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Byline: Jutima Simsiriwong, Rani Warsi Sullivan This paper describes the details of an experimental investigation focusing on the vibration characteristics of a composite fuselage structure of an ultralight unmanned aerial vehicle (UAV). The UAV has a total empty weight of 70.3 kg and 6.3 m in length. The fuselage structure consists of the fuselage body with an integrated vertical stabiliser. All structural components are fabricated from oven-cured laminated carbon composite materials using uniaxial and biaxial prepreg fabric. The modal characteristics of the fuselage structure are determined for a free-free configuration which is simulated by suspending the structure from its wing attachment points through the use of springs. A centrally located shaker system is used to induce vertical oscillations in the structure, which is instrumented with nineteen dual axis accelerometers. Dynamic properties such as the frequency, damping and associated mode shapes are obtained for aeroelastic analysis. The design and implementation of the vibration tests along with the experimental results are presented.

Master modulus curves are developed for a vinyl ester polymer with variability in its material properties. Tensile creep strains were obtained at three temperatures below the Tg through digital image correlation. A spectrum function was used to represent the viscoelastic strain response and modulus. A two-parameter Weibull distribution was used to characterize the probability distribution of the longitudinal modulus. The Weibull probability density functions of the viscoelastic modulus were obtained for each test configuration and found to be time and temperature dependent. Longitudinal modulus curves at constant probabilities were used to develop the master curves using the time-temperature superposition principle. [ABSTRACT FROM AUTHOR]

Analytical and experimental deterministic and statistical protocols are formulated for the constitutive linear relations that characterize viscoelastic media. These proceedings are achieved in the real time space in terms of moduli and/or compliances without any inclusion of viscoelastic Poisson's ratios. Independent experimental determinations of linear viscoelastic material properties of three distinct polymers include single conditions of creep, relaxation, and constant strain and extensometer time rates - except for starting transients. Statistical dynamic data for the instantaneous modulus and quasi-static data for relaxation moduli are analytically and numerically merged to produce relaxation modulus and creep compliance expressions containing properly evaluated parameters. The combined actual starting load and displacement transient and subsequent time histories are tracked, recorded and incorporated into the analyses to produce moduli and compliances based on actual continuous loading-time sequences. Prony series and generalized continuous relaxation time spectral modulus and compliance functions are derived and discussed. Four distinct probability models of material properties and temperature are postulated and evaluated for two sets of real viscoelastic materials (polymers). Statistical sufficiency considerations are included in the analyses. [ABSTRACT FROM AUTHOR]

This paper presents two characterization methods that are used to describe viscoelastic behavior. In the Prony series method, a discrete representation of the generalized Kelvin model is used, whereas the spectral method uses a continuous distribution. The Prony series coefficients are determined through the linear least squares technique. The Elastic-Viscoelastic Correspondence Principle and the Laplace transform are used in the spectral function approach, which utilizes a selected distribution function that has the potential to describe a wide range of viscoelastic material behavior. Characterization exercises from both methods used strain histories from short-term unidirectional tensile creep experiments that were obtained at three temperatures below the glass transition temperature of a vinyl ester polymer. Experimental strains in both the longitudinal and transverse directions were measured using the digital image correlation technique and linear viscoelastic material properties were obtained without using Poisson's ratios. The measured data was subsequently used to determine the tensile creep compliance function for each test configuration. The potential and limitations of each modeling approach are discussed. [ABSTRACT FROM AUTHOR]