Fourier Transform Mechanical Analysis (FTMA) has been used to study the strain amplitude dependent nonlinear dynamic mechanical properties of two elastomer compounds, nitrile rubber (NBR) and Neoprene, at various frequencies up to 750 Hz. Basic theory and experimental results are presented for one‐dimensional isothermal single frequency simple shear deformation. The Green‐Rivlin constitutive equation was used to model the observed behavior. The energy dissipation mechanism and a physical meaning for the material functions in the Green‐Rivlin representation have also been examined. The stress Fourier spectrum contained terms at the input frequency and its higher harmonics. It can qualitatively indicate the type of mathematical mode that best describes the observed behavior. The first harmonic storage and loss moduli showed strong dependence on the strain amplitude and frequency. The FTMA methodology presented can be used to systematically conduct nonlinear dynamic mechanical studies on any polymer. It can provide enough insight which, along with a knowledge of the molecular structure, may indicate a path for developing a better representative continuum constitutive model of these complex materials.

On-line surveillance of the structural integrity of wind turbines is a critical need in this currently fast growing industry. The structural integrity of the turbine blades themselves is critical to the continued operation of a wind turbine.It is well known that the resonant or modal properties of a mechanical structure are directly influenced by its physical properties. Hence, any change in the physical properties of a structure should cause a change in its modal parameters. One question is always apparent though; "Do structural faults cause significant changes in a structure's modal parameters?"In this paper, we present test results from a wind turbine blade with different cracks induced in it. Each result shows that some of the modes of the blade are significantly affected by a crack, and that the modal parameters change more significantly with a more severe crack. Changes in modal frequency, damping, and mode shape are considered.Using changes in modal parameters to indicate physical damage to turbine blades should be implemented in the online continuous monitoring of wind turbines. In such a system, differences between monitored modal parameters and their base-line values could be compared to both absolute and percentage difference warning levels. Comparing changes between operating and baseline modal parameters with warning levels will indicate when the blades of a wind turbine have undergone physical damage. INTRODUCTIONIt is well known that the elastic modes of a structure are strongly affected by its physical properties and boundary conditions. Its physical properties are summarized in its mass, stiffness and damping properties. Its boundary conditions are influenced by its geometric shape and its physical boundary conditions.

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