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The advent of new structural concepts employing composites in primary load carrying aerospace structures in UAVs, MAVs, Boeing 787s, Airbus A380s, etc., necessitates the inclusion of flexibility as well as viscoelasticity in static structural and aero-viscoelastic analyses. Differences and similarities between aeroelasticity and aero-viscoelasticity have been investigated in [2]. An investigation is undertaken as to the dependence and sensitivity of aerodynamic and stability derivatives to elastic and viscoelastic structural flexibility and as to time dependent flight and maneuver velocities. Longitudinal, lateral and directional stabilities are investigated. It has been a well established fact that elastic lifting surfaces are subject to loss of control effectiveness and control reversal at certain flight speeds, which depend on aerodynamic, structural and material properties [5]. Such elastic analyses are extended to linear viscoelastic materials under quasi-static, dynamic, and sudden and gradual loading conditions. In elastic wings one of the critical static parameters is the velocity at which control reversal takes place *** Since elastic formulations constitute viscoelastic initial conditions, viscoelastic reversal may occur at speeds VREV but furthermore does so in time at 0 < tREV ≤ ∞. The influence of the twin effects of viscoelastic and elastic materials and of variable flight velocities on longitudinal, lateral, directional and spin stabilities are also investigated. It has been a well established fact that elastic lifting surfaces are subject to loss of control effectiveness and control reversal at certain flight speeds, which depend on aerodynamic, structural and material properties [5]. Such elastic analyses are here extended to linear viscoelastic materials under quasi-static, dynamic, and sudden and gradual loading conditions. In elastic wings the critical parameter is the velocity at which control reversal takes place ***. Since elastic formulations constitute viscoelastic initial conditions, viscoelastic reversal may occur at speeds VREV *** but furthermore does so in time at 0 < tREV ≤ ∞. This paper reports on analytical analyses and simulations of the effects of flexibility and time dependent material properties (viscoelasticity) on aerodynamic derivatives and on lateral, longitudinal, directional and spin stability derivatives. Cases of both constant and variable flight and maneuver velocities are considered. Analytical results for maneuvers involving constant and time dependent rolling velocities are analyzed, discussed and evaluated. The relationships between rolling velocity p and aileron angular displacement ß as well as control effectiveness are analyzed and discussed in detail for elastic and viscoelastic wings. Such analyses establish the roll effectiveness derivatives ***. Similar studies involving other stability and aerodynamic derivatives are also undertaken. The influence of the twin effects of viscoelastic and elastic materials and of variable flight, rolling, pitching and yawing velocities on longitudinal, lateral and directional are also investigated. Variable flight velocities, encountered during maneuvers, render the usually linear problem at constant velocities into a nonlinear one. [ABSTRACT FROM AUTHOR]

Aeroelastic and aero-viscoelastic phenomena arising from the high flexibility of modern flight vehicles are examined, and governing relations are formulated and solved. In particular, the time dependent flight velocities associated with maneuvers and with in-plane bending are considered, which necessitate new derivations of the Theodorsen function, unsteady aerodynamic relations and equations of motion. Under these conditions, simple harmonic motion (SHM) is no longer achievable and different flutter criteria based directly on motion stability are presented. The viscoelastic problem is formulated in terms of integral partial differential equations with variable nonlinear coefficients. Their solutions and evaluations are discussed in detail. One interesting departure from linear responses emerged, which indicates flutter in one bending while the other bending mode and the torsional are both stable. A detailed and extended treatment of these subjects may be found in [1]. [ABSTRACT FROM AUTHOR]

Protocols are developed for formulating optimal viscoelastic designer functionally graded materials tailored to best respond to prescribed loading and boundary conditions. In essence, an inverse approach is adopted where material properties instead of structures per se are designed and then distributed throughout structural elements. The final measure of viscoelastic material efficacy is expressed in terms of failure probabilities vs. survival times. [ABSTRACT FROM AUTHOR]

Asphalt concrete pavements exhibit severely graded properties through their thickness due to oxidative aging effects, which are most pronounced at the surface of the pavement and decrease rapidly with depth from the surface. Most of the literature to date has focused on use of layered-elastic models for the consideration of age stiffening. In the current work, a graded viscoelastic model has been implemented within a numerical framework for the simulation of asphalt pavement responses under thermal and mechanical loading. The graded viscoelastic work is extension of the previous work by Paulino and Jin [1], Mukherjee and Paulino [2], and Buttlar et al. [3]. A functionally graded generalized Maxwell model has been used in the development of a constitutive model for asphalt concrete considering aging and temperature gradients. The aging gradient data from laboratory test results reported by Apeagyei [4] is used for obtaining material properties for the graded viscoelastic model. Finite element implementation of the constitutive model incorporates the generalized iso-parametric formulation (GIF) proposed by Kim and Paulino [5], which leads to the graded viscoelastic elements used in this work. [ABSTRACT FROM AUTHOR]

The objective of this work is to present the numerical implementation for the alternative determination of vis-coelastic material properties without using Poisson’s ratios as presented in [1]–[3]. The presented method is based on the 3-D generalized constitutive relations viscoelastic materials with hereditary integrals. The numerical procedures are based on experiments using photogrammetric and tensile testing instrumentation, which provide stress data in the 1-D loaded direction and strains in both longitudinal (loaded) and transverse directions. Measurements and data analyses include both starting transient and steady-state loading conditions. The paper presents the implementation of solutions for the linear case, where the relaxation time values are prescribed according to to the scheme presented in [1] and [3]. Convergence of the Prony series representations is evaluated. [ABSTRACT FROM AUTHOR]