On the use of measured rotational degrees of freedom in structural dynamics modification

This work discusses the utilization of experimentally measured rotational degrees-of-freedom for structural modification. The rotational degrees of freedom are measured via a dual beam laser vibrometer capable of measuring one translation (z) and two rotations (Θ_x, Θ_y) from a reflective surface on a vibrating object. Initially, measured rotational residues are used as a basis to examine the difficulty of estimating the rotational characteristics solely from measured translational data. The measured rotational data is then used as a database for the Structural Dynamics Modification (SDM) of a structure with a beam type element. First a complete experimental modal analysis of a cantilever beam is performed extracting both translational and rotational modal parameters. An analytical SDM is then performed with the experimental modal data base to fix the free beam end. The resulting SDM modal parameters are compared to those determined from an experimental modal analysis of another beam with dual fixed-fixed boundary conditions. The comparison shows excellent correlation.