Improvement of Interfacıal Toughness of Layered Composıtes By Usıng Electrostatıc Flockıng Technıque

Abstract

Fiber-reinforced composites have remarkable strength to weight ratio. Hence, their areas of usage are increasing. Solving the delamination problem that affects service life of layered composites has gained importance. As a solution, addition of toughening particles or interleaves (in the form of films or nanofibrous veils laid parallel to the layers) to the brittle interlayer matrix material have been proposed. Z-pinning applied perpendicular and across the whole lamina is another method. It is known that nanofibers of the veils or the pins activate bridging mechanisms between adjacent layers during crack propagation. For the same purpose, perpendicular placement of small chopped fibers (flock) to the interface was experimented by electrostatic flocking, which is mechanism-wise similar to the z-pinning method, but akin to interleaving with veils in terms of the location of modification. In previous studies, the possible advantage of using shorter Nylon 66 (N66) flocks in interlayer modification was demonstrated using 1.3 mm flocks. In this study, it is hypothesized that the use of flocks with sizes approaching the similar size scale of the hills and troughs forming among the twills of the epoxy impregnated carbon fiber fabric (prepreg) can increase the effectiveness of the flocks in their vertical bridging positions. In addition, commercial flocks which have been used at the composite interface had never been functionalized with the purpose of improving the matrix – flock interface, as far as the available literature concerned. Here, 0.4 mm long 0.9 dtex N66 flocks, with or without an amino silane modification, were coated on prepreg surfaces with variable densities and vertical alignments under the effect of different voltage, time and distance values during flocking. In the control of areal density and obtaining maximum vertical alignment, the total flight distance was found to be more effective than the other process parameters. Mode I interlaminar toughness values of the produced composites were determined. The amino silane-treated N66 flocks which showed the highest increases showed an 18% increase in the GIC initiation value and a 35% increase in the GIC propagation value relative to the unflocked reference sample. Examination of the delaminated surfaces after double cantilever beam (DCB) tests with scanning electron microscope (SEM) revealed the fact that N66 flocks were able to activate a debonding based toughening mechanism, but prone to easy-peeling due to insufficient chemical interaction with the matrix epoxy. On the other hand, amino functionalities in the modified flocks increased the debonding resistance and led to bridging mode failure upon this debonding that was becoming harder. With further modifications to the surface treatment and mechanical test procedures, higher gains in interlaminar toughness values can be expected.