1 A DAMAGE IDENTIFICATION NUMERICAL MODEL BASED ON THE SENSITIVITY OF ORTHOGONALITY CONDITIONS AND LEAST SQUARES TECHNIQUES J.V. Araœjo dos Santos, C.M. Mota Soares * , C.A. Mota Soares and H.L.G. Pina IDMEC/IST, Instituto Superior TØcnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Abstract This paper presents a numerical model for damage identification of composite structures. The model is based on the orthogonality conditions sensitivities of the damaged and undamaged structures mode shapes and relies on a discretisation by finite elements. The damage is directly related to the stiffness reduction of the damaged element. A study on the use of eigenvector sensitivities for damage identification and the influence of measurements errors and noise in the modal data is also presented. The model is applied to a laminated rectangular plate, free in space and the efficiency of the numerical tools used to solve the defined set of equations is discussed. Keywords: Damage Identification, Orthogonality Conditions Sensitivities, Finite Element Method, Composite Structures, Least Squares Techniques * To whom correspondence should be addressed