Diagnostics of honeycomb core sandwich panels through modal analysis

The possibility of using a new diagnostic technique to identify the presence of localised debonding in metallic sandwich panels with honeycomb cores is analysed in this paper. The technique is based on the natural frequencies measurement of panels excited with an impulsive load. A preliminary simulation demonstrated that when there is a debonding it is possible to identify peaks relative to the vibrations of the debonded skin seen as a plate clamped at boundary. The tests on panels with artificial debonding, using an accelerometer and a high sensitivity displacement laser sensor, validated the simulation. The no-contact displacement sensor is suitable for its capability to pick up the vibrations of the skin when placed above the debonded area, but its frequency field is limited as it can only identify defects larger than about 60-70 mm. On the contrary the accelerometer reveals the debonded areas if it is placed outside them.