## 1 Introduction  
Structural damage investigation is of a primary importance for improving the safety levels and keeping operation costs under control. Maintenance processes are among the highest operative costs in the aircraft life. In recent years, for many reasons, also related to the enlarging \"globalization\" scenario, attention towards economic aspects has grown. Safety is also increasing its role more than ever since the continuous expansion of the flight market causes the current percentage protection levels should prevent increment of accident. The consolidated introduction of composite technologies in the commercial products, moreover, makes the structural monitoring valuable for future planes competitiveness. From this point of view, in fact, there are many open points, hard to be overcome by the current understanding of the engineering artefacts behavior. Penalties induced by certification authorities' requirements [1] make the promised advantages almost vanish (indeed, there are even technical limitations like \"acoustic transparency\" that in turn require additive weight integration--insulators). Among many others, there is one aspect on which the authors concentrated their efforts in this paper: the bonding line reliability after low/medium energy impact. A system including reliable observability of the bonding state can lead in principle to a \"condition-based\" maintenance and is therefore of absolute interest. Then the impact could be extended to the design segment, such a system allowing the assessment of lighter components. Low/medium energy impact causes internal delamination and sometimes bonding line failure (called BVID [2]) that drop the compressive strength by nearly 50% [3]. Under this topic, in the context of the Italian Aerospace Research Center's (CIRA) SMAF project (SMartAirFrame), funded by PRORA (Italian Program for Aerospace Research), a cost-effective maintenance approach is investigated by using the strain information given by an integrated SHM (structural health monitoring) system. In particular, the detection of damage with size and indentation smaller than that one currently assumed for design and certification, is used as requirement for the tests. Besides, the use of a monitoring system can overcome the current uncertainty of long-term adhesive performance and a fully integrated sensor network with low weight and naturally EM (electromagnetic) shielding, must be a core issue. Many scientists worldwide, indeed, approached the problem of the damage identification in composite structures by using fiber optics, with Bragg grating, long period, chirped grating or even distributed [4; 5; 6], with static or even dynamic methodologies. Much work has been done to develop nondestructive inspection (NDI)/evaluation/ testing techniques, useful for the structural health monitoring (SHM) of composite structures. In aerospace applications [7], for instance, SHM techniques can be operated online during the flight or offline on the ground and can be focused on global inspection of large surface areas or limited region of interest thanks to distributed sensor system like optical fiber [8]. From the bibliographic analysis, many signal processing [9] methodologies for SHM are also resumed. Most of them use the change of the strain curves shape [10; 11]. Others use the differences between a baseline set of measurements and the dataset at an unknown structural state [12; 13; 14]. Hence, the computation of a metric [15] (i.e., Mahalanobis distance--MD) can be performed, producing a damage alert when this difference exceeds a predefined limit. However, it is not trivial to select a \"normal\" or \"healthy\" group before constructing the Mahalanobis space. Modal or frequency filter-based algorithms are other possible approaches [16; 17; 18]. These methodologies could detect multidefects, as long as each defect is located in between a pair of sensors.