Fatigue Behaviour of 6061 and Its Composite

By

Ping Hwa LIM, BEng (Honours)

This thesis is submitted to Dublin C ity University as the fulfilm ent o f the requirement for the award o f the degree o f

Master in Engineering

Supervisor: Dr. Lisa Looney

School of Mechanical and Manufacturing Engineering Dublin City University September 2001 DECLARATION

I hereby certify that this material, which I now submit for assessment on the programme of study leading to the award of Master in Engineering, is entirely my own work and has not been taken from the work of others save and to the extent that such work has been cited and acknowledge within the text of my work.

Date: 23rd September 2001

Ping Hwa LIM ID: 98970917 Acknowledgements

I would like to take this opportunity to acknowledge the contribution of those who assisted me in completing this project. In particular:

My academic supervisor, Dr. Lisa Looney of Dublin City University (DCU) for her guidance, support, encouragement, time and limitless energy for useful discussion and thesis evaluation.

Professor Saleem Hashmi of the School of Mechanical and Manufacturing Engineering DCU, for the funding, without which this project would not have been possible.

Christopher Crouch, Jim Barry, Keith Hickey, Liam Domican, Martin Johnson, Michael May and all the technical and academic staff involved in this work, for their assistance and providing me with all I requested.

All the postgraduates within the department of Mechanical and Manufacturing Engineering, Dublin City University. Special thanks to Amy O’Callaghan, Annmarie Byrne, Anthony Comer, Brian O'Sullivan, Clint Carolan, David Chua and Joseph Stokes for their assistance, support and friendship.

Finally, to my parents, brothers and sisters for their endless source of encouragement and optimism; thanks. Abstract Fatigue Behaviour of 6061 and Its Composite

Ping Hwa LIM, BEng (Honours)

Fatigue behaviour of an artificial aged powder metallurgy 6061 aluminium alloy, and a composite made of this alloy with 15% volume fraction of SiCp was investigated. The alloy was subjected to T6 heat treatment, as was the composite material chosen (which incorporated SiC particles of average size 30pm). An extensive experimental programme was carried out in which fatigue lives were determined using load-controlled axial loading of unnotched cylindrical samples, at stress ratios of-1 and 0.1. Tensile properties of both materials were also established.

The tensile behaviour of the alloy is in agreement with other published data for this material, and the significant influence of mean stress on fatigue behaviour found, is consistent with that normally observed in metals. The relationship between number of cycles to failure and stress found for the alloy is within the range of scatter usually seen in such tests, and corresponds with available data. However, it has been found that while stiffness of the composite material is higher than that of the matrix, tensile strength and fatigue life are lower in the composite, as is ductility. At each stress ratio, the composite material exhibits shorter fatigue life at low and intermediate stresses, compared with the unreinforced aluminium alloy. While a number of these trends are consistent with expectations for ceramic reinforced metal composites, the relatively low strength of the materials is not.

Standard metallographic techniques, as well as fractographic observation under a scanning electron microscope were used to further investigate material behaviour. Particularly large particles of SiC are identified as having contributed to fracture behaviour, along with poor interfacial properties. Table of Contents

Page

Title Page i Declaration ii Acknowledgements Hi Abstract iv Table of Contents v

CHAPTER 1: INTRODUCTION ;