"Fuzzy tap-testing" sensors for material health-state characterization

A smart sensor for structural health monitoring, based on piezoelectric tactile sensors and soft-computing signal-processing techniques, is presented in this paper. The proposed probe, made by tubular translating piezoceramic elements, stimulates the surface of the object under test and analyzes the local viscoelastic response produced at the moment of tapping. An automatic and "real-time" strategy, based on the processing of a class of features, extracted from the time-domain responses of the developed tactile probe, is proposed in order to classify the damages compromising the health status of a material. Suitable software procedures, based on fuzzy logic and implemented on a dedicated microcontroller, elaborate the input data in order to realize the desired classification task. A specific application is reported here in order to validate the proposed sensing device. It consists of the "detection of damages induced in composite structures"; in particular, a coupon of quasi-isotropic carbon epoxy material, subjected to various aggressions, has been considered. Different damages such as delaminations, induced by calibrated impacts, and burning, induced either by contact with a hot body or by simulated lightning impact produced by electric sparks, have been considered. The results obtained are quite interesting in view of the simple structure of the measurement system proposed here with respect to other classical approaches