Improving the mechanical properties of aerospace carbon fibre-epoxy joints by z-pinning

Koh, T 2012, Improving the mechanical properties of aerospace carbon fibre-epoxy joints by z-pinning, Doctor of Philosophy (PhD), Aerospace, Mechanical and Manufacturing Engineering, RMIT University.


Document type: Thesis
Collection: Theses

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Title Improving the mechanical properties of aerospace carbon fibre-epoxy joints by z-pinning
Author(s) Koh, T
Year 2012
Abstract The use of carbon fibre/epoxy laminates and other types of composites in aircraft and helicopters has increased greatly over the past decade. These applications include structural bonded joints and stiffener-skin bonded panels such as ribs, spars and stringers. A long-standing problem with bonded composite structural joints is the low strength and toughness of their bond-line, which makes them susceptible to bond-line cracking under the application of normal (through-thickness) tensile, in-plane secondary bending or impact loads. One promising through-thickness reinforcement method is z-pinning. Z-pins are thin metallic or fibrous rods inserted through the bonded region of a composite joint, and they act as fine nails which hold the adherends together by bridging tractions after the bond-line has fractured.

The general aim of this PhD study is to investigate the structural properties and damage tolerance of z-pinned composite bonded joints. This PhD investigates two types of bonded carbon fibre/epoxy joints commonly used in aircraft (and other engineering) structural composites: T-shaped joints and single lap joints. Specifically, the PhD examines the influences of various parameters on the mechanical performance of bonded T-joints to optimise the z-pinning conditions. The parameters studied include the volume content, diameter and embedded length of the z-pins; the location of the z-pins in the joint; and the direction of the applied load. The PhD also investigates the mechanical performance of z-pinned lap joints, and in particular their creep and stress rupture properties as well as the creep toughening mechanisms.

This thesis presents an experimental investigation into the efficacy of z-pins to improve the structural properties of bonded T-joints under an applied tensile (pull-off) load. This study is the first comprehensive assessment of the effect of z-pin content on the strengthening and toughening of T-joints, and provides important analytical information and experimental data to optimise the z-pinning conditions for maximum structural performance. The study found that z-pinning significantly improves the mechanical performance of T-joints under different pull-off load angles, and when a pre-existing crack is present in the joint. Also, effects of z-pin reinforcement location and z-pin embedded length (i.e. joint thickness) were investigated experimentally and analytically. Lastly, this thesis presents a study into the creep properties, stress rupture and creep mechanisms of single lap joints reinforced with z-pins. The PhD thesis concludes with a summary of the major research findings, a discussion of future research directions into z-pinned composite joints, and the remaining challenges in the certification of z-pinned joints for use in primary aircraft structures.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Aerospace, Mechanical and Manufacturing Engineering
Keyword(s) Composites
Joints
Properties
Z-pins
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Created: Fri, 21 Sep 2012, 11:19:44 EST by Kelly Duong
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