Investigation of Electrical Time Domain Reflectometry for infusion and cure monitoring in combination with electrically conductive fibers and tooling materials

The presented work investigates Differential Electrical Time Domain Reflectometry (D-TDR) as a method to monitor the flow front position and curing degree of resin during manufacturing of composite structures by vacuum infusion technology. The sensor concept is based on the physical principle that a voltage pulse introduced into a transmission line, which serves as a sensor, is reflected at impedance changes, which correlate to changes of the dielectric properties along the line. The method's accuracy under different boundary conditions is investigated by infiltrating glass and carbon fiber preforms on metallic and wooden toolings with epoxy resins. A shielded sensor concept is presented, which eliminates the influence of conductive materials on the signal. For all tested fiber/tooling combinations, the monitoring method shows a good agreement between the flow front position and optically determined flow fronts. Degree of cure measurements at ambient and elevated temperatures show a correlation with Differential Scanning Calorimetry (DSC) and rheology analyses.