An innovative and low-cost system for in situ and real-time cure monitoring using electrical impedancemetry for thermoset and CFRP laminate

Abstract

Abstract This paper is divided into five sections. The first one introduces the context of our study dealing with the monitoring of the cure of structural composites made of carbon fibers and thermoset matrix (CFRPs). The second one presents a brief state of the art. The third one deals with the materials characterized in this study, and the design and the production of a new bench dedicated to electrical impedance (EI) and thermal measurements. The fourth one compares the results obtained on CFRP and unreinforced matrix samples using conventional differential scanning calorimetry and EI measurements. A conclusion and some prospectives close the paper. In a recent previous work, we studied phases transitions in CFRP during their curing by EI showing that changes in the electrical complex impedance Z can easily be related to those of conventional parameters such as the thermoset matrix degree of cure (DOC) α. This work was carried out using a commercial single-channel impedance analyzer: HIOKI. In addition to its high acquisition cost, this device presented certain limitations in terms of measurement channels (only one) and data acquisition rate (60 s for a frequency sweep). These facts led us to develop a new EI measurement bench for monitoring the impedance changes in CFRP part for aeronautical applications (airframe structures). The innovative new multi-channel (8) bench we designed and manufactured is based on the Digilent PmodIA module (never used previously for this purpose) and an analog-front-end developed for EI measurements. It costs only 15% of the HIOKI analyzer and has a data acquisition rate 48 times higher (1,23 s versus 60 s). All this while maintaining an equivalent impedance measurement range: 100 mΩ to 1 MΩ. The proposed approach makes it possible not only to monitor the DOC α, but also to detect potential cure cycle issues. This latter demonstration was carried out on a new composite material (vacuum-bag oven-cured), NC66/1808NA (supplied by CTMI), which, to our knowledge, had never been characterized in the literature. Compared with preliminary results, similar behavior is obtained on two different CFRPs using two different benches. This clearly underlines the value and quality of the study. Experimental validation of our approach will contribute to CFRP structural health monitoring.