In this paper we examine the validity of using proportional damping approximations for systems with repeated eigenvalues. A six degree of freedom cyclic mechanical system is studied as an example, as it has two pairs of repeated roots. Also a rigid body vibration problem with two closely spaced natural frequencies has been investigated. Our results indicate that the errors in frequency response calculations, associated with the commonly used proportional damping models, are large. The errors have been quantified using a numerical index.
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