Air-Coupled Lamb Wave Testing of Buried Air-Voids in Foam-Filled Sandwich Panels

Sandwich panels, composed of two steel faces and a rigid foam core, are an inexpensive and lightweight option for construction industry. However, voids can form in the foam core during the manufacturing process. This paper uses ultrasonic testing to detect such voids in the foam core of sandwich panels, buried a few millimeters below the surface. The testing setup employs both air-coupled and non-contact ultrasonic testing. Different frequencies are investigated for their influence on the detection capabilities. Two air-coupled experimental setups are constructed, one at 40kHz and the other one at 200kHz. Artificial defects are carved into the sandwich panel at different depths. The results are compared to a simulation. We found that detecting buried voids in these sandwich panels is feasible. The 40-kHz setup has a larger penetration depth of 14mm, while the 200-kHz setup has a smaller penetration depth of 2.5mm. The 200-kHz setup shows a better contrast, i.e. the amplitude at the defect increases by 27% compared to 6% with the 40-kHz setup. These methods enable air-coupled, non-contact ultrasonic testing of buried defects in sandwich panels. They have the potential to be integrated into production lines, contributing to improved material efficiency and quality control for these sandwich panels.