Abstract

Abstract

A finite deformation hyper-elastic membrane theory based on inter-atomic potentials for crystalline films composed of a single atomic layer is developed. For this purpose, an extension of the standard Born rule that exploits the differential geometry concept of the exponential map is proposed to deal with the curvature of surfaces. The exponential map is approximated locally and strain measures based on the stretch and the curvature of the membrane arise. The methodology is first particularized to atomic chains in two dimensions, and then to graphene sheets. A reduced model for the transverse mechanics of carbon nanotubes is developed in detail. This model is a hyper-elastic constrained membrane which fully exploits the symmetry of the transverse deformation. Additionally, a continuum version of the non-bonded interactions is provided. The continuum model is discretized using finite elements and very good agreement with molecular mechanics simulations is obtained. Finally, several simulations illustrate the strong effect of the van der Waals interactions in the transverse deformation of carbon nanotubes.

Abstract

This work presents a comparison in the production of electrical conductivity polyester resins by doping it with two types of carbonous fillers; carbon nanotubes (CNT) and carbon black (CB). These polyester resins are used to obtain electrical conductivity powder gel coat. This type of coating reduces cycle times in the manufacture of composites due to the fact that significantly decrease curing time of the gel coat and eliminate secondary operations for the application of the final coating in the composite. Electrical conductivity with different fillers content and percolation thresholds obtained in the simulation phase are detailed. The results obtained in the simulation are validated at the pilot plant level taking into account the influence of the loading percentage on the resin processing. Finally, a comparison between the results obtained with the different percentage of CNT and CB is made. The results of the simulation show that the value of the percolation threshold is much lower in the case of CNT (about 3% in the case of CNT versus 35% in the case of CB). The experimental results indicate the strong dependence of the electrical properties on the production process obtaining variations of up to two orders of magnitude in the experimental results using the same percentage of CNT but different type of polyester resin.

Abstract

In the last years, 3D printing is attracting the interest from the industry and scientific community because of the reduction of manufacturing times and machining processes. Furthermore, it is important to develop materials with new functionalities or enhanced properties regarding the currently available materials

In this context, the addition of nanoparticles, such as carbon nanotubes (CNTs), is arousing much interest. More specifically, CNTs have a vast potential in Structural Health Monitoring (SHM) applications, due to their piezoresistive behaviour and tunnelling conduction mechanisms providing a great electrical sensitivity under strain.

The present study aims to exploit those CNT capabilities for SHM purposes, combining them with the 3D printing manufacturing technology. To achieve this, Digital Light Processing (DLP) 3D printing technique has been used to obtain composite materials based on a commercial resin doped with CNTs, by varying the nanoparticles content.

First results obtained for tensile and flexural tests show an electrical sensitivity slightly higher to the typical values of metallic gauges, thus, proving a high potential for strain monitoring applications.

Abstract

EMAA copolymer neutralized with sodium salt is a type of ionomer with self-healing abilities. This polymer has been used as raw material for the manufacturing of nanocomposite material by the addition of multiwall carbon nanotubes (MWCNTs). The optimization of the extrusion process, as well as the mixing and dispersion of the nano-reinforcement in the polymeric matrix during extrusion, deals with the manufacturing of a homogeneous reinforced filament. The obtained filaments were used afterwards as feeding materials of a 3D polymer printing machine and tensile test samples were manufactured. Mechanical properties of both unreinforced and reinforced samples were tested. The MWCNTs-reinforced samples exhibited higher mechanical properties without affecting its self-healing ability.

Abstract

Currently, due to the great advances achieved in medicine, together with those related to materials science and engineering, in recent years there has been a growing interest in the use of flexible sensors for monitoring the human body. The present work shows the development of flexible piezoresistive sensors based on polyvinylidene fluoride (PVDF) reinforced with carbon nanotubes (CNTs). First, the analysis of the electrical properties shows an increase in the electrical conductivity with CNT content. This increase is less pronounced at higher CNT contents due to an electrical network saturation, where the main electrical transport mechanisms are dominated by the CNT aggregates. In this context, the electromechanical analysis shows a higher sensitivity, by means of Gauge Factor, at lower CNT contents due to a higher prevalence of tunneling transport mechanisms between neighboring nanoparticles, which is reflected in a more exponential electromechanical behavior. Finally, the analysis of the electromechanical properties at cycling loading shows an elevated repeatability of the electrical response between consecutive cycles, which is indicative of the high robustness of the manufactured sensors.

Abstract

Abstract

The aim of the research is focused on the manufacture of multiscale reinforced composites with self-monitoring capabilities. In order to achieve it, reference material based on epoxy resin and glass fiber fabric has been modified by the addition of carbon nanotubes which allow the creation of a nanoreinforcement electrically conductive percolation network. The registration of electrical resistance variations due to strain or damage can be used to monitorize these effects caused by the mechanical loads. The research also studies the modification of the electrical properties (electrical conductivity measured in both transverse and longitudinal directions) and interlaminar mechanical properties caused by the use of nanoreinforced resins or surface modified fabrics. Multiscale reinforced composites manufactured allow the detection and partial location of damage caused during the interlaminar strength test. Moreover, differences in sensitivity due to manufacturing method and nanoreinforcement used have been clarified.

Abstract

Through powder metallurgy techniques, it is intended to obtain a metal matrix composite (MMC) of 7075 aluminum alloy reinforced with 1% of multiple wall carbon nanotubes (MWCNTs), which after hot extrusion present mechanical properties similar or even superior to the conventional AA7075 alloy in condition T6. This MMC should be able to be welded by friction stir welding (FSW), thus avoiding the disadvantages presented by the hardenable alloys by precipitation in the welding process. This work focuses on the optimization of the dispersion stage of the MWCNTs in the prealloyed powders of the matrix. For this, three mechanical ball milling strategies are compared. In the first, the pre-alloyed powders and MWCNTs are milled together following a cycle of variation of low energy rotation speeds (LE) (300-200rpm). The second also performs the milling of the mix of powders and nanotubes  by varying the rotation speed in a high energy cycle (HE) (1300-1200rpm). In the third one (H + LE), powders of 7075 are initially milled at high folowing a cycle of high rotation speed (high energy), followed by a low rotation speed stage (low energy) in which the MWCNTs are mixed. From the morphological characterization by SEM and determination of the crystallite size of the powders (XRD), a complete discussion of each of said pathways is carried out.

Abstract

In the present work, a model of electrical properties of composites with inherent sensing properties is proposed. More specifically, the effect of both geometry and dispersion procedure is analyzed for carbon nanostructured polimers, exploring their influence in its electromechanical properties by developing simple analytical methods. In this context, it is observed that a better CNT distribution (by reducing the aggregate ratio) leads to a higher electrical conductivity as well as a higher electrical sensitivity to mechanical deformation. Therefore, the proposed model allows to get a deepr knoweledge about the influence of different processing parametrs in the final electrical properties of the nanocomposite. Moreover, the experimetal testing of self-sensing structures has been carried out by using a carbon nanotube doped adhesive film. More specifically, it has been observed that the proposed nanostructured adhesive presnts an outstanding capability for early damage detection and propagation, being a promising technique in order to complement other available inspection methods. This tests have been validated at coupon level, as well as a subelement one, by using stiffened panels. In addition, it has been proved a higher capability for detecting the presence of possible weak interfaces, demonstrating the applicability of the developed technique.