This paper describes the application of proper orthogonal decomposition (POD) method in the context of structural health monitoring of a smart structure, bonded with a distributed piezoceramic patches array. Using measurements given by the distributed sensors, we propose a damage index (DI) based on change in angle between subspaces. Furthermore, from statistics, we propose a threshold that automatically decides if the structure is in healthy or damaged state. The efficiency of the proposed approach is validated on composite plate with two different damages (impact damage and additive mass).