beams with delamination are conducted. The influence of the delamination on the vibration characteristics of the stationary and rotating tapered composite beams is comprehensively studied. The Finite Element Analysis tool ANSYS® is used to develop three-dimensional models of the intact and delaminated composite beams. The natural frequencies of the stationary and rotating intact cantilever composite beams are determined for uniform, thickness-tapered and doubly tapered beam profiles using modal analysis and the results are compared with the results available in the literature. The Mode-I and Mode-II delamination tests are performed on the numerical models of the double cantilever beam and end notch flexure test samples based on cohesive zone modeling and the results of the tests are verified with the available results. The critically stressed locations prone to delamination in the stationary and rotating composite beams are determined using the first-ply failure analyses based on Tsai-Wu failure criterion. The free vibration responses of the stationary and rotating composite beams with end and mid-span delaminations of different lengths and with different stacking sequences are obtained and they are verified wherever possible. The delamination length that has minimal effect on the first three natural frequencies of the uniform and thickness-tapered composite beams is determined and is found to be 5% of the total beam length. Higher modes should be investigated for the composite structures with smaller delamination. A basis for the non-destructive evaluation is suggested for the stationary thickness-tapered simply supported composite beams with end and mid-span delaminations. The influences of the delamination length, delamination location, fiber orientation angle, thickness-tapering, double tapering, layer reduction and taper angle on the free vibration response of the stationary and rotating delaminated composite beams are investigated for uniform, thickness-tapered and doubly tapered beam profiles through various parametric studies. The influences of the rotational velocity and hub radius on the natural frequencies of the rotating doubly tapered composite beams with delamination are thoroughly examined. The present thesis contributes towards the safe design of the composite structures. The studies performed are helpful for developing delamination detection techniques based on the free vibration response of tapered composite beams and can aid designers to model optimised tapered composite structures by considering the influences of delamination on their vibrational characteristics.