Corrosion Fatigue of an Aluminium Alloy

Abstract

The corrosion fatigue behaviour of a high strength weldable aluminium-zinc-magnesium alloy (Hiduminium 48) has been investigated under different loading conditions. Further studies were carried out in two regions of the weld heat affected zone. The influences of cycling frequency and waveform shape on crack growth rate in salt water were related to a single parameter, the stress intensity rise time. Long stress intensity rise times enhanced environmental fatigue crack growth rete, while short rise times reduced environmental influence. The relationship between crack growth rate and stress intensity amplitude ( ΔK) was not linear mt followed a two stage process in which the transition from the first to the second stage was affected by environment and yield strength. Decreases in yield strength increased the ΔK factor at which the transition occurred while increase in environmental severity reduced the transition. Crack growth rates in the heat affected zone were up to one order of magnitude slower than in the parent material. However by using the factor ΔK/Ko-Kmin (Q) the different crack growth rates resulting from material toughness and stress ratio effects were normalised to a single design curve. Use of the Q factor showed that in the heat affected zone the influence of salt water on crack growth rate was independent of stress ratio. The mechanism of fatigue crack growth was dependent on ΔK and environments Under low stress intensity factors growth was associated with cleavage-like step wise growth, which was enhanced by the presence of salt water. At medium values of ΔK, in air, growth was controlled by a ductile striation mechanisms In both the parent material and heat affected zone at high ΔK crack growth was predominantly by microvoid coalescence. The occurrence of this mechanism was also independent of the environment used.