P. Karapappas

Enhanced Fracture Properties of Carbon Reinforced Composites by the Addition of Multi-Wall Carbon Nanotubes

In the present study, the fracture energy of hybrid carbon fiber reinforced polymers was investigated. The composites were modified by the addition of multi-walled carbon nanotubes into the matrix material. The interlaminar fracture properties under Mode I and Mode II remote loading were studied as a function of the carbon nanotube content in the matrix. With the addition of carbon nanotubes in the epoxy matrix, a significant increase in the load bearing ability as well as in the fracture energy was observed, for both Mode I and Mode II tests. It is speculated that carbon nanotubes due to their large aspect ratio have a significant toughening effect since extra energy is needed in order to pull them out from the matrix and start the crack propagation following a kinking out pattern at nanoscale.

Damage Monitoring of Carbon Fiber Reinforced Laminates Using Resistance Measurements. Improving Sensitivity Using Carbon Nanotube Doped Epoxy Matrix System

In this study, carbon nanotubes (CNTs) were used as additives in the epoxy matrix of unidirectional (UD) carbon fiber reinforced laminates. CNTs were employed not only for improving the mechanical performance of composite, but also for providing an innovative way for monitoring the damage accumulation during the loading of the laminate via the monitoring of the changes in the conductivity of the material. The CNT inclusion improved the transverse conductivity rendering the UD composite more electrically isotropic. Both plain and modified laminates were subjected to monotonic and cyclic tensile loading with simultaneous monitoring of the longitudinal resistance. The resistance change due to mechanical loading was more pronounced for the laminates with the modified matrices. As it was expected, the presence of the electrically conductive CNT network acted as a direct sensor of matrix related damage phenomena, which was complementary to changes related to failure of the reinforcing phase. This was not the case for the reference composite laminates where the monitored changes of the electrical conductivity mirrored the damage exclusively related to the reinforcing phase i.e., the carbon fibers.

Multistage fatigue life monitoring on carbon fibre reinforced polymers enhanced with multiwall carbon nanotubes

Abstract In this study, CNTs were used as modifiers of the epoxy matrix of quasi-isotropic carbon fibre reinforced laminates. The prepared laminates were subjected to tensile loading and tension–tension fatigue and, the changes in the longitudinal resistance were monitored via a digital multimeter. In addition, acoustic emission and acoustoultrasonic techniques were used for monitoring the fatigue process of the laminates. The nanoenhanced laminates, on the one hand, exhibited superior fatigue properties and on the other hand, they demonstrated the full potential of the material to be used as an integrated sensor to monitor the fatigue life.

Carbon nanotube epoxy modified CFRPs: toward improved mechanical and sensing for multifunctional aerostructures

In aerospace structures, the increase of mechanical performance of materials such as Carbon Fiber Reinforced Polymers (CFRPs) is always a key goal. In parallel, there is a constant demand for multi-functional solutions that provide continuous, integrated damage monitoring in an efficient and cost affordable way. Structural Health Monitoring systems are crucial for a variety of aerospace applications where safety, operational cost and the maintenance have increased significantly. The Electrical Resistance Technique (ERT) as a promising damage monitoring technique uses the CFRP materials themselves as inherent damage sensors. Currently methods medium sensitivity does not allow the identification of early damage stages requested for a potential application. By using highly conductive carbon-nanotubes as filler material into the epoxy matrix of CFRP is expected to increase the sensitivity of the method, allowing for wider field of applications. In addition, it is expected that the introduction of CNTs into the polymer matrix of CFRP laminates will increase the overall mechanical and electrical performance of the composite. This double role of the CNTs is investigated in the present study. Quasi-static tensile, cyclic loading-unloading-reloading with increase load level at each loading cycle and tension-tension fatigue tests with parallel monitoring of the longitudinal resistance performed on CFRP laminates with various contents of CNTs in the epoxy matrix showed that matrix cracking and fiber breakage caused resistance to increase irreversibly. Although the individual damage mechanisms could not be easily distinguished the overall damage state can be reliably characterized. Moreover significant increase in the fracture resistance was shown, for both Mode I and Mode II tests in the case of CNT doped laminates, compared against the reference laminate where neat epoxy matrix was used. Finally, low velocity impact tests showed that the CNT doped laminates appear to have reduced damage area based on C-Scan evidences.

Influence of Carbon Nanofibers and Piezoelectric Particles on the Thermomechanical Behaviour of Epoxy Mixtures

Vapor growth carbon nanofibers (CNFs), lead zirconate titanate piezoelectric (PZT) particles, as well as a combination of these two were added in an epoxy resin (EP), and their influence on the curing reaction was investigated. Moreover, the cured samples were characterised by dynamic scanning calorimetry and dynamic thermal mechanical analysis. The presence of the fillers had no significant effect of the curing reaction of the EP system and the glass transition temperature, Tg.

