Effect of voids on the interlaminar failure of carbon/epoxy composites

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Porosity is a common manufacturing defect in composite materials. It can be caused by inadequate debulk, curing cycles or trapped air, and has significant effects on the matrix-dominated properties of a composite. It is almost impossible to eliminate voids during manufacturing, and they can lead to the development of other defects such as delamination.

Many researchers have investigated the influence of void content on the mechanical performance of composites. However the size, shape and location of voids are important parameters often not characterised. With knowledge of the size, shape and location of voids it is possible to not only qualitatively understand the failure of composites under load, but also to quantitatively compare their effects using numerical modelling techniques.

The main objective of this thesis is to understand the effect of void features on the strength of composite materials, and to identify the void characteristics most influential in affecting the failure of composites. To achieve this goal, it was necessary develop a technique to manufacture samples with a controlled void content. A novel pressure and temperature-controlled method was used, and by varying manufacturing parameters, such as compaction temperature and pressure, it has been shown that samples with a range of void contents can be produced. Furthermore, two different material systems and lay-ups have been investigated.

To characterise the voids, each sample has been analysed by X-ray Computed Tomography, which is a non-destructive technique that is allows the size, morphology and location of every void in a sample to be extracted and post-processed. However, accurate characterisation of the voids requires accurate identification of the void boundary and separation of the void from the surrounding composite material. A new, simple, rigorous, reproducible and accurate CTsegmentation thresholding method is proposed to characterise voids in a wider range of composite systems and with reduced errors than previously known threshold methods.

To characterise the effect of voids on strength, the short beam shear (SBS) test was used, and then results were correlated to the void morphology, size and location. This provided valuable information that helps to better understand the failure behaviour of composites containing void defects, and specifically how the voids influence the failure. Furthermore, a simplified finite element model was developed and compared to the experimental results.

This investigation has been undertaken on two different material systems, revealing useful information on the criticality of particular void features in composites that may be used in more detailed finite element models, such as micro-mechanical models, and may also inform manufacturing tolerances for acceptable engineering design.
Date of Award23 Jan 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorLuiz F Kawashita (Supervisor), Stephen R Hallett (Supervisor) & Robert A Smith (Supervisor)

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