The article describes a study on the use of the electromechanical impedance spectroscopy method for crack-damage detection with piezoelectric wafer active sensors bonded to thin-walled structures. The study combines analytical, finite element method, and experimental methods. The specimens under consideration consisted of circular plates with circular piezoelectric wafer active sensors bonded at the center. Some of the plates were pristine, and others were damaged. The damage consists of simulated cracks (thin laser-machined slits) placed at various locations away from the center. For pristine plates, the electromechanical impedance spectroscopy spectrum was predicted with two numerical methods: (a) analytically using closed-form solutions and (b) Comsol 4.3 Multi-physics software using two-dimensional axisymmetric elements. The results were compared with experimental measurements performed with an HP 4194 impedance analyzer. This comparison allowed us to tune the model parameters. For plates with simulated cracks, the electromechanical impedance spectroscopy spectrum was predicted with three-dimensional finite element method models and then compared with experiments. Overall, the effect of crack damage on electromechanical impedance spectroscopy signature was found to consist of frequency shifts, peak splitting, and appearance of new peaks. These effects were predicted by the numerical models and confirmed by experiments. Modeshapes were also measured using a scanning laser Doppler vibrometer and compared with finite element method predictions. Interesting new modeshape features (local high-frequency modes) were observed near the crack. These features tend to explain some of the new peaks appearing in the electromechanical impedance spectroscopy spectrum. Further refining of the finite element method model was performed to capture variability effects associated with plate thickness, piezoelectric wafer active sensors thickness, adhesive layer thickness, and imperfect adhesive layer. The effect on the piezoelectric wafer active sensors asymmetrically placed electrodes and soldering points was also studied. It was found that these variations modify the electromechanical impedance spectroscopy signatures, but much less than the changes induced by the presence of the crack. Hence, it was concluded that the capability of the electromechanical impedance spectroscopy method to detect actual damage was not affected by these variations. A proper physics-based analysis and interpretation of the electromechanical impedance spectroscopy spectral changes are performed.

This article reports the results of recent complex tests on the survival, in view of space applications, of structural health monitoring (SHM) methodology that uses piezo wafer active sensors (PWAS) and the electromechanical impedance spectroscopy (EMIS) method. Successive and then concomitant actions of the harsh conditions of outer space, including extreme temperatures and radiation, were simulated in a laboratory. The basis of the method consists in the fact that the real part of the bonded PWAS impedance spectrum, the so‐called EMIS structure signature, follows the resonance behaviour of the structure vibrating under the PWAS excitation and, consequently, the onset and progress of structural damage with fidelity. The tests were conducted on the PWAS separately and aluminium discs with PWAS bonded on them as structural specimens. The conclusion of the tests is that the cumulative impact of severe conditions of temperature and radiation did not result in the decommissioning of the sensors or adhesive, which would have meant that the methodology was compromised. This conclusion occurs as a result of applying two new analysis methods to EMIS signatures. The first method, based on systematic observation of EMIS signatures during tests, makes it possible to distinguish between real damage with a mechanical origin and false damage, which is reversible and caused by the harsh environmental factors. A second method, based on the concept of entropy, shows how to identify mechanical damage at a certain distance from the PWAS. Moreover, an offline analysis of the EMIS “entropy” signatures supports the conclusion that the SHM technology survived the harsh environmental conditions.

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Purpose The purpose of this paper is to report the results of a recent project of complex tests on the survival of structural health monitoring (SHM) technology with piezo wafer active sensors (PWAS) and electromechanical impedance spectroscopy (EMIS) at simulating the concomitant action of harsh conditions of outer space: extreme temperatures, radiations, vacuum. Design/methodology/approach The tests were conducted on PWAS, consists in adhesive and aluminium discs as structural specimens, with PWAS bonded on them. The substantiating of PWAS-EMIS-based SHM technique consists the fact that real part of the PWAS electromechanical impedance spectrum follows with fidelity the resonance behaviour of the structure vibrating under the PWAS excitation. This EMIS signature is very sensitive to any structural changes and, on this basis, can be monitored the onset and progress of structural damages such as fatigue, cracks, corrosion, etc. Findings The conclusion of the tests is that the cumulative impact of severe conditions of temperature, radiation and vacuum has not generated decommissioning of sensors or adhesive, which would have meant the compromise of the methodology. A second important outcome is linked to the capability of this methodology to distinguish between the damages of mechanical origin and the false ones, caused by environmental conditions, which are, basically, harmless. Originality/value The question of transfer of PWAS-EMIS-based SHM technology to space vehicles and applications received, as a novelty, a first and encouraging response.