Improved Sensing Properties of Epoxy Resin Polymers and Carbon Fiber Reinforced Epoxy Resin Composites by the Introduction of Multi-Wall Carbon Nanotubes (MWCNT)

The proposed paper will present the manufacturing process of epoxy resin compounds with several multi-wall carbon nanotube (MWCNT) contents per weight and, carbon fibre reinforced polymers with the previous nano-doped epoxy matrix material. Enhanced mechanical properties of the doped specimens both epoxy and carbon reinforced against the neat epoxy testpieces e.g. tensile strength and modulus of elasticity was achieved and attributed to the high surface area and high aspect ratio of the nanotubes. Moreover the dynamic properties of the nano-doped epoxy polymers were investigated and the relation of glass transition temperature with increasing CNT content was found to be inverse. Another goal of the present work was to use the electrical/sensing properties of MWCNTs as a nano- sensor for the damage detection within the matrix material of the CFRPS. Therefore loading-unloading tensile tests were performed, along with on-line conductivity monitoring

Enhancement of the Mechanical Performance of Carbon Fiber Reinforced Epoxy Resin Composites by the Introduction of Carbon Nano-Fibers (CNF)

The proposed paper will present the manufacturing process of CFRP panels with doped epoxy matrix with CNF 1% vol. and their enhanced mechanical properties against the neat. Also a correlation between electrical conductivity and degradation of fatigue properties will be presented. Tensile tests were performed in order to investigate any improvements in the Youngs Modulus. The CNF panel even though had lower Vf, it exhibited greater modulus of elasticity. The next step involved fracture tests in Mode I and Mode II using DCB specimens. It is observed that the toughness increases remarkably after the addition of the CNFs in the matrix of the laminates (100% and 40% increase for Mode I and Mode II, respectively).

The effect of silicon carbide nanoparticles on the multifunctionality of epoxy polymers and CFRPs

In this work the effect of silicon carbide nanoparticles (n-SiC) into an epoxy matrix was investigated. High shear mixing techniques combined with sonication methods were used to homogeneously disperse the Silicon Carbide nanoparticles (Nano-SiC) in bisphenol-A epoxy resin at 1% weight fraction. SEM and AFM were used to evaluate the achieved dispersion in the nanopolymer. Mechanical, thermal and dynamic tests were performed to evaluate the nanopolymer and directly compared with the neat resin. On polymer level the produced materials showed improvement in the mechanical properties reaching up to 25% and 30% in Youngs modulus and failure stress respectively. The nanopolymer exhibited a more brittle behavior through the decrease of the maximum strain at fracture. The thermal properties of the nanocomposite were highly affected leading to an enhancement of the thermal conductivity and thermal effusivity of the material. Meanwhile the glass transition temperature increased up to 28% as measured through DMA tests. The aforementioned material was used as the matrix material in order to produce carbon fibre reinforced panel. The improved properties of the nanopolymer have enhanced the fracture properties of the composite material as the dispersed nanospheres can work as arrestors/deflectors of the propagating cracks through the composite.

Improved Damage Tolerance Properties of Aerospace Structures by the Addition of Carbon Nanotubes

The potential use of carbon nanotubes (CNTs) in aerospace structures is considered in this chapter. Various studies are presented on how carbon nanotubes may be the driving force of a new generation of aerospace structures with superior damage tolerance properties, which in turn will lead to novel composite structures for the aerospace industry. This chapter examines the inclusion of CNTs in aerospace grade resins and their reinforcing mechanisms. The conclusion reached is that the main reinforcing mechanisms of carbon nanotubes are: fibre breakage, fibre pull-out, crack bridging and crazing. These are responsible for the improvement of the mechanical properties of composite materials and their structures. In other words, the use of carbon nanotubes in aerospace composite structures has been proven to increase fracture toughness, impact strength, post-impact properties and the fatigue life of composites, all these attributes making them more damage tolerant. Finally, a new generation of fibres and fabrics with CNTs grafted or grown on them are presented. They are expected to play a key role in evolution of aerospace composite structures, overcoming any processing issues that have risen due to high CNT-polymer viscosities.

Nano-enhanced aerospace composites for increased damage tolerance and service life damage monitoring

This study deals with new generation composite systems which apart from the primary reinforcement at the typical fiber scale (~10 μm) are also reinforced at the nanoscale. This is performed via incorporation of nano-scale additives in typical aerospace matrix systems, such as epoxies. Carbon Nanotubes (CNTs) are ideal candidates as their extremely high aspect ratio and mechanical properties render them advantageous to other nanoscale materials. The result is the significant increase in the damage tolerance of the novel composite systems even at very low CNT loadings. By monitoring the resistance change of the CNT network, information both on the real time deformation state of the composite is obtained as a reversible change in the bulk resistance of the material, and the damage state of the material as an irreversible change in the bulk resistance of the material. The irreversible monotonic increase of the electrical resistance can be related to internal damage in the hybrid composite system and may be used as an index of the remaining lifetime of a structural component.