Methods for the exclusion of the fibre bridging effect in composite structures under cyclic Mode I loading

Master thesis (2023)

Authors

I. Boul Boul Aerospace Engineering

Contributors

J.A. Pascoe Structural Integrity & Composites - Aerospace Engineering (mentor)
R.C. Alderliesten Structural Integrity & Composites - Aerospace Engineering (graduation committee member)
Saullo G.P. Castro Aerospace Structures & Computational Mechanics - Aerospace Engineering (graduation committee member)
Faculty
Aerospace Engineering, Aerospace Engineering
Copyright: © 2023 Idriss Boul Boul
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Published Date

05-07-2023

Language

English

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Faculty

Aerospace Engineering

Abstract

In composite structures, delamination damage is typically the most common failure mechanism. Accurately characterizing the delamination behaviour of composite laminates is therefore crucial for predicting the fatigue safe-life of such structures. To this end, double-cantilever beam (DCB) composite specimens are used to measure the interlaminar fracture toughness and delamination growth rate under cyclic Mode I loading.

In unidirectional (UD) composite laminates, delamination planes may exhibit fibre nesting, leading to the development of the fibre bridging effect during delamination growth. This effect, which resists delamination, significantly increases the apparent fracture toughness of the laminate. However, fibre bridging is usually insignificant in multidirectional (MD) laminates, where delamination occurs between plies with different fibre orientations. Nesting typically does not happen in MD laminates. As a result, MD laminates should not be designed using fatigue resistance data obtained from UD specimens without first accounting for the fibre bridging effect. Neglecting fibre bridging exclusion can result in an overestimation of delamination resistance, leading to unsafe failure predictions.

This research investigated methods to exclude the fibre bridging effect in cyclic Mode I experiments with UD composite specimens. Existing literature suggested different approaches to account for this effect, aiming to create a "zero-bridging" fatigue delamination resistance curve. The study examined methods such as cutting bridging fibres in-situ, constant-SERR experiments, specimen-specific extrapolation, and utilizing the Hartman-Schijve equation to describe fatigue delamination.

By examining different exclusion methods and understanding their limitations, this work contributed to enhancing the reliability of fatigue delamination predictions in composite specimens under laboratory conditions. This study compared methods to exclude the fibre bridging effect and assessed their merits in terms of ease of use, accuracy, and conservative predictions of delamination resistance. The results of this study suggest that a specimen-specific extrapolation method is a suitable approach to account for fibre bridging.