Air-coupled Ultrasonic Inspection of Thermoplastic Composite Structures for Aerospace Vehicles

Abstract

Composites have become the most important type of material used in the manufacturing of aerospace vehicle components. One focus of the German Aerospace Center is to develop cost-efficient manufacturing processes for large-scale composite components such as liquid hydrogen tanks, rocket booster pressure vessels, and aircraft fuselages. To enable a high degree of automation in the manufacturing, robot-assisted tape-laying processes with flash lamp consolidation of thermoplastic prepregs are preferred. Nondestructive inspection is crucial to monitor manufacturing quality, and various forms of ultrasonic inspection have become standard. Air-coupled ultrasonic inspection is applied at the German Aerospace Center, and since access to the component is usually available only from one side, Lamb waves are excited and detected in focused slanted reflection mode. Like the manufacturing process, air-coupled ultrasonic inspection has been automated as well to avoid a significant pause during or after manufacturing. The paper demonstrates the robot-assisted air-coupled ultrasonic inspection of a liquid hydrogen tank. The robotic program is generated using offline programming software, based on the topography of a CAD model of the hydrogen tank. The inspection is carried out by scanning the component while ultrasonic pulses are triggered on a regular grid. The ultrasonic A-scans together with the measurement coordinates are monitored simultaneously. The data are used to generate and evaluate a three-dimensional C-scan.