Performance Evaluation of Carbon Nanotubes Modified Fibre Reinforced Polymer Composites

Abstract

The present study is carried out to reinforce glass fibre polymer composites using nanofillers, which facilitate enhanced mechanical performance as well as durability in hygrothermal environment. Also, the study proposes a non-contact, non-invasive, in-situ damage monitoring methodology to identify the defects introduced in composite laminates either during the manufacturing stage, while in-service or after exposure to hygrothermal conditions. The Glass-fibre reinforced polymer (GFRP) composites form an important topic of research and are widely used for preparing components of marine structures such as canoes, fishing trawlers, patrol boats and naval mine-hunting ships. This is attributed to the fact that GFRP composites possess high strength-to-weight ratio, lower initial and maintenance costs, and excellent corrosion as well as thermal resistance as compared to traditional materials such as steel or aluminum. Apparently, the marine components consist of glass fibre mat strongly bonded by means of a thermosetting matrix, usually the epoxy resin which has excellent properties such as low shrinkage after curing, good resistance to corrosion and stability at higher temperatures. However, with recent advancement in technology, the GFRP composites are being replaced with nanocomposites, which have exceptionally high surface-area-to-volume ratio as well as high aspect ratio. For preparing nanocomposites the nano-fillers are dispersed into the matrix during the processing phase and are usually added by weight percentage of the order of 0.5 – 5%. Studies have confirmed that relatively lower loading of nano-fillers is sufficient to obtain composites with desired properties as compared to those obtained with higher loading of micro-fillers. There are several nano-fillers that are incorporated globally to develop multifunctional composite materials with outstanding properties. The commonly used nanofillers are nanoclay, graphene, carbon nanofibers, and carbon nanotubes. Amongst these, carbon nanotubes (CNTs) have been the most promising nanofiller which appear as long and thin cylinders of carbon, when observed under the microscope. Due to their unique combination of physical and multifunctional properties, CNTs have become a strong candidate for use as tailoring agents in various polymer matrices to develop advanced composite materials for the 21st century. In view of these observations, the present study addresses the enhancement in mechanical properties of GFRP composites at the macro-scale by treating the glass fibres with silane coupling agents and at the nanoscale by reinforcing the matrix using carbon nanotubes. It has been observed experimentally that by treating the glass fibre mat with amino and epoxy silane coupling agents, in neat epoxy (EP) matrix composition, the flexural strength improved by 139% and 125%, respectively. This was possibly due to enhanced interfacial bonding between the fibre and the epoxy matrix. The experimental results also revealed that the silanization of glass fibres benefited the flexural properties more than the tensile properties of neat epoxy and CNT modified epoxy matrix compositions. In addition, the present study also reports an effective and stable methodology for dispersion of carbon nanotubes in epoxy resin. Interestingly, for the first time, the pristine nanotube modified (PCNT) GFRP composites have scored over, amino-functionalized nanotube (ACNT) GFRP composites when prepared at stoichiometric ratio. This is a very significant finding since PCNT is available at a much lower cost than ACNT. Further, it has also been observed that dispersion of ACNT in an amine-rich concentration proved advantageous over EP GFRP and PCNT GFRP composites. This may be due to stronger cross-linking network and interfacial bonding that was evident from the composite mechanical behavior and also supported by SEM images. Lastly, for specific processing conditions the composite prepared by dual reinforcement, i.e at the macro-scale by treating fibre surface with amino silane and at the nanoscale by homogeneously dispersing ACNT in amine-rich resin demonstrated considerable increase in mechanical properties as compared to EP GFRP specimen. The composite materials fabricated in this study were designed as a replacement of existing materials used for marine structures. Generally for marine application, the factors affecting the longevity, durability and performance of GFRP components are based on exposure to hygrothermal conditions. The rate of degradation of composite materials when exposed to different environmental conditions depends upon the intensity, duration and type of exposure, which makes it complex and rather difficult to predict. The physical/mechanical/chemical changes due to water absorption weakens the fibre-matrix interfacial bonding, initially visualized as delamination or micro