Fatigue of cast aluminium alloys under constant and variable-amplitude loading

Three cast aluminium materials, Al 206, Al 319 and Al 390, were fatigue tested under constant- and variable-amplitude loading. A stress ratio of -1 was used for the constant-amplitude tests. The variable-amplitude load history consisted of underloads followed by constant-amplitude small cycles. The stress ratio and the number of the constant-amplitude small cycles following an underload were adjusted so that the crack did not close and remained fully open for all the small cycles. Underloads reduced the fatigue strength of the alloys by 66-77%. A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behaviour of the cast aluminium material under constant- and variable-amplitude loading. The crack growth analysis was based on an effective strain-based intensity factor, elastic and plastic notch strain calculations based on Neuber's formula, and a reference crack growth rate curve obtained during closure-free crack growth. In modelling the fatigue life behaviour of cast aluminium the flaws were modelled as circular notches having the same diameter as the flaws. The notches were assumed to be edge notches, which represents the flaw location with the most detrimental effect on fatigue life. Fatigue life predictions were in good agreement with experimental results. The fatigue life behaviour for an ideal cast material free of flaws was also predicted using the same model for an unnotched specimen. The cast material was then hipped (the material was subjected to a high pressure at high temperature and then slowly cooled) to eliminate internal flaws. The hipped material showed an increase in fatigue strength of almost 40% under constant-amplitude loading with R = -1, which was in good agreement with the results predicted by the model.