Abstract
Syntactic foams comprising hollow glass microspheres (GMS) in an epoxy matrix are critical materials for lightweight structures, being extensively used in marine and aerospace as cores for composite sandwich panels. They are buoyant and crush resistant, but their use is limited by their brittleness. Milled carbon fibres (MCF) were used to increase toughness, by introducing energy absorption mechanisms, to foams comprising ∼60 vol% GMS. Weight ratios of up to 40% MCF:GMS were used. The tensile modulus of the foams increased from 3.36 GPa to 5.41 GPa with the addition of 40% weight ratio of MCF. The tensile strength of the syntactic foam decreased with low loadings of MCF, but then recovers when more MCF particles are added, and the mechanisms responsible are explained for the first time. The fracture energy of the syntactic foam increased by 183%, from 182 J/m2 to 516 J/m2, due to the addition of 40% weight ratio of MCF. Toughening mechanisms were identified as crack deflection, debonding and subsequent plastic void growth, and fibre pull-out. Thus, the simple and cheap addition of MCF greatly increases the toughness of the syntactic foams, enabling lighter or more damage-resistant structures to be produced.
Highlights
The syntactic foams modified with low volume fractions of microfibres showed very little or no increase in tensile failure strength, which is a similar observation to studies which modified bulk polymer matrices [17-19]
This study investigates the effect of milled carbon fibre (MCF) particles on the tensile and fracture properties of a syntactic foam
Syntactic foams comprising hollow glass microspheres in an epoxy matrix are extensively used in lightweight structures for marine and aerospace, often as the core for sandwich composite panels, but their use in limited by their brittleness
Summary
The syntactic foams modified with low volume fractions of microfibres showed very little or no increase in tensile failure strength, which is a similar observation to studies which modified bulk polymer matrices (i.e. with no hollow glass microspheres) [17-19]. These studies even report a decrease in tensile failure strength at very low volume fractions of fibres, followed by a recovery at higher volume fractions. The high modulus and low density of carbon fibre, especially when compared to glass fibres, makes it an ideal candidate for lightweight applications of this type of syntactic foam Such short fibres are very effective at introducing toughening mechanisms into epoxy composites. This work demonstrates that the difficult problem of simultaneously improving the strength and toughness of these materials can be solved by the addition of milled carbon fibre, providing safer and lighter structures enabling more efficient vehicles to be produced
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