4. SUMMARY AND OUTLOOK
Lightweight and compact structures can be designed by merging different functions of carbon fiber reinforced composites. Carbon fibers, besides their mechanical load-bearing capability, can be used for secondary tasks based on their electrical properties. Examples are given in Table 1 and are categorized according to their function. One of the most important aspects in choosing a secondary role is the material used. In most examples, long carbon fiber reinforced epoxy matrix composites were used. In some cases, however, a thermoplastic matrix proved to be a better choice. In order to improve efficiency, nano-sized carbon particles, such as carbon black, CNTs or carbon nanofibers were added to the composite.
Two different methods for connecting the composite part to the electric circuit exist:
induction or direct physical contact. The effect of induction is local; therefore, it is suitable for the heat treatment of the joints (e.g. bonds and welds). By direct contact whole composite parts or structures can be electrified; in this case, low contact resistance is needed between the carbon fibers and the electrical circuit. For this, the fibers first have to be exposed either during fabrication (a dry fiber bundle sticking out, or a laminated conductive foil) or after curing (matrix burning or grinding). For the sake of good contact, usually a metallic block or foil (copper, nickel, gold) is pressed against the fibers or another conductive layer (evaporated gold, silver-filled paint or adhesive) is applied on the fibers.
Table 1 is a useful tool from two aspects. Firstly, it helps the designer of a multifunctional structure to choose a secondary function. Secondly, the necessary layout and materials can be determined with it. The roles, based on the electrical properties of carbon fibers, cover the whole life of a composite part. These multifunctional structures can be used in manufacturing (graphitization of carbon fiber, matrix curing, process control), assembly (bonding, welding) and application (deformation, temperature and humidity sensing) until failure (micro cracks, delamination, fiber breakage), besides structural load bearing.
Resistance-based state monitoring is proven to work well with CFRP, but can be used in insulating glass fiber reinforced composites or even in existing structures. Several researchers [22,76–79] investigated electrically insulating glass fiber reinforced epoxy composite sheets by measuring their resistance. To make the test specimens conductive, they added nanoparticles (multiwalled carbon nanotube, carbon black). They proved that a basically insulating material could be modified to measure the electrical resistance of the samples, thus continuous and in situ structural state monitoring can be achieved. As a consequence, failure can be predicted. In the case of finished parts, state monitoring can be performed by applying a conductive film layer [79].
In order to meet the efficiency standards of the energy and transportation industry, lightweight structures are essential. Merging functions, creating and using multifunctional materials promote weight reduction in these industries. Weight can be reduced significantly
with the use of the electrical properties of the CFRP to substitute electrical instruments.
Adding energy storage functions to body parts (e.g. trunk lid, body stiffener) creates a structural supercapacitor, which merges functions and creates free space in the underhood area [55]. In a car a serious amount of wires are used to operate devices or to gather information from sensors. The reinforcing carbon fibers could be used to transmit electrical signals, therefore a reserve network can be formed, or cable cords could be replaced completely [80]. Adding sensing capability as a secondary function converts conventional materials into raw materials for Industry 4.0 and autonomous vehicles, which will need increasing amounts of multifunctional carbon fiber reinforced composites.
5. ACKNOWLEDGEMENTS
This work was supported by the OTKA (K 116070 and K 120592) and NVKP (NVKP_16-1-2016-0046) projects of the National Research, Development and Innovation Office (NKFIH), and by the Higher Education Excellence Program of the Ministry of Human Capacities in the frame of Nanotechnology research area of Budapest University of Technology and Economics (BME FIKP-NANO).
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