Micromechanical Modeling Of Interphase And Interface Effects In Polymer Nanocomposites Via An Augmented Mori Tanaka Approach PDF Download

Recent initiatives to stimulate development of next-generation rotorcraft featuring leap-ahead improvements in speed, payload, range, and durability, such as Clean Sky and Future Vertical Lift, have revitalized research efforts directed toward advanced, unconventional designs that emphasize lower operations and sustainment costs. Accordingly, soft-inplane damperless bearingless and hingeless rotor concepts have garnered significant interest. However, soft-inplane designs are susceptible to aeromechanical instabilities, such as air and ground resonance, which can potentially induce catastrophic blade vibrations without sufficient blade damping. To ensure stability, current composite blades typically require auxiliary damping sources that incur weight, volume, complexity, and maintenance penalties. Alternatively, one promising approach for achieving new lightweight, low vibration rotorcraft structures is passive damping engineered intrinsically into a structure via polymeric nanocomposites.In this study, a multi-fidelity modeling effort is employed to investigate the interfacial load transfer micromechanics, including strain energy storage and dissipation, of an off-aligned discontinuously-reinforced polymer/carbon nanotube nanocomposite. The effects of off-alignment angle on nanocomposite mechanical properties is of primary interest. The methodology in this study is separated into two independent modeling tracks: a simplified analytical micromechanics model and a high-fidelity 3D finite element model. Both model types explore transverse fixed and transverse free boundary conditions applied to the representative volume element, which correspond to applied strain and applied stress external loading conditions, respectively. Each model accounts for interfacial shear stress variations along the azimuthal direction of the nano-inclusion surface that are a result of nonzero and non-right alignment angles with respect to the applied loading. The analytical micromechanics models examine non-embedded fiber conditions, for which matrix end material effects are neglected, in the preslip and postslip regimes and embedded fiber conditions, for which matrix end material effects are included, in the preslip regime. The non-embedded micromechanics model is based on principles from an extended Cox model for discontinuous fiber reinforcement and generalized shear lag analysis for off-aligned discontinuous fibers; furthermore, the energy dissipation, which is based on principles of a simple amplitude-dependent friction damper, is assumed to be caused only by interfacial slip friction between constituents and is functionally dependent on the interfacial shear force acting over slipped portions of the matrix/nano-inclusion interface. In order to isolate the effects of azimuthal interfacial shear stress variation, a comparison of the current non-embedded model with an alternative non-embedded analytical model that employs an interfacial shear magnitude approach is performed. The embedded analytical micromechanics model is based on principles from a modified Cox model that extends the non-embedded approach to account for finite matrix end material and nonzero fiber end normal stress. The finite element model is implemented in the preslip regime for an embedded fiber with limited off-alignment angle range.The material properties employed by each model reflect those of a realistic multi-walled carbon nanotube/poly-ether-ether-ketone nanocomposite architecture. In the preslip regime, the FEM and analytical model predictions for interfacial shear and nano-inclusion normal stress distributions generally display good agreement, which is improved by including inclusion end stress effects in the analytical models. For the transverse fixed boundary condition, the non-embedded analytical model predicts reduced interfacial slip damping capacity as off-alignment increases, with initiation of slip becoming impossible at relatively high off-alignment angles. However, for the transverse free boundary condition, the non-embedded analytical model predicts that zero interfacial slip damping occurs comparatively at more moderate off-alignment angles, with nonzero damping occurring at both lower and higher off-alignment angles. The phenomena of extrema in interfacial slip damping with respect to alignment angle is due to the relative strain behavior between nanocomposite constituents caused by elastic stiffness mismatch. The alternative azimuthal magnitude non-embedded analytical model generally underpredicts storage modulus and greatly overpredicts loss modulus (for nonzero and non-right off-alignments) compared with the corresponding properties predicted by the current non-embedded analytical model because the alternative azimuthal magnitude approach assumes a greater interfacial slip surface area for a given off-alignment angle and strain magnitude compared to the current approach. Overall, the results demonstrate that nano-inclusion alignment angle substantially affects nanocomposite stiffness and interfacial damping and that azimuthal variation of the interfacial shear is a critical feature of nanocomposite mechanics. The outcome of this multi-fidelity modeling study is an array of qualified nanocomposite mechanical property prediction methods spanning a wide range of practical off-alignment angles, applied dynamic strain amplitudes and static strain magnitudes, loading and fiber embedment conditions, and nano-inclusion geometries and concentrations.