A computationally attractive algorithm is developed to provide an insight to the location and extent of structural damage. The algorithm makes use of an original finite element model and a subset of measured eigenvalues and eigenvectors. The developed theory approaches the damage location and extent problem in a decoupled fashion. First, a theory is developed to determine the location of structural damage. With location determined, an extent algorithm is then developed. The extent algorithm is a minimum rank update, which is consistent with the effects of many classes of structural damage on a finite element model. If the actual damage results in a rank p change to the finite element model, then the extent algorithm produces exact results if p eigenvalues and eigenvectors are measured exactly. In addition, the extent algorithm preserves any rigid body modes of the structure. The algorithms are demonstrated using both numerical and actual experimental data. The effects of eigenvector measurement and expansion errors are demonstrated and techniques to overcome the effects of noise are discussed.

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