RESUMO
After identifying the parameters of the piezoelectric transducer, the mechanical impedance of the front medium is deduced. More particularly, the wave propagation velocity and attenuation are deduced in the inspected plate. By fitting the electrical impedance measurement results, the aging of composite materials is quantified, showing the effectiveness of this means of nondestructive evaluation. A specific measurement tool and protocol are proposed. Several estimators are identified on the basis of Gaussian fits of the resonances observed on the electrical impedance measurements. Those estimators are identified and the obtained results show that both the frequencies and widths of the resonances peaks vary according to the health of the plate, or aging duration. This method is applied successfully on non-perfect sandwich plates, porous, and with non-ideally flat surface. The sensitivity and limits of the most relevant estimators are discussed for the two studied plate families.
RESUMO
The capability of shear horizontal (SH) guided waves, to evaluate geometrical imperfections in a bonding layer, is investigated. SH waves are used in a three-layer structure in which the adhesive layer has variable thickness. It is proven that the SH waves are adapting to the local thickness of the adhesive layer (adiabatic waves). This is particularly useful in case of small thickness variations, which is of technical interest. The influence of thickness and stiffness of the adhesive layer on the wavenumbers are investigated. The selected SH2 mode is proven to be very sensitive to the adhesive layer thickness variation in the given frequency range and considerably less sensitive to the adhesive stiffness variation. This property is due to its specific displacement field and is important in practical applications, such as inspection techniques based on SH waves, in order to avoid false alarms.
