Double function of integrated piezoelectric sensors for ageing and damage monitoring composite structures

The life duration of a structure depends on both the homogeneously distributed degradation of its mechanical properties during its life cycle (ageing) and the presence of localised defects such as cracks or delaminations. Previous studies! 1-21, described an ultrasonic health monitoring method enabling the recovery of both kinds of information. 7 lie method is based upon an integrated piezoelectric element into the plate-1 ike composite structure which avoids coupling reproducibility problems. The element dimensions are determined in order to uncouple the frequency ranges of the thickness and radial vibration modes (Fig. I ). The thick ness mode is then exploited to monitor the homogeneous ageing of the structure through electrical impedance measurement. Whereas the radial vibrations, are used to launch and delect Lamb waves, which are known to propagate over long distances. The guided waves also have the advantage to be multimodal, so the various propagating modes offer a specific sensitivity to the different kinds of defects that a structure can host. The previous papers dealt respectively to each of the two possible monitoring modes of the system. The present work focuses more particularly on the specific problems related to the simultaneous implementation of both aspects.Furthermore, the influence of the material properties upon the limitations of each kind of monitoring will be discussed in relation with the characteristics of the waves that the structure can propagat