Toughening composite laminates using hierarchical electrospun fibrous veils

  • Konstantina Kanari

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Composite laminates are an environmentally friendly approach to the development of toughened lightweight structures. If these materials replace the traditionally used metals in the aerospace and automotive industries, they can lead to reduced fuel consumption and fewer carbon emissions.

Composite laminates are materials that consist of laid-up plies, stacked on top of each other. Two of the main disadvantages of the use of composite laminates in load bearing applications are delamination and low fracture toughness. In order to improve these two mechanical properties, tougheners, like fibrous interleaves, are incorporated between layers of the composite laminates. These interleaves can enhance the fracture properties of the composite laminates by acting as fibrous bridges in crack zones, while adding minimal weight to the laminate. Furthermore, nanoparticles can be integrated inside the fibres for the improvement of the fibres’ mechanical properties.

In this thesis, polystyrene (PS) was used for the production of fibrous interleaves, while cellulose nanocrystals (CNCs) were added inside the fibres as reinforcement. The fibres were produced by electrospinning. Firstly, the production of pure PS fibres was optimised by evaluating parameters such as the concentration of the solution, the applied voltage and the tip-to-collector distance. The concentrations that were used were 10, 15 and 20 wt%, the applied voltage varied between 10 and 25 kV and the tip-to-collector distance was set from 10 to 20 cm. Scanning Electron Microscopy and Fourier-Transform Infrared Spectroscopy were used to analyse the external morphology of the fibres. Atomic Force Microscopy was used for the examination of the internal structure of the fibres.

The amount of the added CNCs was chosen as 1, 5, 10 wt % in relation to the adding weight of the polystyrene. Composite fibres were electrospun with a PS solution of 20 wt% concentration, at a tip-to-collector distance and under applied voltage between 12.5 and 17.5 kV. The fibres had both random and aligned orientation, and they were also tested both as-spun and after an annealing step to consolidate them and remove any internal porosity.

Subsequently, the different interleaves were mechanically tested with tensile testing. It was observed that the incorporation of CNCs mostly improved the mechanical properties of as-spun fibres of random orientation, while it didn't significantly affect or decreased the properties of annealed fibres of random orientation. Furthermore, alignment of the fibres showed the most promising results. When the fibres were as-spun, the modulus of the pure PS interleaf was improved by up to 2500% compared to the random orientation interleaf, and the UTS was improved by 300%. When the fibres were annealed, the modulus of pure PS fibres was improved by 220%, and the UTS by 282% compared to annealed fibres of random orientation.

Finally, these veils were incorporated as interleaves in carbon fibre reinforced composite laminates. Their fracture properties were studied with applied axial (Mode I) and shear load (Mode II). It was shown that the fracture toughness of the laminates was improved only in Mode II testing when the interleaf contained fibres of aligned orientation.
Date of Award25 Jan 2022
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorSteve Eichhorn (Supervisor) & Michael R Wisnom (Supervisor)

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