Effect of nanofillers on thermo-mechanical properties of polymers and composite laminates

Carbon fibre reinforced polymer (CFRP) composites are high performance materials which are widely used in various applications, such as aircraft and aerospace structures, satellites, advanced marine vessels, fuel tanks, sports equipment, high-end automobile structures and many other strength/weight critical applications. It is well known that CFRP composites are stronger in tension (in the fibre direction) than in compression, typically 30-40% higher. This is due to the fact that the compressive strength depends on the properties of the matrix and quality of the laminate, such as alignment of the fibres embedded in the matrix and void content. In theory, stiffer, stronger and tougher matrices provide better support to the carbon fibres (better resistance to fibre instability or microbuckling), hence enhancing the compressive properties of the CFRP composites. The aim of this study is to improve the properties of the CFRP composite by carefully selecting and incorporating nanofillers in the epoxy resin. The nanomodified-epoxy is then combined with continuous carbon fibres, which results in better overall structural response. The thesis is made up of two main parts i.e., examination of the thermal and mechanical properties of nanomodified-epoxies and investigation of mechanical properties of the nanofilled-CFRP composite with an emphasis on compressive behaviour. In the first part, a systematic experimental investigation is conducted in order to identify the optimum content and dispersion of nanofillers in the resin systems to be used in the fabrication of CFRP composite laminates. The effect of silica nanospheres, carbon nanotubes and clay nanoplatelets on the compressive, tensile, flexural and fracture toughness properties of epoxy polymers were studied. Two types of epoxy resin were used: Epikote 828 and Cycom 977-20. In addition, the thermal properties of the nanomodified-epoxies compared to the neat systems were also investigated. The results showed that the addition of nanosilica into the epoxy significantly enhanced the compressive, tensile and flexural moduli. Additionally, strength and fracture toughness properties were also improved without any significant reduction in failure strain and thermal properties of the epoxy. It was found that the mechanical performance of nanosilica-modified Epikote 828 system was comparable to that of the commercial high-performance Cycom 977-20 polymer. The Halpin-Tsai model was modified to include the effect of particle volume fraction on the shape factor ~ that appears in the equation for predicting the Young's modulus of the nanoreinforced-resin. In the second part of the investigation, the effect of nanosilica on the compressive and in-plane shear properties of HTS40/828 CFRP composite was studied. A number of [O]s and [±45b laminates were fabricated using dry filament winding, wet resin impregnation and vacuum bagging techniques. The quality of the laminate such as fibre distribution, fibre misalignment, void content, fibre and nanosilica volume fraction was examined and measured. Static uniaxial compression and tensile tests on [O]s and [±45b laminates were performed. It was found that the compressive and in-plane shear properties of nanomodified CFRP were better than the neat system. For example, the addition of 7 vol% nanosilica improved the unidirectional (UD) compressive modulus and strength of the HTS40/828 composite by 40% and 54%, respectively. The compressive strength was also predicted using several analytical models based on fibre micro buckling and fibre kinking fracture mechanisms. One of the existing fibre microbuckling models was modified in this work to better account for the non-linear resin response. The predicted values showed that the UD nanomodified-FRP laminate exhibited a better compressive strength compared to that of the neat composite system. In addition, the results demonstrated that the performance of the nanosilica-filled HTS40/828 composite was comparable to that of the commercially available HTS40/977-2 system, which is currently used by the aircraft industry.