| Summary: | Fatigue is one of the common types of failures in engineering components. It is defined as the failure of a component under repetitive loading. As the research on fatigue progresses over time, the variety and number of factors contributing to fatigue have developed as well. From the early interest concerning mechanical and materials behaviour, factors such as heat, corrosion, surface finish have also been taken into consideration. This study aims to investigate the interaction between two factors of interest, residual stress and cold work and to correlate their effects to fatigue performance.
Residual stress is defined as the stress that is still present in the equilibrium of the materials when all the loading from machining process are removed or completed. Compressive residual stress has been linked to fatigue life enhancement by inducing crack closure and delaying crack initiation. On the other hand, cold work is defined as the strengthening of a metal by plastic deformation due to dislocations generation and movement within the crystal structure itself. Residual stress and cold work are introduced simultaneously through surface treatment process such as shot peening and deep cold rolling. As a result of that, the effects of residual stress and cold work are hard to discern and differentiate. In addition, the investigation of the already intricate interaction between residual stress and cold work is made even more difficult by residual stress relaxation.
This thesis focused on the correlation between residual stress and cold work to further determine their influence on fatigue life. The investigation begins with materials characterisation and fatigue test. Next, emphasize is given to mechanical cyclic relaxation and thermal relaxation as stress relaxation is an inevitable phenomenon during fatigue. The use of neutron diffraction to investigate the mechanical stress relaxation will be highlighted. A modified Zener-Wert-Avrami method has also been developed to model the thermal stress relaxation.
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