In-situ structural monitoring of fibre-reinforced plastic composites under compressive loading
Continuous structural monitoring of FRP components, especially in complex, changing load scenarios, represents an efficient solution approach to detect potentially occurring fatigue or damage at an early stage. Especially in FRP components, textile-based sensors are an economical solution for continuous in-situ structure monitoring, due to their high structural compatibility and direct textile integration during textile production.
The textile-based sensor concept developed in this research project was electromechanically characterised at the yarn and composite scale and was further processed in multiaxial warp-knitting to manufacture functionalised fabrics. The sensor functionality in CFRP specimen was tested in tensile, pressure and bending tests. Finally, a CFRP profile demonstrator was used to test and prove the practical feasibility and functionality. These "smart composites" not only enable continuous in-situ structural monitoring of FRP components under tensile, bending and, especially, compressive stress, but can also be used to detect cracking and delamination processes. This allows both the understanding of the material behaviour to be improved and taken into account for future designs, as well as necessary measures to be initiated to ensure the functionality of the overall system.
Fibre-reinforced composite structures are currently used in the fields of mechanical engineering, aircraft construction and automotive engineering, among others, due to their excellent mechanical properties combined with a high lightweight construction potential . In the construction sector, high-performance textiles are increasingly being used as a substitute for steel reinforcement in textile reinforced concrete , due to their mechanical and chemical properties and the resulting resource-saving, filigree, lightweight construction potential. The long-term stable functionality and safety of fibre-reinforced composite structures is urgently required due to their frequent use in safety-critical components and structures. A promising practice-oriented approach is the continuous structural monitoring in order to quantify the (residual) load-bearing capacity and to initiate any necessary measures to ensure functional capability. A particularly economical and structurally compatible solution are textile-based sensors that are integrated during the manufacture of the textile reinforcement and used to detect complex load scenarios as well as cracking and delamination processes at the composite scale. [3 – 6]
Due to their operating principle, textile-based strain sensors are mainly used for monitoring composite structures subjected to tensile stress. In order to be able to derive reliable statements about structural changes and critical overload conditions even in complex overlapping stress scenarios (e.g. tensile and compressive stresses), textile-based pressure sensitive sensor systems for continuous in-situ structural monitoring for FRP were developed in IGF project 21169 BR.
Objective and solution
The aim of the IGF research project was the development, characterisation and testing of textile-based pressure sensitive sensor systems and their textile-technical integration in multi-axial warp knitting for the production of sensor-functionalised textile reinforcements for use in FRP. The requirements for the textile sensors were derived simulation-based by analysing a functional demonstrator. The textile sensors were specifically designed to detect structural deformations induced by tensile, bending and especially compressive stresses. Therefore, the approach of increasing the pressure sensitivity of textile sensors by pre-tension was investigated. The sensor behaviour was extensively analysed in electromechanical investigations at fibre and composite scale and tested on the functional demonstrator.
The IGF project 21169 BR of the Research Association Forschungskuratorium Textil e.V., Reinhardtstr. 12-14, 10117 Berlin was funded by the Federal Ministry of Economics and Climate Protection via the AiF within the framework of the programme for the promotion of joint industrial research and development (IGF) on the basis of a resolution of the German Bundestag.
The authors would like to thank the above-mentioned institutions for providing the financial resources. The research report and further information are available from the Institute of Textile Machinery and High Performance Textile Materials Technology at TU Dresden.