The following dissertation details the development of a simple defect-tolerant based model for determining the fatigue properties of a 5383-H321 aluminium alloy and metal inert gas and friction stir welded butt joints in the same alloy. The model considers the fatigue life to consist of three regimes, crack initiation and short and long crack growth, in contrast to the typical defect-tolerant approaches which only consider the long crack growth period. Crack initiation was considered to consist of an initial short crack, present prior to fatigue loading, identical in length to the crack initiator. The short crack growth rate was considered to be a function of the crack length and stress amplitude, whilst the long crack growth regime was described through the Paris equation, dcldN = C(AK)m. The model also considers the effect on the crack initiation and growth rates of the microstructure, macrostructure and residual stresses, with the latter being determined both through the conventional hole-drilling approach and the emerging technique of synchrotron strain scanning. The accuracy of the model was verified through integration of the short and long crack growth regimes, with the predicted lifespan being compared to results obtained from S-N testing of identical welded specimens. A good correlation between the experimental and theoretical results was observed for the parent material and friction stir welded joints for lives in the region 105-106 cycles, although the approach, in its current format, appeared less suitable for determining the fatigue properties of the MIG welded joints. 2

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