Capacitance measurements on integrated conductors for detection of matrix cracks in GFRP

Christina Buggisch,Bodo Fiedler,Abedin Gagani

doi:10.1186/s42252-020-00013-x

Abstract

For the reliable and cost-efficient application of glass fibre polymer composites in structural applications, knowledge of the damage state of the material during operation is necessary. Within this work, a structural health monitoring method based on in-situ electrical capacitance measurements is presented, which enables damage monitoring in glass fibre reinforced polymers. For this purpose, individual glass fibre rovings in a non-crimp fabric were replaced by carbon fibre rovings at regular intervals. Additionally, specimens with solid or stranded copper conductors were manufactured to gain insights into the influences of conductor material and composition. The modified fabrics were implemented as 90 ∘ layers of [0/904]s glass fibre polymer cross-ply laminates. To monitor the progressive damage, conductive rovings were contacted, forming the capacitor walls of interleaved capacitors. Carbon fibre conductors show higher sensitivity of the capacitance to crack formation than solid or stranded copper conductors. Capacitance decrease measured in-situ during tensile tests on specimens with carbon fibre conductors shows a high correlation with crack initiation, further crack formation and speed of crack evolution. An analytical model can describe the correlation based on the assumptions of an ideal plate capacitor. Thus, the structural health monitoring method developed in this work can reveal in-situ knowledge of the material damage state.

Highlights

Glass fibre reinforced polymers (GFRP) are used in numerous structural applications, where the reliability of the structures is essential for operation
An in-situ structural health monitoring (SHM) technique for damage detection and state estimation in GFRP based on capacitance change due to dielectric changes introduced by evolving matrix cracks is established
For embedded carbon fibre (CF) conductors, the capacitance decrease measured with increasing strain shows a high correlation with the number of cracks and the stiffness decrease

Summary

Introduction

Glass fibre reinforced polymers (GFRP) are used in numerous structural applications, where the reliability of the structures is essential for operation. Coplanar electrode pairs without surface contact are used during capacitive imaging to scan across a surface measuring changes in charge for given voltages resulting in a map of electrical property changes enabling detection of cracks, voids or moisture [21]. Capacitative imaging with coplanar non-contact electrodes can be used for moisture or damage detection and localisation in different FRP composites as CFRP, honeycomb structures and GFRP [25, 26]. It was shown that capacitance measurements on embedded piezoelectric transducers are in good agreement with loading and unloading trends in different tensile tests on GFRP [28]. An in-situ SHM technique for damage detection and state estimation in GFRP based on capacitance change due to dielectric changes introduced by evolving matrix cracks is established. Individual rovings of a glass-fibre fabric have been replaced by conductive carbon fibre (CF) rovings, solid and stranded copper wires forming the capacitor walls of an interleaved capacitor enabling monitoring of capacitance change during tensile tests

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Conclusion