Compact Substrate Integrated Wireless Cure Monitoring Sensor for Epoxy Resin

In this paper, a novel compact fully passive wireless cure monitoring sensor for fibre-reinforced plastics (FRP) structures is presented. A completely new concept for a very reliable separation of the radiated measurement signal from the interrogated RF power is achieved by using a passive sensor internal frequency doubler. The frequency doubling improves the separability and reduces the space requirements for the sensing resonators and radiating antenna by using the second harmonic as measurement signal. Thus the sensor enables locally wireless monitoring of the increasing cross-linking level of the polymer molecules and the reaction temperature of epoxy resins during the exothermic curing reaction at particular critical locations inside FRP structures. The temperature and permittivity of the reacting epoxy resin composition is determined by measuring and evaluating the change in resonance frequency for two substrate integrated waveguide (SIW) resonators. The obtained measurement results show an excellent correlation with the data provided by the widely used and established cure monitoring technique differential scanning colorimetry (DSC), as well as with thermocouple temperature measurements.