Ultrasonic non-destructive testing of cardboard tubes using air-coupled transducers

Cardboard tubes are commonly used for industrial plastic film wrapping due to their low cost, high compression strength, reliability and low sensitivity to environmental changes. In order to guarantee the high radial compression strength during the manufacturing process, destructive testing such as manual peeling or non-destructive testing using acoustic impedance measurements are currently performed on a regular basis. In order to achieve a continuous quality control, automatic and non-contact inspection still need to be developed. In this paper, a method and apparatus for non-contact and rapid inspection of cardboard tubes is presented. The principle is based on the use of capacitive air-coupled transducers at frequencies below 20 kHz for generation and measurement of propagative flexural waves in a pitch-and-catch configuration. Sensitivity analysis is performed for different modes and damage types and is validated experimentally for four flaw types typically observed during the manufacturing process. Experimental validation of detection and flaw quantification is demonstrated using both amplitude and time-of-flight of wave packets at different frequencies, allowing automatic quality control of the manufacturing process.