Structural health monitoring (SHM) combines sensors, signal processing and modelling strategies to perform the automated diagnosis of damages and impacts in a wide range of engineering structures, including mechanical, aerospace and civil applications. In particular, impact monitoring adopts both active and passive approaches to estimate impact occurrences. Active monitoring relies on the propagation of ultrasonic Lamb waves inside the damaged specimen, while passive monitoring analyses the strain signals generated by the impact itself to provide information about impact location and force [1,2]. In this latter case, impact localization is typically performed by evaluating the time of arrival of strain signals at different sensor positions. Yet still, it has not been widely addressed how to estimate the kind of impact on the basis of received signals. Indeed, this information is essential to evaluate the severity of the damage and the actual need for an intervention. In this work, fiber optic technology, now increasingly taken into account in SHM, has been adopted to perform passive impact tests and evaluate the peculiarities between different kinds of impacts on GFRP structure. The proposed sensing approach relies on standard optical fibers combined to a highly sensitive coherent detection scheme  whose feasibility for active SHM of ultrasonic strain waves in aluminium specimen has already been proved .
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