This paper presents a new framework to predict a structure's effective prop
erties and sensitivities to multiple simultaneous uncertain endogenous para
meters. The methodology is based on the use of fuzzy sets and this paper ex
tends the fuzzy set theory to a dynamic finite element analysis of engineer
ing systems containing uncertainty on material properties. A general algori
thm, which can resolve the uncertain eigenvalue problem try using a Neumann
expansion, is studied This algorithm is applied to the study of the modal
behavior of structures presenting uncertain material properties. Finally, t
he entropy and the specificity of fuzzy responses lead to the identificatio
n of a plate structure's most sensitive eigenvalue to uncertain sources.