抄録
Flexible packages are used for hermetic packaging of foods and medicines. These packages consist of several seats of film and are sealed by heat sealing. Delamination of the welded part is a critical problem. Even if the diameter of the delamination is less than 100 micrometers, leakage of contents and invasion of bacteria occur. Therefore, detecting the small defect is required. The detection method should be non-destructive and non-contact. So we use air-coupled ultrasound and its coincidence effect. The coincidence effect occurs when the ultrasonic wave is incident to the film with a certain angle. A bending wave is generated on the film by the effect. The bending wave propagates along the film and produces to ultrasonic wave that can be received with air-coupled ultrasonic transducers. The defect reduces bending stiffness of the film. So the velocity of the bending wave decreases. Our new method detects the change of the bending wave velocity as a phase delay of the receipt ultrasonic wave. We demonstrated the detecting method in some sample films. We also simulated the coincidence effect in samples. As a result, influence of the defect is observed experimentally and theoretically.
