Monitoring cure and detecting damage in composites with inductively coupled embedded sensors

Inductively coupled piezoelectric sensors were embedded into fibre reinforced composites and used to monitor the progress of cure during manufacture, and the subsequent damage state of a cured part. The sensors used in this proof of concept study, consist of a single piezoelectric transducer, which is electronically connected to an inductance coil. The electromagnetic coupling between an external probe and the embedded sensors, allowed the ultrasonic measurements recorded by the embedded transducers to be captured wirelessly. Two different inductively coupled transducer systems were used to monitor cure. A system which generated bulk waves monitored the cure of a 10.4 mm thick glass fibre sample, and a system which generated guided elastic waves monitored the cure of a glass fibre plate. The inductively coupled systems followed the progress of cure by measuring the velocity and amplitude of the received ultrasonic waveforms. To characterise the cure monitoring ability of each system, two established cure monitoring techniques were used to record measurements during cure; differential scanning calorimetry and dielectric analysis. In both experiments the inductively coupled sensors were shown to be capable of monitoring the curing process, and the recorded measurements showed significant variation at the end of the curing process, which suggests a high sensitivity to the final stages of cure. The inductively coupled guided wave system was then used to successfully detect barely visible impact damage, created by a 10 J impact at a distance of 300 mm from the sensor embedded in the cured glass fibre plate.