Structural Health Monitoring by Means of Strain Field Pattern Recognition on the basis of PCA and Automatic Clustering Techniques Based on SOM**The research included in this document was partially supported by the "Ministerio de Ciencia e Innovación" in Spain through the coordinated projects DPI2011-28033-C03-02 and DPI2011-28033-C03-03; and by the European Commission through the project SARISTU | Publicación