Ultrasound – All-Round Tool for Composite Process Monitoring

Kurzfassung

The use of fiber reinforced plastics is growing at high rates in branches like aeronautic and automotive as well as wind energy industry with very specific requirements regarding quality, cost and production rate. But the production processes of these composite materials are complex and difficult to automate. The high amount of necessary manual work is one of the main reasons for the high costs of composite structures. A key step in the process chain is the curing of the thermoset polymer matrix, with a significant influence on the final material properties. In this phase process-integrated cure monitoring sensors could play an important role for quality assurance, process optimization and automation. Cure monitoring sensors are amongst others dielectric, resistive and ultrasonic sensors. The drawback of dielectric and resistive sensors is the required direct contact to the resin obtaining only the cure state on the surface. They affect the part surface, are difficult to integrate into the mould and their information content is limited. Ultrasonic sensors on the other hand do not need direct contact to the resin and deliver information about the resin cure through the whole part thickness. The ultrasound waves are sent from one actuator through the mould, pass through the part in production and then are registered by a sensor located on the opposite. With rising degree of cure the sound waves propagation speed increase allowing a highly sensitive cure monitoring. In practice – especially in regard of industrial production conditions – the reliability of state of the art ultrasound transducers is not sufficient for automated processes. In addition they are expensive and rather hard to integrate into the mould. Instead of commonly used ultrasound transducers a method is presented where tool-mounted piezoceramics are used instead. The novel sensors reach significantly higher amplitudes and stability. With their small size the piezoceramics are easy to integrate while they are less expensive than transducers. In addition to the cure progress the resin flow in liquid moulding processes can be monitored by ultrasound sensors as well as the mould wall temperature and even the laminate thickness can be acquired during the process. Process monitoring sensors can be a powerful tool in composite production. They can be used not only for quality assurance and documentation but offer as well a high potential for effective process control and optimization, trouble shooting and short run up time for new processes.