In this paper, the effect of adding silica nanoparticle to epoxy, silica weight percent, particle size and various combinations of epoxy/silica on Young’s modulus, yield strength and energy absorption capability of thin-walled square columns was investigated. Two different sizes of silica nanoparticles, nominally 17 nm and 65 nm in diameter, were used. Nanosilica particles were dispersed almost homogeneously in the epoxy resin from 1.5 wt.% to 6 wt.% in three series of composites. First and second series were composites reinforced with 17 nm and 65 nm particle size, respectively and third series were composites reinforced with combination of both particle sizes. All specimens were tested under quasi-static loading using a servohydraulic Instron machine. A scanning electron microscopy (SEM) was used for fracture surface studies. The results showed that when silica weight percent was increased, the Young’s modulus increased, yield strength remained constant and energy absorption capability of columns decreased, and specimens collapsed under unstable and dangerous mode. It was observed that the reason for this type of collapse was initiation of axial cracks at an early stage of the loading and propagation along the specimen height. Energy absorption capability for columns with height of 60 mm and 90 mm was also investigated and results showed that this parameter for shorter specimens was higher in the same combination of a nanocomposite. Moreover, the effect of particle size on Young’s modulus, yield strength and energy absorption capability was not considerable. Using both particles in a combination did not show any important synergy effect. And finally, fracture surfaces of the specimens showed that surface roughness was increased by increasing the silica nanoparticles.

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