JEC COMPOSITES MAGAZINE - Issue #143 - November/December 2021 - 53

Fibre-reinforced composites with a vitrimer matrix
Fig. 8: Phased array C-scans of an impacted and repaired 3R CFRP laminate made
with an RTM process (provided by the University of Ioannina)
is reached in the joining zone, the inductive heating is controlled for a
defined time, so that the target temperature can be maintained until the
desired end of the process. In the case of 3R CFRP, the vitrimer in the
joining zone is heated above its Tg
and thus reaches a viscoelastic state. In
combination with the applied pressure, the press tool descends, resulting
in a displacement of the consolidation plate, which can be measured
as a joining path. After induction heating has been stopped, the joint
is cooled by applying a consolidation pressure, so that the joining path
continues to increase until the vitrimer is cooled below Tg
Fig. 9: Expected cost savings when manufacturing composite parts with 3R-FRPC
compared to autoclave processes (Source: www.airpoxy.eu)
. After the final
joining path is reached, the press is opened and the joined structure
is removed. A special version of the discontinuous induction welding
process is induction spot welding (Figure 7). The process is based on a
cooled punch with an integrated inductor. The pressure is applied to the
joining partners through a ceramic inlay. Since the ceramic inlay is permeable
to the electromagnetic field, simultaneous application of pressure
and heating prevents deconsolidation. With this technology, 3R
CFRP laminates produced with the RTM process could be successfully
joined. The fracture surfaces of the tested tensile shear specimens show
a cohesive failure, which indicates good bonding of the joining partners.
Repair
The specific properties of the 3R vitrimer also make it possible to repair
defects such as delaminations and detached welded or bonded joints by
applying pressure and temperature on the damaged areas.
Due to the compactness of the welding head shown in Figure 7, this
repair technology could be transferred to sufficiently stiff components
already installed, offering a high cost-saving potential not only for the
aviation industry. Figure 8 shows the NDT images of a specimen that
was subjected to an impact test.
The delamination, which occurred in the impact area in the non-porous
laminate specimen, was repaired by applying pressure and temperature,
so that the initial condition was restored.
The C-scan was conducted with a Dolphicam 2 (Dolphitech) ultrasonic
matrix scanner for non-destructive testing. A transducer with a frequency
of 3.5 MHz and a resolution of 128x128 elements was used to scan
the laminate surface. The individual images were stitched together to
form an overall image.
Outlook
3R CFRP show great potential for the production of high-strength
structural components. They make it possible to use the advantages of
thermosets while accessing efficient automated processes that were previously
reserved for thermoplastic polymers.
3R CFRP offer:
- High production rates through adapted thermoforming processes
- Weight-neutral welding and bonding technologies
- Production of complex parts by resin transfer moulding (RTM)
processes
- Potential for cost-efficient repairs
- Recycling of end-of-life and production waste
This results in efficient process chains that support the objective of
a far-reaching cost reduction and contribute to the development of
3R-CFRP high-performance components for the aerospace industry
(Figure 9).
More information:
www.airpoxy.eu
Partners
This work is the result of the collaboration of a multidisciplinary team
that brings together 11 partners from six EU countries: Fundacion
Cidetec, Leibniz-Institut für Verbundwerkstoffe GmbH, Coexpair SA,
Fundacio Eurecat, Eirecomposites Teoranta, Ingenieria y Desarrollos
en Composite S.L., Societe Nationale de Construction Aerospatiale
Sonaca S.L., Panepistimio Ioanninon, Altair Engineering France Sarl.,
Arttic, and Asociation Espanola de Normalizacion.
Acknowledgements
The research leading to these results received funding from the
European Union's Horizon 2020 research and innovation programme
under grant agreement no. 769274.
References
[1] D. Montarnal, M. Capelot, F. Tournilhac and L. Leibler, " Silica-like
malleable materials from permanent organic networks " , Science
(New York, N.Y.), p. 965-968, 2011.
[2] A. Ruiz de Luzuriaga, R. Martin, N. Markaide, A. Rekondo, G.
Cabañero, J. Rodríguez and I. Odriozola, " Epoxy resin with exchangeable
disulfide crosslinks to obtain reprocessable, repairable and
recyclable fiber-reinforced thermoset composites " , Material Horizons,
2016, Issue 3, p. 241-247.
N°143 November - December 2021 / jec composites magazine
53
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