| Περίληψη: | Carbon fiber reinforced polymer (CFRPs) composites are increasingly used as advanced materials in many structural applications i.e. aerospace, automotive, marine etc. due to their superior in-plane mechanical properties, high specific strength and stiffness and corrosion resistance compared to conventional metals. However, during their service life, these composite materials due to their laminated structure appear poor out-of-plane properties due to the absence of through-thickness reinforcement. Additionally, their low polymer matrix toughness leads to interlaminar failures such as delamination. Delaminations as a primary failure mode are resulted either from fatigue loadings or low-velocity impact which are caused by the extension of multiple matrix cracks. Therefore, composites are highly susceptible to impact damage due to brittle matrix behaviour and poor impact damage tolerance. In light of this issue, the introduction of carbon nano-particles and especially carbon nanotubes into the matrix of composites has shown considerable improvement on the fracture behaviour of the composites by introducing additional energy absorption mechanisms during fracture, thus concluding to composites with improved damage tolerance. Several attempts have been developed for enhancing the fracture toughness and damage tolerance of fibre-reinforced polymers such as stitching, hybridization, z-pinning, interleaving and toughening the polymer matrix using nano-sized particles. Towards this direction, the use of nanoparticles and especially carbon nanotubes (CNTs) into the matrix results in the development of composites with multi-scale reinforcement. CNTs are highly potential fillers due to their extraordinary mechanical, electrical and thermal properties. The nano-phase improves the fracture behaviour of composites by introducing additional energy absorption mechanisms during fracture, thus concluding to composites with improved damage tolerance characteristics. However, a direct introduction of CNTs into matrix (mainly in high CNTs concentrations) leads to increase of resin’s viscosity and an uneven nanofiller distribution and/or filtering effects that block the resin close to the inlet gates. This difficulty makes almost impossible the direct introduction of CNTs during Out-of-Autoclave (OoA) processes such as liquid resin infusion (LRI) or resin transfer molding (RTM).
The present Dissertation aims to the development of Out-of-Autoclave Carbon Fibre Reinforced Polymers (CFRPs) with improved interlaminar fracture toughness and damage tolerance characteristics by the introduction of carbon nanotubes and nano-doped nonwoven interleaf veils on the surface of the main reinforcement (carbon fabrics). More precisely, the deposition of multi-walled carbon nanotubes (MWCNTs) into the structure of CFRPs has been succeeded by using a liquid CNT-enriched sizing agent solution for the pretreatment of main reinforcement prior or after the carbon fabric preform. In addition, the introduction of the copolyamide (Nylon-66, 6/11/12, Griltex 1516A) nonwoven veils enhanced with CNTs (max concentration of 2.5 %wt. MWCNTs) as interleaf materials into the structure of CFRPs was performed as an alternative route of introduction of carbon nanotubes between the carbon fabrics. In this thesis, three manufacturing OoA techniques were developed to produce nano-enhanced reinforced CFRP composite structures with multi-scale reinforcements, as follows:
• Dip-Coating Technique, concerns the nano-modification of the main reinforcement surface (carbon fabrics) by the integration of MWCNTs in a form of CNT-enriched sizing agent. A water-based nano-doped solution was prepared at a given wt.% CNT content and the fabric was immersed through it at a constant speed rate. Then, the fabric was dried and the dry fabric used to prepare the final preform placing it in to the mould.
• Two Step Infusion Technique, concerns the nano-modification of preform by the introduction of a water-based nano-doped solution under vacuum during the first Step of process. At the second Step, the nano-doped preform was dried and the resin matrix was infused under vacuum to impregnate the nano-treated preform.
• Interleaving Technique, concerns the indirect introduction of MWCNTs in to the main reinforcement by the deposition of nano-doped non-woven interleaf veils.
The primary objective of the current thesis is to investigate the suitable concentration of carbon nanotubes leading to the improvement of interlaminar fracture toughness under Mode I and Mode II remote loading as well as the effect of the suitable configuration of non-woven interleaf veils on the low velocity impact and compression after impact behaviour was assessed. Additionally, potential knock-down effects of the in-plane mechanical properties of the CFRP composites by the incorporation of carbon nanotubes and nano-doped veils were investigated. For that reason, three-point bending (3PB) and tensile plain and open-hole tests were performed.
More precisely, at the first two manufacturing techniques (Dip-Coating and Two-Step), composite laminates were produced with various CNTs concentrations (0, 0.5, 1, 1.5 and 2.5 wt.%). During the Interleaving Technique two kinds of nano-doped interleaf veils with different thickness (80 and 200 μm) were studied and directly compared with the non-interleaved (reference) laminates. In addition, the through-thickness electrical and thermal conductivity were also characterized in order to ensure the multi-functional performance of nano-enhanced CFRP composites. Optical and scanning electron microscopy (SEM) examinations of the specimens of each kind of materials for each manufacturing technique were also carried out in order to confirm the synergistic associated mechanisms that carbon nanotubes introduced during fracture.
According to the aforementioned experimental campaign, it was shown that by the direct incorporation of carbon nanotubes to the main reinforcement surface of the composites either their introduction prior or after the carbon fabric preform, the effective CNT concentration was 1.5 wt.% and the nano-modified CFRP composites exhibit a significant increase of the interlaminar fracture toughness under Mode I and Mode II of the order up to 100% and 60%, respectively.
Taking into account the results obtained of the study of interlaminar fracture toughness, at the two first manufacturing techniques, the nano-modified CFRP laminates with 1.5 wt.% MWCNTs were subjected to low velocity impact at three impact energy levels (8J, 15J and 30J) and directly compared with the reference laminates. The compression after impact tests (CAI) were carried out in order to investigate the influence of CNTs on the damage tolerance of the CFRP composite. In terms of the manufacturing of CFRP laminates with the introduction of non-woven co-polyamide interleaf veils, six layers of thin veils (thickness <80μm) were selected and positioned at two outer surfaces between the carbon fabric layers. For impact resistance, the nano-modified composites produced by Dip-Coating and Two Step Infiltration Technique with the introduction of 1.5% concentration of MWCNTs exhibited an excellent improvement at impact energies of 8J and 15J. While in the case of higher energy (30J), the nano-doped composites both for Dip-Coating and Two Step techniques reported an increase of damaged areas of approximately 4% and 30% respectively. However, in the case of Dip Coating Technique, CAI tests revealed that the residual strength of nano-modified composites was significantly improved close to 10% for all energy levels. While in the case of Two Step Technique, On the other hand, the copolyimide interleaved composites show a considerable increase of delaminated areas for all impact energies compared to the non-interleaved and the presence of interleaved veils was not exhibited better CAI characteristics. Therefore, it was concluded that the copolyimide veils are more susceptible to impact damage as the reference non-interleaved composite exhibited more resistance to delamination during LVI test.
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