Oksana Shishkina

Stress distribution around a broken carbon fibre and how it is affected by carbon nanotubes in the interface region

ABSTRACT We performed a numerical study to evaluate the influence of carbon nanotubes (CNTs) on the stress build-up in and around a broken fibre in a single carbon fibre/epoxy composite loaded longitudinally. The CNTs were located in the interface region and mimicked one of the two deposition techniques: 1) direct growth on the fibre or 2) dispersion in the fibre sizing. These techniques are known to produce different alignment, orientation and concentration of CNTs in a composite, which affect stress redistribution upon the fibre failure. The study was performed using a two-scale finite element model based on the embedded regions technique. The most significant effect on the stress build-up was found for CNTs aligned in the fibre direction. In this case, the crack opening displacement and the ineffective length were reduced by 15% and 28%, respectively, thus making the stress recovery in the broken fibre faster. In addition, the shear stress at the fibre-matrix interface decreased by a factor of two. These results indicate that hybridization of microscopic fibres with nanotubes aligned in the fibre direction is a promising strategy to influence the process of fibre fragmentation in a composite. Graphical Abstract

Modeling of elastic properties of cell-wall material in nanoclay-reinforced foams

A new micromechanical model that predicts anisotropic elastic properties of cell-wall material in nanoclay-reinforced polymeric foams is proposed. The model accounts for the reorientation of nanoclays during the foaming process, due to the stretching of the foam cell walls. Elastic moduli of bulk nanocomposites were studied first. It was found that given the high aspect ratio of nanoclays, they cannot be randomly placed in a representative volume element without having them intersect each other at high weight fractions. At 1.5 wt% the nanoclays already align and form clusters resulting in a significant effect on the stiffness of the nanocomposite. The number of silicate layers per cluster stack was also found to influence the Young’s modulus of the nanocomposite. When reinforcing polypropylene (PP) with 2 wt% of nanoclays, the modulus of elasticity decreased by 17% when the number of clays in a cluster went from 1 (fully exfoliated system) to 5. The stretching of the nanocomposite was shown to reorient nanoclays and, consequently, to change the stiffness of the cell wall. For the PP reinforced with 1 wt% of nanoclays and a stretching ratio of 6.5, the modulus increased by 15%.