cracking under the microscope, but on prolonged exposure causes failure of the composites. Hence, in order to investigate the performance of manufactured composites under submerged conditions, the EP GFRP, PCNT GFRP and ACNT GFRP samples were exposed to simulated seawater (SW) environment at relatively lower and higher temperatures (25oC and 55oC). Subsequently, the weight gain, diffusivity rate and change in mechanical properties of the conditioned samples were evaluated. From the diffusion studies carried out at 25oC in SW and distilled water (DW), it was observed that the absorbed equilibrium moisture content and diffusivity rate for EP GFRPs was higher in DW than in SW. This has been attributed to the formation of the salt layer on the surface of samples immersed in SW (also depicted in SEM images) which in turn reduced the capillary flow of water into the interface. The plasticizing effect of water on material‘s stiffness and strength was observed to be significant during first 15-20 days but recovered over prolonged aging. Contrarily at elevated temperature, the water content in the EP GFRP composite increased at a faster rate and reached higher levels than observed at room temperature. Further, in order to assess the performance of CNT modified GFRP composites, the PCNT GFRP and ACNT GFRP samples were also exposed to SW environment. The exposure of the ACNT and PCNT GFRP composites at 55oC also resulted in higher water absorption as observed for EP GFRP samples. The excess water content in the composite proved to be detrimental as it induced swelling of the matrix and weakened the fibre-matrix interfacial bonding strength. It also led to early occurrence of damages such as crack formation, stress concentration sites, fibre-delamination, etc., thus affecting the stability and performance of the composite. Interestingly, from the diffusion study carried out at 25oC in SW it was observed that the absorbed water content and diffusivity rate plunged by about 35% due to the presence of CNTs in an epoxy matrix. This is possibly due to the barrier properties of CNTs wherein, the individual CNTs spread to nano-level in the entire material, thus offer a tortuous path to the diffusing water molecules. Further, the flexural properties of ACNT GFRP samples dropped initially, but, showed a recovery after approximately 30 days of immersion and the possible reasons explaining it have been discussed in the chapters ahead. On the contrary, PCNT GFRP samples illustrated a continuous drop in stiffness, strength, and ductility with an increase in aging time in SW. It may be stated that ACNT GFRP composites showed better performance than EP GFRP composites on immersion in SW. Further, from SEM images it was observed that the exposure of GFRP composites in SW (which represents actual service conditions) caused frequent fibre breakage, delamination, cracks and surface abrasion. In addition to service induced damages, some defects (such as air gaps,missing epoxy) were also introduced during manufacturing of GFRP composites. Thus, a health monitoring tool was needed to assess such damages/defects in GFRPs so that the failure of the composite structures could be prevented. Amongst various non-destructive tools (NDT), ultrasonic guided waves had shown a better response to defects in thin-plate-like structures under submerged conditions and also emerged as an effective health monitoring tool. Interestingly, guided wave mode specific for GFRP laminates was excited and the ultrasonic signal was recorded in healthy and defective regions. The leaky Lamb waves that propagated through the GFRP sheet were able to identify, quantify the presence and severity of defects/damages such as air gaps, missing epoxy, notches and surface abrasion. In general, the ultrasonic signal strength dropped for manufactured as well as usage defects in GFRP laminate. The developed methodology was reliable as it showed reproducibility of results on monitoring usage defects (i.e. notches and surface degradation) in CNT GFRP laminate. In addition to this, the developed in-situ, non-contact and non-invasive structural health monitoring technique was also successful in diagnosing defects introduced in EP GFRP sheet after exposure to warm seawater conditions. It is important to point out that, a linear relation between the drop in ultrasonic voltage amplitude, tensile strength, and increasing water uptake content over immersion time was successfully established in this study. The non-destructive damage monitoring technique developed for composite structures in this study allows the inspection of the component without interfering with its service conditions, thus providing an excellent balance between quality control and cost-effectiveness. Therefore, the in-situ, non-contact and non-invasive Lamb wave technique can serve as an effective method to diagnose the service life of a structural component made up of composite while, they are in operational mode.