In-line Production Testing of Sandwich Panels using an Air-Coupled Ultrasonic System

Lightweight steel-polyisocyanurate (PIR) foam sandwich panels are used in industrial construction of buildings, since they are inexpensive, easy to build, and have good insulation. However, air voids or delaminations in the foam core that form during production can negatively impact their structural properties. Detecting such defects in-process non-destructively during the production remains a challenge. This work uses air-coupled ultrasound for the in-line defect detection in steel-PIR sandwich panels, by exciting and receiving guided Lamb waves propagating in the steel face sheet to identify local amplitude changes caused by air voids in the foam core. Two systems are investigated: First, a laboratory prototype employing a transmission phased array together with a laser Doppler vibrometer (LDV) for reception at 40 kHz for comparison with previous work, and second, an industrial prototype better optimized for the production environment using air-coupled ultrasonic transducers for reception at 216\,kHz. In total 49 production-inserted artificial test defects are evaluated for testing these systems. With the laboratory prototype as reference system employing the LDV, all tested artificial air void defects are detected by an approx. 25% amplitude change at the defect locations. The industrial prototype with the ultrasonic transducer for reception detected an amplitude increase of 10% to 40% at most of the test defects. When using a conservative 4σ threshold, 63% of defects are detected with no false positives during the measurement time. These results demonstrate that air-coupled ultrasound enables non-destructive detection of foam defects under real production conditions.