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Structurally compatible embedded sensors for damage detection in glass fibre reinforced polymers
Citation Link: https://doi.org/10.15480/882.4557
Publikationstyp
Doctoral Thesis
Date Issued
2022
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2022-07-13
TORE-DOI
First published in
Number in series
42
Citation
Technische Universität Hamburg (2022)
Fibre reinforced polymers exhibit a complex failure behaviour due to their multi-scale nature. Even damage that severely influences the material integrity can often not be detected by visual inspection. Therefore, different non-destructive testing and structural health monitoring (SHM) techniques have been developed to ensure composite structures’ reliable and safe operation. Many SHM methods require skilled personnel, expensive measuring equipment and complex sensor networks that reduce the lightweight potential. Especially in carbon fibre reinforced polymers, electrical resistance measurements overcome the limitations. In conductive carbon fibre reinforced polymers, in-situ electrical resistance measurements can monitor the material state during operation without additional sensor networks. However, in unmodified glass fibre reinforced polymers (GFRPs), such resistive monitoring is not possible due to a lack of material conductivity. This dissertation provides two approaches to enable electrical SHM in GFRPs without significantly reducing the mechanical properties. The first method describes a local matrix modification with conductive, fully-integrated, pre-cured single-walled carbon nanotube (SWCNT)/epoxy thin-film sensors. The second technique outlines a local replacement of glass fibre bundles with electrically conductive fibres as conductors for capacitance measurements.
The embedded SWCNT/epoxy thin-film sensors exhibit a piezoresistive effect and enable real-time, local strain monitoring. In general, tensile stresses result in a resistance increase and compressive stresses in a resistance decrease. The integrated thin-film sensors do not significantly reduce the mechanical properties for various lay-ups and load cases. Local stress concentrations due to cracks or around holes result in significant resistance increases. The sensitivity of the sensor films can be tuned by geometry variation. First durability studies prove the integrity of the sensors during cyclic fatigue tests. Buckling of a stringer component can be reliably detected during crippling tests by a resistance increase.
Capacitance measurements on integrated carbon fibre bundles enable damage detection in GFRP composites due to damage-induced material permittivity changes. Developing damages with incorporated air cause an overall lower permittivity and, thus, a drop in capacitance. Matrix crack evolution and capacitance decrease clearly correlate during tensile tests. The capacitance decreases more severely during rapid crack formation and slower the closer to crack saturation. Analytical modelling of the capacitance decrease is possible with assumptions of an ideal plate capacitor. A higher distance of the bundles decreases the sensitivity towards the end of the test, where a more diffuse crack pattern is present. By integrating the bundles in different layers, crack evolution in the respective layers can precisely be monitored. Furthermore, using the measured capacitance decrease, a detection and size estimation of impact damages is possible.
The two presented methods offer highly tailored SHM of GFRPs and can easily be incorporated in industrial processes – carbon fibre bundles can be integrated directly during fabric manufacturing, and pre-cured thin-film sensors can be placed in the desired locations during stacking of dry fabrics. Both methods provide cost-efficient, real-time SHM without significantly altering the material properties and therefore help to enable safer use of GFRP structures.
The embedded SWCNT/epoxy thin-film sensors exhibit a piezoresistive effect and enable real-time, local strain monitoring. In general, tensile stresses result in a resistance increase and compressive stresses in a resistance decrease. The integrated thin-film sensors do not significantly reduce the mechanical properties for various lay-ups and load cases. Local stress concentrations due to cracks or around holes result in significant resistance increases. The sensitivity of the sensor films can be tuned by geometry variation. First durability studies prove the integrity of the sensors during cyclic fatigue tests. Buckling of a stringer component can be reliably detected during crippling tests by a resistance increase.
Capacitance measurements on integrated carbon fibre bundles enable damage detection in GFRP composites due to damage-induced material permittivity changes. Developing damages with incorporated air cause an overall lower permittivity and, thus, a drop in capacitance. Matrix crack evolution and capacitance decrease clearly correlate during tensile tests. The capacitance decreases more severely during rapid crack formation and slower the closer to crack saturation. Analytical modelling of the capacitance decrease is possible with assumptions of an ideal plate capacitor. A higher distance of the bundles decreases the sensitivity towards the end of the test, where a more diffuse crack pattern is present. By integrating the bundles in different layers, crack evolution in the respective layers can precisely be monitored. Furthermore, using the measured capacitance decrease, a detection and size estimation of impact damages is possible.
The two presented methods offer highly tailored SHM of GFRPs and can easily be incorporated in industrial processes – carbon fibre bundles can be integrated directly during fabric manufacturing, and pre-cured thin-film sensors can be placed in the desired locations during stacking of dry fabrics. Both methods provide cost-efficient, real-time SHM without significantly altering the material properties and therefore help to enable safer use of GFRP structures.
Subjects
Structural health monitoring
Electrical measurements
Carbon nanotubes
Piezoresistive effect
Composites
DDC Class
600: Technik
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