| Περίληψη: | During the last decades, fiber reinforced composites (FRPs) are year by year replacing metals due to their high stiffness and high strength in combination with low specific weight and corrosion resistance. However, during their service life, these composites due to their laminated structure (no fibers are present in transverse direction) appear matrix cracking and delaminations between the reinforcing plies. A primary limitation of these composites is the poor interlaminar toughness and strength. The mismatch of anisotropic mechanical and thermal properties in between plies of deferent principal directions promotes out of plane stresses at the edges of the structures as well as in the case of stringer run out, thickness variation, holes and structural stiffeners joined to composite skin, and are only some of the candidate areas for delamination under in plane and out of plane loadings. Delaminations are among the most frequent modes of failure encountered in laminated composites and are resulted either from fatigue loadings or low velocity impact events.
Conventional repair techniques of composites have a lot of drawbacks; are expensive, require extensive human work and cannot repair defects deep inside the material. Self-healing polymers is an approach which has not yet been incorporated to commercial composites but promises to face some principal weak points. This smart technology aims to in-situ repair matrix cracks and matrix/reinforcement debonding and thus to extend the effective life-span of the composites, to reduce the maintenance needs and costs and to improve the damage tolerance and reliability of composite structures. Self-healing composites have previously been developed by embedding healing agents into the matrix using microcapsules or vascular networks, that will release the healing agent upon crack damage. A different approach towards self-healing composites is matrices that comprise reversible polymers that are able to proceed with multiply healing cycles at the same damaged site.
In the present investigation, the utilization of three different reversible polymeric systems (based on their chemistry) as healing agent into CFRPs was studied. More precisely, common thermoplastics such as PET and Polyamides (Nylon-66) based on reversible covalent bonds, Bis-maleimide polymers (pure and blends) based on Diels Alder (DA) and Retro-DA reactions through special covalent bonding and finally Supramolecular polymers based on hydrogen bonds were integrated into aerospace-grade CFRPs. A variety of methodologies (i.e. blending, interleaving, sieving and pre-preging) was utilized for the modification. The assessment of potential knock down effects and the healing capability of the resulting composites were investigated under mode I and mode II fracture tests, low velocity impact (LVI), compression after impact (CAI) and three-point bending (3PB) tests. Optical microscopy, SEM examinations and acoustic emission activity (AE) of the samples was monitored and led to qualitative conclusions regarding the involved failure and healing mechanisms.
According to all these experimental campaign, it was shown that by the incorporation of all these SHAs to the composites the mode I and II fracture toughness characteristics were significantly increased with samples containing supramolecular interleaves to exhibit dramatically increased fracture toughness characteristics (e.g., GIC increased with more than one order of magnitude at approximately 1550%). These modified composites exhibited healing efficiency values from 60% to 100% after the application of the first healing cycle. In addition the effect of the curing regime on the toughening and healing behaviour of CFRPs containing bis-maleimide polymers was investigated. It was shown that curing temperatures lower than the melting point of the healing agent slightly decreased the fracture toughness characteristics while increased the healing capabilities of these samples. LVI tests revealed that samples containing supramolecular prepregs or MWCNT doped nylon electrospun veils as interleaves between the primary layers of the composite exhibited higher resistance to delamination and increased CAI characteristics after the application of the healing cycle. Finally, AE recordings showed that by the incorporation of a ductile phase (i.e., healing agent) into the composite the AE activity in terms of hits is typically reduced while both AE characteristics (hits and energy) was reduced after the application of the healing cycles.
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