Hybrid structural health monitoring on a composite plate manufactured with automatic fibers placement including embedded fiber Bragg gratings and bonded piezoelectric patches

Secondary bonded or embedded sensors are usually adopted in Structural Health Monitoring of composite structures. Each type of sensor has advantages and drawbacks when used separately although their proper integrated combination may improve the overall performance of a SHM system. The aim of the present work is to evaluate the feasibility of an efficient hybrid SHM system able to sense and locate low velocity impacts and to localize eventual impact damages at their early stage with few sensors. A CFRP panel demonstrator with embedded FBGs has been fabricated by NOVOTECH srl through an advanced out-of-autoclave manufacturing process based on the following main phases: production of dry preforms laid down with a laser assisted automated fiber placement robot (AFP); insertion of the FBGs network within the stack of the preform according to the design requirements; high temperature liquid resin infusion process. Disk shaped piezo patches (PZT) are then secondary bonded to the panel. First, an impact sensing mode is enabled resorting to both PZTs and FBGs as receivers, gathering strain waves from the impact location. A material independent technique, that needs neither a priori knowledge of the material properties even for anisotropic plates nor a dense array of sensors, will be used to locate the impact. To assess the integrity of the panel after impact localization, the damage location mode is then performed using the most effective sensors, where the selection of these sensors is possible thanks to the knowledge of the impact location carried out in the previous mentioned impact sensing mode. The damage localization technique is based on guided waves generated by PZTs and sensed by both PZTs and FBGs. Again, thanks to the knowledge of the impact location, a reduced number of PZTs is selected to perform the damage localization. The main advantage of the proposed technique is the capability to sense impact events and to activate the damage localization mode only when required. Moreover, by the knowledge of the impact location, only a few sensors (sources and receivers) need to be used in favor a reduced number of acquisition channels required and acquisition data to be managed.