A novel dielectric sensor for process monitoring of carbon fibre composites manufacturing (Conf paper)

Process monitoring techniques have been developed to monitor critical parameters of manufacturing such as flow front position and cure reaction progress and to identify potential defects. The main technologies proposed include dielectric spectroscopy [1], fibre optics [2], time domain reflectometry [3], and pressure transducers [4]. Monitoring based on impedance/dielectric spectroscopy is considered advantageous due to the sensitivity of response to filling state and cure progress, robustness and relatively low cost of the sensors and measuring setup and capability for incorporation on tooling surfaces. Lineal dielectric flow sensors are appropriate for use with non-conductive reinforcement [1] as presence of carbon would disturb the electric field, whilst setups making use of the conductive carbon reinforcement as one of the electrodes of the sensing system [5] involve significant practical complexity as their operation requires electrical insulation of the reinforcement from the tooling assembly. The solution adopted in cure applications for carbon composites is to cover the sensor with a permeable non-conductive material such as glass cloth or a polymer weave [6-8]. This type of solution increases the intrusiveness of the sensing system and generates some differences between the material monitored and the material of the composite. The present study reports the development of a novel non-intrusive dielectric sensor overcoming the difficulties arising by the conductive properties of carbon reinforcement. The concept is used for the design of two sensor types; a lineal flow front position sensor applied to resin transfer moulding (RTM) and a woven arrangement used to monitor the cure and of a carbon fibre/epoxy composite.