MASS-ADDITIVE MODAL TEST METHOD FOR VERIFICATION OF CONSTRAINED STRUCTURAL MODELS

A method for deriving constrained or fixed-base modes and frequencies from free-free modes of a structure with mass-loaded boundaries is dev eloped. Problems associated with design and development of test fixtur es can be avoided with such an approach. The analytical methodology pr esented is used to assess applicability of the mass-additive method fo r three types of structures and to determine the accuracy of derived c onstrained modes and frequencies. Results show that mass loading of th e boundaries enables local interface modes to be measured within a des ired frequency bandwidth, thus allowing constrained modes to be derive d with considerably fewer free-free modes than for unloaded boundaries . Good convergence was obtained for a simple beam and a truss-like Shu ttle payload, both of which had well-spaced modes and stiff interface support structures. Slow convergence was obtained for a space station module prototype, a shell-like structure having high modal density.