Detection of Air-Voids in Foam-filled Sandwich Panels using Air-Coupled Lamb Waves

We investigate non-contact air-coupled ultrasonic Lamb wave detection of voids in polyisocyanurate (PIR) foam filled sandwich panels with steel face sheets, a material commonly used in lightweight steel construction. These voids, which usually form during manufacturing, can develop into blisters, which pose a risk to structural integrity, and their repair requires significant effort. In order to address this problem, low-frequency A0 mode Lamb waves are used in the steel sheet to test for voids in the foam core even when they are buried, i.e. not in direct contact with the steel-sheet. These Lamb waves are generated using an air-coupled ultrasonic phased array at 40kHz and received by a laser Doppler vibrometer. Our results show that an artificially created, slightly buried 6×8cm void in the foam alters the amplitude of the Lamb wave in the steel sheet. An increase in amplitude of about 6% in the steel sheet is achieved for a 6×8cm void, buried in 0.5mm depth, compared to an intact foam. The technique investigated is promising to enable the early detection of potentially growing defects such as air voids in the foam.