On the efficiency of interleaves in carbon fibre/epoxy composite laminates by the fractal approach

Abstract

It is well documented in the literature how vulner- able laminates composed of unidirectional (UD) carbon fibre/epoxy (CF/EP) prepregs are to trans- verse single or multiple bounce low energy impacts [1-3]. Interleaving, i.e. incorporation of a non- reinforced thin adhesive layer between the prepregs, has been found to be effective to suppress the delamination caused by transverse static or dynamic loading. The adhesive layer (A) or interleaf in CF/ EP composites is generally a modified EP resin, in some cases coated on a textile carrier fabric. The efficiency of the interleaving can be evaluated in different ways. The techniques used are related to one or more of the following: determination of the onset of property degradation, evaluation of the residual mechanical properties after low-energy impact, and direct assessment of the delamination damage by (quasi) non-destructive methods. All of these tests are time-consuming and, in addition, they require skilled personnel. The aim of the study reported here was to find a simple and reliable way of ranking different interleaves in CF/EP laminates. In a damage cone developed due to low-energy transverse impact across the thickness of a laminate the following individual failure events can be resolved: delamination and matrix cracking. From the viewpoint of the latter, shear and transverse type matrix cracking can be distinguished (Fig. 1). It was reported earlier that interleaving suppresses the delamination caused by transverse loading, and thus the cone of damage becomes narrower [4, 5]. At the same time, however, the crack density in the EP