The work presented here focuses on the structural health monitoring (SHM) of a foreign object damage (FOD) composite panel equipped with an innovative printed piezoelectric transducer network. The 3D woven composite FOD panel measures approximately 800 mm × 320 mm, is curved with a cross-sectional thickness varying from approximately 2 mm to 12 mm, and a stainless-steel leading edge is bonded at one of its sides. The core idea explored here is to rely on an innovative screen-printing technology to print a full piezoelectric transducer a flowing to successfully achieve SHM on such a complex composite structure. This work is being carried out within the European project MORPHO - H2020. After printing a 25 elements PZT network, a four points bending fatigue experimental campaign using the PZT network along with other sensor technologies (embedded optical fibres with FBG sensors and acoustic emission sensors) is carried out. This unique experimental campaign allows to generate data and will help to develop diagnostic and prognostic methodologies for remaining life estimation and SHM of the FOD panel. It is demonstrated here through impedance measurements that the printing process associated with the printed PZT transducers is highly repeatable thus validating its use at a larger industrial scale. Furthermore, the printed piezoelectric transducers a re shown to be able to detect foreign object impact and sense Lamb waves signals. This innovative printing technology for PZT transducers network is thus extremely promising. It is furthermore highly advantageous to use the printed transducers for SHM instead of regular ceramic ones as this technology is non-intrusive, add negligible weight, can be printed during the manufacturing process, and arrays of transducers ensure easy availability of another transducer in case of failure of one.

The application of structural health monitoring (SHM) in composite airframe structural elements under long-term realistic fatigue loading needs to consider the structural behavior on the global level, which is an intricate task. The overall structural stiffness is a key design parameter for composite structures and the stiffness degradation under fatigue loading is closely related to the damage accumulation and failure mechanism which can be used as an indicator for the structural degradation. Therefore, this paper investigates the use of guided waves in axial stiffness degradation estimation for stiffened carbon fiber reinforced polymer (CFRP) composite panels under post-buckling compression-compression (C-C) fatigue loads. Impacted or artificially debonded stiffened composite panels are tested under fatigue until failure and guided waves are acquired using a network of piezoelectric (PZT) sensors at fixed cycle intervals. The guided wave phase velocity along the loading direction is extracted to estimate the axial stiffness degradation with the consideration of mode conversion and failure of PZT sensors. The estimated stiffness of five stiffened composite panels matches well with the stiffness calculated from the load–displacement curves. The estimated stiffness is also assessed using prognostic performance metrics and shows good potential for being used as a health indicator for prognostic purposes.