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Abstract
Sandwich composites are lightweight structures with widespread applications in aerospace, offshore and marine industries. The sandwich construction is based on strong, stiff skins, for instance Fibre Reinforced Polymers (FRPs), bonded to either side of a low density core material, such as polymeric foams. However, sandwich structures are susceptible to damage caused by impact. Several researchers have tried to improve the impact resistance of sandwich composites by various methods including toughening the matrix. However toughening the epoxy matrix with micro-rubber particles compromised other favourable properties. Recently, nano-reinforcements such as Carbon Nanotubes, nanoclay and SiO2 nanoparticles have been used to toughen the resin. However, one of the challenges with these nanoparticles is obtaining uniform dispersion. Current research has shown that acrylate triblock copolymers which self-assemble in the nanoscale do not agglomerate and are therefore a promising candidate for improving the impact performance of composites. The objective of this thesis is to investigate the effectiveness of these block copolymers in improving the impact resistance of sandwich structures with FRP facesheets. Normal impact tests are conducted using drop tower impact of sandwich composites made of Kevlar and Glass fibre reinforced epoxy facesheets and Rohacell foam core. The macroscopic behaviour and the microscopic phenomena involved in dissipating impact energy are identified and compared for sandwich plates with and without the nanoparticles. The results from drop tower impact tests show that the addition of 10% M52N Nanostrength substantially improves the impact resistance of the Kevlar- Rohacell sandwich plates. Numerical simulation of the low velocity impact of sandwich plate is conducted using Finite Element Analysis (FEA) software LS-Dyna. Laminated Composite Fabric model (MAT58), a material model based on Continuum Damage Mechanics (CDM) and Hashin failure criteria is used for the facesheets and a Crushable foam model (MAT63) is used for the foam core. The FE models show good correlation with the experimental results. In order to establish that the nano block copolymers are effective in loading conditions other than normal impact, the behaviour of the Kevlar sandwich plate to parabolic impact loading is studied. A modified Stewart platform (Hexapod) was instrumented for parabolic impact testing and the results were compared with numerical simulation.