Development of an inline monitoring system for quality assurance of thermoplastic CFRP tape using air-coupled ultrasound

Besides the aerospace industry, fiber reinforced plastics have also spread towards many further applications such as automotive or civil engineering. Their superior strength and stiffness to mass ratio made them the number one material for achieving high performance. Especially continuous fiber reinforced plastics allow for the construction of structures which are custom tailored to their mechanical loads. The two main constituents of CFRP are carbon fibers and matrix. Two possibilities for matrix material exist: thermosetting and thermoplastic matrix. While thermosetting matrix may yield better properties, thermoplasticity opens a wide range of applications due to weldability and compatibility to e.g. thermoplastic injection molding materials. In this work an inline monitoring system was developed for quality assurance of continuous fiber reinforced, thermoplastic CFRP tapes using air-coupled ultrasound. This system is independent of tape thickness and tape width due to an adaption of the angle of incidence. By slanted incidence of the ultrasound on the tape surface, guided waves were excited in the material in fiber direction and perpendicular to the fiber direction. Artificial defects – fiber cuts, matrix cuts, circular holes, low velocity impacts from tool drop, and sharp bends – were produced. Experiments on a stationary tape showed good detectability of all artificial defects by guided waves. Also the effects of variation in material properties, fiber volume content and fiber matrix adhesion were examined, to allow for quality assessment. For validation of the monitoring system, an apparatus containing one endless loop of CFRP tape was constructed and built to simulate the pultrusion process. Several methods were investigated regarding the scanning path of single channel ultrasonic transducers in order to achieve 100 % testing of the tape. For final validation of the monitoring system a Probability of Detection (POD) analysis was conducted and analyzed.