Interfacial slip mechanisms of strain energy dissipation and vibration damping of highly aligned carbon nanotube (CNT) reinforced polymer composites were studied through experimentation and complementary micromechanics modeling. Experimentally, we have developed CNT-polystyrene (PS) composites with a high degree of CNT alignment via a combination of twin-screw extrusion and hot-drawing. The aligned nanocomposites enabled a focused study of the interfacial slip mechanics associated with shear stress concentrations along the CNT-PS interface induced by the elastic mismatch between the filler and matrix. The variation of storage and loss modulus suggests the initiation of the interfacial slip occurs at axial strains as low as 0.028%, primarily due to shear stress concentration along the CNT-PS interface. Through micromechanics modeling and by matching the model with the experimental results at the onset of slip, the interfacial shear strength was evaluated. The model was then used to provide additional insight into the experimental observations by showing that the nonlinear variation of damping with dynamic strain can be attributed to slip-stick behavior. The dependence of the interfacial load-transfer reversibility on the dynamic strain history and characteristic time scale was experimentally investigated to demonstrate the relative contribution of van der Waals (vdW) interactions, mechanical interlocking, and covalent bonding to shear interactions.
Various approximations to unsteady aerodynamics are examined for the aeroelastic analysis of a thin doublewedge airfoil in hypersonic flow. Flutter boundaries are obtained using classical hypersonic unsteady aerodynamic theories: piston theory, Van Dyke's second-order theory, Newtonian impact theory, and unsteady shock-expansion theory. The theories are evaluated by comparing the flutter boundaries with those predicted using computational fluid dynamics solutions to the unsteady Navier-Stokes equations. In addition, several alternative approaches to the classical approximations are also evaluated: two different viscous approximations based on effective shapes and combined approximate computational approaches that use steady-state computational-fluid-dynamics-based surrogate models in conjunction with piston theory. The results indicate that, with the exception of first-order piston theory and Newtonian impact theory, the approximate theories yield predictions between 3 and 17% of normalized root-mean-square error and between 7 and 40% of normalized maximum error of the unsteady Navier-Stokes predictions. Furthermore, the demonstrated accuracy of the combined steady-state computational fluid dynamics and piston theory approaches suggest that important nonlinearities in hypersonic flow are primarily due to steadystate effects. This implies that steady-state flow analysis may be an alternative to time-accurate Navier-Stokes solutions for capturing complex flow effects.
The effectiveness of surrogate modeling of helicopter vibrations, and the use of the surrogates for optimization of helicopter vibration are studied. The accuracies of kriging, radial basis function interpolation, and polynomial regression surrogates are compared. In addition, the surrogates for the vibratory hub shears and moments are used to generate an objective function which is employed in an optimization study. The design variables consist of the cross-sectional dimensions of the structural member of the blade and the non-structural masses. The optimized blade is compared with a baseline rotor blade which resembles an MBB BO-105 blade. The results indicate that: (a) the kriging surrogates are the best for approximating vibratory hub loads over the entire design space and (b) and the surrogates can be used effectively in helicopter rotor vibration reduction studies. * Ph.D. Candidate, Student Member aiaa. † François-Xavier Bagnoud Professor, Fellow aiaa, ahs ‡ Postdoctoral Researcher, Member aiaa.
The effectiveness of surrogate-based optimization of helicopter rotor blades for vibration reduction at both low-and highspeed forward flight is studied. The efficient global optimization (EGO) algorithm is used to conduct a global search of the design space. Vibration levels in the low-speed regime are caused primarily by blade-vortex interaction (BVI), and vibrations at high advance ratios are caused by dynamic stall. Although the source of high vibrations is different for both flight conditions, the results show that the EGO algorithm is effective in finding reduced vibration designs at both flightconditions. Furthermore, the results indicate that it is possible to design a blade with reduced vibration levels at both flight conditions. However, a single design that is best for both flight conditions does not exist. Therefore, multiobjective function optimization techniques were employed to identify Pareto optimal designs.
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