Examination of Sandwich Materials Using Air-Coupled Ultrasonics

The air-coupled ultrasonic techniques have been improved drastically in recent years. Better equipment has made this technique much more useful. This paper focuses on the examination of sandwich materials used in naval ships. It is more convenient to be able to make the measurements directly in atmospheric air instead of using immersion technique. The air-coupled techniques have been used in through-transmission mode using separate sender and receiver transducer. The frequency was either 50 kHz or 120 kHz. Laboratory tests on glass fibre/PVC foam core sandwich panels showed that debonds between core and skin laminate could be found by the air-coupled technique. It was therefore decided to use this technique in-situ during mechanical testing of sandwich panels. These tests were done in order to verify a developed FEM code using interfacial fracture mechanics and illustrated on a superstructure/deck debond in a representative sandwich panel. The purpose of the ultrasonic scanning was to detect the actual crack front, which can be difficult to detect otherwise. The described ultrasonic set-up has shown to be a reliable technique for measuring crack propagation and it has been successfully used in verifying the model for crack propagation. The techniques work best for core densities of 130 kg/m3 and 200 kg/m3. For 80 kg/m3 the damping of the ultrasonic waves is higher resulting in less well-defined crack detection.