Abstract
Curing rates of an epoxy amine system were varied via different curing cycles, and glass-fiber epoxy composites were prepared using the same protocol, with the aim of investigating the correlation between microstructure and composite properties. It was found that the fast curing cycle resulted in a non-homogenous network, with a larger percentage of a softer phase. Homogenized composite properties, namely storage modulus and quasi-static intra-laminar shear strength, remained unaffected by the change in resin microstructure. However, fatigue tests revealed a significant reduction in fatigue life for composites cured at fast curing rates, while composites with curing cycles that allowed a pre-cure until the critical gel point, were unaffected by the rate of reaction. This result was explained by the increased role of epoxy microstructure on damage initiation and propagation in the matrix during fatigue life. Therefore, local non-homogeneities in the epoxy matrix, corresponding to regions with variable crosslink density, can play a significant role in limiting the fatigue life of composites and must be considered in the manufacturing of large scale components, where temperature gradients and significant exotherms are expected.
Highlights
High performance thermoset composites are ubiquitous all around us
It is essential to study the microstructure of the matrix and understand how local non-homogeneities in the epoxy can affect the overall performance of the composite
We propose changing the microstructure by modifying the curing rate of the epoxy amine reaction
Summary
High performance thermoset composites are ubiquitous all around us. They are selected due to their light weight, high glass transition temperature, high strength and great chemical resistance [1,2,3]. Several studies have examined the microstructure of the epoxy network to get a better idea of the way gelation occurs [21,22,23,24] In each of these studies, a two-phase structure was observed, consisting of a highly cross-linked, high density nodular phase, interspersed with a low density, softer phase. Studies on varying the stoichiometry of the epoxy amine reaction show either a decrease in mechanical properties or a decrease in the glass transition temperature (Tg ), due to the presence of excess amine or epoxy [22,25,26] This does not present an effective way of tuning the microstructure of the epoxy, especially in the case of structural composites where high strength and Tg are required. This work involves studying the effect of three different curing cycles on the evolution of the epoxy network and the implication it has on the final composite properties
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