FEM assisted development of a SHM-piezo-package for damage evaluation in airplane components

Abstract

Structural Health Monitoring (SHM) is a wide spread field for material condition observation of differential structure components. At the IZFP the guided wave (Lamb wave) technology is under higher investigation. Actual investigations are in progress to apply SHM-systems at structures in airplanes to perform condition monitoring. New materials like Carbon Fibre Reinforced Plastics (CFRP) will be placed in airplanes partially, because they provide very high stiffness, high rupture strength and reduced total mass. These SHM-systems are using different damage indicators, which are based on differences in amplitude or phase relation between two measured signals at two different times points (condition). Additionally, these signals are affected by environmental loads, sensor setup and changes in material properties of the adhesive layer. A successful material application can only be achieved by using an integrated reliable SHM-system. The validation of reliability comes along with high probability of detection and high robustness regarding environmental loads. This study tests and analyses the robustness of a novel piezo-sensor-package. The sensor package is very slim and consists of LTCC ceramic, which encloses a PZT piezo ceramic sheet and carries electronic components on its surface. Using the piezoelectric effect the package generates lamb waves and transmits them into a base substrate. The package is assembled on a 2 mm (thick) aluminium plate for study purposes, because aluminium possesses an isotropic material behaviour. Frequency ranges from 25 kHz up to 400 kHz produce excited symmetrical S0 and asymmetrical A0 lamb waves that are guided into the aluminium plate. Subsequently, a FEM-model of the package is calibrated to ensure correct physical behaviour of the simulation using analytical solutions of lamb wave propagation and experimental data. The calibration of the FEM-model provides the base for further investigations. The principle of wave propagation based on the new package configuration is studied and effects resulting from the package shape and construction are defined. Also, influences of the adhesive layer between the ceramic package and the aluminium plate are determined as a function of thickness and temperature depended stiffness and for the case of a delaminating progress.